Condensed Matter
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New submissions for Tue, 2 Dec 08
- [1] arXiv:0812.0004 [ps, pdf, other]
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Title: Ultracold gases far from equilibriumAuthors: Thomas GasenzerComments: 59 pages, 26 figures; Compiled from notes for lectures held at 46. Internationale Universitaetswochen fuer Theoretische Physik 2008 in Schladming, Austria. To be published in Eur. Phys. J. Special TopicsSubjects: Other (cond-mat.other)
Ultracold atomic quantum gases belong to the most exciting challenges of modern physics. Their theoretical description has drawn much from classical field equations. These mean-field approximations are in general reliable for dilute gases in which the atoms collide only rarely with each other, and for situations where the gas is not too far from thermal equilibrium. With present-day technology it is, however, possible to drive and observe a system far away from equilibrium. Functional quantum field theory provides powerful tools to achieve both, analytical understanding and numerical computability, also in higher dimensions, of far-from-equilibrium quantum many-body dynamics. In the article, an outline of these approaches is given, including methods based on the two-particle irreducible effective action as well as on renormalisation-group theory. Their relation to near-equilibrium kinetic theory is discussed, and the distinction between quantum and classical statistical fluctuations is shown to naturally emerge from the functional-integral description. Example applications to the evolution of an ultracold atomic Bose gas in one spatial dimension underline the power of the methods. The article is compiled from the notes for lectures held at 46. Internationale Universitaetswochen fuer Theoretische Physik 2008 in Schladming, Austria.
- [2] arXiv:0812.0015 [ps, pdf, other]
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Title: Andreev Bound states as a phase sensitive probe of the pairing symmetry of the iron pnictide superconductorsComments: 5 pages, 5 figuresSubjects: Superconductivity (cond-mat.supr-con)
A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_\pm$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special phase sensitive probe that is also momentum selective. We show that the sign structure of $s_\pm$ pairing leads to surface Andreev bound states at the sample edge. In the clean limit they only occur when the edge is along the nearest neighbor Fe-Fe bond, but not for a diagonal edge or a surface orthogonal to the c-axis. In contrast to d-wave Andreev bound states, they are not at zero energy and, in general, do not produce a zero bias tunneling peak. Consequences for tunneling measurements are derived, within a simplified two band model and also for a more realistic five band model.
- [3] arXiv:0812.0016 [ps, pdf, other]
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Title: Global Phase Diagram for Magnetism and Lattice Distortion of Fe-pnictide MaterialsComments: 8 pages, 7 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
We study the global phase diagram of magnetic orders and lattice structure in the Fe-pnictide materials at zero temperature within one unified theory, tuned by both doping and pressure. On the low doping and high pressure side of the phase diagram, there is one single transition, which is described by a z = 2 mean field theory with very weak run-away flows; on the high doping and low pressure side the transition is expected to split to two transitions, with one O(3) spin density wave transition followed by a z = 3 quantum Ising transition at larger doping. The fluctuation of the shear strain field which linearly couples to the Ising nematic order parameter effectively increases the dimension, and hence drives the nematic transition more meanfield like, as observed experimentally in BaFe_{2-x}Co_xAs_2.
- [4] arXiv:0812.0024 [ps, pdf, other]
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Title: Electronic Transport at Low Temperatures: Diagrammatic ApproachComments: Submitted to Physics E (FMQT08 Proceedings). Requires Elsevier style file (included)Subjects: Statistical Mechanics (cond-mat.stat-mech)
We prove that a diagrammatic evaluation of the Kubo formula for the electronic transport conductivity due the exchange of bosonic excitations, in the usual conserving ladder approximation, yields a result consistent with the Boltzmann equation. In particular, we show that an uncontrolled approximation that has been used to solve the integral equation for the vertex function is unnecessary. An exact solution of the integral equation yields the same asymptotic low-temperature behavior as the approximate one, albeit with a different prefactor, and agrees with the temperature dependence of the Boltzmann solution. Examples considered are electron scattering from acoustic phonons, and from helimagnons in helimagnets.
- [5] arXiv:0812.0026 [ps, pdf, other]
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Title: Temperature dependent Raman and x-ray studies of spin-ice pyrochlore $Dy_2Ti_2O_7$ and non-magnetic pyrochlore $Lu_2Ti_2O_7$Authors: Surajit Saha, Surjeet Singh, B. Dkhil, S. Dhar, R. Suryanarayanan, G. Dhalenne, A. Revcolevschi, A. K. SoodComments: 28 pages, 9 figures (Accepted for publication in Physical Review B)Subjects: Materials Science (cond-mat.mtrl-sci)
We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound $Dy_2Ti_2O_7$ and its non-magnetic analogue $Lu_2Ti_2O_7$. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in $Dy_2Ti_2O_7$, appearing below a temperature of $T_c=$110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O$^{18}$-isotope substituted $Dy_2Ti_2O_7$ confirm the phonon origin of the "new" mode. These findings, absent in $Lu_2Ti_2O_7$, suggest that the room temperature cubic lattice of the pyrochlore $Dy_2Ti_2O_7$ undergoes a "subtle" structural transformation near $T_c$. We find anomalous \textit{red-shift} of some of the phonon modes in both the $Dy_2Ti_2O_7$ and the $Lu_2Ti_2O_7$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.
- [6] arXiv:0812.0048 [ps, pdf, other]
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Title: Fabrication of graphene nanoribbon by local anodic oxidation lithography using atomic force microscopeComments: 4 pages, 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
We conducted local anodic oxidation (LAO) lithography in single-layer, bilayer, and multilayer graphene using tapping-mode atomic force microscope. The width of insulating oxidized area depends systematically on the number of graphene layers. An 800-nm-wide bar-shaped device fabricated in single-layer graphene exhibits the half-integer quantum Hall effect. We also fabricated a 55-nm-wide graphene nanoribbon (GNR). The conductance of the GNR at the charge neutrality point was suppressed at low temperature, which suggests the opening of an energy gap due to lateral confinement of charge carriers. These results show that LAO lithography is an effective technique for the fabrication of graphene nanodevices.
- [7] arXiv:0812.0055 [ps, pdf, other]
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Title: Membrane simulation models from nm to $\mu$m scaleAuthors: Hiroshi NoguchiComments: 10 pages, 13 figures, review articleSubjects: Soft Condensed Matter (cond-mat.soft)
Recent developments in lipid membrane models for simulations are reviewed. To reduce computational costs, various coarse-grained molecular models have been proposed. Among them, implicit solvent (solvent-free) molecular models are relatively more coarse-grained and efficient for simulating large bilayer membranes. On a $\mu$m scale, the molecular details are typically negligible and the membrane can be described as a continuous curved surface. The theoretical models for fluid and elastic membranes with mesh or meshless discretizations are presented. As examples of applications, the dynamics of vesicles in flows, vesicle formation, and membrane fusion are presented.
- [8] arXiv:0812.0056 [ps, pdf, other]
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Title: Phenomenological theory of phase transitions in epitaxial BaxSr(1-x)TiO3 thin filmsComments: 8 pages 5 figuresSubjects: Superconductivity (cond-mat.supr-con); Other (cond-mat.other)
A phenomenological thermodynamic theory of BaxSr(1-x)TiO3 (BST-x) thin films epitaxially grown on cubic substrates is developed using the Landau-Devonshire approach. The eighth-order thermodynamic potential for BT single crystal and modified fourth-order potential for ST single crystal were used as starting potentials for the end-members of the solid solution with the aim to develop potential of BST-$x$ solid solution valid at high temperatures. Several coefficients of these potentials for BT were changed to obtain reasonable agreement between theory and experimental phase diagram for BST-x (x > 0.2) solid solutions. For low Ba content we constructed the specific phase diagram where five phases converge at the multiphase point (T_N2 = 47 K, x = 0.028) and all transitions are of the second order. The "concentration-misfit strain" phase diagrams for BST-x thin films at room temperature and "temperature-misfit strain" phase diagrams for particular concentrations are constructed and discussed. Near T_N2 coupling between polarization and structural order parameter in the epitaxial film is modified considerably and large number of new phases not present in the bulk materials appear on the phase diagram.
- [9] arXiv:0812.0079 [ps, pdf, other]
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Title: Impurity crystal in a Bose-Einstein condensateComments: 5 pages, 3 color figures; Accepted to Phys. Rev. LettSubjects: Statistical Mechanics (cond-mat.stat-mech); Other (cond-mat.other)
We investigate the behavior of impurity fields immersed in a larger condensate field in 1, 2, and 3 dimensions. We discuss the localization of a single impurity field within a condensate and note the effects of surface energy. We derive the functional form of the attractive interaction between two impurities due to mediation from the condensate. Generalizing the analysis to $N$ impurity fields, we show that within various parameter regimes a crystal of impurity fields can form spontaneously in the condensate. Finally, the system of condensate and crystallized impurity structure is shown to have nonclassical rotational inertia, which is characteristic of superfluidity, i.e. the system can be seen to exhibit supersolid behavior.
- [10] arXiv:0812.0086 [pdf]
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Title: Isotope effect on the E2g phonon and mesoscopic phase separation near the electronic topological transition in Mg1-xAlxB2Authors: L. Simonelli, V. Palmisano, M. Fratini, M. Filippi, P. Parisiades, D. Lampakis, E. Liarokapis, A BianconiComments: 17 pages, 5 figuresSubjects: Superconductivity (cond-mat.supr-con)
We report the boron isotope effect on the E2g phonon mode by micro Raman spectroscopy, on the ternary Mg(1-x)Al(x)B(2) system. The E2g phonon mode shows an isotope coefficient near ~ 0.5 for samples synthesized with boron isotopes 10 and 11. The E2g mode shows the Kohn anomaly related with the electronic topological transition (ETT) around x=0.3 where the sigma band changes from 2D to 3D. We have detected the splitting of the E2g phonon frequency related with a phase separation associated with the proximity to the ETT and the anisotropic internal chemical pressure. Finally the electron-phonon (e-ph) coupling for the electrons in the sigma band has been extracted from the E2g line-width and frequency softening. The results suggest a minor role of the intraband phonon mediated pairing in the control of the high critical temperature in Mg(1-x)Al(x)B(2). The present results and the unconventional multiband superconductivity in this multilayer system show the similarity with the novel FeAs-based superconductors.
- [11] arXiv:0812.0095 [ps, pdf, other]
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Title: Atomistic modeling of the electrostatic and transport properties of a simplified nanoscale field effect transistorAuthors: Li-Na Zhao, Xue-Feng Wang, Zhen-Hua Yao, Zhu-Feng Hou, Marcus Yee, Xing Zhou, Shi-Huan Lin, Teck-Seng LeeComments: 8 pages, 5 figuresJournal-ref: J. Comput. Electron. vol. 7, 500 (2008)Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
A first-principle model is proposed to study the electrostatic properties of a double-gated silicon slab of nano scale in the framework of density functional theory. The applied gate voltage is approximated as a variation of the electrostatic potential on the boundary of the supercell enclosing the system. With the electron density estimated by the real space Green's functions, efficient multigrid and fast Fourier Poisson solvers are employed to calculate the electrostatic potential from the charge density. In the representation of localized SIESTA linear combination of atomic orbitals, the Kohn-Sham equation is established and solved self-consistently for the wavefunction of the system in the local density approximation. The transmission for ballistic transport across the atomic silicon slab at small bias is calculated. The charge distribution and electrostatic potential profile in the silicon slab versus the gate voltage are then analyzed with the help of the equivalent capacitive model. Quantum confinement and short gate effects are observed and discussed.
- [12] arXiv:0812.0099 [ps, pdf, other]
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Title: The Zero Temperature Phase Diagram of the Kitaev ModelComments: 7 pages with 3 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)
We show that the zero temperature phase diagram of the vortex free sector of the Kitaev model is in one to one correspondence with that of the classical dimer model on the corresponding lattice. We find that the model generically has three distinct phases. On a honeycomb lattice with a $3\times3$ fundamental domain all three phases are accessible. As the couplings are varied there are two distinct transitions. The new transition is one to a gapped phase that opens up in the interior of the $B$ phase of the Kitaev model.
- [13] arXiv:0812.0101 [pdf, other]
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Title: Graphene in the Quantum Hall Regime: Effects of Vacancies, Sublattice Polarization and DisorderComments: 8 pages, 6 figures, to be published in the Int. Journal of Modern Physics B - Proceedings of the HMF18Subjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)
We investigate the effects of vacancies, disorder and sublattice polarization on the electronic properties of a monolayer graphene in the quantum Hall regime. Energy spectra as a function of magnetic field and the localization properties of the states within the graphene Landau levels (LLs) are calculated through a tight-binding model. We first discuss our results considering vacancies in the lattice, where we show that vacancies introduce extra levels (or well-defined bands) between consecutive LLs. An striking consequence here is that extra Hall resistance plateaus are expected to emerge when an organized vacancy superlattice is considered. Secondly, we discuss the anomalous localization properties we have found for the lowest LL, where an increasing disorder is shown to enhance the wave functions delocalization (instead of inducing localization). This unexpected effect is shown to be directly related to the way disorder increasingly destroys the sublattice (valley) polarization of the states in the lowest LL. The reason why this anomalous disorder effect occurs only for the zero-energy LL is that, in absence of disorder, only for this level all the states are sublattice polarized, i.e., their wave functions have amplitudes in only one of the sublattices.
- [14] arXiv:0812.0103 [ps, pdf, other]
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Title: Modeling the isotropic/smectic-C tilted lamellar liquid crystalline transitionComments: manuscript submitted for presentation at 8th World Congress of Chemical Engineering August 23-27, 2009, Montreal, Quebec, CanadaSubjects: Soft Condensed Matter (cond-mat.soft); Materials Science (cond-mat.mtrl-sci)
Extensions of a previously presented Landau-de Gennes type liquid crystalline phase transition model for the direct isotropic/smectic-A (lamellar) a phase transition to the direct isotropic/smectic-C (tilted lamellar) transition are studied. Two different proposed extensions to the model are studied both in the context of ideal ordering (point volume) and full three-dimensional uniaxial scalar/vector decomposition. Recommendations based upon the inclusion of essential physics and computational feasibility are made, distinguishing each of the two proposed extensions based upon these criteria. Additionally, it is found that the approach used for the isotropic/smectic-A model to deterministically compute phase diagram data is not possible for either of the two proposed isotropic/smectic-C models.
- [15] arXiv:0812.0109 [ps, pdf, other]
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Title: Path integral analysis of Jarzynski's equality: Analytical resultsComments: 10 pages, 1 figureSubjects: Statistical Mechanics (cond-mat.stat-mech)
We apply path integrals to study nonequilibrium work theorems in the context of Brownian dynamics, deriving in particular the equations of motion governing the most typical and most dominant trajectories. For the analytically soluble cases of a moving harmonic potential and a harmonic oscillator with time-dependent natural frequency, we find such trajectories, evaluate the work-weighted propagators, and validate Jarzynski's equality.
- [16] arXiv:0812.0122 [ps, pdf, other]
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Title: Geometric strong segregation theory for compositionally asymmetric diblock copolymer meltsComments: 5 pages, 1 figureSubjects: Soft Condensed Matter (cond-mat.soft); Materials Science (cond-mat.mtrl-sci)
We have identified the effect of the Wigner-Seitz cell geometry in the strong segregation limit of diblock copolymer melts with strong composition asymmetry. A variational problem is proposed describing the distortions of the chain paths due to the geometric constraints imposed by the cell shape. We computed the geometric excess energies for cylindrical phases arranged into hexagonal, square, and triangular lattices and explicitly demonstrated that the hexagonal lattice has the lowest energy for a fixed cell area.
- [17] arXiv:0812.0123 [ps, pdf, other]
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Title: Electronic-structure Modulation Transistor with few kT operationSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
We propose a novel transistor concept with self gain and high ON/OFF current ratio using a few kT supply voltage for post-CMOS logic applications. The idea is based on electronic structure modulation of the channel material due to an applied electric field. A single-band tight-binding method coupled with non-equilibrium Green's function formalism for quantum transport is employed to predict the IV characteristics. We also report an experimental proof-of-concept device using a 20 nm long and 10 um wide channel consisting of Au nanocrystals and nitride traps with Au source and drain.
- [18] arXiv:0812.0127 [ps, pdf, other]
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Title: Phase diagrams of the Ising-Heisenberg chain with S=1/2 triangular XXZ clustersAuthors: Vadim OhanyanComments: 9 pages, 6 figures, Contribution to the proceedings of the XXVII Internatiotal Colloquium "Group Theoretical Methods in Physics" (Group27), Yerevan, 13-19 August 2008Subjects: Statistical Mechanics (cond-mat.stat-mech)
The one dimensional spin system consisted of triangular $S=1/2$ $XXZ$ Heisenberg clusters alternating with single Ising spins is considered. Partition function of the system is calculated exactly within the transfer--matrix formalism. T=0 ground state phase diagrams, corresponding to different regions of the values of system parameters are obtained.
- [19] arXiv:0812.0128 [ps, pdf, other]
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Title: Quantum transfer matrix method for one-dimensional disordered electronic systemsComments: 7 pages, 10 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Disordered Systems and Neural Networks (cond-mat.dis-nn)
We develop a novel quantum transfer matrix method to study thermodynamic properties of one-dimensional (1D) disordered electronic systems. It is shown that the partition function can be expressed as a product of $2\times2$ local transfer matrices. We demonstrate this method by applying it to the 1D disordered Anderson model. Thermodynamic quantities of this model are calculated and discussed.
- [20] arXiv:0812.0133 [ps, pdf, other]
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Title: Diamond chains with multiple-spin exchange interactionsComments: 6 pages, 8 figures, submitted to Rhys. Rev. BSubjects: Strongly Correlated Electrons (cond-mat.str-el)
We study the phase diagram of a symmetric spin-1/2 Heisenberg diamond chain with additional cyclic four-spin exchange interactions. The presented analysis supplemented by numerical exact-diagonalization results for finite periodic clusters implies a rich phase diagram containing, apart from standard magnetic and spin-liquid phases, two different tetramer-dimer phases as well as an exotic four-fold degenerate dimerized phase. The characteristics of the established spin phases as well as the nature of quantum phase transitions are discussed, as well.
- [21] arXiv:0812.0137 [pdf]
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Title: Two dimensional frustrated magnetic state in superconducting RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222)Comments: submitted to Phys. Rev. BSubjects: Superconductivity (cond-mat.supr-con)
In this paper we investigate the magnetic state and the role of the crystalline structure in RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222). Measurements were made in the isomorphic series (Nb1-xRux)Sr2Eu1.5Ce0.5Cu2O10 [(Nb,Ru)-1222], with x between 0 and 1. 3D XY fluctuations above the magnetic transition were not observed in Ru-1222, suggesting a weak inter-plane coupling between the RuO2 layers. The compositional dependence of the magnetic susceptibility shows a rapid broadening with increasing Nb content, explained in terms of a cluster-glass state. The variation of several superconducting parameters as a function of Ru content is linear in the whole concentration range, with no jumps at the critical concentration for which percolation of long range order is expected. 3D Arrhenius and Vogel-Fulcher-type dependencies fail to describe the dynamic properties. Fitting of a generalized Vogel-Fulcher-type dependence, with ln(tau/tau0) = A(T-T0)^-B, yield B = 2.0, in excellent agreement with Monte Carlo simulations for 2D systems. The value deduced for T0 agrees well with the re-opening of hysteresis in the M(H) curves. The observed superconducting and magnetic features are explained in terms of a scenario of 2D magnetic islands at the RuO2 layers, with no long range magnetic order.
- [22] arXiv:0812.0139 [ps, pdf, other]
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Title: Spin control in semiconductor quantum wiresComments: 4 pages, 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
We show that spin-flip rotation in a semiconductor quantum wire, caused by the Rashba and the Dresselhaus interactions (both of arbitrary strengths), can be suppressed by dint of an in-plane magnetic field. We found a new type of symmetry, which arises at a particular set of intensity and orientation of the magnetic field and explains this suppression. Based on our findings, we propose a transport experiment to measure the strengths of the Rashba and the Dresselhaus interactions.
- [23] arXiv:0812.0148 [pdf]
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Title: Silicon as a model ion trap: time domain measurements of donor Rydberg statesAuthors: N Q Vinh, P T Greenland, K Litvinenko, B Redlich, A F G van der Meer, S A Lynch, M Warner, A M Stoneham, G Aeppli, D J Paul, C R Pidgeon, B N MurdinJournal-ref: PNAS Aug 5 2008 vol 105 no 31 10649-10653Subjects: Other (cond-mat.other); Materials Science (cond-mat.mtrl-sci)
One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr-Sommerfeld formula, shifted to the far-infrared due to the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multi-photon processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus- and arsenic-doped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent the first step towards coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times key to many well-known schemes for quantum computing qubits for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.
- [24] arXiv:0812.0155 [ps, pdf, other]
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Title: Universal versus Material-Dependent Two-Gap Behaviors in the High-Tc Cuprates: Angle-Resolved Photoemission Study of La2-xSrxCuO4Authors: T. Yoshida, M. Hashimoto, S. Ideta, A. Fujimori, K. Tanaka, N. Mannella, Z. Hussain, Z.-X. Shen, M. Kubota, K. Ono, Seiki Komiya, Yoichi Ando, H. Eisaki, S. UchidaComments: 5 pages, 4 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
We have investigated the doping and temperature dependences of the pseudogap/superconducting gap in the single-layer cuprate La$_{2-x}$Sr$_x$CuO$_4$ by angle-resolved photoemission spectroscopy. The results clearly exhibit two distinct energy and temperature scales, namely, the gap around ($\pi$,0) of magnitude $\Delta^*$ and the gap around the node characterized by the d-wave order parameter $\Delta_0$, like the double-layer cuprate Bi2212. In comparison with Bi2212 having higher $T_c$'s, $\Delta_0$ is smaller, while $\Delta^*$ and $T^*$ are similar. This result suggests that $\Delta^*$ and $T^*$ are approximately material-independent properties of a single CuO$_2$ plane, in contrast the material-dependent $\Delta_0$, representing the pairing strength.
- [25] arXiv:0812.0158 [ps, pdf, other]
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Title: Effect of a in-plane magnetic field on the microwave assisted magnetotransport in a two-dimensional electron systemComments: 4 pages 4 figures. Published in PRBJournal-ref: Physical Review B, 78, 193310, (2008)Subjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
In this work we present a theoretical approach to study the effect of an in-plane (parallel) magnetic field on the microwave-assisted transport properties of a two-dimensional electron system. Previous experimental evidences show that microwave-induced resistance oscillations and zero resistance states are differently affected depending on the experimental set-up: two magnetic fields (two-axis magnet) or one tilted magnetic field. In the first case, experiments report a clear quenching of resistance oscillations and zero resistance states. In a tilted field, one obtains oscillations displacement and quenching but the latter is unbalanced and less intense. In our theoretical proposal we explain these results in terms of the microwave-driven harmonic motion performed by the electronic orbits and how this motion is increasingly damped by the in-plane field.
- [26] arXiv:0812.0181 [ps, pdf, other]
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Title: Universal relation for critical percolation thresholds of kagome-like latticesSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech)
Lattices that can be represented in a kagome-like form satisfy a universal percolation criticality condition, expressed as a relation between P_3, the probability that all three vertices in the triangle connect, and P_0, the probability that none connect. An approximate linear relation for P_3(P_0) is conjectured to provide a rigorous bound for critical thresholds. A more accurate relation is found numerically, and its predictions for the kagome, site-bond honeycomb, (3-12^2) lattice, and "stack-of-triangle" lattices compare favorably with numerical results.
- [27] arXiv:0812.0194 [pdf]
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Title: Response to "Comment on "Conjectures on exact solution of three-dimensional (3D) simple orthorhombic Ising lattices " " by PerkAuthors: Z.D. ZhangComments: 8 pages, 0 figureSubjects: Statistical Mechanics (cond-mat.stat-mech)
The error of eq. (15b) in my article [Z.D. Zhang, Phil. Mag. 87, 5309 (2007)] in the application of the Jordan-Wigner transformation does not affect the validity of the putative exact solution, since the solution is not derived directly from it. Other objections of Perk's Comment [J.H.H. Perk, Phil. Mag. 88, (2008) in press, also see arXiv:0811.1802v2] are the same as those in Wu et al's Comments [F.Y. Wu et al., Phil. Mag. 88, (2008) in press; in press], which do not stand on solid ground and have been rejected in my previous Response [Z.D. Zhang, Phil. Mag. 88, (2008) in press]. The conjectured solution can be utilized to understand critical phenomena in various systems, while the conjectures are open to prove rigorously.
- [28] arXiv:0812.0203 [ps, pdf, other]
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Title: Universal entanglement entropy in 2D conformal quantum critical pointsComments: 14 pages, 3 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We study the scaling behavior of the entanglement entropy of two dimensional conformal quantum critical systems, i.e. systems with scale invariant wave functions. They include two-dimensional generalized quantum dimer models on bipartite lattices and quantum loop models, as well as the quantum Lifshitz model and related gauge theories. We show that, under quite general conditions, the entanglement entropy of a large and simply connected sub-system of an infinite system with a smooth boundary has a universal finite contribution, as well as scale-invariant terms for special geometries. The universal finite contribution to the entanglement entropy is computable in terms of the properties of the conformal structure of the wave function of these quantum critical systems. The calculation of the universal term reduces to a problem in boundary conformal field theory.
- [29] arXiv:0812.0207 [ps, pdf, other]
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Title: Entangling photons by means of the nonlinear response of quantum wells to an ultrashort pulseComments: 9 pages, 2 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
Polarization-entangled photons can be produced from semiconductor bulk crystals made of CuCl through resonant hyperparametric scattering off the bound biexciton state with a yield exceeding $10^{-5}$, much higher than yields $<10^{-9}$ achieved with bulk nonlinear crystals. Here we show a different method to produce pairs of entangled photons in the short time response of a quantum well excited by a short intense pulse. At time scales where the biexciton effect is not yet pronounced, the Pauli exclusion principle is responsible for many-body correlations among excitons, giving rise to the production of entangled photons with a yield of around $10^{-2}$. We make use of a quantum-field theoretical two-particle density matrix in second quantization to calculate the entanglement for arbitrary emission angles of the entangled pairs of photons. At time scales where the heavy-light hole splitting is resolved, the resonances corresponding to different two-exciton states are developing, so that a simple kinematic theory can be presented which relates the states of the outgoing photons to the respective two-exciton states. The resonant response can be expected at symmetric emission angles for resonances with the heavy-heavy and light-light two-exciton states, while for heavy-light hole two-exciton states the resonant response occurs at asymmetric angles. We study the remarkably nontrivial dependence of entanglement on the emission angles of the entangled photons and on the ellipticity parameters of the incident photons. We show that the emitted entangled 2-photon states are always in a triplet state, in contrast to the entangled 2-photon state emitted from a quantum dot, which is a singlet state.
- [30] arXiv:0812.0211 [ps, pdf, other]
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Title: Localization in Correlated Bi-Layer Structures: From Photonic Cristals to Metamaterials and Electron SuperlatticesComments: 4 pages, 3 figuresSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn)
In a unified approach, we study the transport properties of periodic-on-average bi-layered photonic crystals, metamaterials and electron superlattices. Our consideration is based on the analytical expression for the localization length derived for the case of weakly fluctuating widths of layers, that also takes into account possible correlations in disorder. We analyze how the correlations lead to anomalous properties of transport. In particular, we show that for quarter stack layered media specific correlations can result in a $\omega^2$-dependence of the Lyapunov exponent in all spectral bands.
- [31] arXiv:0812.0216 [ps, pdf, other]
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Title: Study of phase stability of MnCr using the augmented space recursion based orbital peeling techniqueComments: 11 pagesSubjects: Materials Science (cond-mat.mtrl-sci)
In an earlier communication we have developed a recursion based approach to the study of phase stability and transition of binary alloys. We had combined the recursion method introduced by Haydock, Heine and Kelly and the our augmented space approach with the orbital peeling technique proposed by Burke to determine the small energy differences involved in the discussion of phase stability. We extend that methodology for the study of MnCr alloys.
- [32] arXiv:0812.0218 [ps, pdf, other]
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Title: Quantum trajectory equation for multiple qubits in circuit QED: Generating entanglement by measurementComments: 7 pages, 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
In this paper we derive an effective master equation and quantum trajectory equation for multiple qubits in a single resonator and in the large resonator decay limit. We show that homodyne measurement of the resonator transmission is a weak measurement of the collective qubit inversion. As an example of this result, we focus on the case of two qubits and show how this measurement can be used to generate an entangled state from an initially separable state. This is realized without relying on an entangling Hamiltonian. We show that, for {\em current} experimental values of both the decoherence and measurement rates, this approach can be used to generate highly entangled states. This scheme takes advantage of the fact that one of the Bell states is decoherence-free under Purcell decay.
- [33] arXiv:0812.0220 [ps, pdf, other]
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Title: Qubit dispersive readout scheme with a microstrip squid amplifierAuthors: Sebastien MichotteComments: 4 pages, 3 figuresSubjects: Superconductivity (cond-mat.supr-con)
A flux qubit readout scheme involving a dispersive technique coupled to a microstrip squid amplifier has been tested experimentally. Thanks to the almost quantum limited noise of this amplifier at low temperature, this readout device is very promising for a design with actual qubits. It's proof of principle and low noise performance have been tested by simulating the qubit presence applying a tiny flux change in the input squid. The resonant frequency of the amplifier is adjusted in-situ with a varactor diode to approach the frequency of the tank circuit. Depending on the sign of the transfer function, two operating mode (crossing or anticrossing regime) have been identified.
- [34] arXiv:0812.0223 [ps, pdf, other]
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Title: Universal Correlations and Dynamic Disorder in a Nonlinear Periodic 1D SystemComments: 4 pages, 4 figures. Comments welcomeSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn)
When a periodic 1D system described by a tight-binding model is uniformly initialized with equal amplitudes at all sites, yet with completely random phases, it evolves into a thermal distribution with no spatial correlations. However, when the system is nonlinear, correlations are spontaneously formed. We find that for strong nonlinearities, the intensity histograms approach a narrow Gaussian distributed around their mean and phase correlations are formed between neighboring sites. Sites tend to be out-of-phase for a positive nonlinearity and in-phase for a negative one. The field correlations take a universal shape independent of parameters. This nonlinear evolution produces an effectively dynamically disordered potential which exhibits interesting diffusive behavior.
- [35] arXiv:0812.0226 [ps, pdf, other]
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Title: Modelling two-dimensional Crystals with Defects under Stress: Superelongation of Carbon Nanotubes at high TemperaturesComments: 10 opages, 4 eps figures, 1 pstricks pictureSubjects: Soft Condensed Matter (cond-mat.soft); Materials Science (cond-mat.mtrl-sci)
We calculate analytically the phase diagram of a two-dimensional square crystal with defects under external homogeneous stress as a function of temperature using a simple elastic lattice model that allows for defect formation. The temperature dependence turns out to be very weak. The results are relevant for recent stress experiments on Carbon nanotubes. Under increasing stress, we find a crossover regime which we identify with a cracking transition that is almost independent of temperature. Furthermore, we find an almost stress-independent melting point. In addition, we derive an enhanced ductility with relative strains before cracking between 200-400%, in agreement with Carbon nanotube experiments. The specific values depend on the Poisson ratio and the angle between the external force and the crystal axes.
- [36] arXiv:0812.0233 [pdf]
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Title: Spontaneous Breaking of the Rotation Symmetry of Edge States in Zigzag Carbon NanotubesComments: 14 pages, 4 figuresSubjects: Materials Science (cond-mat.mtrl-sci); Other (cond-mat.other)
The electron edge states of carbon nanotubes with chiral index (N, 0) are studied. The total energy is lowered when the hydrogen ring at the end of the nanotube has distortion. The axial symmetry is thereby broken spontaneously. The prediction is supported via quantum chemistry simulation for (7,0) and (8,0) carbon nanotubes.
- [37] arXiv:0812.0234 [ps, pdf, other]
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Title: Tuning the bond order wave (BOW) phase of half-filled extended Hubbard modelsComments: 9 pages, 7 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Other (cond-mat.other)
Theoretical and computational studies of the quantum phase diagram of the one-dimensional half-filled extended Hubbard model (EHM) indicate a narrow bond order wave (BOW) phase with finite magnetic gap $E_m$ for on-site repulsion $U < U^*$, the critical point, and nearest neighbor interaction $V_c \approx U/2$ near the boundary of the charge density wave (CDW) phase. Potentials with more extended interactions that retain the EHM symmetry are shown to have a less cooperative CDW transition with higher $U^*$ and wider BOW phase. Density matrix renormalization group (DMRG) is used to obtain $E_m$ directly as the singlet-triplet gap, with finite $E_m$ marking the BOW boundary $V_s(U)$. The BOW/CDW boundary $V_c(U)$ is obtained from exact finite-size calculations that are consistent with previous EHM determinations. The kinetic energy or bond order provides a convenient new estimate of $U^*$ based on a metallic point at $V_c(U)$ for $U < U^*$. Tuning the BOW phase of half-filled Hubbard models with different intersite potentials indicates a ground state with large charge fluctuations and magnetic frustration. The possibility of physical realizations of a BOW phase is raised for Coulomb interactions.
- [38] arXiv:0812.0239 [ps, pdf, other]
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Title: Collective modes of trapped Fermi gases with in-medium interactionComments: 17 pagesSubjects: Other (cond-mat.other)
Due to Pauli blocking of intermediate states, the scattering matrix (or T matrix) of two fermionic atoms in a Fermi gas becomes different from that of two atoms in free space. This effect becomes particularly important near a Feshbach resonance, where the interaction in free space is very strong but becomes effectively suppressed in the medium. We calculate the in-medium T matrix in ladder approximation and study its effects on the properties of collective modes of a trapped gas in the normal-fluid phase. We introduce the in-medium interaction on both sides of the Boltzmann equation, namely in the calculation of the mean field and in the calculation of the collision rate. With mean field inclusion, we can considerably improve the agreement with measured temperature dependence of frequency and damping rate of the scissors mode. In addition, we compute frequencies and damping rates of the radial quadrupole and compression modes.
- [39] arXiv:0812.0244 [ps, pdf, other]
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Title: Efficient implementation of a van der Waals density functional: Application to double-wall carbon nanotubesComments: 4 pages, 1 figure, 1 tableSubjects: Materials Science (cond-mat.mtrl-sci)
We present an efficient implementation of the van der Waals density functional of Dion et al [Phys. Rev. Lett. 92, 246401 (2004)], which expresses the nonlocal correlation energy as a double spacial integral. We factorize the integration kernel and use fast Fourier transforms to evaluate the selfconsistent potential, total energy, and atomic forces, in N log(N) operations. The resulting overhead in total computational cost, over semilocal functionals, is very moderate for medium and large systems. We apply the method to calculate the binding energies and the barriers for relative translation and rotation in double-wall carbon nanotubes.
- [40] arXiv:0812.0245 [ps, pdf, other]
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Title: Nano-scale brushes: How to build a smart surface coatingSubjects: Soft Condensed Matter (cond-mat.soft)
Via computer simulations, we demonstrate how a densely grafted layer of polymers, a {\it brush}, could be turned into an efficient switch through chemical modification of some of its end-monomers. In this way, a surface coating with reversibly switchable properties can be constructed. We analyze the fundamental physical principle behind its function, a recently discovered surface instability, and demonstrate that the combination of a high grafting density, an inflated end-group size and a high degree of monodispersity are conditions for an optimal functionality of the switch.
- [41] arXiv:0812.0246 [ps, pdf, other]
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Title: Bose-Einstein condensate in a quartic potential: Static and Dynamic propertiesComments: 10 pages, including figuresSubjects: Other (cond-mat.other); Statistical Mechanics (cond-mat.stat-mech)
In this paper, we present a theoretical study of a Bose-Einstein condensate of interacting bosons in a quartic trap in one, two, and three dimensions. Using Thomas-Fermi approximation, suitably complemented by numerical solutions of the Gross-Pitaevskii equation, we study the ground sate condensate density profiles, the chemical potential, the effects of cross-terms in the quartic potential, temporal evolution of various energy components of the condensate, and width oscillations of the condensate. Results obtained are compared with corresponding results for a bose condensate in a harmonic confinement.
- [42] arXiv:0812.0253 [pdf]
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Title: Calcualtion of geometrical and spin features of a series of metall-endofullerenesAuthors: V.S. GurinComments: 9 pages including 1 figureSubjects: Materials Science (cond-mat.mtrl-sci); Other (cond-mat.other)
A series of endofullerenes M@C60 were calculated from the first principles (unrestricted Hartree Fock and DFT B3LYP) with effective core potential (M = Ag, Cs) and all-electronic basis set (M = Li, Na, Cu). An arbitrary symmetry distortion (down to C1 point group) was assumed. The geometrical and electronic properties are compared for this series of endoatoms including the off-centre position of enoatoms within C60, effective charges and spin density. The latter values are featured for Ag@C60 and Cu@C60
- [43] arXiv:0812.0255 [ps, pdf, other]
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Title: From Vicious Walkers to TASEPComments: 37 pages, 4 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech)
We propose a model of semi-vicious walkers, which interpolates between the totally asymmetric simple exclusion process and the vicious walkers model, having the two as limiting cases. For this model we calculate the asymptotics of the survival probability for $m$ particles and obtain a scaling function, which describes the transition from one limiting case to another. Then, we use a fluctuation-dissipation relation allowing us to reinterpret the result as the particle current generating function in the totally asymmetric simple exclusion process. Thus we obtain the particle current distribution asymptotically in the large time limit as the number of particles is fixed. The results apply to the large deviation scale as well as to the diffusive scale. In the latter we obtain a new universal distribution, which has a skew non-Gaussian form. For $m$ particles its asymptotic behavior is shown to be $e^{-\frac{y^{2}}{2m^{2}}}$ as $y\to -\infty $ and $e^{-\frac{y^{2}}{2m}}y^{-\frac{m(m-1)}{2}}$ as $y\to \infty $.
- [44] arXiv:0812.0261 [ps, pdf, other]
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Title: Cooling a micro-mechanical resonator by quantum back-action from a noisy qubitComments: 5 pages, 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
We study the different roles of qubit dephasing and relaxation in the process of cooling a mechanical resonator by quantum back-action. With a superconducting flux qubit as a specific example, we show that ground-state cooling of a mechanical resonator is possible under present experimental conditions. Our investigation suggests that the cooling limit is primarily determined by the dissipative nature of the qubit, including both relaxation and dephasing.
- [45] arXiv:0812.0268 [ps, pdf, other]
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Title: Drag force on an oscillating object in quantum turbulenceComments: 7 pages, 5 figures, 1 tableSubjects: Other (cond-mat.other)
This paper reports results of the computation of the drag force exerted on an oscillating object in quantum turbulence in superfluid $^4$He.
The drag force is calculated on the basis of numerical simulations of quantum turbulent flow about the object. The drag force is proportional to the square of the magnitude of the oscillation velocity, which is similar to that in classical turbulence at high Reynolds number. The drag coefficient is also calculated, and its value is found to be of the same order as that observed in previous experiments. The correspondence between quantum and classical turbulence is further clarified by examining the turbulence created by oscillating objects. - [46] arXiv:0812.0278 [ps, pdf, other]
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Title: Ferrofluidity in a two-component dipolar Bose-Einstein CondensateComments: 4 pages, 6 figuresSubjects: Other (cond-mat.other)
It is shown that the interface in a two-component Bose-Einstein condensate (BEC) in which one component exhibits a dipole-dipole interaction spontaneously forms patterns similar to those formed in a magnetic liquid subject to a magnetic field. A hexagonal pattern, hysteretic behavior, and soliton-like structure are numerically demonstrated. A phenomenon similar to the labyrinthine instability is also found. These phenomena may be realized using a 52Cr BEC. The periodic density modulation in the superfluid ground state offers evidence of supersolidity.
- [47] arXiv:0812.0282 [ps, pdf, other]
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Title: Measurement of the shot noise in a single electron transistorComments: 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)
We have systematically measured the shot noise in a single electron transistor (SET) as a function of bias and gate voltages. By embedding a SET in a resonance circuit we have been able to measure its shot noise at the resonance frequency 464 MHz, where the 1/f noise is negligible. We can extract the Fano factor which varies between 0.5 and 1 depending on the amount of Coulomb blockade in the SET, in very good agreement with the theory.
- [48] arXiv:0812.0290 [ps, pdf, other]
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Title: Electrothermal feedback in superconducting nanowire single-photon detectorsSubjects: Superconductivity (cond-mat.supr-con)
We investigate the role of electrothermal feedback in the operation of superconducting nanowire single-photon detectors (SNSPDs). It is found that the desired mode of operation for SNSPDs is only achieved if this feedback is unstable, which happens naturally through the slow electrical response associated with their relatively large kinetic inductance. If this response is sped up in an effort to increase the device count rate, the electrothermal feedback becomes stable and results in an effect known as latching, where the device is locked in a resistive state and can no longer detect photons. We present a set of experiments which elucidate this effect, and a simple model which quantitatively explains the results.
- [49] arXiv:0812.0300 [ps, pdf, other]
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Title: Ion-ion correlation and charge reversal at titrating solid interfacesComments: 4 pages, 4 figures, submitted to PRLSubjects: Soft Condensed Matter (cond-mat.soft); Statistical Mechanics (cond-mat.stat-mech)
Confronting grand canonical titration Monte Carlo simulations (MC) with recently published titration and charge reversal (CR) experiments on silica surfaces by Dove et al. and van der Heyden it et al, we show that ion-ion correlations quantitatively explain why divalent counterions strongly promote surface charge which, in turn, eventually causes a charge reversal (CR). Titration and CR results from simulations and experiments are in excellent agreement without any fitting parameters. This is the first unambiguous evidence that ion-ion correlations are instrumental in the creation of highly charged surfaces and responsible for their CR. Finally, we show that charge correlations result in "anomalous" charge regulation in strongly coupled conditions in qualitative desagreement with its classical treatment.
- [50] arXiv:0812.0301 [ps, pdf, other]
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Title: Optical Properties and Modal Gain of InGaN Quantum Dot StacksAuthors: Joachim Kalden, Kathrin Sebald, Juergen Gutowski, Christian Tessarek, Timo Aschenbrenner, Stephan Figge, Detlef HommelComments: 9 Pages, 4 FiguresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
We present investigations of the optical properties of stacked InGaN quantum dot layers and demonstrate their advantage over single quantum dot layer structures. Measurements were performed on structures containing a single layer with quantum dots or threefold stacked quantum dot layers, respectively. A superlinear increase of the quantum dot related photoluminescence is detected with increasing number of quantum dot layers while other relevant GaN related spectral features are much less intensive when compared to the photoluminescence of a single quantum dot layer. The quantum dot character of the active material is verified by microphotoluminescence experiments at different temperatures. For the possible integration within optical devices in the future the threshold power density was investigated as well as the modal gain by using the variable stripe length method.
- [51] arXiv:0812.0302 [ps, pdf, other]
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Title: High - Temperature Superconductivity in Iron Based Layered CompoundsAuthors: M.V. SadovskiiComments: 43 pages, 30 figures, review talk on 90th anniversary of Physics UspekhiSubjects: Superconductivity (cond-mat.supr-con)
We present a review of basic experimental facts on the new class of high - temperature superconductors - iron based layered compounds like REOFeAs (RE=La,Ce,Nd,Pr,Sm...), AFe_2As_2 (A=Ba,Sr...), AFeAs (A=Li,...) and FeSe(Te). We discuss electronic structure, including the role of correlations, spectrum and role of collective excitations (phonons, spin waves), as well as the main models, describing possible types of magnetic ordering and Cooper pairing in these compounds.
- [52] arXiv:0812.0309 [pdf, other]
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Title: Vibrational nonequilibrium effects in the conductance of single-molecules with multiple electronic statesComments: 5 pages, 4 figuresSubjects: Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall)
Vibrational nonequilibrium effects in charge transport through single-molecule junctions are investigated. Focusing on molecular bridges with multiple electronic states, it is shown that electronic-vibrational coupling triggers a variety of vibronic emission and absorption processes, which influence the conductance properties and mechanical stability of single-molecule junctions profoundly. Employing a master equation and a nonequilibrium Green's function approach, these processes are analyzed in detail for a generic model of a molecular junction and for benzenedibutanethiolate bound to gold electrodes.
- [53] arXiv:0812.0316 [ps, pdf, other]
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Title: A graph theoretical analysis of the energy landscape of a model proteinComments: 14 pages, 15 figuresSubjects: Soft Condensed Matter (cond-mat.soft); Statistical Mechanics (cond-mat.stat-mech)
In systems characterized by a rough potential energy landscape, local minima and saddles define a network of metastable states whose topology strongly influences the dynamics of the system. Changes in temperature, causing the merging and splitting of metastable states, have non trivial effects on such networks and must be taken into account by means of suitable renormalization procedures. In this paper we analyze the topology of the network of metastable states for different polypeptidic sequences in a minimalist protein model. A smaller spectral dimension emerges as a hallmark of stability of the native configuration and highlights a non-obvious link between dynamic and thermodynamic properties.
- [54] arXiv:0812.0331 [ps, pdf, other]
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Title: Coarse-grained models for fluids and their mixtures: Comparison of Monte Carlo studies of their phase behavior with perturbation theory and experimentComments: J. Chem. Phys., acceptedSubjects: Statistical Mechanics (cond-mat.stat-mech)
The prediction of the equation of state and the phase behavior of simple fluids (noble gases, carbon dioxide, benzene, methane, short alkane chains) and their mixtures by Monte Carlo computer simulation and analytic approximations based on thermodynamic perturbation theory is discussed. Molecules are described by coarse grained (CG) models, where either the whole molecule (carbon dioxide, benzene, methane) or a group of a few successive CH_2 groups (in the case of alkanes) are lumped into an effective point particle. Interactions among these point particles are fitted by Lennard-Jones (LJ) potentials such that the vapor-liquid critical point of the fluid is reproduced in agreement with experiment; in the case of quadrupolar molecules a quadrupole-quadrupole interaction is included. These models are shown to provide a satisfactory description of the liquid-vapour phase diagram of these pure fluids. Investigations of mixtures, using the Lorentz-Berthelot (LB) combining rule, also produce satisfactory results if compared with experiment, while in some previous attempts (in which polar solvents were modelled without explicitly taking into account quadrupolar interaction), strong violations of the LB rules were required. For this reason, the present investigation is a step towards predictive modelling of polar mixtures at low computational cost. These very simple coarse-grained models of small molecules developed here should be useful e.g. for simulations of polymer solutions with such molecules as solvent.
- [55] arXiv:0812.0343 [ps, pdf, other]
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Title: Near-degeneracy of several pairing channels in multiorbital models for the Fe-pnictidesComments: 17 pages, 23 figuresSubjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
Weak-coupling approaches to the pairing problem in the iron pnictide superconductors have predicted a wide variety of superconducting ground states. We argue here that this is due both to the inadequacy of certain approximations to the effective low-energy band structure, and to the natural near-degeneracy of different pairing channels in superconductors with many distinct Fermi surface sheets. In particular, we review attempts to construct two-orbital effective band models, the argument for their fundamental inconsistency with the symmetry of these materials, and the comparison of the dynamical susceptibilities in two- and five-band models. We then present results for the magnetic properties, pairing interactions, and pairing instabilities within a five-band Random Phase Approximation model. We discuss the robustness of these results for different dopings, interaction strengths, and variations in band structure. Within the parameter space explored, an anisotropic, sign-changing s-wave state and a d_x2-y2 state are nearly degenerate, due to the near nesting of Fermi surface sheets.
- [56] arXiv:0812.0356 [ps, pdf, other]
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Title: Shining a Light on Bose--Hubbard MixturesComments: 6 pages, 3 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)
Motivated by recent experiments on cold atomic gases in ultra high finesse optical cavities, we consider the problem of a two-band Bose--Hubbard model coupled to quantum light. Photoexcitation promotes carriers between the bands and we study the non-trivial interplay between Mott insulating behavior and superfluidity. The model displays a global U(1) X U(1) symmetry which supports the coexistence of Mott insulating and superfluid phases, and yields a rich phase diagram with multicritical points. This symmetry property is shared by several other problems of current experimental interest, including two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover problem for atom-molecule mixtures induced by a Feshbach resonance. We corroborate our findings by numerical simulations.
- [57] arXiv:0812.0357 [ps, pdf, other]
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Title: Comment on "Twofold Advance in the Theoretical Understanding of Far-From-Equilibrium Properties of Interacting Nanostructures"Comments: 1 page 1 figureSubjects: Strongly Correlated Electrons (cond-mat.str-el)
Boulat, Saleur and Schmitteckert (BSS) reported results on the full $I-V$ characteristics of the interacting resonant level model (IRLM) exhibiting region with unexpected negative differential conductance (NDC). Using time-dependent density matrix renormalization group complemented with the exact solution performed at a special point (the self-dual point) in the parameter space BSS have shown that at nonzero Coulomb interaction $U$ the current flowing through the impurity level (IL) exhibits a power-law asymptotics as a function of large applied bias voltage. Similar conclusion was earlier reached by Doyon. Even though their results are solid and supported by both analytic and numeric arguments, BSS concluded that "the NDC at large voltage seems a truly nonperturbative behavior, with unclear physical origin". On the contrary, the remarkable physics of NDC can be explained by simple physical arguments and, as we shall show in this Comment, in certain circumstances can be calculated in the framework of perturbation theory.
Cross-lists for Tue, 2 Dec 08
- [58] arXiv:0811.2262 (cross-list from hep-th) [ps, pdf, other]
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Title: Holographic Responses of Fermion MatterComments: 21 page, 1 figure, harvmacSubjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Phenomenology (hep-ph)
We consider the $D4-D8-\overline{D8}$ brane system which serves as ultraviolet completion of the Nambu-Jona-Lasinio model, where the only degrees of freedom carrying baryon charge are fermions. By turning on chemical potential for this charge one may expect the formation of the Fermi liquid ground state. At strong coupling we use the dual holographic description to investigate the responses of the system to small perturbations. In the chirally symmetric phase we find that the density dependent part of the heat capacity vanishes linearly with temperature. We also observe a zero sound excitation in the collisionless regime, whose speed is equal to that of normal sound in the hydrodynamic regime. Both the linear dependence of the heat capacity and the existence of zero sound are properties of the Fermi liquid ground state. We also compute the two-point function of the currents at vanishing frequency but do not find any singularities at finite values of the momentum.
- [59] arXiv:0811.2472 (cross-list from math-ph) [ps, pdf, other]
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Title: Interaction-Flip Identities in Spin GlassesSubjects: Mathematical Physics (math-ph); Disordered Systems and Neural Networks (cond-mat.dis-nn)
We study the properties of fluctuation for the free energies and internal energies of two spin glass systems that differ for having some set of interactions flipped. We show that their difference has a variance that grows like the volume of the flipped region. Using a new interpolation method, which extends to the entire circle the standard interpolation technique, we show by integration by parts that the bound imply new overlap identities for the equilibrium state. As a side result the case of the random field is analyzed and the triviality of its overlap distribution proved.
- [60] arXiv:0811.3050 (cross-list from hep-ph) [ps, pdf, other]
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Title: Gauge Covariance Relations and the Fermion Propagator in Maxwell-Chern-Simons QED3Comments: 13 pages, 4 figures. Published version. V2, References correctedJournal-ref: J. Phys. A: Math. Theor. 41 (2008) 505401Subjects: High Energy Physics - Phenomenology (hep-ph); Mesoscopic Systems and Quantum Hall Effect (cond-mat.mes-hall); High Energy Physics - Theory (hep-th)
We study the gauge covariance of the fermion propagator in Maxwell-Chern-Simons planar quantum electrodynamics (QED$_3$) considering four-component spinors with parity-even and parity-odd mass terms both for fermions and photons. Starting with its tree level expression in the Landau gauge, we derive a non perturbative expression for this propagator in an arbitrary covariant gauge by means of its Landau-Khalatnikov-Fradkin transformation (LKFT). We compare our findings in the weak coupling regime with the direct one-loop calculation of the two-point Green function and observe perfect agreement up to a gauge independent term. We also reproduce results derived in earlier works as special cases of our findings.
- [61] arXiv:0811.3967 (cross-list from quant-ph) [pdf, other]
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Title: Dynamical Coupling between a Bose-Einstein Condensate and a Cavity Optical LatticeAuthors: Stephan Ritter, Ferdinand Brennecke, Christine Guerlin, Kristian Baumann, Tobias Donner, Tilman EsslingerComments: 5 pages, 4 figuresSubjects: Quantum Physics (quant-ph); Other (cond-mat.other)
A Bose-Einstein condensate is dispersively coupled to a single mode of an ultra-high finesse optical cavity. The system is governed by strong interactions between the atomic motion and the light field even at the level of single quanta. While coherently pumping the cavity mode the condensate is subject to the cavity optical lattice potential whose depth depends nonlinearly on the atomic density distribution. We observe bistability already below the single photon level and strong back-action dynamics which tunes the system periodically out of resonance.
- [62] arXiv:0811.4593 (cross-list from physics.flu-dyn) [pdf, other]
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Title: General on-site velocity boundary conditions for lattice BoltzmannComments: 10 pages, 4 figuresSubjects: Fluid Dynamics (physics.flu-dyn); Other (cond-mat.other); Computational Physics (physics.comp-ph)
We present a general on-site velocity boundary condition for lattice Boltzmann simulations with variable inflow direction on a D3Q19 lattice. The boundary condition we propose acts locally on each lattice site, is independent on the details of the relaxation process during collision and contains no artificial slip. With this boundary condition it is possible to specify the velocity exactly on the boundary nodes. In particular, the case of an on-site no-slip boundary condition is naturally included if the velocity is chosen to be zero. The boundary condition is tested and compared to other approaches in the case of a Poiseuille flow through a tilted channel, laminar shear flow and Poiseuille flow between two planar plates. By the numerical results, it is confirmed that the boundary condition is of second order accuracy and and does not contain any numerical slip.
- [63] arXiv:0811.4618 (cross-list from physics.atom-ph) [ps, pdf, other]
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Title: Ultracold polar molecules near quantum degeneracyAuthors: S. Ospelkaus, K.-K. Ni, M. H. G. de Miranda, B. Neyenhuis, D. Wang, S. Kotochigova, P. S. Julienne, D. S. Jin, J. YeSubjects: Atomic Physics (physics.atom-ph); Other (cond-mat.other)
We report the creation and characterization of a near quantum-degenerate gas of polar $^{40}$K-$^{87}$Rb molecules in their absolute rovibrational ground state. Starting from weakly bound heteronuclear KRb Feshbach molecules, we implement precise control of the molecular electronic, vibrational, and rotational degrees of freedom with phase-coherent laser fields. In particular, we coherently transfer these weakly bound molecules across a 125 THz frequency gap in a single step into the absolute rovibrational ground state of the electronic ground potential. Phase coherence between lasers involved in the transfer process is ensured by referencing the lasers to two single components of a phase-stabilized optical frequency comb. Using these methods, we prepare a dense gas of $4\cdot10^4$ polar molecules at a temperature below 400 nK. This fermionic molecular ensemble is close to quantum degeneracy and can be characterized by a degeneracy parameter of $T/T_F=3$. We have measured the molecular polarizability in an optical dipole trap where the trap lifetime gives clues to interesting ultracold chemical processes. Given the large measured dipole moment of the KRb molecules of 0.5 Debye, the study of quantum degenerate molecular gases interacting via strong dipolar interactions is now within experimental reach.
- [64] arXiv:0811.4695 (cross-list from quant-ph) [ps, pdf, other]
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Title: Information Transferring Ability of the Different Phases of a finite XXZ Spin ChainComments: 5 pages, 4 figures. Comments are welcomeSubjects: Quantum Physics (quant-ph); Other (cond-mat.other)
We study the transmission of information, particularly one half of an entangled state through all the phases of a finite XXZ spin chain. This characterizes the merit of the different phases in terms of their ability to act as a quantum wire. We find the isotropic antiferromagnetic spin chain to be the best point of the phase diagram in terms of highest entanglement, fastest transmission and the robustness to temperature and decoherence. A curious feature is a hopping mode of entanglement transfer which skips odd numbered sites and manifests as a curious behavior of two time correlation functions during the non-equilibrium dynamics. Our predictions are potentially testable in several physical systems.
- [65] arXiv:0811.4749 (cross-list from nlin.CD) [ps, pdf, other]
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Title: Extremum complexity in the monodimensional ideal gas: the piecewise uniform density distribution approximationComments: 11 pages, 1 table, 16 figuresSubjects: Chaotic Dynamics (nlin.CD); Statistical Mechanics (cond-mat.stat-mech); Data Analysis, Statistics and Probability (physics.data-an)
In this work, it is suggested that the extremum complexity distribution of a high dimensional dynamical system can be interpreted as a piecewise uniform distribution in the phase space of its accessible states. When these distributions are expressed as one--particle distribution functions, this leads to piecewise exponential functions. It seems plausible to use these distributions in some systems out of equilibrium, thus greatly simplifying their description. In particular, here we study an isolated ideal monodimensional gas far from equilibrium that presents an energy distribution formed by two non--overlapping Gaussian distribution functions. This is demonstrated by numerical simulations. Also, some previous laboratory experiments with granular systems seem to display this kind of distributions.
Replacements for Tue, 2 Dec 08
- [66] arXiv:cond-mat/0608472 (replaced) [ps, pdf, other]
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Title: Conserving Gapless Mean-Field Theory of a Multi-Component Bose-Einstein CondensateComments: 8 pages, 7 figures Minor errors removedJournal-ref: J. Phys. Soc. Jpn. 76, 024008 (2007)Subjects: Statistical Mechanics (cond-mat.stat-mech)
- [67] arXiv:0801.3802 (replaced) [ps, pdf, other]
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Title: Dichotomy Results for Fixed-Point Existence Problems for Boolean Dynamical SystemsAuthors: Sven KosubComments: 17 pages; this version corrects an error/typo in the 2008/01/24 versionJournal-ref: Mathematics in Computer Science, 1(3):487-505, 2008, special issue on Modeling and Analysis of Complex SystemsSubjects: Computational Complexity (cs.CC); Disordered Systems and Neural Networks (cond-mat.dis-nn); Discrete Mathematics (cs.DM); Adaptation and Self-Organizing Systems (nlin.AO); Cellular Automata and Lattice Gases (nlin.CG)
- [68] arXiv:0801.4068 (replaced) [ps, pdf, other]
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Title: What Is There Between Any Two Nodes in a Complex Network?Autho