Mar 28 - Apr 1, Chungwei Lin
Apr 1
1. arXiv:1103.6144 [pdf, ps, other]
Title: Interaction of half-quantized vortices in two-component Bose-Einstein condensates
Authors: Minoru Eto, Kenichi Kasamatsu, Muneto Nitta, Hiromitsu Takeuchi, Makoto Tsubota
Comments: 9 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
We study the asymptotic interaction between two half-quantized vortices in two-component Bose-Einstein condensates. When two vortices in different components are placed at distance 2R, the leading order of the force between them is found to be (log R/\xi-1/2)/R^3, in contrast to 1/R between vortices placed in the same component. We derive it analytically using the Abrikosov ansatz and the profile functions of the vortices, confirmed numerically with the Gross-Pitaevskii model. We also find that the short-range cutoff of the inter-vortex potential linearly depends on the healing length.
2. arXiv:1103.6183 [pdf, ps, other]
Title: FFLO- and Néel States in finite-size Systems
Authors: T. Gottwald, P. G. J. van Dongen
Comments: 6 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
The general structure of the $s$-wave fermionic superfluid pairing order parameter is discussed for systems in thermal equilibrium. We demonstrate that for finite-size systems with fixed boundary conditions the pairing-amplitude may always be chosen as a {\it real} function in space, in contrast to systems underlying periodic boundary conditions, with drastical consequences for several postulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states. Using a simple mapping, we also investigate the consequences of our results for antiferromagnetic equilibrium states in a repulsive Hubbard model.
3. arXiv:1103.6285 [pdf, other]
Title: Phase diagram for the $\nu=0$ quantum Hall state in monolayer graphene
Authors: Maxim Kharitonov
Comments: 25 pages, 19 figs
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
The $\nu=0$ quantum Hall state in a defect-free graphene sample is studied within the framework of quantum Hall ferromagnetism. We perform a systematic analysis of the pseudospin anisotropies, which arise from the valley and sublattice asymmetric short-range electron-electron (e-e) and electron-phonon (e-ph) interactions. The phase diagram, obtained in the presence of generic pseudospin anisotropy and the Zeeman effect, consists of four phases characterized by the following orders: spin-polarized ferromagnetic, canted antiferromagnetic, charge density wave, and Kekul\'{e} distortion. We take into account the Landau level mixing effects and show that they result in the key renormalizations of parameters. First, the absolute values of the anisotropy energies become greatly enhanced and can significantly exceed the Zeeman energy. Second, the signs of the anisotropy energies due to e-e interactions can change upon renormalization. A crucial consequence of the latter is that the short-range e-e interactions alone could favor any state on the phase diagram, depending on the details of interactions at the lattice scale. On the other hand, the leading e-ph interactions always favor the Kekul\'{e} distortion order. The possibility of inducing phase transitions by tilting the magnetic field is discussed.
March 31
1. arXiv:1103.5804 (cross-list from cond-mat.quant-gas) [pdf, ps, other]
Title: The lowest scattering state of one-dimensional Bose gas with attractive interactions
Authors: Yajiang Hao, Hongli Guo, Yunbo Zhang, Shu Chen
Comments: 6 pages, 6figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We investigate the lowest scattering state of one-dimensional Bose gas with attractive interactions trapped in a hard wall trap. By solving the Bethe ansatz equation numerically we determine the full energy spectrum and the exact wave function for different attractive interaction parameters. The resultant density distribution, momentum distribution, reduced one body density matrix and two body correlation show that the decreased attractive interaction induces rich density profiles and specific correlation properties in the weakly attractive Bose gas.
2. arXiv:1103.5964 (cross-list from cond-mat.quant-gas) [pdf, other]
Title: Time-reversal symmetry breaking of fermions in the p-band of an optical lattice
Authors: Philipp Hauke, Erhai Zhao, Krittika Goyal, Ivan H. Deutsch, W. Vincent Liu, Maciej Lewenstein
Comments: 4+epsilon pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
Orbital physics plays an important in strongly-correlated electron systems, but coupling to other degrees of freedom often obscures its effects. A tunable system for exploring orbital physics alone is provided by ultracold spinless fermionic atoms in the p-band of an optical lattice. Instead of relying on deep optical lattice potentials, which often means experimentally inaccessible low temperature, the strongly-correlated regime can be reached without suffering significant atom loss by using an optical p-wave Feshbach resonance, as recently proposed by Goyal \emph{et al.}\ [PRA 82, 062704 (2010)]. We investigate this system (which is equivalent to a three-color fermion model with color-dependent interaction, and spatially anisotropic and color-dependent tunneling) in strong coupling at 1/3 filling, employing a Gutzwiller ansatz to search for competing orbital orderings. Beside a phase without tunneling and an `orbital N\'eel' phase where $p_x$ and $p_y$ orbitals alternate, we find a novel phase of alternating $p_z$ and $p_x+ip_y$ orbits on the lattice, which breaks spatial and time-reversal symmetry.
3. arXiv:1103.5787 [pdf, other]
Title: Two energy scales in the magnetic resonance spectrum of electron and hole doped pnictide superconductors
Authors: Tanmoy Das, A. V. Balatsky
Comments: 4.1 pages, Accepted in Phys. Rev. Lett., Please refer to the publication link for supplementary material
Subjects: Superconductivity (cond-mat.supr-con)
We argue that a multiband superconductor with sign-changing gaps may have multiple spin resonances. We calculate the RPA-based spin resonance spectra of a pnictide superconductor by using the five band tight-binding model or angle-resolved photoemission spectroscopy (ARPES) Fermi surface (FS) and experimental values of superconducting (SC) gaps. The resonance spectra split in both energy and momenta due to the effects of multiband and multiple gaps in $s^{\pm}-$pairing; the higher energy peak appears around the commensurate momenta due to scattering between $\alpha-$FS to $\gamma/\delta-$FS pockets. The second resonance is incommensurate coming from $\beta-$FS to $\gamma/\delta-$FS scatterings and its $q-$vector is doping-dependent and hence on the FS topology. Energies of both resonances $\omega^{1,2}_{res}$ are strongly doping dependent and are proportional to the gap amplitudes at the contributing FSs. We also discuss the evolution of the spin excitation spectra with various other possible gap symmetries, which may be relevant when either both the electron pockets or both the hole pockets completely disappear.
March 30
1. arXiv:1103.5725 [pdf, ps, other]
Title: Repulsive Fermions in Optical Lattices: Phase separation versus Coexistence of Antiferromagnetism and d-Superfluidity
Authors: S. Y. Chang, S. Pathak, N. Trivedi
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate a system of fermions on a two-dimensional optical square lattice in the strongly repulsive coupling regime. In this case, the interactions can be controlled by laser intensity as well as by Feshbach resonance. We compare the energetics of states with resonating valence bond d-wave superfluidity, antiferromagnetic long range order and a homogeneous state with coexistence of superfluidity and antiferromagnetism. We show that the energy density of a hole $e_{hole}(x)$ has a minimum at doping $x=x_c$ that signals phase separation between the antiferromagnetic and d-wave paired superfluid phases. The energy of the phase-separated ground state is however found to be very close to that of a homogeneous state with coexisting antiferromagnetic and superfluid orders. We explore the dependence of the energy on the interaction strength and on the three-site hopping terms and compare with the nearest neighbor hopping {\it t-J} model.
2. arXiv:1103.5603 [pdf, ps, other]
Title: Competition between Kondo and RKKY correlations in the presence of strong randomness
Authors: Minh-Tien Tran, Ki-Seok Kim
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We propose that competition between Kondo and magnetic correlations results in a novel universality class for heavy fermion quantum criticality in the presence of strong randomness. Starting from an Anderson lattice model with disorder, we derive an effective local field theory in the dynamical mean-field theory (DMFT) approximation, where randomness is introduced into both hybridization and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Performing the saddle-point analysis in the U(1) slave-boson representation, we reveal its phase diagram which shows a quantum phase transition from a spin liquid state to a local Fermi liquid phase. In contrast with the clean limit of the Anderson lattice model, the effective hybridization given by holon condensation turns out to vanish, resulting from the zero mean value of the hybridization coupling constant. However, we show that the holon density becomes finite when variance of hybridization is sufficiently larger than that of the RKKY coupling, giving rise to the Kondo effect. On the other hand, when the variance of hybridization becomes smaller than that of the RKKY coupling, the Kondo effect disappears, resulting in a fully symmetric paramagnetic state, adiabatically connected with the spin liquid state of the disordered Heisenberg model.
March 29
1. arXiv:1103.5422 [pdf, ps, other]
Title: Three Dimensional Topological Insulators with Landau Levels
Authors: Yi Li, Congjun Wu
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study the three dimensional topological insulators of spin-1/2 fermions coupling to the Aharanov-Casher SU(2) gauge field. They exhibit flat Landau levels in which orbital angular momentum and spin are coupled with a fixed helicity. In spite of the intrinsic spatial inhomogeneity, magnetic translations can be defined for the highest weight states of the total angular momentum. Each Landau level contributes one branch of gapless helical Dirac channel to the surface spectra, whose topological properties belong to the $\mathbb{Z}_{2}$-class. The flat Landau levels can be generalized to $N$ dimensions with the fundamental SO(N) spinor fermions coupling to the SO(N) gauge field.
2. arXiv:1103.5391 [pdf, other]
Title: Time Evolution with the DMRG Algorithm: A Generic Implementation for Strongly Correlated Electronic Systems
Authors: G. Alvarez, L. G. G. V. Dias da Silva, E. Ponce, E. Dagotto
Comments: RevTeX4, 9 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
A detailed description of the time-step-targetting time evolution method within the DMRG algorithm is presented. The focus of this publication is on the implementation of the algorithm, and on its generic application. The case of one-site excitations within a Hubbard model is analyzed as a test for the algorithm, using open chains and two-leg ladder geometries. The accuracy of the procedure in the case of the recently discussed holon-doublon photo excitations of Mott insulators is also analyzed. Performance and parallelization issues are discussed. In addition, the full open-source code is provided as supplementary material.
March 28
1. arXiv:1103.4985 [pdf, other]
Title: Impurity Effects in Highly Frustrated Diamond Lattice Antiferromagnets
Authors: Lucile Savary, Emanuel Gull, Simon Trebst, Jason Alicea, Doron Bergman, Leon Balents
Comments: 13 pages, 7 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We consider the effects of local impurities in highly frustrated diamond lattice antiferromagnets, which exhibit large but non-extensive ground state degeneracies. Such models are appropriate to many A-site magnetic spinels. We argue very generally that sufficiently dilute impurities induce an ordered magnetic ground state, and provide a mechanism of degeneracy breaking. The states which are selected can be determined by a "swiss cheese model" analysis, which we demonstrate numerically for a particular impurity model in this case. Moreover, we present criteria for estimating the stability of the resulting ordered phase to a competing frozen (spin glass) one. The results may explain the contrasting finding of frozen and ordered ground states in CoAl2O4 and MnSc2S4, respectively.
Mar 21 - Mar 25, Xiaopeng Li
March 25
1. arXiv:1103.4627 [pdf, ps, other]
Title: Symmetries of multifractal spectra and field theories of Anderson localization
Authors: I. A. Gruzberg, A. W. W. Ludwig, A. D. Mirlin, M. R. Zirnbauer
We uncover field-theoretic underpinnings of symmetry relations for multifractal spectra at Anderson transitions and at critical points of other disordered systems. We show that such relations follow from the conformal invariance of the critical theory, which implies their general character. We also demonstrate that for the Anderson localization problem the entire probability distribution for the local density of states possesses a symmetry arising from the invariance of correlation functions of the underlying non-linear $\sigma$-model with respect to the Weyl group of the target space of the model.
2. arXiv:1103.4630 [pdf, ps, other]
Title:Strongly Interacting Isotopic Bose-Fermi Mixture Immersed in a Fermi Sea
Authors: Cheng-Hsun Wu, Ibon Santiago, Jee Woo Park, Peyman Ahmadi, Martin W. Zwierlein
We have created a triply quantum degenerate mixture of bosonic $^{41}$K and two fermionic species $^{40}$K and $^6$Li. The boson is shown to be an efficient coolant for the two fermions, spurring hopes for the observation of fermionic superfluids with imbalanced masses. We observe multiple heteronuclear Feshbach resonances, in particular a wide s-wave resonance for the combination $^{41}$K-$^{40}$K, opening up studies of strongly interacting {\it isotopic} Bose-Fermi mixtures. For large imbalance, we enter the polaronic regime of dressed impurities immersed in a bosonic or fermionic bath.
3.arXiv:1103.4775 [pdf, other]
Title: Nonequilibrium entropy production for open quantum systems
Authors: Sebastian Deffner, Eric Lutz
We consider open quantum systems weakly coupled to a heat reservoir and driven by arbitrary time-dependent parameters. We derive exact microscopic expressions for the nonequilibrium entropy production and entropy production rate, valid arbitrarily far from equilibrium. By using the two-point energy measurement statistics for system and reservoir, we further obtain a quantum generalization of the integrated fluctuation theorem put forward by Seifert [PRL 95, 040602 (2005)].
4.arXiv:1103.4776 [pdf, other]
Title: Hartree shift in unitary Fermi gases
Author: J. J. Kinnunen
The Hartree energy shift is calculated for a unitary Fermi gas. By including the momentum dependence of the scattering amplitude explicitly, the Hartree energy shift remains finite even at unitarity. Extending the theory also for spin-imbalanced systems allows calculation of polaron properties. The results are in good agreement with more involved theories and experiments.
March 24
1. arXiv:1103.4473 [pdf, ps, other]
Title: Time-reversal-symmetry-broken quantum spin Hall effect
Author: Yunyou Yang, Zhong Xu, L. Sheng, Baigeng Wang, D. Y. Xing, D. N. Sheng
We study the fate of the quantum spin Hall (QSH) effect in the presence of an exchange field. It is found that the topological QSH state characterized by nonzero spin Chern numbers $C_{\pm}=\pm 1$ persists when the time reversal (TR) symmetry is broken. The corresponding counterpropagating spin-filtered edge states can remain to be gapless when the TR symmetry breaking term is turned off in the vicinity the sample edge. A topological phase transition from the TR symmetry-broken QSH phase to a quantum anomalous Hall phase occurs at a critical exchange field, where the bulk band gap just closes.
March 23
1. arXiv:1103.4232 [pdf, ps, other]
Title:Rydberg atoms in one-dimensional optical lattices
Author: M. Viteau, M. G. Bason, J. Radogostowicz, N. Malossi, D. Ciampini, O. Morsch, E. Arimondo
We experimentally realize Rydberg excitations in Bose-Einstein condensates of rubidium atoms loaded into quasi one-dimensional traps and in optical lattices. Our results for condensates expanded to different sizes in the one-dimensional trap agree well with the intuitive picture of a chain of Rydberg excitations. We also find that the Rydberg excitations in the optical lattice do not destroy the phase coherence of the condensate, and our results in that system agree with the picture of localized collective Rydberg excitations including nearest-neighbour blockade.
March 22
1. arXiv:1103.4087 [pdf, ps, other]
Title: Trimer liquids and crystals of polar molecules in coupled wires
Author: M. Dalmonte, P. Zoller, G. Pupillo
We investigate the pairing and crystalline instabilities of bosonic and fermionic polar molecules confined to a ladder geometry. By means of analytical and quasi-exact numerical techniques, we show that gases of composite molecular dimers as well as trimers can be stabilized as a function of the density difference between the wires. A shallow optical lattice can pin both liquids, realizing crystals of composite bosons or fermions. We show that these exotic quantum phases should be realizable under current experimental conditions in finite-size confining potentials.
2. arXiv:1103.4003 [pdf, other]
Title: How and why does statistical mechanics work
Author: Navinder Singh
As the title says we want to answer the question; how and why does statistical mechanics work? As we know from the most used prescription of Gibbs we calculate the phase space averages of dynamical quantities and we find that these phase averages agree very well with experiments. Clearly actual experiments are not done on a hypothetical ensemble they are done on the actual system in the laboratory and these experiments take a finite amount of time. Thus it is usually argued that actual measurements are time averages and they are equal to phase averages due to ergodicity. Aim of the present review is to show that ergodicity is not relevant for equilibrium statistical mechanics (with Tolman and Landau). We will see that the solution of the problem is in the very peculiar nature of the macroscopic observables and with the very large number of the degrees of freedom involved in macroscopic systems as first pointed out by Khinchin. Similar arguments are used by Landau based upon the approximate property of "Statistical Independence". We review these ideas in detail and in some cases present a critique. We review the role of chaos (classical and quantum) where it is important and where it is not important. We criticise the ideas of E. T. Jaynes who says that the ergodic problem is conceptual one and is related to the very concept of ensemble itself which is a by-product of frequency theory of probability, and the ergodic problem becomes irrelevant when the probabilities of various micro-states are interpreted with Laplace-Bernoulli theory of Probability (Bayesian viewpoint). In the end we critically review various quantum approaches to the foundations of statistical mechanics. It is argued that the eigenstate thermalization hypothesis is a special case of von Neumann's quantum ergodic theory.
March 21
1. arXiv:1103.3522 [pdf, other]
Title: A spin-orbit coupled Bose-Einstein condensate
Author: Y.-J. Lin, K. Jiménez-García, I. B. Spielman
Spin-orbit (SO) coupling -- the interaction between a quantum particle's spin and its momentum -- is ubiquitous in nature, from atoms to solids. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators, which are of extensive interest; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Ultracold atoms, quantum many-body systems under precise experimental control, would seem to be an ideal platform to study these fascinating SO coupled systems. While an atom's intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the center-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose-Einstein condensate by dressing two atomic spin states with a pair of lasers. Not only is this the first SO coupling realized in ultracold atomic gases, it is also the first ever for bosons. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase separated state (above a critical laser intensity). The location of this transition is in quantitative agreement with our theory. This SO coupling -- equally applicable for bosons and fermions -- sets the stage to realize topological insulators in fermionic neutral atom systems.
2. arXiv:1103.3694 [pdf]
Title: Quantum criticality in an Ising chain: experimental evidence for emergent E8 symmetry
Author: R. Coldea, D.A. Tennant, E.M. Wheeler, E. Wawrzynska, D. Prabhakaran, M. Telling, K. Habicht, P. Smeibidl, K. Kiefer
Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of 8 particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by tuning the quasi-one-dimensional Ising ferromagnet CoNb2O6 through its critical point using strong transverse magnetic fields. The spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean as predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviours.
Mar 14 - Mar 18, Zixu Zhang
Mar 18
]1. arXiv:1103.3315 [pdf, ps, other]
Title: Phases of the infinite U Hubbard model
Authors:Li Liu, Hong Yao, Erez Berg, Steven A. Kivelson
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We apply the density matrix renormalization group (DMRG) to study the phasediagram of the infinite U Hubbard model on 2-, 4-, and 6-leg ladders. Where theresults are largely insensitive to the ladder width, we consider the resultsrepresentative of the 2D square lattice model. We find a fully polarizedferromagnetic Fermi liquid phase when n, the density of electrons per site, isin the range 1>n>n_F ~ 4/5. For n=3/4 we find an unexpected commensurateinsulating "checkerboard" phase with coexisting bond density order with 4 sitesper unit cell and block spin antiferromagnetic order with 8 sites per unitcell. For 3/4 > n, the wider ladders have unpolarized groundstates, which issuggestive that the same is true in 2D.
]2. arXiv:1103.3323 [pdf, other]
Title: Towards a complete classification of 1D gapped quantum phases in interacting spin systems
Authors:Xie Chen, Zheng-Cheng Gu, Xiao-Gang WenComments: 15 pages, 3 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Quantum phases with different orders exist with or without breaking thesymmetry of the system. Recently, a classification of gapped quantum phaseswhich do not break time reversal, parity or on-site unitary symmetry has beengiven for 1D spin systems in [X. Chen, Z.-C. Gu, and X.-G. Wen, Phys. Rev. B83, 035107 (2011); arXiv:1008.3745]. It was found that, such symmetry protectedtopological (SPT) phases are labeled by the projective representations of thesymmetry group which can be viewed as a symmetry fractionalization. In thispaper, we extend the classification of 1D gapped phases by considering SPTphases with combined time reversal, parity, and/or on-site unitary symmetriesand also considering the possibility of symmetry breaking. In this way, weobtain a complete classification of gapped quantum phases in 1D spin systems.We find that in general, symmetry fractionalization, symmetry breaking and longrange entanglement(present in 2 or higher dimensions) represent three mainmechanisms to generate a very rich set of gapped quantum phases. As anapplication of our classification, we study the possible SPT phases in 1Dfermionic systems, which can be mapped to spin systems by Jordan-Wignertransformation.
Mar 17
]1. arXiv:1103.3106 [pdf, ps, other]
Title: Gutzwiller study of extended Hubbard models with fixed boson densities
Authors:Takashi Kimura
Comments: 13 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We studied all possible ground states, including supersolid (SS) phases andphase separations of hard-core and soft-core extended Bose--Hubbard models withfixed boson densities by using the Gutzwiller variational wave function. Wefound that the phase diagram of the soft-core model strongly depends on itstransfer integral. Furthermore, for a large transfer integral, we showed that aSS phase can be the ground state even below half filling. Finally, we foundthat the density difference between nearest-neighbor sites, which shows thedensity order of the SS phase, strongly depends on the boson density,especially for a small transfer integral.
]2. arXiv:1103.3145 [pdf, ps, other]
Title: Ultracold Dipolar Gases in Optical Lattices
Authors:C. Trefzger, C. Menotti, B. Capogrosso-Sansone, M. Lewenstein
Comments: 56 pages, 26 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
This tutorial is a theoretical work, in which we study the physics ofultra-cold dipolar bosonic gases in optical lattices. Such gases consist ofbosonic atoms or molecules that interact via dipolar forces, and that arecooled below the quantum degeneracy temperature, typically in the nK range.When such a degenerate quantum gas is loaded into an optical lattice producedby standing waves of laser light, new kinds of physical phenomena occur. Thesesystems realize then extended Hubbard-type models, and can be brought to astrongly correlated regime. The physical properties of such gases, dominated bythe long-range, anisotropic dipole-dipole interactions, are discussed using themean-field approximations, and exact Quantum Monte Carlo techniques (the Wormalgorithm).
Mar 16
]1. arXiv:1103.2770 [pdf, ps, other]
Title: Topological periodic superconductor-nanowire structures
Authors:Jay D. Sau, Chien Hung Lin, Hoi-Yin Hui, S. Das Sarma
Comments: 4.5 pages, 3 Figures
Subjects: Superconductivity (cond-mat.supr-con)
Semiconducting nanowires in proximity to superconductors are among promisingcandidates to search for Majorana fermions which may ultimately be used asbuilding blocks for topological quantum computers. A serious challenge in theexperimental realization of the Majorana fermion in thesesemiconductor-superconductor sandwich structures is the problem of tuning thesemiconductor chemical potential in close proximity to the metallicsuperconductor. This poses a central challenge to the field. In this work, we show how toovercome this challenge, thus resolving a crucial barrier to the solid staterealization of a topological system containing the Majorana fermion. We propose a new topological superconducting array structure where it is notnecessary to gate segments of the nanowire in direct contact with asuperconductor, thus making it easy to tune the chemical potential in thenanowire.
]2. arXiv:1103.2851 [pdf, ps, other]
Title: The Unitary Gas and its Symmetry Properties
Authors:Yvan Castin (LKB - Lhomond), Félix Werner
Comments: 63 pages, 8 figures. Contribution to Lecture Notes in Physics "BEC-BCS Crossover and the Unitary Fermi gas" edited by Wilhelm Zwerger
Subjects: Quantum Gases (cond-mat.quant-gas)
The physics of atomic quantum gases is currently taking advantage of apowerful tool, the possibility to fully adjust the interaction strength betweenatoms using a magnetically controlled Feshbach resonance. For fermions with twointernal states, formally two opposite spin states, this allows to prepare longlived strongly interacting three-dimensional gases and to study the BEC-BCScrossover. Of particular interest along the BEC-BCS crossover is the so-calledunitary gas, where the atomic interaction potential between the opposite spinstates has virtually an infinite scattering length and a zero range. Thisunitary gas is the main subject of the present chapter: It has fascinatingsymmetry properties, from a simple scaling invariance, to a more subtledynamical symmetry in an isotropic harmonic trap, which is linked to aseparability of the N-body problem in hyperspherical coordinates. Otheranalytical results, valid over the whole BEC-BCS crossover, are presented,establishing a connection between three recently measured quantities, the tailof the momentum distribution, the short range part of the pair distributionfunction and the mean number of closed channel molecules.
]3. arXiv:1103.2962 [pdf, ps, other]
Title: FFLO and domain structure of the repulsive Hubbard model with correlated hopping
Authors:A. Anfossi, C. Degli Esposti Boschi, A. Montorsi
Comments: RevTeX 4, 9 pages, 13 composite *.eps figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
We observe the effect of non-zero magnetization m onto the superconductingground state of the one dimensional repulsive Hubbard model with correlatedhopping X. For t/2 < X < 2t/3, the system first manifestsFulde-Farrel-Larkin-Ovchinnikov (FFLO) oscillations in the pair-paircorrelations. For m = m1 a kinetic energy driven macroscopic phase separationinto low-density superconducting domains and high-density polarized walls takesplace. For m > m2 the domains fully localize, and the system eventually becomesa ferrimagnetic insulator.
4.
Mar 15
]1. arXiv:1103.2420 [pdf, ps, other]
Title: Spontaneous quantum Hall effect in quarter doped Hubbard model on honeycomb lattice and its possible realization in quarter doped graphene system
Authors:Tao LiComments: 5 pages, 5 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)
We show as the result of the nesting property of the Fermi surface, thequarter doped Hubbard model on honeycomb lattice is unstable toward theformation of a magnetic insulating state with nonzero spin chirality forinfinite small value of electron correlation. The insulating state is found tobe topological and exhibit quantized Hall effect with$\sigma_{xy}=\frac{e^{2}}{h}$. Interestingly, we find the nesting of the Fermisurface is robust for arbitrary value of the next-nearest neighboring hoppingintegrals. It is thus very possible that the quarter doped graphene system willexhibit such an exotic ground state.
]2. arXiv:1103.2473 [pdf, ps, other]
Title: Orbital multicriticality in spin gapped quasi-1D antiferromagnets
Authors:Eran Sela, R. G. Pereira
Comments: 5 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Motivated by the quasi-1D antiferromagnet CaV$_2$O$_4$, we explore a newroute to realize exotic orbital phases in spin-orbital systems in which thespin modes are gapped but orbitals are near a macroscopically degenerateclassical transition. Within a simplified model we show that orbital liquidphases possessing power-law correlations may occur without the strict conditionof a continuous orbital symmetry. For the model proposed for CaV$_2$O$_4$, wefind that an orbital phase with coexisting order parameters emerges from amulticritical point. The effective orbital model consists of nontrivialzigzag-coupled transverse field Ising chains. The corresponding global phasediagram is conjectured using field theory methods and analyzed near themulticritical point with the aid of an exact solution of a zigzag XXZ model.
Mar 14
]1. arXiv:1103.2129 [pdf, ps, other]
Title: Engineering a p+ip Superconductor: Comparison of Topological Insulator and Rashba Spin-Orbit Coupled Materials
Authors: Andrew C. Potter, Patrick A. Lee
Comments: 10 pages, 4 Figures
Subjects: Superconductivity (cond-mat.supr-con); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We compare topological insulator materials and Rashba coupled surfaces as candidates for engineering p+ip superconductivity. Specifically, in each type of material we examine 1) the limitations to inducing superconductivity by proximity to an ordinary s-wave superconductor, and 2) the robustness of the resulting superconductivity against disorder. We find that topological insulators have strong advantages in both regards: there are no fundamental barriers to inducing superconductivity, and the induced superconductivity is immune to disorder. In contrast, for Rashba coupled quantum wires or surface states, the the achievable gap from induced superconductivity is limited unless the Rashba coupling is large. Furthermore, for small Rashba coupling the induced superconductivity is strongly susceptible to disorder. These features pose serious difficulties for realizing p+ip superconductors in semiconductor materials due to their weak spin-orbit coupling, and suggest the need to seek alternatives. Some candidate materials are discussed.
]2.arXiv:1103.2337 [pdf, other]
Title: Spin Transport in Polaronic and Superfluid Fermi Gases
Authors: Ariel Sommer, Mark Ku, Martin W. Zwierlein
Comments: 14 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We present measurements of spin transport in ultracold gases of fermionic lithium-6 in a mixture of two spin states at a Feshbach resonance. In particular, we study the spin dipole mode, where the two spin components are displaced from each other against a harmonic restoring force. We prepare a highly-imbalanced, or polaronic, spin mixture with a spin dipole excitation and observe strong, unitarity limited damping of the spin dipole mode. In gases with small spin imbalance, below the Pauli limit for superfluidity, we observe strongly damped spin flow despite the presence of a superfluid core.
Mar 7 - Mar 11, Chungwei Lin
Mar 11
1. arXiv:1103.1926 [pdf, ps, other]
Title: Quantum magnetism of ultra-cold fermion systems with the symplectic symmetry
Authors: Hsiang-hsuan Hung, Yupeng Wang, Congjun Wu
Comments: 16 pages
Subjects: Quantum Gases (cond-mat.quant-gas)
We numerically study quantum magnetism of ultra-cold alkali and alkaline-earth fermion systems with large hyperfine spin $F=3/2$, which are characterized by a generic $Sp(N)$ symmetry with N=4. The methods of exact diagonalization (ED) and density-matrix-renormalization-group are employed for the large size one-dimensional (1D) systems, and ED is applied to a two-dimensional (2D) square lattice on small sizes. We focus on the magnetic exchange models in the Mott-insulating state at quarter-filling. Both 1D and 2D systems exhibit rich phase diagrams depending on the ratio between the spin exchanges $J_0$ and $J_2$ in the bond spin singlet and quintet channels, respectively. In 1D, the ground states exhibit a long-range-ordered dimerization with a finite spin gap at $J_0/J_2>1$, and a gapless spin liquid state at $J_0/J_2 \le 1$, respectively. In the former and latter cases, the correlation functions exhibit the two-site and four-site periodicities, respectively. In 2D, various spin correlation functions are calculated up to the size of $4\times 4$. The Neel-type spin correlation dominates at large values of $J_0/J_2$, while a $2\times 2$ plaquette correlation is prominent at small values of this ratio. Between them, a columnar spin-Peierls dimerization correlation peaks. We infer the competitions among the plaquette ordering, the dimer ordering, and the Neel ordering in the 2D system.
2. arXiv:1103.2039 [pdf, other]
Title: Self-similar expansion of the density profile in a turbulent Bose-Einstein condensate
Authors: Mônica Caracanhas, Alexander L. Fetter, Sérgio R. Muniz, K. M. F. Magalhães, G. Roati, G. Bagnato, Vanderlei S. Bagnato
Comments: 8 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Fluid Dynamics (physics.flu-dyn)
In a recent study we have demonstrated the emergence of a turbulent regime in a trapped Bose-Einstein condensate of Rb-87 atoms. An intriguing observation in such system is the behavior of the turbulent cloud during free expansion.The aspect ratio of the cloud sizes does not change in the way one would expect for an ordinary nonrotating (vortex-free) condensate. Here we show that this self-similar expansion of the density profile can be understood, at least qualitatively, in terms of the presence of rotation (vortices) distributed in the cloud. Using a hydrodynamic approach we demonstrate the deceleration of the aspect ratio change due to the presence of this distributed vorticity.
3. arXiv:1103.2115 [pdf, ps, other]
Title: Fermi-surface reconstruction in a smectic phase of a high temperature superconductor
Authors: Hong Yao, Dung-Hai Lee, Steven A. Kivelson
Comments: 4 pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
It is shown that, in the presence of a moderately strong C_4 symmetry breaking (which could be produced either by lattice orthorhombicity or the presence of an electron nematic phase), a weak, period 4, unidirectional charge density wave ("charge stripe") order can reconstruct the Fermi surface of a typical hole-doped cuprate to produce a small electron pocket. This form of charge density wave order is consistent with that adduced from recent high field NMR experiments in YBCO. The Fermi pocket has an area and effective mass which is a rough caricature of those seen in recent high field quantum oscillation experiments.
Mar 10
1. arXiv:1103.1638 [pdf, ps, other]
Title: Quantum Phase Transitions around the Staggered Valence Bond Solid
Authors: Cenke Xu, Leon Balents
Comments: 9 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Motivated by recent numerical results, we study the quantum phase transitions between Z_2 spin liquid, Neel ordered, and various valence bond solid (VBS) states on the honeycomb and square lattices, with emphasis on the staggered VBS. In contrast to the well-understood columnar VBS order, the staggered VBS is not described by an XY order parameter with Z_N anisotropy close to these quantum phase transitions. Instead, we demonstrate that on the honeycomb lattice, the staggered VBS is more appropriately described as an O(3) or CP(2) order parameter with cubic anisotropy, while on the square lattice it is described by an O(4) or CP(3) order parameter.
2. arXiv:1103.1860 [pdf, other]
Title: Strongly interacting bosons in a 1D optical lattice at incommensurate densities
Authors: Achilleas Lazarides, Olivier Tieleman, Cristiane Morais Smith
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate quantum phase transitions occurring in a system of strongly interacting ultracold bosons in a 1D optical lattice. After discussing the commensurate-incommensurate transition, we focus on the phases appearing at incommensurate filling. We find a rich phase diagram, with superfluid, supersolid and solid (kink-lattice) phases. Supersolids generally appear in theoretical studies of systems with long-range interactions; our results break this paradigm and show that they may also emerge in models including only short-range (contact) interactions, provided that quantum fluctuations are properly taken into account.
3. arXiv:1103.1686 (cross-list from cond-mat.str-el) [pdf, ps, other]
Title: Fractional Quantum Hall Effect of Hard-Core Bosons in Topological Flat Bands
Authors: Yi-Fei Wang, Zheng-Cheng Gu, Chang-De Gong, D. N. Sheng
Comments: 4 pages, 6 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
Recent proposals of topological flat band (TFB) models have provided a new route to realize the fractional quantum Hall effect (FQHE) without Landau levels. We study hard-core bosons with short-range interactions in two representative TFB models, one of which is the well known Haldane model (but with different parameters). We demonstrate that FQHE states emerge with signatures of even number of quasi-degenerate ground states on a torus and a robust spectrum gap separating these states from higher energy spectrum. We also establish quantum phase diagrams for the filling factor 1/2 and illustrate quantum phase transitions to other competing symmetry-breaking phases.
Mar 9
1. arXiv:1103.1549 (cross-list from quant-ph) [pdf, ps, other]
Title: Model independence in two dimensions and polarized cold dipolar molecules
Authors: A. G. Volosniev, D. V. Fedorov, A. S. Jensen, N. T. Zinner
Comments: 5 pages, 3 figures, Revtex4
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Mathematical Physics (math-ph)
We prove rigorously that two particles interacting via arbitrary anisotropic potentials with negative or zero net volume always have at least one bound state. We derive general analytic expressions for energies and wave functions in the weak coupling limit where universality or model independence are approached. The monopole part of anisotropic potentials is crucial in the universal limit. We illustrate the analytic results with a system of two arbitrarily polarized cold dipolar molecules in a bilayer. We discuss the transition to universality as function of polarization and binding energy, and compare analytic and numerical results obtained by the stochastic variational method. The universal limit is essentially reached for experimentally accessible strengths.
2. arXiv:1103.1557 (cross-list from nlin.PS) [pdf, other]
Title: Solitons supported by localized nonlinearities in periodic media
Authors: Nir Dror, Boris A. Malomed
Subjects: Pattern Formation and Solitons (nlin.PS); Quantum Gases (cond-mat.quant-gas); Optics (physics.optics)
Nonlinear periodic systems, such as photonic crystals and Bose-Einstein condensates (BECs) loaded into optical lattices, are often described by the nonlinear Schr\"odinger/Gross-Pitaevskii equation with a sinusoidal potential. Here, we consider a model based on such a periodic potential, with the nonlinearity (attractive or repulsive) concentrated either at a single point or at a symmetric set of two points, which are represented, respectively, by a single {\delta}-function or a combination of two {\delta}-functions. This model gives rise to ordinary solitons or gap solitons (GSs), which reside, respectively, in the semi-infinite or finite gaps of the system's linear spectrum, being pinned to the {\delta}-functions. Physical realizations of these systems are possible in optics and BEC, using diverse variants of the nonlinearity management. First, we demonstrate that the single {\delta}-function multiplying the nonlinear term supports families of stable regular solitons in the self-attractive case, while a family of solitons supported by the attractive {\delta}-function in the absence of the periodic potential is completely unstable. We also show that the {\delta}-function can support stable GSs in the first finite gap in both the self-attractive and repulsive models. The stability analysis for the GSs in the second finite gap is reported too, for both signs of the nonlinearity. Alongside the numerical analysis, analytical approximations are developed for the solitons in the semi-infinite and first two finite gaps, with the single {\delta}-function positioned at a minimum or maximum of the periodic potential. In the model with the symmetric set of two {\delta}-functions, we study the effect of the spontaneous symmetry breaking of the pinned solitons. Two configurations are considered, with the {\delta}-functions set symmetrically with respect to the minimum or maximum of the potential.
3. arXiv:1103.1395 (cross-list from cond-mat.supr-con) [pdf, ps, other]
Title: Electronic States at the Paraelectric and Superconducting Interface
Authors: J.T. Haraldsen, S.A. Trugman, A.V. Balatsky
Comments: 4 pages, 3 figures, Submitted to Phys. Rev. B
Subjects: Superconductivity (cond-mat.supr-con); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)
We examine the modified electronic states at the interface between superconducting and ferro(para)-electric heterostructures. We find that electric polarization $P$ and superconducting $\psi$ order parameters can be significantly modified due to coupling through linear terms brought about by explicit symmetry breaking at the interface. Using an effective action and a Ginzburg-Landau formalism, we show that an interaction term linear in the electric polarization will modify the superconducting order parameter $\psi$ at the interface. This also produces modulation of a ferroelectric polarization. It is shown that a paraelectric-superconductor interaction will produce an interface-induced ferroelectric polarization.
Mar 8
1. arXiv:1103.1372 [pdf, other]
Title: Quantum Simulation of an Antiferromagnetic Spin Chain in an Optical Lattice
Authors: Jonathan Simon, Waseem S. Bakr, Ruichao Ma, M. Eric Tai, Philipp M. Preiss, Markus Greiner
Comments: 12 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications from high temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers due to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we employ a degenerate Bose gas confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary an applied field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase the interaction between the spins is overwhelmed by the applied field which aligns the spins. In the antiferromagnetic phase the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through both in-situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, improving our understanding of real magnetic materials.
2. arXiv:1103.1022 [pdf, ps, other]
Title: Orbital elementary excitations as probes of entanglement and quantum phase transitions of collective spins in an entangled Bose-Einstein condensate
Authors: Rukuan Wu, Yu Shi
Comments: 5 pages, received by PRA on 22 December 2010
Journal-ref: Phys. Rev. A 83, 025601 (2011)
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
A mixture of two species of pseudospin-1/2 Bose gases exhibits interesting interplay between spin and orbital degrees of freedom. Expectation values of various quantities of the collective spins of the two species play crucial roles in the Gross-Pitaevskii-like equations governing the four orbital wave functions in which Bose-Einstein condensation occurs. Consequently, the elementary excitations of these orbital wave functions reflect properties of the collective spins. When the coupling between the two collective spins is isotropic, the energy gap of the gapped orbital excitation peaks, while there is a quantum phase transition in the ground state of the effective Hamiltonian of the two collective spins, which have previously been found to be maximally entangled.
Mar 7
1. arXiv:1103.0787 (cross-list from cond-mat.dis-nn) [pdf, ps, other]
Title: Quantum Quenches in Disordered Systems: Approach to Thermal Equilibrium without a Typical Relaxation Time
Authors: Antonio M. Garcia-Garcia, Armando Relaño, Marcos Rigol
Comments: 4 pages, 4 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We study spectral properties and the dynamics after a quench of one-dimensional spinless fermions with short-range interactions and long-range random hopping. We show that a sufficiently fast decay of the hopping term promotes localization effects at finite temperature, which prevent thermalization even if the classical motion is chaotic. For slower decays, we have found that this model falls in a new universality class: power-law approach to thermal equilibrium, i.e., it is not possible to define a finite time scale for which corrections to the expectation values of observables with respect to thermal equilibrium predictions are exponentially small.
2. arXiv:1103.0836 (cross-list from quant-ph) [pdf, ps, other]
Title: Diverging equilibration times in long-range quantum spin models
Authors: Michael Kastner
Comments: 4+ pages, 1 figure
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)
Approach to equilibrium is studied for long-range quantum Ising models where the interaction strength decays like r^{-\alpha} at large distances r with an exponent $\alpha$ not exceeding the lattice dimension. For a large class of observables and initial states, the time evolution of expectation values can be calculated. We prove analytically that, at a given instant of time t and for sufficiently large system size N, the expectation value of some observable <A>(t) will practically be unchanged from its initial value <A>(0). This finding implies that, for large enough N, equilibration effectively occurs on a time scale beyond the experimentally accessible one and will not be observed in practice.
3. arXiv:1103.0936 [pdf, other]
Title: Time-dependent variational principle for quantum lattices
Authors: Jutho Haegeman, J. Ignacio Cirac, Tobias J. Osborne, Iztok Pizorn, Henri Verschelde, Frank Verstraete
Comments: main text (4 pages, 2 figures, 1 table) + supplementary material (13 pages, 2 figures)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We develop a new algorithm based on the time-dependent variational principle applied to matrix product states to efficiently simulate the real- and imaginary time dynamics for infinite one-dimensional quantum lattice systems. This procedure: (1) is argued to be optimal; (2) does not rely on the Trotter decomposition and thus has no Trotter error; (3) explicitly preserves all symmetries and conservation laws; and (4) has low computational complexity. The algorithm is illustrated using both imaginary time and real-time examples.
4. arXiv:1103.0965 [pdf, ps, other]Title: Surface effects in doping a Mott insulator
Authors: Reza Nourafkan, Frank Marsiglio
Comments: accepted in Phys. Rev. B
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
The physics of doping a Mott insulator is investigated in the presence of a solid-vacuum interface. Using the embedding approach for dynamical mean field theory we show that the change in surface spectral evolution in a doped Mott insulator is driven by a combination of charge transfer effects and enhanced correlation effects. Approaching a Mott insulating phase from the metallic side, we show that a dead layer forms at the surface of the solid, where quasiparticle amplitudes are exponentially suppressed. Surface correlation and charge transfer effects can be strongly impacted by changes of the hopping integrals at the surface.
