Aug 29 - Sep 2, Zixu Zhang
Sep 2
1. arXiv:1109.0017 [pdf, ps, other]
Title: Correlated topological insulators of Cooper pairs induced by proximity effect
Authors:Predrag Nikolic, Tanja Duric
Comments: 4 pages, 1 figure
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We argue that a two-dimensional spin-orbit topological band-insulator with induced pairing interaction exhibits a superconductor-insulator quantum phase transition with inert high-energy fermionic excitations. The resulting paired insulator can be a topologically ordered "pseudogap" state of p-wave Cooper pairs, whose excitations have fractional charge, spin and generally non-Abelian exchange statistics. We discuss the qualitative properties of this unconventional state and propose a heterostructure device that could host it.
2. arXiv:1109.0145 [pdf, ps, other]
Title: Tan's distributions and Fermi-Huang pseudopotential in momentum space
Authors:Manuel Valiente
Comments: 4 pages, no figures
Subjects: Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)
The long-standing question of finding the momentum representation for the s-wave zero-range interaction in three spatial dimensions is here solved. This is done by expressing a certain distribution, introduced in a formal way by S. Tan [Ann. Phys. 323, 2952 (2008)], explicitly. The resulting form of the Fourier-transformed pseudopotential remains very simple. Operator forms for the so-called Tan's selectors which, together with Fermi-Huang pseudopotential, largely simplify the derivation of Tan's universal relations for the Fermi gas, are here derived and are also very simple. A momentum cut-off version of the pseudopotential is also provided, and with this no apparent contradiction with the notion of integrals in Tan's methods is left. The equivalence, even at the intermediate step level, between the pseudopotential approach and momentum-space renormalization of the bare Dirac delta interaction is then shown by using the explicit form of the cut-off pseudopotential.
Sep 1
1. arXiv:1108.6064 [pdf, other]
Title: Quantum criticality of dipolar spin chains
Authors:Aldo Isidori, Annika Ruppel, Andreas Kreisel, Peter Kopietz, Alexander Mai, Reinhard M. Noack
Comments: 4 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech)
We show that a chain of Heisenberg spins interacting with long-range dipolar forces in a magnetic field h perpendicular to the chain exhibits a quantum critical point belonging to the two-dimensional Ising universality class. Within linear spin-wave theory the magnon dispersion for small momenta k is [Delta^2 + v_k^2 k^2]^{1/2}, where Delta^2 \propto |h - h_c| and v_k^2 \propto |ln k|. For fields close to h_c linear spin-wave theory breaks down and we investigate the system using density-matrix and functional renormalization group methods. The Ginzburg regime where non-Gaussian fluctuations are important is found to be rather narrow on the ordered side of the transition, and very broad on the disordered side.
Aug 31
1. arXiv:1108.5993 [pdf, other]
Title: A Strongly Dipolar Bose-Einstein Condensate of Dysprosium
Authors:Mingwu Lu, Nathaniel Q. Burdick, Seo Ho Youn, Benjamin L. Lev
Comments: 4 pages plus refs on fifth, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
We report the Bose-Einstein condensation (BEC) of the most magnetic atom, dysprosium. The Dy BEC is the first for an open f-shell lanthanide (rare-earth) element and is produced via forced evaporation in a crossed optical dipole trap loaded by an unusual, blue-detuned and spin-polarized narrow-line magneto-optical trap. Nearly pure condensates of 1.5x10^4 164Dy atoms form below T = 15 nK. We observe that stable BEC formation depends on the relative angle of a small polarizing magnetic field to the axis of the oblate trap, a property of trapped condensates only expected in the strongly dipolar regime. This regime was heretofore only attainable in Cr BECs via a Feshbach resonance accessed at high magnetic field.
2. arXiv:1108.6047 [pdf, other]
Title: Effective multi-body induced tunneling and interactions in the Bose-Hubbard model of the lowest dressed band of an optical lattice
Authors:U. Bissbort, F. Deuretzbacher, W. Hofstetter
Comments: 12 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We construct the effective lowest-band Bose-Hubbard model incorporating interaction-induced on-site correlations. The model is based on ladder operators for local correlated states, which deviate from the usual Wannier creation and annihilation, allowing for a systematic construction of the most appropriate single-band low-energy description in form of the extended Bose-Hubbard model. A formulation of this model in terms of ladder operators not only naturally contains the previously found effective multi-body interactions, but also contains \emph{multi-body induced} single particle tunneling, pair tunneling and nearest-neighbor interaction processes of higher orders. An alternative description of the same model can be formulated in terms of occupation-dependent Bose-Hubbard parameters. These multi-particle effects can be enhanced using Feshbach resonances, leading to corrections which are well within experimental reach and of significance to the phase diagram of ultracold bosonic atoms in an optical lattice. We analyze the energy reduction mechanism of interacting atoms on a local lattice site and show that this cannot be explained only by a spatial broadening of Wannier orbitals on a single particle level.
Aug 30
1. arXiv:1108.5408 [pdf, other]
Title: Bragg Scattering as a Probe of Atomic Wavefunctions and Quantum Phase Transitions in Optical Lattices
Authors:Hirokazu Miyake, Georgios A. Siviloglou, Graciana Puentes, David E. Pritchard, Wolfgang Ketterle, David M. Weld
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We have observed Bragg scattering of photons from quantum degenerate $^{87}$Rb atoms in a three-dimensional optical lattice. Bragg scattered light directly probes the microscopic crystal structure and atomic wavefunction whose position and momentum width is Heisenberg-limited. The spatial coherence of the wavefunction leads to revivals in the Bragg scattered light due to the atomic Talbot effect. The decay of revivals across the superfluid to Mott insulator transition indicates the loss of superfluid coherence.
2. arXiv:1108.5650 [pdf, ps, other]
Title: Isotropic Landau levels of Dirac fermions in high dimensions
Authors:Yi Li, Kenneth Intriligator, Yue Yu, Congjun Wu
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We generalize the Landau levels of two-dimensional Dirac fermions to three dimensions and above with the full rotational symmetry. Similar to the two dimensional case, there exists a branch of zero energy Landau levels of fractional fermion modes for the massless Dirac fermions. The spectra of other Landau levels distribute symmetrically with respect to the zero energy scaling with the square root of the Landau level indices. This mechanism is a non-minimal coupling of Dirac fermions to the background fields. This high dimensional relativistic Landau level problem is a square root problem of its previous studied non-relativistic version investigated in Ref. [1].
3. arXiv:1108.5712 [pdf, ps, other]
Title: Density of States of Quantum Spin Systems from Isotropic Entanglement
Authors:Ramis Movassagh, Alan Edelman
Comments: 4+ pages, content is as in the published version
Journal-ref: Phys. Rev. Lett. 107, 097205 (2011)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We propose a method which we call "Isotropic Entanglement" (IE), that predicts the eigenvalue distribution of quantum many body (spin) systems (QMBS) with generic interactions. We interpolate between two known approximations by matching fourth moments. Though, such problems can be QMA-complete, our examples show that IE provides an accurate picture of the spectra well beyond what one expects from the first four moments alone. We further show that the interpolation is universal, i.e., independent of the choice of local terms.
Aug 29
1. arXiv:1108.5207 [pdf, other]
Visibility of the Amplitude (Higgs) Mode in Condensed Matter
Daniel Podolsky, Assa Auerbach, Daniel P. Arovas
Comments: 16 pages, 8 figures
Subjects: Superconductivity (cond-mat.supr-con); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el)
The amplitude mode is a ubiquitous collective excitation in condensed matter systems with broken continuous symmetry. It is expected in antiferromagnets, short coherence length superconductors, charge density waves, and lattice Bose condensates. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the amplitude mode can decay into low energy Goldstone modes, its experimental visibility has been questioned. Here we show that the visibility depends on the symmetry of the measured susceptibility. The longitudinal susceptibility diverges at low frequency as \chi_{\sigma\sigma} ~ \omega^{-1} (d=2) or \log(1/|\omega|) (d=3), which can completely obscure the amplitude peak. In contrast, the scalar susceptibility is suppressed by four extra powers of frequency which exposes the amplitude peak throughout the ordered phase. We discuss experimental setups for measuring the scalar susceptibility. The conductivity of the O(2) theory (relativistic superfluid), is a scalar response, and therefore exhibits suppressed absorption below the Higgs mass threshold, \sigma ~ \omega^{2d+1}. In layered, short coherence length superconductors, (relevant e.g. to cuprates) this threshold is raised by the interlayer plasma frequency.
2. arXiv:1108.5380 [pdf, ps, other]
Thermalization in a one-dimensional integrable system
Pjotrs Grisins, Igor E. Mazets
Comments: 4 pages revtex4, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We present numerical results demonstrating the possibility of thermalization of single-particle observables in a one-dimensional integrable system (a quasicondensate of ultra-cold, weakly-interacting bosonic atoms being studied as a definite example). These results may seem counterintuitive because the physical system is integrable in both the quantum and classical (mean-field) descriptions. However, we find a class of initial conditions that admits the relaxation of distributions of single-particle observables to the equilibrium state very close to the Bose-Einstein thermal distribution of Bogoliubov quasiparticles. Our numerical results allow us to explain experimentally observed thermalization in one-dimensional ultracold atomic gases on a short (~1 ms) time scale.
Aug 22 - 26, Chungwei Lin
Aug 26
1. arXiv:1108.5000 [pdf, ps, other]
Exact dark state solutions of the coupled atomic-molecular Bose-Einstein condensates in an external potential
Xiao-Fei Zhang, Jun-Chao Chen, Biao Li, Lin Wen, W. M. Liu
Comments: 6 pages,5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We consider a coupled nonlinear Schr\"{o}dinger equations describing an atomic Bose-Einstein condensates coupled to a molecular condensates through the stimulated Raman adiabatic passage loaded in an external potential. The existence of dark state are investigated within the full parameter space accounts for all the nonlinear collisions, together with the atom-molecule conversion coupling and external potential. The results show that there are a class of external potentials such as double-well, periodical and double periodical, and harmonic potentials, where the exact dark solutions can be formed. Our results may raise the possibility of relative experiments for dark state in the coupled atomic-molecular Bose-Einstein condensates.
2. arXiv:1108.5075 [pdf, ps, other]
Nonadiabatic creation of macroscopic superpositions with strongly correlated 1D bosons on a ring trap
C. Schenke, A. Minguzzi, F.W.J. Hekking
Comments: 4 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We consider a strongly interacting quasi-one dimensional Bose gas on a tight ring trap subjected to a localized barrier potential which is suddenly set into motion. Using an exact solution for the dynamical evolution in the impenetrable-boson (Tonks-Girardeau) limit, we predict the formation of a macroscopic superposition of a rotating and a nonrotating state. We find that the barrier velocity should be tuned close to multiples of integer or half-integer number of Coriolis flux quanta to maximize the nonadiabatic excitation, and it should be larger than the sound velocity to better discriminate the two components of the superposition.
3. arXiv:1108.5121 [pdf, ps, other]
Multistage Kondo effect as a manifestation of dynamical symmetries in the single- and two-electron tunneling
K. Kikoin
Comments: 13 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
The concept of dynamical symmetries is used for formulation of the renormalization group approach to the Kondo effect in the Anderson model with repulsive and attractive interaction $U$. It is shown that the generic local symmetry of the Anderson Hamiltonian is determined by the SU(4) Lie group. The Anderson Hamiltonian is rewritten in terms of the Gell-Mann matrices of the 4-th rank, which form the set of group generators and the basis for construction of irreducible vector operators describing the excitation spectra in the charge and spin sectors. The multistage Kondo sceening is described in terms of the local SU(4) dynamical symmetry. It is shown that the similarity between the conventional Kondo cotunneling effect for spin 1/2 in the positive $U$ model and the Kondo resonance for pair tunneling in the negative $U$ model is a direct manifestation of implicit SU(4) symmetry of the Anderson/Kondo model.
Aug 25
1. arXiv:1108.4695 [pdf, ps, other]
Quantum phase transitions in optical lattices beyond Bogoliubov approximation
H. Kleinert, Z. Narzikulov, Axel Pelster, Abdulla Rakhimov
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the quantum phase transition in optical lattices using ordinary Bose Hubbard Hamiltonian within two loop approximation in variational perturbation theory. We have shown that this approximation can reproduce superfluid Mott insulator transition in contrast to the simple Bogoliubov or Hartee - Fock - Popov approximations. The superfluid fraction and ground state energy per particle vs input parameters of the model are studied.
2. arXiv:1108.4786 [pdf, ps, other]
Cavity-induced switching between localized and extended states in a non-interacting Bose-Einstein condensate
Lu Zhou, Han Pu, Keye Zhang, Xing-Dong Zhao, Weiping Zhang
Subjects: Quantum Gases (cond-mat.quant-gas)
We study an ultracold atom-cavity coupling system, which had been implemented in experiment to display weak light nonlinearity [S. Gupta \textit{et al}., Phys. Rev. Lett. \textbf{99}, 213601 (2007)]. The model is described by a non-interacting Bose-Einstein condensate contained in a Fabry-P\'{e}rot optical resonator, in which two incommensurate standing-wave modes are excited and thus form a quasiperiodic optical lattice potential for the atoms. Special emphasis are paid to the variation of atomic wavefunction induced by the cavity light field. We show that bistability between the atomic localized and extended states can be generated under appropriate conditions.
3. arXiv:1108.4732 [pdf, ps, other]
Textured Superconducting State in the Heavy Fermion CeRhIn5
Tuson Park, H. lee, I. Martin, X. Lu, V. A. Sidorov, F. Ronning, E. D. Bauer, J. D. Thompson
Comments: 8 pages, 9 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
Anisotropic, spatially textured electronic states often emerge when the symmetry of the underlying crystalline structure is lowered. However, the possibility recently has been raised that novel electronic quantum states with real-space texture could arise in strongly correlated systems even without changing the underlying crystalline structure. Here we report evidence for such texture in the superconducting quantum fluid that is induced by pressure in the heavy-fermion compound CeRhIn5. When long-range antiferromagnetic order coexists with unconventional superconductivity, there is a significant temperature difference between resistively- and thermodynamically-determined transitions into the superconducting state, but this difference disappears in the absence of magnetism. Anisotropic transport behaviour near the superconducting transition in the coexisting phase signals the emergence of textured superconducting planes that are nucleated preferentially along the {100} planes and that appear without a change in crystal symmetry. We show that CeRhIn5 is not unique in exhibiting a difference between resistive and bulk superconducting transition temperatures, indicating that textured superconductivity may be a general consequence of coexisting orders.
4. arXiv:1108.4788 [pdf, ps, other]
Diagonal composite order in two-channel Kondo lattice
Shintaro Hoshino, Junya Otsuki, Yoshio Kuramoto
Comments: 5pages, 4figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
A novel type of symmetry breaking is reported for the two-channel Kondo lattice where conduction electrons have spin and orbital (channel) degrees of freedom. Using the continuous-time quantum Monte Carlo and the dynamical mean-field theory, a spontaneous breaking of the orbital symmetry is observed. The tiny breakdown of orbital occupation number, however, vanishes if the conduction electrons have the particle-hole symmetry. The proper order parameter instead is identified as a composite quantity representing the orbital-selective Kondo effect. The single-particle spectrum of the selected orbital shows insulating property, while the other orbital behaves as Fermi liquid. This composite order is the first example of odd-frequency order other than off-diagonal order (superconductivity), and is a candidate of hidden order in $f$-electron systems.
Aug 24
1. arXiv:1108.4409 [pdf, other]
Mottness and magnetism of fermions in a double-well optical lattice
Xin Wang, Qi Zhou, S. Das Sarma
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We theoretically investigate, using nonperturbative strong correlation techniques, Mott insulating phases and magnetic ordering of two-component fermions in a two-dimensional double-well optical lattice. At filling of two fermions per site, there are two types of Mott insulators, one of which is characterized by spin-1 antiferromagnetism below the Neel temperature. The super-exchange interaction in this system is induced by the interplay between the inter-band interaction and the spin degree of freedom. A great advantage of the double-well optical lattice is that the magnetic quantum phase diagram and the Neel temperature can be easily controlled by tuning the orbital energy splitting of the two-level system. Particularly, the Neel temperature can be one order of magnitude larger than that in standard optical lattices, facilitating the experimental search for magnetic ordering in optical lattice systems.
2. arXiv:1108.4574 [pdf, ps, other]
Trapped Ultracold Bosons in Periodic Lattice Modulations
Jia-Wei Huo, Fu-Chun Zhang, Weiqiang Chen, M. Troyer, U. Schollwöck
Comments: 7 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Motivated by the recent rapid development of the field of quantum gases in optical lattices, we present a comprehensive study of the spectrum of ultracold atoms in a one-dimensional optical lattice subjected to a periodic lattice modulation. Using the time-dependent density-matrix renormalization group method, we study the dynamical response due to lattice modulations in different quantum phases of the system with varying density. For the Mott insulating state, we identify several excitation processes, which provide important information about the density profile of the gases. For the superfluid, the dynamical response can be well described in a local density approximation. This simplification can be valuable in understanding the strong-correlated superfluid in a slow-varying harmonic potential. All these spectroscopic features of an inhomogeneous system can be used as a test for the validity of the Bose-Hubbard model in a parabolic trapping potential.
3. arXiv:1108.4483 [pdf, ps, other]
Signatures of Fractional Quantum Hall States in Topological Insulators
Dong-Xia Qu, Y. S. Hor, R. J. Cava, N. P. Ong
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
The fractional quantum Hall (FQH) state is a topological state of matter resulting from the many-body effect of interacting electrons and is of vast interest in fundamental physics. Recent theoretical studies suggest that the FQH effect might be observable in topological insulators, which can host a single Dirac valley with no spin degeneracy. However, due to the dominant bulk conduction it is difficult to probe the strong correlation effect in topological insulators from resistivity measurements. Here we report the discovery of FQH states in Bi2Te3 from thermopower measurements. The surface thermopower is ten times greater than that of bulk, which makes possible the observation of fractional-filled Landau levels in surface states. Hence, thermopower provides a powerful tool to investigate correlated Dirac fermions in topological insulators.
Aug 23
1. arXiv:1108.4038 [pdf, ps, other]
Entanglement Entropy of Gapped Phases and Topological Order in Three dimensions
Tarun Grover, Ari M. Turner, Ashvin Vishwanath
Comments: 16.5 pages, 10 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We discuss entanglement entropy of gapped ground states in different dimensions, obtained on partitioning space into two regions. For trivial phases without topological order, we argue that the entanglement entropy may be obtained by integrating an `entropy density' over the partition boundary that admits a gradient expansion in the curvature of the boundary. This constrains the expansion of entanglement entropy as a function of system size, and points to an even-odd dependence on dimensionality. For example, in contrast to the familiar result in two dimensions, a size independent constant contribution to the entanglement entropy can appear for trivial phases in any odd spatial dimension. We then discuss phases with topological entanglement entropy (TEE) that cannot be obtained by adding local contributions. We find that in three dimensions there is just one type of TEE, as in two dimensions, that depends linearly on the number of connected components of the boundary (the `zeroth Betti number'). In D > 3 dimensions, new types of TEE appear which depend on the higher Betti numbers of the boundary manifold. We construct generalized toric code models that exhibit these TEEs and discuss ways to extract TEE in D >=3.
2. arXiv:1108.4068 [pdf, ps, other]
Topological phase transitions in ultra-cold Fermi superfluids: the evolution from BCS to BEC under artificial spin-orbit fields
Kangjun Seo, Li Han, C. A. R. Sá de Melo
Comments: 4 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We discuss topological phase transitions in ultra-cold Fermi superfluids induced by interactions and artificial spin orbit fields. We construct the phase diagram for population imbalanced systems at zero and finite temperatures, and analyze spectroscopic and thermodynamic properties to characterize various phase transitions. For balanced systems, the evolution from BCS to BEC superfluids in the presence of spin-orbit effects is only a crossover as the system remains fully gapped, even though a triplet component of the order parameter emerges. However, for imbalanced populations, spin-orbit fields induce a triplet component in the order parameter that produces nodes in the quasiparticle excitation spectrum leading to bulk topological phase transitions of the Lifshitz type. Additionally a fully gapped phase exists, where a crossover from indirect to direct gap occurs, but a topological transition to a gapped phase possessing Majorana fermions edge states does not occur.
3. arXiv:1108.4212 [pdf, other]
Vortices in spin-orbit-coupled Bose-Einstein condensates
Juraj Radic, Tigran Sedrakyan, Ian Spielman, Victor Galitski
Comments: 16 pages, 12 color figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
Realistic methods to create vortices in spin-orbit-coupled Bose-Einstein condensates are discussed. It is shown that, contrary to common intuition, rotation of the trap containing a spin-orbit condensate does not lead to an equilibrium state with static vortex structures, but gives rise instead to non-equilibrium behavior described by an intrinsically time-dependent Hamiltonian. We propose here the following alternative methods to induce thermodynamically stable static vortex configurations: (1) to rotate both the lasers and the anisotropic trap; and (2) to impose a synthetic Abelian field on top of synthetic spin-orbit interactions. Effective Hamiltonians for spin-orbit condensates under such perturbations are derived for most currently known realistic laser schemes that induce synthetic spin-orbit couplings. The Gross-Pitaevskii equation is solved for several experimentally relevant regimes. The new interesting effects include spatial separation of left- and right-moving spin-orbit condensates, the appearance of unusual vortex arrangements, and parity effects in vortex nucleation where the topological excitations are predicted to appear in pairs. All these phenomena are shown to be highly non-universal and depend strongly on a specific laser scheme and system parameters.
4. arXiv:1108.4233 [pdf, ps, other]
Spin-orbit coupled weakly interacting Bose-Einstein condensates in harmonic traps
Hui Hu, Han Pu, Xia-Ji Liu
Comments: 4 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate theoretically the phase diagram of a spin-orbit coupled Bose gas in two-dimensional harmonic traps. We show that discrete Landau levels develop at strong spin-orbit coupling. For a weakly interacting gas, quantum states with skyrmion lattice patterns emerge spontaneously and preserve either parity symmetry or combined parity-time-reversal symmetry. These phases can be readily observed by experimentally engineering spin-orbit coupling and interatomic interactions for a cloud of $^{87}$Rb atoms in a highly oblate trap.
Aug 22
1. arXiv:1108.3862 (cross-list from cond-mat.str-el) [pdf, ps, other]
Topological liquids and valence cluster states in two-dimensional SU(N) magnets
Michael Hermele, Victor Gurarie
Comments: 26 pages, 7 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We study the zero temperature phase diagram of a class of two-dimensional SU(N) antiferromagnets. These models are characterized by having the same type of SU(N) spin placed at each site of the lattice, and share the property that, in general, more than two spins must be combined to form a singlet. An important motivation to study these systems is that they may be realized naturally in Mott insulators of alkaline earth atoms placed on optical lattices; indeed, such Mott insulators have already been obtained experimentally, although the temperatures are still high compared to the magnetic exchange energy. We study these antiferromagnets in a large-N limit, finding a variety of ground states. Some of the models studied here have a valence bond solid ground state, as was found in prior studies, yet we find that many others have a richer variety of ground states. Focusing on the two-dimensional square lattice, in addition to valence cluster states (which are analogous to valence bond states), we find both Abelian and non-Abelian chiral spin liquid ground states, which are magnetic counterparts of the fractional quantum Hall effect. We also find a "doubled" chiral spin liquid ground state that preserves time reversal symmetry. These results are based on a combination of rigorous lower bounds on the large-N ground state energy, and a systematic numerical ground state search. We conclude by discussing whether experimentally relevant SU(N) antiferromagnets -- away from the large-N limit -- may be chiral spin liquids.
2. arXiv:1108.3858 (cross-list from cond-mat.mes-hall) [pdf, other]
Magneto-Optical Faraday and Kerr Effects in Topological Insulator Films and in Other Layered Quantized Hall Systems
Wang-Kong Tse, A. H. MacDonald
Comments: 14 pages, 5 figures. This work expands on our previous papers [Phys. Rev. Lett. 105, 057401 (2010); Phys. Rev. B 82, 161104(R) (2010)] by including thick topological insulator films and layered quantized Hall systems in general, as well as the effects of bulk conduction, substrate, and oblique light incidence that are relevant to experiments
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)
We present a theory of the magneto-optical Faraday and Kerr effects of topological insulator (TI) films. For film thicknesses short compared to wavelength, we find that the low-frequency Faraday effect in ideal systems is quantized at integer multiples of the fine structure constant, and that the Kerr effect exhibits a giant $\pi/2$ rotation for either normal or oblique incidence. For thick films that contain an integer number of half wavelengths, we find that the Faraday and Kerr effects are both quantized at integer multiples of the fine structure constant. For TI films with bulk parallel conduction, we obtain a criterion for the observability of surface-dominated magneto-optical effects. For thin samples supported by a substrate, we find that the universal Faraday and Kerr effects are present when the substrate is thin compared to the optical wavelength or when the frequency matches a thick-substrate cavity resonance. Our theory applies equally well to any system with two conducting layers that exhibit quantum Hall effects.
Aug 15 - 19, Xiaopeng Li
Aug 19
1. arXiv:1108.3608 [pdf, ps, other]
Title: Yang-Yang thermometry and momentum distribution of a trapped one-dimensional Bose gas
Authors: M. J. Davis, P. B. Blakie, A. H. van Amerongen, N. J. van Druten, K. V. Kheruntsyan
Comments: 4 pages, 3 figures
We describe the use of the exact Yang-Yang solutions for the one-dimensional Bose gas to enable accurate kinetic-energy thermometry based on the root-mean-square width of an experimentally measured momentum distribution. Furthermore, we use the stochastic projected Gross-Pitaevskii theory to provide the first quantitative description of the full momentum distribution measurements of Van Amerongen et al., Phys. Rev. Lett. 100, 090402 (2008). We find the fitted temperatures from the stochastic projected Gross-Pitaevskii approach are in excellent agreement with those determined by Yang-Yang kinetic-energy thermometry.
2. arXiv:1108.3712 [pdf, ps, other]
Title: Manipulating and probing Majorana fermions using superconducting circuits
Authors: J. Q. You, Z. D. Wang, Wenxian Zhang, Franco Nori
Majorana fermions are long-sought exotic particles that are their own antiparticles and satisfy non-Abelian statistics. Here we utilize superconducting circuits to construct two superconducting-qubit arrays where Majorana modes can occur. Moreover, we propose to use four superconducting qubits as the smallest system to demonstrate the braiding of Majorana modes and show how the states before and after braiding Majoranas can be discriminated. This provides an experimentally realizable, relatively simple system for manipulating and probing Majorana fermions.
Aug 18
1. arXiv:1108.3562 [pdf, other]
Possibility of Skyrmion Superconductivity in Doped Antiferromagnet K$_2$Fe$_4$Se$_5$
Authors: G. Baskaran
Abstract: Intercalated Fe chalcogenide K$_2$Fe$_4$Se$_5$ family exhibits high Tc ($\sim$ 30 K) superconductivity and spin-8 high T$_{\rm N}$ ($\sim$ 560 K) antiferromagnetism (AFM). We present a model Hamiltonian and suggest \textit{Skyrmion superconductivity}. A doped electron creates an orbitally non degenerate S=7\half state in a F$_4$Se$_9$ cluster and moves in a single correlated band, exchange coupled to a robust S=8 AFM order. Skyrmion, a topological excitation of 2 dimensional AFM order acquires an \textit{induced charge -2e} through a quantum anomaly and becomes a Cooper pair. Superconductivity emerges for a range of doping. Fluctuating superconductivity, arising from preformed Cooper pairs (stable Skyrmions), around room temperatures is predicted. We offer hope for higher Tc's in large-S Mott insulators.
2. arXiv:1108.3371 [pdf, other]
A theory of Topological Kondo Insulators
Authors: Maxim Dzero, Kai Sun, Piers Coleman, Victor Galitski
We examine how the properties of the Kondo insulators change when the symmetry of the underlying crystal field multiplets is taken into account. We employ the Anderson lattice model and consider its low-energy physics. We show that in a large class of crystal field configurations, Kondo insulators can develop a topological non-trivial ground-state. Such topological Kondo insulators are adiabatically connected to non-interacting insulators with unphysically large spin-orbit coupling, and as such may be regarded as interaction-driven topological insulators. We analyze the entanglement entropy of the Anderson lattice model of Kondo insulators by evaluating its entanglement spectrum. Our results for the entanglement spectrum are consistent with the surface state calculations. Lastly, we discuss the construction of the maximally localized Wannier wave functions for generic Kondo insulators.
Aug 17
1. arXiv:1108.3317 [pdf, other]
Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators
Authors: M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M.C. Banuls, L. Pollet, I. Bloch, S. Kuhr
Abstract: Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body systems possesses a hidden non-local order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particle-hole pairs that lead to the emergence of a non-local string order in one dimension. Using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case.
2. arXiv:1108.3076 [pdf, other]
Transport in three-dimensional topological insulators: theory and experiment
Authors: Dimitrie Culcer
Abstract: This article reviews recent theoretical and experimental work on transport due to the surface states of three-dimensional topological insulators. The theoretical focus is on longitudinal transport in the presence of an electric field, including Boltzmann transport, quantum corrections and weak localization, as well as longitudinal and Hall transport in the presence of both electric and magnetic fields and/or magnetizations. Special attention is paid to transport at finite doping, to the $\pi$-Berry phase, which leads to the absence of backscattering, Klein tunneling and half-quantized Hall response. Signatures of surface states in ordinary transport and magnetotransport are clearly identified. The review also covers transport experiments of the past years, reviewing the initial obscuring of surface transport by bulk transport, and the way transport due to the surface states has increasingly been identified experimentally. Current and likely future experimental challenges are given prominence and the current status of the field is assessed.
Aug 16
1. arXiv:1108.2929 [pdf, other]
Topologically Protected Extended States in Disordered Quantum Spin-Hall Systems without Time-Reversal Symmetry
Authors: Zhong Xu, L. Sheng, D. Y. Xing, Emil Prodan, D. N. Sheng
Abstract: We demonstrate the existence of robust bulk extended states in the disordered Kane-Mele model with vertical and horizontal Zeeman fields, in the presence of a large Rashba coupling. The phase diagrams are mapped out by using level statistics analysis and computations of the localization length and spin-Chern numbers $C_\pm$. $C_\pm$ are protected by the finite energy and spin mobility gaps. The latter is shown to stay open for arbitrarily large vertical Zeeman fields, or for horizontal Zeeman fields below a critical strength or at moderate disorder. In such cases, a change of $C_\pm$ is necessarily accompanied by the closing of the mobility gap at the Fermi level. The numerical simulations reveal sharp changes in the quantized values of $C_\pm$ when crossing the regions of bulk extended states, indicating that the topological nature of the extended states is indeed linked to the spin-Chern numbers. For large horizontal Zeeman fields, the spin-gap closes at strong disorder prompting a change in the quantized spin-Chern numbers without a closing of the energy mobility gap.
2. arXiv:1108.2811 [pdf, ps, other]
Theory of a Fermi-Liquid-to-Non-Fermi-Liquid Quantum Phase Transition in Dimensions d>1
Authors: T.R. Kirkpatrick, D. Belitz
Abstract: We develop a theory for a generic instability of a Fermi liquid in dimension d>1 against the formation of a Luttinger-liquid-like state. The density of states at the Fermi level is the order parameter for the ensuing quantum phase transition, which is driven by the effective interaction strength. A scaling theory in conjunction with an effective field theoy for clean electrons is used to obtain the critical behavior of observables. In the Fermi-liquid phase the order-parameter susceptibility, which is measurable by tunneling, is predicted to diverge for 1<d<3.
Aug 15
1. arXiv:1108.2672 [pdf, other]
Quantum phase transition of ultracold bosons in the presence of a non-Abelian synthetic gauge field
Authors: T. Grass, K. Saha, K. Sengupta, M. Lewenstein
Abstract: We study the Mott phases and the superfluid-insulator transition of two-component ultracold bosons on a square optical lattice in the presence of a non-Abelian synthetic gauge field, which renders a SU(2) hopping matrix for the bosons. Using a resummed hopping expansion, we calculate the excitation spectra in the Mott insulating phases and demonstrate that the superfluid-insulator phase boundary displays a non-monotonic dependence on the gauge field strength. We also compute the momentum distribution of the bosons in the presence of the non-Abelian field and show that they develop peaks at non-zero momenta as the superfluid-insulator transition point is approached from the Mott side. Finally, we study the superfluid phases near the transition and discuss the induced spatial pattern of the superfluid density due to the presence of the non-Abelian gauge potential.
2. arXiv:1108.2507 [pdf, ps, other]
Topological Phase Transitions for Interacting Finite Systems
Authors: Christopher N. Varney, Kai Sun, Marcos Rigol, Victor Galitski
Abstract: In this Letter, we investigate the signatures of the topological phase transition in interacting systems. The key signature is the existence of a topologically protected level crossing, which is robust and sharply defines the topological transition, even in finite-size systems. We discuss the theoretical implications of this discovery and utilize exact diagonalization to demonstrate this signature in the Haldane-Fermi-Hubbard model. This result provides a new, efficient way to detect topological transitions in experiments and also in numerical methods that cannot access the ground state wavefunction.
Aug 8 - 12, Zixu Zhang
Aug 12
1. arXiv:1108.2272 [pdf, ps, other]
Title: Phase diagram of the half-filled one-dimensional $t$-$V$-$V'$ model
Authors:Tapan Mishra, Juan Carrasquilla, Marcos Rigol
Comments: 7 pages, 9 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We study the phase diagram of spinless fermions with nearest and next-nearest-neighbor interactions in one dimension utilizing the (finite-size) density-matrix renormalization group (DMRG) method. The competition between nearest and next-nearest-neighbor interactions and nearest-neighbor hopping generates four phases in this model: two charge-density-wave insulators, a Luttinger liquid phase, and a bond-order phase. We use finite-size scaling of the gap and various structure factors to determine the phase diagram.
2. arXiv:1108.2327 [pdf, other]
Title: SU(N) magnetism in chains of ultracold alkaline earth atoms: Mott transitions and quantum correlations
Authors:Salvatore R. Manmana, Kaden R. A. Hazzard, Gang Chen, Adrian E. Feiguin, Ana Maria Rey
Comments: 19 pages, 13 figures and 2 tables; submitted to Phys. Rev. A
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We investigate one dimensional SU$(N)$ Hubbard chains at zero temperature, which can be emulated with ultracold alkaline earth atoms, by using the density matrix renormalization group (DMRG), Bethe ansatz (BA), and bosonization. We use the DMRG to benchmark the accuracy of the Bethe ansatz for $N>2$ where the BA is only approximate. In the worst case, we find a relative error $\epsilon \lesssim 4%$ in the BA ground state energy for $N \leq 4$ at filling 1/N, which is due to the fact that BA improperly treats the triply and higher occupied states. Using the DMRG for $N \leq 4$ and the BA for large $N$, we determine the regimes of validity of strong- and weak-coupling perturbation theory for all values of $N$ and in particular, the parameter range in which the system is well described by a SU(N) Heisenberg model at filling 1/N. We find this depends only weakly on $N$. We investigate the Berezinskii-Kosterlitz-Thouless phase transition from a Luttinger liquid to a Mott-insulator by computing the fidelity susceptibility and the Luttinger parameter $K_\rho$ at 1/N filling. The numerical findings give strong evidence that the fidelity susceptibility develops a minimum at a critical interaction strength which is found to occur at a finite positive value for $N>2$.
Aug 11
1. arXiv:1108.2097 [pdf, ps, other]
Title: Quantized vortices in a rotating Bose-Einstein condensate with spatiotemporally modulated interaction
Authors:Deng-Shan Wang, Shu-Wei Song, Bo Xiong, W. M. Liu
Subjects: Quantum Gases (cond-mat.quant-gas)
We present theoretical analysis and numerical studies of the quantized vortices in a rotating Bose-Einstein condensate with spatiotemporally modulated interaction in harmonic and anharmonic potentials, respectively. The exact quantized vortex and giant vortex solutions are constructed explicitly by similarity transformation. Their stability behavior has been examined by numerical simulation, which shows that a new series of stable vortex states (defined by radial and angular quantum numbers) can be supported by the spatiotemporally modulated interaction in this system. We find that there exist stable quantized vortices with large topological charges in repulsive condensates with spatiotemporally modulated interaction. We also give an experimental protocol to observe these vortex states in future experiments.
Aug 10
1. arXiv:1108.1909 [pdf, ps, other]
Title: Efimov Resonances in Ultracold Quantum Gases
Authors:F. Ferlaino, A. Zenesini, M. Berninger, B. Huang, H.-C. Nägerl, R. Grimm
Comments: 23 pages, 13 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Ultracold atomic gases have developed into prime systems for experimental studies of Efimov three-body physics and related few-body phenomena, which occur in the universal regime of resonant interactions. In the last few years, many important breakthroughs have been achieved, confirming basic predictions of universal few-body theory and deepening our understanding of such systems. We review the basic ideas along with the fast experimental developments of the field, focussing on ultracold cesium gases as a well-investigated model system. Triatomic Efimov resonances, atom-dimer Efimov resonances, and related four-body resonances are discussed as central observables. We also present some new observations of such resonances, supporting and complementing the set of available data.
2. arXiv:1108.1980 [pdf, ps, other]
Title: Evolution of edge states in topological superfluids during the quantum phase transition
Authors:M.A. Silaev, G.E.Volovik
Comments: 4 pages, 2 figures, JETP Letters style
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Other Condensed Matter (cond-mat.other)
The quantum phase transition between topological and non-topological insulators or between fully gapped superfluids/superconductors can occur without closing the gap. We consider the evolution of the Majorana edge states on the surface of topological superconductor during transition to the topologically trivial superconductor on example of non-interacting Hamiltonian describing the spin-triplet superfluid 3He-B. In conventional situation when the gap is nullified at the transition, the spectrum of Majorana fermions shrinks and vanishes after the transition to the trivial state. If the topological transition occurs without the gap closing, the Majorana fermion spectrum disappears by escaping to ultraviolet, where Green's function approaches zero. This demonstrates the close connection between the topological transition without closing the gap and zeroes in the Green's function. Similar connection takes place in interacting systems where zeroes may occur due to interaction.
3. arXiv:1108.2017 [pdf, other]
Title: Correlations and Pair Formation in a Repulsively Interacting Fermi Gas
Authors:Christian Sanner, Edward J. Su, Wujie Huang, Aviv Keshet, Jonathon Gillen, Wolfgang Ketterle
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
A degenerate Fermi gas is rapidly quenched into the regime of strong effective repulsion near a Feshbach resonance. The spin fluctuations are monitored using speckle imaging and, contrary to several theoretical predictions, the samples remain in the paramagnetic phase for arbitrarily large scattering length. Over a wide range of interaction strengths a rapid decay into bound pairs is observed over times on the order of 10\hbar/E_F, preventing the study of equilibrium phases of strongly repulsive fermions. Our work suggests that a Fermi gas with strong short-range repulsive interactions does not undergo a ferromagnetic phase transition.
Aug 9
1. arXiv:1108.1408 [pdf, ps, other]
Title: The bosonic superfluid-insulator transition in continuous space
Authors:S. Pilati, M. Troyer
Comments: 4+ pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate the zero-temperature phase diagram of interacting Bose gases in the presence of a simple cubic optical lattice, going beyond the regime where the mapping to the single-band Bose-Hubbard model is reliable. Our computational approach is a new hybrid quantum Monte Carlo method which combines algorithms used to simulate homogeneous quantum fluids in continuous space with those used for discrete lattice models of strongly correlated systems. We determine the critical interaction strength and optical lattice intensity where the superfluid-to-insulator transition takes place, considering also the regime of shallow optical lattices and strong inter-atomic interactions. The implications of our findings for the super-solid state of matter are discussed.
2. arXiv:1108.1734 [pdf, ps, other]
Title: Exact Zero Modes in Closed Systems of Interacting Fermions
Authors:Garry Goldstein, Claudio Chamon
Comments: 4 pages, 1 figure
Subjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We show that for closed finite sized systems with an odd number of real fermionic modes there are always at least two fermionic operators that commute with the Hamiltonian, even in the presence of many-body interactions. There is a zero mode corresponding to the fermion parity operator, as shown by Akhmerov, as well as at least one additional linearly independent zero mode, which is 1) the one that is continuously connected to the Majorana mode solution in the non-interacting limit, and 2) less prone to decoherence when the system is opened to contact with an infinite bath. We also show that in the idealized situation where there are two or more well separated zero modes each associated with a finite number of fermions at a localized vortex, these modes have non-Abelian Ising statistics under braiding.
3. arXiv:1108.1779 [pdf, other]
Title: Quantum Shock Waves and Domain Walls in the Real-Time Dynamics of a Superfluid Unitary Fermi Gas
Authors:Aurel Bulgac, Yuan-Lung (Alan)Luo, Kenneth J. Roche
Comments: 4 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Nuclear Theory (nucl-th); Quantum Physics (quant-ph)
We show that in the collision of two superfluid fermionic atomic clouds one observes the formation of quantum shock waves as discontinuities in the number density and collective flow velocity. Domain walls, which are topological excitations of the superfluid order parameter, are also generated and exhibit abrupt phase changes by $\pi$ and slower motion than the shock waves. The domain walls are distinct from the gray soliton train or number density ripples formed in the wake of the shock waves and observed in the collisions of superfluid bosonic atomic clouds. Domain walls with opposite phase jumps collide elastically.
Aug 8
1. arXiv:1108.1192 [pdf, ps, other]
Title: Expansion of Bose-Hubbard Mott insulators in optical lattices
Authors:Mark Jreissaty, Juan Carrasquilla, F. Alexander Wolf, Marcos Rigol
Comments: 11 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study the expansion of bosonic Mott insulators in the presence of an optical lattice after switching off a confining potential. We use the Gutzwiller mean-field approximation and consider two different set ups. In the first one, the expansion is restricted to one direction. We show that this leads to the emergence of two condensates with well defined momenta, and argue that such a construct can be used to create atom lasers in optical lattices. In the second set up, we study Mott insulators that are allowed to expand in all directions within the lattice. In this case, a single condensate develops in the system. However, its constituent bosons are found to populate many nonzero momentum modes. An analytic understanding of both phenomena in terms of the exact dispersion relation in the hard-core limit is presented.
2. arXiv:1108.1197 [pdf, other]
Title: What can gauge-gravity duality teach us about condensed matter physics?
Authors:Subir Sachdev
Comments: 33 pages, 10 figures + 16 pages of Supplementary Material with 4 figures; to appear in Annual Reviews of Condensed Matter Physics
Subjects: Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
I discuss the impact of gauge-gravity duality on our understanding of two classes of systems: conformal quantum matter and compressible quantum matter.
The first conformal class includes systems, such as the boson Hubbard model in two spatial dimensions, which display quantum critical points described by conformal field theories. Questions associated with non-zero temperature dynamics and transport are difficult to answer using conventional field theoretic methods. I argue that many of these can be addressed systematically using gauge-gravity duality, and discuss the prospects for reliable computation of low frequency correlations.
Compressible quantum matter is characterized by the smooth dependence of the charge density, associated with a global U(1) symmetry, upon a chemical potential. Familiar examples are solids, superfluids, and Fermi liquids, but there are more exotic possibilities involving deconfined phases of gauge fields in the presence of Fermi surfaces. I survey the compressible systems studied using gauge-gravity duality, and discuss their relationship to the condensed matter classification of such states. The gravity methods offer hope of a deeper understanding of exotic and strongly-coupled compressible quantum states.
3. arXiv:1108.1222 [pdf, other]
Title: Extremely Anisotropic Wigner Crystal in a Spin-Orbit Coupled Two-Dimensional Electron Gas
Authors:Erez Berg, Mark S. Rudner, Steven A. Kivelson
Comments: 9 pages, 7 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We argue that the ground state of a two-dimensional electron gas with Rashba spin-orbit coupling is a Wigner crystal in the low-density limit, even for short-range repulsive electron-electron interactions (i.e. which decay with distance with a power larger than 2). In this case, the unit cell of the Wigner crystal becomes increasingly anisotropic as the density decreases. In contrast, with long range interactions, the usual hexagonal Wigner crystal is recovered. We discuss the magnetic properties of the ground state in either case. Melting of the anisotropic Wigner via thermal or quantum fluctuations likely gives rise to a nematic phase.
4. arXiv:1108.1238 [pdf, ps, other]
Title: Vortex structures of rotating spin-orbit coupled Bose-Einstein condensates
Authors:Xiang-Fa Zhou, Jing Zhou, Congjun Wu
Comments: 6 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We consider the quasi-2D two-component Bose-Einstein condensates with Rashba spin-orbit (SO) coupling in a rotating trap. An external Zeeman term favoring spin polarization along the radial direction is also considered, which has the same form as the non-canonical part of the mechanical angular momentum. The rotating condensate exhibits rich structures as varying the strengths of trapping potential and interaction. With a strong trapping potential, the condensate exhibits a half-quantum vortex-lattice configuration. Such a configuration is driven to the normal one by introducing the external radial Zeeman field. In the case of a weak trap potential, the condensate exhibits a multi-domain pattern of plane-wave states under the external radial Zeeman field.
5. arXiv:1108.1326 [pdf, other]
Title: Tunable multiple layered Dirac cones in optical lattices
Authors:Z. Lan, A. Celi, W. Lu, P. Ohberg, M. Lewenstein
Comments: 4 papges, 2 figures
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
We show that multiple layered Dirac cones can emerge in the band structure of properly addressed multi-component cold fermionic gases in optical lattices. The layered Dirac cones contain multiple copies of the massless spin-1/2 Dirac fermions at the same location in momentum space, whose different Fermi velocity can be tuned at will. On-site microwave Raman transitions can further lead to the mixing of the different Dirac species, resulting in a splitting of the multiple layered cones into several single layered cones. The tunability of the multiple layered Dirac cones allows to simulate a number of fundamental phenomena in modern physics, such as neutrino oscillations and exotic particle dispersions with $E\sim p^n $ for arbitrary integer n.
Aug 1 - 5, Chungwei Lin
Aug 5
1. arXiv:1108.0928 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Alternatives to Eigenstate Thermalization
Marcos Rigol, Mark Srednicki
Comments: 5 pages, 3 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
An isolated quantum many-body system in an initial pure state will come to thermal equilibrium if it satisfies the eigenstate thermalization hypothesis (ETH). We consider alternatives to ETH that have been proposed. We first show that von Neumann's quantum ergodic theorem (QET) relies on an assumption that is essentially equivalent to ETH. We also investigate whether initial states produced by a quench can lead to thermal behavior in systems that do not obey ETH after the quench, namely, integrable systems. We find examples of this, but only for initial states that obeyed ETH before the quench.
2. arXiv:1108.1132 [pdf, ps, other]
Condensate fraction of a resonant Fermi gas with spin-orbit coupling in three and two dimensions
L. Dell'Anna, G. Mazzarella, L. Salasnich
Comments: 6 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the effects of laser-induced Rashba-like spin-orbit coupling along the BCS-BEC crossover of a Feshbach resonance for a two-spin-component Fermi gas. We calculate the condensate fraction in three and two dimensions and find that this quantity characterizes the crossover better than other quantities, like the chemical potential or the pairing gap. We show in fact that a large enough spin-orbit interaction enhances the condensate fraction in the BCS side while suppressing it on the BEC side.
3. arXiv:1108.1168 [pdf, ps, other]
Origin of the high Neel temperature in SrTcO3
J. Mravlje, M. Aichhorn, A. Georges
Comments: 4+ pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We investigate the origin of the high Neel temperature recently found in Tc perovskites. The electronic structure in the magnetic state of SrTcO3 and its 3d analogue SrMnO3 is calculated within a framework combining band-structure and many-body methods. In agreement with experiment, the Neel temperature of SrTcO3 is found to be four times larger than that of SrMnO3. We show that this is because the Tc-compound lies on the verge of the itinerant-to-localized transition, and also has a larger bandwidth, while the Mn-compound lies deeper into the localized side. The smaller magnetic moment is also reproduced and explained by the ensuing larger charge fluctuations.
Aug 4
1. arXiv:1108.0886 [pdf, other]
Quantum Capillary Waves at the Superfluid--Mott Insulator Interface
Steffen Patrick Rath, Boris Spivak, Wilhelm Zwerger
Comments: 4+epsilon pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We discuss quantum fluctuations of the interface between a superfluid and a Mott-insulating state of ultracold atoms in a trap. The fluctuations of the boundary are due to a new type of surface modes, whose spectrum is similar (but not identical) to classical capillary waves. The corresponding quantum capillary length sets the scale for the penetration of the superfluid into the Mott-insulating regime by the proximity effect and may be on the order of several lattice spacings. It determines the typical magnitude of the interface width due to quantum fluctuations, which may be inferred from single site imaging of ultracold atoms in an optical lattice.
2. arXiv:1108.0869 [pdf]
A Tunable Two-impurity Kondo system in an atomic point contact
Jakob Bork, Yong-hui Zhang, Lars Diekhöner, László Borda, Pascal Simon, Johann Kroha, Peter Wahl, Klaus Kern
Comments: 22 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Two magnetic atoms, one attached to the tip of a Scanning Tunneling Microscope (STM) and one adsorbed on a metal surface, each constituting a Kondo system, have been proposed as one of the simplest conceivable systems potentially exhibiting quantum critical behaviour. We have succeeded in implementing this concept experimentally for cobalt dimers clamped between an STM tip and a gold surface. Control of the tip-sample distance with sub-picometer resolution allows us to tune the interaction between the two cobalt atoms with unprecedented precision. Electronic transport measurements on this two-impurity Kondo system reveal a rich physical scenario which is governed by a crossover from local Kondo screening to non-local singlet formation due to antiferromagnetic coupling as a function of separation of the cobalt atoms.
Aug 3
1. arXiv:1108.0516 [pdf, ps, other]
Persistent currents in dipolar Bose-Einstein condensates confined in annular potentials
F. Malet, G. M. Kavoulakis, S. M. Reimann
Comments: 7 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We consider a dipolar Bose-Einstein condensate confined in an annular potential, with all the dipoles being aligned along some arbitrary direction. In addition to the dipole-dipole interaction, we also assume a zero-range hard-core potential. We investigate the stability of the system against collapse, as well as the stability of persistent currents as a function of the orientation of the dipoles and of the strength of the hard-core interaction.
2. arXiv:1108.0651 [pdf, other]
Interplay of quantum and classical fluctuations near quantum critical points
Mucio A. Continentino
Comments: 10 pages, 6 figures, to be published in Brazilian Journal of Physics
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
For a system near a quantum critical point (QCP), above its lower critical dimension $d_L$, there is in general a critical line of second order phase transitions that separates the broken symmetry phase at finite temperatures from the disordered phase. The phase transitions along this line are governed by thermal critical exponents that are different from those associated with the quantum critical point. We point out that, if the effective dimension of the QCP, $d_{eff}=d+z$ ($d$ is the Euclidean dimension of the system and $z$ the dynamic quantum critical exponent) is above its upper critical dimension $d_C$, there is an intermingle of classical (thermal) and quantum critical fluctuations near the QCP. This is due to the breakdown of the generalized scaling relation $\psi=\nu z$ between the shift exponent $\psi$ of the critical line and the crossover exponent $\nu z$, for $d+z>d_C$ by a \textit{dangerous irrelevant interaction}. This phenomenon has clear experimental consequences, like the suppression of the amplitude of classical critical fluctuations near the line of finite temperature phase transitions as the critical temperature is reduced approaching the QCP.
3. arXiv:1108.0487 (cross-list from cond-mat.supr-con) [pdf]
Topological Defects Coupling Smectic Modulations to Intra-unit-cell Nematicity in Cuprate
A. Mesaros, K. Fujita, H. Eisaki, S. Uchida, J. C. Davis, S. Sachdev, J. Zaanen, M. J. Lawler, Eun-Ah Kim
Journal-ref: Science 333, 426 (2011)
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
We study the coexisting smectic modulations and intra-unit-cell nematicity in the pseudogap states of underdoped Bi2Sr2CaCu2O8+{\delta}. By visualizing their spatial components separately, we identified 2\pi topological defects throughout the phase-fluctuating smectic states. Imaging the locations of large numbers of these topological defects simultaneously with the fluctuations in the intra-unit-cell nematicity revealed strong empirical evidence for a coupling between them. From these observations, we propose a Ginzburg-Landau functional describing this coupling and demonstrate how it can explain the coexistence of the smectic and intra-unit-cell broken symmetries and also correctly predict their interplay at the atomic scale. This theoretical perspective can lead to unraveling the complexities of the phase diagram of cuprate high-critical-temperature superconductors.
Aug 2
1. arXiv:1108.0145 [pdf, ps, other]
Exact three-body local correlations for arbitrary excited states of the 1D Bose gas
Marton Kormos, Yang-Zhi Chou, Adilet Imambekov
Comments: 4 pages, 3 figures + supplementary material
Subjects: Quantum Gases (cond-mat.quant-gas)
We derive an exact analytic expression for the three-body local correlations in the Lieb-Liniger model of 1D Bose gas with contact repulsion. The local three-body correlations control the thermalization and particle loss rates in the presence of terms which break integrability, as is realized in the case of 1D ultracold bosons. Our result is valid at finite temperature and for arbitrary excited states. We present calculations in the presence of external harmonic confinement within local density approximation, and for a highly excited state that resembles an experimentally realized configuration.
2. arXiv:1108.0270 (cross-list from quant-ph) [pdf, other]
Thermalisation of a closed quantum system: From many-body dynamics to a Fokker-Planck equation
C. Ates, J. P. Garrahan, I. Lesanovsky
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Atomic Physics (physics.atom-ph)
How a closed quantum system reaches a steady state in which observables assume time-independent expectation values is both an intriguing and fundamental question in physics. When such a steady state can be described by an equilibrium thermodynamic ensemble we speak of "thermalisation". Thermalisation has been shown to occur in a number of many-body systems, but the question of how a closed quantum system relaxes dynamically to the thermalised state remains open. Here we show that in a non-integrable closed quantum spin system this relaxation is accurately described by a Fokker-Planck equation derived from the underlying quantum dynamics, without relying on a weak coupling assumption between subsystems. Transitions within narrow energy shells lead to a dynamics which is entropy dominated and establishes detailed balance conditions that determine both the eventual equilibrium state and the non-equilibrium relaxation to it. Our study treats the relaxation problem from a dynamical perspective that is complementary to current work in which thermalisation is related to the structure of the energy level spectrum.
3. arXiv:1108.0281 [pdf, ps, other]
Reentrant behaviour in Landau Fermi liquids with spin-split Pomeranchuk instabilities
P. Rodriguez Ponte, D.C. Cabra, N. Grandi
Comments: 5 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study the effects of spin-antisymmetric interactions on the stability of a Landau-Fermi liquid on the square lattice, using the generalized Pomeranchuk method for two-dimensional lattice systems. In particular, we analyze interactions that could induce instabilities of the so called spin-split type, that is when spin-up and spin-down Fermi surfaces are displaced with respect to each other. The phase space is studied as a function of the strength of the interaction $V$, the electron chemical potential $\mu$ and an external magnetic field $h$. We find that such interactions produce in general an enhancement of the instability region of the Landau-Fermi liquid. More interestingly, in certain regions of the $V$-$\mu$ phase space, we find a reentrant behaviour as a function of the magnetic field $h$, similar to that found in recent experiments, e.g. in URu$_2$Si$_2$ and Sr$_3$Ru$_2$O$_7$.
Aug 1
1. arXiv:1107.5937 [pdf, ps, other]
Extracting information from non adiabatic dynamics: excited symmetric states of the Bose-Hubbard model
M. Lacki, D. Delande, J. Zakrzewski
Comments: inv. talk at 5th Workshop on Quantum Chaos and Localization Phenomena, Warsaw 2011
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
Using Fourier transform on a time series generated by unitary evolution, we extract many-body eigenstates of the Bose-Hubbard model corresponding to low energy excitations, which are generated when the insulator-superfluid phase transition is realized in a typical experiment. The analysis is conducted in a symmetric external potential both without and with and disorder. A simple classification of excitations in the absence disorder is provided. The evolution is performed assuming the presence of the parity symmetry in the system rendering many-body quantum states either symmetric or antisymmetric. Using symmetry-breaking technique, those states are decomposed into elementary one-particle processes.
2. arXiv:1107.5848 [pdf, other]
Auxiliary Field quantum Monte Carlo for Strongly Paired Fermions
J. Carlson, Stefano Gandolfi, Kevin E. Schmidt, Shiwei Zhang
Comments: 5 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Lattice (hep-lat); Nuclear Theory (nucl-th)
We demonstrate that the inclusion of a BCS importance function dramatically increases the efficiency of the auxiliary field method for strong pairing. We calculate the ground-state energy of an unpolarized fermi gas at unitarity with up to 66 particles and lattices of up to $27^3$ sites. The method has no fermion sign problem, and an accurate result is obtained for the universal parameter $\xi$. Several different forms of the kinetic energy adjusted to the unitary limit but with different effective ranges extrapolate to the same continuum limit within error bars. The finite effective range results for different interactions are consistent with a linear term proportional to the Fermi momentum times the effective range. The new method described herein will have many applications in superfluid cold atom systems and in both electronic and nuclear structures when pairing is important.
3. arXiv:1107.5920 [pdf, ps, other]
Hallmark of strong electronic correlations in LaNiO$_3$: photoemission kink and broadening of fully occupied bands
Xiaoyu Deng, Michel Ferrero, Jernej Mravlje, Markus Aichhorn, Antoine Georges
Comments: 5 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Recent angular-resolved photoemission experiments on LaNiO$_3$ reported a renormalization of the Fermi velocity of $e_g$ quasiparticles, a kink in their dispersion at $ -0.2$ eV and a large broadening and weakened dispersion of the occupied $t_{2g}$ states. We show here that all these features result from electronic correlations and are quantitatively reproduced by calculations combining density-functional theory and dynamical mean-field theory. The importance and general relevance of correlation effects in filled bands coupled by inter-orbital interactions to a partially-filled band are pointed out.
