June 27 - July 1, Zixu Zhang
July 1
1. arXiv:1106.6116 [pdf, ps, other]
Title: Inhomogeneous Pseudogap Phenomenon in the BCS-BEC Crossover Regime of a Trapped Superfluid Fermi Gas
Authors:Ryota Watanabe, Shunji Tsuchiya, Yoji Ohashi
Comments: 4 pages, 2 figures, proceedings of 26th International Conference on Low Temperature Physics (LT26)
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate pseudogap phenomena in the unitarity limit of a trapped superfluid Fermi gas. Including effect of strong pairing fluctuations within a $T$-matrix approximation, as well as effects of a harmonic trap within the local density approximation (LDA), we calculate the local superfluid density of states below the superfluid phase transition temperature $T_{\rm c}$. We show that the spatial region where single-particle excitations are dominated by the pseudogap may still exist even below $T_{\rm c}$, due to inhomogeneous pairing fluctuations caused by the trap potential. From the temperature dependence of the pseudogapped density of states, we identify the pseudogap regime of the unitarity Fermi gas with respect to the temperature and spatial position. We also show that the combined $T$-matrix theory with the LDA can quantitatively explain the local pressure which was recently observed in the unitarity limit of a $^6$Li Fermi gas.
2. arXiv:1106.6193 [pdf, ps, other]
Title: Dispersions, weights, and widths of the single-particle spectral function in the normal phase of a Fermi gas
Authors:F. Palestini, A. Perali, P. Pieri, G. C. Strinati
Comments: 17 pages, 20 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
The dispersions, weights, and widths of the peaks of the single-particle spectral function in the presence of pair correlations, for a Fermi gas with either attractive or repulsive short-range inter-particle interaction, are determined in the normal phase over a wide range of wave vectors, with a twofold purpose. The first one is to determine how these dispersions identify both an energy scale known as the pseudo-gap near the Fermi wave vector, as well as an additional energy scale related to the contact C at large wave vectors. The second one is to differentiate the behaviors of the repulsive gas from the attractive one in terms of crossing versus avoided crossing of thedispersions near the Fermi wave vector. An analogy will also be drawn between the occurrence of the pseudo-gap physics in a Fermi gas subject to pair fluctuations and the persistence of local spin waves in the normal phase of magnetic materials.
3. arXiv:1106.6311 [pdf, ps, other]
Title: Modulated superconductivity near Pomeranchuk instabilities in the spin channel
Authors:Daniel G. Barci, Paulo S. A. Bonfim
Comments: 4 pages, 3 figures
Subjects: Superconductivity (cond-mat.supr-con)
We study the competition between a Pomeranchuk instability in spin-channel with angular momentum $\ell=1$ and an attractive BCS interaction, favoring Cooper pair formation. We compute the phase diagram at mean-field level, finding a transition between two superconductor phases: a homogeneous and amodulated superconductor, with similar characteristics of the FFLO state. Differently from FFLO, time-reversal symmetry is preserved due to the spontaneous generation of a spin-orbit coupling. An experimental signature for this phase transition could be the simultaneous observation of anisotropic superconductivity and an anomalous Hall effect.
Jun 30
1. arXiv:1106.5842 [pdf, ps, other]
Title: Ferromagnetism, spiral magnetic structures and phase separation in the two-dimensional Hubbard model
Authors:P. A. Igoshev, A.V. Zarubin, A. A. Katanin, V. Yu. Irkhin
Comments: Report on the Moscow International Symposium on Magnetism MISM-2011; 4 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
The quasistatic approximation and equation-of-motion decoupling for the electron Green's functions are applied to trace the effect of electronic dispersion and electron correlations on the ferromagnetism of two-dimensional itinerant-electron systems. It is found that next-nearest-neighbor hopping t' is of crucial importance for ferromagnetism formation yielding the magnetic phase diagram which is strongly asymmetric with respect to half-filling. At small t' in the vicinity of half-filling the ferromagnetic phase region is restricted by the spin-density wave instability, and far from half-filling by one-particle (spin-polaron) instability. At t' close to t/2 ferromagnetism is stabilized at moderate Hubbard U due to substantial curvature of the Fermi surface which passes in the vicinity of the van Hove singularity points. The results obtained are of possible importance for high-T_c compounds and layered ruthenates.
2. arXiv:1106.5843 [pdf, ps, other]
Title: Phase separation and pattern formation in a binary Bose-Einstein condensate
Authors:Jacopo Sabbatini, Wojciech H. Zurek, Matthew J. Davis
Comments: 4 pages, 3 figure
Subjects: Quantum Gases (cond-mat.quant-gas)
The miscibility-immiscibility phase transition in binary Bose-Einstein condensates (BECs) can be controlled by a coupling between the two components. Here we propose a new scheme that uses coupling-induced pattern formation to test the Kibble-Zurek mechanism (KZM) of topological-defect formation in a quantum phase transition. For a binary BEC in a ring trap we find that the number of domains forming the pattern scales with the coupling quench rate with an exponent as predicted by the KZM. For a binary BEC in an elongated harmonic trap we find a different scaling law due to the transition being spatially inhomogeneous. We perform a "simulation" of the harmonically trapped system in a ring trap to verify the scaling exponent.
3. Xiv:1106.5925 [pdf, ps, other]
Title: From Anderson to anomalous localization in cold atomic gases with effective spin-orbit coupling
Authors:M. J. Edmonds, J. Otterbach, R. G. Unanyan, M. Fleischhauer, M. Titov, P. Ohberg
Comments: 4 pages and 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn)
We study the dynamics of a one-dimensional spin-orbit coupled Schrodinger particle with two internal components moving in a random potential. We show that this model can be implemented by the interaction of cold atoms with external lasers and additional Zeeman and Stark shifts. By direct numerical simulations a crossover from an exponential Anderson-type localization to an anomalous power-law behavior of the intensity correlation is found when the spin-orbit coupling becomes large. The power-law behavior is connected to a Dyson singularity in the density of states emerging at zero energy when the system approaches the quasi-relativistic limit of the random mass Dirac model. We discuss conditions under which the crossover is observable in an experiment with ultracold atoms and construct explicitly the zero-energy state, thus proving its existence under proper conditions.
4. arXiv:1106.5964 [pdf, other]
Title: Mechanism for a Pairing State with Time-Reversal Symmetry Breaking in Iron-Based Superconductors
Authors:Christian Platt, Ronny Thomale, Carsten Honerkamp, Shou-Cheng Zhang, Werner Hanke
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
The multipocket Fermi surfaces of iron-based superconductors promote pairing states with both s_{+-}-wave and d_{x^2-y^2}-wave symmetry. We argue that the competition between these two order parameters could lead to a time-reversal-symmetry breaking state with s+id-pairing symmetry in the iron-based superconductors, and propose serveral scenarios in which this phase may be found. To understand the emergence of such a pairing state on a more rigorous footing, we start from a microscopic 5-orbital description representative for the pnictides. Using a combined approach of functional renormalization group and mean-field analysis, we identify the microscopic parameters of the s+id-pairing state. There, we find the most promising region for s+id-pairing in the electron doped regime with an enhanced pnictogen height.
Jun 29
1. arXiv:1106.5506 [pdf, ps, other]
Title: Probing topological quantum critical points
Authors:Sumanta Tewari, J. D. Sau, V. W. Scarola, Chuanwei Zhang, S. Das Sarma
Comments: 7 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Quantum ground states on the non-trivial side of a topological quantum critical point (TQCP) have unique properties that make them attractive candidates for quantum memory and quantum information applications. A recent example is provided by s-wave superconductivity on a semiconductor platform, which is tuned through a TQCP to a topological superconducting (TS) state by an external Zeeman field Gamma > Gamma_c. Despite many attractive features of TS states, TQCPs themselves do not break any symmetries, making it impossible to distinguish the TS state at large Gamma > Gamma_c from a regular superconductor at Gamma < Gamma_c in conventional bulk measurements. Here we show that this problem can be overcome by tracking suitable bulk transport properties with increasing Gamma across the topological quantum critical (QC) regime, the physics of which is very similar to the bulk d-wave superconductivity of high-T_c cuprates. The low-energy finite-temperature effective theory and the scaling form of the relevant susceptibilities also provide a useful theoretical framework in which to understand the topological superconducting transitions in semiconductor heterostructures.
2. arXiv:1106.5585 [pdf, other]
Title: Nonperturbative renormalization-group approach to strongly-correlated lattice bosons
Authors:A. Rancon, N. Dupuis
Comments: 27 pages, 30 figures
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
We present a nonperturbative renormalization-group approach to the Bose-Hubbard model. By taking as initial condition of the renormalization-group flow the (local) limit of decoupled sites, we take into account both local and long-distance fluctuations in a nontrivial way. This approach yields a phase diagram in very good quantitative agreement with quantum Monte Carlo simulations, and reproduces the two universality classes of the superfluid--Mott-insulator transition. The critical behavior near the multicritical points, where the transition takes place at constant density, agrees with the original predictions of Fisher {\it et al.} [Phys. Rev. B {\bf 40}, 546 (1989)] based on simple scaling arguments. At a generic transition point, the critical behavior is mean-field like with logarithmic corrections in two dimensions. In the weakly-correlated superfluid phase (far away from the Mott insulating phase), the renormalization-group flow is controlled by the Bogoliubov fixed point down to a characteristic (Ginzburg) momentum scale $k_G$ which is much smaller than the inverse healing length $k_h$. In the vicinity of the multicritical points, when the density is commensurate, we identify a sharp crossover from a weakly- to a strongly-correlated superfluid phase where the condensate density and the superfluid stiffness are strongly suppressed and both $k_G$ and $k_h$ are of the order of the inverse lattice spacing.
3. arXiv:1106.5604 [pdf, ps, other]
Title: Separation induced resonances in quasi-1D ultracold atomic gases
Authors:Wenbo Fu, Zhenhua Yu, Xiaoling Cui
Comments: 4+2 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study quasi-1D effective scattering and bound states for two atoms confined individually by {\it separated} transverse traps. With equal trapping frequency for two atoms, we find that tuning the separation can induce {\it double} 1D scattering resonances when the ratio between the confinement length and s-wave scattering length is within $(0.791,1.46]$. The sign and amplitude of resonance width show nontrivial dependences on the resonance position. The universal properties of a many-body system at double resonances are manifested by studying the interaction effect of a localized impurity immersed in a Fermi sea of light atoms.
4. arXiv:1106.5667 [pdf, other]
Title: Phase separation in a polarized Fermi gas with spin-orbit coupling
Authors:W. Yi, G.-C. Guo
Comments: 4.2 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the phase separation of a spin polarized Fermi gas with spin-orbit coupling near a wide Feshbach resonance. As a result of the competition between spin-orbit coupling and population imbalance, the phase diagram for a uniform gas develops a rich structure of phase separated states involving gapless superfluid states which are topologically non-trivial. We find that these novel gapless phases can be stabilized by intermediate spin-orbit coupling strengths. We then demonstrate the phase separation induced by an external trapping potential and discuss the optimal parameter region for the experimental observation of the gapless superfluid phases.
5. arXiv:1106.5765 [pdf, ps, other]
Title: Finite-momentum superfluidity and phase transitions in a p-wave resonant Bose gas
Authors:Sungsoo Choi, Leo Radzihovsky
Comments: 32 pages, 21 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study a degenerate two-species gas of bosonic atoms interacting through a p-wave Feshbach resonance as for example realized in a Rb85-Rb87 mixture. We show that in addition to a conventional atomic and a p-wave molecular spinor-1 superfluidity at large positive and negative detunings, respectively, the system generically exhibits a finite momentum atomic-molecular superfluidity at intermediate detuning around the unitary point. We analyze the detailed nature of the corresponding phases and the associated quantum and thermal phase transitions.
6. arXiv:1106.5773 [pdf, ps, other]
Title: Collision of two spin polarized fermionic clouds
Authors:Olga Goulko, Frédéric Chevy, Carlos Lobo
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the collision of two spin polarized Fermi clouds in a harmonic trap using a simulation of the Boltzmann equation. As observed in recent experiments we find three distinct regimes of behavior. For weak interactions the clouds pass through each other. If interactions are increased they approach each other exponentially and for strong interactions they bounce off each other several times. We show that these changes in behavior are associated with an increasing collision rate. We then analyze the oscillation of the clouds in terms of a nonlinear coupling between the spin dipole mode and the axial breathing mode which is enforced by collisions. We also determine the frequency of the bounce as a function of the final temperature of the equilibrated system.
7. arXiv:1106.5725 (cross-list from hep-lat) [pdf, other]
Title: Unitary fermions on the lattice I: in a harmonic trap
Authors:Michael G. Endres, David B. Kaplan, Jong-Wan Lee, Amy N. Nicholson
Comments: 47 pages, 19 figures
Subjects: High Energy Physics - Lattice (hep-lat); Quantum Gases (cond-mat.quant-gas); Nuclear Theory (nucl-th); Atomic Physics (physics.atom-ph)
We present a new lattice Monte Carlo approach developed for studying large numbers of strongly interacting nonrelativistic fermions, and apply it to a dilute gas of unitary fermions confined to a harmonic trap. Our lattice action is highly improved, with sources of discretization and finite volume errors systematically removed; we are able to demonstrate the expected volume scaling of energy levels of two and three untrapped fermions, and to reproduce the high precision calculations published previously for the ground state energies for N = 3 unitary fermions in a box (to within our 0.3% uncertainty), and for N = 3, . . ., 6 unitary fermions in a harmonic trap (to within our ~ 1% uncertainty). We use this action to determine the ground state energies of up to 70 unpolarized fermions trapped in a harmonic potential on a lattice as large as 64^3 x 72; our approach avoids the use of importance sampling or calculation of a fermion determinant and employs a novel statistical method for estimating observables, allowing us to generate ensembles as large as 10^8 while requiring only relatively modest computational resources.
Jun 28
1. arXiv:1106.5160 [pdf, ps, other]
Title: Fermi surface reconstruction in strongly correlated Fermi systems as a first order phase transition
Authors:S.S.Pankratov, M.V.Zverev, M.Baldo
Comments: 6 pages, 7 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
A quantum phase transition in strongly correlated Fermi systems beyond the topological quantum critical point is studied within the Fermi liquid approach. The transition occurs between two topologically equivalent states, each with three sheets of the Fermi surface. One of these states possesses a quasiparticle halo in the quasiparticle momentum distribution $n(p)$, while the other, the hole pocket. The transition is found to be of the first order with respect to both the coupling constant $g$ and the temperature $T$. The phase diagram of the system in the vicinity of this transition is constructed.
2. arXiv:1106.5206 [pdf, ps, other]
Title: Thermalization and Quantum Correlations in Exactly Solvable Models
Authors:Miguel A. Cazalilla, A. Iucci, Ming-Chiang Chung
Comments: 4 pages
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas)
The generalized Gibbs ensemble introduced for describing few body correlations in exactly solvable systems following a quantum quench is related to the way in which operators sample, in the limit of infinite time after the quench, the quantum correlations present in the initial state. The emergence of the generalized Gibbs ensemble is thus \emph{analytically} demonstrated for the quantum Ising and XX chains in the thermodynamic limit. For these models and a broad class of initial states, which includes both translationally and non-translationally invariant states, the validity of the generalized Gibbs ensemble for simple correlation functions of both local and nonlocal operators is established.
3. Xiv:1106.5340 [pdf, ps, other]
Title: Spontaneous symmetry breaking in linearly coupled disk-shaped Bose-Einstein condensates
Authors:Luca Salasnich (Padua Univ.), Boris A. Malomed (Tel Aviv Univ.)
Comments: 13 pages, 8 figures, accepted for publication in Molecular Physics, special issue "Luciano Reatto Festschrift" [LS thanks Luciano Reatto for 9 years of fruitful scientific collaboration at the Physics Department of the University of Milano]
Subjects: Quantum Gases (cond-mat.quant-gas); Pattern Formation and Solitons (nlin.PS)
We study effects of tunnel coupling on a pair of parallel disk-shaped Bose-Einstein condensates with the self-attractive intrinsic nonlinearity. Each condensate is trapped in a combination of in-plane and transverse harmonic-oscillator potentials. It is shown that, depending on the self-interaction strength and tunneling coupling, the ground state of the system exhibits a phase transition which links three configurations: a symmetric one with equal numbers of atoms in the coupled condensates, an asymmetric configuration with a population imbalance (a manifestation of the macroscopic quantum self-trapping), and the collapsing state. A modification of the phase diagram of the system in the presence of vortices in the disk-shaped condensates is reported too. The study of dynamics around the stationary configurations reveals properties which strongly depend on the symmetry of the configuration.
Jun 27
1. arXiv:1106.4906 [pdf, ps, other]
Title: Characterization of excited states of ultracold atoms in optical lattices
Authors:Mateusz Lacki, Dominique Delande, Jakub Zakrzewski
Comments: remarks to kuba@if.uj.edu.pl please
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
Loading ultracold atoms in an optical lattice from a Bose-Einstein condensate is generally a non-adiabatic process resulting in the dynamical excitation of a wavepacket, a combination of several eigenstates. Using the time evolving block decimation algorithm, we show how to extract information on these excited states, and how their properties differ from those of the ground state. This allows for a deeper understanding of nonadiabaticity in experimental realizations of insulating phases.
2. arXiv:1106.4936 [pdf, other]
Title: Probing the BCS-BEC crossover with photons in a nonlinear optical fiber
Authors:Mingxia Huo, Changsuk Noh, B. M. Rodríguez-Lara, Dimitris G. Angelakis
Comments: 4 and bit pages, 4 figures, comments welcome
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
We propose a scheme where strongly correlated photons generated inside a hollow-core one-dimensional fiber filled with two cold atomic species can be used to simulate the BCS-BEC crossover. We first show how stationary light-matter excitations (polaritons) in the system can realize an optically tunable two component Bose-Hubbard model, and then analyze the optical parameters regime necessary to generate an effective Fermi-Hubbard model of photons exhibiting Cooper pairing. The characteristic correlated phases of the system can be efficiently observed due to the {\it in situ} accessibility of the photon correlations with standard optical technology.
3. arXiv:1106.5018 [pdf, ps, other]
Title: Ferromagnetic frustrated spin systems on the square lattice: a Schwinger boson study
Authors:H. Feldner, D.C. Cabra, G.L. Rossini
Comments: 7 pages, 11 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study a ferromagnetic Heisenberg spin system on the square lattice, with nearest neighbors interaction J_1 frustrated by second J_2 and third J_3 neighbors antiferromagnetic interactions, using a mean field theory for theSchwinger boson representation of spins. For J_3=0 we find that the boundary between the ferromagnetic and the collinear classical phases shifts to smaller values of J_2 when quantum fluctuations are included. Along the line J_2/|J_1|= 1 the boundaries between the collinear and incommensurate regions are strongly shifted to larger values with respect to the classical case. We do not find clear evidence for spin gapped phases within the present approximation.
June 20-24, Chungwei Lin
Jun 24
1. arXiv:1106.4559 [pdf, ps, other]
Low temperature properties of the infinite-dimensional attractive Hubbard model
Akihisa Koga, Philipp Werner
Comments: 8 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate the attractive Hubbard model in infinite spatial dimensions by combining dynamical mean-field theory with a strong-coupling continuous-time quantum Monte Carlo method. By calculating the superfluid order parameter and the density of states, we discuss the stability of the superfluid state. In the intermediate coupling region above the critical temperature, the density of states exhibits a heavy fermion behavior with a quasi-particle peak in the dense system, while a dip structure appears in the dilute system. The formation of the superfluid gap is also addressed.
2. arXiv:1106.4686 [pdf, ps, other]
Nonlinear Bose-Einstein-condensate Dynamics Induced by a Harmonic Modulation of the s-wave Scattering Length
Ivana Vidanovic, Antun Balaz, Hamid Al-Jibbouri, Axel Pelster
Comments: 12 pages, 13 figures, accepted for publication in Phys. Rev. A
Subjects: Quantum Gases (cond-mat.quant-gas); Computational Physics (physics.comp-ph)
In a recent experiment, a Bose-Einstein condensate of Li7 has been excited by a harmonic modulation of the atomic s-wave scattering length via Feshbach resonance. Combining an analytical perturbative approach with extensive numerical simulations we analyze the emerging nonlinear dynamics of the system on the mean-field Gross-Pitaevskii level at zero temperature. Resulting excitation spectra are presented and prominent nonlinear features are found: mode coupling, higher harmonics generation and significant shifts in the frequencies of collective modes. We indicate how nonlinear dynamical properties could be made clearly observable in future experiments and compared to our results.
Jun 23
1. arXiv:1106.4345 [pdf, other]
Collective phenomena in quasi-two-dimensional fermionic polar molecules: band renormalization and excitons
Mehrtash Babadi, Eugene Demler
Comments: 21 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We theoretically analyze a quasi-two-dimensional system of fermionic polar molecules in a harmonic transverse confining potential. The renormalized energy bands are calculated by solving the Hartree-Fock equation numerically for various trap and dipolar interaction strengths. The inter-subband excitations of the system are studied in the conserving time-dependent Hartree-Fock (TDHF) approximation from the perspective of lattice modulation spectroscopy experiments. We find that the excitation spectrum consists of both inter-subband particle-hole excitation continuums and anti-bound excitons, arising from the anisotropic nature of dipolar interactions. The excitonic modes capture the majority of the spectral weight. We also evaluate the inter-subband transition rates in order to investigate the nature of the excitonic modes and find that they are anti-bound states formed from particle-hole excitations arising from several subbands. Our results indicate that the excitonic effects are present for interaction strengths and temperatures accessible in current experiments with polar molecules.
2. arXiv:1106.4438 [pdf, ps, other]
Numerical study of Kondo impurity models with strong potential scattering: - reverse Kondo effect and antiresonance -
Annamaria Kiss, Yoshio Kuramoto, Shintaro Hoshino
Comments: 10 pages, 7 figures, submitted to PRB
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Accurate numerical results are derived for transport properties of Kondo impurity systems with potential scattering and orbital degeneracy. Using the continuous-time quantum Monte Carlo (CT-QMC) method, static and dynamic physical quantities are derived in a wide temperature range across the Kondo temperature T_K. With strong potential scattering, the resistivity tends to decrease with decreasing temperature, in contrast to the ordinary Kondo effect. Correspondingly, the quasi-particle density of states obtains the antiresonance around the Fermi level. Thermopower also shows characteristic deviation from the standard Kondo behavior, while magnetic susceptibility follows the universal temperature dependence even with strong potential scattering. It is found that the t-matrix in the presence of potential scattering is not a relevant quantity for the Friedel sum rule, for which a proper limit of the f-electron Green's function is introduced. The optical theorem is also discussed in the context of Kondo impurity models with potential scattering. It is shown that optical theorem holds not only in the Fermi-liquid range but also for large energies, and therefore is less restrictive than the Friedel sum rule.
3. arXiv:1106.4405 [pdf]
Spin-Triplet Vortex State in the Topological Superconductor CuxBi2Se3
Pradip Das, Yusuke Suzuki, Masashi Tachiki, Kazuo Kadowaki
Comments: 11 pages, 4 figures, to be published in PRB Rapid Communications
Subjects: Superconductivity (cond-mat.supr-con)
We report on the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of CuxBi2Se3. The magnitude of the magnetization in the Meissner state is very small and the magnetic-field dependence of the magnetization just above the lower critical field Hc1 is very different from those of usual type-II superconductors. We studied the character of the vortex state theoretically in a spin-triplet pairing superconductor and compared it with the experimental results. The results showed that, the superconductivity observed in CuxBi2Se3 is consistent with the spin-triplet pairing superconductivity with odd parity. We also observed a rapid relaxation phenomenon of the superconducting diamagnetism.
Jun 22
1. arXiv:1106.4245 [pdf, ps, other]
Colliding clouds of strongly interacting spin-polarized fermions
Edward Taylor, Shizhong Zhang, William Schneider, Mohit Randeria
Comments: 4 pages, 3 figures + 2 pages Supplemental material
Subjects: Quantum Gases (cond-mat.quant-gas)
Motivated by the recent experiment of Sommer {\it et al.} \cite{Sommer11}, we consider the collision of two clouds of atomic Fermi gases prepared in different hyperfine-Zeeman states close to a Feshbach resonance. We argue that, upon coming into contact with each other, the fermions in the strongly interacting regime are well described by hydrodynamics. Solving Euler's equations, we show that the clouds \emph{bounce} off each other, with a damped oscillatory motion of the two centers of mass at short time, followed by phase separation of the two species at intermediate times. This dynamics, which is in excellent agreement with the experiment, reflects a metastable many-body state on the "upper branch" of the resonance where the effective interaction is repulsive, despite the underlying attraction.
2. arXiv:1106.4031 [pdf, other]
Non-equilibrium dynamics of Bosonic Mott insulators in an electric field
Michael Kolodrubetz, David Pekker, Bryan K. Clark, Krishnendu Sengupta
Comments: 4+epsilon pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study the non-equilibrium dynamics of one-dimensional Mott insulating bosons in the presence of a tunable effective electric field E which takes the system across a quantum critical point (QCP) separating a disordered and a translation symmetry broken ordered phase. We provide an exact numerical computation of the residual energy Q, the log-fidelity F, the excess defect density D, and the order parameter correlation function for a linear-in-time variation of E with a rate v. We discuss the temporal and spatial variation of these quantities for a range of v and for finite system sizes as relevant to realistic experimental setups [J. Simon et al., Nature 472, 307 (2011)]. We show that in finite-sized systems Q, F, and D obey Kibble-Zurek scaling, and suggest further experiments within this setup to test our theory.
3. arXiv:1106.4296 (cross-list from cond-mat.mtrl-sci) [pdf, ps, other]
Interface engineering of quantum Hall effects in digital heterostructures of transition-metal oxides
Di Xiao, Wenguang Zhu, Ying Ran, Naoto Nagaosa, Satoshi Okamoto
Comments: Main text 7.1 pages with 4 figures. Supplementary materials 6 pages with 2 figures and 1 table
Subjects: Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)
Topological insulators are new states of quantum matter characterized by nontrivial band topology driven by the spin-orbit coupling. To fully explore the fundamental science and application of topological insulators, material realization is indispensable. Here we predict, based on tight-binding modeling and first-principles calculations, that bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be fine-tuned by changing dopant ions, substrates, and external gate voltages, and even the topological phase transition could be manipulated. In particular, we predict that LaAuO$_3$ bilayer has a topologically-nontrivial energy gap about 0.15 eV, which is sufficiently large to realize the room-temperature quantum spin-Hall effect. Intriguing phenomena, such as high-temperature fractional quantum Hall effect, associated with the nearly-flat topologically-nontrivial bands found in $e_g$ systems are also discussed. This may pave a new route to the topological electronics in oxide heterostructures.
Jun 21
1. arXiv:1106.3567 [pdf, other]
Potential insights into non-equilibrium behavior from atomic physics
Austen Lamacraft, Joel Moore
Comments: Chapter to appear in the forthcoming volume "Ultracold Bosonic and Fermionic Gases", (Contemporary Concepts of Condensed Matter Science (Elsevier)), edited by Alexander Fetter, Katherine Levin, and Dan Stamper-Kurn
Subjects: Quantum Gases (cond-mat.quant-gas)
This chapter seeks to outline a few basic problems in quantum statistical physics where recent experimental advances from the atomic physics community offer the hope of dramatic progress. The focus is on nonequilibrium situations where the powerful concepts and methods of equilibrium statistical physics and "linear response" theory (for small deviations from equilibrium) are not applicable. The problems discussed here are chosen in part because they have a high degree of "universality" or generality across different microscopic situations, as the major challenge in nonequilibrium statistical physics, both quantum and classical, has been to find principles as general as the basic principles of equilibrium statistical physics or linear response.
2. arXiv:1106.3613 [pdf, ps, other]
Evolution from BCS to BEC superfluidity in the presence of spin-orbit coupling
Li Han, C. A. R. Sá de Melo
Subjects: Quantum Gases (cond-mat.quant-gas)
We discuss the evolution from BCS to BEC superfluids in the presence of spin-orbit coupling, and show that this evolution is just a crossover in the balanced case. The dependence of several thermodynamic properties, such as the chemical potential, order parameter, pressure, entropy, isothermal compressibility and spin susceptibility tensor on the spin-orbit coupling and interaction parameter at low temperatures are analyzed. We studied both the case of equal Rashba and Dresselhaus (ERD) and the Rashba-only (RO) spin-orbit coupling. Comparisons between the two cases reveal several striking differences in the corresponding thermodynamic quantities. Finally we propose measuring the spin susceptibility as a means to detect the spin-orbit coupling effect.
3. arXiv:1106.3993 [pdf, other]
Spin-orbit coupling in $d^{2}$ ordered double perovskites
Gang Chen, Leon Balents
Comments: 13 pages, 16 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We construct and analyze a microscopic model for insulating rock salt ordered double perovskites, with the chemical formula A$_2$BB'O$_6$, where the magnetic ion B' has a 4d$^2$ or 5d$^2$ electronic configuration and forms a face centered cubic (fcc) lattice. For these B' ions, the combination of the triply-degenerate antisymmetric two-electron orbital states and strong spin-orbit coupling forms local quintuplets with an effective spin moment $j=2$. Moreover, due to strongly orbital-dependent exchange, the effective spins have substantial biquadratic and bicubic interactions (fourth and sixth order in the spins, respectively). This leads, at the mean field level, to a rich ground state phase diagram which includes seven different phases: a uniform ferromagnetic phase with an ordering wavevector ${\bf p} = {\bf 0}$ and uniform magnetization along $[111]$ direction, four two-sublattice phases with an ordering wavevector ${\bf p} = 2\pi(001)$ and two four-sublattice antiferromagnetic phases. Amongst the two-sublattice phases there is a quadrupolar ordered phase which preserves time reversal symmetry. Extending the mean field theory to finite temperatures, we find ten different magnetization processes with different magnetic thermal transitions. In particular, we find that thermal fluctuations stabilize the two-sublattice quadrupolar ordered phase in a large portion of phase diagram. Existing and possible future experiments are discussed in light of these theoretical predictions.
4. arXiv:1106.3728 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
Towards a description of the Kondo effect using time-dependent density functional theory
G. Stefanucci, S. Kurth
Comments: 5 pages, 3 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)
We demonstrate that the zero-temperature conductance of the Anderson model can be calculated within the Landauer formalism combined with static density functional theory (DFT). The proposed approximate functional is based on finite-temperature DFT and yields the exact Kohn-Sham potential at the particle-hole symmetric point. Furthermore, in the limit of zero temperature it correctly exhibits a derivative discontinuity which is shown to be essential to reproduce the conductance plateau. On the other hand, at the Kondo temperature the exact Kohn-Sham conductance overestimates the real one by an order of magnitude. To understand the failure of DFT we resort to its time-dependent version and conclude that the suppression of the Kondo resonance with increasing temperature must be attibuted to dynamical exchange-correlation corrections.
Jun 20
1. arXiv:1106.3499 [pdf, other]
Nonlinear localized modes in dipolar Bose-Einstein condensates in optical lattices
S. Rojas-Rojas, R. A. Vicencio, M. I. Molina, F. Kh. Abdullaev
Comments: 12 pages, 13 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Pattern Formation and Solitons (nlin.PS)
The modulational instability and discrete matter wave solitons in dipolar BEC, loaded into a deep optical lattice, are investigated analytically and numerically. The process of modulational instability of nonlinear plane matter waves in a dipolar nonlinear lattice is studied and the regions of instability are established. The existence and stability of bulk discrete solitons are analyzed analytically and confirmed by numerical simulations. In a marked contrast with the usual DNLS behavior (no dipolar interactions), we found a region where the two fundamental modes are simultaneously unstable allowing enhanced mobility across the lattice for large norm values. To study the existence and properties of surface discrete solitons, an analysis of the dimer configuration is performed. The properties of symmetric and antisymmetric modes including the stability diagrams and bifurcations are investigated in closed form. For the case of a bulk medium, properties of fundamental on-site and inter-site localized modes are analyzed. On-site and inter-site surface localized modes are studied finding that they do not exist when nonlocal interactions predominate with respect to local ones.
2. arXiv:1102.1445 (cross-list from cond-mat.supr-con) [pdf, ps, other]
Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides
Andrew Nicholson, Weihao Ge, Xiaotian Zhang, Jose Riera, Maria Daghofer, Andrzej M. Oles, George B. Martins, Adriana Moreo, Elbio Dagotto
Comments: submitted PRL 10/5/10
Journal-ref: Phys. Rev. Lett. 106, 217002 (2011)
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
We introduce and study an extended "t-U-J" two-orbital model for the pnictides that includes Heisenberg terms deduced from the strong coupling expansion. Including these J terms explicitly allows us to enhance the strength of the (pi, 0)-(0, pi) spin order which favors the presence of tightly bound pairing states even in the small clusters that are here exactly diagonalized. The A1g and B2g pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying B1g states, suggesting that small changes in parameters may render any of the three channels stable.
3. arXiv:1106.3470 [pdf, ps, other]
Magnetic susceptibility of Ce: an LDA+DMFT study
S.V. Streltsov, E. Gull, A.O. Shorikov, M. Troyer, V.I. Anisimov, P. Werner
Comments: 4 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
The magnetic properties of Ce in the $\alpha$ and $\gamma$ phase are calculated within the LDA+DMFT approach. The magnetic susceptibility in these two phases shows a similar behavior over a wide temperature range: a Curie-Weiss law at high temperatures, indicating the presence of local moments, followed by a maximum in a crossover regime, and a saturation characteristic of a state with screened local moments at low temperature. The difference in experimentally observable magnetic properties is caused by the shift of the susceptibility to higher temperatures in the $\alpha-$phase compared to the $\gamma-$phase, due to the larger hybridization between localized $4f-$states and conductive $spd-$electrons.
June 13-17, Xiaopeng Li
Jun 17
1. arXiv:1106.3078 [pdf, other]
Title: Majorana Fermions in Semiconductor Nanowires
Authors: Tudor Stanescu, Roman M. Lutchyn, S. Das Sarma
We study multiband semiconducting nanowires proximity-coupled with an s-wave superconductor and calculate the topological phase diagram as a function of the chemical potential and magnetic field. Here the non-trivial topological state corresponds to a superconducting phase supporting Majorana modes localized at the ends of the wire whereas non-topological phase corresponds to a trivial phase with no Majoranas. Our key finding is that multiband occupancy not only lifts the stringent constraint of one-dimensionality but also allows having higher carrier density in the nanowire and as such multiband nanowires are better-suited for observing the Majorana physics. We present a detailed study of the parameter space for multiband semiconductor nanowires focusing on understanding the key experimental conditions required for the realization and detection of Majorana fermions in solid-state systems. We include various sources of disorder and characterize their effects on the stability of the topological phase. Finally, we calculate local density of states as well as differential tunneling conductance as a function of external parameters, predicting in the process the experimental results which would establish the existence of emergent Majorana zero-energy modes in solid-state systems.
2. arXiv:1106.3108 [pdf, ps, other]
A Proposal for Detecting Spin-Chirality Terms in Mott Insulators via RIXS
Authors: Wing-Ho Ko, Patrick A. Lee
We consider the question of whether resonant inelastic x-ray scattering (RIXS) can be used to detect many-body excitations that are coupled to the spin-chirality terms S_i . (S_j x S_k) in a Mott insulator. We find that while such terms are in general absent in the usual experimental setups, in which the connection between the photon polarization and the direction of virtual electron hop in the lattice is lost, there is prospect in realizing such terms in RIXS if one instead considers scattering near a pre-edge. Analysis is then performed in details for the square and the kagome lattices, and briefly for the triangular and the honeycomb lattices, in which we show that the spin-chirality terms are indeed present in all cases, but that they occurred at a higher order in our expansion for the kagome and the honeycomb case. The merit of considering RIXS in addition to Raman spectroscopy for detecting excitations coupled to the spin-chirality terms is also briefly discussed in the context of the emergent gauge boson in the U(1) Dirac spin liquid.
3. arXiv:1106.3148 [pdf, ps, other]
Title: Field Evolution of FFLO State in a Superconductor with Strong Pauli Effects
Authors: Kenta M. Suzuki, Yasumasa Tsutsumi, Noriyuki Nakai, Masanori Ichioka, Kazushige Machida
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the vortex state is quantitatively studied by selfconsistent Eilenberger theory in three dimensional (3D) space of vortex lattice and FFLO modulation. We estimate free energy to determine the FFLO phase diagram in $T$-$H$ plane and stable FFLO wave number for typical and canonical example of 3D Fermi sphere with s-wave pairing. To facilitate the experimental identification of FFLO state, we investigate the field evolutionof NMR spectra and flux line lattice form factors by neutron scattering in the FFLO vortex states. Possible applications to CeCoIn$_5$ is mentioned.
4. arXiv:1106.3231 [pdf, ps, other]
Title: Microscopic structure and thermodynamics of a core-softened model fluid from the second-order integral equations theory
Authors: O. Pizio, Z. Sokolowska, S. Sokolowski
We have studied the structure and thermodynamic properties of isotropic three-dimensional core-softened fluid by using the second-order Ornstein-Zernike integral equations completed by the hypernetted chain and Percus-Yevick closures. The radial distribution functions are compared with those from singlet integral equations and with computer simulation data. The limits of the region of density anomaly resulting from different approximate theories are established. The obtained results show that the second-order hypernetted chain approximation can be used to describe both the structure and the density anomaly of this model fluid. Moreover, we present the results of calculations of the bridge functions.
Jun 16
1. arXiv:1106.2997 [pdf, ps, other]
Phase diagram of Rydberg atoms with repulsive van der Waals interaction
Authors: O. N. Osychenko, G. E. Astrakharchik, Y. Lutsyshyn, Yu. E. Lozovik, J. Boronat
Abstract: We report a quantum Monte Carlo calculation of the phase diagram of bosons interacting with a repulsive inverse sixth power pair potential, a model for assemblies of Rydberg atoms in the local van der Waals blockade regime. The model can be parametrized in terms of just two parameters, the reduced density and temperature. Solidification happens to an fcc phase. At zero temperature the transition density is found with the diffusion Monte Carlo method at density $\rho = 3.9 (\hbar^2/m C_6)^{3/4} $, where $C_6$ is the strength of the interaction. The solidification curve at non-zero temperature is studied with the path integral Monte Carlo approach and is compared with transitions in corresponding harmonic and classical crystals. Relaxation mechanisms are considered in relation to present experiments, especially pertaining to hopping of the Rydberg excitation.
2. arXiv:1106.2931 [pdf, ps, other]
Title: Role of d-wave pairing in $A15$ superconductors
Authors: S. Mukherjee, D. F. Agterberg
Abstract: We argue that the recent Raman spectroscopy observation of a sharp mode in s-wave superconducting V$_3$Si is due to a competing d-wave pairing state. We present microscopic arguments for the origin of this d-wave order. We further argue that the d-wave order explains the observed shrinking of the vortex core structure at anomalously low magnetic fields and the large anisotropy observed in the upper critical field.
Jun 15
1. arXiv:1106.2614 [pdf, ps, other]
Title: Majorana Edge States in Interacting Two-chain Ladders of Fermions
Authors: Meng Cheng, Hong-Hao Tu
Abstract: In this work we study interacting spinless fermions on a two-chain ladder with inter-chain pair tunneling while single-particle tunneling is suppressed at low energy. The model embodies a $\mathbb{Z}_2$ symmetry associated with the fermion parity on each chain. We find that when the system is driven to the strong-coupling phase by the pair tunneling, this fermion parity is effectively "fractionalized", signified by appearance of Majorana excitations on the boundary. Such Majorana edge states correspond to two-fold degeneracy of ground states distinguished by different fermion parity on each chain, thus representing a generalization of one-dimensional topological superconductors. Lattice fermion models realizing such effective field theory are discussed.
Jun 14
1. arXiv:1106.2439 [pdf, ps, other]
Title: Orbital-order melting in rare-earth manganites: the role of super-exchange
Authors: Andreas Flesch, Guoren Zhang, Erik Koch, Eva Pavarini
Abstract: We study the mechanism of orbital-order melting observed at temperature T_OO in the series of rare-earth manganites. We find that many-body super-exchange yields a transition-temperature T_KK that decreases with decreasing rare-earth radius, and increases with pressure, opposite to the experimental T_OO. We show that the tetragonal crystal-field splitting reduces T_KK further increasing the discrepancies with experiments. This proves that super-exchange effects, although very efficient, in the light of the experimentally observed trends, play a minor role for the melting of orbital ordering in rare-earth manganites.
2. arXiv:1106.2306 [pdf, other]
Critical Behavior of AC Antiferromagnetic and Ferromagnetic Susceptibilities of a Spin-1/2 Metamagnetic Ising System
Authors: Gul Gulpinar, Vatansever Erol
Abstract: In this study, the temperature variations of the equilibrium and the non-equilibrium antiferromagnetic and ferromagnetic susceptibilities of a metamagnetic system are examined near the critical point. The kinetic equations describing the time dependencies of the total and staggered magnetizations are derived by utilizing linear response theory. In order to obtain dynamic magnetic relaxation behavior of the system, the stationary solutions of the kinetic equations in existence of sinusoidal staggered and physical external magnetic fields are performed. In addition, the static and dynamical mean field critical exponents are calculated in order to formulate the critical behavior of antiferromagnetic and ferromagnetic magnetic response of a metamagnetic system. Finally, a comparison of the findings of this study with previous theoretical and experimental studies is represented and it is shown that a good agreement is found with our results.
Jun 13
1. arXiv:1106.1930 [pdf, other]
Title: Chirality and Orbital Order in Charge Density Waves
Authors: Jasper van Wezel
Abstract: We show that the recently observed chirality in the charge ordered phase of TiSe2 can be understood as a form of orbital ordering. The microscopic mechanism driving the transition between the novel chiral state and the non-chiral charge density wave is discussed, and shown to be of a general form, thus allowing for a broad class of materials to display this type of orbitally ordered chiral charge density wave.
June 6-10, Zixu Zhang
Jun 10
1. Xiv:1106.1644 [pdf, other]
Title: Tunable Superfluidity and Quantum Magnetism with Ultracold Polar Molecules
Authors:Alexey V. Gorshkov, Salvatore R. Manmana, Gang Chen, Jun Ye, Eugene Demler, Mikhail D. Lukin, Ana Maria Rey
Comments: 4 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
By selecting two dressed rotational states of ultracold polar molecules in an optical lattice, we obtain a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. In addition to XXZ spin exchange, the model features density-density interactions and novel density-spin interactions; all interactions are dipolar. We show that full control of all interaction parameters in both magnitude and sign can be achieved independently of each other and of the tunneling. As a first step towards demonstrating the potential of the system, we apply the density matrix renormalization group method (DMRG) to obtain the 1D phase diagram of the simplest experimentally realizable case. Specifically, we show that the tunability and the long-range nature of the interactions in the t-J-V-W model enable enhanced superfluidity. Finally, we show that Bloch oscillations in a tilted lattice can be used to probe the phase diagram experimentally.
2. arXiv:1106.1655 [pdf, other]
Title: Quantum Magnetism with Polar Alkali Dimers
Authors:Alexey V. Gorshkov, Salvatore R. Manmana, Gang Chen, Eugene Demler, Mikhail D. Lukin, Ana Maria Rey
Comments: 24 pages, 8 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)
We show that dipolar interactions between ultracold polar alkali dimers in optical lattices can be used to realize a highly tunable generalization of the t-J model, which we refer to as the t-J-V-W model. The model features long-range spin-spin interactions J_z and J_perp of XXZ type, long-range density-density interaction V, and long-range density-spin interaction W, all of which can be controlled in both magnitude and sign independently of each other and of the tunneling t. The "spin" is encoded in the rotational degree of freedom of the molecules, while the interactions are controlled by applied static electric and continuous-wave microwave fields. Furthermore, we show that nuclear spins of the molecules can be used to implement an additional (orbital) degree of freedom that is coupled to the original rotational degree of freedom in a tunable way. The presented system is expected to exhibit exotic physics and to provide insights into strongly correlated phenomena in condensed matter systems. Realistic experimental imperfections are discussed.
3. arXiv:1106.1685 [pdf, ps, other]
Title: Conventional and charge six superfluids from melting hexagonal Fulde-Ferrell-Larkin-Ovchinnikov phases in two dimensions
Authors:D.F. Agterberg, M Geracie, H. Tsunetsugu
Comments: 8 pages, 3 figures
Subjects: Superconductivity (cond-mat.supr-con); Atomic Physics (physics.atom-ph)
We consider defect mediated melting of Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and pair density wave (PDW) phases in two dimensions. Examining mean-field ground states in which the spatial oscillations of the FFLO/PDW superfluid order parameter exhibit hexagonal lattice symmetry, we find that thermal melting leads to a variety of novel phases. We find that a spatially homogeneous charge six superfluid can arise from melting a hexagonal vortex-anitvortex lattice FFLO/PDW phase. The charge six superfluid has an order parameter corresponding to a bound state of six fermions. We further find that a hexagonal vortex-free FFLO/PDW phase can melt to yield a conventional (charge two) homogeneous superfluid. A key role is played by topological defects that combine fractional vortices of the superfluid order and fractional dislocations of the lattice order.
4. arXiv:1106.1718 [pdf, other]
Title: Spectroscopy of dipolar fermions in 2D pancakes and 3D lattices
Authors:Kaden R. A. Hazzard, Alexey V. Gorshkov, Ana Maria Rey
Comments: 14 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)
Motivated by ongoing measurements at JILA, we calculate the recoil-free spectra of dipolar interacting fermions, for example ultracold heteronuclear molecules, in a one-dimensional lattice of two-dimensional pancakes, spectroscopically probing transitions between different internal (e.g., rotational) states. We additionally incorporate p-wave interactions and losses, which are important for reactive molecules such as KRb. Moreover, we consider other sources of spectral broadening: interaction-induced quasiparticle lifetimes and the different polarizabilities of the different rotational states used for the spectroscopy. Although our main focus is molecules, some of the calculations are also useful for optical lattice atomic clocks. For example, understanding the p-wave shifts between identical fermions and small dipolar interactions coming from the excited clock state are necessary to reach future precision goals. Finally, we consider the spectra in a deep 3D lattice and show how they give a great deal of information about static correlation functions, including \textit{all} the moments of the density correlations between nearby sites. The range of correlations measurable depends on spectroscopic resolution and the dipole moment.
Jun 9
1. arXiv:1106.1437 [pdf, ps, other]
Title: On the Validity of the Tomonaga Luttinger Liquid Relations for the One-dimensional Holstein Model
Authors:Ka-Ming Tam, S.-W. Tsai, D. K. Campbell
Comments: 7 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
For the one-dimensional Holstein model, we show that the relations among the scaling exponents of various correlation functions of the Tomonaga Luttinger liquid (LL), while valid in the thermodynamic limit, are significantly modified by finite size corrections. We obtain analytical expressions for these corrections and find that they decrease very slowly with increasing system size. The interpretation of numerical data on finite size lattices in terms of LL theory must therefore take these corrections into account. As an important example, we re-examine the proposed metallic phase of the zero-temperature, half-filled one-dimensional Holstein model without employing the LL relations. In particular, using quantum Monte Carlo calculations, we study the competition between the singlet pairing and charge ordering. Our results do not support the existence of a dominant singlet pairing state.
2. arXiv:1106.1605 [pdf, other]
Title: Triplet pair correlations in {\it s-}wave superfluids as a signature of the FFLO state
Authors:I. Zapata, F. Sols, E. Demler
Comments: 4 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
We show that triplet pairing correlations are generated in purely {\it s-}waves superfluids whenever population imbalance enforces anisotropic Fulde-Ferrell (FF) or inhomogeneous Larkin-Ovchinikov (LO) states. The same set of quasiparticle states contributes to the triplet component and to the polarization, thus spatially correlating them. In the LO case, this set forms a narrow band of Andreev states centered on the nodes of the {\it s-}wave order parameter. This picture naturally provides a unifying explanation of previous findings that attractive {\it p}-wave interaction stabilizes FFLO states. We also study a similar triplet mixing which occurs when a balanced two-component system displays FFLO type oscillations due to a spin-dependent optical lattice. We discuss how this triplet component can be measured in systems of ultra-cold atoms using a rapid ramp across a {\it p-}wave Feshbach resonance. This should provide a smoking gun signature of FFLO states.
3. arXiv:1106.1628 [pdf, ps, other]
Title: Adiabatic preparation of a Heisenberg antiferromagnet using an optical superlattice
Authors:Michael Lubasch, Valentin Murg, Ulrich Schneider, J. Ignacio Cirac, Mari-Carmen Bañuls
Comments: 4 pages, 4 figures + Supplementary Material (5 pages, 6 figures)
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We analyze the possibility to prepare a Heisenberg antiferromagnet with cold fermions in optical lattices, starting from a band insulator and adiabatically changing the lattice potential. The numerical simulation of the dynamics in 1D allows us to identify the conditions for success, and to study the influence that the presence of holes in the initial state may have on the protocol. We also extend our results to two-dimensional systems.
Jun 8
1. arXiv:1106.1333 [pdf, ps, other]
Title: Doublon production rate in modulated optical lattices
Authors:Akiyuki Tokuno, Eugene Demler, Thierry Giamarchi
Comments: 5 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study theoretically lattice modulation experiments with ultracold fermions in optical lattices. We focus on the regime relevant to current experiments when interaction strength is larger than the bandwidth and temperature is higher than magnetic superexchange energy. We obtain analytical expressions for the rate of doublon production as a function of modulation frequency, filling factor, and temperature. We use local density approximation to average over inhomogeneous density for atoms in a parabolic trap and find excellent agreement with experimentally measured values. Our results suggest that lattice modulation experiments can be used for thermometry of strongly interacting fermionic ensembles in optical lattices.
2. arXiv:1106.1345 [pdf, ps, other]
Title: Non-Perturbative Prediction of the Ferromagnetic Transition in Repulsive Fermi Gases
Authors:Lianyi He, Xu-Guang Huang
Comments: 4+epsilon pages, 4 figures. Comments are welcome
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Nuclear Theory (nucl-th)
It is generally believed that a dilute spin-1/2 Fermi gas with repulsive interactions can undergo a ferromagnetic phase transition to a spin-polarized state at a critical gas parameter $(k_{\text F}a)_c$. The perturbation theory fails to predict quantitatively the ferromagnetic transition since $(k_{\text F}a)_c$ is not small. In this Letter we study the non-perturbative effects on the ferromagnetic transition by summing the particle-particle ladder diagrams to all orders in the gas parameter. To the leading order of the effective range expansion, such a resummation predicts a second order ferromagnetic phase transition. The predicted critical gas parameter $(k_{\text F}a)_c=0.858$ is in good agreement with recent Quantum Monte Carlo result $(k_{\text F}a)_c=0.86$ for a nearly zero-range potential [S. Pilati, \emph{et al}., Phys. Rev. Lett. {\bf 105}, 030405 (2010)].
3. arXiv:1101.5694 (cross-list from nucl-th) [pdf, ps, other]
Title: Critical Zeeman Splitting of a Unitary Fermi Superfluid
Authors:Lianyi He, Pengfei Zhuang
Comments: Version to appear in Physical Review B
Journal-ref: Phys. Rev. B 83, 174504 (2011)
Subjects: Nuclear Theory (nucl-th); Quantum Gases (cond-mat.quant-gas)
We determine the critical Zeeman energy splitting of a homogeneous Fermi superfluid at unitary in terms of the Fermi energy $\epsilon_{\text F}$ according to recent experimental results in LKB-Lhomond. Based on the universal equations of state for the superfluid and normal phases, we show that there exist two critical fields $H_{c1}$ and $H_{c2}$, between which a superfluid-normal mixed phase is energetically favored. Universal formulae for the critical fields and the critical population imbalance $P_c$ are derived. We have found a universal relation between the critical fields and the critical imbalances: $H_{c1}=\gamma\xi\epsilon_{\text F}$ and $H_{c2}=(1+\gamma P_c)^{2/3}H_{c1}$ where $\xi$ is the universal constant and $\gamma$ is the critical value of the chemical potential imbalance in the grand canonical ensemble. Since $\xi$, $\gamma$ and $P_c$ have been measured in the experiments, we can determine the critical Zeeman fields without the detail information of the equation of state for the polarized normal phase. Using the experimental data from LKB-Lhomond, we have found $H_{c1}\simeq0.37\epsilon_{\text F}$ and $H_{c2}\simeq0.44\epsilon_{\text F}$. Our result of the polarization $P$ as a function of the Zeeman field $H/\epsilon_{\text F}$ is in good agreement with the data extracted from the experiments. We also give an estimation of the critical magnetic field for dilute neutron matter at which the matter gets spin polarized, assuming the properties of the dilute neutron matter are close to those of the unitary Fermi gas.
4. arXiv:1106.1321 (cross-list from nucl-th) [pdf, ps, other]
Title: BCS-BEC crossover in spatially modulated fermionic condensates
Authors:Armen Sedrakian
Comments: 6 pages, 6 figures. to appear in Journal of Physics: Conference Series (JPCS); contribution to "Many-body correlations from dilute to dense nuclear systems (MBC2011)"
Subjects: Nuclear Theory (nucl-th); Quantum Gases (cond-mat.quant-gas)
Several novel multi-component fermionic condensates show universal behavior under imbalance in the number of fermionic species. Here I discuss their phase structure, thermodynamics, and the transition from the weak (BCS) to strong (BEC) coupling regime. The inhomogeneous superconducting phases are illustrated on the example of the Fulde-Ferrell phase which appears in the weak coupling regime, at low temperatures and large asymmetries. The inhomogeneous phases persist through the crossover up to (and possibly beyond) the transition to the strong coupling regime.
Jun 7
1. arXiv:1106.0822 [pdf, ps, other]
Title: Dipole-dipole interaction and polarization mode in BEC
Authors:P. A. Andreev, L. S. Kuzmenkov
Comments: 8 pages, 0 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
Here we propose the construction of a set of quantum hydrodynamics equations for a Bose-Einstein condensate (BEC) where atoms have electric dipole moment (EDM). The contribution of the dipole-dipole interactions (DDI) to the Euler equation is estimated. Quantum equations for the evolution of medium polarization are constructed for the first time. The mathematical method we developed allows studying the effect of interactions on the evolution of polarization. The developed method may be applied to various physical systems in which dynamics are affected by DDI. A problem of elementary excitations in BEC, either affected or not affected by the uniform external electric field is addressed using the our method. We show that the evolution of polarization leads to the formation of a novel type of elementary excitations in BEC. Also, we consider the process of wave generation in polarized BEC by means of monoenergetic beam of neutral polarized particles and demonstrate the possibilities of the generation of Bogoliubov's and polarization modes by thedipole beam.
2. arXiv:1106.0891 [pdf, ps, other]
Title: Thermal-assisted Anisotropy and Thermal-driven Instability in the Superfluidity state of Two-Species Polar Fermi Gas
Authors:Renyuan Liao, Wu-Ming Liu
Comments: 7 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the superfluid state of two-species heteronuclear Fermi gases with isotropic contact and anisotropic long-range dipolar interactions. By explicitly taking account of Fock exchange contribution, we derive self-consistent equations describing the pairing states in the system. Exploiting the symmetry of the system, we developed an efficient way of solving the self-consistent equations by exploiting the symmetries. We find that the temperature tends to increase the anisotropy of the pairing state, which is rather counterintuitive. We study the anisotropic properties of the system by examining the angular dependence of the number density distribution, the excitation spectrum and the pair correlation function. The competing effects of the contact interaction and the dipolar interaction upon the anisotropy are revealed. We derive and compute the superfluid mass density $\rho_{ij}$ for the system. Astonishingly, we find that $\rho_{zz}$ becomes negative above some certain temperature $T^*$($T<T_c$), signaling some instability of the system. This suggests that the elusive FFLO state may be observed in experiments, due to an anisotropic state with a spontaneously generated superflow.
3. arXiv:1106.0904 [pdf, other]
Title: Fermions in 3D Optical Lattices: Cooling Protocol to Obtain Antiferromagnetism
Authors:Thereza Paiva (1), Yen Lee Loh (2), Mohit Randeria (2), Richard T. Scalettar (3), Nandini Trivedi (2) ((1) Instituto de Fisica, Universidade Federal do Rio de Janeiro, Brazil, (2) Department of Physics, Ohio State University, (3) Department of Physics, University of California)
Comments: 4 pages; 5 figures; also see supplementary material in 2 pages with 1 figure
Subjects: Quantum Gases (cond-mat.quant-gas)
A major challenge in realizing antiferromagnetic (AF) and superfluid phases in optical lattices is the ability to cool fermions. We determine the equation of state for the 3D repulsive Fermi-Hubbard model as a function of the chemical potential, temperature and repulsion using unbiased determinantal quantum Monte Carlo methods, and we then use the local density approximation to model a harmonic trap. We show that increasing repulsion leads to cooling, but only in a trap, due to the redistribution of entropy from the center to the metallic wings. Thus, even when the average entropy per particle is larger than that required for antiferromagnetism in the homogeneous system, the trap enables the formation of an AF Mott phase.
4. arXiv:1106.0922 [pdf, ps, other]
Title: Checkerboards, stripes and corner energies in spin models with competing interactions
Authors:Alessandro Giuliani, Joel L. Lebowitz, Elliott H. Lieb
Comments: 21 pages, 3 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph)
We study the zero temperature phase diagram of Ising spin systems in two dimensions in the presence of competing interactions, long range antiferromagnetic and nearest neighbor ferromagnetic of strength J. We first introduce the notion of a "corner energy" which shows, when the antiferromagnetic interaction decays faster than the fourth power of the distance, that a striped state is favored with respect to a checkerboard state when J is close to J_c, the transition to the ferromagnetic state, i.e., when the length scales of the uniformly magnetized domains become large. Next, we perform detailed analytic computations on the energies of the striped andcheckerboard states in the cases of antiferromagnetic interactions with exponential decay and with power law decay r^{-p}, p>2, that depend on the Manhattan distance instead of the Euclidean distance. We prove that the striped phase is always favored compared to the checkerboard phase when the scale of the ground state structure is very large. This happens for J\lesssim J_c if p>3, and for J sufficiently large if 2<p<=3. Many of our considerations involving rigorous bounds carry over to dimensions greater than two and to more general short-range ferromagnetic interactions.
5. arXiv:1106.0960 [pdf, other]
Title: The Nature and Properties of a Repulsive Fermi Gas in the "Upper Branch"
Authors:Vijay B. Shenoy, T.-L. Ho
Comments: 4+ pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We generalize the Nozi\'eres-Schmitt-Rink (NSR) method to study the repulsive Fermi gas in the absence of molecule formation, i.e., in the so-called "upper branch". We find that the system remains stable except close to resonance at sufficiently low temperatures. With increasing scattering length, the energy density of the system attains a maximum at a positive scattering length before resonance. This is shown to arise from Pauli blocking which causes the bound states of fermion pairs of different momenta to disappear at different scattering lengths. At the point of maximum energy, the compressibility of the system is substantially reduced, leading to a sizable uniform density core in a trapped gas. The change in spin susceptibility with increasing scattering length is moderate and does not indicate any magnetic instability. These features should also manifest in Fermi gases with unequal masses and/or spin populations.
6. arXiv:1106.1121 [pdf, ps, other]
Title: Complex and real unconventional Bose-Einstein condensations in high orbital bands
Authors:Zi Cai, Congjun Wu
Comments: 4.2 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other)
We perform the theoretical study on the unconventional Bose-Einstein condensations (UBEC) in the high bands of optical lattices observed by Hemmerich's group. These exotic states are characterized by complex-valued condensate wavefunctions with nodal points, or real-valued ones with nodal lines, thus are beyond the {\it "no-node"} paradigm of the conventional BECs. A quantum phase transition is driven by the competition between the single particle band and interaction energies. The complex UBECs spontaneously break time-reversal symmetry, exhibiting a vortex-antivortex lattice structure.
Jun 6
1. arXiv:1106.0575 [pdf, ps, other]
Title: Collective modes of a strongly interacting Bose gas: probing the Mott transition
Authors:Michiel Snoek
Comments: 9 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We analyze the collective modes of a harmonically trapped, strongly interacting Bose gas in an optical lattice in the vicinity of the Mottinsulator transition. For that aim we employ the dynamical Gutzwiller equations, by performing real-time evolution and by solving the equations in linear response. We find a strong dependence on the spatial dimension of the system: while in one dimension the frequency of the dipole mode vanishes at theMott transition, in higher dimensions the dominant dipole mode is featureless and we only find a signature in the breathing mode. We discuss implications for experiments with bosonic and fermionic atoms.
