Sep 2015

Sep 7-Sep 11 Bo Liu, Sep 14-Sep 18 Haiyuan Zou, Sep 21-Sep 25 Ahmet Keles, Sep 28-Oct 2, Xuguang Yue

Sep 25
arXiv:1509.07488 [pdf, other]
Anomalous supersolidity in a weakly interacting dipolar Bose mixture on a square lattice
Ryan M. Wilson, Wilbur E. Shirley, Stefan S. Natu
Comments: 4+ pages, 4 figures, comments welcome
Subjects: Quantum Gases (cond-mat.quant-gas)
We calculate the mean-field phase diagram of a zero-temperature, binary Bose mixture on a square optical lattice, where one species possesses a non-negligible dipole moment. Remarkably, this system exhibits supersolidity for anomalously weak dipolar interaction strengths, which are readily accessible with current experimental capabilities. The supersolid phases are robust, in that they occupy large regions in the parameter space. Further, we identify a first-order quantum phase transition between supersolid and superfluid phases. Our results demonstrate the rich features of the dipolar Bose mixture, and open the door for exploring the exotic supersolid phase in a realistic experimental setting.

arXiv:1509.07439 [pdf, ps, other]
Suppression of two-body collisional loss in an ultracold gas via the Fano effect
Jianwen Jie, Yawen Zhang, Peng Zhang
Comments: 14 pages,4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
The Fano effect (U. Fano, Phys. Rev. \textbf{15},1866 (1961) shows that an inelastic scattering process can be suppressed when the output channel (OC) is coupled to an isolated bound state. In this paper we investigate the application of this effect for the suppression of two-body collisional losses of ultracold atoms. The Fano effect is originally derived via a first-order perturbation treatment for coupling between the incident channel (IC) and the OC. We generalize the Fano effect to systems with arbitrarily strong IC--OC couplings. We analytically prove that, in a system with one IC and one OC, when the inter-atomic interaction potentials are real functions of the inter-atomic distance, the exact s-wave inelastic scattering amplitude can always be suppressed to \emph{zero} by coupling between the IC or the OC (or both of them) and an extra isolated bound state. We further show that when the low-energy inelastic collision between two ultracold atoms is suppressed by this effect, the real part of the elastic scattering length between the atoms is still possible to be much larger than the range of inter-atomic interaction.In addition, when open scattering channels are coupled to two bound states, with the help of the Fano effect, independent control of the elastic and inelastic scattering amplitudes of two ultracold atoms can be achieved. Possible experimental realizations of our scheme are also discussed.

 arXiv:1509.07362 [pdf, other]
Competition between valence bond and symmetry breaking Mott states of four component fermions on a honeycomb optical lattice
D. Jakab, E. Szirmai, M. Lewenstein, G. Szirmai
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We investigate the magnetic properties of strongly interacting, four component, spin-3/2 ultracold fermionic atoms in the Mott insulator limit with one particle per site in an optical lattice with honeycomb symmetry. In this limit, atomic tunneling is frozen, and only the atomic spins can exchange. We find a competition between symmetry breaking and liquid like disordered phases. Particularly interesting are valence bond states with bond centered magnetizations, situated between the ferromagnetic and conventional valence bond phases. In the framework of a mean-field theory, we calculate the phase diagram and identify the parameter region where a homogeneous SU(4) symmetric Affleck-Kennedy-Lieb-Tasaki-like valence bond state is present.

 arXiv:1509.07328 [pdf, other]
Fractional Local Moment and High Temperature Kondo Effect in Rashba-Fermi Gases
Adhip Agarwala, Vijay B. Shenoy
Comments: 5 pages, 4 figures, and supplemental material(4 pages)
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)
We investigate the new physics that arises when a correlated quantum impurity hybridizes with Fermi gas with a generalized Rashba spin-orbit coupling produced via a uniform synthetic non-Abelian gauge field. We show that the impurity develops a {\it fractional} local moment which couples anti-ferromagnetically to the Rashba-Fermi gas. This results in a concomitant {\it Kondo effect with a high temperature scale} that can be tuned by the strength of the Rashba spin-orbit coupling.

arXiv:1509.07232 [pdf, ps, other]
Specific heat and effects of pairing fluctuations in the BCS-BEC crossover regime of an ultracold Femi gas
Pieter van Wyk, Hiroyuki Tajima, Ryo Hanai, Yoji Ohashi
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)
We investigate the specific heat at constant volume $C_V$ in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas above the superfluid phase transition temperature $T_{\rm c}$. Within the framework of the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink, we show that this thermodynamic quantity is sensitive to the stability of preformed Cooper pairs. That is, while $C_V(T\gesim T_{\rm c})$ in the unitary regime is remarkably enhanced by {\it metastable} preformed Cooper pairs or pairing fluctuations, it is well described by that of an ideal Bose gas of long-lived {\it stable} molecules in the strong-coupling BEC regime. Using these results, we identify the region where the system may be viewed as an almost ideal Bose gas of stable pairs, as well as the pseudogap regime where the system is dominated by metastable preformed Cooper pairs, in the phase diagram of an ultracold Fermi gas with respect to the strength of a pairing interaction and the temperature. We also show that the calculated specific heat agrees with the recent experiment on a $^6$Li unitary Fermi gas. Since the formation of preformed Cooper pairs is a crucial key in the BCS-BEC crossover phenomenon, our results would be helpful in considering how fluctuating preformed Cooper pairs appear in a Fermi gas, to eventually become stable, as one passes through the BCS-BEC crossover region.

Sep 18

arXiv:1509.05050 [pdf, other]
Fulde-Ferrell-Larkin-Ovchinnikov pairing as leading instability on the square lattice
Jan Gukelberger, Sebastian Lienert, Evgeny Kozik, Lode Pollet, Matthias Troyer
Comments: 6 pages, 4 figures
Subjects: Superconductivity (cond-mat.supr-con); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study attractively interacting spin-1/2 fermions on the square lattice subject to a spin population imbalance. Using unbiased diagrammatic Monte Carlo simulations we find an extended region in the parameter space where the Fermi liquid is unstable towards formation of Cooper pairs with non-zero center-of-mass momentum, known as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The highest temperature where the FFLO instability can be observed is about half of the superfluid transition temperature in the unpolarized system.


Sep 16
arXiv:1509.04358 [pdf, ps, other]
Featureless quantum insulator on the honeycomb lattice
Hyunyong Lee, Shenghan Jiang, Jung Hoon Han, Ying Ran
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We construct fully symmetric, gapped states without topological order on a honeycomb lattice at half filling in terms of projected entangled pair states (PEPS). One is a system of physical spin-1/2s with the virtual bond dimension D = 4. Four distinct states differing by lattice quantum numbers are found by applying the systematic classification scheme introduced by two of the authors [S. Jiang and Y. Ran, arXiv:1505.03171 (2015)]. The other state has half a spinless fermion per site. Lack of topological degeneracy or other conventional form of symmetry breaking, and the existence of the energy gap in both wave functions are checked by numerical calculation of the entanglement entropy and various correlation functions.

Sep 15
arXiv:1509.04257 [pdf, other]
Observation of an orbital interaction-induced Feshbach resonance in 173-Yb
M. Höfer, L. Riegger, F. Scazza, C. Hofrichter, D.R. Fernandes, M. M. Parish, J. Levinsen, I. Bloch, S. Fölling
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We report on the experimental observation of a novel inter-orbital Feshbach resonance in ultracold 173-Yb atoms, which opens the possibility of tuning the interactions between the 1S0 and 3P0 metastable state, both possessing vanishing total electronic angular momentum. The resonance is observed at experimentally accessible magnetic field strengths and occurs universally for all hyperfine state combinations. We characterize the resonance in the bulk via inter-orbital cross-thermalization as well as in a three-dimensional lattice using high-resolution clock-line spectroscopy.


Sep 14
arXiv:1509.03314 [pdf, other]
Fractionalized Fermi liquid on the surface of a topological Kondo insulator
Alex Thomson, Subir Sachdev
Comments: 18 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We argue that topological Kondo insulators can also have 'intrinsic' topological order associated with fractionalized excitations on their surfaces. The hydridization between the local moments and conduction electrons can weaken near the surface, and this enables the local moments to form spin liquids. This co-exists with the conduction electron surface states, realizing a surface fractionalized Fermi liquid. We present mean-field solutions of a Kondo-Heisenberg model which display such surfaces.



Sep 11

1 arXiv:1509.02993 [pdf, other]
Characterising the topological superradiant state
Jian-Song Pan, Xiong-Jun Liu, Wei Zhang, Wei Yi
Coherently driven atomic gases inside optical cavities hold great promises in generating rich dynamics and exotic states of matter. Recently, it has been shown that a novel topological superradiant state exists in a two-component degenerate Fermi gas coupled to a cavity, where local order parameters coexist with global topological invariants. In this work, we characterise in detail various properties of this exotic state, focusing on the feedback interactions between the atoms and the cavity field. In particular, we demonstrate that the cavity-induced inter-band couplings play a crucial role in inducing the topological phase transition between the conventional and the topological superradiant state. Furthermore, we analyse how the closing and reopening of the atomic bulk gap across the topological phase boundary leaves interesting signatures in the cavity field. We also discuss the robustness of the topological superradiant state by investigating the steady-state phase diagram under various circumstances. Our work provides many valuable insights into the unique atom-cavity hybrid system under study, and is helpful for future experimental exploration of the topological superradiant state.


Sep 10
1. arXiv:1509.02788 [pdf, other]
Freed by interaction kinetic states in the Harper model
Klaus M. Frahm, Dima L. Shepelyansky
We study the problem of two interacting particles in a one-dimensional quasiperiodic lattice of the Harper model. We show that a short or long range interaction between particles leads to emergence of delocalized pairs in the non-interacting localized phase. The properties of these Freed by Interaction Kinetic States (FIKS) are analyzed numerically including the advanced Arnoldi method. We find that the number of sites populated by FIKS pairs grows algebraically with the system size with the maximal exponent b=1, up to a largest lattice size N=10946 reached in our numerical simulations, thus corresponding to a complete delocalization of pairs. For delocalized FIKS pairs the spectral properties of such quasiperiodic operators represent a deep mathematical problem. We argue that FIKS pairs can be detected in the framework of recent cold atom experiments [M.~Schreiber {\it et al.} Science {\bf 349}, 842 (2015)] by a simple setup modification. We also discuss possible implications of FIKS pairs for electron transport in the regime of charge-density wave and high Tc superconductivity.

2. arXiv:1509.02339 (cross-list from nlin.CD) [pdf, other]
Spectral statistics of chaotic many-body systems
Rémy Dubertrand, Sebastian Müller
We derive a trace formula that expresses the level density of chaotic many-body systems as a smooth term plus a sum over contributions associated to solutions of the nonlinear Schr\"odinger equation. Our formula applies to bosonic systems with discretised positions, such as the Bose-Hubbard model, in the semiclassical limit as well as in the limit where the number of particles is taken to infinity. We use the trace formula to investigate the spectral statistics of these systems, by studying interference between solutions of the nonlinear Schr\"odinger equation. We show that in the limits taken the statistics of fully chaotic many-particle systems becomes universal and agrees with predictions from the Wigner-Dyson ensembles of random matrix theory. The conditions for Wigner-Dyson statistics involve a gap in the spectrum of the Frobenius-Perron operator, leaving the possibility of different statistics for systems with weaker chaotic properties.


Sep 9

1. arXiv:1509.02185 [pdf, other]
Experimental reconstruction of Wilson lines in Bloch bands
Tracy Li, Lucia Duca, Martin Reitter, Fabian Grusdt, Eugene Demler, Manuel Endres, Monika Schleier-Smith, Immanuel Bloch, Ulrich Schneider
Topology and geometry are essential to our understanding of modern physics, underlying many foundational concepts from a variety of fields. In condensed matter systems, the electronic properties of a solid are determined not only by the scalar dispersion of the bands, but also by the geometry of the band eigenstates. While this information is encoded in the Berry connection and the corresponding Berry phase in the specific case of a single band, the geometry of general multi-band systems is described by the matrix-valued Wilczek-Zee connection and the corresponding Wilson lines. Whereas Berry phases have been directly observed in Bloch bands, Wilson lines have solely been employed as a theoretical construct. Here, we report on the realization of strong-force dynamics in Bloch bands that are described by Wilson lines. The resulting evolution of band populations is purely geometric in origin and can directly reveal both the geometric structure and dispersion of the bands. Our techniques enable a full determination of band eigenstates, Berry curvature, and topological invariants, including Chern and Z2 numbers.




 Sep 8

1. arXiv:1509.01803 [pdf, ps, other]
Traveling Majorana solitons in a one-dimensional spin-orbit coupled Fermi superfluid
Peng Zou, Joachim Brand, Xia-Ji Liu, Hui Hu
We investigate traveling solitons of a one-dimensional spin-orbit coupled Fermi superfluid in both topologically trivial and non-trivial regimes by solving the static and time-dependent Bogoliubov-de Gennes equations. We find a critical velocity vh for traveling solitons that is much smaller than the value predicted using the Landau criterion due to the presence of spin-orbit coupling, which strongly upshifts the energy level of the soliton-induced Andreev bound states towards the quasi-particle scattering continuum. Above vh, our time-dependent simulations in harmonic traps indicate that traveling solitons decay by radiating sound waves. In the topological phase, we predict the existence of peculiar Majorana solitons, which host two Majorana fermions and feature a phase jump of π across the soliton, irrespective of the velocity of travel. These unusual properties of Majorana solitons may open an alternative way to manipulate Majorana fermions for fault-tolerant topological quantum computations.