Apr 2017
Apr 1-Apr 10 Biao Huang, Apr 11-Apr 20 Haiyuan Zou, Apr 21-Apr 28 Zehan Li
Apr 20 arXiv:1704.05803 [pdf, other] Monitoring and manipulating Higgs and Goldstone modes in a supersolid quantum gas Julian Léonard, Andrea Morales, Philip Zupancic, Tobias Donner, Tilman Esslinger Comments: 5+6 pages, 4+4 figures Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) Access to collective excitations lies at the heart of our understanding of quantum many-body systems. We study the Higgs and Goldstone modes in a supersolid quantum gas that is created by coupling a Bose-Einstein condensate symmetrically to two optical cavities. The cavity fields form a U(1)-symmetric order parameter that can be modulated and monitored along both quadratures in real time. This enables us to measure the excitation energies across the superfluid-supersolid phase transition, establish their amplitude and phase nature, as well as characterize their dynamics from an impulse response. Furthermore, we can give a tunable mass to the Goldstone mode at the crossover between continuous and discrete symmetry by changing the coupling of the quantum gas with either cavity. Apr 19 arXiv:1704.05410 [pdf, ps, other] Hydrodynamics of Quantum Vortices in Two Dimensions Xiaoquan Yu, Ashton S. Bradley Comments: 7 pages Subjects: Quantum Gases (cond-mat.quant-gas); Fluid Dynamics (physics.flu-dyn) We show that in two dimensional superfluids a large number of quantum vortices with positive and negative circulations behave as an inviscid fluid on large scales. Two hydrodynamical velocities are introduced to describe this emergent binary vortex fluid, via vortex number current and vortex change current. The velocity field associated with the vortex number current evolves according to a hydrodynamic equation, subject to an anomalous stress absent from Euler's equation. In contrast to the chiral vortex fluid containing only like-sign vortices, the binary vortex fluid is compressible and the orbital angular momentum is not conserved, as characterized by an asymmetric Cauchy stress tensor. Dissipation effects due to thermal friction and vortex-sound interactions induce an effective damping rate, an anomalous viscous stress tensor, and a coefficient of second viscosity. Apr 17 arXiv:1704.04498 [pdf, other] Mobility Edges in 1D Bichromatic Incommensurate Potentials Xiao Li, Xiaopeng Li, S. Das Sarma Comments: 16 pages, 12 figures Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el) We theoretically study a one-dimensional (1D) mutually incommensurate bichromatic lattice system which has been implemented in ultracold atoms to study quantum localization. It has been universally believed that the tight-binding version of this bichromatic incommensurate system is represented by the well-known Aubry-Andre model. Here we establish that this belief is incorrect and that the Aubry-Andre model description, which applies only in the extreme tight-binding limit of very deep primary lattice potential, generically breaks down near the localization transition due to the unavoidable appearance of single-particle mobility edges (SPME). In fact, we show that the 1D bichromatic incommensurate potential system manifests generic mobility edges which disappear in the tight-binding limit, leading to the well-studied Aubry-Andre physics. We carry out an extensive study of the localization properties of the 1D incommensurate optical lattice without making any tight-binding approximation. We find that, for the full lattice system, an intermediate phase between completely localized and completely delocalized regions appears due to the existence of the SPME, making the system qualitatively distinct from the Aubry-Andre prediction. Using the Wegner flow approach, we show that the SPME in the real lattice system can be attributed to significant corrections of higher-order harmonics in the lattice potential which are absent in the strict tight-binding limit. We calculate the dynamical consequences of the intermediate phase in detail to guide future experimental investigations for the observation of 1D SPME and the associated intermediate phase. We consider effects of interaction numerically, and conjecture the stability of SPME to weak interaction effects, thus leading to the exciting possibility of an experimentally viable nonergodic extended phase in interacting 1D optical lattices. Apr 14 arXiv:1704.04229 [pdf, ps, other] Hyper-invariant tensor networks and holography Glen Evenbly Comments: Main text: 5 pages, 4 figures. Supplemental material: 6 pages, 11 figures Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el) We propose a new class of tensor network state as a model for the AdS/CFT correspondence and holography. This class is demonstrated to retain key features of the multi-scale entanglement renormalization ansatz (MERA), in that they describe quantum states with algebraic correlation functions, have free variational parameters, and are efficiently contractible. Yet, unlike MERA, they are built according to a uniform tiling of hyperbolic space, without inherent directionality or preferred locations in the holographic bulk, and thus circumvent key arguments made against the MERA as a model for AdS/CFT. Novel holographic features of this tensor network class are examined, such as an equivalence between the causal cones C(R) and the entanglement wedges E(R) of connected boundary regions R. Apr 12 arXiv:1704.02997 [pdf, other] Symmetry protected topological Luttinger liquids and the phase transition between them Hong-Chen Jiang, Zi-Xiang Li, Alexander Seidel, Dung-Hai Lee Comments: 5.2 pages, 4 figures + 2.8 pages supplemental materials Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) We show that a doped spin-1/2 ladder with antiferromagnetic intra-chain and ferromagnetic inter-chain coupling is a symmetry protected topologically non-trivial Luttinger liquid. Turning on a large easy-plane spin anisotropy drives the system to a topologically-trivial Luttinger liquid. Both phases have full spin gaps and exhibit power-law superconducting pair correlation. The Cooper pair symmetry is singlet dxy in the non-trivial phase and triplet Sz=0 in the trivial phase. The topologically non-trivial Luttinger liquid exhibits gapless spin excitations in the presence of a boundary, and it has no non-interacting or mean-field theory analog even when the fluctuating phase in the charge sector is pinned. As a function of the strength of spin anisotropy there is a topological phase transition upon which the spin gap closes. We speculate these Luttinger liquids are relevant to the superconductivity in metalized integer spin ladders or chains. Apr 11 arXiv:1704.02606 [pdf, other] Bose polarons in ultracold atoms in one dimension: beyond the Fröhlich paradigm Fabian Grusdt, Gregory E. Astrakharchik, Eugene A. Demler Comments: 21 pages, 18 figures, 4 pages appendices Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) Mobile impurity atoms immersed in Bose-Einstein condensates provide a new platform for exploring Bose polarons. Recent experimental advances in the field of ultracold atoms make it possible to realize such systems with highly tunable microscopic parameters and to explore equilibrium and dynamical properties of polarons using a rich toolbox of atomic physics. In this paper we present a detailed theoretical analysis of Bose polarons in one dimensional systems of ultracold atoms. By combining a non-perturbative renormalization group approach with numerically exact diffusion Monte Carlo calculations we obtain not only detailed numerical results over a broad range of parameters but also qualitative understanding of different regimes of the system. We find that an accurate description of Bose polarons requires the inclusion of two-phonon scattering terms which go beyond the commonly used Fr\"ohlich model. Furthermore we show that when the Bose gas is in the strongly interacting regime, one needs to include interactions between the phonon modes. We use several theoretical approaches to calculate the polaron energy and its effective mass. The former can be measured using radio-frequency spectroscopy and the latter can be studied experimentally using impurity oscillations in a harmonic trapping potential. We compare our theoretical results for the effective mass to the experiments by Catani et al. [PRA 85, 023623 (2012)]. In the weak-to-intermediate coupling regimes we obtain excellent quantitative agreement between theory and experiment, without any free fitting parameter. We supplement our analysis by full dynamical simulations of polaron oscillations in a shallow trapping potential. We also use our renormalization group approach to analyze the full phase diagram and identify regions that support repulsive and attractive polarons, as well as multi-particle bound states. arXiv:1704.02605 [pdf, other] Strong coupling Bose polarons in a BEC Fabian Grusdt, Richard Schmidt, Yulia E. Shchadilova, Eugene A. Demler Comments: 17 pages, 9 figures, 9 pages appendicesSubjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)We use a non-perturbative renormalization group approach to develop a unified picture of the Bose polaron problem, where a mobile impurity is strongly interacting with a surrounding Bose-Einstein condensate (BEC). A detailed theoretical analysis of the phase diagram is presented and the polaron-to-molecule transition is discussed. For attractive polarons we argue that a description in terms of an effective Fr\"ohlich Hamiltonian with renormalized parameters is possible. Its strong coupling regime is realized close to a Feshbach resonance, where we predict a sharp increase of the effective mass. Already for weaker interactions, before the polaron mass diverges, we predict a transition to a regime where states exist below the polaron energy and the attractive polaron is no longer the ground state. On the repulsive side of the Feshbach resonance we recover the repulsive polaron, which has a finite lifetime because it can decay into low-lying molecular states. We show for the entire range of couplings that the polaron energy has logarithmic corrections in comparison with predictions by the mean-field approach. We demonstrate that they are a consequence of the polaronic mass renormalization which is due to quantum fluctuations of correlated phonons in the polaron cloud.
