Apr 2018

Apr 1- Apr 4 Haiyuan Zou, Apr 5-Apr 9 Zehan Li, Apr 10-Apr 14 Jiansong Pan, Apr 15-Apr 19 Ahmet Keles, Apr 20-Apr 24 Max Aarzamazovs, Apr 25-Apr 29 Xuguang Yue, Apr 30 Biao Huang

Apr 12
arXiv:1804.03772 (cross-list from physics.optics) [pdf, ps, other]
Topological Floquet edge states in periodically curved waveguides
Bo Zhu, Honghua Zhong, Yongguan Ke, Xizhou Qin, Andrey A. Sukhorukov, Yuri S. Kivshar, Chaohong Lee
Comments: 6 pages, 4 figures
Subjects: Optics (physics.optics); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)



We study the Floquet edge states in arrays of periodically curved optical waveguides described by the modulated Su-Schrieffer-Heeger model. Beyond the bulk-edge correspondence, our study explores the interplay between band topology and periodic modulations. By analysing the quasi-energy spectra and Zak phase, we reveal that, although topological and non-topological edge states can exist for the same parameters, \emph{they can not appear in the same spectral gap}. In the high-frequency limit, we find analytically all boundaries between the different phases and study the coexistence of topological and non-topological edge states. In contrast to unmodulated systems, the edge states appear due to either band topology or modulation-induced defects. This means that periodic modulations may not only tune the parametric regions with nontrivial topology, but may also support novel edge states.




Apr 11
arXiv:1804.01291 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
Spatial-Translation-Induced Discrete Time Crystals
Kaoru Mizuta, Kazuaki Takasan, Masaya Nakagawa, Norio Kawakami
Comments: 11 pages, 10 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)

A discrete time crystal is a phase unique to nonequilibrium systems, where discrete time translation symmetry is spontaneously broken. Most of conventional time crystals proposed so far rely on spontaneous breaking of on-site symmetries and their corresponding on-site symmetry operations. In this Letter, we propose a new time crystal dubbed "spatial-translation-induced discrete time crystal (STI-DTC)", which is realized by spatial translation and its symmetry breaking. Owing to the properties of spatial translation, in this new time crystal, various time crystal orders can emerge only by changing the filling but not changing the driving protocol. We demonstrate that local transport of charges or spins shows a nontrivial oscillation, enabling detection and applications of time crystal orders. Our proposal opens up a new avenue of realizing time crystal orders by spatial translation.




Apr 10
arXiv:1804.02957 [pdf, other]
Bosonic Topological Excitations from the Instability of a Quadratic Band Crossing
Guang-Quan Luo, Andreas Hemmerich, Zhi-Fang Xu
Comments: 6+6 pages, 4+4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate the interaction-driven instability of a quadratic band crossing arising for ultracold bosonic atoms loaded into a two-dimensional optical lattice. We consider the case when the degenerate point becomes a local minimum of both crossing energy bands such that it can support a stable Bose-Einstein condensate. Repulsive contact interaction among the condensed bosons induces a spontaneously time-reversal symmetry broken superfluid phase and a topological gap is opened in the excitation spectrum. We propose two concrete realizations of the desired quadratic band crossing in lattices with either fourfold or sixfold rotational symmetries via suitable tuning of the unit cell leading to reduced Brillouin zones and correspondingly folded bands. In either case, topologically protected edge excitations are found for a finite system.

Apr 9

arXiv:1804.02236 (cross-list from cond-mat.str-el) [pdf, ps, other]
Dirac points, spinons and spin liquid in twisted bilayer graphene
V. Yu. Irkhin, Yu. N. Skryabin
Comments: 4 pages
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
Twisted bilayer graphene is an excellent example of highly correlated system demonstrating a nearly flat electron band, the Mott transition and probably a spin liquid state. Besides the one-electron picture, analysis of Dirac points is performed in terms of spinon Fermi surface in the limit of strong correlations. Application of gauge field theory to describe deconfined spin liquid phase is treated. Topological quantum transitions, including those from small to large Fermi surface in the presence of van Hove singularities, are discussed.

Apr 8
arXiv:1804.02376 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
q-deformed Fermion in Many-Particle Systems and Its Application to BCS Theory
Xu-Yang Hou, Xun Huang, Yan He, Hao Guo
Comments: 16 pages, 4 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas)In recent decades, there have been increasing interests in quantum statistics beyond the standard Fermi-Dirac and Bose-Einstein statistics, such as the fractional statistics, quon statistics, anyon statistics and quantum groups, since they can provide some new insights into the cosmology, nuclear physics and condensed matter. In this paper, we study the many-particle system formed by the q-deformed fermions (q-fermion), which is realized by deforming the quantum algebra of the anticommutation relations. We investigate from a standard perspective of the finite temperature field theory and try to construct the finite temperature Green’s function formalism for the free many-q-fermion system, then generalize it to the well known interacting fermionic system, the superconductor, and finally obtain a consistent q-deformed BCS (qBCS) theory. At low temperature, this theory predicts a Sarma-like ordered phase, and we call it the q-deformed Sarma phase. It also presents a symmetric phase diagram in the parameter space and new thermodynamic relations.




Apr 4
arXiv:1804.01114 [pdf, other]
Rényi generalization of the operational entanglement entropy
Hatem Barghathi, C. M. Herdman, Adrian Del Maestro
Comments: Main text: 6 pages, 3 figures + Supplementary material: 5 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
(Submitted on 3 Apr 2018)
Operationally accessible entanglement in bipartite systems of indistinguishable particles could be reduced due to restrictions on the allowed local operations as a result of particle number conservation. In order to quantify such effects, Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)] introduced an operational measure of the von Neumann entanglement entropy. Motivated by advances in measuring R\'enyi entropies in quantum many-body systems subject to conservation laws, we introduce a generalization of the operational entanglement that is computationally and experimentally accessible. Using the Widom theorem, we investigate its scaling with the size of a spatial subregion for free fermions and find a logarithmically violated area law scaling, similar to the spatial entanglement entropy, with at most, a double-log leading-order correction. A modification of the correlation matrix method confirms our findings in systems of up to 105 particles.



Apr 3
arXiv:1804.00402 [pdf, other]
Bipolarons in a Bose-Einstein condensate
A. Camacho-Guardian, L. A. Peña Ardila, T. Pohl, G. M. Bruun
Subjects: Quantum Gases (cond-mat.quant-gas)
Mobile impurities in a Bose-Einstein condensate form quasiparticles called polarons. Here, we show that two such polarons can bind to form a bound bipolaron state. Its emergence is caused by an induced nonlocal interaction mediated by density oscillations in the condensate, and we derive using field theory an effective Schr\"odinger equation describing this for arbitrarily strong impurity-boson interaction. We furthermore compare with Quantum Monte Carlo simulations finding remarkable agreement, which underlines the predictive power of the developed theory. It is found that bipolaron formation typically requires strong impurity interactions beyond the validity of more commonly used weak-coupling approaches that lead to local Yukawa-type interactions. We predict that the bipolarons are observable in present experiments and describe a procedure to probe their properties.



Apr 2
arXiv:1804.00010 [pdf, other]
Entanglement structure of current driven quantum many-body systems
Michael J. Gullans, David A. Huse
Comments: 23 pages, 7 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
When an extended system is coupled at its opposite boundaries to two reservoirs at different temperatures or chemical potentials, it cannot achieve a global thermal equilibrium and is instead driven to a set of current carrying non-equilibrium states. Motivated by developments in the understanding of thermalization in closed quantum many-body systems, we find conditions under which such current-driven systems can achieve, or violate, local thermal equilibrium by investigating their entropy, mutual information, and entanglement at long times. We focus on a specific model consisting of a two-parameter family of random unitary circuits acting locally on a chain of spin-1/2s (equivalently, qubits) that exhibits quantum chaotic behavior in most of its parameter space. The only conserved quantity is the total magnetization of the spins. We choose the model so that for all parameter values the time-averaged correlation functions agree and are close to local equilibrium. However, computing the total von Neumann entropy of the system shows that there are in fact three distinct "phases" of the driven problem, with local equilibrium only emerging in the quantum chaotic regime, while one of the other phases exhibits volume-law mutual information and entanglement. We extend these results to the three-dimensional, non-interacting Anderson model in the diffusive regime, showing that the non-equilibrium steady-state for fermions realizes the volume-law mutual information phase of the random circuit. Our results suggest a generic picture for the emergence of local equilibrium in current-driven quantum chaotic systems, as well as provide insights into methods to stabilize highly-entangled many-body states out of equilibrium.