Nov 2017
Nov 1-Nov 4 Xuguang Yue, Nov 5-Nov 9 Biao Huang, Nov 10-Nov 14 Haiyuan Zou, Nov 15-Nov 19 Zehan Li, Nov 20-Nov 24 Jiansong Pan, Nov 25-Nov 29 Ahmet Keles, Nov 30- Max Arzamasovs
NOV 23 arXiv:1711.07988 (cross-list from cond-mat.quant-gas) [pdf, other] Coupling two order parameters in a quantum gas Andrea Morales, Philip Zupancic, Julian Léonard, Tilman Esslinger, Tobias Donner Comments: 8 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph) Controlling matter to simultaneously support multiple coupled properties is of fundamental and technological importance. For example, the simultaneous presence of magnetic and ferroelectric orders in multiferroic materials leads to enhanced functionalities. In high-temperature superconductors, intertwining between charge- and spin-order can form superconducting states at high transition temperatures. However, pinning down the microscopic mechanisms responsible for the simultaneous presence of different orders is difficult, making it hard to predict the phenomenology of a material or to experimentally modify its properties. Here we use a quantum gas to engineer an adjustable interaction at the microscopic level between two orders, and demonstrate scenarios of competition, coexistence and coupling between them. In the latter case, intriguingly, the presence of one order lowers the critical point of the other. Our system is realized by a Bose-Einstein condensate which can undergo self-organization phase transitions in two optical resonators, resulting in two distinct crystalline density orders. We characterize the intertwining between these orders by measuring the composite order parameter and the elementary excitations. We explain our results with a mean-field free energy model, which is derived from a microscopic Hamiltonian. Our system is ideally suited to explore properties of quantum tricritical points as recently realized in and can be extended to study the interplay of spin and density orders also as a function of temperature. NOV 22 arXiv:1711.07889 [pdf, other] Resonant thermalization of periodically driven strongly correlated electrons Francesco Peronaci, Marco Schiró, Olivier Parcollet Comments: 5+3 pages Subjects: Strongly Correlated Electrons (cond-mat.str-el) We study the dynamics of the Fermi-Hubbard model driven by a time-periodic modulation of the interaction within nonequilibrium Dynamical Mean-Field Theory. For moderate interaction, we find clear evidence of thermalization to a genuine infinite-temperature state with no residual oscillations. Quite differently, in the strongly correlated regime, we find a quasi-stationary extremely long-lived state with oscillations synchronized with the drive (Floquet prethermalization). Remarkably, the nature of this state dramatically changes upon tuning the drive frequency. In particular, we show the existence of a critical frequency at which the system rapidly thermalizes despite the large interaction. We characterize this resonant thermalization and provide an analytical understanding in terms of a break down of the periodic Schrieffer-Wolff transformation. NOV 21 arXiv:1711.07262 [pdf, other] Exposing the quantum geometry of spin-orbit coupled Fermi superfluids M. Iskin Comments: 5 pages with 3 figures in the Main text, and 1 additional figure Online Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) The coupling between a quantum particle's intrinsic angular momentum and its center-of-mass motion gives rise to the so-called helicity states that are characterized by the projection of the spin onto the direction of momentum. In this paper, by unfolding the superfluid-density tensor into its intra-helicity and inter-helicity components, we reveal that the latter contribution is directly linked with the total quantum metric of the helicity bands. We consider both Rashba and Weyl spin-orbit couplings across the BCS-BEC crossover, and show that the geometrical inter-helicity contribution is responsible for up to a quarter of the total superfluid density. We believe this is one of those elusive effects that may be measured within the highly-tunable realm of cold Fermi gases. NOV 20 arXiv:1711.06622 [pdf, other] Strongly Interacting Bose Gases near a d-wave Shape Resonance Xing-Can Yao, Ran Qi, Xiang-Pei Liu, Xiao-Qiong Wang, Yu-Xuan Wang, Yu-Ping Wu, Hao-Ze Chen, Peng Zhang, Hui Zhai, Yu-Ao Chen, Jian-Wei Pan Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph) Many unconventional quantum matters, such as fractional quantum Hall effect and d-wave high-Tc superconductor, are discovered in strongly interacting systems. Understanding quantum many-body systems with strong interaction and the unconventional phases therein is one of the most challenging problems in physics nowadays. Cold atom systems possess a natural way to create strong interaction by bringing the system to the vicinity of a scattering resonance. Although this has been a focused topic in cold atom physics for more than a decade, these studies have so far mostly been limited for s-wave resonance. Here we report the experimental observation of a broad d-wave shape resonance in degenerate 41K gas. We further measure the molecular binding energy that splits into three branches as a hallmark of d-wave molecules, and find that the lifetime of this many-body system is reasonably long at strongly interacting regime. From analyzing the breathing mode excited by ramping through this resonance, it suggests that a quite stable low-temperature atom and molecule mixture is produced. Putting all the evidence together, our system offers great promise to reach a d-wave molecular superfluid. arXiv:1711.03673 (cross-list from physics.atom-ph) [pdf, other] Cavity Mediated Collective Spin Exchange Interactions in a Strontium Superradiant Laser Matthew A. Norcia, Robert J. Lewis-Swan, Julia R.K. Cline, Bihui Zhu, Ana M. Rey, James K. Thompson Comments: 7 pages, 4 figures plus supplement Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) Laser cooled and quantum degenerate atoms are widely being pursued as quantum simulators that may explain the behavior of strongly correlated material systems, and as the basis of today's most precise sensors. A key challenge towards these goals is to understand and control coherent interactions between the atoms. Here, we observe long-range exchange interactions mediated by an optical cavity, which manifest as tunable spin-spin interactions on the pseudo spin-1/2 system composed of the millihertz linewidth clock transition in strontium. We observe the so-called one axis twisting dynamics, the emergence of a many-body energy gap, and signatures of gap protection of the optical coherence against certain sources of decoherence. These effects manifest in the output of a pulsed, superradiant laser operating on the millihertz linewidth transition. Our observations will aid in the future design of versatile quantum simulators that take advantage of the unique control and probing capabilities of cavity QED and the rich internal structure of long-lived Sr atoms. They also open a route for the next generation of atomic clocks that utilize quantum correlations for enhanced metrology. NOV 19 arXiv:1711.06416 [pdf, other] Ginzburg-Landau-type theory of non-polarized spin superconductivity Peng Lv, Zhi-qiang Bao, Ai-Min Guo, X. C. Xie, Qing-Feng Sun Comments: 9 pages, 5 figures Journal-ref: Phys. Rev. B 95, 014501 (2017) Subjects: Superconductivity (cond-mat.supr-con) Since the concept of spin superconductor was proposed, all the related studies concentrate on spin-polarized case. Here, we generalize the study to spin-non-polarized case. The free energy of non-polarized spin superconductor is obtained, and the Ginzburg-Landau-type equations are derived by using the variational method. These Ginzburg-Landau-type equations can be reduced to the spinpolarized case when the spin direction is fixed. Moreover, the expressions of super linear and angular spin currents inside the superconductor are derived. We demonstrate that the electric field induced by super spin current is equal to the one induced by equivalent charge obtained from the second Ginzburg-Landau-type equation, which shows self-consistency of our theory. By applying these Ginzburg-Landau-type equations, the effect of electric field on the superconductor is also studied. These results will help us get a better understanding of the spin superconductor and the related topics such as Bose-Einstein condensate of magnons and spin superfluidity NOV 18 arXiv:1711.05803 (cross-list from cond-mat.quant-gas) [pdf, other] Suppressed solitonic cascade in spin-imbalanced superfluid Fermi gas Gabriel Wlazłowski, Kazuyuki Sekizawa, Maciej Marchwiany, Piotr Magierski Comments: Manuscript with included Supplementary Material. Movies available at this http URL Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con) Cold atoms experiments offer invaluable information on superfluid dynamics, including decay cascades of topological defects. While the cascade properties are well established for Bose systems, our understanding of their behavior in Fermi counterparts is very limited, in particular in spinimbalanced systems, where superfluid (paired) and normal (unpaired) particles naturally coexist giving rise to complex spatial structure of the atomic cloud. Here we show, based on a newly developed microscopic approach, that the decay cascades of topological defects are dramatically modified by the spin-polarization. We demonstrate that decay cascades end up at different stages: “dark soliton”, “vortex ring” or “vortex line”, depending on the polarization. We reveal that it is caused by sucking of unpaired particles into the solitonic internal structure. As a consequence vortex reconnections are hindered and we anticipate that quantum turbulence phenomenon can be significantly affected, indicating new physics induced by polarization effects. NOV 16 arXiv:1711.05241 [pdf, ps, other] Spontaneous surface magnetization and chiral Majorana modes in the p±is superconductors Wang Yang, Chao Xu, Congjun Wu Subjects: Superconductivity (cond-mat.supr-con) Majorana fermions are often proposed to be realized by first singling out a single non-degenerate Fermi surface in spin-orbit coupled systems, and then imposing boundaries or defects. We take a different route starting with two degenerate Fermi surfaces without spin-orbit coupling. By the method of “boundary of boundary”, both the zero energy Majorana modes and the dispersive chiral ones are formed in superconducting systems with the mixed p ± is pairings. Their surfaces develop spontaneous magnetizations with directions determined by the boundary orientations and the phase difference between the p and s-component gap functions. Along the magnetic domain walls on the surface, the chiral Majorana modes propagate unidirectionally, which can be controlled by external magnetic fields. The surface magnetization is a magneto-electric effect which can be analyzed based on the Ginzburg-Landau free energy analysis. Nov 15 arXiv:1711.05020 [pdf, ps, other] Autoresonant excitation of Bose-Einstein condensates S.V. Batalov, A.G. Shagalov, L. Friedland Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph) Controlling the state of a Bose-Einstein condensate driven by a chirped frequency perturbation in a one-dimensional anharmonic trapping potential is discussed. By identifying four characteristic time scales in this chirped-driven problem, three dimensionless parameters P1,2,3 are defined describing the driving strength, the anharmonicity of the trapping potential, and the strength of the particles interaction, respectively. As the driving frequency passes the linear resonance in the problem, and depending on the location in the P1,2,3 parameter space, the system may exhibit two very different evolutions, i.e. the quantum energy ladder climbing (LC) and the classical autoresonance (AR). These regimes are analysed both in theory and simulations with the emphasis on the effect of the interaction parameter P3. In particular, the transition thresholds on the driving parameter P1 and their width in P1 in both the AR and LC regimes are discussed. Different driving protocols are also illustrated, showing efficient control of excitation and de-excitation of the condensate. Nov 13 arXiv:1711.03915 [pdf, other] Title: Generalized classes of continuous symmetries in two-mode Dicke models Authors: Ryan I. Moodie, Kyle E. Ballantine, Jonathan Keeling Comments: 10 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) As recently realized experimentally [L\'eonard et al., Nature 543, 87 (2017)], one can engineer models with continuous symmetries by coupling two cavity modes to trapped atoms, via a Raman pumping geometry. Considering specifically cases where internal states of the atoms couple to the cavity, we show an extended range of parameters for which continuous symmetry breaking can occur, and we classify the distinct steady states and time-dependent states that arise for different points in this extended parameter regime. Nov 10 arXiv:1711.03478 [pdf, other] Title: Strong coupling Bose polarons out of equilibrium: Dynamical RG approach Authors: Fabian Grusdt, Kushal Seetharam, Yulia Shchadilova, Eugene Demler Comments: 19 pages, 7 figures, 2 appendices Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) When a mobile impurity interacts with a surrounding bath of bosons, it forms a polaron. Numerous methods have been developed to calculate how the energy and the effective mass of the polaron are renormalized by the medium for equilibrium situations. Here we address the much less studied non-equilibrium regime and investigate how polarons form dynamically in time. To this end, we develop a time-dependent renormalization group approach which allows calculations of all dynamical properties of the system and takes into account the effects of quantum fluctuations in the polaron cloud. We apply this method to calculate trajectories of polarons following a sudden quench of the impurity-boson interaction strength, revealing how the polaronic cloud around the impurity forms in time. Such trajectories provide additional information about the polaron's properties which are challenging to extract directly from the spectral function measured experimentally using ultracold atoms. At strong couplings, our calculations predict the appearance of trajectories where the impurity wavers back at intermediate times as a result of quantum fluctuations. Our method is applicable to a broader class of non-equilibrium problems. We also apply it to calculate the spectral function and find good agreement with experimental results. At very strong couplings, we predict that quantum fluctuations lead to the appearance of a dark continuum with strongly suppressed spectral weight at low energies. While our calculations start from an effective Fr\"ohlich Hamiltonian describing impurities in a three-dimensional Bose-Einstein condensate, we also calculate the effects of additional terms in the Hamiltonian beyond the Fr\"ohlich paradigm. We demonstrate that the main effect of these additional terms on the attractive side of a Feshbach resonance is to renormalize the coupling strength of the effective Fr\"ohlich model. arXiv:1711.03118 [pdf, other] Title: Efimov Effect in the Dirac Semi-metals Authors: Pengfei Zhang, Hui Zhai Comments: 7 pages, 5 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas) Efimov effect refers to quantum states with discrete scaling symmetry and a universal scaling factor, and has attracted considerable interests from nuclear to atomic physics communities. In a Dirac semi-metal, when an electron interacts with a static impurity though a Coulomb interaction, the same scaling of the kinetic and interaction energies also gives rise to such a Efimov effect. However, even when the Fermi energy exactly lies at the Dirac point, the vacuum polarization of electron-hole pair fluctuation can still screen the Coulomb interaction, which leads to derivation from this scaling symmetry and eventually breakdown of the Efimov effect. This distortion of the Efimov bound state energy due to vacuum polarization is a relativistic electron analogy of the Lamb shift for the hydrogen atom. Motivated by recent experimental observations in two- and three-dimensional Dirac semi-metals, in this paper we investigate this many-body correction to the Efimov effect, and answer the question that under what condition a good number of Efimov-like bound states can still be observed in these condensed matter experiments. Nov. 06 arXiv:1711.01272 [pdf, other] Topological two-body bound states in the interacting Haldane model Grazia Salerno, Marco Di Liberto, Chiara Menotti, Iacopo Carusotto Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el) We study the topological properties of the two-body bound states in an interacting Haldane model as a function of inter-particle interactions. In particular, we identify topological phases where the two-body edge states have either the same or the opposite chirality as compared to single-particle edge states. We highlight that in the moderately-interacting regime, which is relevant for the experimental realization with ultracold atoms, the topological transition is affected by the internal structure of the bound state and the phase boundaries are deformed. arXiv:1711.01549 [pdf, other] DC-current induced domain wall in a chiral p-wave superconductor Thibaut Jonckheere, Takeo Kato Comments: 7 pages, 3 figures Subjects: Superconductivity (cond-mat.supr-con); Mesoscale and Nanoscale Physics (cond-mat.mes-hall) We study theoretically the impact of an applied DC-current on a mesoscopic chiral p-wave superconductor. Performing quasiclassical calculations on a two-dimensional system, with an external magnetic flux to generate a DC current, we show that the current can trigger a transition to a state with a domain wall between regions of different chiralities. The system shows an hysteretic behavior, as different domain wall configurations are possible for a given current. This domain wall creation mechanism can give new insights on recent experiments observing anomalous current variations in Sr2RuO4 junctions. arXiv:1711.02061 [pdf, other] Correlated Spin-Flip Tunneling in a Fermi Lattice Gas Wenchao Xu, William Morong, Hoi-Yun Hui, Vito W. Scarola, Brian DeMarco Subjects: Quantum Gases (cond-mat.quant-gas) We report the realization of correlated, density-dependent tunneling for fermionic 40K atoms trapped in an optical lattice. By appropriately tuning the frequency difference between a pair of Raman beams applied to a spin-polarized gas, simultaneous spin transitions and tunneling events are induced that depend on the relative occupations of neighboring lattice sites. Correlated spin-flip tunneling is spectroscopically resolved using gases prepared in opposite spin states, and the inferred Hubbard interaction energy is compared with a tight-binding prediction. We show that the laser-induced correlated tunneling process generates doublons via loss induced by light-assisted collisions. Furthermore, by controllably introducing vacancies to a spin-polarized gas, we demonstrate that correlated tunneling is suppressed when neighboring lattice sites are unoccupied. Nov. 3 arXiv:1711.00650 [pdf, other] Superadiabatic quantum friction suppression in finite-time thermodynamics Shujin Deng, Aurélia Chenu, Pengpeng Diao, Fang Li, Shi Yu, Ivan Coulamy, Adolfo del Campo, Haibin Wu Comments: 13 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph) Optimal performance of thermal machines is reached by suppressing friction. Friction in quantum thermodynamics results from fast driving schemes that generate nonadiabatic excitations. The far-from-equilibrium dynamics of quantum devices can be tailored by shortcuts to adiabaticity to suppress quantum friction. We experimentally demonstrate friction-free superadiabatic strokes with a trapped unitary Fermi gas as a working substance and establish the equivalence between the superadiabatic mean work and its adiabatic value. arXiv:1711.00519 (cross-list from cond-mat.stat-mech) [pdf, other] Low-Temperature Transport in Out-of-Equilibrium XXZ Chains Bruno Bertini, Lorenzo Piroli Comments: 38 pages, 8 figures Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) We study the low-temperature transport properties of out-of-equilibrium XXZ spin-1/2 chains. We consider the protocol where two semi-infinite chains are prepared in two thermal states at small but different temperatures and suddenly joined together. We focus on the qualitative and quantitative features of the profiles of local observables, which at large times t and distances x from the junction become functions of the ratio ζ=x/t. By means of the generalized hydrodynamic equations, we analyse the rich phenomenology arising by considering different regimes of the phase diagram. In the gapped phases, variations of the profiles are found to be exponentially small in the temperatures but described by non-trivial functions of ζ. We provide analytical formulae for the latter, which give accurate results also for small but finite temperatures. In the gapless regime, we show how the three-step conformal predictions for the profiles of energy density and energy current are naturally recovered from the hydrodynamic equations. Moreover, we also recover the recent non-linear Luttinger liquid predictions for low-temperature transport: universal peaks of width Δζ∝T emerge at the edges of the light cone in the profiles of generic observables. Such peaks are described by the same function of ζfor all local observables. Nov. 2 arXiv:1711.00263 [pdf, ps, other] Tunable topological phases with fermionic atoms in a one-dimensional flux lattice Y. Deng, R. Lü, L. You Comments: 6 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) We present a simple scheme for implementing a one-dimensional (1D) magnetic-flux lattice of ultracold fermionic spin-1/2 atoms. The resulting tight-binding model supports gapped and gapless topological phases, and chiral currents for Meissner and vortex phases. Its single-particle spectra exhibit topological flat bands at small flux, and the flatness sensitively depends on hopping strength. An effective p-wave interaction arises in a s-wave paired superfluid. Treating atomic internal states as forming a synthetic dimension and balancing the interplay of magnetic flux and Zeeman field, our model describes a tunable topological Fermi superfluid, which paves the way towards experimental explorations of non-Abelian topological matter in 1D atomic quantum gases. arXiv:1711.00015 (cross-list from cond-mat.stat-mech) [pdf, other] Irreversible dynamics in quantum many-body systems Markus Schmitt, Stefan Kehrein Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph) Irreversibility, despite being a necessary condition for thermalization, still lacks a sound understanding in the context of quantum many-body systems. In this work we approach this question by studying the behavior of generic many-body systems under imperfect effective time reversal, where the imperfection is introduced as a perturbation of the many-body state at the point of time reversal. Based on numerical simulations of the full quantum dynamics we demonstrate that observable echos occurring in this setting decay exponentially with a rate that is intrinsic to the system meaning that the dynamics is effectively irreversible. arXiv:1711.00348 (cross-list from cond-mat.str-el) [pdf, other] Dipolar quantum phase transition in the Dicke model with infinitely coordinated frustrating interaction Sergei I. Mukhin, Nikolay V. Gnezdilov Comments: 30 pages, 5 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)We propose the realization of a quantum fluid of spinful fermionic atoms interacting via a cavity-photon mediating mechanism, whose effective coupling g can be tuned in strength and sign after detuning the cavity frequency from the atomic transition. Cavity photons are able to drive strong correlations at all coupling strengths, with fermionic character for g>0 and bosonic character for g<0. Using a combination of mean-field and exact diagonalization methods supported by bosonization analysis,we explore the quantum phases emerging from the interplay between internal and motional degrees of freedom, finding spin-density and atomic-density wave ordering, and inferring the occurrence of superfluidity for g>0. arXiv:1711.00014 [pdf, other] Topological Floquet-Thouless energy pump Michael H. Kolodrubetz, Frederik Nathan, Snir Gazit, Takahiro Morimoto, Joel E. Moore Comments: 4 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Physics (quant-ph)We explore adiabatic pumping in the presence of periodic drive, finding a new phase in which the topologically quantized pumped quantity is energy rather than charge. The topological invariant is given by the winding number of the micromotion with respect to time within each cycle, momentum, and adiabatic tuning parameter. We show numerically that this pump is highly robust against both disorder and interactions, breaking down at large values of either in a manner identical to the Thouless charge pump. Finally, we suggest experimental protocols for measuring this phenomenon.
