May 2016

May 2-May 6 Bo Liu, May 9-May 13 Biao Huang, May 16-May 20 Haiyuan Zou, May 23-May 27 Ahmet Kel

May 20
arXiv:1605.05738 [pdf, other]
String order via Floquet interactions in atomic systems
Tony E. Lee, Yogesh N. Joglekar, Philip Richerme
Comments: 7 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
We study the transverse-field Ising model with interactions that are modulated in time. In a rotating frame, the system is described by a time-independent Hamiltonian with many-body interactions, similar to the cluster Hamiltonians of measurement-based quantum computing. In one dimension, there is a three-body interaction, which leads to string order instead of conventional magnetic order. We show that the string order is robust to power-law interactions that decay with the cube of distance. In two and three dimensions, there are five- and seven-body interactions. We discuss adiabatic preparation of the ground state as well as experimental implementation with trapped ions, Rydberg atoms, and dipolar molecules.


May 19
arXiv:1605.05661 [pdf, other]
Spin and Charge Resolved Quantum Gas Microscopy of Antiferromagnetic Order in Hubbard Chains
Martin Boll, Timon A. Hilker, Guillaume Salomon, Ahmed Omran, Immanuel Bloch, Christian Gross
Comments: 9 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
The repulsive Hubbard Hamiltonian is one of the foundational models describing strongly correlated electrons and is believed to capture essential aspects of high temperature superconductivity. Ultracold fermions in optical lattices allow for the simulation of the Hubbard Hamiltonian with a unique control over kinetic energy, interactions and doping. A great challenge is to reach the required low entropy to observe antiferromagnetic spin correlations beyond nearest neighbors, for which quantum gas microscopes are ideal. Here we report on the direct, single-site resolved detection of antiferromagnetic correlations extending up to three sites in spin-1/2 Hubbard chains, which requires an entropy well below s= ln(2). Finally, our simultaneous detection of spin and charge opens the route towards the study of the interplay between magnetic ordering and doping in various dimensions.


May 18
arXiv:1605.04916 [pdf, other]
Topological edge states in single layers of honeycomb materials with strong spin-orbit coupling
Andrei Catuneanu, Heung-Sik Kim, Oguzhan Can, Hae-Young Kee
Comments: 5 pages, 2 tables, 4 figures

Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study possible edge states in single layers of honeycomb materials such as α-RuCl3 and A2IrO3 (A=Li, Na) with strong spin-orbit coupling (SOC). These two dimensional systems exhibit linearly dispersing one-dimensional (1D) edge states when their 1D boundary forms a zig-zag shape. Using an effective tight-binding model based on first principles band structure calculations including Hubbard U and SOC, we find degenerate edge states at the zone center and zone boundary. The roles of chiral symmetry and time-reversal symmetry are presented. The implications to experimental signatures and the effects of disorder are also discussed.

May 17
arXiv:1605.04363 [pdf, other]
Universal One-dimensional Atomic Gases Near Odd-wave Resonance
Xiaoling Cui
Comments: 5+5 pages, 3 figures, 1 table
Subjects: Quantum Gases (cond-mat.quant-gas)

We show the renormalization of contact interaction for odd-wave scattering in one-dimension(1D). Based on the renormalized interaction, we exactly solve the two-body problem in a harmonic trap, and further explore the universal properties of spin-polarized fermions near odd-wave resonance using the operator product expansion method. It is found that the high-momentum distribution behaves as $C/k^2$, with $C$ the odd-wave contact. Various universal relations are derived. Our work suggests a new universal system emergent in 1D with large odd-wave scattering length.

May 16
arXiv:1605.04223 [pdf, other]
Quantum correlations, separability and quantum coherence length in equilibrium many-body systems
Daniele Malpetti, Tommaso Roscilde
Comments: 4+5 pages, 2+1 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

Non-locality is a fundamental trait of quantum many-body systems, both at the level of pure states, as well as at the level of mixed states. Due to non-locality, mixed states of any two subsystems are correlated in a stronger way than what can be accounted for by considering correlated probabilities of occupying some microstates. In the case of equilibrium mixed states, we explicitly build two-point quantum correlation functions, which capture the specific, superior correlations of quantum systems at finite temperature, and which are directly { accessible to experiments when correlating measurable properties}. When non-vanishing, these correlation functions rule out a precise form of separability of the equilibrium state. In particular, we show numerically that quantum correlation functions generically exhibit a finite \emph{quantum coherence length}, dictating the characteristic distance over which degrees of freedom cannot be considered as separable. This coherence length is completely disconnected from the correlation length of the system -- as it remains finite even when the correlation length of the system diverges at finite temperature -- and it unveils the unique spatial structure of quantum correlations.


May 9
arXiv:1605.02704 [pdf]

Site-resolved observations of antiferromagnetic correlations in the Hubbard model

Maxwell F. Parsons, Anton Mazurenko, Christie S. Chiu, Geoffrey Ji, Daniel Greif, Markus Greiner
Comments: 6 + 11 pages, 4 + 6 figures, 1 table

Subjects: Quantum Gases (cond-mat.quant-gas)
Quantum many-body systems exhibiting magnetic correlations underlie a wide variety of phenomena. High-temperature superconductivity, for example, can arise from the correlated motion of holes on an antiferromagnetic (AFM) Mott insulator. Ultracold fermionic atoms in optical lattices provide realizations of strongly correlated many-body systems with a tunability that is unparalleled in conventional solid-state systems. Recent experiments exploring the Hubbard model with cold atoms are accessing temperatures where AFM correlations form, but have only observed these correlations via measurements that were averages over inhomogeneous systems. With the advance of quantum gas microscopy we can now take a snapshot of the real-space correlations in a single quantum many-body state at the atomic scale. Here we report site-resolved observations of AFM correlations in a Hubbard-regime optical lattice. The ability to locally measure spin correlations for a many-body state allows us to make unprecedented comparisons to theoretical predictions. We measure the in-situ distributions of the particle density and magnetic correlations, extract thermodynamic quantities from comparisons to theory, directly measure the correlation length, and study how lattice loading dynamics affect our ability to prepare samples in thermal equilibrium. The largest nearest-neighbor spin correlator that we observe is 53 % of the value predicted by quantum Monte Carlo calculations in the zero-temperature limit. Our results demonstrate that quantum gas microscopy is a powerful tool for studying fermionic quantum magnetism. Direct access to many-body physics at the single-particle level and the microscopic study of quantum dynamics will further our understanding of how new states of matter emerge from the interplay of motion and magnetism in quantum many-body systems.



May 6
1. arXiv:1605.01479 [pdf, other]
Two-dimensional spin-imbalanced Fermi gases at non-zero temperature: Phase separation of a non-condensate
Chien-Te Wu, Brandon M. Anderson, Rufus Boyack, K. Levin
We study a trapped two-dimensional spin-imbalanced Fermi gas over a range of temperatures. In the moderate temperature regime, associated with current experiments, we find reasonable semi-quantitative agreement with the measured density profiles as functions of varying spin imbalance and interaction strength. Our calculations show that, in contrast to the three-dimensional case, the phase separation which appears as a spin balanced core, can be associated with non-condensed fermion pairs. We present predictions at lower temperatures where a quasi-condensate will first appear, based on the pair momentum distribution and following the protocols of Jochim and collaborators. While these profiles also indicate phase separation, they exhibit distinctive features which may aid in identifying the condensation regime.




May 5
1. arXiv:1605.01237 [pdf, other]
Quantum domain walls induce incommensurate supersolid phase on the anisotropic triangular lattice
Xue-Feng Zhang, Shijie Hu, Axel Pelster, Sebastian Eggert
We investigate the extended hard-core Bose-Hubbard model on the triangular lattice as a function of spatial anisotropy with respect to both tunneling and nearest-neighbor interaction strength. At half-filling the system can be tuned from decoupled one-dimensional chains to a two-dimensional solid phase with alternating density order by adjusting the anisotropic coupling. At intermediate anisotropy, however, frustration effects dominate and an incommensurate supersolid phase emerges, which is characterized by incommensurate density order as well as an anisotropic superfluid density. We demonstrate that this intermediate phase results from the proliferation of topological defects in the form of quantum bosonic domain walls. Accordingly, the structure factor has peaks at wave vectors, which are linearly related to the number of domain walls in a finite system in agreement with extensive quantum Monte Carlo simulations. We discuss possible connections with the supersolid behavior in the high-temperature superconducting striped phase.


May 4
1. arXiv:1605.01027 [pdf, ps, other]
Mechanism of stimulated Hawking radiation in a laboratory Bose-Einstein condensate
Yi-Hsieh Wang, Ted Jacobson, Mark Edwards, Charles W. Clark
We model a sonic black hole analog in a quasi one-dimensional Bose-Einstein condensate, using a Gross-Pitaevskii equation matching the configuration of a recent experiment by Steinhauer. The model agrees well with the experimental observations, with no adjustable parameters, demonstrating their hydrodynamic nature. With enhanced but experimentally feasible parameters we establish by spectral analysis that a growing bow wave is generated at the inner (white hole) horizon, stimulating the emission of Hawking radiation. The black hole laser effect plays no role.


2. arXiv:1605.01023 [pdf, other]
Universal space-time scaling symmetry in the dynamics of bosons across a quantum phase transition
Logan W. Clark, Lei Feng, Cheng Chin

The dynamics of many-body systems spanning condensed matter, cosmology, and beyond is hypothesized to be universal when the systems cross continuous phase transitions. The universal dynamics is expected to satisfy a scaling symmetry of space and time with the crossing rate, inspired by the Kibble-Zurek mechanism. We test this symmetry based on Bose condensates in a shaken optical lattice. Shaking the lattice drives condensates across an effectively ferromagnetic quantum phase transition. After crossing the critical point, the condensates manifest delayed growth of spin fluctuations and develop anti-ferromagnetic spatial correlations resulting from sub-Poisson generation of topological defects. The characteristic times and lengths scale as power-laws of the crossing rate, yielding the temporal exponent 0.50(2) and the spatial exponent 0.26(2), consistent with theory. Furthermore, the fluctuations and correlations are invariant in scaled space-time coordinates, in support of the scaling symmetry of quantum critical dynamics.


May 3
1. arXiv:1605.00597 [pdf, other]
Quasi-Long-Range Order in Trapped 2D Bose Gases
Igor Boettcher, Markus Holzmann
We study the fate of algebraic decay of correlations in a harmonically trapped two-dimensional degenerate Bose gas. The analysis is inspired by recent experiments on ultracold atoms where power-law correlations have been observed despite the presence of the external potential. We generalize the spin wave description of phase fluctuations to the trapped case and obtain an analytical expression for the one-body density matrix within this approximation. We show that algebraic decay of the central correlation function persists to lengths of about 20% of the Thomas--Fermi radius. We establish that the trap-averaged correlation function decays algebraically with a strictly larger exponent weakly changing with trap size and find indications that the recently observed enhanced scaling exponents receive significant contributions from the normal component of the gas. We discuss radial and angular correlations and propose a local correlation approximation which captures the correlations very well. Our analysis goes beyond the usual local density approximation and the developed summation techniques constitute a powerful tool to investigate correlations in inhomogeneous systems.


May 2
1. arXiv:1604.08870 [pdf, other]
Momentum distribution in the unitary Bose gas from first principles
Tommaso Comparin, Werner Krauth
Ultracold atomic gases have over the last decades boosted the understanding of quantum physics in the whole range from weak interactions to the infinite-interaction unitary limit. The latter has lead to a revival of the celebrated Efimov effect, that had only been hypothesized in nuclear matter. Here we present first-principles quantum Monte Carlo results for a realistic bosonic N-particle model with infinite, unitary, interactions. We compute the critical temperature for Bose-Einstein condensation and determine the full momentum distribution, including its universal asymptotic behavior. We compare this crucial observable to recent experimental data. The weak dependence of physical observables on the sole parameter of the model, the three-body cutoff, supports its universality. We argue that the thermodynamic instability from the atomic gas towards an Efimov liquid remains hidden by the experimental dynamical instability caused by three-body losses.