Dec 2015

Nov 30-Dec 4 Max, Dec 7-Dec 11 Bo Liu, Dec 14-Dec 18 Haiyuan Zou, Dec 28-Jan 1 Ahmet Keles

Dec 18
arXiv:1512.05645 [pdf, other]
Parametric Excitation and Squeezing in a Many-Body Spin System
T.M. Hoang, M. Anquez, B.A. Robbins, X.Y. Yang, B.J. Land, C.D. Hamley, M.S. Chapman
Subjects: Quantum Gases (cond-mat.quant-gas)

We demonstrate a new method to coherently excite and control the quantum spin states of an atomic Bose gas using parametric excitation of the collective spin by time varying the relative strength of the Zeeman and spin-dependent collisional interaction energies at multiples of the natural frequency of the system. Compared to the usual single-particle quantum control techniques used to excite atomic spins (e.g. Rabi oscillations using rf or microwave fields), the method demonstrated here is intrinsically many-body, requiring inter-particle interactions. While parametric excitation of a classical system is ineffective from the ground state, we show that in our quantum system, parametric excitation from the quantum ground state leads to the generation of quantum squeezed states.
Dec 17
arXiv:1512.05262 [pdf, other]
Quantum Turbulence in Trapped Atomic Bose-Einstein condensates
Marios C. Tsatsos, Pedro E. S. Tavares, Amilson R. Fritsch, Andre Cidrim, Monica A. Caracanhas, F. Ednilson A. dos Santos, Carlo F. Barenghi, Vanderlei S. Bagnato
Comments: Review paper. 57 pages, 33 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Turbulence, the complicated fluid behavior of nonlinear and statistical nature, arises in many physical systems across various disciplines, from tiny laboratory scales to geophysical and astrophysical ones. The notion of turbulence in the quantum world was conceived long ago by Onsager and Feynman, but the occurrence of turbulence in ultracold gases has been studied in the laboratory only very recently. Albeit new as a field, it already offers new paths and perspectives on the problem of turbulence. Herein we review the general properties of quantum gases at ultralow temperatures paying particular attention to vortices, their dynamics and turbulent behavior. We review the recent advances both from theory and experiment. We highlight, moreover, the difficulties of identifying and characterizing turbulence in gaseous Bose-Einstein condensates compared to ordinary turbulence and turbulence in superfluid liquid helium and spotlight future possible directions.
Dec 16
arXiv:1512.04550 [pdf, ps, other]
First-order superfluid to Mott-insulator phase transitions in spinor condensates
J. Jiang, L. Zhao, S.-T. Wang, Z. Chen, T. Tang, L.-M. Duan, Y. Liu
Subjects: Quantum Gases (cond-mat.quant-gas)

We observe evidence of first-order superfluid to Mott-insulator quantum phase transitions in a lattice-confined antiferromagnetic spinor Bose-Einstein condensate. The observed signatures include hysteresis effect and significant heatings across the phase transitions. The nature of the phase transitions is found to strongly depend on the ratio of the quadratic Zeeman energy to the spin-dependent interaction. Our observations are qualitatively understood by the mean field theory, and in addition suggest tuning the quadratic Zeeman energy is a new approach to realize superfluid to Mott-insulator phase transitions.



arXiv:1512.04938 [pdf, other]
Loop optimization for tensor network renormalization
Shuo Yang, Zheng-Cheng Gu, Xiao-Gang Wen

Comments: 14 pages, 10 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We introduce a tensor renormalization group scheme for coarse-graining a two-dimensional tensor network, which can be successfully applied to both classical and quantum systems on and off criticality. The key idea of our scheme is to deform a 2D tensor network into small loops and then optimize tensors on each loop. In this way we remove short-range entanglement at each iteration step, and significantly improve the accuracy and stability of the renormalization flow. We demonstrate our algorithm in the classical Ising model and a frustrated 2D quantum model.
Dec 15
arXiv:1512.03994 [pdf, ps, other]
Mott insulating states and quantum phase transitions of correlated SU(2N) Dirac fermions
Zhichao Zhou, Da Wang, Zi Yang Meng, Yu Wang, Congjun Wu
Comments: 18 pages, 24 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

The interplay between charge and spin degrees of freedom in strongly correlated fermion systems, in particular of Dirac fermions, is a long-standing problem in condensed matter physics. We investigate the competing orders of the half-filled SU(2N) Hubbard model on a honeycomb lattice, which can be accurately realized in optical lattices with large-spin ultra-cold alkaline-earth fermions. Employing large-scale projector determinant quantum Monte Carlo simulations, we have explored quantum phase transitions from the gapless Dirac semi-metals to the gapped Mott-insulating phases in the SU(4) and SU(6) cases. Both of these Mott-insulating states are found to be columnar valence bond solid (cVBS) and to be absence of the antiferromagnetic Neel ordering and the loop current ordering. Inside the cVBS phases, the dimer ordering is enhanced by increasing fermion components and behaves non-monotonically as the interaction strength increases. The transitions can be either weakly first order due to a cubic invariance composed by the cVBS order, or, be softened to second order by quantum fluctuations. Our simulations provide important guidance for the experimental explorations of novel states of matter with ultra-cold alkaline earth fermions.
Dec 14
arXiv:1512.03431 [pdf, other]
Equation of state of the two-dimensional Hubbard model
Eugenio Cocchi, Luke A. Miller, Jan H. Drewes, Marco Koschorreck, Daniel Pertot, Ferdinand Brennecke, Michael Köhl
Comments: 8 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Understanding the phases of strongly correlated quantum matter is challenging because they arise from the subtle interplay between kinetic energy, interactions, and dimensionality. In this quest it has turned out that even conceptually simple models of strongly correlated fermions, which often only approximately represent the physics of the solid state, are very hard to solve. Since the conjecture by P. W. Anderson that the two-dimensional Hubbard model describes the main features of high-T$_c$ superconductivity in the cuprates, there has been a major, yet inconclusive, research effort on determining its fundamental thermodynamic properties. Here we present an experimental determination of the equation of state of the repulsive two-dimensional Hubbard model over a broad range of interactions, $0\leq U/t \lesssim 20$, and temperatures, down to $k_BT/t=0.63(2)$, using high-resolution imaging of ultracold atoms in optical lattices. The equation of state fully characterizes the thermodynamics of the Hubbard model, and our results constitute benchmarks for state-of-the-art theoretical approaches.

Dec 11
1. arXiv:1512.03407 [pdf, other]
Interferometric Measurements of Many-body Topological Invariants using Mobile Impurities
Fabian Grusdt, Norman Y. Yao, Dmitry A. Abanin, Michael Fleischhauer, Eugene A. Demler
Topological quantum phases cannot be characterized by Ginzburg-Landau type order parameters, and are instead described by non-local topological invariants. Experimental platforms capable of realizing such exotic states now include "synthetic" many-body systems such as ultracold atoms or photons. Unique tools available in these systems enable a new characterization of strongly correlated many-body states. Here we propose a general scheme for detecting topological order using interferometric measurements of elementary excitations. The key ingredient is the use of mobile impurities which bind to quasiparticles of a host many-body system. Specifically we show how fractional charges can be probed in the bulk of fractional quantum Hall systems. We demonstrate that combining Ramsey interference with Bloch oscillations can be used to measure Chern numbers of individual quasiparticles, which gives a direct probe of their fractional charges. We discuss possible extensions of our method to other topological many-body systems, such as spin liquids.


2. arXiv:1512.03218 (cross-list from quant-ph) [pdf, other]
Quantum Transport of Energy in Controlled Synthetic Quantum Magnets
Alejandro Bermudez, Tobias Schaetz
We introduce a scheme that exploits laser cooling and phonon-mediated spin-spin interactions in crystals of trapped atomic ions to explore the transport of energy through a quantum magnet. We show how to implement an effective transport window to control the flow of energy through the magnet even in the absence of fermionic statistics for the carriers. This is achieved by shaping the density of states of the effective thermal reservoirs that arise from the interaction with the external bath of the modes of the electromagnetic field, and can be experimentally controlled by tuning the laser frequencies and intensities appropriately. The interplay of this transport window with the spin-spin interactions is exploited to build an analogue of the Coulomb-blockade effect in nano-scale electronic devices, and opens new possibilities to study quantum effects in energy transport


Dec 10
1. arXiv:1512.02641 (cross-list from hep-th) [pdf, ps, other]
Retarded Correlators in Kinetic Theory: Branch Cuts, Poles and Transport Phase Transitions
Paul Romatschke
In this work the collective modes of an effective kinetic theory description based on the Boltzmann equation in relaxation time approximation applicable to gauge theories at weak but finite coupling and low frequencies are studied. Real time retarded two-point correlators of the energy-momentum tensor and the R-charge current are calculated at finite temperature in flat space-times for large N gauge theories. It is found that the real time correlators possess logarithmic branch cuts which in the limit of large coupling disappear and give rise to non-hydrodynamic poles that are reminiscent of quasi-normal modes in black holes. In addition to branch cuts, correlators can have simple hydrodynamic poles, generalizing the concept of hydrodynamic modes to intermediate wavelength. Surprisingly, the hydrodynamic poles cease to exist for some critical value of the wavelength and coupling reminiscent of the properties of phase transitions.


Dec 9
1. arXiv:1512.02273 [pdf, other]
A continuum compass model on the honeycomb lattice: phase diagram from tensor networks
Haiyuan Zou, Bo Liu, Erhai Zhao, W. Vincent Liu
Quantum spin models with spatially dependent interactions, known as compass models, play an important role in the study of frustrated quantum magnetism and Mott insulators with orbital degrees of freedom. One example is the Kitaev model on the honeycomb lattice with spin liquid ground states and anyonic excitations. It is highly desirable to go beyond this exactly solvable limit by generalizing the Kitaev model to connect with specific materials or other known models. A conventional way to achieve this is through hybridization with other models. For example, the Kitaev-Heisenberg model was proposed to describe the iridates. Here we pursue an alternative direction and propose a new model, dubbed "the tripod model", which contains a continuum of compass-type models. It smoothly interpolates the Ising model, the Kitaev model, and the quantum 120^\circ model by tuning a single parameter \theta', the angle between the three legs of a tripod in the spin space. Not only does the tripod model establish a natural connection between the three paradigmatic models, but it also enables the study of the quantum phase transitions between them. We solve this model numerically by tensor networks in the thermodynamic limit and find a spin-disordered (spin liquid) phase between an antiferromagnetic phase and a dimer phase. Our results unambiguously establish the long-range order of the geometrically frustrated 120^\circ model and provide an intuitive picture for the phase transition between the spin-liquid phase and the dimer phase. The tripod model proposed here could be realized in cold atoms experiments by introducing non-Abelian artificial gauge fields for the fermionic Hubbard model.


Dec 8
1. arXiv:1512.01849 [pdf, ps, other]
Mixed parity pairing in a dipolar gas
G. M. Bruun, C. Hainzl, M. Laux
We show that fermionic dipoles in a two-layer geometry form Cooper pairs with both singlet and triplet components, when they are tilted with respect to the normal of the planes. The mixed parity pairing arises because the interaction between dipoles in the two different layers is not inversion symmetric. We use an efficient eigenvalue approach to calculate the zero temperature phase diagram of the system as a function of the dipole orientation and the layer distance. The phase diagram contains purely triplet as well as mixed singlet and triplet superfluid phases. We show in detail how the pair wave function for dipoles residing in different layers smoothly changes from singlet to triplet symmetry as the orientation of the dipoles is changed. Our results indicate that dipolar quantum gases can be used to unambiguously observe mixed parity pairing.



2. arXiv:1512.02043 (cross-list from cond-mat.dis-nn) [pdf, other]
Construction of Wannier Functions in Disordered Systems
Junbo Zhu, Zhu Chen, Biao Wu
In this work, we propose a general method of constructing Wannier functions in disordered system out of energy eigenstates directly. This method consists of two successive operations: (i) a phase transformation setting the proper localization center; (ii) the mixing of adjacent states in energy space which sufficiently minimize the spread of the Wannier functions. The latter operation can be well approximated by a band matrix, further facilitating the calculation. Detailed illustration is given in terms of examples in one dimensional system. The generalization to higher dimensions is straightforward.


Dec 7
1 arXiv:1512.01392 [pdf, other]
Itinerant ferromagnetism in 1D two-component Fermi gases
Yuzhu Jiang, D.V. Kurlov, Xi-Wen Guan, F. Schreck, G.V. Shlyapnikov
We study a one-dimensional two-component atomic Fermi gas with an infinite intercomponent contact repulsion. It is found that adding an attractive resonant odd-wave interaction breaking the rotational symmetry one can make the ground state ferromagnetic. A promising system for the observation of this itinerant ferromagnetic state is a 1D gas of ^{40}K atoms, where 3D s-wave and p-wave Feshbach resonances are very close to each other and the 1D confinement significantly reduces the inelastic decay.