Arxiv Selection Dec 2018

Dec 1-Dec 7 Zehan Li, Dec 8-Dec 14 Jiansong Pan, Dec 15-Dec 21 Ahmet Keles, Dec 22- Dec 28 Haiping Hu

Dec. 7

arXiv:1812.02701 (cross-list from cond-mat.stat-mech) [pdf, other]

Kinetic theory of spin superdiffusion in Heisenberg spin chains at high temperature

Sarang Gopalakrishnan, Romain Vasseur

Comments: 6 pages, 1 figure

Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)

We address the nature of spin transport in the integrable XXZ spin chain, focusing on the isotropic Heisenberg limit. We calculate the diffusion constant using a kinetic picture based on generalized hydrodynamics: we find that it diverges, and show that a self-consistent treatment of this divergence gives superdiffusion, with an effective time-dependent diffusion constant that scales as D(t) ∼ t 1/3 . This exponent had previously been observed in large-scale numerical simulations, but had not been theoretically explained. Our results also make clear why the anomalous diffusion in the present case is a qualitatively different phenomenon from Levy flights and other phenomena in random systems. We briefly discuss XXZ models with easy-axis anisotropy ∆ > 1; for these our treatment predicts diffusion, with a diffusion constant D that saturates at large anisotropy, and diverges as the Heisenberg limit is approached, as D ∼ (∆ − 1)−1/2 .

Dec. 6

arXiv:1812.01976 [pdf, other]

Spin-charge separation in strongly interacting multicomponent few-body systems

Rafael Emilio Barfknecht, Angela Foerster, Nikolaj Thomas Zinner

Comments: 11 pages, 8 figures

Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

The effect of spin-charge separation is known to happen in one-dimensional many-body systems in the presence of interactions between particles. In a few-body regime, however, little is known about this phenomenon. To address this problem, we describe the time-evolution of a small system of strongly interacting fermions with SU(N) symmetry after a sudden change in the trapping geometry. The presence of strong contact interactions allows for a mapping between a multicomponent repulsive gas and an effective spin chain, where the internal degrees of freedom of the atoms play the role of different spin projections. This model captures the dynamical behavior of the system by taking into account a set of time-dependent exchange coefficients, which are determined by the instantaneous spatial densities of the system. We show how changing the trapping potential influences the spatial distribution of each atomic species in the ground state of the spin chain. We then obtain the dynamics of the spin densities after a sudden change in the trap. Even at the few-body level, the excitation spectrum for this quantity presents clear separate signatures of both spin and charge dynamics. Moreover, as the number of internal components is increased, we show that the spin excitations vanish, and the dynamics can be predicted by the excitation frequencies expected for a gas of impenetrable bosons. Additionally, we include the description of the dynamics under the same quench protocol in a system where SU(N) symmetry is broken.

Dec. 5

arXiv:1812.00387 [pdf, other]

The four-body scale in universal few-boson systems

Betzalel Bazak, Johannes Kirscher, Sebastian König, Manuel Pavón Valderrama, Nir Barnea, Ubirajara van Kolck

Subjects: Quantum Gases (cond-mat.quant-gas); Nuclear Theory (nucl-th); Quantum Physics (quant-ph)

The role of an intrinsic four-body scale in universal few-boson systems is the subject of active debate. We study these systems within the framework of effective field theory. For systems of up to six bosons we establish that no four-body scale appears at leading order. However, we find that at next-to-leading order a four-body force is needed to obtain renormalized results for binding energies. With the associated parameter fixed to the binding energy of the four-boson system, this force is shown to renormalize the five- and six-body systems as well. As a specific example, calculations are presented for clusters of helium atoms. Our results apply however more generally to other few-body systems governed by a large scattering length, such as low-energy properties of light nuclei and halo states the character of which does not depend on the detailed structure of their cores.

Dec. 4

arXiv:1812.00473 [pdf, other]

Quantum vortex reconnections: crossover from interaction to driven regimes

Luca Galantucci, Andrew W. Baggaley, Nick G. Parker, Carlo F. Barenghi

Comments: 9 pages, 8 Figures main manuscript; 7 pages, 3 Figures Supporting Information

Subjects: Quantum Gases (cond-mat.quant-gas)

Reconnections of coherent filamentary structures play a key role in the dynamics of fluids, redistributing energy and helicity among the length scales, triggering dissipative effects and inducing fine-scale mixing. Unlike ordinary (classical) fluids where vorticity is a continuous field, in superfluid helium and in atomic Bose-Einstein condensates (BECs) vorticity takes the form of isolated quantised vortex lines, which are conceptually easier to study. New experimental techniques now allow visualisation of individual vortex reconnections in helium and condensates. It has long being suspected that reconnections obey universal laws, particularly a universal scaling with time of the minimum distance between vortices δ. Here we perform a comprehensive analysis of this scaling across a range of scenarios relevant to superfluid helium and trapped condensates, combining our own numerical simulations with the previous results in the literature. We reveal that the scaling exhibit two distinct regimes: a δ ∼ t 1/2 scaling arising from the mutual interaction of the reconnecting strands and a δ ∼ t scaling when extrinsic factors drive the individual vortices

Dec. 3

arXiv:1811.12743 [pdf, ps, other]

Superfluidity in dipolar binary Bose mixtures

Abdelaali Boudjemaa

Comments: 7 pages, 4 figures

Subjects: Quantum Gases (cond-mat.quant-gas)

We study the superfluidity and the coherence in dipolar binary Bose mixtures using the hydrodynamic approach. Useful analytical formula for the excitations spectrum, the correlation function and the superfluid fraction are derived. We find that in the case of highly imbalanced mixture, the superfluidity can occur in the dilute component only at extremely low temperatures. The behavior of the first-order correlation function for both dipolar and nondipolar Bose mixtures is deeply analyzed. Then we face the two-dimensional case which encodes a non-trivial physics due to the roton modes.