arXiv subjects

Stefan K. Baur

Publications and source records attributed to Stefan K. Baur.

12 recordsLinked to original sources

arXiv2014

Dynamic Optical Superlattices with Topological Bands

We introduce an all-optical approach to producing high-flux synthetic magnetic fields for neutral atoms or molecules by designing intrinsically time-periodic optical superlattices. A single laser source, modulated to generate two frequencies, suffices to create dynamically modulated interference patterns which have topological Floquet energy bands. In particular, we propose a simple laser setup that realizes a tight-binding model with uniform flux per plaquette and well-separated Chern bands. Our method relies only on the particles' scalar polarizability and far detuned light.

cond-mat.other↗

arXiv2013

Adiabatic preparation of vortex lattices

By engineering appropriate artificial gauge potentials, a Bose-Einstein condensate can be adiabatically loaded into a current carrying state that resembles a vortex lattice of a rotating uniform Bose gas. We give two explicit, experimentally feasible protocols by which vortex lattices can be smoothly formed from a condensate initially at rest. In the first example we show how this can be achieved by adiabatically loading a uniform BEC into an optical flux lattice, formed from coherent optical coupling of internal states of the atom. In the second example we study a tight binding model that is continuously manipulated in parameter space such that it smoothly transforms into the Harper-Hofstadter model with 1/3 flux per plaquette.

cond-mat.quant-gas↗

arXiv2013

Collective modes of a two-dimensional spin-1/2 Fermi gas in a harmonic trap

We derive analytical expressions for the frequency and damping of the lowest collective modes of a two-dimensional Fermi gas using kinetic theory. For strong coupling, we furthermore show that pairing correlations overcompensate the effects of Pauli blocking on the collision rate for a large range of temperatures, resulting in a rate which is larger than that of a classical gas. Our results agree well with experimental data, and they recover the observed cross-over from collisionless to hydrodynamic behaviour with increasing coupling for the quadruple mode. Finally, we show that a trap anisotropy within the experimental bounds results in a damping of the breathing mode which is comparable to what is observed, even for a scale invariant system.

cond-mat.quant-gas↗

arXiv2013

Coupled Ferromagnetic and Nematic Ordering of Fermions in an Optical Flux Lattice

Ultracold atoms in Raman-dressed optical lattices allow for effective momentum-dependent interactions among single-species fermions originating from short-range s-wave interactions. These dressed-state interactions combined with very flat bands encountered in the recently introduced optical flux lattices push the Stoner instability towards weaker repulsive interactions, making it accessible with current experiments. As a consequence of the coupling between spin and orbital degrees of freedom, the magnetic phase features Ising nematic order.

cond-mat.quant-gas↗

arXiv2012

High-polarization limit of the quasi-two-dimensional Fermi gas

We demonstrate that the theoretical description of current experiments of quasi-2D Fermi gases requires going beyond usual 2D theories. We provide such a theory for the highly spin-imbalanced quasi-2D Fermi gas. For typical experimental conditions, we find that the location of the recently predicted polaron-molecule transition is shifted to lower values of the vacuum binding energy due to the interplay between transverse confinement and many-body physics. The energy of the attractive polaron is calculated in the 2D-3D crossover and displays a series of cusps before converging towards the 3D limit. The repulsive polaron is shown to be accurately described by a 2D theory with a single interaction parameter.

cond-mat.quant-gas↗

arXiv2012

Radio frequency spectra of Feshbach molecules in quasi-two dimensional geometries

The line shape of radio frequency spectra of tightly bound Feshbach molecules in strong transverse confinement can be described by a simple analytic formula that includes final state interactions. By direct comparison to experimental data, we clarify the role of effective range corrections to two-body bound-state energies in lower dimensions.

cond-mat.quant-gas↗

arXiv2010

Two-body recombination in a quantum mechanical lattice gas: Entropy generation and probing of short-range magnetic correlations

We study entropy generation in a one-dimensional (1D) model of bosons in an optical lattice experiencing two-particle losses. Such heating is a major impediment to observing exotic low temperature states, and "simulating" condensed matter systems. Developing intuition through numerical simulations, we present a simple empirical model for the entropy produced in this 1D setting. We also explore the time evolution of one and two particle correlation functions, showing that they are robust against two-particle loss. Because of this robustness, induced two-body losses can be used as a probe of short range magnetic correlations.

cond-mat.quant-gas↗

arXiv2010

Spin-Imbalance in a One-Dimensional Fermi Gas

Superconductivity and magnetism generally do not coexist. Changing the relative number of up and down spin electrons disrupts the basic mechanism of superconductivity, where atoms of opposite momentum and spin form Cooper pairs. Nearly forty years ago Fulde and Ferrell and Larkin and Ovchinnikov proposed an exotic pairing mechanism (FFLO) where magnetism is accommodated by formation of pairs with finite momentum. Despite intense theoretical and experimental efforts, however, polarized superconductivity remains largely elusive. Here we report experimental measurements of density profiles of a two spin mixture of ultracold 6Li atoms trapped in an array of one dimensional (1D) tubes, a system analogous to electrons in 1D wires. At finite spin imbalance, the system phase separates with an inverted phase profile in comparison to the three-dimensional case. In 1D we find a partially polarized core surrounded by wings composed of either a completely paired BCS superfluid or a fully polarized Fermi gas, depending on the degree of polarization. Our observations are in quantitative agreement with theoretical calculations in which the partially polarized phase is found to be a 1D analogue of the FFLO state. This study demonstrates how ultracold atomic gases in 1D may be used to create non-trivial new phases of matter, and also paves the way for direct observation and further study of the FFLO phase.

physics.atom-ph↗

arXiv2010

FFLO vs Bose-Fermi mixture in polarized 1D Fermi gas on a Feshbach resonance: a 3-body study

We study the three-fermion problem within a 1D model of a Feshbach resonance in order to gain insight into how the FFLO-like state at small negative scattering lengths evolves into a Bose-Fermi mixture at small positive scattering lengths. The FFLO state possesses an oscillating superfluid correlation function, while in a Bose-Fermi mixture correlations are monotonic. We find that this behavior is is already present at the 3-body level. We present an exact study of the 3-body problem, and gain extra insights by considering worldlines of a path integral Monte-Carlo calculation.

cond-mat.supr-con↗

arXiv2009

Stirring trapped atoms into fractional quantum Hall puddles

We theoretically explore the generation of few-body analogs of fractional quantum Hall states. We consider an array of identical few-atom clusters (n=2,3,4), each cluster trapped at the node of an optical lattice. By temporally varying the amplitude and phase of the trapping lasers, one can introduce a rotating deformation at each site. We analyze protocols for coherently transferring ground state clusters into highly correlated states, producing theoretical fidelities in excess of 99%.

cond-mat.quant-gas↗

arXiv2009

Theory of the Normal/Superfluid interface in population imbalanced Fermi gases

We present a series of theoretical studies of the boundary between a superfluid and normal region in a partially polarized gas of strongly interacting fermions. We present mean-field estimates of the surface energy in this boundary as a function of temperature and scattering length. We discuss the structure of the domain wall, and use a previously introduced phenomonological model to study its influence on experimental observables. Our microscopic mean-field calculations are not consistent with the magnitude of the surface tension found from our phenomonological modelling of data from the Rice experiments. We conclude that one must search for novel mechanisms to explain the experiments.

cond-mat.quant-gas↗

arXiv2007

Quasi-one-dimensional polarized Fermi superfluids

We calculate the zero temperature phase diagram of a polarized two-component Fermi gas in an array of weakly-coupled parallel one-dimensional (1D) 'tubes' produced by a two-dimensional optical lattice. Increasing the lattice strength drives a crossover from three-dimensional (3D) to 1D behavior, stabilizing the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superfluid phase. We argue that the most promising regime for observing the FFLO phase is in the quasi-1D regime, where the atomic motion is largely 1D but there is weak tunneling in the other directions that stabilizes long range order. In the FFLO phase, we describe a phase transition where the quasiparticle spectrum changes from gapless near the 3D regime to gapped in quasi-1D.

cond-mat.other↗