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Theory of bosons in two-leg ladders with large magnetic fields

Cornell Affiliated Author(s)

Author

R. Wei
E.J. Mueller

Abstract

We calculate the ground state of a Bose gas trapped on a two-leg ladder where Raman-induced hopping mimics the effect of a large magnetic field. In the mean-field limit, where there are large numbers of particles per site, this maps onto a uniformly frustrated two-leg ladder classical spin model. The net particle current always vanishes in the ground state, but generically there is a finite "chiral current," corresponding to equal and opposite flow on the two legs. We vary the strength of the hopping across the rungs of the ladder and the interaction between the bosons. We find the following three phases. (1) A "saturated chiral current phase" (SCCP), where the density is uniform and the chiral current is simply related to the strength of the magnetic field. In this state the only broken symmetry is the U(1) condensate phase. (2) A "biased ladder phase" (BLP), where the density is higher on one leg than the other. The fluid velocity is higher on the lower density leg, so the net current is zero. In addition to the U(1) condensate phase, this has a broken Z2 reflection symmetry. (3) A "modulated density phase" (MDP), where the atomic density is modulated along the ladder. In addition to the U(1) condensate phase, this has a second broken U(1) symmetry corresponding to translations of the density wave. We further study the fluctuations of the condensate in the BLP, finding a roton-maxon-like excitation spectrum. Decreasing the hopping along the rungs softens the spectrum. As the energy of the "roton" reaches to zero, the BLP becomes unstable. We describe the experimental signatures of these phases, including the response to changing the frequency of the Raman transition. © 2014 American Physical Society.

Date Published

Journal

Physical Review A - Atomic, Molecular, and Optical Physics

Volume

89

Issue

6

URL

https://www.scopus.com/inward/record.uri?eid=2-s2.0-84903538430&doi=10.1103%2fPhysRevA.89.063617&partnerID=40&md5=d86971be0f3901152bc54657c7aed73f

DOI

10.1103/PhysRevA.89.063617

Funding Source

PHY-1068165
1068165

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