89 research outputs found

### Quantum Hall fractions in ultracold atomic vapors

Atomic vapors can be prepared and manipulated at very low densities and
temperatures. When they are rotating, they can reach a quantum Hall regime in
which there should be manifestations of the fractional quantum Hall effect. We
discuss the appearance of the principal sequence of fractions nu =p/(p+- 1) for
bosonic atoms. The termination point of this series is the paired Moore-Read
Pfaffian state. Exotic states fill the gap between the paired state and the
vortex lattice expected at high filling of the lowest Landau level. In
fermionic vapors, the p-wave scattering typical of ultralow energy collisions
leads to the hard-core model when restricted to the lowest Landau level.Comment: 7 pages, 2 figs, brief review submitted to Modern Physics Letters

### Generalized nonlinear sigma model approach to alternating spin chains and ladders

We generalize the nonlinear sigma model treatment of quantum spin chains to
cases including ferromagnetic bonds. When these bonds are strong enough, the
classical ground state is no longer the standard Neel order and we present an
extension of the known formalism to deal with this situation. We study the
alternating ferromagnetic-antiferromagnetic spin chain introduced by Hida. The
smooth crossover between decoupled dimers and the Haldane phase is
semi-quantitatively reproduced. We study also a spin ladder with diagonal
exchange couplings that interpolates between the gapped phase of the two-leg
spin ladder and the Haldane phase. Here again we show that there is good
agreement between DMRG data and our analytical results.Comment: 10 pages, 5 encapsulated figures, REVTeX 3.

### Parafermionic states in rotating Bose-Einstein condensates

We investigate possible parafermionic states in rapidly rotating ultracold
bosonic atomic gases at lowest Landau level filling factor nu=k/2. We study how
the system size and interactions act upon the overlap between the true ground
state and a candidate Read-Rezayi state. We also consider the quasihole states
which are expected to display non-Abelian statistics. We numerically evaluate
the degeneracy of these states and show agreement with a formula given by E.
Ardonne. We compute the overlaps between low-lying exact eigenstates and
quasihole candidate wavefunctions. We discuss the validity of the parafermion
description as a function of the filling factor.Comment: 23 pages, 10 figure

### Field-Induced Disorder Point in Non-Collinear Ising Spin Chains

We perform a theoretical study of a non-collinear Ising ferrimagnetic spin
chain inspired by the compound Co(hfac)2NITPhOMe. The basic building block of
its structure contains one Cobalt ion and one organic radical each with a spin
1/2. The exchange interaction is strongly anisotropic and the corresponding
axes of anisotropy have a period three helical structure. We introduce and
solve a model Hamiltonian for this spin chain. We show that the present
compound is very close to a so-called disorder point at which there is a
massive ground state degeneracy. We predict the equilibrium magnetization
process and discuss the impact of the degeneracy on the dynamical properties by
using arguments based on the Glauber dynamics.Comment: revtex 4, 10 pages, 7 figure

### Pairing in ultracold Fermi gases in the lowest Landau level

We study a rapidly rotating gas of unpolarized spin-1/2 ultracold fermions in
the two-dimensional regime when all atoms reside in the lowest Landau level.
Due to the presence of the spin degree of freedom both s-wave and p-wave
interactions are allowed at ultralow temperatures. We investigate the phase
diagram of this system as a function of the filling factor in the lowest Landau
level and in terms of the ratio between s- and p-wave interaction strengths. We
show that the presence of attractive interactions induces a wide regime of
phase separation with formation of maximally compact droplets that are either
fully polarized or composed of spin-singlets. In the regime with no phase
separation, we give evidence for fractional quantum Hall states. Most notably,
we find two distinct singlet states at the filling nu =2/3 for different
interactions. One of these states is accounted for by the composite fermion
theory while the other one is a paired state for which we identify two
competing descriptions with different topological structure. This paired state
may be an Abelian liquid of composite spin-singlet Bose molecules with Laughlin
correlations. Alternatively, it may be a known non-Abelian paired state,
indicated by good overlaps with the corresponding trial wavefunction. By fine
tuning of the scattering lengths it is possible to create the non-Abelian
critical Haldane-Rezayi state for nu =1/2 and the permanent state of Moore and
Read for nu =1. For purely repulsive interactions, we also find evidence for a
gapped Halperin state at nu=2/5.Comment: 12 pages, 9 figs (best viewed in color), published version with
additional evidence for a non-Abelian spin singlet state at filling nu=2/

### Phases of random antiferromagnetic spin-1 chains

We formulate a real-space renormalization scheme that allows the study of the
effects of bond randomness in the Heisenberg antiferromagnetic spin-1 chain.
There are four types of bonds that appear during the renormalization flow. We
implement numerically the decimation procedure. We give a detailed study of the
probability distributions of all these bonds in the phases that occur when the
strength of the disorder is varied. Approximate flow equations are obtained in
the weak-disorder regime as well as in the strong disorder case where the
physics is that of the random singlet phase.Comment: 29 pages, 12 encapsulated Postscript figures, REVTeX 3.

### Incommensurability in the magnetic excitations of the bilinear-biquadratic spin-1 chain

We study the magnetic excitation spectrum of the S=1 quantum Heisenberg spin
chain with Hamiltonian : H = sum_i cos(theta) S_i S_i+1 + sin(theta) (S_i
S_i+1)^2. We focus on the range -pi/4 < theta < +pi/4 where the spin chain is
in the gapped Haldane phase. The excitation spectrum and static structure
factor is studied using direct Lanczos diagonalization of small systems and
density-matrix renormalization group techniques combined with the single-mode
approximation. The magnon dispersion has a minimum at q=pi until a critical
value theta_c = 0.38 is reached at which the curvature (velocity) vanishes.
Beyond this point, which is distinct from the VBS point and the Lifshitz point,
the minimum lies at an incommensurate value that goes smoothly to 2pi/3 when
theta approaches pi/4, the Lai-Sutherland point. The mode remains isolated from
the other states: there is no evidence of spinon deconfinement before the point
theta =+pi/4. These findings explain recent observation of the magnetization
curve M approx (H -H_c)^1/4 for theta =theta_c.Comment: 14 pages, 8 encapsulated figures, REVTeX 3.

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