283 research outputs found
Higgs Mode and Magnon Interactions in 2D Quantum Antiferromagnets from Raman Scattering
We present a theory for Raman scattering on 2D quantum antiferromagnets. The
microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective
- model. Well within the N\'eel ordered phase, the Raman spectrum
contains a two-magnon and a two-Higgs contribution, which are calculated
diagramatically. The vertex functions for both the Higgs and magnon
contributions are determined from a numerical solution of the corresponding
Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in
the relevant symmetry, the contribution from the Higgs mode is
strongly suppressed. Except for intermediate values of the Higgs mass, it does
not show up as separate peak in the spectrum but gives rise to a broad
continuum above the dominant contribution from two-magnon excitations. The
latter give rise to a broad, asymmetric peak at , which
is a result of magnon-magnon interactions mediated by the Higgs mode. The full
Raman spectrum is determined completely by the antiferromagnetic exchange
coupling and a dimensionless Higgs mass. Experimental Raman spectra of
undoped cuprates turn out to be in very good agreement with the theory only
with inclusion of the Higgs contribution. They thus provide a clear signature
of the presence of a Higgs mode in spin one-half 2D quantum antiferromagnets.Comment: 12 pages, 15 figure
Density matrix renormalization group for disordered bosons in one dimension
We calculate the zero-temperature phase diagram of the disordered
Bose-Hubbard model in one dimension using the density matrix renormalization
group. For integer filling the Mott insulator is always separated from the
superfluid by a Bose glass phase. There is a reentrance of the Bose glass both
as a function of the repulsive interaction and of disorder. At half-filling
where no Mott insulator exists, the superfluid density has a maximum where the
kinetic and repulsive energies are about the same. Superfluidity is suppressed
both for small and very strong repulsion but is always monotonic in disorder.Comment: 4 pages, 2 eps figures, uses RevTe
Exponential localization in one-dimensional quasiperiodic optical lattices
We investigate the localization properties of a one-dimensional bichromatic
optical lattice in the tight binding regime, by discussing how exponentially
localized states emerge upon changing the degree of commensurability. We also
review the mapping onto the discrete Aubry-Andre' model, and provide evidences
on how the momentum distribution gets modified in the crossover from extended
to exponentially localized states. This analysis is relevant to the recent
experiment on Anderson localization of a noninteracting Bose-Einstein
condensate in a quasiperiodic optical lattice [G. Roati et al., Nature 453, 895
(2008)].Comment: 13 pages, 6 figure
Commensurate-incommensurate transition of cold atoms in an optical lattice
An atomic gas subject to a commensurate periodic potential generated by an
optical lattice undergoes a superfluid--Mott insulator transition. Confining a
strongly interacting gas to one dimension generates an instability where an
arbitrary weak potential is sufficient to pin the atoms into the Mott state;
here, we derive the corresponding phase diagram. The commensurate pinned state
may be detected via its finite excitation gap and the Bragg peaks in the static
structure factor.Comment: 4 pages, 2 figure
Ramping fermions in optical lattices across a Feshbach resonance
We study the properties of ultracold Fermi gases in a three-dimensional
optical lattice when crossing a Feshbach resonance. By using a zero-temperature
formalism, we show that three-body processes are enhanced in a lattice system
in comparison to the continuum case. This poses one possible explanation for
the short molecule lifetimes found when decreasing the magnetic field across a
Feshbach resonance. Effects of finite temperatures on the molecule formation
rates are also discussed by computing the fraction of double-occupied sites.
Our results show that current experiments are performed at temperatures
considerably higher than expected: lower temperatures are required for
fermionic systems to be used to simulate quantum Hamiltonians. In addition, by
relating the double occupancy of the lattice to the temperature, we provide a
means for thermometry in fermionic lattice systems, previously not accessible
experimentally. The effects of ramping a filled lowest band across a Feshbach
resonance when increasing the magnetic field are also discussed: fermions are
lifted into higher bands due to entanglement of Bloch states, in good agreement
with recent experiments.Comment: 9 pages, 7 figure
Bose-Einstein Quantum Phase Transition in an Optical Lattice Model
Bose-Einstein condensation (BEC) in cold gases can be turned on and off by an
external potential, such as that presented by an optical lattice. We present a
model of this phenomenon which we are able to analyze rigorously. The system is
a hard core lattice gas at half-filling and the optical lattice is modeled by a
periodic potential of strength . For small and temperature,
BEC is proved to occur, while at large or temperature there is no
BEC. At large the low-temperature states are in a Mott insulator
phase with a characteristic gap that is absent in the BEC phase. The
interparticle interaction is essential for this transition, which occurs even
in the ground state. Surprisingly, the condensation is always into the
mode in this model, although the density itself has the periodicity of the
imposed potential.Comment: RevTeX4, 13 pages, 2 figure
- …