255 research outputs found
BCS-BEC crossover in bilayers of cold fermionic polar molecules
We investigate the quantum and thermal phase diagram of fermionic polar molecules loaded in a bilayer trapping potential with perpendicular dipole moment. We use both a BCS-theory approach that is most reliable at weak coupling and a strong-coupling approach that considers the two-body bound dimer states with one molecule in each layer as the relevant degree of freedom. The system ground state is a Bose-Einstein condensate (BEC) of dimer bound states in the low-density limit and a paired superfluid (BCS) state in the high-density limit. At zero temperature, the intralayer repulsion is found to broaden the regime of BCS-BEC crossover and can potentially induce system collapse through the softening of roton excitations. The BCS theory and the strongly coupled dimer picture yield similar predictions for the parameters of the crossover regime. The Berezinskii-Kosterlitz-Thouless transition temperature of the dimer superfluid is also calculated. The crossover can be driven by many-body effects and is strongly affected by the intralayer interaction which was ignored in previous studies
A Mott Glass to Superfluid Transition for Random Bosons in Two Dimensions
We study the zero temperature superfluid-insulator transition for a
two-dimensional model of interacting, lattice bosons in the presence of
quenched disorder and particle-hole symmetry. We follow the approach of a
recent series of papers by Altman, Kafri, Polkovnikov, and Refael, in which the
strong disorder renormalization group is used to study disordered bosons in one
dimension. Adapting this method to two dimensions, we study several different
species of disorder and uncover universal features of the superfluid-insulator
transition. In particular, we locate an unstable finite disorder fixed point
that governs the transition between the superfluid and a gapless, glassy
insulator. We present numerical evidence that this glassy phase is the
incompressible Mott glass and that the transition from this phase to the
superfluid is driven by percolation-type process. Finally, we provide estimates
of the critical exponents governing this transition.Comment: (24 pages + 7 page appendix, 28 figures) This version has been
accepted to PRB. We have acquired new data that resolves the contradiction
between two estimates of the critical exponents in the earlier version of the
pape
Relaxation of Fermionic Excitations in a Strongly Attractive Fermi Gas in an Optical Lattice
We theoretically study the relaxation of high energy single particle
excitations into molecules in a system of attractive fermions in an optical
lattice, both in the superfluid and the normal phase. In a system characterized
by an interaction scale and a tunneling rate , we show that the
relaxation rate scales as in the large
limit. We obtain explicit expressions for the exponent , both in the
low temperature superfluid phase and the high temperature phase with pairing
but no coherence between the molecules. We find that the relaxation rate
decreases both with temperature and deviation of the fermion density from
half-filling. We show that quasiparticle and phase degrees of freedom are
effectively decoupled within experimental timescales allowing for observation
of ordered states even at high total energy of the system.Comment: 5 pages, 3 figure
Modulation Spectroscopy and Dynamics of Double Occupancies in a Fermionic Mott Insulator
We calculate the rate of creation of double occupancies in a 3D
Fermionic Mott insulator near half-filling by modulation of optical lattice
potential. At high temperatures, incoherent holes lead to a broad response
peaked at the Hubbard repulsion . At low temperatures, antiferromagnetic
order leads to a coherent peak for the hole along with broad features
representing spin wave shake-off processes. This is manifested in the doublon
creation rate as a sharp absorption edge and oscillations as a function of
modulating frequency. Thus, modulation spectroscopy can be used as a probe of
antiferromagnetic order and nature of quasiparticle excitations in the system.Comment: 4 pages 2 figure
Nanosegregation in Na2C60
There is continuous interest in the nature of alkali metal fullerides containing C(4)(60) and C(2)(60),
because these compounds are believed to be nonmagnetic Mott–Jahn–Teller insulators. This idea
could be verified in the case of A(4)C(60), but Na(2)C(60) is more controversial. By comparing the results
of infrared spectroscopy and X-ray diffraction, we found that Na(2)C(60) is segregated into 3-10 nm
large regions. The two main phases of the material are insulating C(60) and metallic Na(3)C(60). We
found by neutron scattering that the diffusion of sodium ions becomes faster on heating. Above
470 K Na(2)C(60) is homogeneous and we show IR spectroscopic evidence of a Jahn–Teller distorted
C(2)(60) anion
Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps
We investigate the equilibrium and dynamical properties of the Bose-Hubbard
model and the related particle-hole symmetric spin-1 model in the vicinity of
the superfluid to Mott insulator quantum phase transition. We employ the
following methods: exact-diagonalization, mean field (Gutzwiller), cluster
mean-field, and mean-field plus Gaussian fluctuations. In the first part of the
paper we benchmark the four methods by analyzing the equilibrium problem and
give numerical estimates for observables such as the density of double
occupancies and their correlation function. In the second part, we study
parametric ramps from the superfluid to the Mott insulator and map out the
crossover from the regime of fast ramps, which is dominated by local physics,
to the regime of slow ramps with a characteristic universal power law scaling,
which is dominated by long wavelength excitations. We calculate values of
several relevant physical observables, characteristic time scales, and an
optimal protocol needed for observing universal scaling.Comment: 23 pages, 13 figure
Majorana Fermions in Equilibrium and Driven Cold Atom Quantum Wires
We introduce a new approach to create and detect Majorana fermions using
optically trapped 1D fermionic atoms. In our proposed setup, two internal
states of the atoms couple via an optical Raman transition---simultaneously
inducing an effective spin-orbit interaction and magnetic field---while a
background molecular BEC cloud generates s-wave pairing for the atoms. The
resulting cold atom quantum wire supports Majorana fermions at phase boundaries
between topologically trivial and nontrivial regions, as well as `Floquet
Majorana fermions' when the system is periodically driven. We analyze
experimental parameters, detection schemes, and various imperfections.Comment: 4 pages, 3 figures; references adde
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
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