112 research outputs found
Phase sensitive measurements of order parameters for ultracold atoms through two particles interferometry
Nontrivial symmetry of order parameters is crucial in some of the most
interesting quantum many-body states of ultracold atoms and condensed matter
systems. Examples in cold atoms include p-wave Feshbach molecules and d-wave
paired states of fermions that could be realized in optical lattices in the
Hubbard regime. Identifying these states in experiments requires measurements
of the relative phase of different components of the entangled pair
wavefunction.
We propose and discuss two schemes for such phase sensitive measurements,
based on two-particle interference revealed in atom-atom or atomic density
correlations. Our schemes can also be used for relative phase measurements for
non-trivial particle-hole order parameters, such as d-density wave order.Comment: 4 pages, 4 figure
Antiferromagnetic noise correlations in optical lattices
We analyze how noise correlations probed by time-of-flight (TOF) experiments
reveal antiferromagnetic (AF) correlations of fermionic atoms in
two-dimensional (2D) and three-dimensional (3D) optical lattices. Combining
analytical and quantum Monte Carlo (QMC) calculations using experimentally
realistic parameters, we show that AF correlations can be detected for
temperatures above and below the critical temperature for AF ordering. It is
demonstrated that spin-resolved noise correlations yield important information
about the spin ordering. Finally, we show how to extract the spin correlation
length and the related critical exponent of the AF transition from the noise.Comment: 4 pages, 4 figure
Few-Body Bound Complexes in One-dimensional Dipolar Gases and Non-Destructive Optical Detection
We consider dipolar interactions between heteronuclear molecules in
low-dimensional geometries. The setup consists of two one-dimensional tubes. We
study the stability of possible few-body complexes in the regime of repulsive
intratube interaction, where the binding arises from intertube attraction. The
stable dimers, trimers, and tetramers are found and we discuss their properties
for both bosonic and fermionic molecules. To observe these complexes we propose
an optical non-destructive detection scheme that enables in-situ observation of
the creation and dissociation of the few-body complexes. A detailed description
of the expected signal of such measurements is given using the numerically
calculated wave functions of the bound states. We also discuss implications on
the many-body physics of dipolar systems in tubular geometries, as well as
experimental issues related to the external harmonic confinement along the tube
and the prospect of applying an in-tube optical lattice to increase the
effective dipole strength.Comment: 16 pages, 15 figures, published versio
Mapping of Coulomb gases and sine-Gordon models to statistics of random surfaces
We introduce a new class of sine-Gordon models, for which interaction term is
present in a region different from the domain over which quadratic part is
defined. We develop a novel non-perturbative approach for calculating partition
functions of such models, which relies on mapping them to statistical
properties of random surfaces. As a specific application of our method, we
consider the problem of calculating the amplitude of interference fringes in
experiments with two independent low dimensional Bose gases. We calculate full
distribution functions of interference amplitude for 1D and 2D gases with
nonzero temperatures.Comment: final published versio
Anisotropic transport for FQH state at intermediate magnetic field
The state is spin-unpolarized at weak magnetic field and fully
polarized at strong field. At intermediate field, a plateau of half the maximal
polarization is observed. We study this phenomenon in the frame of composite
fermion theory. Due to the mixing of the composite fermion Landau levels, the
unidirectional charge/spin density wave state of composite fermions is lower in
energy than the Wigner crystal. It means that transport anisotropy, similar to
those for electrons in higher Landau levels at half fillings, may take place at
this fractional quantum Hall state when the external magnetic field is in an
appropriate range. When the magnetic field is tilted an angle, the easy
transport direction is perpendicular to the direction of the in-plane field.
Varying the partial filling factor of composite fermion Landau level from 0 to
1, we find that the energy minimum occurs in the vicinity of one-half.Comment: 2 figure
Superfluid-insulator transition in Fermi-Bose mixtures and the orthogonality catastrophe
The superfluid-insulator transition of bosons is strongly modified by the
presence of Fermions. Through an imaginary-time path integral approach, we
derive the self-consistent mean-field transition line, and account for both the
static and the dynamic screening effects of the fermions. We find that an
effect akin to the fermionic orthogonality catastrophe, arising from the
fermionic screening fluctuations, suppresses superfluidity. We analyze this
effect for various mixture parameters and temperatures, and consider possible
signatures of the orthogonality catastrophe effect in other measurables of the
mixture.Comment: 11 pages, 4 figure
Phase diagram of two-component bosons on an optical lattice
We present a theoretical analysis of the phase diagram of two--component
bosons on an optical lattice. A new formalism is developed which treats the
effective spin interactions in the Mott and superfluid phases on the same
footing. Using the new approach we chart the phase boundaries of the broken
spin symmetry states up to the Mott to superfluid transition and beyond. Near
the transition point, the magnitude of spin exchange can be very large, which
facilitates the experimental realization of spin-ordered states. We find that
spin and quantum fluctuations have a dramatic effect on the transition making
it first order in extended regions of the phase diagram. For Mott states with
even occupation we find that the competition between effective Heisenberg
exchange and spin-dependent on--site interaction leads to an additional phase
transition from a Mott insulator with no broken symmetries into a spin-ordered
insulator
Competing orders in a magnetic field: spin and charge order in the cuprate superconductors
We describe two-dimensional quantum spin fluctuations in a superconducting
Abrikosov flux lattice induced by a magnetic field applied to a doped Mott
insulator. Complete numerical solutions of a self-consistent large N theory
provide detailed information on the phase diagram and on the spatial structure
of the dynamic spin spectrum. Our results apply to phases with and without
long-range spin density wave order and to the magnetic quantum critical point
separating these phases. We discuss the relationship of our results to a number
of recent neutron scattering measurements on the cuprate superconductors in the
presence of an applied field. We compute the pinning of static charge order by
the vortex cores in the `spin gap' phase where the spin order remains
dynamically fluctuating, and argue that these results apply to recent scanning
tunnelling microscopy (STM) measurements. We show that with a single typical
set of values for the coupling constants, our model describes the field
dependence of the elastic neutron scattering intensities, the absence of
satellite Bragg peaks associated with the vortex lattice in existing neutron
scattering observations, and the spatial extent of charge order in STM
observations. We mention implications of our theory for NMR experiments. We
also present a theoretical discussion of more exotic states that can be built
out of the spin and charge order parameters, including spin nematics and phases
with `exciton fractionalization'.Comment: 36 pages, 33 figures; for a popular introduction, see
http://onsager.physics.yale.edu/superflow.html; (v2) Added reference to new
work of Chen and Ting; (v3) reorganized presentation for improved clarity,
and added new appendix on microscopic origin; (v4) final published version
with minor change
Spin and charge order in the vortex lattice of the cuprates: experiment and theory
I summarize recent results, obtained with E. Demler, K. Park, A. Polkovnikov,
M. Vojta, and Y. Zhang, on spin and charge correlations near a magnetic quantum
phase transition in the cuprates. STM experiments on slightly overdoped BSCCO
(J.E. Hoffman et al., Science 295, 466 (2002)) are consistent with the
nucleation of static charge order coexisting with dynamic spin correlations
around vortices, and neutron scattering experiments have measured the magnetic
field dependence of static spin order in the underdoped regime in LSCO (B. Lake
et al., Nature 415, 299 (2002)) and LaCuO_4+y (B. Khaykovich et al., Phys. Rev.
B 66, 014528 (2002)). Our predictions provide a semi-quantitative description
of these observations, with only a single parameter measuring distance from the
quantum critical point changing with doping level. These results suggest that a
common theory of competing spin, charge and superconducting orders provides a
unified description of all the cuprates.Comment: 18 pages, 7 figures; Proceedings of the Mexican Meeting on
Mathematical and Experimental Physics, Mexico City, September 2001, to be
published by Kluwer Academic/Plenum Press; (v2) added clarifications and
updated reference
Planar SFS Josephson Junctions Made by Focused Ion Beam Etching
Superconductor-Ferromagnet-Superconductor (S-F-S) Josephson junctions were
fabricated by making a narrow cut through a S-F double layer using direct
writing by Focused Ion Beam (FIB). Due to a high resolution (spot size smaller
than 10 nm) of FIB, junctions with a small separation between superconducting
electrodes ( 30 nm) can be made. Such a short distance is sufficient for
achieving a considerable proximity coupling through a diluted CuNi ferromagnet.
We have successfully fabricated and studied S-F-S (Nb-CuNi-Nb) and S-S'-S
(Nb-Nb/CuNi-Nb) junctions. Junctions exhibit clear Fraunhofer modulation of the
critical current as a function of magnetic field, indicating good uniformity of
the cut. By changing the depth of the cut, junctions with the product
ranging from 0.5 mV to V were fabricated.Comment: 5 pages, 5 figures, presentation at EUCAS-2003, to be published in
Physica
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