375 research outputs found
Evolution of Fermion Pairing from Three to Two Dimensions
We follow the evolution of fermion pairing in the dimensional crossover from
3D to 2D as a strongly interacting Fermi gas of Li atoms becomes confined
to a stack of two-dimensional layers formed by a one-dimensional optical
lattice. Decreasing the dimensionality leads to the opening of a gap in
radio-frequency spectra, even on the BCS-side of a Feshbach resonance. The
measured binding energy of fermion pairs closely follows the theoretical
two-body binding energy and, in the 2D limit, the zero-temperature mean-field
BEC-BCS theory.Comment: 5 pages, 4 figure
Spin-Injection Spectroscopy of a Spin-Orbit Coupled Fermi Gas
The coupling of the spin of electrons to their motional state lies at the
heart of recently discovered topological phases of matter. Here we create and
detect spin-orbit coupling in an atomic Fermi gas, a highly controllable form
of quantum degenerate matter. We reveal the spin-orbit gap via spin-injection
spectroscopy, which characterizes the energy-momentum dispersion and spin
composition of the quantum states. For energies within the spin-orbit gap, the
system acts as a spin diode. To fully inhibit transport, we open an additional
spin gap, thereby creating a spin-orbit coupled lattice whose spinful band
structure we probe. In the presence of s-wave interactions, such systems should
display induced p-wave pairing, topological superfluidity, and Majorana edge
states
Mott Transition and Spin Structures of Spin-1 Bosons in Two-Dimensional Optical Lattice at Unit Filling
We study the ground state properties of spin-1 bosons in a two-dimensional
optical lattice, by applying a variational Monte Carlo method to the S=1
Bose-Hubbard model on a square lattice at unit filling. A doublon-holon binding
factor introduced in the trial state provides a noticeable improvement in the
variational energy over the conventional Gutzwiller wave function and allows us
to deal effectively with the inter-site correlations of particle densities and
spins. We systematically show how spin-dependent interactions modify the
superfluid-Mott insulator transitions in the S=1 Bose-Hubbard model due to the
interplay between the density and spin fluctuations of bosons. Furthermore,
regarding the magnetic phases in the Mott region, the calculated spin structure
factor elucidates the emergence of nematic and ferromagnetic spin orders for
antiferromagnetic () and ferromagnetic () couplings,
respectively.Comment: 5 pages, 5 figures, to appear in Journal of the Physical Society of
Japa
Single-particle-sensitive imaging of freely propagating ultracold atoms
We present a novel imaging system for ultracold quantum gases in expansion.
After release from a confining potential, atoms fall through a sheet of
resonant excitation laser light and the emitted fluorescence photons are imaged
onto an amplified CCD camera using a high numerical aperture optical system.
The imaging system reaches an extraordinary dynamic range, not attainable with
conventional absorption imaging. We demonstrate single-atom detection for
dilute atomic clouds with high efficiency where at the same time dense
Bose-Einstein condensates can be imaged without saturation or distortion. The
spatial resolution can reach the sampling limit as given by the 8 \mu m pixel
size in object space. Pulsed operation of the detector allows for slice images,
a first step toward a 3D tomography of the measured object. The scheme can
easily be implemented for any atomic species and all optical components are
situated outside the vacuum system. As a first application we perform
thermometry on rubidium Bose-Einstein condensates created on an atom chip.Comment: 24 pages, 10 figures. v2: as publishe
Hybrid 2D surface trap for quantum simulation
We demonstrate a novel optical trapping scheme for ultracold atoms. Using a
combination of evanescent wave, standing wave, and magnetic potentials we
create a deeply 2D Bose-Einstein condensate (BEC) at a few microns from a glass
surface. Using techniques such as broadband "white" light to create evanescent
and standing waves, we realize a smooth potential with a trap frequency aspect
ratio of 300:1:1 and long lifetimes. This makes the setup suitable for
many-body quantum simulations and applications such as high precision
measurements close to surfaces.Comment: 5 pages, 4 figure
Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x
We report a detailed Raman scattering study of the lattice dynamics in
detwinned single crystals of the underdoped high temperature superconductor
YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the
phonon spectra of these compounds are similar to that of optimally doped
YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200
K in underdoped crystals. The temperature dependence of these new features
indicates that they are associated with the incommensurate charge density wave
state recently discovered using synchrotron x-ray scattering techniques on the
same single crystals. Raman scattering has thus the potential to explore the
evolution of this state under extreme conditions.Comment: 12 pages, 11 figure
Applications of Space Mapping Optimization Technology to Filter Design
One of the frontiers that remains in the optimization of large engineering systems is the successful application of optimization
procedures in problems where direct optimization is not practical. The recent exploitation of surrogates in conjunction with “true”
models, the development of artificial neural network approaches to device modeling and the implementation of space mapping are
attempts to address this issue.
Our original “Space Mapping” concept, first conceived in 1993, and the subsequent Aggressive Space Mapping approach to
engineering design optimization will be discussed, along with new variations. Aggressive space mapping optimization closely follows
the traditional experience and intuition of designers. It has been amply demonstrated as a very natural and flexible way of
systematically optimizing microwave filters.
Space mapping optimization intelligently links companion “coarse” and “fine” models of different complexities, e.g., full-wave
electromagnetic simulations and empirical circuit-theory based simulations, to accelerate iterative design optimization of engineering
structures. New trust region space mapping optimization algorithms will be mentioned.
We briefly review the Expanded Space Mapping Design Framework (ESMDF) concept in which we allow preassigned parameters, not
used in optimization, to change in some components of the coarse model. Other recent developments include the introduction of the
object oriented SMX system to facilitate implementation of our algorithms in conjunction with certain commercial simulators.
Extensive filter design examples complement the presentation.ITESO, A.C
Topological superfluid of spinless Fermi gases in p-band honeycomb optical lattices with on-site rotation
In this paper, we put forward to another route realizing topological
superfluid (TS). In contrast to conventional method, spin-orbit coupling and
external magnetic field are not requisite. Introducing an experimentally
feasible technique called on-site rotation (OSR) into p-band honeycomb optical
lattices for spinless Fermi gases and considering CDW and pairing on the same
footing, we investigate the effects of OSR on superfluidity. The results
suggest that when OSR is beyond a critical value, where CDW vanishes, the
system transits from a normal superfluid (NS) with zero TKNN number to TS
labeled by a non-zero TKNN number. In addition, phase transitions between
different TS are also possible
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