12,803 research outputs found
Spectrum of a duality-twisted Ising quantum chain
The Ising quantum chain with a peculiar twisted boundary condition is
considered. This boundary condition, first introduced in the framework of the
spin-1/2 XXZ Heisenberg quantum chain, is related to the duality
transformation, which becomes a symmetry of the model at the critical point.
Thus, at the critical point, the Ising quantum chain with the duality-twisted
boundary is translationally invariant, similar as in the case of the usual
periodic or antiperiodic boundary conditions. The complete energy spectrum of
the Ising quantum chain is calculated analytically for finite systems, and the
conformal properties of the scaling limit are investigated. This provides an
explicit example of a conformal twisted boundary condition and a corresponding
generalised twisted partition function.Comment: LaTeX, 7 pages, using IOP style
Exciting a d-density wave in an optical lattice with driven tunneling
Quantum phases with unusual symmetries may play a key role for the
understanding of solid state systems at low temperatures. We propose a
realistic scenario, well in reach of present experimental techniques, which
should permit to produce a stationary quantum state with -symmetry
in a two-dimensional bosonic optical square lattice. This state, characterized
by alternating rotational flux in each plaquette, arises from driven tunneling
implemented by a stimulated Raman scattering process. We discuss bosons in a
square lattice, however, more complex systems involving other lattice
geometries appear possible.Comment: 4 pages, 3 figure
On Flux Quantization in F-Theory II: Unitary and Symplectic Gauge Groups
We study the quantization of the M-theory G-flux on elliptically fibered
Calabi-Yau fourfolds with singularities giving rise to unitary and symplectic
gauge groups. We seek and find its relation to the Freed-Witten quantization of
worldvolume fluxes on 7-branes in type IIB orientifold compactifications on
Calabi-Yau threefolds. By explicitly constructing the appropriate four-cycles
on which to calculate the periods of the second Chern class of the fourfolds,
we find that there is a half-integral shift in the quantization of G-flux
whenever the corresponding dual 7-brane is wrapped on a non-spin submanifold.
This correspondence of quantizations holds for all unitary and symplectic gauge
groups, except for SU(3), which behaves mysteriously. We also perform our
analysis in the case where, in addition to the aforementioned gauge groups,
there is also a 'flavor' U(1)-gauge group.Comment: 33 pages, 4 figure
Efficient creation of molecules from a cesium Bose-Einstein condensate
We report a new scheme to create weakly bound Cs molecules from an atomic
Bose-Einstein condensate. The method is based on switching the magnetic field
to a narrow Feshbach resonance and yields a high atom-molecule conversion
efficiency of more than 30%, a factor of three higher than obtained with
conventional magnetic-field ramps. The Cs molecules are created in a single
-wave rotational quantum state. The observed dependence of the conversion
efficiency on the magnetic field and atom density shows scattering processes
beyond two-body coupling to occur in the vicinity of the Feshbach resonance.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
The dynamics of condensate shells: collective modes and expansion
We explore the physics of three-dimensional shell-shaped condensates,
relevant to cold atoms in "bubble traps" and to Mott insulator-superfluid
systems in optical lattices. We study the ground state of the condensate
wavefunction, spherically-symmetric collective modes, and expansion properties
of such a shell using a combination of analytical and numerical techniques. We
find two breathing-type modes with frequencies that are distinct from that of
the filled spherical condensate. Upon trap release and subsequent expansion, we
find that the system displays self-interference fringes. We estimate
characteristic time scales, degree of mass accumulation, three-body loss, and
kinetic energy release during expansion for a typical system of Rb87
How Do Quasicrystals Grow?
Using molecular simulations, we show that the aperiodic growth of
quasicrystals is controlled by the ability of the growing quasicrystal
`nucleus' to incorporate kinetically trapped atoms into the solid phase with
minimal rearrangement. In the system under investigation, which forms a
dodecagonal quasicrystal, we show that this process occurs through the
assimilation of stable icosahedral clusters by the growing quasicrystal. Our
results demonstrate how local atomic interactions give rise to the long-range
aperiodicity of quasicrystals.Comment: 4 pages, 4 figures. Figures and text have been updated to the final
version of the articl
Fluxes and Warping for Gauge Couplings in F-theory
We compute flux-dependent corrections in the four-dimensional F-theory
effective action using the M-theory dual description. In M-theory the 7-brane
fluxes are encoded by four-form flux and modify the background geometry and
Kaluza-Klein reduction ansatz. In particular, the flux sources a warp factor
which also depends on the torus directions of the compactification fourfold.
This dependence is crucial in the derivation of the four-dimensional action,
although the torus fiber is auxiliary in F-theory. In M-theory the 7-branes are
described by an infinite array of Taub-NUT spaces. We use the explicit metric
on this geometry to derive the locally corrected warp factor and M-theory
three-from as closed expressions. We focus on contributions to the 7-brane
gauge coupling function from this M-theory back-reaction and show that terms
quadratic in the internal seven-brane flux are induced. The real part of the
gauge coupling function is modified by the M-theory warp factor while the
imaginary part is corrected due to a modified M-theory three-form potential.
The obtained contributions match the known weak string coupling result, but
also yield additional terms suppressed at weak coupling. This shows that the
completion of the M-theory reduction opens the way to compute various
corrections in a genuine F-theory setting away from the weak string coupling
limit.Comment: 46 page
Spin Hall Effect in Atoms
We propose an optical means to realize a spin hall effect (SHE) in neutral
atomic system by coupling the internal spin states of atoms to radiation. The
interaction between the external optical fields and the atoms creates effective
magnetic fields that act in opposite directions on "electrically" neutral atoms
with opposite spin polarizations. This effect leads to a Landau level structure
for each spin orientation in direct analogy with the familiar SHE in
semiconductors. The conservation and topological properties of the spin
current, and the creation of a pure spin current are discussed.Comment: 4 pages, 2 figure; Final versio
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