4,519 research outputs found
Introduction: The history and scope of the sociology of higher education
No abstract available
Replica study of pinned bubble crystals
In higher Landau levels (), the ground state of the two-dimensional
electron gas in a strong perpendicular magnetic field evolves from a Wigner
crystal for small filling of the partially filled Landau level, into a
succession of bubble states with increasing number of guiding centers per
bubble as increases, to a modulated stripe state near . In
this work, we compute the frequency-dependent longitudinal conductivity of the Wigner and bubble crystal states in the presence
of disorder. We apply an elastic theory to the crystal states which is
characterized by a shear and a bulk modulus. We obtain both moduli from the
microscopic time-dependent Hartree-Fock approximation. We then use the replica
and Gaussian variational methods to handle the effects of disorder. Within the
semiclassical approximation we get the dynamical conductivity as well as the
pinning frequency as functions of the Landau level filling factor and compare
our results with recent microwave experiments.Comment: 19 pages and 6 eps figure
Skyrme Crystal In A Two-Dimensional Electron Gas
The ground state of a two-dimensional electron gas at Landau level filling
factors near is a Skyrme crystal with long range order in the
positions and orientations of the topologically and electrically charged
elementary excitations of the ferromagnetic ground state. The lowest
energy Skyrme crystal is a square lattice with opposing postures for
topological excitations on opposite sublattices. The filling factor dependence
of the electron spin-polarization, calculated for the square lattice Skyrme
crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from
[email protected]
Controllable binding of polar molecules and meta-stability of 1-D gases with attractive dipole forces
We explore one-dimensional (1-D) samples of ultracold polar molecules with
attractive dipole-dipole interactions and show the existence of a repulsive
barrier due to a strong quadrupole interaction between molecules. This barrier
can stabilize a gas of ultracold KRb molecules and even lead to long-range
wells supporting bound states between molecules. The properties of these wells
can be controlled by external electric fields, allowing the formation of long
polymer-like chains of KRb, and studies of quantum phase transitions by varying
the effective interaction between molecules. We discuss the generalization of
those results to other systems
Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems
In higher Landau levels (N>0) and around filling factors nu =4N+1, a
two-dimensional electron gas in a double-quantum-well system supports a stripe
groundstate in which the electron density in each well is spatially modulated.
When a parallel magnetic field is added in the plane of the wells, tunneling
between the wells acts as a spatially rotating effective Zeeman field coupled
to the ``pseudospins'' describing the well index of the electron states. For
small parallel fields, these pseudospins follow this rotation, but at larger
fields they do not, and a commensurate-incommensurate transition results.
Working in the Hartree-Fock approximation, we show that the combination of
stripes and commensuration in this system leads to a very rich phase diagram.
The parallel magnetic field is responsible for oscillations in the tunneling
matrix element that induce a complex sequence of transitions between
commensurate and incommensurate liquid or stripe states. The homogeneous and
stripe states we find can be distinguished by their collective excitations and
tunneling I-V, which we compute within the time-dependent Hartree-Fock
approximation.Comment: 23 pages including 8 eps figure
Orbital order in bilayer graphene at filling factor
In a graphene bilayer with Bernal stacking both and orbital
Landau levels have zero kinetic energy. An electronic state in the N=0 Landau
level consequently has three quantum numbers in addition to its guiding center
label: its spin, its valley index or , and an orbital quantum
number The two-dimensional electron gas (2DEG) in the bilayer supports
a wide variety of broken-symmetry states in which the pseudospins associated
these three quantum numbers order in a manner that is dependent on both filling
factor and the electric potential difference between the layers. In this
paper, we study the case of in an external field strong enough to
freeze electronic spins. We show that an electric potential difference between
layers drives a series of transitions, starting from interlayer-coherent states
(ICS) at small potentials and leading to orbitally coherent states (OCS) that
are polarized in a single layer. Orbital pseudospins carry electric dipoles
with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya
interactions that can lead to spiral instabilities. We show that the microwave
absorption spectra of ICSs, OCSs, and the mixed states that occur at
intermediate potentials are sharply distinct.Comment: 21 pages, 14 figure
Collective Modes of Quantum Hall Stripes
The collective modes of striped phases in a quantum Hall system are computed
using the time-dependent Hartree-Fock approximation. Uniform stripe phases are
shown to be unstable to the formation of modulations along the stripes, so that
within the Hartree-Fock approximation the groundstate is a stripe crystal. Such
crystalline states are generically gapped at any finite wavevector; however, in
the quantum Hall system the interactions of modulations among different stripes
is found to be remarkably weak, leading to an infinite collection of collective
modes with immeasurably small gaps. The resulting long wavelength behavior is
derivable from an elastic theory for smectic liquid crystals. Collective modes
for the phonon branch are computed throughout the Brillouin zone, as are spin
wave and magnetoplasmon modes. A soft mode in the phonon spectrum is identified
for partial filling factors sufficiently far from 1/2, indicating a second
order phase transition. The modes contain several other signatures that should
be experimentally observable.Comment: 36 pages LaTex with 11 postscript figures. Short animations of the
collective modes can be found at
http://www.physique.usherb.ca/~rcote/stripes/stripes.ht
Radiative charge transfer lifetime of the excited state of (NaCa)
New experiments were proposed recently to investigate the regime of cold
atomic and molecular ion-atom collision processes in a special hybrid
neutral-atom--ion trap under high vacuum conditions. The collisional cooling of
laser pre-cooled Ca ions by ultracold Na atoms is being studied. Modeling
this process requires knowledge of the radiative lifetime of the excited
singlet A state of the (NaCa) molecular system. We calculate
the rate coefficient for radiative charge transfer using a semiclassical
approach. The dipole radial matrix elements between the ground and the excited
states, and the potential curves were calculated using Complete Active Space
Self-Consistent field and M\"oller-Plesset second order perturbation theory
(CASSCF/MP2) with an extended Gaussian basis, 6-311+G(3df). The semiclassical
charge transfer rate coefficient was averaged over a thermal Maxwellian
distribution. In addition we also present elastic collision cross sections and
the spin-exchange cross section. The rate coefficient for charge transfer was
found to be cm/sec, while those for the elastic and
spin-exchange cross sections were found to be several orders of magnitude
higher ( cm/sec and cm/sec,
respectively). This confirms our assumption that the milli-Kelvin regime of
collisional cooling of calcium ions by sodium atoms is favorable with the
respect to low loss of calcium ions due to the charge transfer.Comment: 4 pages, 5 figures; v.2 - conceptual change
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