3,349 research outputs found
Dynamics of quantum Hall stripes in double-quantum-well systems
The collective modes of stripes in double layer quantum Hall systems are
computed using the time-dependent Hartree-Fock approximation. It is found that,
when the system possesses spontaneous interlayer coherence, there are two
gapless modes, one a phonon associated with broken translational invariance,
the other a pseudospin-wave associated with a broken U(1) symmetry. For large
layer separations the modes disperse weakly for wavevectors perpendicular to
the stripe orientation, indicating the system becomes akin to an array of
weakly coupled one-dimensional XY systems. At higher wavevectors the collective
modes develop a roton minimum associated with a transition out of the coherent
state with further increasing layer separation. A spin wave model of the system
is developed, and it is shown that the collective modes may be described as
those of a system with helimagnetic ordering.Comment: 16 pages including 7 postscript figure
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
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]
Electron-phonon coupling in the C60 fullerene within the many-body GW approach
We study the electron-phonon coupling in the C60 fullerene within the
first-principles GW approach, focusing on the lowest unoccupied t1u three-fold
electronic state which is relevant for the superconducting transition in
electron doped fullerides. It is shown that the strength of the coupling is
significantly enhanced as compared to standard density functional theory
calculations with (semi)local functionals, with a 48% increase of the
electron-phonon potential Vep. The calculated GW value for the contribution
from the Hg modes of 93 meV comes within 4% of the most recent experimental
values. The present results call for a reinvestigation of previous density
functional based calculations of electron-phonon coupling in covalent systems
in general.Comment: 4 pages, 0 figur
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
The velocity dispersion and mass profile of the Milky Way
We re-analyse the velocity-dispersion profile sigma(r) at radii r>10 kpc in
the Galactic stellar halo, recently derived by Battaglia et al. (2005), who
concluded that, for constant velocity anisotropy of the tracers, these data
rule out a flat circular-speed curve for the Milky Way. However, we demonstrate
that if one makes the natural assumption that the tracer density is truncated
at r_t >~ 160 kpc and falls off significantly more steeply than r^-3.5 at r >~
80 kpc, then these data are consistent with a flat circular-speed curve and a
constant velocity anisotropy comparable to that observed for halo stars in the
Solar neighbourhood. We also consider a more detailed mass model with an
exponential stellar disc and an extended non-isothermal dark-matter halo. In
this two-component model, the Milky Way's virial radius and mass are r_vir =
200 kpc and M_vir = 1.5 x 10^12 M_sun. Still assuming the tracers' velocity
anisotropy to be constant (at beta=0.5) we again find good agreement with the
observed sigma(r), so long as the tracer density is truncated near the virial
radius. These data by themselves do not allow to differentiate between
different dark-halo or total-mass models for the Milky Way, nor between
different velocity-anisotropy profiles for the tracers.Comment: 5 pages, 4 figures, accepted for publication in MNRA
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
Long-range potentials and molecular resonances in an ultracold rydberg gas
We have calculated long-range molecular potentials of the ,
and symmetries between highly-excited rubidium atoms. Strong
potentials characterized by these symmetries are important in
describing interaction-induced phenomena in the excitation spectra of high
Rydberg states. Long-range molecular resonances are such phenomena and they
were first reported in S.M. Farooqi {\it et al.}, Phys. Rev. Lett. {\bf 91}
183002. One class of these resonances occurs at energies corresponding to
excited atom pairs . Such resonances are attributed to -mixing
due to Rydberg-Rydberg interactions so that otherwise forbidden molecular
transitions become allowed. We calculate molecular potentials in Hund's case
(c), use them to find the resonance lineshape and compare to experimental
results.Comment: 11 pages, 7 figure
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