2,715 research outputs found
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]
Solitonic Excitations in Linearly Coherent Channels of Bilayer Quantum Hall Stripes
In some range of interlayer distances, the ground state of the
two-dimensional electron gas at filling factor nu =4N+1 with N=0,1,2,... is a
coherent stripe phase in the Hartree-Fock approximation. This phase has
one-dimensional coherent channels that support charged excitations in the form
of pseudospin solitons. In this work, we compute the transport gap of the
coherent striped phase due to the creation of soliton-antisoliton pairs using a
supercell microscopic unrestricted Hartree-Fock approach. We study this gap as
a function of interlayer distance and tunneling amplitude. Our calculations
confirm that the soliton-antisoliton excitation energy is lower than the
corresponding Hartree-Fock electron-hole pair energy. We compare our results
with estimates of the transport gap obtained from a field-theoretic model valid
in the limit of slowly varying pseudospin textures.Comment: 15 pages, 8 figure
Skyrme and Wigner crystals in graphene
At low-energy, the band structure of graphene can be approximated by two
degenerate valleys about which the electronic spectra of the
valence and conduction bands have linear dispersion relations. An electronic
state in this band spectrum is a linear superposition of states from the
and sublattices of the honeycomb lattice of graphene. In a quantizing
magnetic field, the band spectrum is split into Landau levels with level N=0
having zero weight on the sublattice for the valley.
Treating the valley index as a pseudospin and assuming the real spins to be
fully polarized, we compute the energy of Wigner and Skyrme crystals in the
Hartree-Fock approximation. We show that Skyrme crystals have lower energy than
Wigner crystals \textit{i.e.} crystals with no pseudospin texture in some range
of filling factor around integer fillings. The collective mode spectrum
of the valley-skyrmion crystal has three linearly-dispersing Goldstone modes in
addition to the usual phonon mode while a Wigner crystal has only one extra
Goldstone mode with a quadratic dispersion. We comment on how these modes
should be affected by disorder and how, in principle, a microwave absorption
experiment could distinguish between Wigner and Skyrme crystals.Comment: 14 pages with 11 figure
Dynamics of electrons in the quantum Hall bubble phases
In Landau levels N > 1, the ground state of the two-dimensional electron gas
(2DEG) in a 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 the filling
increases, to a modulated stripe state near half filling. In this work, we show
that these first-order phase transitions between the bubble states lead to
measurable discontinuities in several physical quantities such as the density
of states and the magnetization of the 2DEG. We discuss in detail the behavior
of the collective excitations of the bubble states and show that their spectra
have higher-energy modes besides the pinned phonon mode. The frequencies of
these modes, at small wavevector k, have a discontinuous evolution as a
function of filling factor that should be measurable in, for example, microwave
absorption experiments.Comment: 13 pages, 7 figures. Corrected typos in eqs. (38),(39),(40
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
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
Dynamical matrix of two-dimensional electron crystals
In a quantizing magnetic field, the two-dimensional electron (2DEG) gas has a
rich phase diagram with broken translational symmetry phases such as Wigner,
bubble, and stripe crystals. In this paper, we derive a method to get the
dynamical matrix of these crystals from a calculation of the density response
function performed in the Generalized Random Phase Approximation (GRPA). We
discuss the validity of our method by comparing the dynamical matrix calculated
from the GRPA with that obtained from standard elasticity theory with the
elastic coefficients obtained from a calculation of the deformation energy of
the crystal.Comment: Revised version published in Phys. Rev. B. 12 pages with 11
postscripts figure
Macrodimers: ultralong range Rydberg molecules
We study long range interactions between two Rydberg atoms and predict the
existence of ultralong range Rydberg dimers with equilibrium distances of many
thousand Bohr radii. We calculate the dispersion coefficients ,
and for two rubidium atoms in the same excited level , and find
that they scale like , and , respectively. We show that
for certain molecular symmetries, these coefficients lead to long range
potential wells that can support molecular bound levels. Such macrodimers would
be very sensitive to their environment, and could probe weak interactions. We
suggest experiments to detect these macrodimers.Comment: 4 pages, submitted to PR
Polarization of the Charge-Exchange X-Rays Induced in the Heliosphere
We report results of a theoretical investigation of polarization of the X-ray emissions induced in charge-exchange collisions of fully stripped solar wind ions C6+and O8+ with the heliospheric hydrogen atoms. The polarization of X-ray emissions has been computed for line-of-sight observations within the ecliptic plane as a function of solar wind ion velocities, including a range of velocities corresponding to the slow and fast solar wind, and Coronal Mass Ejections. To determine the variability of polarization of heliospheric X-ray emissions, the polarization has been computed for solar minimum conditions with self-consistent parameters of the solar wind plasma and heliospheric gas and compared with the polarization calculated for an averaged solar activity. We predict the polarization of charge-exchange X-rays to be between 3% and 8%, depending on the line-of-sight geometry, solar wind ion velocity, and the selected emission lines
THz emission from coherently controlled photocurrents in GaAs
We report broadband terahertz radiation from ballistic photocurrents generated via quantum interference of one- and two-photon absorption in low-temperature-grown and semi-insulating GaAs at 295 K. For 90 fs, 1550 and 775 nm optical pulses, we obtain phase-controllable near-single cycle 4 THz radiation. Higher frequency THz emission should be achievable with shorter pulses. At a 250 kHz repetition rate and average powers of 10 mW (1550 nm) and 400 μμW (775 nm), we measure 3 nW of THz power, limited mainly by phase walkoff of the optical beams within the 1.5-μμm-thick sample and collection efficiency. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70293/2/APPLAB-75-25-3959-1.pd
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