3,152 research outputs found
Composite Fermions in Modulated Structures: Transport and Surface Acoustic Waves
Motivated by a recent experiment of Willett et al. [Phys. Rev. Lett. 78, 4478
(1997)], we employ semiclassical composite-fermion theory to study the effect
of a periodic density modulation on a quantum Hall system near Landau level
filling factor nu=1/2. We show that even a weak density modulation leads to
dramatic changes in surface-acoustic-wave (SAW) propagation, and propose an
explanation for several key features of the experimental observations. We
predict that properly arranged dc transport measurements would show a structure
similar to that seen in SAW measurements.Comment: Version published in Phys. Rev. Lett. Figures changed to show SAW
velocity shift. LaTeX, 5 pages, two included postscript figure
Magneto-Acoustic Spectroscopy in Superfluid 3He-B
We have used the recently discovered acoustic Faraday effect in superfluid
3He to perform high resolution spectroscopy of an excited state of the
superfluid condensate. With acoustic cavity interferometry we measure the
rotation of the plane of polarization of a transverse sound wave propagating in
the direction of magnetic field from which we determine the Zeeman energy of
the excited state. We interpret the Lande g-factor, combined with the
zero-field energies of the state, using the theory of Sauls and Serene to
calculate the strength of f -wave interactions in 3He.Comment: 4 pages, 5 figures, submitted to PRL, Aug 30th, 200
Absorption/Expulsion of Oligomers and Linear Macromolecules in a Polymer Brush
The absorption of free linear chains in a polymer brush was studied with
respect to chain size and compatibility with the brush by means of
Monte Carlo (MC) simulations and Density Functional Theory (DFT) /
Self-Consistent Field Theory (SCFT) at both moderate, , and
high, , grafting densities using a bead-spring model.
Different concentrations of the free chains are
examined. Contrary to the case of when all species are almost
completely ejected by the polymer brush irrespective of their length , for
we find that the degree of absorption (absorbed amount)
undergoes a sharp crossover from weak to strong () absorption,
discriminating between oligomers, , and longer chains. For a
moderately dense brush, , the longer species, ,
populate predominantly the deep inner part of the brush whereas in a dense
brush they penetrate into the "fluffy" tail of the dense
brush only. Gyration radius and end-to-end distance of absorbed
chains thereby scale with length as free polymers in the bulk. Using both
MC and DFT/SCFT methods for brushes of different chain length , we demonstrate the existence of unique {\em critical} value of
compatibility . For the energy of free
chains attains the {\em same} value, irrespective of length whereas the
entropy of free chain displays a pronounced minimum. At all density
profiles of absorbing chains with different intersect at the same distance
from the grafting plane. The penetration/expulsion kinetics of free chains into
the polymer brush after an instantaneous change in their compatibility
displays a rather rich behavior. We find three distinct regimes of penetration
kinetics of free chains regarding the length : I (), II (), and III (), in which the time of absorption grows with
at a different rate. During the initial stages of penetration into the
brush one observes a power-law increase of with power
whereby penetration of the free chains into the
brush gets {\em slower} as their concentration rises
Composite Fermions with Orbital Magnetization
For quantum Hall systems, in the limit of large magnetic field (or
equivalently small electron band mass ), the static response of electrons
to a spatially varying magnetic field is largely determined by kinetic energy
considerations. This response is not correctly given in existing approximations
based on the Fermion Chern-Simons theory of the partially filled Landau level.
We remedy this problem by attaching an orbital magnetization to each fermion to
separate the current into magnetization and transport contributions, associated
with the cyclotron and guiding center motions respectively. This leads to a
Chern-Simons Fermi liquid description of the state which
correctly predicts the dependence of the static and dynamic response in
the limit .Comment: 4 pages, RevTeX, no figure
The half-filled Landau level - composite fermions and dipoles
The composite-fermion approach as formulated in the fermion Chern-Simons
theory has been very successful in describing the physics of the lowest Landau
level near Landau level filling factor 1/2. Recent work has emphasized the fact
that the true quasiparticles at these filling factors are electrically neutral
and carry an electric dipole moment. In a previous work, we discussed at length
two formulations in terms of dipolar quasiparticles. Here we briefly review one
approach - termed electron-centered quasiparticles - and show how it can be
extended from 1/2 to nearby filling factors where the quasiparticles carry both
an electric dipole moment and an overall charge.Comment: 10 pages, minor improvements of notation and referencin
Theory of Incompressible States in a Narrow Channel
We report on the properties of a system of interacting electrons in a narrow
channel in the quantum Hall effect regime. It is shown that an increase in the
strength of the Coulomb interaction causes abrupt changes in the width of the
charge-density profile of translationally invariant states. We derive a phase
diagram which includes many of the stable odd-denominator states as well as a
novel fractional quantum Hall state at lowest half-filled Landau level. The
collective mode evaluated at the half-filled case is strikingly similar to that
for an odd-denominator fractional quantum Hall state.Comment: 4 pages, REVTEX, and 4 .ps file
A new broken U(1)-symmetry in extreme type-II superconductors
A phase transition within the molten phase of the Abrikosov vortex system
without disorder in extreme type-II superconductors is found via large-scale
Monte-Carlo simulations. It involves breaking a U(1)-symmetry, and has a
zero-field counterpart, unlike vortex lattice melting. Its hallmark is the loss
of number-conservation of connected vortex paths threading the entire system
{\it in any direction}, driving the vortex line tension to zero. This tension
plays the role of a generalized ``stiffness'' of the vortex liquid, and serves
as a probe of the loss of order at the transition, where a weak specific heat
anomaly is found.Comment: 5 pages, 3 figure
Spontaneous Fluxon Production in Annular Josephson Tunnel Junctions in the Presence of a Magnetic Field
We report on the spontaneous production of fluxons in the presence of a
symmetry-breaking magnetic field for annular Josephson tunnel junctions during
a thermal quench. The dependence on field intensity of the probability
to trap a single defect during the N-S phase transition drastically
depends on the sample circumferences. We show that the data can be understood
in the framework of the Kibble-Zurek picture of spontaneous defect formation
controlled by causal bounds.Comment: Submitted to Phys. Rev. B with 5 figures on Nov. 15, 200
Free Energies of Isolated 5- and 7-fold Disclinations in Hexatic Membranes
We examine the shapes and energies of 5- and 7-fold disclinations in
low-temperature hexatic membranes. These defects buckle at different values of
the ratio of the bending rigidity, , to the hexatic stiffness constant,
, suggesting {\em two} distinct Kosterlitz-Thouless defect proliferation
temperatures. Seven-fold disclinations are studied in detail numerically for
arbitrary . We argue that thermal fluctuations always drive
into an ``unbuckled'' regime at long wavelengths, so that
disclinations should, in fact, proliferate at the {\em same} critical
temperature. We show analytically that both types of defects have power law
shapes with continuously variable exponents in the ``unbuckled'' regime.
Thermal fluctuations then lock in specific power laws at long wavelengths,
which we calculate for 5- and 7-fold defects at low temperatures.Comment: LaTeX format. 17 pages. To appear in Phys. Rev.
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