544 research outputs found
Microwave Absorption of Surface-State Electrons on Liquid He
We have investigated the intersubband transitions of surface state electrons
(SSE) on liquid He induced by microwave radiation at temperatures from 1.1
K down to 0.01 K. Above 0.4 K, the transition linewidth is proportional to the
density of He vapor atoms. This proportionality is explained well by Ando's
theory, in which the linewidth is determined by the electron - vapor atom
scattering. However, the linewidth is larger than the calculation by a factor
of 2.1. This discrepancy strongly suggests that the theory underestimates the
electron - vapor atom scattering rate. At lower temperatures, the absorption
spectrum splits into several peaks. The multiple peak structure is partly
attributed to the spatial inhomogeneity of the static holding electric field
perpendicular to the electron sheet.Comment: 15 pages, 7 figures, submitted to J. Phys. Soc. Jp
Rotation-induced 3D vorticity in 4He superfluid films adsorbed on a porous glass
Detailed study of torsional oscillator experiments under steady rotation up
to 6.28 rad/sec is reported for a 4He superfluid monolayer film formed in 1
micrometer-pore diameter porous glass. We found a new dissipation peak with the
height being in proportion to the rotation speed, which is located to the lower
temperature than the vortex pair unbinding peak observed in the static state.
We propose that 3D coreless vortices ("pore vortices") appear under rotation to
explain this new peak. That is, the new peak originates from dissipation close
to the pore vortex lines, where large superfluid velocity shifts the vortex
pair unbinding dissipation to lower temperature. This explanation is confirmed
by observation of nonlinear effects at high oscillation amplitudes.Comment: 4pages, 5figure
Low-Temperature Mobility of Surface Electrons and Ripplon-Phonon Interaction in Liquid Helium
The low-temperature dc mobility of the two-dimensional electron system
localized above the surface of superfluid helium is determined by the slowest
stage of the longitudinal momentum transfer to the bulk liquid, namely, by the
interaction of surface and volume excitations of liquid helium, which rapidly
decreases with temperature. Thus, the temperature dependence of the
low-frequency mobility is \mu_{dc} = 8.4x10^{-11}n_e T^{-20/3} cm^4 K^{20/3}/(V
s), where n_e is the surface electron density. The relation
T^{20/3}E_\perp^{-3} << 2x10^{-7} between the pressing electric field (in
kV/cm) and temperature (in K) and the value \omega < 10^8 T^5 K^{-5}s^{-1} of
the driving-field frequency have been obtained, at which the above effect can
be observed. In particular, E_\perp = 1 kV/cm corresponds to T < 70 mK and
\omega/2\pi < 30 Hz.Comment: 4 pages, 1 figur
Magneto-shear modes and a.c. dissipation in a two-dimensional Wigner crystal
The a.c. response of an unpinned and finite 2D Wigner crystal to electric
fields at an angular frequency has been calculated in the dissipative
limit, , where is the scattering rate. For
electrons screened by parallel electrodes, in zero magnetic field the
long-wavelength excitations are a diffusive longitudinal transmission line mode
and a diffusive shear mode. A magnetic field couples these modes together to
form two new magneto-shear modes. The dimensionless coupling parameter where and are the
speeds of transverse and longitudinal sound in the collisionless limit and
and are the tensor components of the
magnetoconductivity. For , both the coupled modes contribute
to the response of 2D electrons in a Corbino disk measurement of
magnetoconductivity. For , the electron crystal rotates rigidly in
a magnetic field. In general, both the amplitude and phase of the measured a.c.
currents are changed by the shear modulus. In principle, both the
magnetoconductivity and the shear modulus can be measured simultaneously.Comment: REVTeX, 7 pp., 4 eps figure
Annealing Effect for Supersolid Fraction in He
We report on experimental confirmation of the non-classical rotational
inertia (NCRI) in solid helium samples originally reported by Kim and Chan. The
onset of NCRI was observed at temperatures below ~400 mK. The ac velocity for
initiation of the NCRI suppression is estimated to be ~10 m/sec. After an
additional annealing of the sample at K for 12 hours, ~ 10% relative
increase of NCRI fraction was observed. Then after repeated annealing with the
same conditions, the NCRI fraction was saturated. It differs from Reppy's
observation on a low pressure solid sample.Comment: to be published in J. of Low Temp. Phys. (QFS2006 proceedings
Binding of molecules to DNA and other semiflexible polymers
A theory is presented for the binding of small molecules such as surfactants
to semiflexible polymers. The persistence length is assumed to be large
compared to the monomer size but much smaller than the total chain length. Such
polymers (e.g. DNA) represent an intermediate case between flexible polymers
and stiff, rod-like ones, whose association with small molecules was previously
studied. The chains are not flexible enough to actively participate in the
self-assembly, yet their fluctuations induce long-range attractive interactions
between bound molecules. In cases where the binding significantly affects the
local chain stiffness, those interactions lead to a very sharp, cooperative
association. This scenario is of relevance to the association of DNA with
surfactants and compact proteins such as RecA. External tension exerted on the
chain is found to significantly modify the binding by suppressing the
fluctuation-induced interaction.Comment: 15 pages, 7 figures, RevTex, the published versio
Decay of Superflow Confined in Thin Torus: A Realization of Tunneling Quantum Fields
The quantum nucleation of phase slips in neutral superfluids confined in a
thin torus is investigated by means of the collective coordinate method. We
have devised, with numerical justification, a certain collective coordinate to
describe the quantum nucleation process of a phase slip. Considering the
quantum fluctuation around the local minimum of the action, we calculate the
effective mass of the phase slip. Due to the coherence of the condensate
throughout the torus, the effective mass is proportional to the circumference L
of the torus, and the decay rate has a strong exponential L-dependence.Comment: 4 pages, 2 figures, REVTe
Study of Kosterlitz-Thouless transition of Bose systems governed by a random potential using quantum Monte Carlo simulations
We perform quantum Monte Carlo simulations to study the 2D hard-core
Bose-Hubbard model in a random potential. Our motivation is to investigate the
effects of randomness on the Kosterlitz--Thouless (KT) transition. The chemical
potential is assumed to be random, by site, with a Gaussian distribution. The
KT transition is confirmed by a finite-size analysis of the superfluid density
and the power-law decay of the correlation function. By varying the variance of
the Gaussian distribution, we find that the transition temperature decreases as
the variance increases. We obtain the phase diagram showing the superfluid and
disordered phases, and estimate the quantum critical point (QCP). Our results
on the ground state reveal the existence of the Bose glass phase. Finally, we
discuss what the value of the variance at the QCP indicates from the viewpoint
of percolation.Comment: 7 pages, 9 figures, accepted for publication in JPS
Time-dependent approach to three-body rearrangement collisions: Application to the capture of heavy negatively charged particles by hydrogen atoms
We present a theoretical method for Coulomb three-body rearrangement collisions solving a Chew-Goldberger-type integral equation directly. The scattering boundary condition is automatically satisfied by adiabatically switching on the interaction between the projectile and hydrogen atom. Hence the outgoing wave function is obtained without the tedious procedure of adjusting the total wave function in the asymptotic region. All the dynamical information can be derived from the outgoing wave function obtained on pseudospectral grids numerically. Taking µ−+H(1s) and [overline p]+H(1s) collisions as examples, we demonstrate the usefulness and powerfulness of the method and present the state-specified capture cross sections of heavy negatively charged particles by hydrogen atoms. The convergence and accuracy of the numerical procedure are examined with sufficient care
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