2,484 research outputs found
Characterization of the Torsional Piezoelectric-like Response of Tantalum Trisulfide Associated with Charge-Density-Wave Depinning
We have studied the frequency and voltage dependence of voltage-induced
torsional strains in orthorhombic TaS3 [V. Ya. Pokrovskii, et al, Phys. Rev.
Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency
of an RF cavity containing the sample. The strain has an onset voltage below
the charge-density-wave (CDW) threshold voltages associated with changes in
shear compliance and resistance, suggesting that the strain is associated with
polarization of the CDW rather than CDW current. Measurements with square-wave
voltages show that the strain is very sluggish, not even reaching its dc value
at a frequency of 0.1 Hz, but the dynamics appear to be very sample dependent.
By applying oscillating torque while biasing the sample with a dc current, we
have also looked for strain induced voltage in the sample; none is observed at
the low biases where the voltage-induced strains first occur, but an induced
voltage is observed at higher biases, probably associated with strain-dependent
CDW conductance.Comment: 11 pages, including 3 figures, to be published in Phys. Rev. B (Rapid
Comm.
Dynamics of Charge Flow in the Channel of a Thin-Film Field-Effect Transistor
The local conductivity in the channel of a thin-film field-effect transistor
is proportional to the charge density induced by the local gate voltage. We
show how this determines the frequency- and position-dependence of the charge
induced in the channel for the case of "zero applied current": zero
drain-source voltage with charge induced by a square-wave voltage applied to
the gate, assuming constant mobility and negligible contact impedances. An
approximate expression for the frequency dependence of the induced charge in
the center of the channel can be conveniently used to determine the charge
mobility. Fits of electro-optic measurements of the induced charge in organic
transistors are used as examples.Comment: 9 pages including table + 3 figures; submitted to Jnl. Appl. Phy
Initial Data for General Relativity with Toroidal Conformal Symmetry
A new class of time-symmetric solutions to the initial value constraints of
vacuum General Relativity is introduced. These data are globally regular,
asymptotically flat (with possibly several asymptotic ends) and in general have
no isometries, but a group of conformal isometries. After
decomposing the Lichnerowicz conformal factor in a double Fourier series on the
group orbits, the solutions are given in terms of a countable family of
uncoupled ODEs on the orbit space.Comment: REVTEX, 9 pages, ESI Preprint 12
Fate of Kaluza-Klein Bubble
We numerically study classical time evolutions of Kaluza-Klein bubble
space-time which has negative energy after a decay of vacuum. As the zero
energy Witten's bubble space-time, where the bubble expands infinitely, the
subsequent evolutions of Brill and Horowitz's momentarily static initial data
show that the bubble will expand in terms of the area. At first glance, this
result may support Corley and Jacobson's conjecture that the bubble will expand
forever as well as the Witten's bubble. The irregular signatures, however, can
be seen in the behavior of the lapse function in the maximal slicing gauge and
the divergence of the Kretchman invariant. Since there is no appearance of the
apparent horizon, we suspect an appearance of a naked singularity as the final
fate of this space-time.Comment: 13 pages including 10 figures, RevTeX, epsf.sty. CGPG-99/12-8,
RESCEU-6/00 and DAMTP-2000-30. To appear in Phys. Rev.
Evaluation of the synoptic and mesoscale predictive capabilities of a mesoscale atmospheric simulation system
The overall performance characteristics of a limited area, hydrostatic, fine (52 km) mesh, primitive equation, numerical weather prediction model are determined in anticipation of satellite data assimilations with the model. The synoptic and mesoscale predictive capabilities of version 2.0 of this model, the Mesoscale Atmospheric Simulation System (MASS 2.0), were evaluated. The two part study is based on a sample of approximately thirty 12h and 24h forecasts of atmospheric flow patterns during spring and early summer. The synoptic scale evaluation results benchmark the performance of MASS 2.0 against that of an operational, synoptic scale weather prediction model, the Limited area Fine Mesh (LFM). The large sample allows for the calculation of statistically significant measures of forecast accuracy and the determination of systematic model errors. The synoptic scale benchmark is required before unsmoothed mesoscale forecast fields can be seriously considered
Holographic fermions in external magnetic fields
We study the Fermi level structure of 2+1-dimensional strongly interacting
electron systems in external magnetic field using the AdS/CFT correspondence.
The gravity dual of a finite density fermion system is a Dirac field in the
background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit
the magnetic system can be reduced to the non-magnetic one, with
Landau-quantized momenta and rescaled thermodynamical variables. We find that
at strong enough magnetic fields, the Fermi surface vanishes and the
quasiparticle is lost either through a crossover to conformal regime or through
a phase transition to an unstable Fermi surface. In the latter case, the
vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi
liquid regime with unstable quasiparticles and a change in transport properties
of the system. We associate it with a metal-"strange metal" phase transition.
Next we compute the DC Hall and longitudinal conductivities using the
gravity-dressed fermion propagators. For dual fermions with a large charge,
many different Fermi surfaces contribute and the Hall conductivity is quantized
as expected for integer Quantum Hall Effect (QHE). At strong magnetic fields,
as additional Fermi surfaces open up, new plateaus typical for the fractional
QHE appear. The somewhat irregular pattern in the length of fractional QHE
plateaus resemble the outcomes of experiments on thin graphite in a strong
magnetic field. Finally, motivated by the absence of the sign problem in
holography, we suggest a lattice approach to the AdS calculations of finite
density systems.Comment: 34 pages, 14 figure
Coupling of the lattice and superlattice deformations and hysteresis in thermal expansion for the quasi one-dimensional conductor TaS
An original interferometer-based setup for measurements of length of
needle-like samples is developed, and thermal expansion of o-TaS crystals
is studied. Below the Peierls transition the temperature hysteresis of length
is observed, the width of the hysteresis loop being up to . The behavior of the loop is anomalous: the length changes so
that it is in front of its equilibrium value. The hysteresis loop couples with
that of conductivity. The sign and the value of the length hysteresis are
consistent with the strain dependence of the charge-density waves (CDW) wave
vector. With lowering temperature down to 100 K the CDW elastic modulus grows
achieving a value comparable with the lattice Young modulus. Our results could
be helpful in consideration of different systems with intrinsic
superstructures.Comment: 4 pages, 3 figures. Phys. Rev. Lett., accepted for publicatio
Gravitational Geons Revisited
A careful analysis of the gravitational geon solution found by Brill and
Hartle is made. The gravitational wave expansion they used is shown to be
consistent and to result in a gauge invariant wave equation. It also results in
a gauge invariant effective stress-energy tensor for the gravitational waves
provided that a generalized definition of a gauge transformation is used. To
leading order this gauge transformation is the same as the usual one for
gravitational waves. It is shown that the geon solution is a self-consistent
solution to Einstein's equations and that, to leading order, the equations
describing the geometry of the gravitational geon are identical to those
derived by Wheeler for the electromagnetic geon. An appendix provides an
existence proof for geon solutions to these equations.Comment: 18 pages, ReVTeX. To appear in Physical Review D. Significant changes
include more details in the derivations of certain key equations and the
addition of an appendix containing a proof of the existence of a geon
solution to the equations derived by Wheeler. Also a reference has been added
and various minor changes have been mad
Charge Order Superstructure with Integer Iron Valence in Fe2OBO3
Solution-grown single crystals of Fe2OBO3 were characterized by specific
heat, Mossbauer spectroscopy, and x-ray diffraction. A peak in the specific
heat at 340 K indicates the onset of charge order. Evidence for a doubling of
the unit cell at low temperature is presented. Combining structural refinement
of diffraction data and Mossbauer spectra, domains with diagonal charge order
are established. Bond-valence-sum analysis indicates integer valence states of
the Fe ions in the charge ordered phase, suggesting Fe2OBO3 is the clearest
example of ionic charge order so far.Comment: 4 pages, 5 figures. Fig. 3 is available in higher resolution from the
authors. PRL in prin
Dynamics of the hysteretic voltage-induced torsional strain in tantalum trisulfide
We have studied how the hysteretic voltage-induced torsional strain,
associated with charge-density-wave depinning, in orthorhombic tantalum
trisulfide depends on square-wave and triangle-wave voltages of different
frequencies and amplitudes. The strains are measured by placing the sample,
with a wire glued to the center as a transducer, in a radio frequency cavity
and measuring the modulated response of the cavity. From the triangle waves, we
map out the time dependence of the hysteresis loops, and find that the
hysteresis loops broaden for waves with periods less than 30 seconds. The
square-wave response shows that the dynamic response to positive and negative
voltages can be quite different. The overall frequency dependence is
relaxational, but with multiple relaxation times which typically decrease with
increasing voltage. The detailed dynamic response is very sample dependent,
suggesting that it depends in detail on interactions of the CDW with sample
defects.Comment: 13 pages, 6 figures, to be published in Journal of Physics: Cond.
Mat
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