296 research outputs found
The Inverse Problem of Analog Gravity Systems
Analog gravity models of black holes and exotic compact objects provide a
unique opportunity to study key properties of such systems in controlled
laboratory environments. In contrast to astrophysical systems, analog gravity
systems can be prepared carefully and their dynamical aspects thus investigated
in unprecedented ways. While gravitational wave scattering properties of
astrophysical compact objects are more connected to quasi-normal modes,
laboratory experiments can also access the transmission and reflection
coefficients, which are otherwise mostly relevant for Hawking radiation related
phenomena. In this work, we report two distinct results. First, we outline a
semi-classical, non-parametric method that allows for the reconstruction of the
effective perturbation potential from the knowledge of transmission and
reflection coefficients for certain types of potentials in the Schr\"odinger
wave equation admitting resonant tunneling. Second, we show how to use our
method by applying it to an imperfect draining vortex, which has been suggested
as analog of extreme compact objects. Although the inverse problem is in
general not unique, choosing physically motivated assumptions and requiring the
validity of semi-classical theory, we demonstrate that the method provides
efficient and accurate results.Comment: 11 pages, 7 figure
Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets
In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are
non-planar vortices which exhibit a localized structure of the z-components of
the spins around the vortex center. We study how thermal noise induces a
transition of this structure from one polarization to the opposite one. We
describe the vortex core by a discrete Hamiltonian and consider a stationary
solution of the Fokker-Planck equation. We find a bimodal distribution function
and calculate the transition rate using Langer's instanton theory (1969). The
result is compared with Langevin dynamics simulations for the full many-spin
model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres
Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)
The transport properties of high-performance thin-film transistors (TFT) made
with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The
room-temperature field-effect mobility of the devices varied between 0.004
cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the
material, which modified the structural order. The transport properties of TFTs
were studied as a function of temperature. The field-effect mobility is
thermally activated in all films at T<200 K and the activation energy depends
on the charge density in the channel. The experimental data is compared to
theoretical models for transport, and we argue that a model based on the
existence of a mobility edge and an exponential distribution of traps provides
the best interpretation of the data. The differences in room-temperature
mobility are attributed to different widths of the shallow localized state
distribution at the edge of the valence band due to structural disorder in the
film. The free carrier mobility of the mobile states in the ordered regions of
the film is the same in all structural modifications and is estimated to be
between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure
Cloning, expression and functional characterization of the full-length murine ADAMTS13
Functional deficiency or absence of the human von Willebrand factor (VWF)-cleaving protease (VWF-cp), recently termed ADAMTS13, has been shown to cause acquired and congenital thrombotic thrombocytopenic purpura (TTP), respectively. As a first step towards developing a small animal model of TTP, we have cloned the complete (non-truncated) murine Adamts13 gene from BALB/c mice liver poly A + mRNA. Murine ADAMTS13 is a 1426-amino-acid protein with a high homology and similar structural organization to the human ortholog. Transient expression of the murine Adamts13 cDNA in HEK 293 cells yielded a protein with a molecular weight of approximately 180 kDa which degraded recombinant murine VWF (rVWF) in a dose-dependent manner. The cleavage products of murine rVWF had the expected size of 140 and 170 kDa. Murine ADAMTS13 was inhibited by EDTA and the plasma from a TTP patient.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73592/1/j.1538-7836.2005.01246.x.pd
Evidence for Different Freeze-Out Radii of High- and Low-Energy Pions Emitted in Au+Au Collisions at 1 GeV/nucleon
Double differential production cross sections of negative and positive pions
and the number of participating protons have been measured in central Au+Au
collisions at 1 GeV per nucleon incident energy. At low pion energies the pi^-
yield is strongly enhanced over the pi^+ yield. The energy dependence of the
pi^-/pi^+ ratio is assigned to the Coulomb interaction of the charged pions
with the protons in the reaction zone. The deduced Coulomb potential increases
with increasing pion c.m. energy. This behavior indicates different freeze-out
radii for different pion energies in the c.m.~frame.Comment: IKDA is the Institute for Nuclear Physics in Darmstadt/German
Magnetic Vortex Core Reversal by Excitation of Spin Waves
Micron-sized magnetic platelets in the flux closed vortex state are
characterized by an in-plane curling magnetization and a nanometer-sized
perpendicularly magnetized vortex core. Having the simplest non-trivial
configuration, these objects are of general interest to micromagnetics and may
offer new routes for spintronics applications. Essential progress in the
understanding of nonlinear vortex dynamics was achieved when low-field core
toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was
established. At frequencies more than an order of magnitude higher vortex state
structures possess spin wave eigenmodes arising from the magneto-static
interaction. Here we demonstrate experimentally that the unidirectional vortex
core reversal process also occurs when such azimuthal modes are excited. These
results are confirmed by micromagnetic simulations which clearly show the
selection rules for this novel reversal mechanism. Our analysis reveals that
for spin wave excitation the concept of a critical velocity as the switching
condition has to be modified.Comment: Minor corrections and polishing of previous versio
Evidence of the Jahn-Teller splitting of C60- in C60- tetraphenylphosphoniumchloride from an electron-spin-relaxation study
Pulsed EPR measurements of the transverse and longitudinal relaxation times of the C60 - anion radical in crystalline C60-tetraphenylphosphoniumchloride were done at temperatures from 4 to 40 K. Above 40 K to room temperature the longitudinal relaxation time was taken from the cw-EPR linewidth. The low-temperature data are explained in terms of local magnetic fluctuations, slow C60- motion, and localized two-level states related with the local disorder. The relaxation data at higher temperatures reveal experimental evidence for the Jahn-Teller distortion of the C60 - anion radical and allow to determine the Jahn-Teller splitting between the a2u electronic ground state and the excited e1u state of the unpaired electron. © 1995 The American Physical Society
Dynamics of ferroelectric nano cluster in BaTiO3 observed as a real time correlation between two soft X-ray laser pulses
We carry out a theoretical investigation to clarify the dynamic property of
photo-created nano-sized ferroelectric cluster observed in the paraelectric
BaTiO3 as a real time correlation of speckle pattern between two soft X-ray
laser pulses, at just above the paraelectric-ferroelectric phase transition
temperature. Based on a model with coupled soft X-ray photon and ferroelectric
phonon mode, we study the time dependence of scattering probability by using a
perturbative expansion approach. The cluster-associated phonon softening as
well as central peak effects are well reproduced in the phonon spectral
function via quantum Monte Carlo simulation. Besides, it is found that the time
dependence of speckle correlation is determined by the relaxation dynamics of
ferroelectric clusters. Near the transition point, cluster excitation is
stable, leading to a long relaxation time. While, at high temperature, cluster
structure is subject to the thermal fluctuation, ending up with a short
relaxation time.Comment: 9 pages, 3 figure
Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide
We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]
- …