24,674 research outputs found
Large magnetic field-induced spectral weight enhancement of high-energy spin excitations in
We report electronic Raman scattering experiments on a superconducting single crystal in a magnetic field. At low
temperatures, the spectral weight of the high-energy two-magnon peak increases
linearly with field and is amplified by a factor of more than two at 14 T. The
effect disappears at elevated temperatures and is not present in undoped . This observation is discussed in terms of an electronically
inhomogeneous state in which the field enhances the volume fraction of a phase
with local antiferromagnetic order at the expense of the superconducting phase.Comment: to appear in PR
Nearly Instantaneous Alternatives in Quantum Mechanics
Usual quantum mechanics predicts probabilities for the outcomes of
measurements carried out at definite moments of time. However, realistic
measurements do not take place in an instant, but are extended over a period of
time. The assumption of instantaneous alternatives in usual quantum mechanics
is an approximation whose validity can be investigated in the generalized
quantum mechanics of closed systems in which probabilities are predicted for
spacetime alternatives that extend over time. In this paper we investigate how
alternatives extended over time reduce to the usual instantaneous alternatives
in a simple model in non-relativistic quantum mechanics. Specifically, we show
how the decoherence of a particular set of spacetime alternatives becomes
automatic as the time over which they extend approaches zero and estimate how
large this time can be before the interference between the alternatives becomes
non-negligible. These results suggest that the time scale over which coarse
grainings of such quantities as the center of mass position of a massive body
may be extended in time before producing significant interference is much
longer than characteristic dynamical time scales.Comment: 12 pages, harvmac, no figure
Localization Properties of Electronic States in Polaron Model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers
We numerically investigate localization properties of electronic states in a
static model of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA polymers with
realistic parameters obtained by quantum-chemical calculation. The randomness
in the on-site energies caused by the electron-phonon coupling are completely
correlated to the off-diagonal parts. In the single electron model, the effect
of the hydrogen-bond stretchings, the twist angles between the base pairs and
the finite system size effects on the energy dependence of the localization
length and on the Lyapunov exponent are given. The localization length is
reduced by the influence of the fluctuations in the hydrogen bond stretchings.
It is also shown that the helical twist angle affects the localization length
in the poly(dG)-poly(dC) DNA polymer more strongly than in the
poly(dA)-poly(dT) one. Furthermore, we show resonance structures in the energy
dependence of the localization length when the system size is relatively small.Comment: 6 pages, 6 figure
Fermi edge singularity in a non-equilibrium system
We report exact results for the Fermi Edge Singularity in the absorption
spectrum of an out-of-equilibrium tunnel junction. We consider two metals with
chemical potential difference V separated by a tunneling barrier containing a
defect, which exists in one of two states. When it is in its excited state,
tunneling through the otherwise impermeable barrier is possible. We find that
the lineshape not only depends on the total scattering phase shift as in the
equilibrium case but also on the difference in the phase of the reflection
amplitudes on the two sides of the barrier. The out-of-equilibrium spectrum
extends below the original threshold as energy can be provided by the power
source driving current across the barrier. Our results have a surprisingly
simple interpretation in terms of known results for the equilibrium case but
with (in general complex-valued) combinations of elements of the scattering
matrix replacing the equilibrium phase shifts.Comment: 4 page
Magnetoresistance and surface roughness study of the initial growth of electrodeposited Co/Cu multilayers
The giant magnetoresistance (GMR) effect has been widely investigated on electrodeposited ferromagnetic/non-magnetic (FM/NM) multilayers generally containing a large number of bilayers. In most applications of the GMR effect, layered structures consisting of a relatively small number of consecutive FM and NM layers are used. It is of great interest, therefore, to investigate the initial stages of GMR multilayer film growth by electrodeposition. In the present work we have extended our previous studies on ED GMR multilayers to layered structures with a total thickness ranging from a few nanometers up to 70 nm. The evolution of the surface roughness and electrical transport properties of such ultrathin ED Co/Cu layered structures was investigated. Various layer combinations were produced including both Co and Cu either as starting or top layers in order (i) to see differences in the nucleation of the first layer and (ii) to trace out the effect of the so called exchange reaction. Special attention was paid to measure the field dependence of the magnetoresistance, MR(H) in order to derive information for the appearance of superparamagnetic regions in the magnetic layers. This proved to be helpful for monitoring the evolution of the layer microstructure at each step of the deposition sequence
Measurement of temperature profiles in hot gases by emission-absorption spectroscopy Final report
Measurement of spectral radiances and absorptances in hot gase
Measurement of temperature profiles in hot gases and flames
Computer program was written for calculation of molecular radiative transfer from hot gases. Shape of temperature profile was approximated in terms of simple geometric forms so profile could be characterized in terms of few parameters. Parameters were adjusted in calculations using appropriate radiative-transfer expression until best fit was obtained with observed spectra
Quantum shutter approach to tunneling time scales with wave packets
The quantum shutter approach to tunneling time scales (G. Garc\'{\i
}a-Calder\'{o}n and A. Rubio, Phys. Rev. A \textbf{55}, 3361 (1997)), which
uses a cutoff plane wave as the initial condition, is extended in such a way
that a certain type of wave packet can be used as the initial condition. An
analytical expression for the time evolved wave function is derived. The
time-domain resonance, the peaked structure of the probability density (as the
function of time) at the exit of the barrier, originally found with the cutoff
plane wave initial condition, is studied with the wave packet initial
conditions. It is found that the time-domain resonance is not very sensitive to
the width of the packet when the transmission process is in the tunneling
regime.Comment: 6 page
Universal Phase Diagram for High-Piezoelectric Perovskite Systems
Strong piezoelectricity in the perovskite-type PbZr(1-x)TixO3 (PZT) and
Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) systems is generally associated with the
existence of a morphotropic phase boundary (MPB) separating regions with
rhombohedral and tetragonal symmetry. An x-ray study of PZN-9%PT has revealed
the presence of a new orthorhombic phase at the MPB, and a near-vertical
boundary between the rhombohedral and orthorhombic phases, similar to that
found for PZT between the rhombohedral and monoclinic phases. We discuss the
results in the light of a recent theoretical paper by Vanderbilt and Cohen,
which attributes these low-symmetry phases to the high anharmonicity in these
oxide systems.Comment: REVTeX file. 4 pages,=A0 4 figures embedde
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