266 research outputs found
Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+
Frequency and dc magnetic field dependences of dynamic susceptibility in
diluted paramagnets LiYF:Ho have been measured at liquid helium
temperatures in the ac and dc magnetic fields parallel to the symmetry axis of
a tetragonal crystal lattice. Experimental data are analyzed in the framework
of microscopic theory of relaxation rates in the manifold of 24
electron-nuclear sublevels of the lowest non-Kramers doublet and the first
excited singlet in the Ho ground multiplet split by the crystal
field of S symmetry. The one-phonon transition probabilities were computed
using electron-phonon coupling constants calculated in the framework of
exchange charge model and were checked by optical piezospectroscopic
measurements. The specific features observed in field dependences of the in-
and out-of-phase susceptibilities (humps and dips, respectively) at the
crossings (anti-crossings) of the electron-nuclear sublevels are well
reproduced by simulations when the phonon bottleneck effect and the cross-spin
relaxation are taken into account
Dynamics of Anderson localization in open 3D media
We develop a self-consistent theoretical approach to the dynamics of Anderson
localization in open three-dimensional (3D) disordered media. The approach
allows us to study time-dependent transmission and reflection, and the
distribution of decay rates of quasi-modes of 3D disordered slabs near the
Anderson mobility edge.Comment: 4 pages, 4 figure
Nonuniversal dynamic conductance fluctuations in disordered systems
Sample-to-sample fluctuations of the time-dependent conductance of a system
with static disorder have been studied by means of diagrammatic theory and
microwave pulsed transmission measurements. The fluctuations of time-dependent
conductance are not universal, i.e., depend on sample parameters, in contrast
to the universal conductance fluctuations in the steady-state regime. The
variance of normalized conductance, determined by the infinite-range intensity
correlation C_3(t), is found to increase as a third power of delay time from an
exciting pulse, t. C_3(t) grows larger than the long-range intensity
correlation C_2(t) after a time t_q ~ ^{1/2} t_D (t_D being the diffusion
time, being the average dimensionless conductance).Comment: Revised version, 6 pages, 5 figure
EPR studies of manganese centers in SrTiO3: Non-Kramers Mn3+ ions and spin-spin coupled Mn4+ dimers
X- and Q-band electron paramagnetic resonance (EPR) study is reported on the
SrTiO3 single crystals doped with 0.5-at.% MnO. EPR spectra originating from
the S = 2 ground state of Mn3+ ions are shown to belong to the three distinct
types of Jahn-Teller centres. The ordering of the oxygen vacancies due to the
reduction treatment of the samples and consequent formation of oxygen vacancy
associated Mn3+ centres are explained in terms of the localized charge
compensation. The EPR spectra of SrTiO3: Mn crystals show the presence of next
nearest neighbor exchange coupled Mn4+ pairs in the directions.Comment: 17 pages, 8 figure
Effect of gas flow on electronic transport in a DNA-decorated carbon nanotube
We calculate the two-time current correlation function using the experimental
data of the current-time characteristics of the Gas-DNA-decorated carbon
nanotube field effect transistor. The pattern of the correlation function is a
measure of the sensitivity and selectivity of the sensors and suggest that
these gas flow sensors may also be used as DNA sequence detectors. The system
is modelled by a one-dimensional tight-binding Hamiltonian and we present
analytical calculations of quantum electronic transport for the system using
the time-dependent nonequilibrium Green's function formalism and the adiabatic
expansion. The zeroth and first order contributions to the current
and are calculated, where is the Landauer formula. The formula for the time-dependent current
is then used to compare the theoretical results with the experiment.Comment: 14 pages, 5 figures and 2 table
The effect of the spin-orbit geometric phase on the spectrum of Aharonov-Bohm oscillations in a semiconductor mesoscopic ring
Taking into account the spin precession caused by the spin-orbit splitting of
the conduction band in semiconductor quantum wells, we have calculated the
Fourier spectra of conductance and state-density correlators in a 2D ring, in
order to investigate the structure of the main peak corresponding to
Aharonov-Bohm oscillations. In narrow rings the peak structure is determined by
the competition between the spin-orbit and the Zeeman couplings. The latter
leads to a peak broadening, and produces the peak splitting in the
state-density Fourier spectrum. We have found an oscillation of the peak
intensity as a function of the spin-orbit coupling constant, and this effect of
the quantum interference caused by the spin geometric phase is destroyed with
increasing Zeeman coupling.Comment: 4 pages, 3 figures, uses epsfig.st
Energy relaxation in the spin-polarized disordered electron liquid
The energy relaxation in the spin-polarized disordered electron systems is
studied in the diffusive regime. We derived the quantum kinetic equation in
which the kernel of electron-electron collision integral explicitly depends on
the electron magnetization. As the consequence, the inelastic scattering rate
is found to have non-monotonic dependence on the spin polarization of the
electron system
Electron-electron interaction in carbon nanostructures
The electron-electron interaction in carbon nanostructures was studied. A new
method which allows to determine the electron-electron interaction constant
from the analysis of quantum correction to the magnetic
susceptibility and the magnetoresistance was developed. Three types of carbon
materials: arc-produced multiwalled carbon nanotubes (arc-MWNTs), CVD-produced
catalytic multiwalled carbon nanotubes (c-MWNTs) and pyrolytic carbon were used
for investigation. We found that =0.2 for arc-MWNTs (before and
after bromination treatment); = 0.1 for pyrolytic graphite;
0 for c-MWNTs. We conclude that the curvature of graphene layers
in carbon nanostructures leads to the increase of the electron-electron
interaction constant .Comment: 12 pages, 18 figures, to be published in the Proceedings of the NATO
Advanced Research Workshop on Electron Correlation in New Materials and
Nanosystems, NATO Science Series II, Springer, 200
On the Phase Boundaries of the Integer Quantum Hall Effect. II
It is shown that the statements about the observation of the transitions
between the insulating phase and the integer quantum Hall effect phases with
the quantized Hall conductivity made in a
number of works are unjustified. In these works, the crossing points of the
magnetic field dependences of the diagonal resistivity at different
temperatures at have been misidentified as the
critical points of the phase transitions. In fact, these crossing points are
due to the sign change of the derivative owing to the quantum
corrections to the conductivity.Comment: 3 pages, 2 figure
Odd-frequency Pairs and Josephson Current through a Strong Ferromagnet
We study Josephson current in superconductor / diffusive ferromagnet
/superconductor junctions by using the recursive Green function method. When
the exchange potential in a ferromagnet is sufficiently large as compared to
the pair potential in a superconductor, an ensemble average of Josephson
current is much smaller than its mesoscopic fluctuations. The Josephson current
vanishes when the exchange potential is extremely large so that a ferromagnet
is half-metallic. Spin-flip scattering at junction interfaces drastically
changes the characteristic behavior of Josephson current. In addition to
spin-singlet Cooper pairs, equal-spin triplet pairs penetrate into a half
metal. Such equal-spin pairs have an unusual symmetry property called
odd-frequency symmetry and carry the Josephson current through a half metal.
The penetration of odd-frequency pairs into a half metal enhances the low
energy quasiparticle density of states, which could be detected experimentally
by scanning tunneling spectroscopy. We will also show that odd-frequency pairs
in a half metal cause a nonmonotonic temperature dependence of the critical
Josephson current.Comment: 12 pages 14 figures embedde
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