368 research outputs found
Alternative approach to computing transport coefficients: application to conductivity and Hall coefficient of hydrogenated amorphous silicon
We introduce a theoretical framework for computing transport coefficients for
complex materials. As a first example, we resolve long-standing inconsistencies
between experiment and theory pertaining to the conductivity and Hall mobility
for amorphous silicon and show that the Hall sign anomaly is a consequence of
localized states. Next, we compute the AC conductivity of amorphous
polyanaline. The formalism is applicable to complex materials involving defects
and band-tail states originating from static topological disorder and extended
states. The method may be readily integrated with current \textit{ab initio}
methods.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
Quantum oscillations of rectified dc voltage as a function of magnetic field in an "almost" symmetric superconducting ring
Periodic quantum oscillations of a rectified dc voltage Vdc(B) vs the
perpendicular magnetic field B were measured near the critical temperature Tc
in a single superconducting aluminum almost symmetric ring (without specially
created circular asymmetry) biased by alternating current with a zero dc
component. With varying bias current and temperature, these Vdc(B) oscillations
behave like the Vdc(B) oscillations observed in a circular-asymmetric ring but
are of smaller amplitude. The Fourier spectra of the Vdc(B) functions exhibit a
fundamental frequency, corresponding to the ring area, and its higher
harmonics. Unexpectedly, satellite frequencies depending on the structure
geometry and external parameters were found next to the fundamental frequency
and around its higher harmonics.Comment: author english version, 2 pages, 3 figires, Proc. of the XXXIV
Conference on Low-Temperature Physics "NT-34" (Russia, 2006
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First CRDS-measurements of water vapour continuum in the 940nm absorption band
Measurements of near-infrared water vapour continuum using continuous wave cavity ring down spectroscopy (cw-
CRDS) have been performed at around 10611.6 and 10685:2 cm1. The continuum absorption coefficients for N2-
broadening have been determined for two temperatures and wavenumbers.
These results represent the first near-IR continuum laboratory data determined within the complex spectral environment in the 940nm water vapour band and are in reasonable agreement with simulations using the semiempirical CKD formulation
A Variational Approach to Nonlocal Exciton-Phonon Coupling
In this paper we apply variational energy band theory to a form of the
Holstein Hamiltonian in which the influence of lattice vibrations (optical
phonons) on both local site energies (local coupling) and transfers of
electronic excitations between neighboring sites (nonlocal coupling) is taken
into account. A flexible spanning set of orthonormal eigenfunctions of the
joint exciton-phonon crystal momentum is used to arrive at a variational
estimate (bound) of the ground state energy for every value of the joint
crystal momentum, yielding a variational estimate of the lowest polaron energy
band across the entire Brillouin zone, as well as the complete set of polaron
Bloch functions associated with this band. The variation is implemented
numerically, avoiding restrictive assumptions that have limited the scope of
previous assaults on the same and similar problems. Polaron energy bands and
the structure of the associated Bloch states are studied at general points in
the three-dimensional parameter space of the model Hamiltonian (electronic
tunneling, local coupling, nonlocal coupling), though our principal emphasis
lay in under-studied area of nonlocal coupling and its interplay with
electronic tunneling; a phase diagram summarizing the latter is presented. The
common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure
Nanofabricated media with negative permeability at visible frequencies
We report a nanofabricated medium made of electromagnetically coupled pairs
of gold dots with geometry carefully designed at a 10-nm level. The medium
exhibits strong magnetic response at visible-light frequencies, including bands
with negative \mu. The magnetism arises due to the excitation of quadrupole
plasmon resonances. Our approach shows for the first time the feasibility of
magnetism at optical frequencies and paves a way towards magnetic and
left-handed components for visible optics.Comment: 16 pages, 4 figures. submitted to Nature on 1 April 200
Induction of dc voltage, proportional to the persistent current, by external ac current on system of inhomogeneous superconducting loops
A dc voltage induced by an external ac current is observed in system of
asymmetric mesoscopic superconducting loops. The value and sign of this dc
voltage, like the one of the persistent current, depend in a periodical way on
a magnetic field with period corresponded to the flux quantum within the loop.
The amplitude of the oscillations does not depend on the frequency of the
external ac current (in the investigated region 100 Hz - 1 MHz) and depends on
its amplitude. The latter dependence is not monotonous. The observed phenomenon
of rectification is interpreted as a consequence of a dynamic resistive state
induced by superposition of the external current and the persistent current. It
is shown that the dc voltage can be added in system of loops connected in
series: the dc voltage oscillations with amplitude up to 0.00001 V were
observed in single loop, up to 0.00004 V in a system of 3 loops and up to
0.0003 V in a system of 20 loops.Comment: 5 pages, 6 figure
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