586 research outputs found
Recalculation of an artificially released avalanche with SAMOS and validation with measurements from a pulsed Doppler radar
A joint experiment was carried out on 10 February 1999 by the Swiss Federal Institute for Snow and Avalanche Research (SFISAR) and the Austrian Institute for Avalanche and Torrent Research (AIATR, of the Federal Office and Re-search Centre for Forests, BFW) to measure forces and velocities at the full scale experimental site CRÊTA BESSE in VALLÉE DE LA SIONNE, Canton du Valais, Switzerland. A huge avalanche could be released artificially, which permitted extensive investigations (dynamic measurements, im-provement of measurement systems, simulation model verification, design of protective measures, etc.). The results of the velocity measurements from the dual frequency pulsed Doppler avalanche radar of the AIATR and the recalculation with the numerical simulation model SAMOS are explained in this paper
Absence of weak antilocalization in ferromagnetic films
We present magnetoresistance measurements performed on ultrathin films of
amorphous Ni and Fe. In these films the Curie temperature drops to zero at
small thickness, making it possible to study the effect of ferromagnetism on
localization. We find that non-ferromagnetic films are characterized by
positive magnetoresistance. This is interpreted as resulting from weak
antilocalization due to strong Bychkov-Rashba spin orbit scattering. As the
films become ferromagnetic the magnetoresistance changes sign and becomes
negative. We analyze our data to identify the individual contributions of weak
localization, weak antilocalization and anisotropic magnetoresistance and
conclude that the magnetic order suppresses the influence of spin-orbit effects
on localization phenomena in agreement with theoretical predictions.Comment: 6 pages, 6 figure
Weak localization effects in granular metals
The weak localization correction to the conductivity of a granular metal is
calculated using the diagrammatic technique in the reciprocal grain lattice
representation. The properties of this correction are very similar to that one
in disordered metal, with the replacement of the electron mean free path by the grain diameter and the dimensionless conductance by the
tunnelling dimensionless conductance . In particular, we demonstrate
that at zero temperature no conducting phase can exist for dimensions . We also analyze the WL correction to magnetoconductivity in the weak field
limit.Comment: 4 pages, 3 figures; minor corrections adde
A semiclassical theory of the Anderson transition
We study analytically the metal-insulator transition in a disordered
conductor by combining the self-consistent theory of localization with the one
parameter scaling theory. We provide explicit expressions of the critical
exponents and the critical disorder as a function of the spatial
dimensionality, . The critical exponent controlling the divergence of
the localization length at the transition is found to be . This result confirms that the upper critical dimension is
infinity. Level statistics are investigated in detail. We show that the two
level correlation function decays exponentially and the number variance is
linear with a slope which is an increasing function of the spatial
dimensionality.Comment: 4 pages, journal versio
Hot electrons in low-dimensional phonon systems
A simple bulk model of electron-phonon coupling in metals has been
surprisingly successful in explaining experiments on metal films that actually
involve surface- or other low-dimensional phonons. However, by an exact
application of this standard model to a semi-infinite substrate with a free
surface, making use of the actual vibrational modes of the substrate, we show
that such agreement is fortuitous, and that the model actually predicts a
low-temperature crossover from the familiar T^5 temperature dependence to a
stronger T^6 log T scaling. Comparison with existing experiments suggests a
widespread breakdown of the standard model of electron-phonon thermalization in
metals
Spin-orbit scattering in quantum diffusion of massive Dirac fermions
Effect of spin-orbit scattering on quantum diffusive transport of
two-dimensional massive Dirac fermions is studied by the diagrammatic
technique. The quantum diffusion of massive Dirac fermions can be viewed as a
singlet Cooperon in the massless limit and a triplet Cooperon in the large-mass
limit. The spin-orbit scattering behaves like random magnetic fields only to
the triplet Cooperon, and suppresses the weak localization of Dirac fermions in
the large-mass regime. This behavior suggests an experiment to detect the weak
localization of bulk subbands in topological insulator thin films, in which a
narrowing of the cusp of the negative magnetoconductivity is expected after
doping heavy-element impurities. Finally, a detailed comparison between the
conventional two-dimensional electrons and Dirac fermions is presented for
impurities of orthogonal, symplectic, and unitary symmetries.Comment: 5 pages, 3 figures, 2 tables. To be submitted, comments are welcom
GW approximations and vertex corrections on the Keldysh time-loop contour: application for model systems at equilibrium
We provide the formal extension of Hedin's GW equations for single-particle
Green's functions with electron-electron interaction onto the Keldysh time-loop
contour. We show an application of our formalism to the plasmon model of a core
electron within the plasmon-pole approximation. We study in detail the
diagrammatic perturbation expansion of the core-electron/plasmon coupling on
the spectral functions of the so-called S-model which provides an exact
solution, concentrating especially on the effects of self-consistency and
vertex corrections on the GW self-energy. For the S-model, self-consistency is
essential for GW-like calculations to obtain the full spectral information. The
second- order exchange diagram (i.e. a vertex correction) is crucial to obtain
a better spectral description of the plasmon peak and side-band peaks in
comparison to GW-like calculations. However, the vertex corrections are well
reproduced within a non-self-consistent calculation. We also consider
conventional equilibrium GW calculations for the pure jellium model. We find
that with no second-order vertex correction, we cannot obtain the full set of
plasmon side-band peaks. Finally, we address the issues of formal connection
for the Dyson equations of the time-ordered Green's function and the Keldysh
Green's functions at equilibrium in the cases of zero and finite temperature.Comment: Published in PRB November 22 201
Localization of Matter Waves in 2D-Disordered Optical Potentials
We consider ultracold atoms in 2D-disordered optical potentials and calculate
microscopic quantities characterizing matter wave quantum transport in the
non-interacting regime. We derive the diffusion constant as function of all
relevant microscopic parameters and show that coherent multiple scattering
induces significant weak localization effects. In particular, we find that even
the strong localization regime is accessible with current experimental
techniques and calculate the corresponding localization length.Comment: 4 pages, 3 figures, figures changed, references update
Muon Spin Relaxation Studies of Superconductivity in a Crystalline Array of Weakly Coupled Metal Nanoparticles
We report Muon Spin Relaxation studies in weak transverse fields of the
superconductivity in the metal cluster compound,
Ga[N(SiMe)]-LiBr(thf)2toluene. The temperature and field dependence of the muon spin relaxation
rate and Knight shift clearly evidence type II bulk superconductivity below
K, with T,
T, and weak flux pinning. The data
are well described by the s-wave BCS model with weak electron-phonon coupling
in the clean limit. A qualitative explanation for the conduction mechanism in
this novel type of narrow band superconductor is presented.Comment: 4 figures, 5 page
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