4,134 research outputs found
Itinerant Ferromagnetism in the electronic localization limit
We present Hall effect, , and magnetoresistance, ,
measurements of ultrathin films of Ni, Co and Fe with thicknesses varying
between 0.2-8 nm and resistances between 1 M - 100 Both
measurements show that films having resistance above a critical value, ,
(thickness below a critical value, ) show no signs for ferromagnetism.
Ferromagnetism appears only for films with , where is material
dependent. We raise the possibility that the reason for the absence of
spontaneous magnetization is suppression of itinerant ferromagnetism by
electronic disorder in the strong localization regime.Comment: 4 pages, 4 figure
Optical counterparts of cosmological GRBs due to heating of ISM in the parent galaxy
We investigated influence of cosmological GRB on the surrounding interstellar
medium. It was shown that \gamma-radiation from the burst heats interstellar
gas to the temperatures > 10^4 K up to the distance \sim 10 pc. For high
density ISM optical and UV radiation of the heated gas can be observed on the
Eath several years as a GRB`s counterpartComment: 2 pages, 1 figure; presented at the Rome Conference on Gamma Ray
Bursts in the Afterglow Ag
Continuum Limits of ``Induced QCD": Lessons of the Gaussian Model at d=1 and Beyond
We analyze the scalar field sector of the Kazakov--Migdal model of induced
QCD. We present a detailed description of the simplest one dimensional
{()} model which supports the hypothesis of wide applicability of the
mean--field approximation for the scalar fields and the existence of critical
behaviour in the model when the scalar action is Gaussian. Despite the
ocurrence of various non--trivial types of critical behaviour in the
model as , only the conventional large- limit is
relevant for its {\it continuum} limit. We also give a mean--field analysis of
the model in {\it any} and show that a saddle point always exists in
the region . In it exhibits critical behaviour as
. However when there is no critical
behaviour unless non--Gaussian terms are added to the scalar field action. We
argue that similar behaviour should occur for any finite thus providing a
simple explanation of a recent result of D. Gross. We show that critical
behaviour at and can be obtained by adding a
term to the scalar potential. This is equivalent to a local
modification of the integration measure in the original Kazakov--Migdal model.
Experience from previous studies of the Generalized Kontsevich Model implies
that, unlike the inclusion of higher powers in the potential, this minor
modification should not substantially alter the behaviour of the Gaussian
model.Comment: 31 page
Neutrino dispersion in external magnetic fields
We calculate the neutrino self-energy operator Sigma (p) in the presence of a
magnetic field B. In particular, we consider the weak-field limit e B <<
m_\ell^2, where m_\ell is the charged-lepton mass corresponding to the neutrino
flavor \nu_\ell, and we consider a "moderate field" m_\ell^2 << e B << m_W^2.
Our results differ substantially from the previous literature. For a moderate
field, we show that it is crucial to include the contributions from all Landau
levels of the intermediate charged lepton, not just the ground-state. For the
conditions of the early universe where the background medium consists of a
charge-symmetric plasma, the pure B-field contribution to the neutrino
dispersion relation is proportional to (e B)^2 and thus comparable to the
contribution of the magnetized plasma.Comment: 9 pages, 1 figure, revtex. Version to appear in Phys. Rev. D
(presentation improved, reference list revised, numerical error in Eq.(41)
corrected, conclusions unchanged
Spin polarization control by electric stirring: proposal for a spintronic device
We propose a spintronic device to generate spin polarization in a mesoscopic
region by purely electric means. We show that the spin Hall effect in
combination with the stirring effect are sufficient to induce measurable spin
polarization in a closed geometry. Our device structure does not require the
application of magnetic fields, external radiation or ferromagnetic leads, and
can be implemented in standard semiconducting materials
Area Law and Continuum Limit in "Induced QCD"
We investigate a class of operators with non-vanishing averages in a
D-dimensional matrix model recently proposed by Kazakov and Migdal. Among the
operators considered are ``filled Wilson loops" which are the most reasonable
counterparts of Wilson loops in the conventional Wilson formulation of lattice
QCD. The averages of interest are represented as partition functions of certain
2-dimensional statistical systems with nearest neighbor interactions. The
``string tension" , which is the exponent in the area law for the
``filled Wilson loop" is equal to the free energy density of the corresponding
statistical system. The continuum limit of the Kazakov--Migdal model
corresponds to the critical point of this statistical system. We argue that in
the large limit this critical point occurs at zero temperature. In this
case we express in terms of the distribution density of eigenvalues
of the matrix-valued master field. We show that the properties of the continuum
limit and the description of how this limit is approached is very unusual and
differs drastically from what occurs in both the Wilson theory () and in the ``adjoint'' theory (). Instead, the continuum limit of the model appears to be
intriguingly similar to a string theory.Comment: 38 page
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
On the Possible Enhancement of the Magnetic Field by Neutrino Reemission Processes in the Mantle of a Supernova
URCA neutrino reemission processes under the conditions in the mantle of a
supernova with a strong toroidal magnetic field are investigated. It is shown
that parity violation in these processes can be manifested macroscopically as a
torque that rapidly spins up the region of the mantle occupied by such a field.
Neutrino spin-up of the mantle can strongly affect the mechanism of further
generation of the toroidal field, specifically, it can enhance the field in a
small neighborhood of the rigid-body-rotating core of the supernova remnant.Comment: 8 pages, late
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