42 research outputs found
Massless Minimally Coupled Fields in De Sitter Space: O(4)-Symmetric States Versus De Sitter Invariant Vacuum
The issue of de Sitter invariance for a massless minimally coupled scalar
field is revisited. Formally, it is possible to construct a de Sitter invariant
state for this case provided that the zero mode of the field is quantized
properly. Here we take the point of view that this state is physically
acceptable, in the sense that physical observables can be computed and have a
reasonable interpretation. In particular, we use this vacuum to derive a new
result: that the squared difference between the field at two points along a
geodesic observer's space-time path grows linearly with the observer's proper
time for a quantum state that does not break de Sitter invariance. Also, we use
the Hadamard formalism to compute the renormalized expectation value of the
energy momentum tensor, both in the O(4) invariant states introduced by Allen
and Follaci, and in the de Sitter invariant vacuum. We find that the vacuum
energy density in the O(4) invariant case is larger than in the de Sitter
invariant case.Comment: TUTP-92-1, to appear in Phys. Rev.
Squeezed States in the de Sitter Vacuum
We discuss the treatment of squeezed states as excitations in the Euclidean
vacuum of de Sitter space. A comparison with the treatment of these states as
candidate no-particle states, or alpha-vacua, shows important differences
already in the free theory. At the interacting level alpha-vacua are
inconsistent, but squeezed state excitations seem perfectly acceptable. Indeed,
matrix elements can be renormalized in the excited states using precisely the
standard local counterterms of the Euclidean vacuum. Implications for
inflationary scenarios in cosmology are discussed.Comment: 15 pages, no figures. One new citation in version 3; no other change
Andreev conductance of a domain wall
At low temperatures, the transport through a superconductor-ferromagnet
tunnel interface is due to tunneling of electrons in pairs. Exchange field of a
monodomain ferromagnet aligns electron spins and suppresses the two electron
tunneling. The presence of the domain walls at the SF interface strongly
enhances the subgap current. The Andreev conductance is proven to be
proportional to the total length of domain walls at the SF interface.Comment: 4 pages and 1 figur
A Model of Curvature-Induced Phase Transitions in Inflationary Universe
Chiral phase transitions driven by space-time curvature effects are
investigated in de Sitter space in the supersymmetric Nambu-Jona-Lasinio model
with soft supersymmetry breaking. The model is considered to be suitable for
the analysis of possible phase transitions in inflationary universe. It is
found that a restoration of the broken chiral symmetry takes place in two
patterns for increasing curvature : the first order and second order phase
transition respectively depending on initial settings of the four-body
interaction parameter and the soft supersymmetry breaking parameter. The
critical curves expressing the phase boundaries in these parameters are
obtained. Cosmological implications of the result are discussed in connection
with bubble formations and the creation of cosmic strings during the
inflationary era.Comment: 12 pages, 3 figures, REVTe
Theory of proximity effect in superconductor/ferromagnet heterostructures
We present a microscopic theory of proximity effect in the
ferromagnet/superconductor/ferromagnet (F/S/F) nanostructures where S is s-wave
low-T_c superconductor and F's are layers of 3d transition ferromagnetic metal.
Our approach is based on the solution of Gor'kov equations for the normal and
anomalous Green's functions together with a self-consistent evaluation of the
superconducting order parameter. We take into account the elastic
spin-conserving scattering of the electrons assuming s-wave scattering in the S
layer and s-d scattering in the F layers. In accordance with the previous
quasiclassical theories, we found that due to exchange field in the ferromagnet
the anomalous Green's function F(z) exhibits the damping oscillations in the
F-layer as a function of distance z from the S/F interface. In the given model
a half of period of oscillations is determined by the length \xi_m^0 = \pi
v_F/E_ex, where v_F is the Fermi velocity and E_ex is the exchange field, while
damping is governed by the length l_0 = (1/l_{\uparrow} +
1/l_{\downarrow})^{-1} with l_{\uparrow} and l_{\downarrow} being
spin-dependent mean free paths in the ferromagnet. The superconducting
transition temperature T_c(d_F) of the F/S/F trilayer shows the damping
oscillations as a function of the F-layer thickness d_F with period \xi_F =
\pi/\sqrt{m E_ex}, where m is the effective electron mass. We show that strong
spin-conserving scattering either in the superconductor or in the ferromagnet
significantly suppresses these oscillations. The calculated T_c(d_F)
dependences are compared with existing experimental data for Fe/Nb/Fe trilayers
and Nb/Co multilayers.Comment: 13 pages, REVTeX4, 8 PS-figures; improved version, submitted to PR
Tall tales from de Sitter space II: Field theory dualities
We consider the evolution of massive scalar fields in (asymptotically) de
Sitter spacetimes of arbitrary dimension. Through the proposed dS/CFT
correspondence, our analysis points to the existence of new nonlocal dualities
for the Euclidean conformal field theory. A massless conformally coupled scalar
field provides an example where the analysis is easily explicitly extended to
'tall' background spacetimes.Comment: 31 pages, 2 figure
Josephson current in superconductor-ferromagnet structures with a nonhomogeneous magnetization
We calculate the dc Josephson current for two types of
superconductor-ferromagnet (S/F) Josephson junctions. The junction of the first
type is a S/F/S junction. On the basis of the Eilenberger equation, the
Josephson current is calculated for an arbitrary impurity concentration. If the expression for the Josephson critical current is reduced
to that which can be obtained from the Usadel equation ( is the exchange
energy, is the momentum relaxation time). In the opposite limit
the superconducting condensate oscillates with period and
penetrates into the F region over distances of the order of the mean free path
. For this kind of junctions we also calculate in the case when the F
layer presents a nonhomogeneous (spiral) magnetic structure with the period
. It is shown that for not too low temperatures, the -state which
occurs in the case of a homogeneous magnetization (Q=0) may disappear even at
small values of . In this nonhomogeneous case, the superconducting
condensate has a nonzero triplet component and can penetrate into the F layer
over a long distance of the order of . The junction
of the second type consists of two S/F bilayers separated by a thin insulating
film. It is shown that the critical Josephson current depends on the
relative orientation of the effective exchange field of the bilayers. In
the case of an antiparallel orientation, increases with increasing .
We establish also that in the F film deposited on a superconductor, the
Meissner current created by the internal magnetic field may be both diamagnetic
or paramagnetic.Comment: 13 pages, 11 figures. To be published in Phys. Rev.
Proximity effects and characteristic lengths in ferromagnet-superconductor structures
We present an extensive theoretical investigation of the proximity effects
that occur in Ferromagnet/Superconductor () systems. We use a numerical
method to solve self consistently the Bogoliubov-de Gennes equations in the
continuum. We obtain the pair amplitude and the local density of states (DOS),
and use these results to extract the relevant lengths characterizing the
leakage of superconductivity into the magnet and to study spin splitting into
the superconductor. These phenomena are investigated as a function of
parameters such as temperature, magnet polarization, interfacial scattering,
sample size and Fermi wavevector mismatch, all of which turn out to have
important influence on the results. These comprehensive results should help
characterize and analyze future data and are shown to be in agreement with
existing experiments.Comment: 24 pages, including 26 figure
Spontaneous Spin Polarized Currents in Superconductor-Ferromagnetic Metal Heterostructures
We study a simple microscopic model for thin, ferromagnetic, metallic layers
on semi-infinite bulk superconductor. We find that for certain values of the
exchange spliting, on the ferromagnetic side, the ground states of such
structures feature spontaneously induced spin polarized currents. Using a
mean-field theory, which is selfconsistent with respect to the pairing
amplitude , spin polarization and the spontaneous current
, we show that not only there are Andreev bound states in the
ferromagnet but when their energies are near zero they support
spontaneous currents parallel to the ferromagnetic-superconducting interface.
Moreover, we demonstrate that the spin-polarization of these currents depends
sensitively on the band filling.Comment: 4 pages, 5 Postscript figures (included
Layered ferromagnet-superconductor structures: the state and proximity effects
We investigate clean mutilayered structures of the SFS and SFSFS type, (where
the S layer is intrinsically superconducting and the F layer is ferromagnetic)
through numerical solution of the self-consistent Bogoliubov-de Gennes
equations for these systems. We obtain results for the pair amplitude, the
local density of states, and the local magnetic moment. We find that as a
function of the thickness of the magnetic layers separating adjacent
superconductors, the ground state energy varies periodically between two stable
states. The first state is an ordinary "0-state", in which the order parameter
has a phase difference of zero between consecutive S layers, and the second is
a "-state", where the sign alternates, corresponding to a phase difference
of between adjacent S layers. This behavior can be understood from simple
arguments. The density of states and the local magnetic moment reflect also
this periodicity.Comment: 12 pages, 10 Figure