45 research outputs found
Inherent temperature effects in magnetic tunnel junctions
Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. ©2001 American Institute of Physics
Inherent temperature effects in magnetic tunnel junctions
Theoretical studies of the temperature dependence of the tunneling magnetoresistance ratio (TMR) are presented. A successful elastic tunneling model has been extended to handle temperature dependence. It treats Fermi smearing and applies Stoner-like behavior to the exchange split band structure in the electrodes to calculate TMR(T). As expected, the effects of Fermi smearing are small, but small changes in the magnetic band structure produce large changes in TMR. For a Co/I/Co junction produced by LeClair et al. [Phys. Rev. Lett. 84, 2933 (2000)], calculations using bulk magnetization predicted 33% of the experimental loss of TMR from 0 to 300 K with only a 1.5% change in magnetization. A mere 3.2% change in magnetization produced 100% of the observed drop in TMR. These results imply larger than imagined intrinsic temperature dependence for TMR. ©2001 American Institute of Physics
Effect of interface bonding on spin-dependent tunneling from the oxidized Co surface
We demonstrate that the factorization of the tunneling transmission into the
product of two surface transmission functions and a vacuum decay factor allows
one to generalize Julliere's formula and explain the meaning of the ``tunneling
density of states'' in some limiting cases. Using this factorization we
calculate spin-dependent tunneling from clean and oxidized fcc Co surfaces
through vacuum into Al using the principal-layer Green's function approach. We
demonstrate that a monolayer of oxygen on the Co (111) surface creates a
spin-filter effect due to the Co-O bonding which produces an additional
tunneling barrier in the minority-spin channel. This changes the minority-spin
dominated conductance for the clean Co surface into a majority spin dominated
conductance for the oxidized Co surface.Comment: 7 pages, revtex4, 4 embedded eps figure
Kappa symmetric OSp(2|2) WZNW model
We construct a kappa symmetric WZNW model for the OSp(2|2) supergroup, whose
bosonic part is AdS3xS1 space. The field equation gives the chiral current
conservation and the right/left factorization is shown after the kappa symmetry
is fixed. The right-moving modes contain both bosons and fermions while the
left-moving modes contain only bosons.Comment: 18 pages; reference and comments added, version to appear in JHE
Quantum oscillation of magnetoresistance in tunneling junctions with a nonmagnetic spacer
We make a theoretical study of the quantum oscillations of the tunneling
magnetoresistance (TMR) as a function of the spacer layer thickness. Such
oscillations were recently observed in tunneling junctions with a nonmagnetic
metallic spacer at the barrier-electrode interface. It is shown that momentum
selection due to the insulating barrier and conduction via quantum well states
in the spacer, mediated by diffusive scattering caused by disorder, are
essential features required to explain the observed period of oscillation in
the TMR ratio and its asymptotic value for thick nonmagnetic spacer.Comment: 4 pages, 5 figures, two column, REVTex4 styl
Quasi-particle re-summation and integral gap equation in thermal field theory
A new approach to quantum field theory at finite temperature and density in
arbitrary space-time dimension D is developed. We focus mainly on relativistic
theories, but the approach applies to non-relativistic ones as well.
In this quasi-particle re-summation, the free energy takes the free-field
form but with the one-particle energy replaced by \vep
(\vec{k}), the latter satisfying a temperature-dependent integral equation
with kernel related to a zero temperature form-factor of the trace of
stress-energy tensor. For 2D integrable theories the approach reduces to the
thermodynamic Bethe ansatz. For relativistic theories, a thermal c-function
is defined for any based on the coefficient of the black
body radiation formula. Thermodynamical constraints on it's flow are presented,
showing that it can violate a ``c-theorem'' even in 2D. At a fixed point
is a function of thermal gap parameters which generalizes Roger's
dilogarithm to higher dimensions. This points to a strategy for classifying
rational theories based on ``polylogarithmic ladders'' in mathematics, and many
examples are worked out. An argument suggests that the 3D Ising model has
. (In 3D a free fermion has .) Other
applications are discussed, including the free energy of anyons in 2D and 3D,
phase transitions with a chemical potential, and the equation of state for
cosmological dark energy.Comment: Version 4: Published versio
Functional Integral Bosonization for Impurity in Luttinger Liquid
We use a functional integral formalism developed earlier for the pure
Luttinger liquid (LL) to find an exact representation for the electron Green
function of the LL in the presence of a single backscattering impurity. This
allows us to reproduce results (well known from the bosonization techniques)
for the suppression of the electron local density of states (LDoS) at the
position of the impurity and for the Friedel oscillations at finite
temperature. In addition, we have extracted from the exact representation an
analytic dependence of LDoS on the distance from the impurity and shown how it
crosses over to that for the pure LL.Comment: 7 pages, 1 LaTeX produced figur
Topological String on OSP(1|2)/U(1)
We propose an equivalence between topological string on OSP(1|2)/U(1) and
\hat{c} \leq 1 superstring with N=1 world-sheet supersymmetry. We examine this
by employing a free field representation of OSP(1|2) WZNW model and find an
agreement on the spectrum. We also analyze this proposal at the level of
scattering amplitudes by applying a map between correlation functions of
OSP(1|2) WZNW model and those of N=1 Liouville theory.Comment: 25 pages, refereces adde
Entanglement between a qubit and the environment in the spin-boson model
The quantitative description of the quantum entanglement between a qubit and
its environment is considered. Specifically, for the ground state of the
spin-boson model, the entropy of entanglement of the spin is calculated as a
function of , the strength of the ohmic coupling to the environment,
and , the level asymmetry. This is done by a numerical
renormalization group treatment of the related anisotropic Kondo model. For
, the entanglement increases monotonically with , until it
becomes maximal for . For fixed , the entanglement
is a maximum as a function of for a value, .Comment: 4 pages, 3 figures. Shortened version restricted to groundstate
entanglemen
Closed String Field Theory with Dynamical D-brane
We consider a closed string field theory with an arbitrary matter current as
a source of the closed string field. We find that the source must satisfy a
constraint equation as a consequence of the BRST invariance of the theory. We
see that it corresponds to the covariant conservation law for the matter
current, and the equation of motion together with this constraint equation
determines the classical behavior of both the closed string field and the
matter. We then consider the boundary state (D-brane) as an example of a
source. We see that the ordinary boundary state cannot be a source of the
closed string field when the string coupling g turns on. By perturbative
expansion, we derive a recursion relation which represents the bulk
backreaction and the D-brane recoil. We also make a comment on the rolling
tachyon boundary state.Comment: 30 pages, LaTeX2e, no figures. Typos are correcte