213 research outputs found
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
Conductance Oscillations in Transition Metal Superlattices
We present a numerical study of conductance oscillations of transition metal
multilayers as a function of layer thickness. Using a material-specific
tight-binding model, we show that for disorder-free layers with random
thicknesses but clean interfaces, long-period oscillations in the conductance
can occur, which are reminiscent of those found in structures exhibiting GMR.
Using a heuristic effective mass model, we argue that these oscillations arise
from beating between the Fermi wavevector and a class of wavevectors
characteristic of the superlattice structure.Comment: 4 pages, 4 figure
Sub-gap conductance in ferromagnetic-superconducting mesoscopic structures
We study the sub-gap conductance of a ferromagnetic mesoscopic region
attached to a ferromagnetic and a superconducting electrode by means of tunnel
junctions. In the absence of the exchange field, the ratio of the two tunnel junction resistances determines the behaviour of
the sub-gap conductance which possesses a zero-bias peak for and for
a peak at finite voltage. We show that the inclusion of the exchange
field leads to a peak splitting for , while it shifts the zero-bias
anomaly to finite voltages for .Comment: 5 pages revte
Influence of s-d interfacial scattering on the magnetoresistance of magnetic tunnel junctions
We propose the two-band s-d model to describe theoretically a diffuse regime
of the spin-dependent electron transport in magnetic tunnel junctions (MTJ's)
of the form F/O/F where F's are 3d transition metal ferromagnetic layers and O
is the insulating spacer. We aim to explain the strong interface sensitivity of
the tunneling properties of MTJ's and investigate the influence of electron
scattering at the nonideal interfaces on the degradation of the TMR magnitude.
The generalized Kubo formalism and the Green's functions method were used to
calculate the conductance of the system. The vertex corrections to the
conductivity were found with the use of "ladder" approximation combined with
the coherent-potential approximation (CPA) that allowed to consider the case of
strong electron scattering. It is shown that the Ward identity is satisfied in
the framework of this approximation that provides the necessary condition for a
conservation of a tunneling current. Based on the known results of ab-initio
calculations of the TMR for ballistic junctions, we assume that exchange split
quasi-free s-like electrons with the density of states being greater for the
majority spin sub-band give the main contribution to the TMR effect. We show
that, due to interfacial inter-band scattering, the TMR can be substantially
reduced even down to zero value. This is related to the fact that delocalized
quasi-free electrons can scatter into the strongly localized d sub-band with
the density of states at the Fermi energy being larger for minority spins
compared to majority spins. It is also shown that spin-flip electron scattering
on the surface magnons within the interface leads to a further decrease of the
TMR at finite temperature.Comment: REVTeX4, 20 pages, 9 figures, 1 table, submitted to Phys.Rev.B; In
Version 2 the text is substantially improved, the main results and
conclusions left the sam
Spin waves in ultrathin ferromagnetic overlayers
The influence of a non-magnetic metallic substrate on the spin wave
excitations in ultrathin ferromagnetic overlayers is investigated for different
crystalline orientations. We show that spin wave dumping in these systems occur
due to the tunneling of holes from the substrate into the overlayer, and that
the spin wave energies may be considerably affected by the exchange coupling
mediated by the substrate.Comment: RevTeX 4, 7 pages, 5 figures; submitted to Phys. Rev.
Studies of concentration and temperature dependencies of precipitation kinetics in iron-copper alloys using kinetic monte carlo and stochastic statistical simulations
The earlier-developed ab initio model and the kinetic Monte Carlo method
(KMCM) are used to simulate precipitation in a number of iron-copper alloys
with different copper concentrations x and temperatures T. The same simulations
are also made using the improved version of the earlier-suggested stochastic
statistical method (SSM). The results obtained enable us to make a number of
general conclusions about the dependencies of the decomposition kinetics in
Fe-Cu alloys on x and T. We also show that the SSM describes the precipitation
kinetics in a fair agreement with the KMCM, and employing the SSM in
conjunction with the KMCM enables us to extend the KMC simulations to the
longer evolution times. The results of simulations seem to agree with available
experimental data for Fe-Cu alloys within statistical errors of simulations and
the scatter of experimental results. Comparison of results of simulations to
experiments for some multicomponent Fe-Cu-based alloys enables us to make
certain conclusions about the influence of alloying elements in these alloys on
the precipitation kinetics at different stages of evolution.Comment: 18 pages, 17 postscript figures, LaTe
Evaluation of effective resistances in pseudo-distance-regular resistor networks
In Refs.[1] and [2], calculation of effective resistances on distance-regular
networks was investigated, where in the first paper, the calculation was based
on the stratification of the network and Stieltjes function associated with the
network, whereas in the latter one a recursive formula for effective
resistances was given based on the Christoffel-Darboux identity. In this paper,
evaluation of effective resistances on more general networks called
pseudo-distance-regular networks [21] or QD type networks \cite{obata} is
investigated, where we use the stratification of these networks and show that
the effective resistances between a given node such as and all of the
nodes belonging to the same stratum with respect to
(, belonging to the -th stratum with respect
to the ) are the same. Then, based on the spectral techniques, an
analytical formula for effective resistances such that
(those nodes , of
the network such that the network is symmetric with respect to them) is given
in terms of the first and second orthogonal polynomials associated with the
network, where is the pseudo-inverse of the Laplacian of the network.
From the fact that in distance-regular networks,
is satisfied for all nodes
of the network, the effective resistances
for ( is diameter of the network which
is the same as the number of strata) are calculated directly, by using the
given formula.Comment: 30 pages, 7 figure
Fluctuation Induced Non-Fermi Liquid Behavior near a Quantum Phase Transition in Itinerant Electron Systems
The signature for a non-Fermi liquid behavior near a quantum phase transition
has been observed in thermal and transport properties of many metallic systems
at low temperatures. In the present work we consider specific examples of
itinerant ferromagnet as well as antiferromagnet in the limit of vanishing
transition temperature. The temperature variation of spin susceptibility,
electrical resistivity, specific heat, and NMR relaxation rates at low
temperatures is calculated in the limit of infinite exchange enhancement within
the frame work of a self consistent spin fluctuation theory. The resulting
non-Fermi liquid behavior is due to the presence of the low lying critically
damped spin fluctuations in these systems. The theory presented here gives the
leading low temperature behavior, as it turns out that the fluctuation
correlation term is always smaller than the mean fluctuation field term in
three as well as in two space dimensions. A comparison with illustrative
experimental results of these properties in some typical systems has been done.
Finally we make some remarks on the effect of disorder in these systems.Comment: File RevTex, 7 Figures available on request, Abstract and text
modified, To appear in Phys. Rev.
Strongly coupled quantum criticality with a Fermi surface in two dimensions: fractionalization of spin and charge collective modes
We describe two dimensional models with a metallic Fermi surface which
display quantum phase transitions controlled by strongly interacting critical
field theories below their upper critical dimension. The primary examples
involve transitions with a topological order parameter associated with
dislocations in collinear spin density wave ("stripe") correlations: the
gapping of the order parameter fluctuations leads to a fractionalization of
spin and charge collective modes, and this transition has been proposed as a
candidate for the cuprates near optimal doping. The coupling between the order
parameter and long-wavelength volume and shape deformations of the Fermi
surface is analyzed by the renormalization group, and a runaway flow to a
non-perturbative regime is found in most cases. A phenomenological scaling
analysis of simple observable properties of possible second order quantum
critical points is presented, with results quite similar to those near quantum
spin glass transitions and to phenomenological forms proposed by Schroeder et
al. (cond-mat/0011002).Comment: 16 pages, 4 figures; (v2) additional clarifying remark
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