136 research outputs found
Temperature dependent asymmetry of the nonlocal spin-injection resistance: evidence for spin non-conserving interface scattering
We report nonlocal spin injection and detection experiments on mesoscopic
Co-Al2O3-Cu spin valves. We have observed a temperature dependent asymmetry in
the nonlocal resistance between parallel and antiparallel configurations of the
magnetic injector and detector. This strongly supports the existence of a
nonequilibrium resistance that depends on the relative orientation of the
detector magnetization and the nonequilibrium magnetization in the normal metal
providing evidence for increasing interface spin scattering with temperature.Comment: 5 pages, 4 figures, accepted for publication in PRL, minor
corrections (affiliation, acknowledgements, typo
Dynamical Susceptibility in KDP-type Crysals above and below Tc II
The path probability method (PPM) in the tetrahedron-cactus approximation is
applied to the Slater-Takagi model with dipole-dipole interaction for
KH2PO4-type hydrogen-bonded ferroelectric crystals in order to derive a small
dip structure in the real part of dynamical susceptibility observed at the
transition temperature Tc. The dip structure can be ascribed to finite
relaxation times of electric dipole moments responsible for the first order
transition with contrast to the critical slowing down in the second order
transition. The light scattering intensity which is related to the imaginary
part of dynamical susceptibility is also calculated above and below the
transition temperature and the obtained central peak structure is consistent
with polarization fluctuation modes in Raman scattering experiments.Comment: 8 pages, 11 figure
Theory of thermal spin-charge coupling in electronic systems
The interplay between spin transport and thermoelectricity offers several
novel ways of generating, manipulating, and detecting nonequilibrium spin in a
wide range of materials. Here we formulate a phenomenological model in the
spirit of the standard model of electrical spin injection to describe the
electronic mechanism coupling charge, spin, and heat transport and employ the
model to analyze several different geometries containing ferromagnetic (F) and
nonmagnetic (N) regions: F, F/N, and F/N/F junctions which are subject to
thermal gradients. We present analytical formulas for the spin accumulation and
spin current profiles in those junctions that are valid for both tunnel and
transparent (as well as intermediate) contacts. For F/N junctions we calculate
the thermal spin injection efficiency and the spin accumulation induced
nonequilibrium thermopower. We find conditions for countering thermal spin
effects in the N region with electrical spin injection. This compensating
effect should be particularly useful for distinguishing electronic from other
mechanisms of spin injection by thermal gradients. For F/N/F junctions we
analyze the differences in the nonequilibrium thermopower (and chemical
potentials) for parallel and antiparallel orientations of the F magnetizations,
as evidence and a quantitative measure of the spin accumulation in N.
Furthermore, we study the Peltier and spin Peltier effects in F/N and F/N/F
junctions and present analytical formulas for the heat evolution at the
interfaces of isothermal junctions.Comment: to be published in PRB (in press), 19 pages, 19 figure
Delocalized Nature of the E'-delta Center in Amorphous Silicon Dioxide
We report an experimetal study by Electron Paramagnetic Resonance (EPR) of
E'-delta point defect induced by gamma ray irradiation in amorphous SiO2. We
obtained an estimetion of the intensity of the 10 mT doublet characterizing the
EPR spectrum of such a defect arising from hyperfine interaction of the
unpaired electron with a 29Si (I=1/2) nucleus. Moreover, determining the
intensity ratio between this hyperfine doublet and the main resonance line of
E'-delta center, we pointed out that unpaired electron wave function of this
center is actually delocalized over four nearly equivalent silicon atoms.Comment: approved for publication in Physical Review Letter
Restrictions on modeling spin injection by resistor networks
Because of the technical difficulties of solving spin transport equations in
inhomogeneous systems, different resistor networks are widely applied for
modeling spin transport. By comparing an analytical solution for spin injection
across a ferromagnet - paramagnet junction with a resistor model approach, its
essential limitations stemming from inhomogeneous spin populations are
clarified.Comment: To be published in a special issue of Semicond. Sci. Technol., Guest
editor Prof. G. Landweh
Drift-diffusion model for spin-polarized transport in a non-degenerate 2DEG controlled by a spin-orbit interaction
We apply the Wigner function formalism to derive drift-diffusion transport
equations for spin-polarized electrons in a III-V semiconductor single quantum
well. Electron spin dynamics is controlled by the linear in momentum spin-orbit
interaction. In a studied transport regime an electron momentum scattering rate
is appreciably faster than spin dynamics. A set of transport equations is
defined in terms of a particle density, spin density, and respective fluxes.
The developed model allows studying of coherent dynamics of a non-equilibrium
spin polarization. As an example, we consider a stationary transport regime for
a heterostructure grown along the (0, 0, 1) crystallographic direction. Due to
the interplay of the Rashba and Dresselhaus spin-orbit terms spin dynamics
strongly depends on a transport direction. The model is consistent with results
of pulse-probe measurement of spin coherence in strained semiconductor layers.
It can be useful for studying properties of spin-polarized transport and
modeling of spintronic devices operating in the diffusive transport regime.Comment: 16 pages, 3 figure
Spin transport in inhomogeneous magnetic fields: a proposal for Stern-Gerlach-like experiments with conduction electrons
Spin dynamics in spatially inhomogeneous magnetic fields is studied within
the framework of Boltzmann theory. Stern-Gerlach-like separation of spin up and
spin down electrons occurs in ballistic and diffusive regimes, before spin
relaxation sets in. Transient dynamics and spectral response to time-dependent
inhomogeneous magnetic fields are investigated, and possible experimental
observations of our findings are discussed.Comment: 7 pages, 4 figures; revised and extended version, to appear in PR
Proof of the thermodynamical stability of the E' center in SiO2
The E' center is a paradigmatic radiation-induced defect in SiO2 whose
peculiar EPR and hyperfine activity has been known since over 40 years. This
center has been traditionally identified with a distorted, positively-charged
oxygen vacancy V_O+. However, no direct proof of the stability of this defect
has ever been provided, so that its identification is still strongly
incomplete. Here we prove directly that distorted V_O+ is metastable and that
it satisfies the key requirements for its identification as E', such as thermal
and optical response, and activation-deactivation mechanisms.Comment: RevTeX 4 pages, 2 figure
Optical orientation in bipolar spintronic devices
Optical orientation is a highly efficient tool for the generation of
nonequilibrium spin polarization in semiconductors. Combined with
spin-polarized transport it offers new functionalities for conventional
electronic devices, such as pn junction bipolar diodes or transistors. In
nominally nonmagnetic junctions optical orientation can provide a source for
spin capacitance--the bias-dependent nonequilibrium spin accumulation--or for
spin-polarized current in bipolar spin-polarized solar cells. In magnetic
junctions, the nonequilibrium spin polarization generated by spin orientation
in a proximity of an equilibrium magnetization gives rise to the spin-voltaic
effect (a realization of the Silsbee-Johnson coupling), enabling efficient
control of electrical properties such as the I-V characteristics of the
junctions by magnetic and optical fields. This article reviews the main results
of investigations of spin-polarized and magnetic pn junctions, from spin
capacitance to the spin-voltaic effect.Comment: 9 pages, 10 figures; appeared in the special issue of Semicond. Sci.
Technol. on Optical Orientation, in honor of B. P. Zakharcheny
Structural relaxation of E' gamma centers in amorphous silica
We report experimental evidence of the existence of two variants of the E'
gamma centers induced in silica by gamma rays at room temperature. The two
variants are distinguishable by the fine features of their line shapes in
paramagnetic resonance spectra. These features suggest that the two E' gamma
differ for their topology. We find a thermally induced interconversion between
the centers with an activation energy of about 34 meV. Hints are also found for
the existence of a structural configuration of minimum energy and of a
metastable state.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
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