886 research outputs found
Generation of spin currents via Raman scattering
We show theoretically that stimulated spin flip Raman scattering can be used
to inject spin currents in doped semiconductors with spin split bands. A pure
spin current, where oppositely oriented spins move in opposite directions, can
be injected in zincblende crystals and structures. The calculated spin current
should be detectable by pump-probe optical spectroscopy and anomalous Hall
effect measurement
Frustrated square lattice with spatial anisotropy: crystal structure and magnetic properties of PbZnVO(PO4)2
Crystal structure and magnetic properties of the layered vanadium phosphate
PbZnVO(PO4)2 are studied using x-ray powder diffraction, magnetization and
specific heat measurements, as well as band structure calculations. The
compound resembles AA'VO(PO4)2 vanadium phosphates and fits to the extended
frustrated square lattice model with the couplings J(1), J(1)' between
nearest-neighbors and J(2), J(2)' between next-nearest-neighbors. The
temperature dependence of the magnetization yields estimates of averaged
nearest-neighbor and next-nearest-neighbor couplings, J(1) ~ -5.2 K and J(2) ~
10.0 K, respectively. The effective frustration ratio alpha=J(2)/J(1) amounts
to -1.9 and suggests columnar antiferromagnetic ordering in PbZnVO(PO4)2.
Specific heat data support the estimates of J(1) and J(2) and indicate a likely
magnetic ordering transition at 3.9 K. However, the averaged couplings
underestimate the saturation field, thus pointing to the spatial anisotropy of
the nearest-neighbor interactions. Band structure calculations confirm the
identification of ferromagnetic J(1), J(1)' and antiferromagnetic J(2), J(2)'
in PbZnVO(PO4)2 and yield J(1)'-J(1) ~ 1.1 K in excellent agreement with the
experimental value of 1.1 K, deduced from the difference between the expected
and experimentally measured saturation fields. Based on the comparison of
layered vanadium phosphates with different metal cations, we show that a
moderate spatial anisotropy of the frustrated square lattice has minor
influence on the thermodynamic properties of the model. We discuss relevant
geometrical parameters, controlling the exchange interactions in these
compounds, and propose a new route towards strongly frustrated square lattice
materials.Comment: 14 pages, 9 figures, 5 table
Electron-Electron Relaxation Effect on Auger Recombination in Direct Band Semiconductors
Influence of electron-electron relaxation processes on Auger recombination
rate in direct band semiconductors is investigated. Comparison between
carrier-carrier and carrier-phonon relaxation processes is provided. It is
shown that relaxation processes are essential if the free path length of
carriers doesn't exceed a certain critical value, which exponentially increases
with temperature. For illustration of obtained results a typical InGaAsP
compound is used
Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7
We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that
was controversially described with respect to its structural organization and
magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction,
electron diffraction, transmission electron microscopy, and band structure
calculations, we solve the room-temperature structure of this compound
[alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The
gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889
A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7
structure, the Cu and Cl atoms are randomly displaced from the special
positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x
c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub),
space group Pbam] are stable between 640 K and 500 K and below 500 K,
respectively. The structural changes at 500 K and 640 K are identified as
order-disorder phase transitions. The displacement of the Cl atoms is frozen
upon the gamma --> beta transformation, while a cooperative tilting of the NbO6
octahedra in the alpha-phase further eliminates the disorder of the Cu atoms.
The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types
of the atomic displacements that interfere due to the bonding between the Cu
atoms and the apical oxygens of the NbO6 octahedra. The precise structural
information resolves the controversy between the previous computation-based
models and provides the long-sought input for understanding the magnetic
properties of (CuCl)LaNb2O7.Comment: 12 pages, 10 figures, 5 tables; crystallographic information (cif
files) include
Correlation effects in sequential energy branching: an exact model of the Fano statistics
Correlation effects in in the fluctuation of the number of particles in the
process of energy branching by sequential impact ionizations are studied using
an exactly soluble model of random parking on a line. The Fano factor F
calculated in an uncorrelated final-state "shot-glass" model does not give an
accurate answer even with the exact gap-distribution statistics. Allowing for
the nearest-neighbor correlation effects gives a correction to F that brings F
very close to its exact value. We discuss the implications of our results for
energy resolution of semiconductor gamma detectors, where the value of F is of
the essence. We argue that F is controlled by correlations in the cascade
energy branching process and hence the widely used final-state model estimates
are not reliable -- especially in the practically relevant cases when the
energy branching is terminated by competition between impact ionization and
phonon emission.Comment: 11 pages, 4 figures. Submitted to Physical Review
Theory of transient spectroscopy of multiple quantum well structures
A theory of the transient spectroscopy of quantum well (QW) structures under
a large applied bias is presented. An analytical model of the initial part of
the transient current is proposed. The time constant of the transient current
depends not only on the emission rate from the QWs, as is usually assumed, but
also on the subsequent carrier transport across QWs. Numerical simulation was
used to confirm the validity of the proposed model, and to study the transient
current on a larger time scale. It is shown that the transient current is
influenced by the nonuniform distribution of the electric field and related
effects, which results in a step-like behavior of the current. A procedure of
extraction of the QW emission time from the transient spectroscopy experiments
is suggested.Comment: 5 pages, 4 figures, to be published in J. Appl. Phy
Spiral ground state against ferroelectricity in the frustrated magnet BiMnFe2O6
The spiral magnetic structure and underlying spin lattice of BiMnFe2O6 are
investigated by low-temperature neutron powder diffraction and density
functional theory band structure calculations. In spite of the random
distribution of the Mn3+ and Fe3+ cations, this compound undergoes a transition
into an incommensurate antiferromagnetically ordered state below TN ~ 220 K.
The magnetic structure is characterized by the propagation vector k=[0,beta,0]
with beta ~ 0.14 and the P22_12_11'(0 \beta 0)0s0s magnetic superspace
symmetry. It comprises antiferromagnetic helixes propagating along the b-axis.
The magnetic moments lie in the ac plane and rotate about pi*(1+beta) ~ 204.8
deg angle between the adjacent magnetic atoms along b. The spiral magnetic
structure arises from the peculiar frustrated arrangement of exchange couplings
in the ab plane. The antiferromagnetic coupling along the c-axis leads to the
cancellation of electric polarization, and results in the lack of
ferroelectricity in BiMnFe2O6.Comment: 11 pages, 8 figures, 8 table
(CuCl)LaTa2O7 and quantum phase transition in the (CuX)LaM2O7 family (X = Cl, Br; M = Nb, Ta)
We apply neutron diffraction, high-resolution synchrotron x-ray diffraction,
magnetization measurements, electronic structure calculations, and quantum
Monte-Carlo simulations to unravel the structure and magnetism of
(CuCl)LaTa2O7. Despite the pseudo-tetragonal crystallographic unit cell, this
compound features an orthorhombic superstructure, similar to the Nb-containing
(CuX)LaNb2O7 with X = Cl and Br. The spin lattice entails dimers formed by the
antiferromagnetic fourth-neighbor coupling J4, as well as a large number of
nonequivalent interdimer couplings quantified by an effective exchange
parameter Jeff. In (CuCl)LaTa2O7, the interdimer couplings are sufficiently
strong to induce the long-range magnetic order with the Neel temperature TN~7 K
and the ordered magnetic moment of 0.53 mu_B, as measured with neutron
diffraction. This magnetic behavior can be accounted for by Jeff/J4~1.6 and
J4~16 K. We further propose a general magnetic phase diagram for the
(CuCl)LaNb2O7-type compounds, and explain the transition from the gapped
spin-singlet (dimer) ground state in (CuCl)LaNb2O7 to the long-range
antiferromagnetic order in (CuCl)LaTa2O7 and (CuBr)LaNb2O7 by an increase in
the magnitude of the interdimer couplings Jeff/J_4, with the (CuCl)LaM2O7 (M =
Nb, Ta) compounds lying on different sides of the quantum critical point that
separates the singlet and long-range-ordered magnetic ground states.Comment: 13 pages, 13 figures, 4 tables + Supplementary informatio
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