5,431 research outputs found
Ab-initio study of disorder effects on the electronic and magnetic structures of SrFeMoO
We have investigated the electronic structure of ordered and disordered
SrFeMoO using {\it ab-initio} band structure methods. The effect of
disorder was simulated within super-cell calculations to realize several
configurations with mis-site disorders. It is found that such disorder effects
destroy the half-metallic ferro-magnetic state of the ordered compound. It also
leads to a substantial reduction of the magnetic moments at the Fe sites in the
disordered configurations. Most interestingly, it is found for the disordered
configurations, that the magnetic coupling within the Fe sub-lattice as well as
that within the Mo sub-lattice always remain ferro-magnetic, while the two
sub-lattices couple anti-ferromagnetically, in close analogy to the magnetic
structure of the ordered compound, but in contrast to recent suggestions.Comment: 7 pages, 3 figure
Impurity effects on semiconductor quantum bits in coupled quantum dots
We theoretically consider the effects of having unintentional charged
impurities in laterally coupled two-dimensional double (GaAs) quantum dot
systems, where each dot contains one or two electrons and a single charged
impurity in the presence of an external magnetic field. Using molecular orbital
and configuration interaction method, we calculate the effect of the impurity
on the 2-electron energy spectrum of each individual dot as well as on the
spectrum of the coupled-double-dot 2-electron system. We find that the
singlet-triplet exchange splitting between the two lowest energy states, both
for the individual dots and the coupled dot system, depends sensitively on the
location of the impurity and its coupling strength (i.e. the effective charge).
A strong electron-impurity coupling breaks down equality of the two
doubly-occupied singlets in the left and the right dot leading to a mixing
between different spin singlets. As a result, the maximally entangled qubit
states are no longer fully obtained in zero magnetic field case. Moreover, a
repulsive impurity results in a triplet-singlet transition as the impurity
effective charge increases or/and the impurity position changes. We comment on
the impurity effect in spin qubit operations in the double dot system based on
our numerical results.Comment: published version on Physical Review B journal, 25 pages, 26 figure
Origin of Ferromagnetism and its pressure and doping dependence in TlMnO
Using NMTO-{\it downfolding} technique, we explore and establish the origin
of ferromagnetism in the pyrochlore system, TlMnO. It is
found to be driven by hybridization induced spin-polarization of the
delocalized charge carriers derived from Tl- and O- states. The
mean-field estimate of the ferromagnetic transition temperature, T,
estimated using computed exchange integrals are found to be in good agreement
with the measurements. We find an enhancement of T for moderate doping
with nonmagnetic Sb and a suppression of T upon application of pressure,
both in agreement with experimental findings.Comment: Accepted for publication in PR
Screening of charged impurities with multi-electron singlet-triplet spin qubits in quantum dots
Charged impurities in semiconductor quantum dots comprise one of the main
obstacles to achieving scalable fabrication and manipulation of singlet-triplet
spin qubits. We theoretically show that using dots that contain several
electrons each can help to overcome this problem through the screening of the
rough and noisy impurity potential by the excess electrons. We demonstrate how
the desired screening properties turn on as the number of electrons is
increased, and we characterize the properties of a double quantum dot
singlet-triplet qubit for small odd numbers of electrons per dot. We show that
the sensitivity of the multi-electron qubit to charge noise may be an order of
magnitude smaller than that of the two-electron qubit.Comment: 17 pages, 11 figures; typos corrected, minor revision
Analytically solvable driven time-dependent two-level quantum systems
Analytical solutions to the time-dependent Schrodinger equation describing a
driven two-level system are invaluable to many areas of physics, but they are
also extremely rare. Here, we present a simple algorithm that generates an
unlimited number of exact analytical solutions. We show that a general
single-axis driving term and its corresponding evolution operator are
determined by a single real function which is constrained only by a certain
inequality and initial conditions. Any function satisfying these constraints
yields an exact analytical solution. We demonstrate this method by presenting
several new exact solutions to the time-dependent Schrodinger equation. Our
general method and many of the new solutions we present are particularly
relevant to qubit control in quantum computing applications.Comment: 4.5 pages, 4 figures, PRL versio
Spin-polarized transport in inhomogeneous magnetic semiconductors: theory of magnetic/nonmagnetic p-n junctions
A theory of spin-polarized transport in inhomogeneous magnetic semiconductors
is developed and applied to magnetic/nonmagnetic p-n junctions. Several
phenomena with possible spintronic applications are predicted, including
spinvoltaic effect, spin valve effect, and giant magnetoresistance. It is
demonstrated that only nonequilibrium spin can be injected across the
space-charge region of a p-n junction, so that there is no spin injection (or
extraction) at low bias.Comment: Minor Revisions. To appear in Phys. Rev. Let
Analysis of the resistance in p-SiGe over a wide temperature range
The temperature dependence of a system exhibiting a `metal-insulator
transition in two dimensions at zero magnetic field' (MIT) is studied up to
90K. Using a classical scattering model we are able to simulate the
non-monotonic temperature dependence of the resistivity in the metallic high
density regime. We show that the temperature dependence arises from a complex
interplay of metallic and insulating contributions contained in the calculation
of the scattering rate 1/\td(E,T), each dominating in a limited temperature
range.Comment: 4 pages with 5 figure
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