4,601 research outputs found
Quantum Tunneling in Half-Integer Spin Systems
Motivated by the experimental observations of resonant tunnelings in the
systems with half-integer spin, such as V and Mn, we study the
mechanism of adiabatic change of the magnetization in systems with the
time-reversal symmetry. Within the time-reversal symmetric models, effects of
several types of perturbations are investigated. Although tunneling between the
ground states is suppressed in a simple Kramers doublet, we show that the
nonadiabatic transition governed by the Landau-Zener-St\"uckelberg mechanism
occurs in many cases due to the additional degeneracy of the ground state. We
also found more general cases where LZS mechanism can not be applied directly
even the system shows a kind of adiabatic change of the magnetization
Ground-State Decay Rate for the Zener Breakdown in Band and Mott Insulators
Non-linear transport of electrons in strong electric fields, as typified by
dielectric breakdown, is re-formulated in terms of the ground-state decay rate
originally studied by Schwinger in non-linear QED. We discuss the effect of
electron interaction on Zener tunneling by comparing the dielectric breakdown
of the band insulator and the Mott insulator, where the latter is studied by
the time-dependent density-matrix renormalization group (DMRG). The relation
with the Berry's phase theory of polarization is also established.Comment: 5 pages 2 figures, revised text, version to appear in Phys. Rev. Let
Electronic states in a magnetic quantum-dot molecule: phase transitions and spontaneous symmetry breaking
We show that a double quantum-dot system made of diluted magnetic
semiconductor behaves unlike usual molecules. In a semiconductor double quantum
dot or in a diatomic molecule, the ground state of a single carrier is
described by a symmetric orbital. In a magnetic material molecule, new ground
states with broken symmetry can appear due the competition between the
tunnelling and magnetic polaron energy. With decreasing temperature, the ground
state changes from the normal symmetric state to a state with spontaneously
broken symmetry. Interestingly, the symmetry of a magnetic molecule is
recovered at very low temperatures. A magnetic double quantum dot with
broken-symmetry phases can be used a voltage-controlled nanoscale memory cell.Comment: 4 pages, 5 figure
Lattice and superexchange effects in doped CMR manganites
We report on the influence of the lattice degrees of freedom on charge,
orbital and spin correlations in colossal magnetoresistance (CMR) manganites.
For the weakly doped compounds we demonstrate that the electron-phonon coupling
promotes the trapping of charge carriers, the disappearance of the orbital
polaron pattern and the breakdown of ferromagnetism at the CMR transition. The
role of different superexchange interactions is explored.Comment: 2 pages, 1 figure, submitted to ICM 200
Magnetic and Transport Properties of (La,Sr)MnO
Magnetic and transport properties of the perovskite-type
transition-metal oxide (La,Sr)MnO are theoretically studied using the
double-exchange model in infinite dimension. Magnetoresistance properties as
well as the magnetic transition temperatures are in good agreement with the
experimental data.Comment: 5 pages, LaTeX, including 2 PS figures. To be published in Proc.
International Symposium 'Frontiers of High Tc Superconductivity' (Oct. 1995,
Morioka, Japan
Role of Correlation and Exchange for Quasi-particle Spectra of Magnetic and Diluted Magnetic Semiconductors
Theoretical foundation and application of the generalized spin-fermion (sp-d)
exchange lattice model to magnetic and diluted magnetic semiconductors are
discussed. The capabilities of the model to describe spin quasi-particle
spectra are investigated. The main emphasis is made on the dynamic behavior of
two interacting subsystems, the localized spins and spin density of itinerant
carriers. A nonperturbative many-body approach, the Irreducible Green Functions
(IGF) method, is used to describe the quasi-particle dynamics. Scattering
states are investigated and three branches of magnetic excitations are
calculated in the regime, characteristic of a magnetic semiconductor. For a
simplified version of the model (Kondo lattice model) we study the spectra of
quasi-particle excitations with special attention given to diluted magnetic
semiconductors. For this, to include the effects of disorder, modified mean
fields are determined self-consistently. The role of the Coulomb correlation
and exchange is clarified by comparing of both the cases.Comment: 34 page
Different origin of the ferromagnetic order in (Ga,Mn)As and (Ga,Mn)N
The mechanism for the ferromagnetic order of (Ga,Mn)As and (Ga,Mn)N is
extensively studied over a vast range of Mn concentrations. We calculate the
electronic structures of these materials using density functional theory in
both the local spin density approximation and the LDA+U scheme, that we have
now implemented in the code SIESTA.
For (Ga,Mn)As, the LDA+U approach leads to a hole mediated picture of the
ferromagnetism, with an exchange constant =~ -2.8 eV. This is smaller
than that obtained with LSDA, which overestimates the exchange coupling between
Mn ions and the As holes.
In contrast, the ferromagnetism in wurtzite (Ga,Mn)N is caused by the
double-exchange mechanism, since a hole of strong character is found at the
Fermi level in both the LSDA and the LDA+U approaches. In this case the
coupling between the Mn ions decays rapidly with the Mn-Mn separation. This
suggests a two phases picture of the ferromagnetic order in (Ga,Mn)N, with a
robust ferromagnetic phase at large Mn concentration coexisting with a diluted
weak ferromagnetic phase.Comment: 12 pages, 11 figure
MICROWAVE-INDUCED RESONANT REFLECTION AND LOCALIZATION OF BALLISTIC ELECTRONS IN QUANTUM MICROCHANNELS
We show that electron transport in a ballistic microchannel supporting both
propagating and reflected modes can be completely blocked by applying a
microwave electromagnetic field. The effect is due to resonant reflection
caused by multiple coherent electron-photon scattering involving at least two
spatially localized scattering centers in the channel. With many such
scattering centers present the conductance is shown to have an irregular
dependence on bias voltage, gate voltage and frequency with irregularily spaced
dips corresponding to resonant reflection. When averaged over bias, gate
voltage or frequency the conductance will decay exponentially with channel
length in full analogy with the localization of 1D electrons caused by impurity
scattering.Comment: 4 pages, latex, 1 figure available on reques
Y(Ni, Mn)O3 epitaxial thin films prepared by pulsed laser deposition
High–quality epitaxial YNixMn1-xO3 thin films have been successfully
grown on SrTiO3 (100) (STO) by pulsed laser deposition. X-ray diffraction
studies showed that the films deposited on STO are fully c-axis oriented and
exhibit in-plane alignment. The magnetic transition temperatures (Tc) of the
films (both x=0.33 and 0.5) are equivalent to the values of the corresponding
bulk samples. However, when x=0.5, the films show magnetic properties quite
different from those of bulk samples. This difference may be caused by the
structure distortion in these films.Comment: 14 pages, 4 figures. to be publishe
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