7,957 research outputs found
Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion
With a variational three--body calculation we study the role of the interplay
between the onsite Coulomb, Hund's rule, and superexchange interactions on the
spinwave excitation spectrum of itinerant ferromagnets. We show that
correlations between a Fermi sea electron--hole pair and a magnon result in a
very pronounced zone boundary softening and strong deviations from the
Heisenberg spinwave dispersion. We show that this spin dynamics depends
sensitively on the Coulomb and exchange interactions and discuss its possible
relevance to experiments in the manganites.Comment: 4 pages, 4 figures, published in Physical Review B as rapid
communication
Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors
We develop a theory of the magnetization dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. We describe the effects of
the strong carrier spin relaxation on the nonlinear optical response by using
the Lindblad semigroup method. We demonstrate magnetization control during
femtosecond timescales via the interplay between circularly polarized optical
excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin
interactions. Our results show a light-induced magnetization precession and
relaxation for the duration of the optical pulse.Comment: 4 pages, 2 figure
Faraday-rotation fluctuation spectroscopy with static and oscillating magnetic fields
By Faraday-rotation fluctuation spectroscopy one measures the spin noise via
Faraday-induced fluctuations of the polarization plane of a laser transmitting
the sample. In the fist part of this paper, we present a theoretical model of
recent experiments on alkali gas vapors and semiconductors, done in the
presence of a {\em static} magnetic field. In a static field, the spin noise
shows a resonance line, revealing the Larmor frequency and the spin coherence
time of the electrons. Second, we discuss the possibility to use an {\em
oscillating} magnetic field in the Faraday setup. With an oscillating field
applied, one can observe multi-photon absorption processes in the spin noise.
Furthermore an oscillating field could also help to avoid line broadening due
to structural or chemical inhomogeneities in the sample, and thereby increase
the precision of the spin-coherence time measurement.Comment: 5 pages, 7 figure
Measurement of the energy dependence of phase relaxation by single electron tunneling
Single electron tunneling through a single impurity level is used to probe
the fluctuations of the local density of states in the emitter. The energy
dependence of quasi-particle relaxation in the emitter can be extracted from
the damping of the fluctuations of the local density of states (LDOS). At
larger magnetic fields Zeeman splitting is observed.Comment: 2 pages, 4 figures; 25th International Conference on the Physics of
Semiconductors, Osaka, Japan, September 17-22, 200
Signatures of few-body resonances in finite volume
We study systems of bosons and fermions in finite periodic boxes and show how
the existence and properties of few-body resonances can be extracted from
studying the volume dependence of the calculated energy spectra. Using a
plane-wave-based discrete variable representation to conveniently implement
periodic boundary conditions, we establish that avoided level crossings occur
in the spectra of up to four particles and can be linked to the existence of
multi-body resonances. To benchmark our method we use two-body calculations,
where resonance properties can be determined with other methods, as well as a
three-boson model interaction known to generate a three-boson resonance state.
Finding good agreement for these cases, we then predict three-body and
four-body resonances for models using a shifted Gaussian potential. Our results
establish few-body finite-volume calculations as a new tool to study few-body
resonances. In particular, the approach can be used to study few-neutron
systems, where such states have been conjectured to exist.Comment: 13 pages, 10 figures, 2 tables, published versio
Non-collinear single-electron spin-valve transistors
We study interaction effects on transport through a small metallic cluster
connected to two ferromagnetic leads (a single-electron spin-valve transistor)
in the "orthodox model" for the Coulomb blockade. The non-local exchange
between the spin accumulation on the island and the ferromagnetic leads is
shown to affect the transport properties such as the electric current and
spin-transfer torque as a function of the magnetic configuration, gate voltage,
and applied magnetic field.Comment: 4 pages, 3 figure
A strong converse for classical channel coding using entangled inputs
A fully general strong converse for channel coding states that when the rate
of sending classical information exceeds the capacity of a quantum channel, the
probability of correctly decoding goes to zero exponentially in the number of
channel uses, even when we allow code states which are entangled across several
uses of the channel. Such a statement was previously only known for classical
channels and the quantum identity channel. By relating the problem to the
additivity of minimum output entropies, we show that a strong converse holds
for a large class of channels, including all unital qubit channels, the
d-dimensional depolarizing channel and the Werner-Holevo channel. This further
justifies the interpretation of the classical capacity as a sharp threshold for
information-transmission.Comment: 9 pages, revte
EPR and ferromagnetism in diluted magnetic semiconductor quantum wells
Motivated by recent measurements of electron paramagnetic resonance (EPR)
spectra in modulation-doped CdMnTe quantum wells, [F.J. Teran {\it et al.},
Phys. Rev. Lett. {\bf 91}, 077201 (2003)], we develop a theory of collective
spin excitations in quasi-two-dimensional diluted magnetic semiconductors
(DMSs). Our theory explains the anomalously large Knight shift found in these
experiments as a consequence of collective coupling between Mn-ion local
moments and itinerant-electron spins. We use this theory to discuss the physics
of ferromagnetism in (II,Mn)VI quantum wells, and to speculate on the
temperature at which it is likely to be observed in n-type modulation doped
systems.Comment: 4 pages, 1 figur
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