374 research outputs found
Negative Giant Longitudinal Magnetoresistance in NiMnSb/InSb: An interface effect
We report on the electrical and magneto-transport properties of the contact
formed between polycrystalline NiMnSb thin films grown using pulsed laser
deposition (PLD) and n-type degenerate InSb (100) substrates. A negative giant
magnetoresistance (GMR) effect is observed when the external magnetic field is
parallel to the surface of the film and to the current direction. We attribute
the observed phenomenon to magnetic precipitates formed during the magnetic
film deposition and confined to a narrow layer at the interface. The effect of
these precipitates on the magnetoresistance depends on the thermal processing
of the system.Comment: 14 pages, 4 figure
Mass enhancement in narrow band systems
A perturbative study of the Holstein Molecular Crystal Model which accounts
for lattice structure and dimensionality effects is presented. Antiadiabatic
conditions peculiar of narrow band materials and an intermediate to strong
electron-phonon coupling are assumed. The polaron effective mass depends
crucially in all dimensions on the intermolecular coupling strengths which also
affect the size of the lattice deformation associated with the small polaron
formation.Comment: Istituto Nazionale di Fisica della Materia - Dipartimento di
Matematica e Fisica, Istituto Nazionale di Fisica della Materia Universita'
di Camerino, 62032 Camerino, Ital
Polaron self-trapping in a honeycomb net
Small polaron behavior in a two dimensional honeycomb net is studied by
applying the strong coupling perturbative method to the Holstein molecular
crystal model. We find that small optical polarons can be mobile also if the
electrons are strongly coupled to the lattice. Before the polarons localize and
become very heavy, there is infact a window of {\it e-ph} couplings in which
the polarons are small and have masses of order times the bare
band mass according to the value of the adiabaticity parameter. The 2D
honeycomb net favors the mobility of small optical polarons in comparison with
the square lattice.Comment: 6 pages, 3 figures, to appear in J.Phys.:Condensed Matter {PACS:
63.10.+a, 63.20.Dj, 71.38.+i
Many-body large polaron optical conductivity in SrTiNbO
Recent experimental data on the optical conductivity of niobium doped
SrTiO are interpreted in terms of a gas of large polarons with effective
coupling constant . The {theoretical approach takes into
account} many-body effects, the electron-phonon interaction with multiple
LO-phonon branches, and the degeneracy and the anisotropy of the Ti t
conduction band. {Based on the Fr\"{o}hlich interaction, the many-body
large-polaron theory} provides an interpretation for the essential
characteristics, except -- interestingly -- for the unexpectedly large
intensity of a peak at meV, of the observed optical conductivity
spectra of SrTiNbO \textit{without} any adjustment of
material parameters.Comment: to appear in Phys. Rev.
Negative Magnetoresistance in (In,Mn)As
The magnetotransport properties of an In0.95Mn0.05As thin film grown by
metal-organic vapor phase epitaxy were measured. Resistivity was measured over
the temperature range of 5 to 300 K. The resistivity decreased with increasing
temperature from 90 ohm-cm to 0.05 ohm-cm. The field dependence of the low
temperature magnetoresistance was measured. A negative magnetoresistance was
observed below 17 K with a hysteresis in the magnetoresistance observed at 5 K.
The magnetoresistance as a function of applied field was described by the
Khosla-Fischer model for spin scattering of carriers in an impurity band.Comment: 8 pages, 4 figures, accepted to Physical Review
Intersublevel Polaron Dephasing in Self-Assembled Quantum Dots
Polaron dephasing processes are investigated in InAs/GaAs dots using
far-infrared transient four wave mixing (FWM) spectroscopy. We observe an
oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant
excitation of the lowest energy conduction band transition due to coherent
acoustic phonon generation. The subsequent single exponential decay yields long
intraband dephasing times of 90 ps. We find excellent agreement between our
measured and calculated FWM dynamics, and show that both real and virtual
acoustic phonon processes are necessary to explain the temperature dependence
of the polarization decay.Comment: 10 pages, 4 figures, submitted to Phys Rev Let
Polaron Effective Mass, Band Distortion, and Self-Trapping in the Holstein Molecular Crystal Model
We present polaron effective masses and selected polaron band structures of
the Holstein molecular crystal model in 1-D as computed by the Global-Local
variational method over a wide range of parameters. These results are augmented
and supported by leading orders of both weak- and strong-coupling perturbation
theory. The description of the polaron effective mass and polaron band
distortion that emerges from this work is comprehensive, spanning weak,
intermediate, and strong electron-phonon coupling, and non-adiabatic, weakly
adiabatic, and strongly adiabatic regimes. Using the effective mass as the
primary criterion, the self-trapping transition is precisely defined and
located. Using related band-shape criteria at the Brillouin zone edge, the
onset of band narrowing is also precisely defined and located. These two lines
divide the polaron parameter space into three regimes of distinct polaron
structure, essentially constituting a polaron phase diagram. Though the
self-trapping transition is thusly shown to be a broad and smooth phenomenon at
finite parameter values, consistency with notion of self-trapping as a critical
phenomenon in the adiabatic limit is demonstrated. Generalizations to higher
dimensions are considered, and resolutions of apparent conflicts with
well-known expectations of adiabatic theory are suggested.Comment: 28 pages, 15 figure
Theory of optical spectra of polar quantum wells: Temperature effects
Theoretical and numerical calculations of the optical absorption spectra of
excitons interacting with longitudinal-optical phonons in quasi-2D polar
semiconductors are presented. In II-VI semiconductor quantum wells, exciton
binding energy can be tuned on- and off-resonance with the longitudinal-optical
phonon energy by varying the quantum well width. A comprehensive picture of
this tunning effect on the temperature-dependent exciton absorption spectrum is
derived, using the exciton Green's function formalism at finite temperature.
The effective exciton-phonon interaction is included in the Bethe-Salpeter
equation. Numerical results are illustrated for ZnSe-based quantum wells. At
low temperatures, both a single exciton peak as well as a continuum resonance
state are found in the optical absorption spectra. By contrast, at high enough
temperatures, a splitting of the exciton line due to the real phonon absorption
processes is predicted. Possible previous experimental observations of this
splitting are discussed.Comment: 10 pages, 9 figures, to appear in Phys. Rev. B. Permanent address:
[email protected]
The Yellow Excitonic Series of Cu2O Revisited by Lyman Spectroscopy
We report on the observation of the yellow exciton Lyman series up to the
fourth term in Cu2O by time-resolved mid-infrared spectroscopy. The dependence
of the oscillator strength on the principal quantum number n can be well
reproduced using the hydrogenic model including an AC dielectric constant, and
precise information on the electronic structure of the 1s exciton state can be
obtained. A Bohr radius a_{1s}=7.9 A and a 1s-2p transition dipole moment
\mu_{1s-2p}= 4.2 eA were found
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