2,006 research outputs found
Chirality Dependence of the -Momentum Dark Excitons in Carbon Nanotubes
Using a collection of twelve semiconducting carbon nanotube samples, each
highly enriched in a single chirality, we study the chirality dependence of the
-momentum dark singlet exciton using phonon sideband optical spectroscopy.
Measurements of bright absorptive and emissive sidebands of this finite
momentum exciton identify its energy as 20 - 38 meV above the bright singlet
exciton, a separation that exhibits systematic dependencies on tube diameter,
family, and semiconducting type. We present calculations that explain
how chiral angle dependence in this energy separation relates to the Coulomb
exchange interaction, and elaborate the dominance of the phonon
sidebands over the zone-center phonon sidebands over a wide range of
chiralities. The Kataura plot arising from these data is qualitatively well
described by theory, but the energy separation between the sidebands shows a
larger chiral dependence than predicted. This latter observation may indicate a
larger dispersion for the associated phonon near the point than expected
from finite distance force modeling.Comment: 24 pages, 12 figures, 1 table; slight title change, Figures 1 and 11
added, reference added, presentation improved throughout documen
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
Spin Precession and Oscillations in Mesoscopic Systems
We compare and contrast magneto-transport oscillations in the fully quantum
(single-electron coherent) and classical limits for a simple but illustrative
model. In particular, we study the induced magnetization and spin current in a
two-terminal double-barrier structure with an applied Zeeman field between the
barriers and spin disequilibrium in the contacts. Classically, the spin current
shows strong tunneling resonances due to spin precession in the region between
the two barriers. However, these oscillations are distinguishable from those in
the fully coherent case, for which a proper treatment of the electron phase is
required. We explain the differences in terms of the presence or absence of
coherent multiple wave reflections.Comment: 9 pages, 5 figure
Coulomb interaction effects in spin-polarized transport
We study the effect of the electron-electron interaction on the transport of
spin polarized currents in metals and doped semiconductors in the diffusive
regime. In addition to well-known screening effects, we identify two additional
effects, which depend on many-body correlations and exchange and reduce the
spin diffusion constant. The first is the "spin Coulomb drag" - an intrinsic
friction mechanism which operates whenever the average velocities of up-spin
and down-spin electrons differ. The second arises from the decrease in the
longitudinal spin stiffness of an interacting electron gas relative to a
noninteracting one. Both effects are studied in detail for both degenerate and
non-degenerate carriers in metals and semiconductors, and various limiting
cases are worked out analytically. The behavior of the spin diffusion constant
at and below a ferromagnetic transition temperature is also discussed.Comment: 9 figure
Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)
Here we report the magnetic properties of the layered cobalt oxide system,
LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on
dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR
observations, the electronic phase diagram of LixCoO2 has been established. As
in the related material NaxCoO2, a magnetic critical point is found to exist
between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a
Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the
antiferromagnetic spin correlations systematically increase with decreasing x.
Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole
temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other
hand, various phase transitions are observed. For x = 0.40, a susceptibility
hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump,
which is likely to be triggered by charge ordering, is clearly observed at Tt =
175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink
appears at T = 210 K in the sample with an intermediate Li composition, x =
0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the
electronic properties are sensitively influenced by the Li content (x).Comment: 29 pages, 1 table, 9 figure
Electric-field dependent spin diffusion and spin injection into semiconductors
We derive a drift-diffusion equation for spin polarization in semiconductors
by consistently taking into account electric-field effects and nondegenerate
electron statistics. We identify a high-field diffusive regime which has no
analogue in metals. In this regime there are two distinct spin diffusion
lengths. Furthermore, spin injection from a ferromagnetic metal into a
semiconductor is enhanced by several orders of magnitude and spins can be
transported over distances much greater than the low-field spin diffusion
length.Comment: 5 pages, 3 eps figure
Oscillating magnetoresistance in diluted magnetic semiconductor barrier structures
Ballistic spin polarized transport through diluted magnetic semiconductor
(DMS) single and double barrier structures is investigated theoretically using
a two-component model. The tunneling magnetoresistance (TMR) of the system
exhibits oscillating behavior when the magnetic field are varied. An
interesting beat pattern in the TMR and spin polarization is found for
different NMS/DMS double barrier structures which arises from an interplay
between the spin-up and spin-down electron channels which are splitted by the
s-d exchange interaction.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
Spin diffusion in doped semiconductors
The behavior of spin diffusion in doped semiconductors is shown to be
qualitatively different than in undoped (intrinsic) ones. Whereas a spin packet
in an intrinsic semiconductor must be a multiple-band disturbance, involving
inhomogeneous distributions of both electrons and holes, in a doped
semiconductor a single-band disturbance is possible. For n-doped nonmagnetic
semiconductors the enhancement of diffusion due to a degenerate electron sea in
the conduction band is much larger for these single-band spin packets than for
charge packets, and can exceed an order of magnitude at low temperatures even
for equilibrium dopings as small as 10^16 cm^-3. In n-doped ferromagnetic and
semimagnetic semiconductors the motion of spin packets polarized antiparallel
to the equilibrium carrier spin polarization is predicted to be an order of
magnitude faster than for parallel polarized spin packets. These results are
reversed for p-doped semiconductors.Comment: 8 pages, 4 figure
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