664 research outputs found
Theory of Orbital Ordering, Fluctuation and Resonant X-ray Scattering in Manganites
A theory of resonant x-ray scattering in perovskite manganites is developed
by applying the group theory to the correlation functions of the pseudospin
operators for the orbital degree of freedom. It is shown that static and
dynamical informations of the orbital state are directly obtained from the
elastic, diffuse and inelastic scatterings due to the tensor character of the
scattering factor. We propose that the interaction and its anisotropy between
orbitals are directly identified by the intensity contour of the diffuse
scattering in the momentum space.Comment: 4 pages, 1 figur
Spin relaxation in (110) and (001) InAs/GaSb superlattices
We report an enhancement of the electron spin relaxation time (T1) in a (110)
InAs/GaSb superlattice by more than an order of magnitude (25 times) relative
to the corresponding (001) structure. The spin dynamics were measured using
polarization sensitive pump probe techniques and a mid-infrared, subpicosecond
PPLN OPO. Longer T1 times in (110) superlattices are attributed to the
suppression of the native interface asymmetry and bulk inversion asymmetry
contributions to the precessional D'yakonov Perel spin relaxation process.
Calculations using a nonperturbative 14-band nanostructure model give good
agreement with experiment and indicate that possible structural inversion
asymmetry contributions to T1 associated with compositional mixing at the
superlattice interfaces may limit the observed spin lifetime in (110)
superlattices. Our findings have implications for potential spintronics
applications using InAs/GaSb heterostructures.Comment: 4 pages, 2 figure
Electron Spin Decoherence in Bulk and Quantum Well Zincblende Semiconductors
A theory for longitudinal (T1) and transverse (T2) electron spin coherence
times in zincblende semiconductor quantum wells is developed based on a
non-perturbative nanostructure model solved in a fourteen-band restricted basis
set. Distinctly different dependences of coherence times on mobility,
quantization energy, and temperature are found from previous calculations.
Quantitative agreement between our calculations and measurements is found for
GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb quantum wells.Comment: 11 pages, 3 figure
Ferredoxin containing bacteriocins suggest a novel mechanism of iron uptake in <i>Pectobacterium spp</i>
In order to kill competing strains of the same or closely related bacterial species, many bacteria produce potent narrow-spectrum protein antibiotics known as bacteriocins. Two sequenced strains of the phytopathogenic bacterium <i>Pectobacterium carotovorum</i> carry genes encoding putative bacteriocins which have seemingly evolved through a recombination event to encode proteins containing an N-terminal domain with extensive similarity to a [2Fe-2S] plant ferredoxin and a C-terminal colicin M-like catalytic domain. In this work, we show that these genes encode active bacteriocins, pectocin M1 and M2, which target strains of <i>Pectobacterium carotovorum</i> and <i>Pectobacterium atrosepticum</i> with increased potency under iron limiting conditions. The activity of pectocin M1 and M2 can be inhibited by the addition of spinach ferredoxin, indicating that the ferredoxin domain of these proteins acts as a receptor binding domain. This effect is not observed with the mammalian ferredoxin protein adrenodoxin, indicating that <i>Pectobacterium spp.</i> carries a specific receptor for plant ferredoxins and that these plant pathogens may acquire iron from the host through the uptake of ferredoxin. In further support of this hypothesis we show that the growth of strains of <i>Pectobacterium carotovorum</i> and <i>atrosepticum</i> that are not sensitive to the cytotoxic effects of pectocin M1 is enhanced in the presence of pectocin M1 and M2 under iron limiting conditions. A similar growth enhancement under iron limiting conditions is observed with spinach ferrodoxin, but not with adrenodoxin. Our data indicate that pectocin M1 and M2 have evolved to parasitise an existing iron uptake pathway by using a ferredoxin-containing receptor binding domain as a Trojan horse to gain entry into susceptible cells
Dynamics of metallic stripes in cuprates
We study the dynamics of metallic vertical stripes in cuprates within the
three-band Hubbard model based on a recently developed time dependent
Gutzwiller approximation. As doping increases the optical conductivity shows
transfer of spectral weight from the charge transfer band towards i) an
incoherent band centered at 1.3eV, {ii} a Drude peak, mainly due to motion
along the stripe, {iii} a low energy collective mode which softens with doping
and merges with ii} at optimum doping in good agreement with experiment. The
softening is related to the quasidegeneracy between Cu centered and O centered
mean-field stripe solutions close to optimal doping.Comment: 4 pages, 5 figures, corrections to Fig.
Resonant X-ray Scattering in Manganites - Study of Orbital Degree of Freedom -
Orbital degree of freedom of electrons and its interplay with spin, charge
and lattice degrees of freedom are one of the central issues in colossal
magnetoresistive manganites. The orbital degree of freedom has until recently
remained hidden, since it does not couple directly to most of experimental
probes. Development of synchrotron light sources has changed the situation; by
the resonant x-ray scattering (RXS) technique the orbital ordering has
successfully been observed . In this article, we review progress in the recent
studies of RXS in manganites. We start with a detailed review of the RXS
experiments applied to the orbital ordered manganites and other correlated
electron systems. We derive the scattering cross section of RXS where the
tensor character of the atomic scattering factor (ASF) with respect to the
x-ray polarization is stressed. Microscopic mechanisms of the anisotropic
tensor character of ASF is introduced and numerical results of ASF and the
scattering intensity are presented. The azimuthal angle scan is a unique
experimental method to identify RXS from the orbital degree of freedom. A
theory of the azimuthal angle and polarization dependence of the RXS intensity
is presented. The theoretical results show good agreement with the experiments
in manganites. Apart from the microscopic description of ASF, a theoretical
framework of RXS to relate directly to the 3d orbital is presented. The
scattering cross section is represented by the correlation function of the
pseudo-spin operator for the orbital degree of freedom. A theory is extended to
the resonant inelastic x-ray scattering and methods to observe excitations of
the orbital degree of freedom are proposed.Comment: 47 pages, 24 figures, submitted to Rep. Prog. Phy
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