3,053 research outputs found
Superconductivity and Lattice Instability in Compressed Lithium from Fermi Surface Hot Spots
The highest superconducting temperature T observed in any elemental metal
(Li with T ~ 20 K at pressure P ~ 40 GPa) is shown to arise from critical
(formally divergent) electron-phonon coupling to the transverse T phonon
branch along intersections of Kohn anomaly surfaces with the Fermi surface.
First principles linear response calculations of the phonon spectrum and
spectral function reveal (harmonic) instability already at
25 GPa. Our results imply that the fcc phase is anharmonically stabilized in
the 25-38 GPa range.Comment: 4 pages, 3 embedded figure
Fermi surface of the colossal magnetoresistance perovskite La_{0.7}Sr_{0.3}MnO_{3}
Materials that exhibit colossal magnetoresistance (CMR) are currently the
focus of an intense research effort, driven by the technological applications
that their sensitivity lends them to. Using the angular correlation of photons
from electron-positron annihilation, we present a first glimpse of the Fermi
surface of a material that exhibits CMR, supported by ``virtual crystal''
electronic structure calculations. The Fermi surface is shown to be
sufficiently cubic in nature that it is likely to support nesting.Comment: 5 pages, 5 PS figure
Theory of Superconducting of doped fullerenes
We develop the nonadiabatic polaron theory of superconductivity of
taking into account the polaron band narrowing and realistic
electron-phonon and Coulomb interactions. We argue that the crossover from the
BCS weak-coupling superconductivity to the strong-coupling polaronic and
bipolaronic superconductivity occurs at the BCS coupling constant independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's
theorem except small polarons for any realistic electron-phonon interaction. By
the use of the polaronic-type function and the ``exact'' diagonalization in the
truncated Hilbert space of vibrons (``phonons'') we calculate the ground state
energy and the electron spectral density of the molecule. This
allows us to describe the photoemission spectrum of in a wide
energy region and determine the electron-phonon interaction. The strongest
coupling is found with the high-frequency pinch mode and with the
Frenkel exciton. We clarify the crucial role of high-frequency bosonic
excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb
repulsion allowing the intermediate and low-frequency phonons to couple two
small polarons in a Cooper pair. The Eliashberg-type equations are solved for
low-frequency phonons. The value of the superconducting , its pressure
dependence and the isotope effect are found to be in a remarkable agreement
with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques
Half-metallic antiferromagnets in double perovskites: LaAVRuO (A=Ca, Sr, and Ba)
Based on the theoretical exploration of electronic structures, we propose
that the ordered double perovskites LaAVRuO and LaVO/ARuO (001)
superlattice (A = Ca, Sr and Ba) are strong candidates for half-metallic (HM)
antiferromagnets (AFMs). %LaAVRuO and LaVO/ARuO have the %100% spin
polarizations at the Fermi level but with zero %total magnetic moments. We have
shown that the HM-AFM nature in LaAVRuO is very robust regardless of (i)
divalent ion replacement at A-sites, (ii) oxygen site relaxation, (iii) the
inclusion of the Coulomb correlation, and (iv) cation disorder. A type of the
double exchange interaction is expected to be responsible for the
half-metallicity and the antiferromagnetism in these systems.Comment: 4 pages, 4 figure
Spin Gaps and Bilayer Coupling in YBaCuO and YBaCuO
We investigate the relevance to the physics of underdoped
YBaCuO and YBaCuO of the quantum critical point
which occurs in a model of two antiferromagnetically coupled planes of
antiferromagnetically correlated spins. We use a Schwinger boson mean field
theory and a scaling analysis to obtain the phase diagram of the model and the
temperature and frequency dependence of various susceptibilities and relaxation
rates. We distinguish between a low coupled-planes regime in which
the optic spin excitations are frozen out and a high
decoupled-planes regime in which the two planes fluctuate independently. In the
coupled-planes regime the yttrium nuclear relaxation rate at low temperatures
is larger relative to the copper and oxygen rates than would be naively
expected in a model of uncorrelated planes. Available data suggest that in
YBaCuO the crossover from the coupled to the decoupled planes
regime occurs at or . The predicted correlation length is
of order 6 lattice constants at . Experimental data related to the
antiferromagnetic susceptibility of YBaCuO may be made consistent
with the theory, but available data for the uniform susceptibility are
inconsistent with the theory.Comment: RevTex 3.
Origin of Drastic Change of Fermi Surface and Transport Anomalies in CeRhIn5 under Pressure
The mechanism of drastic change of Fermi surfaces as well as transport
anomalies near P=Pc=2.35 GPa in CeRhIn5 is explained theoretically. The key
mechanism is pointed out to be the interplay of magnetic order and Ce-valence
fluctuations. We show that the antiferromagnetic state with "small" Fermi
surfaces changes to the paramagnetic state with "large" Fermi surfaces with
huge enhancement of effective mass of electrons with keeping finite c-f
hybridization. This explains the drastic change of the de Haas-van Alphen
signals. Furthermore, it is also consistent with the emergence of T-linear
resistivity simultaneous with the residual resistivity peak at P=Pc in CeRhIn5.Comment: 5 pages, 3 figures, submitted to Journal of Physical Society of Japa
Priming of production in maize of volatile organic defence compounds by the natural plant activator cis- jasmone
cis-Jasmone (CJ) is a natural plant product that activates defence against herbivores in model and crop plants. In this study, we investigated whether CJ could prime defence in maize, Zea mays, against the leafhopper, Cicadulina storeyi, responsible for the transmission of maize streak virus (MSV). Priming occurs when a pre-treatment, in this case CJ, increases the potency and speed of a defence response upon subsequent attack on the plant. Here, we tested insect responses to plant volatile organic compounds (VOCs) using a Y-tube olfactometer bioassay. Our initial experiments showed that, in this system, there was no significant response of the herbivore to CJ itself and no difference in response to VOCs collected from unexposed plants compared to CJ exposed plants, both without insects. VOCs were then collected from C. storeyi-infested maize seedlings with and without CJ pre-treatment. The bioassay revealed a significant preference by this pest for VOCs from infested seedlings without the CJ pre-treatment. A timed series of VOC collections and bioassays showed that the effect was strongest in the first 22 h of insect infestation, i.e. before the insects had themselves induced a change in VOC emission. Chemical analysis showed that treatment of maize seedlings with CJ, followed by exposure to C. storeyi, led to a significant increase in emission of the defensive sesquiterpenes (E)-(1R,9S)-caryophyllene, (E)-α-bergamotene, (E)-β-farnesene and (E)-4,8-dimethyl-1,3,7-nonatriene, known to act as herbivore repellents. The chemical analysis explains the behavioural effects observed in the olfactometer, as the CJ treatment caused plants to emit a blend of VOCs comprising more of the repellent components in the first 22 h of insect infestation than control plants. The speed and potency of VOC emission was increased by the CJ pre-treatment. This is the first indication that CJ can prime plants for enhanced production of defensive VOCs antagonist towards herbivores
Double-exchange is not the cause of ferromagnetism in doped manganites
The coexistence of ferromagnetism and metallic conduction in doped manganites
has long been explained by a double-exchange model in which the ferromagnetic
exchange arises from the carrier hopping. We evaluate the zero-temperature spin
stiffness D(0) and the Curie temperature T_{C} on the basis of the
double-exchange model using the measured values of the bare bandwidth W and the
Hund's rule coupling J_{H}. The calculated D(0) and T_{C} values are too small
compared with the observed ones even in the absence of interactions. A
realistic onsite interorbital Coulomb repulsion can reduce D(0) substantially
in the case of a 2-orbital model. Furthermore, experiment shows that D(0) is
simply proportional to x in La_{1-x}Sr_{x}MnO_{3} system, independent of
whether the ground state is a ferromagnetic insulator or metal. These results
strongly suggest that the ferromagnetism in manganites does not originate from
the double-exchange interaction. On the other hand, an alternative model based
on the d-p exchange can semi-quantitatively explain the ferromagnetism of doped
manganites at low temperatures.Comment: 6 pages, 3 figures, some modifications in scientific content
Electron-Like Fermi Surface and Remnant (pi,0) Feature in Overdoped La1.78Sr0.22CuO4
We have performed an angle-resolved photoemission study of overdoped
La1.78Sr0.22CuO4, and have observed sharp nodal quasiparticle peaks in the
second Brillouin zone that are comparable to data from Bi2Sr2CaCu2O8+d. The
data analysis using energy distribution curves, momentum distribution curves
and intensity maps all show evidence of an electron-like Fermi surface, which
is well explained by band structure calculations. Evidence for many-body
effects are also found in the substantial spectral weight remaining below the
Fermi level around (pi,0), where the band is predicted to lie above EF.Comment: 4 pages, 4 figure
Magnetic Susceptibility for
We examine experimental magnetic susceptibility for
CaVO by fitting with fitting function .
The function is a power series of 1/T and the lowest order
term is fixed as , where is the Curie constant as determined by the
experimental -value (g=1.96). Fitting parameters are , and
expansion coefficients except for the first one in .
We determine and as 0.73 and 0 for an
experimental sample. We interpret as the volume fraction of
CaVO in the sample and as the susceptibility for the
pure CaVO. The result of means that the sample includes
nonmagnetic components. This interpretation consists with the result of a
perturbation theory and a neutron scattering experiment.Comment: 4pages, 4figure
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