1,533 research outputs found
Optical conductivity in A3C60 (A=K, Rb)
We study the optical conductivity in A3C60 (A =K, Rb). The effects of the
electron-phonon interaction is included to lowest order in the coupling
strength lambda. It is shown that this leads to a narrowing of the Drude peak
by a factor 1+lambda and a transfer of weight to a mid-infrared peak at
somewhat larger energies than the phonon energy. Although this goes in the
right direction, it is not sufficient to describe experiment.Comment: 5 pages, revtex, 2 figures more information at
http://librix.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Nonlocal density functionals and the linear response of the homogeneous electron gas
The known and usable truly nonlocal functionals for exchange-correlation
energy of the inhomogeneous electron gas are the ADA (average density
approximation) and the WDA (weighted density approximation). ADA, by design,
yields the correct linear response function of the uniform electron gas. WDA is
constructed so that it is exact in the limit of one-electron systems. We derive
an expression for the linear response of the uniform gas in the WDA, and
calculate it for several flavors of WDA. We then compare the results with the
Monte-Carlo data on the exchange-correlation local field correction, and
identify the weak points of conventional WDA in the homogeneous limit. We
suggest how the WDA can be modified to improve the response function. The
resulting approximation is a good one in both opposite limits, and should be
useful for practical nonlocal density functional calculations.Comment: 4 pages, two eps figures embedde
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Crystal-field splittings in CeX (X= N, P, As, Sb, Bi) compounds
The unusual and interesting physical properties of rare earth intemetallic
compounds have their origin in the combination of strongly correlated 4f states
and their hybridization with the conduction electron sea, which gives rise to
their complex low temperature Kondo behavior. In particular, Ce compounds are
very sensitive to the crystalline and chemical environment, as compared to
other rare earth systems. The interaction of the 4f state with the conduction
band plays an important role in the determination of the different magnetic,
structural and transport properties of these systems. Among the cerium
compounds, those of the type CeX, which crystallize in the rock salt structure,
exhibit extremely unusual magnetic properties. By making use of the mixed
LDA-NCA calculation technique we analyse the crystal-field splittings of CeX
compounds (X=N, P, As, Sb, Bi). The obtained ab-initio hybridization functions
are taken as imputs to calculate the crystal-field splittings within NCA (non
crossing approximation) and the tendencies are contrasted with experiments. KEY
WORDS: Highly correlated systems, crystal fields, p-electron.Comment: 8 pages, 2 figure
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Magnetic anisotropy in strained manganite films and bicrystal junctions
Transport and magnetic properties of LSMO manganite thin films and bicrystal
junctions were investigated. Manganite films were epitaxially grown on STO,
LAO, NGO and LSAT substrates and their magnetic anisotropy were determined by
two techniques of magnetic resonance spectroscopy. Compare with cubic
substrates a small (about 0.3 persentage), the anisotropy of the orthorhombic
NGO substrate leads to a uniaxial anisotropy of the magnetic properties of the
films in the plane of the substrate. Samples with different tilt of
crystallographic basal planes of manganite as well as bicrystal junctions with
rotation of the crystallographic axes (RB - junction) and with tilting of basal
planes (TB - junction) were investigated. It was found that on vicinal NGO
substrates the value of magnetic anisotropy could be varied by changing the
substrate inclination angle from 0 to 25 degrees. Measurement of magnetic
anisotropy of manganite bicrystal junction demonstrated the presence of two
ferromagnetically ordered spin subsystems for both types of bicrystal
boundaries RB and TB. The magnitude of the magnetoresistance for TB - junctions
increased with decreasing temperature and with the misorientation angle even
misorientation of easy axes in the parts of junction does not change. Analysis
of the voltage dependencies of bicrystal junction conductivity show that the
low value of the magnetoresistance for the LSMO bicrystal junctions can be
caused by two scattering mechanisms with the spin- flip of spin - polarized
carriers due to the strong electron - electron interactions in a disordered
layer at the bicrystal boundary at low temperatures and the spin-flip by anti
ferromagnetic magnons at high temperatures.Comment: 26 pages, 10 figure
Electronic thermal conductivity at high temperatures: Violation of the Wiedemann-Franz law in narrow band metals
We study the electronic part of the thermal conductivity kappa of metals. We
present two methods for calculating kappa, a quantum Monte-Carlo (QMC) method
and a method where the phonons but not the electrons are treated
semiclassically (SC). We compare the two methods for a model of alkali-doped
C60, A3C60, and show that they agree well. We then mainly use the SC method,
which is simpler and easier to interpret. We perform SC calculations for Nb for
large temperatures T and find that kappa increases with T as kappa(T)=a+bT,
where a and b are constants, consistent with a saturation of the mean free
path, l, and in good agreement with experiment. In contrast, we find that for
A3C60, kappa(T) decreases with T for very large T. We discuss the reason for
this qualitatively in the limit of large T. We give a quantum-mechanical
explanation of the saturation of l for Nb and derive the Wiedemann-Franz law in
the limit of T much smaller than W, where W is the band width. In contrast, due
to the small W of A3C60, the assumption T much smaller than W can be violated.
We show that this leads to kappa(T) \sim T^{-3/2} for very large T and a strong
violation of the Wiedemann-Franz law.Comment: 8 pages, 4 figure
Optimized Effective Potential Model for the Double Perovskites Sr2-xYxVMoO6 and Sr2-xYxVTcO6
In attempt to explore half-metallic properties of the double perovskites
Sr2-xYxVMoO6 and Sr2-xYxVTcO6, we construct an effective low-energy model,
which describes the behavior of the t2g-states of these compounds. All
parameters of such model are derived rigorously on the basis of
first-principles electronic structure calculations. In order to solve this
model we employ the optimized effective potential method and treat the
correlation interactions in the random phase approximation. Although
correlation interactions considerably reduce the intraatomic exchange splitting
in comparison with the Hartree-Fock method, this splitting still substantially
exceeds the typical values obtained in the local-spin-density approximation
(LSDA), which alters many predictions based on the LSDA. Our main results are
summarized as follows: (i) all ferromagnetic states are expected to be
half-metallic. However, their energies are generally higher than those of the
ferrimagnetic ordering between V- and Mo/Tc-sites (except Sr2VMoO6); (ii) all
ferrimagnetic states are metallic (except fully insulating Y2VTcO6) and no
half-metallic antiferromagnetism has been found; (iii) moreover, many of the
ferrimagnetic structures appear to be unstable with respect to the spin-spiral
alignment. Thus, the true magnetic ground state of the most of these systems is
expected to be more complex. In addition, we discuss several methodological
issues related to the nonuniqueness of the effective potential for the magnetic
half-metallic and insulating states.Comment: 15 pages, 9 figure
Observation of quantum capacitance in the Cooper-pair transistor
We have fabricated a Cooper-pair transistor (CPT) with parameters such that
for appropriate voltage biases, the sub-gap charge transport takes place via
slow tunneling of quasiparticles that link two Josephson-coupled charge
manifolds. In between the quasiparticle tunneling events, the CPT behaves
essentially like a single Cooper-pair box (SCB). The effective capacitance of a
SCB can be defined as the derivative of the induced charge with respect to gate
voltage. This capacitance has two parts, the geometric capacitance, C_geom, and
the quantum capacitance C_Q. The latter is due to the level anti-crossing
caused by the Josephson coupling. It depends parametrically on the gate voltage
and is dual to the Josephson inductance. Furthermore, it's magnitude may be
substantially larger than C_geom. We have been able to detect C_Q in our CPT,
by measuring the in-phase and quadrature rf-signal reflected from a resonant
circuit in which the CPT is embedded. C_Q can be used as the basis of a charge
qubit readout by placing a Cooper-pair box in such a resonant circuit.Comment: 3 figure
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