2,340 research outputs found
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
Important role of alkali atoms in A4C60
We show that hopping via the alkali atoms plays an important role for the t1u
band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is
broadened by more than 40 % by the presence of the alkali atoms. The difference
between A4C60 and A3C60 is in particular due to the less symmetric location of
the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Ex-situ Tunnel Junction Process Technique Characterized by Coulomb Blockade Thermometry
We investigate a wafer scale tunnel junction fabrication method, where a
plasma etched via through a dielectric layer covering bottom Al electrode
defines the tunnel junction area. The ex-situ tunnel barrier is formed by
oxidation of the bottom electrode in the junction area. Room temperature
resistance mapping over a 150 mm wafer give local deviation values of the
tunnel junction resistance that fall below 7.5 % with an average of 1.3 %. The
deviation is further investigated by sub-1 K measurements of a device, which
has one tunnel junction connected to four arrays consisting of N junctions (N =
41, junction diameter 700 nm). The differential conductance is measured in
single-junction and array Coulomb blockade thermometer operation modes. By
fitting the experimental data to the theoretical models we found an upper limit
for the local tunnel junction resistance deviation of ~5 % for the array of
2N+1 junctions. This value is of the same order as the minimum detectable
deviation defined by the accuracy of our experimental setup
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
Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices
We report on the observation of a non-local voltage in a ballistic
one-dimensional conductor, realized by a single-wall carbon nanotube with four
contacts. The contacts divide the tube into three quantum dots which we control
by the back-gate voltage . We measure a large \emph{oscillating} non-local
voltage as a function of with zero mean. Though a classical
resistor model can account for a non-local voltage including change of sign, it
fails to describe the magnitude properly. The large amplitude of is
due to quantum interference effects and can be understood within the
scattering-approach of electron transport
Salmonella in Sheep in Iceland
In 1995 several outbreaks of food poisoning in humans occurred in Iceland, that were traced to salmonella contamination of singed sheep heads. This prompted us to study the prevalence of salmonella infection in sheep and to trace where and how infection might have occurred. Faecal, intestinal contents and tonsillar samples were collected in the spring and autumn from sheep on 50 farms in the southwestern part of the country, where salmonellosis had been detected and from 5 farms in the northwestern part of the country. All faecal samples from the southwest were negative, whereas samples from 3 farms obtained in the autumn in the northwest were positive. Tonsillae taken in the autumn were positive in sheep from 3 farms in the southwest and 2 in the northwest. Our results show that salmonella infection is rare in Icelandic sheep but healthy carriers may harbour the bacteria in tonsillae. Salmonella was not detected in drainage from slaughterhouses nor in singed sheep heads
The transition from the adiabatic to the sudden limit in core level photoemission: A model study of a localized system
We consider core electron photoemission in a localized system, where there is
a charge transfer excitation. The system is modelled by three electron levels,
one core level and two outer levels. The model has a Coulomb interaction
between these levels and the continuum states into which the core electron is
emitted. The model is simple enough to allow an exact numerical solution, and
with a separable potential an analytic solution. We calculate the ratio
r(omega) between the weights of the satellite and the main peak as a function
of the photon energy omega. The transition from the adiabatic to the sudden
limit takes place for quite small photoelectron kinetic energies. For such
small energies, the variation of the dipole matrix element is substantial and
described by the energy scale Ed. Without the coupling to the photoelectron,
the corresponding ratio r0(omega) is determined by Ed and the satellite
excitation energy dE. When the interaction potential with the continuum states
is introduced, a new energy scale Es=1/(2Rs^2) enters, where Rs is a length
scale of the interaction potential. At threshold there is typically a (weak)
constructive interference between intrinsic and extrinsic contributions, and
the ratio r(omega)/r0(omega) is larger than its limiting value for large omega.
The interference becomes small or weakly destructive for photoelectron energies
of the order Es. For larger energies r(omega)/r0(omega) therefore typically has
a weak undershoot. If this undershoot is neglected, r(omega)/r0(omega) reaches
its limiting value on the energy scale Es.Comment: 18 pages, latex2e, 13 eps figure
Ground State and Spectral Properties of a Quantum Impurity in d-Wave Superconductors
The variational approach of Gunnarsson and Sch\"onhammer to the Anderson
impurity model is generalized to study d-wave superconductors in the presence
of dilute spin-1/2 impurities. We show that the local moment is screened when
the hybridization exceeds a nonzero critical value at which the ground state
changes from a spin doublet to a spin singlet. The electron spectral functions
are calculated in both phases. We find that while a Kondo resonance develops
above the Fermi level in the singlet phase, the spectral function exhibits a
low-energy spectral peak below the Fermi level in the spin doublet phase. The
origin of such a ``virtual Kondo resonance'' is the existence of low-lying
collective excitations in the spin-singlet sector. We discuss our results in
connection to recent spectroscopic experiments on Zn doped high-T
superconductors.Comment: 5 pages, 4figures, revised versio
Magnetic properties of Ruddlesden-Popper phases SrY(FeNi)O: A combined experimental and theoretical investigation
We present a comprehensive study of the magnetic properties of
SrY(FeNi)O ().
Experimentally, the magnetic properties are investigated using superconducting
quantum interference device (SQUID) magnetometry and neutron powder diffraction
(NPD). This is complemented by the theoretical study based on density
functional theory as well as the Heisenberg exchange parameters. Experimental
results show an increase in the N\'eel temperature () with the increase of
Y concentrations and O occupancy. The NPD data reveals all samples are
antiferromagnetically ordered at low temperatures, which has been confirmed by
our theoretical simulations for the selected samples. Our first-principles
calculations suggest that the 3D magnetic order is stabilized due to finite
inter-layer exchange couplings. The latter give rise to a finite inter-layer
spin correlations which disappear above the
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