240 research outputs found
Calculation of the static and dynamical correlation energy of pseudo-one-dimensional beryllium systems via a many-body expansion
Low-dimensional beryllium systems constitute interesting case studies for the
test of correlation methods because of the importance of both static and
dynamical correlation in the formation of the bond. Aiming to describe the
whole dissociation curve of extended Be systems we chose to apply the method of
increments (MoI) in its multireference (MR) formalism. However, in order to do
so an insight into the wave function was necessary. Therefore we started by
focusing on the description of small Be chains via standard quantum chemical
methods and gave a brief analysis of the main characteristics of their wave
functions. We then applied the MoI to larger beryllium systems, starting from
the Be6 ring. First, the complete active space formalism (CAS-MoI) was employed
and the results were used as reference for local MR calculations of the whole
dissociation curve. Despite this approach is well established for the
calculation of systems with limited multireference character, its application
to the description of whole dissociation curves still requires further testing.
After discussing the role of the basis set, the method was finally applied to
larger rings and extrapolated to an infinite chain
Proximity-driven source of highly spin-polarized ac current on the basis of superconductor/weak ferromagnet/superconductor voltage-biased Josephson junction
We theoretically investigate an opportunity to implement a source of highly
spin-polarized ac current on the basis of superconductor/weak
ferromagnet/superconductor (SFS) voltage-biased junction in the regime of
essential proximity effect and calculate the current flowing through the probe
electrode tunnel coupled to the ferromagnetic interlayer region. It is shown
that while the polarization of the dc current component is generally small in
case of weak exchange field of the ferromagnet, there is an ac component of the
current in the system. This ac current is highly spin-polarized and entirely
originated from the non-equilibrium proximity effect in the interlayer. The
frequency of the current is controlled by the voltage applied to SFS junction.
We discuss a possibility to obtain a source of coherent ac currents with a
certain phase shift between them by tunnel coupling two probe electrodes at
different locations of the interlayer region.Comment: 8 pages, 5 figure
Low-energy renormalization of the electron dispersion of high-T superconductors
High-resolution ARPES studies in cuprates have detected low-energy changes in
the dispersion and absorption of quasi-particles at low temperatures, in
particular, in the superconducting state. Based on a new 1/N expansion of the
t-J-Holstein model, which includes collective antiferromagnetic fluctuations
already in leading order, we argue that the observed low-energy structures are
mainly caused by phonons and not by spin fluctuations, at least, in the optimal
and overdoped regime.Comment: 6 pages, 3 figure
Effect of an Electron-phonon Interaction on the One-electron Spectral Weight of a d-wave Superconductor
We analyze the effects of an electron-phonon interaction on the one-electron
spectral weight A(k,omega) of a d_{x^2-y^2} superconductor. We study the case
of an Einstein phonon mode with various momentum-dependent electron-phonon
couplings and compare the structure produced in A(k,omega) with that obtained
from coupling to the magnetic pi-resonant mode. We find that if the strength of
the interactions are adjusted to give the same renormalization at the nodal
point, the differences in A(k,omega) are generally small but possibly
observable near k=(pi,0).Comment: 10 pages, 14 figures (color versions of Figs. 2,4,10,11,12 available
upon request
Theory of Flux-Flow Resistivity near for s-wave Type-II Superconductors
This paper presents a microscopic calculation of the flux-flow resistivity
for s-wave type-II superconductors with arbitrary impurity
concentrations near the upper critical field . It is found that, as the
mean free path becomes longer, increases gradually from the
dirty-limit result of Thompson [Phys. Rev. B{\bf 1}, 327 (1970)] and Takayama
and Ebisawa [Prog. Theor. Phys. {\bf 44}, 1450 (1970)]. The limiting behaviors
suggest that at low temperatures may change from convex downward
to upward as increases, thus deviating substantially from the linear
dependence predicted by the Bardeen-Stephen theory
[Phys. Rev. {\bf 140}, A1197 (1965)]
Electronic and magnetic properties of KCuPO - a model S=1/2 Heisenberg chain system
The electronic and magnetic properties of KCuPO were
investigated by means of susceptibility, specific heat and P nuclear
magnetic resonance (NMR) measurements and by LDA band structure calculations.
The temperature dependence of the NMR shift is well described by the
Heisenberg antiferromagnetic chain (HAF) model with nearest neighbor
exchange K. The corresponding mapping of an
LDA-derived tight-binding model leads to 196 K. The spin
lattice relaxation rate decreases with temperature below 300 K but
becomes nearly temperature independent between 30 K and 2 K as theoretically
expected for an HAF chain. None of the investigated properties give
any evidence for long range magnetic order above 2K, in agreement with the
results of the band structure calculation, which yield extremely weak exchange
to the next nearest neighbor (NNN) and a very small and frustrated inter-chain
exchange. Thus, KCuPO seems to be a better realization of a
nearest neighbor HAF chain than the compounds reported so far.Comment: 9 pages, 8 figures. Accepted in PRB 200
Quasiparticle spectrum of the cuprate BiSrCaCuO: Possible connection to the phase diagram
We previously introduced [T. Cren et al., Europhys. Lett. 52, 203 (2000)] an
energy-dependant gap function, , that fits the unusual shape of the
quasiparticle (QP) spectrum for both BiSrCaCuO and YBaCuO. A simple
anti-resonance in accounts for the pronounced QP peaks in the
density of states, at an energy , and the dip feature at a higher
energy, . Here we go a step further : our gap function is consistent
with the () phase diagram, where is the carrier density. For large QP
energies (), the total spectral gap is , where is tied to the condensation
energy. From the available data, a simple -dependance of and
is found, in particular .
These two distinct energy scales of the superconducting state are interpreted
by comparing with the normal and pseudogap states. The various forms of the QP
density of states, as well as the spectral function , are discussed
Energy density functional on a microscopic basis
In recent years impressive progress has been made in the development of
highly accurate energy density functionals, which allow to treat medium-heavy
nuclei. In this approach one tries to describe not only the ground state but
also the first relevant excited states. In general, higher accuracy requires a
larger set of parameters, which must be carefully chosen to avoid redundancy.
Following this line of development, it is unavoidable that the connection of
the functional with the bare nucleon-nucleon interaction becomes more and more
elusive. In principle, the construction of a density functional from a density
matrix expansion based on the effective nucleon-nucleon interaction is
possible, and indeed the approach has been followed by few authors. However, to
what extent a density functional based on such a microscopic approach can reach
the accuracy of the fully phenomenological ones remains an open question. A
related question is to establish which part of a functional can be actually
derived by a microscopic approach and which part, on the contrary, must be left
as purely phenomenological. In this paper we discuss the main problems that are
encountered when the microscopic approach is followed. To this purpose we will
use the method we have recently introduced to illustrate the different aspects
of these problems. In particular we will discuss the possible connection of the
density functional with the nuclear matter Equation of State and the distinct
features of finite size effects proper of nuclei.Comment: 20 pages, 6 figures,Contribution to J. Phys G, Special Issue, Focus
Section: Open Problems in Nuclear Structur
Tight-binding parameters and exchange integrals of Ba_2Cu_3O_4Cl_2
Band structure calculations for Ba_2Cu_3O_4Cl_2 within the local density
approximation (LDA) are presented. The investigated compound is similar to the
antiferromagnetic parent compounds of cuprate superconductors but contains
additional Cu_B atoms in the planes. Within the LDA, metallic behavior is found
with two bands crossing the Fermi surface (FS). These bands are built mainly
from Cu 3d_{x^2-y^2} and O 2p_{x,y} orbitals, and a corresponding tight-binding
(TB) model has been parameterized. All orbitals can be subdivided in two sets
corresponding to the A- and B-subsystems, respectively, the coupling between
which is found to be small. To describe the experimentally observed
antiferromagnetic insulating state, we propose an extended Hubbard model with
the derived TB parameters and local correlation terms characteristic for
cuprates. Using the derived parameter set we calculate the exchange integrals
for the Cu_3O_4 plane. The results are in quite reasonable agreement with the
experimental values for the isostructural compound Sr_2Cu_3O_4Cl_2.Comment: 5 pages (2 tables included), 4 ps-figure
- …