199 research outputs found
Magnetic properties of -FeCr alloy as calculated with the charge and spin self-consistent KKR(CPA) method
Magnetic properties of a FeCr alloy calculated with
the charge and spin self- consistent Korringa-Kohn-Rostoker (KKR) and combined
with coherent potential approximation (KKR-CPA) methods are reported.
Non-magnetic state as well as various magnetic orderings were considered, i.e.
ferromagnetic (FM) and more complex anti-parallel (called APM) arrangements for
selected sublattices, as follows from the symmetry analysis. It has been shown
that the Stoner criterion applied to non-magnetic density of states at the
Fermi energy, is satisfied for Fe atoms situated on all five lattice
sites, while it is not fulfilled for all Cr atoms. In FM and APM states, the
values of magnetic moments on Fe atoms occupying various sites are dispersed
between 0 and 2.5 , and they are proportional to the number of Fe atoms
in the nearest-neighbor shell. Magnetic moments of Cr atoms havin much smaller
values were found to be coupled antiparallel to those of Fe atoms. The average
value of the magnetic moment per atom was found to be that
is by a factor of 4 larger than the experimental value found for a
FeCr sample. Conversely, admitting an anti-
parallel ordering (APM model) on atoms situated on C and D sites, according to
the group theory and symmetry analysis results, yielded a substantial reduction
of to 0.20 $\mu_B$. Further diminution of to 0.15 ,
which is very close to the experimental value of 0.14 , has been
achieved with the KKR-CPA calculations by considering a chemical disorder on
sites B, C and D
Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems
A new type of perturbation expansion in the mixing of localized orbitals
with a conduction-electron band in the Anderson model is
presented. It is built on Feynman diagrams obeying standard rules. The local
correlations of the unperturbed system (the atomic limit) are included exactly,
no auxiliary particles are introduced. As a test, an infinite-order ladder-type
resummation is analytically treated in the Kondo regime, recovering the correct
energy scale. An extension to the Anderson-lattice model is obtained via an
effective-site approximation through a cumulant expansion in on the
lattice. Relation to treatments in infinite spatial dimensions are indicated.Comment: selfextracting postscript file containing entire paper (10 pages)
including 3 figures, in case of trouble contact author for LaTeX-source or
hard copies (prep0994
Quasiparticle states of the Hubbard model near the Fermi level
The spectra of the t-U and t-t'-U Hubbard models are investigated in the
one-loop approximation for different values of the electron filling. It is
shown that the four-band structure which is inherent in the case of
half-filling and low temperatures persists also for some excess or deficiency
of electrons. Besides, with some departure from half-filling an additional
narrow band of quasiparticle states arises near the Fermi level. The dispersion
of the band, its bandwidth and the variation with filling are close to those of
the spin-polaron band of the t-J model. For moderate doping spectral
intensities in the new band and in one of the inner bands of the four-band
structure decrease as the Fermi level is approached which leads to the
appearance of a pseudogap in the spectrum.Comment: 8 pages, 7 figure
Unified decoupling scheme for exchange and anisotropy contributions and temperature-dependent spectral properties of anisotropic spin systems
We compute the temperature-dependent spin-wave spectrum and the magnetization
for a spin system using the unified decoupling procedure for the high-order
Green's functions for the exchange coupling and anisotropy, both in the
classical and quantum case. Our approach allows us to establish a clear
crossover between quantum-mechanical and classical methods by developing the
classical analog of the quantum Green's function technique. The results are
compared with the classical spectral density method and numerical modeling
based on the stochastic Landau-Lifshitz equation and the Monte Carlo technique.
As far as the critical temperature is concerned, there is a full agreement
between the classical Green's functions technique and the classical spectral
density method. However, the former method turns out to be more straightforward
and more convenient than the latter because it avoids any \emph{a priori}
assumptions about the system's spectral density. The temperature-dependent
exchange stiffness as a function of magnetization is investigated within
different approaches
Disentanglement of the electronic and lattice parts of the order parameter in a 1D Charge Density Wave system probed by femtosecond spectroscopy
We report on the high resolution studies of the temperature (T) dependence of
the q=0 phonon spectrum in the quasi one-dimensional charge density wave (CDW)
compound K0.3MoO3 utilizing time-resolved optical spectroscopy. Numerous modes
that appear below Tc show pronounced T-dependences of their amplitudes,
frequencies and dampings. Utilizing the time-dependent Ginzburg-Landau theory
we show that these modes result from linear coupling of the electronic part of
the order parameter to the 2kF phonons, while the (electronic) CDW amplitude
mode is overdamped.Comment: 4 pages, 3 figures + supplementary material, accepted for publication
in Phys. Rev. Let
Conductance through a potential barrier embedded in a Luttinger liquid: nonuniversal scaling at strong coupling
We calculate the linear response conductance of electrons in a Luttinger
liquid with arbitrary interaction g_2, and subject to a potential barrier of
arbitrary strength, as a function of temperature. We map the Hamiltonian in the
basis of scattering states into an effective low energy Hamiltonian in current
algebra form. First the renormalization group (RG) equation for weak
interaction is derived in the current operator language both using the operator
product expansion and the equation of motion method. To access the strong
coupling regime, two methods of deducing the RG equation from perturbation
theory, based on the scaling hypothesis and on the Callan-Symanzik formulation,
are discussed. The important role of scale independent terms is emphasized. The
latter depend on the regulaization scheme used (length versus temperature
cutoff). Analyzing the perturbation theory in the fermionic representation, the
diagrams contributing to the renormalization group beta-function are
identified. A universal part of the beta-function is given by a ladder series
and summed to all orders in g_2. First non-universal corrections beyond the
ladder series are discussed and are shown to differ from the exact solutions
obtained within conformal field theory which use a different regularization
scheme. The RG equation for the temperature dependent conductance is solved
analytically. Our result agrees with known limiting cases.Comment: 24 pages, 5 figures, REVTEX, final versio
Neel to Spin-Glass-like Phase Transition versus Dilution in Geometrically Frustrated ZnCr_{2-2x}Ga_{2x}O_4
ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K
from a cubic spin liquid phase to a tetragonal Neel state. Using powder
diffraction and single crystal polarized neutron scattering, we determined the
complex spin structure of the Neel phase. This phase consisted of several
magnetic domains with different characteristic wave vectors. This indicates
that the tetragonal phase of ZnCr2O4 is very close to a critical point
surrounded by many different Neel states. We have also studied, using elastic
and inelastic neutron scattering techniques, the effect of nonmagnetic dilution
on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05,
the magnetic correlations do not change qualitatively from those in the pure
material, except that the phase transition becomes second order. For x= 0.3,
the spin-spin correlations become short range. Interestingly, the spatial
correlations of the frozen spins in the x=0.3 material are the same as those of
the fluctuating moments in the pure and the weakly diluted materials
The problem of "macroscopic charge quantization" in single-electron devices
In a recent Letter by the authors [I.S. Burmistrov and A.M.M. Pruisken, Phys.
Rev. Lett. 101, 056801 (2008)] it was shown that single-electron devices
(single electron transistor or SET) display "macroscopic charge quantization"
which is completely analogous to the quantum Hall effect observed on very
different electron systems. In this investigation we present more detail on
these new findings. Based on the Ambegaokar-Eckern-Schoen (AES) theory of the
Coulomb blockade we introduce a general response theory that probes the
sensitivity of SET to changes in the boundary conditions. This response theory
defines a new set of physical observables and we establish the contact with the
standard results obtained from ordinary linear response theory. The response
parameters generally define the renormalization behavior of the SET in the
entire regime from weak coupling with large values of the tunneling conductance
all the way down to the strong coupling phase where the system displays the
Coulomb blockade. We introduce a general criterion for charge quantization that
is analogous to the Thouless criterion for Anderson localization. We present
the results of detailed computations on the weak coupling side of the theory,
i.e. both perturbative and non-perturbative (instantons). Based on an effective
theory in terms of quantum spins we study the quantum critical behavior of the
AES model on the strong coupling side. Consequently, a unifying scaling diagram
of the SET is obtained. This diagram displays all the super universal
topological features of the theta-angle concept that previously arose in the
theory of the quantum Hall effect.Comment: RevTex, 22 pages, 10 figure
Fast electrochemical membrane actuator:Design, fabrication and preliminary testing
An actuator based on water electrolysis with a fast change of voltage polarity is presented. It demonstrates a new actuation principle allowing significant increase the operation frequency of the device due to fast termination of the produced gas. The actuator consists of a working chamber with metallic electrodes and supplying channels filled with an electrolyte. The chamber is formed in a layer of SU-8 and covered by a flexible polydimethylsiloxane membrane, which deforms as the pressure in the chamber increases. Design, fabrication procedure, and first tests of the actuator are described
Diagrammatic analysis of the Hubbard model:Stationary property of the thermodynamic potential
Diagrammatic approach proposed many years ago for strong correlated Hubbard
model is developed for analyzing of the thermodynamic potential properties. The
new exact relation between such renormalized quantities as thermodynamic
potential, one-particle propagator and correlation function is established.
This relation contains additional integration of the one-particle propagator by
the auxiliary constant. The vacuum skeleton diagrams constructed from
irreducible Green's functions and tunneling propagator lines are determined and
special functional is introduced. The properties of such functional are
investigated and its relation to the thermodynamic potential is established.
The stationary properties of this functional with respect to first order
changing of the correlation function is demonstrated and as a consequence the
stationary properties of the thermodynamic potential is proved.Comment: 6 pages, 4 figure
- …