929 research outputs found
Nuclear magnetic susceptibility of metals with magnetic impurities
We consider the contribution of magnetic impurities to the nuclear magnetic
susceptibility and to the specific heat of a metal. The impurity
contribution to the magnetic susceptibility has a behaviour, and the
impurity contribution to the specific heat has a behaviour, both in an
extended region of temperatures . In the case of a dirty metal the RKKY
interaction of nuclear spins and impurity spins is suppressed for low
temperatures and the main contribution to and is given by their
dipole-dipole interaction.Comment: 9 pages, 4 figures, REVTE
Excitations in Spin Chains and Specific-Heat Anomalies in Yb(4)As(3)
An explanation is given for the observed magnetic-field dependence of the
low-temperature specific heat coefficient of Yb(4)As(3). It is based on a
recently developed model for that material which can explain the observed
heavy-fermion behaviour. According to it the Yb(3+)-ions are positioned in a
net of parallel chains with an effective spin coupling of the order of J = 25
K. The magnetic-field dependence can be understood by including a weak magnetic
coupling J' between adjacent chains. The data require a ratio J'/J of about
10^{-4}. In that case the experimental results can be reproduced very well by
the theory.Comment: 5 pages, 5 PostScript-figures, needs LaTeX2e and the graphics-packag
Orbital Localization and Delocalization Effects in the U 5f^2 Configuration: Impurity Problem
Anderson models, based on quantum chemical studies of the molecule of
U(C_8H_8)_2, are applied to investigate the problem of an U impurity in a
metal. The special point here is that the U 5f-orbitals are divided into two
subsets: an almost completely localized set and a considerably delocalized one.
Due to the crystal field, both localized and delocalized U 5f-orbitals affect
the low-energy physics. A numerical renormalization group study shows that
every fixed point is characterized by a residual local spin and a phase shift.
The latter changes between 0 and \pi/2, which indicates the competition between
two different fixed points. Such a competition between the different local
spins at the fixed points reflects itself in the impurity magnetic
susceptibility at high temperatures. These different features cannot be
obtained if the special characters of U 5f-orbitals are neglected.Comment: 4 pages, REVTeX, email to [email protected]
Fast computation of the Kohn-Sham susceptibility of large systems
For hybrid systems, such as molecules grafted onto solid surfaces, the
calculation of linear response in time dependent density functional theory is
slowed down by the need to calculate, in N^4 operations, the susceptibility of
N non interacting Kohn-Sham reference electrons. We show how this
susceptibility can be calculated N times faster within finite precision. By
itself or in combination with previous methods, this should facilitate the
calculation of TDDFT response and optical spectra of hybrid systems.Comment: submitted 25/1/200
Fermiology of Cuprates from First Principles: From Small Pockets to the Luttinger Fermi surface
Fermiology, the shape and size of the Fermi surface, underpins the
low-temperature physical properties of a metal. Recent investigations of the
Fermi surface of high-Tc superconductors, however, show a most unusual
behavior: upon addition of carriers, ``Fermi'' pockets appear around nodal
(hole doping) and antinodal (electron doping) regions of the Brillouin zone in
the ``pseudogap'' state. With progressive doping, p, these evolve into
well-defined Fermi surfaces around optimal doping (p_opt), with no pseudogap.
Correspondingly, various physical responses, including d-wave
superconductivity, evolve from highly anomalous, up to p_opt, to more
conventional beyond. Describing this evolution holds the key to understanding
high-temperature superconductivity. Here, we present ab initio quantum chemical
results for cuprates, providing a quantitative description of the evolution of
the Fermi surface with doping. Our results constitute an ab initio
justification for several, hitherto proposed semiphenomenological theories,
offering an unified basis for understanding of various, unusual physical
responses of doped cuprates
Eliashberg theory of superconductivity and inelastic rare-earth impurity scattering in filled skutterudite LaPrOsSb
We study the influence of inelastic rare-earth impurity scattering on
electron-phonon mediated superconductivity and mass renormalization in
(LaPr)OsSb compounds. Solving the strong coupling
Eliashberg equations we find that the dominant quadrupolar component of the
inelastic scattering on Pr impurities yields an enhancement of the
superconducting transition temperature T in LaOsSb and
increases monotonically as a function of Pr concentration. The calculated
results are in good agreement with the experimentally observed T
dependence. Our analysis suggests that phonons and quadrupolar excitations
cause the attractive electron interaction which results in the formation of
Cooper pairs and singlet superconductivity in PrOsSb.Comment: 5 pages,4 figures, revised title suggested by editor, original fig.4
and fig.5 combined together, discussion added before conclusio
Interaction of a Magnetic Impurity with Strongly Correlated Conduction Electrons
We consider a magnetic impurity which interacts by hybridization with a
system of strongly correlated conduction electrons. The latter are described by
a Hubbard Hamiltonian. By means of a canconical transformation the charge
degrees of freedom of the magnetic impurity are eliminated. The resulting
effective Hamiltonian is investigated and various limiting cases
are considered. If the Hubbard interaction between the conduction electrons
is neglected reduces to a form obtained by the Schrieffer-Wolff
transformation, which is essentially the Kondo Hamiltonian. If is large and
the correlations are strong is changed. One modification concerns
the coefficient of the dominant exchange coupling of the magnetic impurity with
the nearest lattice site. When the system is hole doped, there is also an
antiferromagnetic coupling to the nearest neighbors of that site involving
additionally a hole. Furthermore, it is found that the magnetic impurity
attracts a hole. In the case of electron doping, double occupancies are
repelled by the impurity. In contrast to the hole-doped case, we find no
magnetic coupling which additionally involves a doubly occupied site.Comment: 16 pages, Revtex 3.
Phase diagram of the one dimensional anisotropic Kondo-necklace model
The one dimensional anisotropic Kondo-necklace model has been studied by
several methods. It is shown that a mean field approach fails to gain the
correct phase diagram for the Ising type anisotropy. We then applied the spin
wave theory which is justified for the anisotropic case. We have derived the
phase diagram between the antiferromagnetic long range order and the Kondo
singlet phases. We have found that the exchange interaction (J) between the
itinerant spins and local ones enhances the quantum fluctuations around the
classical long range antiferromagnetic order and finally destroy the ordered
phase at the critical value, J_c. Moreover, our results show that the onset of
anisotropy in the XY term of the itinerant interactions develops the
antiferromagnetic order for J<J_c. This is in agreement with the qualitative
feature which we expect from the symmetry of the anisotropic XY interaction. We
have justified our results by the numerical Lanczos method where the structure
factor at the antiferromagnetic wave vector diverges as the size of system goes
to infinity.Comment: 9 pages and 9 eps figure
Field induced quantum phase transition in the anisotropic Kondo necklace model
The anisotropic Kondo necklace model in 2D and 3D is treated as a genuine
model for magnetic to Kondo singlet quantum phase transitions in the heavy
fermion (HF) compounds. The variation of the quantum critical point (QCP) with
anisotropy parameters has been investigated previously in the zero field case
[1]. Here we extend the treatment to finite fields using a generalised bond
operator representation including all triplet states. The variation of critical
tc with external field H and the associated phase diagram is derived. The
influence of anisotropies and the different g-factors for localised and
itinerant spins on tc(H) is also investigated. It is found that three different
types of behaviour may appear: (i) Destruction of antiferromangetism and
appearance of a singlet state above a critical field. (ii) The inverese
behaviour, namely field induced antiferromagnetism out of the Kondo singlet
phase. (iii) Reentrance behaviour of the Kondo singlet phase as function of
field strength.Comment: 12 pages, 5 figure
Analytical solution of the Gross-Neveu model at finite density
Recent numerical calculations have shown that the ground state of the
Gross-Neveu model at finite density is a crystal. Guided by these results, we
can now present the analytical solution to this problem in terms of elliptic
functions. The scalar potential is the superpotential of the non-relativistic
Lame Hamiltonian. This model can also serve as analytically solvable toy model
for a relativistic superconductor in the Larkin-Ovchinnikov-Fulde-Ferrell
phase.Comment: 5 pages, no figures, revtex; vs2: appendix with analytical proof of
self-consistency adde
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