754 research outputs found
Application of the S=1 underscreened Anderson lattice model to Kondo uranium and neptunium compounds
Magnetic properties of uranium and neptunium compounds showing the
coexistence of Kondo screening effect and ferromagnetic order are investigated
within the Anderson lattice Hamiltonian with a two-fold degenerate -level in
each site, corresponding to electronic configuration with spins. A
derivation of the Schrieffer-Wolff transformation is presented and the
resulting Hamiltonian has an effective -band term, in addition to the
regular exchange Kondo interaction between the -spins and the
spins of the conduction electrons. The obtained effective Kondo lattice model
can describe both the Kondo regime and a weak delocalization of -electron.
Within this model we compute the Kondo and Curie temperatures as a function of
model parameters, namely the Kondo exchange interaction constant , the
magnetic intersite exchange interaction and the effective -bandwidth.
We deduce, therefore, a phase diagram of the model which yields the coexistence
of Kondo effect and ferromagnetic ordering and also accounts for the pressure
dependence of the Curie temperature of uranium compounds such as UTe.Comment: 9 pages, 4 figure
Reply to Millis et al. on "A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems"
In a recent paper (cond-mat/0411197) we showed the equivalence of two
seemingly contradictory theories on Griffiths-McCoy singularities (GMS) in
metallic antiferromagnets close to a quantum critical point (QCP). In a recent
comment, Millis {\it et al.} (cond-mat/0411738) argue that in heavy-fermion
materials the electronic damping is large leading to the freezing of locally
magnetically ordered droplets at high temperatures. In this reply we show that
this erroneous conclusion is based on a treatment of the problem of disorder
close to a QCP which is not self-consistent. We argue that a self-consistent
treatment of the ordered droplets must lead to weak damping and to a large
region of GMS behavior, in agreement with the our ealier results.Comment: 2 pages. Updated versio
High-pressure study of non-Fermi liquid and spin-glass-like behavior in CeRhSn
We present measurements of the temperature dependence of electrical
resistivity of CeRhSn up to ~ 27 kbar. At low temperatures, the electrical
resistivity varies linearly with temperature for all pressures, indicating
non-Fermi liquid behavior. Below a temperature Tf ~ 6 K, the electrical
resistivity deviates from a linear dependence. We found that the
low-temperature feature centered at T = Tf shows a pressure dependence dTf/dP ~
30 mK/kbar which is typical of canonical spin glasses. This interplay between
spin-glass-like and non-Fermi liquid behavior was observed in both CeRhSn and a
Ce0.9La0.1RhSn alloy.Comment: 5 pages, 3 figures, accepted for publication to Journal of Physics:
Condensed Matte
Coexisting Kondo singlet state with antiferromagnetic long-range order: A possible ground state for Kondo insulators
The ground-state phase diagram of a half-filled anisotropic Kondo lattice
model is calculated within a mean-field theory. For small transverse exchange
coupling , the ground state shows an antiferromagnetic
long-range order with finite staggered magnetizations of both localized spins
and conduction electrons. When , the long-range order
is destroyed and the system is in a disordered Kondo singlet state with a
hybridization gap. Both ground states can describe the low-temperature phases
of Kondo insulating compounds. Between these two distinct phases, there may be
a coexistent regime as a result of the balance between local Kondo screening
and magnetic interactions.Comment: four pages, Revtex, one figure; to be published in Phys. Rev. B, 1
July issue, 200
Heavy fermion fluid in high magnetic fields: an infrared study of CeRuSb
We report a comprehensive infrared magneto-spectroscopy study of
CeRuSb compound revealing quasiparticles with heavy effective mass
m, with a detailed analysis of optical constants in fields up to 17 T. We
find that the applied magnetic field strongly affects the low energy
excitations in the system. In particular, the magnitude of m 70
m (m is the quasiparticle band mass) at 10 K is suppressed by as much
as 25 % at 17 T. This effect is in quantitative agreement with the mean-field
solution of the periodic Anderson model augmented with a Zeeman term
Flux melting in BSCCO: Incorporating both electromagnetic and Josephson couplings
Multilevel Monte Carlo simulations of a BSCCO system are carried out
including both Josephson as well as electromagnetic couplings for a range of
anisotropies. A first order melting transition of the flux lattice is seen on
increasing the temperature and/or the magnetic field. The phase diagram for
BSCCO is obtained for different values of the anisotropy parameter .
The best fit to the experimental results of D. Majer {\it et al.} [Phys. Rev.
Lett. {\bf 75}, 1166 (1995)] is obtained for provided one
assumes a temperature dependence of the
penetration depth with . Assuming a dependence
the best fit is obtained for . For finite anisotropy the data is shown to collapse on a straight line
when plotted in dimensionless units which shows that the melting transition can
be satisfied with a single Lindemann parameter whose value is about 0.3. A
different scaling applies to the case. The energy jump is
measured across the transition and for large values of it is found to
increase with increasing anisotropy and to decrease with increasing magnetic
field. For infinite anisotropy we see a 2D behavior of flux droplets with a
transition taking place at a temperature independent of the magnetic field. We
also show that for smaller values of anisotropy it is reasonable to replace the
electromagnetic coupling with an in-plane interaction represented by a Bessel
function of the second kind (), thus justifying our claim in a previous
paper.Comment: 12 figures, revtex
Multi-Channel Kondo Necklace
A multi--channel generalization of Doniach's Kondo necklace model is
formulated, and its phase diagram studied in the mean--field approximation. Our
intention is to introduce the possible simplest model which displays some of
the features expected from the overscreened Kondo lattice. The conduction
electron channels are represented by sets of pseudospins \vt_{j}, , which are all antiferromagnetically coupled to a periodic array of
|\vs|=1/2 spins. Exploiting permutation symmetry in the channel index
allows us to write down the self--consistency equation for general . For
, we find that the critical temperature is rising with increasing Kondo
interaction; we interpret this effect by pointing out that the Kondo coupling
creates the composite pseudospin objects which undergo an ordering transition.
The relevance of our findings to the underlying fermionic multi--channel
problem is discussed.Comment: 29 pages (2 figures upon request from [email protected]), LATEX,
submitted for publicatio
Phase transition in the massive Gross-Neveu model in toroidal topologies
We use methods of quantum field theory in toroidal topologies to study the
-component -dimensional massive Gross-Neveu model, at zero and finite
temperature, with compactified spatial coordinates. We discuss the behavior of
the large- coupling constant (), investigating its dependence on the
compactification length () and the temperature (). For all values of the
fixed coupling constant (), we find an asymptotic-freedom type of
behavior, with as and/or . At T=0, and for
(the strong coupling regime), we show that,
starting in the region of asymptotic freedom and increasing , a divergence
of appears at a finite value of , signaling the existence of a phase
transition with the system getting spatially confined. Such a spatial
confinement is destroyed by raising the temperature. The confining length,
, and the deconfining temperature, , are determined
as functions of and the mass () of the fermions, in the case of
. Taking as the constituent quark mass (), the
results obtained are of the same order of magnitude as the diameter () and the estimated deconfining temperature () of
hadrons.Comment: 14 pages, 10 figures, 1 table, to appear in Phys. Rev.
Pairing via Index theorem
This work is motivated by a specific point of view: at short distances and
high energies the undoped and underdoped cuprates resemble the -flux phase
of the t-J model. The purpose of this paper is to present a mechanism by which
pairing grows out of the doped -flux phase. According to this mechanism
pairing symmetry is determined by a parameter controlling the quantum tunneling
of gauge flux quanta. For zero tunneling the symmetry is ,
while for large tunneling it is . A zero-temperature critical
point separates these two limits
Theory of the Resistive Transition in Overdoped : Implications for the angular dependence of the quasiparticle scattering rate in High- superconductors
We show that recent measurements of the magnetic field dependence of the
magnetization, specific heat and resistivity of overdoped
in the vicinity of the superconducting
imply that the vortex viscosity is anomalously small and that the material
studied is inhomogeneous with small, a few hundred , regions in which the
local is much higher than the bulk . The anomalously small
vortex viscosity can be derived from a microscopic model in which the
quasiparticle lifetime varies dramatically around the Fermi surface, being
small everywhere except along the zone diagonal (``cold spot''). We propose
experimental tests of our results.Comment: 4 pages, LaTex, 2 EPS figure
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