528 research outputs found
Heavy-fermion and spin-liquid behavior in a Kondo lattice with magnetic frustration
We study the competition between the Kondo effect and frustrating exchange
interactions in a Kondo-lattice model within a large- dynamical
mean-field theory. We find a T=0 phase transition between a heavy Fermi-liquid
and a spin-liquid for a critical value of the exchange , the
single-impurity Kondo temperature. Close to the critical point, the Fermi
liquid coherence scale is strongly reduced and the effective mass
strongly enhanced. The regime is characterized by spin-liquid
magnetic correlations and non-Fermi-liquid properties. It is suggested that
magnetic frustration is a general mechanism which is essential to explain the
large effective mass of some metallic compounds such as LiVO.Comment: 7 pages, 1 figure. Late
Scientific research work on marine mammals of the northern Pacific Ocean in 1984/85 [translated from Russian]
Non-Fermi liquid behavior from two-dimensional antiferromagnetic fluctuations: a renormalization-group and large-N analysis
We analyze the Hertz-Moriya-Millis theory of an antiferromagnetic quantum
critical point, in the marginal case of two dimensions (d=2,z=2). Up to
next-to-leading order in the number of components (N) of the field, we find
that logarithmic corrections do not lead to an enhancement of the Landau
damping. This is in agreement with a renormalization-group analysis, for
arbitrary N. Hence, the logarithmic effects are unable to account for the
behavior reportedly observed in inelastic neutron scattering experiments on
CeCu_{6-x}Au_x. We also examine the extended dynamical mean-field treatment
(local approximation) of this theory, and find that only subdominant
corrections to the Landau damping are obtained within this approximation, in
contrast to recent claims.Comment: 15 pages, 8 figure
Critical properties of the Fermi-Bose Kondo and pseudogap Kondo models: Renormalized perturbation theory
Magnetic impurities coupled to both fermionic and bosonic baths or to a
fermionic bath with pseudogap density of states, described by the Fermi-Bose
Kondo and pseudogap Kondo models, display non-trivial intermediate coupling
fixed points associated with critical local-moment fluctuations and local
non-Fermi liquid behavior. Based on renormalization group together with a
renormalized perturbation expansion around the free-impurity limit, we
calculate various impurity properties in the vicinity of those
intermediate-coupling fixed points. In particular, we compute the conduction
electron T matrix, the impurity susceptibility, and the residual impurity
entropy, and relate our findings to certain scenarios of local quantum
criticality in strongly correlated lattice models.Comment: 16 pages, 5 figs; (v2) large-N results for entropy of Bose-Kondo
model added; (v3) final version as publishe
Correlation gap in the heavy-fermion antiferromagnet UPd_2Al_3
The optical properties of the heavy-fermion compound UPdAl have been
measured in the frequency range from 0.04 meV to 5 meV (0.3 to 40 cm) at
temperatures K. Below the coherence temperature K, the hybridization gap opens around 10 meV. As the temperature decreases
further ( K), a well pronounced pseudogap of approximately 0.2 meV
develops in the optical response; we relate this to the antiferromagnetic
ordering which occurs below K. The frequency dependent mass and
scattering rate give evidence that the enhancement of the effective mass mainly
occurs below the energy which is associated to the magnetic correlations
between the itinerant and localized 5f electrons. In addition to this
correlation gap, we observe a narrow zero-frequency conductivity peak which at
2 K is less than 0.1 meV wide, and which contains only a fraction of the
delocalized carriers. The analysis of the spectral weight infers a loss of
kinetic energy associated with the superconducting transition.Comment: RevTex, 15 pages, 7 figure
Ginzburg-Landau functional for nearly antiferromagnetic perfect and disordered Kondo lattices
Interplay between Kondo effect and trends to antiferromagnetic and spin glass
ordering in perfect and disordered bipartite Kondo lattices is considered.
Ginzburg-Landau equation is derived from the microscopic effective action
written in three mode representation (Kondo screening, antiferromagnetic
correlations and spin liquid correlations). The problem of local constraint is
resolved by means of Popov-Fedotov representation for localized spin operators.
It is shown that the Kondo screening enhances the trend to a spin liquid
crossover and suppresses antiferromagnetic ordering in perfect Kondo lattices
and spin glass ordering in doped Kondo lattices. The modified Doniach's diagram
is constructed, and possibilities of going beyond the mean field approximation
are discussed.Comment: 18 pages, RevTeX, 7 EPS figures include
Local fluctuations in quantum critical metals
We show that spatially local, yet low-energy, fluctuations can play an
essential role in the physics of strongly correlated electron systems tuned to
a quantum critical point. A detailed microscopic analysis of the Kondo lattice
model is carried out within an extended dynamical mean-field approach. The
correlation functions for the lattice model are calculated through a
self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled
both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field).
A renormalization-group treatment of this impurity problem--perturbative in
, where is an exponent characterizing the spectrum
of the bosonic bath--shows that competition between the two couplings can drive
the local-moment fluctuations critical. As a result, two distinct types of
quantum critical point emerge in the Kondo lattice, one being of the usual
spin-density-wave type, the other ``locally critical.'' Near the locally
critical point, the dynamical spin susceptibility exhibits scaling
with a fractional exponent. While the spin-density-wave critical point is
Gaussian, the locally critical point is an interacting fixed point at which
long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau
description for the locally critical point is discussed. It is argued that
these results are robust, that local criticality provides a natural description
of the quantum critical behavior seen in a number of heavy-fermion metals, and
that this picture may also be relevant to other strongly correlated metals.Comment: 20 pages, 12 figures; typos in figure 3 and in the main text
corrected, version as publishe
Measurement of the p-pbar -> Wgamma + X cross section at sqrt(s) = 1.96 TeV and WWgamma anomalous coupling limits
The WWgamma triple gauge boson coupling parameters are studied using p-pbar
-> l nu gamma + X (l = e,mu) events at sqrt(s) = 1.96 TeV. The data were
collected with the DO detector from an integrated luminosity of 162 pb^{-1}
delivered by the Fermilab Tevatron Collider. The cross section times branching
fraction for p-pbar -> W(gamma) + X -> l nu gamma + X with E_T^{gamma} > 8 GeV
and Delta R_{l gamma} > 0.7 is 14.8 +/- 1.6 (stat) +/- 1.0 (syst) +/- 1.0 (lum)
pb. The one-dimensional 95% confidence level limits on anomalous couplings are
-0.88 < Delta kappa_{gamma} < 0.96 and -0.20 < lambda_{gamma} < 0.20.Comment: Submitted to Phys. Rev. D Rapid Communication
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt{s} = 1.96 TeV using Kinematic Characteristics of Lepton + Jets Events
We present a measurement of the top quark pair ttbar production cross section
in ppbar collisions at a center-of-mass energy of 1.96 TeV using 230 pb**{-1}
of data collected by the DO detector at the Fermilab Tevatron Collider. We
select events with one charged lepton (electron or muon), large missing
transverse energy, and at least four jets, and extract the ttbar content of the
sample based on the kinematic characteristics of the events. For a top quark
mass of 175 GeV, we measure sigma(ttbar) = 6.7 {+1.4-1.3} (stat) {+1.6- 1.1}
(syst) +/-0.4 (lumi) pb, in good agreement with the standard model prediction.Comment: submitted to Phys.Rev.Let
Measurement of the ttbar Production Cross Section in ppbar Collisions at sqrt(s)=1.96 TeV using Lepton + Jets Events with Lifetime b-tagging
We present a measurement of the top quark pair () production cross
section () in collisions at TeV
using 230 pb of data collected by the D0 experiment at the Fermilab
Tevatron Collider. We select events with one charged lepton (electron or muon),
missing transverse energy, and jets in the final state. We employ
lifetime-based b-jet identification techniques to further enhance the
purity of the selected sample. For a top quark mass of 175 GeV, we
measure pb, in
agreement with the standard model expectation.Comment: 7 pages, 2 figures, 3 tables Submitted to Phys.Rev.Let
- …