56 research outputs found
Quasi-Kondo Phenomenon due to Dynamical Jahn-Teller Effect
A mechanism of non-magnetic Kondo effect is proposed on the basis of a
multiorbital Anderson model coupled with dynamical Jahn-Teller (JT) phonons. An
electron system coupled dynamically with JT phonons has a vibronic ground state
with double degeneracy due to clockwise and anti-clockwise rotational modes
with entropy of . When a temperature is lower than a characteristic
energy to turn the rotational direction, the rotational degree of freedom is
eventually suppressed and the corresponding entropy is released,
leading to quasi-Kondo behavior. We discuss possible relevance of this
quasi-Kondo phenomenon to electronic properties of filled skutterudites.Comment: 4 pages, 3 figure
Transport through a quantum dot with SU(4) Kondo entanglement
We investigate a mesoscopic setup composed of a small electron droplet (dot)
coupled to a larger quantum dot (grain) also subject to Coulomb blockade as
well as two macroscopic leads used as source and drain. An exotic Kondo ground
state other than the standard SU(2) Fermi liquid unambiguously emerges: an
SU(4) Kondo correlated liquid. The transport properties through the small dot
are analyzed for this regime, through boundary conformal field theory, and
allow a clear distinction with other regimes such as a two-channel spin state
or a two-channel orbital state.Comment: 13 pages, 3 figure
Elementary excitations in homogeneous superfluid neutron star matter: Role of the proton component
The thermal evolution of neuron stars depends on the elementary excitations
affecting the stellar matter. In particular, the low-energy excitations, whose
energy is proportional to the transfered momentum, can play a major role in the
emission and propagation of neutrinos. In this paper, we focus on the density
modes associated with the proton component in the homogeneous matter of the
outer core of neutron stars (at density between one and three times the nuclear
saturation density, where the baryonic constituants are expected to be neutrons
and protons). In this region, it is predicted that the protons are
superconductor. We study the respective roles of the proton pairing and Coulomb
interaction in determining the properties of the modes associated with the
proton component. This study is performed in the framework of the Random Phase
Approximation, generalized in order to describe the response of a superfluid
system.The formalism we use ensures that the Generalized Ward's Identities are
satisfied. An important conclusion of this work is the presence of a
pseudo-Goldstone mode associated with the proton superconductor in neutron-star
matter. Indeed, the Goldstone mode, which characterizes a pure superfluid, is
suppressed in usual superconductors due to the long-range Coulomb interaction,
which only allows a plasmon mode. However, for the proton component of stellar
matter, the Coulomb field is screened by the electrons and a pseudo-Goldstone
mode occurs, with a velocity increased by the Coulomb interaction.Comment: Submitted for publicatio
Conductance of a spin-1 quantum dot: the two-stage Kondo effect
We discuss the physics of a of a spin-1 quantum dot, coupled to two metallic
leads and develop a simple model for the temperature dependence of its
conductance. Such quantum dots are described by a two-channel Kondo model with
asymmetric coupling constants and the spin screening of the dot by the leads is
expected to proceed via a two-stage process. When the Kondo temperatures of
each channel are widely separated, on cooling, the dot passes through a broad
cross-over regime dominated by underscreened Kondo physics. A singular, or
non-fermi liquid correction to the conductance develops in this regime. At the
lowest temperatures, destructive interference between resonant scattering in
both channels leads to the eventual suppression of the conductance of the dot.
We develop a model to describe the growth, and ultimate suppression of the
conductance in the two channel Kondo model as it is screened successively by
its two channels. Our model is based upon large-N approximation in which the
localized spin degrees of freedom are described using the Schwinger boson
formalism.Comment: 16 pages, 10 figure
Dynamic response of 1D bosons in a trap
We calculate the dynamic structure factor S(q,omega) of a one-dimensional
(1D) interacting Bose gas confined in a harmonic trap. The effective
interaction depends on the strength of the confinement enforcing the 1D motion
of atoms; interaction may be further enhanced by superimposing an optical
lattice on the trap potential. In the compressible state, we find that the
smooth variation of the gas density around the trap center leads to softening
of the singular behavior of S(q,omega) at Lieb-1 mode compared to the behavior
predicted for homogeneous 1D systems. Nevertheless, the density-averaged
response remains a non-analytic function of q and omega at Lieb-1 mode in the
limit of weak trap confinement. The exponent of the power-law non-analyticity
is modified due to the inhomogeneity in a universal way, and thus, bears
unambiguously the information about the (homogeneous) Lieb-Liniger model. A
strong optical lattice causes formation of Mott phases. Deep in the Mott
regime, we predict a semi-circular peak in S(q,\omega) centered at the on-site
repulsion energy, omega=U. Similar peaks of smaller amplitudes exist at
multiples of U as well. We explain the suppression of the dynamic response with
entering into the Mott regime, observed recently by D. Clement et al., Phys.
Rev. Lett. v. 102, p. 155301 (2009), based on an f-sum rule for the
Bose-Hubbard model.Comment: 24 pages, 11 figure
The electronic structure of the heavy fermion metal
The electronic structure of the first reported heavy fermion compound without
f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the
result of the trigonal splitting and d-d Coulomb interaction one electron of
the configuration of V ion is localized and the rest partially fills
a relatively broad conduction band. The effective Anderson impurity model was
solved by Non-Crossing-Approximation method, leading to an estimation for the
single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion
phenomenon of Nozi\`eres for the Kondo lattice leads to a remarkable decrease
of the heavy-fermion (or coherence) energy scale (D
is the typical bandwidth), comparable to the experimental result.Comment: 4 pages, RevTeX; 3 figures in format .eps. submitted to PR
Anderson-Yuval approach to the multichannel Kondo problem
We analyze the structure of the perturbation expansion of the general
multichannel Kondo model with channel anisotropic exchange couplings and in the
presence of an external magnetic field, generalizing to this case the
Anderson-Yuval technique. For two channels, we are able to map the Kondo model
onto a generalized resonant level model. Limiting cases in which the equivalent
resonant level model is solvable are identified. The solution correctly
captures the properties of the two channel Kondo model, and also allows an
analytic description of the cross-over from the non Fermi liquid to the Fermi
liquid behavior caused by the channel anisotropy.Comment: 23 pages, ReVTeX, 4 figures av. on reques
Simple description of the anisotropic two-channel Kondo problem
We adapt strong-coupling methods first used in the one-channel Kondo model to
develop a simple description of the spin- two-channel Kondo model
with channel anisotropy. Our method exploits spin-charge decoupling to develop
a compactified Hamiltonian that describes the spin excitations. The structure
of the fixed-point Hamiltonian and quasiparticle impurity S-matrix are
incompatible with a Fermi liquid description.Comment: 4 pages, latex (uses revtex and epsf macros) with 3 figures - all in
a self unpacking uuencoded file. Revisions include changes to Fig. 1(a) and
detailed discussion of the spin excitation
Kondo effect in a Luttinger liquid: nonuniversality of the Wilson ratio
Using a precise coset Ising-Bose representation, we show how backscattering
of electrons off a magnetic impurity destabilizes the two-channel Kondo fixed
point and drives the system to a new fixed point, in agreement with previous
results. In addition, we verify the scaling proposed by Furusaki and Nagaosa
and prove that the other possible critical fixed point, namely the local Fermi
liquid class, is not completely universal when backscattering is included
because the Wilson ratio is not well-defined in the spinon basis.Comment: 4 pages, RevTeX; to appear in Physical Review
Crystal Distortion and the Two-Channel Kondo Effect
We study a simple model of the two-channel Kondo effect in a distorted
crystal. This model is then used to investigate the interplay of the Kondo and
Jahn-Teller effects, and also the Kondo effect in an impure crystal. We find
that the Jahn-Teller interaction modifies the characteristic energy scale of
the system below which non-Fermi-liquid properties of the model become
apparent. The modified energy scale tends to zero as the limit of a purely
static Jahn-Teller effect is approached. We find also that the non-Fermi-liquid
properties of the quadrupolar Kondo effect are not stable against crystal
distortion caused by impurities.Comment: 11 page
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