3,852 research outputs found
Schwinger Boson approach to the fully screened Kondo model
We apply the Schwinger boson scheme to the fully screened Kondo model and
generalize the method to include antiferromagnetic interactions between ions.
Our approach captures the Kondo crossover from local moment behavior to a Fermi
liquid with a non-trivial Wilson ratio. When applied to the two impurity model,
the mean-field theory describes the "Varma Jones" quantum phase transition
between a valence bond state and a heavy Fermi liquid.Comment: 4 pages, 4 figures. Changes to references and text in v
Low-temperature electrical resistivity in paramagnetic spinel LiV2O4
The 3d electron spinel compound LiV2O4 exhibits heavy fermion behaviour below
30K which is related to antiferromagnetic spin fluctuations strongly enhanced
in an extended region of momentum space. This mechanism explains enhanced
thermodynamic quantities and nearly critical NMR relaxation in the framework of
the selfconsistent renormalization (SCR) theory. Here we show that the low-T
Fermi liquid behaviour of the resistivity and a deviation from this behavior
for higher T may also be understood within that context. We calculate the
temperature dependence of the electrical resistivity \rho(T) assuming that two
basic mechanisms of the quasiparticle scattering, resulting from impurities and
spin-fluctuations, operate simultaneously at low temperature. The calculation
is based on the variational principle in the form of a perturbative series
expansion for \rho(T). A peculiar behavior of \rho(T) in LiV2O4 is related to
properties of low-energy spin fluctuations whose T-dependence is obtained from
SCR theory.Comment: 10 pages, 3 figures, to appear in Phys. Rev.
Fermi surfaces in general co-dimension and a new controlled non-trivial fixed point
Traditionally Fermi surfaces for problems in spatial dimensions have
dimensionality , i.e., codimension along which energy varies.
Situations with arise when the gapless fermionic excitations live at
isolated nodal points or lines. For weak short range interactions are
irrelevant at the non-interacting fixed point. Increasing interaction strength
can lead to phase transitions out of this Fermi liquid. We illustrate this by
studying the transition to superconductivity in a controlled
expansion near . The resulting non-trivial fixed point is shown to
describe a scale invariant theory that lives in effective space-time dimension
. Remarkably, the results can be reproduced by the more familiar
Hertz-Millis action for the bosonic superconducting order parameter even though
it lives in different space-time dimensions.Comment: 4 page
On kinetic energy stabilized superconductivity in cuprates
The possibility of kinetic energy driven superconductivity in cuprates as was
recently found in the model is discussed. We argue that the violation of
the virial theorem implied by this result is serious and means that the
description of superconductivity within the model is pathological.Comment: 3 pages, v2 includes additional reference
Non-Fermi liquid behavior in nearly charge ordered layered metals
Non-Fermi liquid behavior is shown to occur in two-dimensional metals which
are close to a charge ordering transition driven by the Coulomb repulsion. A
linear temperature dependence of the scattering rate together with an increase
of the electron effective mass occur above T*, a temperature scale much smaller
than the Fermi temperature. It is shown that the anomalous temperature
dependence of the optical conductivity of the quasi-two-dimensional organic
metal alpha-(BEDT-TTF)2MHg(SCN)4, with M=NH4 and Rb, above T*=50-100 K, agrees
qualitatively with our predictions for the electronic properties of nearly
charge ordered two-dimensional metals.Comment: accepted in Phys. Rev. Let
Reduced dimensionality in layered quantum dimer magnets: Frustration vs. inhomogeneous condensates
Motivated by recent experiments on BaCuSi2O6, we investigate magnetic
excitations and quantum phase transitions of layered dimer magnets with
inter-layer frustration. We consider two scenarios, (A) a lattice with one
dimer per unit cell and perfect inter-layer frustration, and (B) an enlarged
unit cell with inequivalent layers, with and without perfect frustration. In
all situations, the critical behavior at asymptotically low temperatures is
three-dimensional, but the corresponding crossover scale may be tiny. Magnetic
ordering in case (B) can be discussed in terms of two condensates; remarkably,
perfect frustration renders the proximity effect ineffective. Then, the
ordering transition will be generically split, with clear signatures in
measurable properties. Using a generalized bond-operator method, we calculate
the low-temperature magnetic properties in the paramagnetic and
antiferromagnetic phases. Based on the available experimental data on
BaCuSi2O6, we propose that scenario (B) with inequivalent layers and imperfect
frustration is realized in this material, likely with an additional modulation
of the inter-layer couling along the c axis.Comment: 23 pages, 18 figs, (v2) new fig for bandwidths, (v3) triplon binding
energy discussed, (v4) small changes for clarification, accepted (PRB
Fermi surface and antiferromagnetism in the Kondo lattice: an asymptotically exact solution in d>1 Dimensions
Interest in the heavy fermion metals has motivated us to examine the quantum
phases and their Fermi surfaces within the Kondo lattice model. We demonstrate
that the model is soluble asymptotically exactly in any dimension d>1, when the
Kondo coupling is small compared with the RKKY interaction and in the presence
of antiferromagnetic ordering. We show that the Kondo coupling is exactly
marginal in the renormalization group sense, establishing the stability of an
ordered phase with a small Fermi surface, AFs. Our results have implications
for the global phase diagram of the heavy fermion metals, suggesting a Lifshitz
transition inside the antiferromagnetic region and providing a new perspective
for a Kondo-destroying antiferromagnetic quantum critical point.Comment: 4 pages, 4 figures; (v2) corrected typos and added
reference/acknowledgment; (v3) version as published in Physical Review
Letters (July, 2007
Theory of non-Fermi liquid near a diagonal electronic nematic state on a square lattice
We study effects of Fermi surface fluctuations on a single-particle life time
near the diagonal electronic nematic phase on a two-dimensional square lattice.
It has been shown that there exists a quantum critical point (QCP) between the
diagonal nematic and isotropic phases. We study the longitudinal fluctuations
of the order parameter near the critical point, where the singular forward
scattering leads to a non-Fermi liquid behavior over the whole Fermi surface
except along the k_x- and k_y-directions. We will also discuss the temperature
and chemical potential dependence of the single-particle decay rate.Comment: 4 pages, 3 figures, revtex
Interaction Correction of Conductivity Near a Ferromagnetic Quantum Critical Point
We calculate the temperature dependence of conductivity due to interaction
correction for a disordered itinerant electron system close to a ferromagnetic
quantum critical point which occurs due to a spin density wave instability. In
the quantum critical regime, the crossover between diffusive and ballistic
transport occurs at a temperature ,
where is the parameter associated with the Landau damping of the spin
fluctuations, is the impurity scattering time, and is the Fermi
energy. For a generic choice of parameters, is few orders of
magnitude smaller than the usual crossover scale . In the ballistic
quantum critical regime, the conductivity has a temperature
dependence, where is the dimensionality of the system. In the diffusive
quantum critical regime we get dependence in three dimensions, and
dependence in two dimensions. Away from the quantum critical regime
we recover the standard results for a good metal.Comment: 15 pages, 8 figure
Origin and roles of a strong electron-phonon interaction in cuprate oxide superconductors
A strong electron-phonon interaction arises from the modulation of the
superexchange interaction by phonons. As is studied in Phys. Rev. B 70, 184514
(2004), Cu-O bond stretching modes can be soft around (pm pi/a, 0) and (0, pm
pi/a), with a the lattice constant of CuO_2 planes. In the critical region of
SDW, where antiferromagnetic spin fluctuations are developed around nesting
wave numbers Q of the Fermi surface, the stretching modes can also be soft
around 2Q. Almost symmetric energy dependences of the 2Q component of the
density of states, which are observed in the so called stripe and checker-board
states, cannot be explained by CDW with 2Q following the complete softening of
the 2Q modes, but they can be explained by a second-harmonic effect of SDW with
Q. The strong electron-phonon interaction can play no or only a minor role in
the occurrence of superconductivity.Comment: 5 pages, 1 fugur
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