360 research outputs found
Multi-scale fluctuations near a Kondo Breakdown Quantum Critical Point
We study the Kondo-Heisenberg model using a fermionic representation for the
localized spins. The mean-field phase diagram exhibits a zero temperature
quantum critical point separating a spin liquid phase where the f-conduction
hybridization vanishes, and a Kondo phase where it does not. Two solutions can
be stabilized in the Kondo phase, namely a uniform hybridization when the band
masses of the conduction electrons and the f spinons have the same sign, and a
modulated one when they have opposite sign. For the uniform case, we show that
above a very small Fermi liquid temperature scale (~1 mK), the critical
fluctuations associated with the vanishing hybridization have dynamical
exponent z=3, giving rise to a specific heat coefficient that diverges
logarithmically in temperature, as well as a conduction electron inverse
lifetime that has a T log T behavior. Because the f spinons do not carry
current, but act as an effective bath for the relaxation of the current carried
by the conduction electrons, the latter result also gives rise to a T log T
behavior in the resistivity. This behavior is consistent with observations in a
number of heavy fermion metals.Comment: 17 pages, 10 figure
Selective Mott transition and heavy fermions
Starting with an extended version of the Anderson lattice where the
f-electrons are allowed a weak dispersion, we examine the possibility of a Mott
localization of the f-electrons, for a finite value of the hybridization .
We study the fluctuations at the quantum critical point (QCP) where the
f-electrons localize. We find they are in the same universality class as for
the Kondo breakdown QCP, with the following notable features.
The quantum critical regime sees the appearance of an additional energy scale
separating two universality classes. In the low energy regime, the fluctuations
are dominated by massless gauge modes, while in the intermediate energy regime,
the fluctuations of the modulus of the order parameter are the most relevant
ones. In the latter regime, electric transport simplifies drastically, leading
to a quasi-linear resistivity in 3D and anomalous exponents lower than T in 2
D. This rather unique feature of the quantum critical regime enables us to make
experimentally testable predictions.Comment: 27 pages, 5 figure
Low energy excitations and singular contributions in the thermodynamics of clean Fermi liquids
Using a recently suggested method of bosonization in an arbitrary dimension,
we study the anomalous contribution of the low energy spin and charge
excitations to thermodynamic quantities of a two-dimensional (2D) Fermi liquid.
The method is slightly modified for the present purpose such that the effective
supersymmetric action no longer contains the high energy degrees of freedom but
still accounts for effects of the finite curvature of the Fermi surface.
Calculating the anomalous contribution to the specific heat, we
show that the leading logarithmic in temperature corrections to can be obtained in a scheme combining a summation of ladder diagrams
and renormalization group equations. The final result is represented as the sum
of two separate terms that can be interpreted as coming from singlet and
triplet superconducting excitations. The latter may diverge in certain regions
of the coupling constants, which should correspond to the formation of triplet
Cooper pairs.Comment: 29 pages, 13 figure
Density of states for dirty d-wave superconductors: A unified and dual approach for different types of disorder
A two-parameter field theoretical representation is given of a 2-dimensional
dirty d-wave superconductor that interpolates between the Gaussian limit of
uncorrelated weak disorder and the unitary limit of a dilute concentration of
resonant scatterers. It is argued that a duality holds between these two
regimes from which follows that a linearly vanishing density of states in the
Gaussian limit transforms into a diverging one in the unitary limit arbitrarily
close to the Fermi energy
Fermi-Surface Reconstruction in the Periodic Anderson Model
We study ground state properties of periodic Anderson model in a
two-dimensional square lattice with variational Monte Carlo method. It is shown
that there are two different types of quantum phase transition: a conventional
antiferromagnetic transition and a Fermi-surface reconstruction which
accompanies a change of topology of the Fermi surface. The former is induced by
a simple back-folding of the Fermi surface while the latter is induced by
localization of electrons. The mechanism of these transitions and the
relation to the recent experiments on Fermi surface are discussed in detail.Comment: 8 pages, 7 figures, submitted to Journal of the Physical Society of
Japa
Temperature and ac Effects on Charge Transport in Metallic Arrays of Dots
We investigate the effects of finite temperature, dc pulse, and ac drives on
the charge transport in metallic arrays using numerical simulations. For finite
temperatures there is a finite conduction threshold which decreases linearly
with temperature. Additionally we find a quadratic scaling of the
current-voltage curves which is independent of temperature for finite
thresholds. These results are in excellent agreement with recent experiments on
2D metallic dot arrays. We have also investigated the effects of an ac drive as
well as a suddenly applied dc drive. With an ac drive the conduction threshold
decreases for fixed frequency and increasing amplitude and saturates for fixed
amplitude and increasing frequency. For sudden applied dc drives below
threshold we observe a long time power law conduction decay.Comment: 6 pages, 7 postscript figure
How do Fermi liquids get heavy and die?
We discuss non-Fermi liquid and quantum critical behavior in heavy fermion
materials, focussing on the mechanism by which the electron mass appears to
diverge at the quantum critical point. We ask whether the basic mechanism for
the transformation involves electron diffraction off a quantum critical spin
density wave, or whether a break-down in the composite nature of the heavy
electron takes place at the quantum critical point. We show that the Hall
constant changes continously in the first scenario, but may ``jump''
discontinuously at a quantum critical point where the composite character of
the electron quasiparticles changes.Comment: Revised version with many new references added. To appear as a
topical review in Journal of Physics: Condensed Matter Physics. Two column
version http://www.physics.rutgers.edu/~coleman/online/questions.ps.g
Quasiparticle Localization in Disordered d-Wave Superconductors
An extensive numerical study is reported on disorder effect in
two-dimensional d-wave superconductors with random impurities in the unitary
limit. It is found that a sharp resonant peak shows up in the density of states
at zero energy and correspondingly the finite-size spin conductance is strongly
enhanced which results in a non-universal feature in one-parameter scaling.
However, all quasiparticle states remain localized, indicating that the
resonant density peak alone is not sufficient to induce delocalization. In the
weak disorder limit, the localization length is so long that the spin
conductance at small sample size is close to the universal value predicted by
Lee (Phys. Rev. Lett. {\bf 71}, 1887 (1993)).Comment: 4 pages, 3 figure
Order from Disorder: Non Magnetic Impurities in the Spin-gap Phase of the Cuprates
We solve the problem of non magnetic impurities in the staggered flux
phase of the Heisenberg model which we assume to be a good mean-field
approximation for the spin-gap phase of the cuprates. The density of states is
evaluated exactly in the unitary limit and is porportional to 1/\left (\omega
\ln^2(|\omega|/D)), in analogy with the 1D case of doped spin-Peierls and
two-leg ladders compounds. We argue that the system exhibits a quasi long-range
order at T=0 with instantaneous spin-spin correlations decreasing as n_i/
\ln^2\left (n_i R_{ij}) for large distances and we predict enhanced
low energy fluctuations in Neutron Scattering.Comment: 4 pages, corrected typos, references adde
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