478 research outputs found
The thermopower as a fingerprint of the Kondo breakdown quantum critical point
We propose that the thermoelectric power distinguishes two competing
scenarios for quantum phase transitions in heavy fermions : the
spin-density-wave (SDW) theory and breakdown of the Kondo effect. In the Kondo
breakdown scenario, the Seebeck coefficient turns out to collapse from the
temperature scale , associated with quantum fluctuations of the Fermi
surface reconfiguration. This feature differs radically from the physics of the
SDW theory, where no reconstruction of the Fermi surface occurs, and can be
considered as the hallmark of the Kondo breakdown theory. We test these ideas,
upon experimental results for YbRhSi
Quantum Correction to Conductivity Close to Ferromagnetic Quantum Critical Point in Two Dimensions
We study the temperature dependence of the conductivity due to quantum
interference processes for a two-dimensional disordered itinerant electron
system close to a ferromagnetic quantum critical point. Near the quantum
critical point, the cross-over between diffusive and ballistic regimes of
quantum interference effects 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
smaller than the nominal crossover scale . In the ballistic quantum
critical regime, the conductivity behaves as .Comment: 5 pages, 1 figur
Violation of Wiedemann-Franz law at the Kondo breakdown quantum critical point
We study both the electrical and thermal transport near the heavy-fermion
quantum critical point (QCP), identified with the breakdown of the Kondo effect
as an orbital selective Mott transition. We show that the contribution to the
electrical conductivity comes mainly from conduction electrons while the
thermal conductivity is given by both conduction electrons and localized
fermions (spinons), scattered with dynamical exponent . This scattering
mechanism gives rise to a quasi-linear temperature dependence of the electrical
and thermal resistivity. The characteristic feature of the Kondo breakdown
scenario turns out to be emergence of additional entropy carriers, that is,
spinon excitations. As a result, we find that the Wiedemann-Franz ratio should
be larger than the standard value, a fact which enables to differentiate the
Kondo breakdown scenario from the Hertz-Moriya-Millis framework
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
Exact bosonization for an interacting Fermi gas in arbitrary dimensions
We present an exact mapping of models of interacting fermions onto boson
models. The bosons correspond to collective excitations in the initial
fermionic models. This bosonization is applicable in any dimension and for any
interaction between fermions. We show schematically how the mapping can be used
for Monte Carlo calculations and argue that it should be free from the sign
problem. Introducing superfields we derive a field theory that may serve as a
new way of analytical study.Comment: Basic equations are derived more carefully and in a simpler wa
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
Scaling approach to itinerant quantum critical points
Based on phase space arguments, we develop a simple approach to metallic
quantum critical points, designed to study the problem without integrating the
fermions out of the partition function. The method is applied to the
spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for
the conduction and the spin fluids are derived by scaling at the tree level. We
conclude that anomalous exponents may be generated for the fermion self-energy
and the spin-spin correlation functions below , in spite of the spin fluid
being above its upper critical dimension.Comment: 3 pages, 2 figures; discussion of the phase space restriction
modified and, for illustrative purposes, restricted to the tree-level
analysis of the ferromagnetic transitio
Quantum critical behavior in itinerant electron systems -- Eliashberg theory and instability of a ferromagnetic quantum-critical point
We consider the problem of fermions interacting with gapless long-wavelength
collective bosonic modes. The theory describes, among other cases, a
ferromagnetic quantum-critical point (QCP) and a QCP towards nematic ordering.
We construct a controllable expansion at the QCP in two steps: we first create
a new, non Fermi-liquid ``zero-order'' Eliashberg-type theory, and then
demonstrate that the residual interaction effects are small. We prove that this
approach is justified under two conditions: the interaction should be smaller
than the fermionic bandwidth, and either the band mass should be much
smaller than , or the number of fermionic flavors should be
large. For an SU(2) symmetric ferromagnetic QCP, we find that the Eliashberg
theory itself includes a set of singular renormalizations which can be
understood as a consequence of an effective long-range dynamic interaction
between quasi-particles, generated by the Landau damping term. These singular
renormalizations give rise to a negative non-analytic correction to
the static spin susceptibility, and destroy a ferromagnetic QCP. We demonstrate
that this effect can be understood in the framework of the theory of
quantum criticality. We also show that the non-analytic correction to
the bosonic propagator is specific to the SU(2) symmetric case. For systems
with a scalar order parameter, the contributions from individual
diagrams cancel out in the full expression of the susceptibility, and the QCP
remains stable.Comment: 37 pages, 10 fig
Atomic Model of Susy Hubbard Operators
We apply the recently proposed susy Hubbard operators to an atomic model. In
the limiting case of free spins, we derive exact results for the entropy which
are compared with a mean field + gaussian corrections description. We show how
these results can be extended to the case of charge fluctuations and calculate
exact results for the partition function, free energy and heat capacity of an
atomic model for some simple examples. Wavefunctions of possible states are
listed. We compare the accuracy of large N expansions of the susy spin
operators with those obtained using `Schwinger bosons' and `Abrikosov
pseudo-fermions'. For the atomic model, we compare results of slave boson,
slave fermion, and susy Hubbard operator approximations in the physically
interesting but uncontrolled limiting case of N->2. For a mixed representation
of spins we estimate the accuracy of large N expansions of the atomic model. In
the single box limit, we find that the lowest energy saddle-point solution
reduces to simply either slave bosons or slave fermions, while for higher boxes
this is not the case. The highest energy saddle-point solution has the
interesting feature that it admits a small region of a mixed representation,
which bears a superficial resemblance to that seen experimentally close to an
antiferromagnetic quantum critical point.Comment: 17 pages + 7 pages Appendices, 14 figures. Substantial revision
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