438 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
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
Nasal carriage of Staphylococcus aureus in dairy sheep.
International audienceThe purpose of this study was to assess the prevalence of Staphylococcus aureus nasal carriage of dairy sheep in farms producing cheeses manufactured with raw ewe's milk. The study showed that 29% of ewes carried S. aureus in their nares. The genetic diversity of the 136 isolates recovered from the anterior nares of the ewes, from the ambient air of the milking parlour and from cheeses was investigated using pulsed-field gel electrophoresis (PFGE) of DNA SmaI digests. The genotyping results showed that 75 out of 106 isolates recovered from nasal carriage in dairy sheep belonged to a dominant pattern (previously named OV) and a genetically related pattern (named OV'). The same profile (OV or OV') was found in the ambient air and cheeses, suggesting a continuum between isolates within these different compartments
Evaluation of Arsenic Leaching Potential in Gold Mine Tailings Amended with Peat and Mine Drainage Treatment Sludge
Peat and mine drainage treatment sludge can be valorized as amendments on mine sites to stabilize gold mine tailings and reduce the potential leaching of contaminants in pore water. However, the influence of organic amendments on the mobility of metalloids and/or metals in the tailings must be validated, as the leached contaminants may vary according to their type, nature, and origin. The objective of the present study was to evaluate over time the effect of peat- and/or Fe-rich sludge amendments on the mobility of As and metallic cations in the drainage water of tailings potentially producing contaminated neutral drainage. Ten duplicated weathering cell experiments containing tailings alone or amended with peat and/or Fe-rich sludge (5-10% dry weight) were performed and monitored for 112 d. The results showed that as low as 5% peat amendment would promote As mobility in tailings' pore water, with As concentrations exceeding Quebec discharge criteria (>0.2 mg L). In addition, As(III), the most mobile and toxic form, was predominant with 10% peat, whereas organic species were negligible in all cells. The use of peat alone as organic amendment for the stabilization of tailing contaminants could increase the risk of generating As-rich contaminated neutral drainage. Conversely, the mix of only 5% Fe-rich sludge with or without peat decreased As concentrations in leachates by 65 to 80%. Further studies on the use of "peat" or "peat + Fe-rich sludge" as cover or amendment should be conducted with a focus on Fe/As and Ca/As ratios
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
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
Impurity-induced spin polarization and NMR line broadening in underdoped cuprates
We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in
underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments
on Cu sites in the proximity of the impurity, a process which is intimately
related to the spin gap phenomenon in cuprates. Below a characteristic Kondo
temperature, the Ni spin is partially screened by the Cu moments, resulting in
an effective impurity spin S=1/2. We further analyze the
Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a
polarization of the effective impurity moments and derive expressions for the
corresponding ^{17}O NMR line broadening. The peculiar aspects of recent
experimental NMR data can be traced back to different spatial characteristics
of Ni and Zn moments as well as to an inherent temperature dependence of local
antiferromagnetic correlations.Comment: PRB B1 01June9
Non Fermi Liquid behavior in the under-screened Kondo model
Using the Schwinger boson spin representation, we reveal a new aspect to the
physics of a partially screened magnetic moment in a metal, as described by the
spin Kondo model. We show that the residual ferromagnetic interaction
between a partially screened spin and the electron sea destabilizes the Landau
Fermi liquid, forming a singular Fermi liquid with a divergence in the low temperature specific heat coefficient
. A magnetic field tunes this system back into Landau Fermi liquid
with a Fermi temperature proportional to . We discuss a
possible link with field-tuned quantum criticality in heavy electron materials.Comment: References corrected. Minor changes to tex
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