54 research outputs found
Dimensional Crossover in Heavy Fermions
Recently we have shown that a one-parameter scaling, the Coherence
Temperature, describes the physical behavior of several heavy fermions in a
region of their phase diagram. In this paper we fully characterize this region,
obtaining the uniform susceptibility, the resistivity and the specific heat.
This allows for an explicit evaluation of the Wilson and the Kadowaki-Woods
ratios in this regime. These quantities turn out to be independent of the
distance to the critical point. The theory of the one-parameter scaling
corresponds to a zero dimensional approach. Although spatial correlations are
irrelevant in this case, time fluctuations are critically correlated and the
quantum hyperscaling relation is satisfied for . The crossover from
to is smooth. It occurs at a lenght scale which is inversely related to
the stiffness of the lifetime of the spin fluctuations.Comment: 4 pages, revtex, no figures, submitted to Physical Review
Use of the Complex Zeros of the Partition Function to Investigate the Critical Behavior of the Generalized Interacting Self-Avoiding Trail Model
The complex zeros of the canonical (fixed walk-length) partition function are calculated for both the self-avoiding trails model and the vertex-interacting self-avoiding walk model, both in bulk and in the presence of an attractive surface. The finite-size behavior of the zeros is used to estimate the location of phase transitions: the collapse transition in the bulk and the adsorption transition in the presence of a surface. The bulk and surface cross-over exponents, Ď• and Ď• S , are estimated from the scaling behavior of the leading partition function zeros
On the numerical modelling of the Jet Erosion Test
International audienceEvaluating the erodibility of a soil, both in terms of erosion threshold (initiation) and erosion rate (progression), is critical for the evaluation of the safety of water retaining structures. Indeed different soils can erode at different rates. However, the relationship between the erosion parameters and the geotechnical and chemical properties of soils remains largely unknown. The jet erosion test appears to be an efficient and simple means for quantifying the two erosion parameters involved. The first parameter is the critical stress while the second parameter is the erosion coefficient. A simplified model of this test has been drawn up by G. Hanson et al. to interpret the experimental curves. Few attempts have been made so far to model the whole process, however. The aim of this study is to simulate the impinging jet and to take into account the erosion of the soil by means of computational fluid dynamics (CFD) numerical modelling. The key point was the time dependence of the problem, due to erosion processes, however the turbulent flow could be considered as steady because of the assumption of low kinetics erosion assumption. The results of the present modelling study are compared to the simplified model and to experimental data. This comparison is a first confirmation of the validity of the simplified model as a means of assessing the critical stress and the erosion coefficient with jet erosion tests
On the origin of heavy quasiparticles in LiV_2O_4
An explanation is provided for the heavy quasiparticle excitations in
LiV_2O_4. It differs considerably from that of other known heavy-fermion
systems. Main ingredients of our theory are the cubic spinel structure of the
material and strong short-range correlations of the d electrons. The large
gamma-coefficient is shown to result from excitations of Heisenberg spin 1/2
rings and chains. The required coupling constant is calculated from LDA+U
calculations and is found to be of the right size. Also the calculated
Sommerfeld-Wilson ratio is reasonably close to the observed one.Comment: REVTEX, 5 pages, 2 figure
Mode-Coupling Model of Mott Gap Collapse in the Cuprates: Natural Phase Boundary for Quantum Critical Points
A simple antiferromagnetic approach to the Mott transition was recently shown
to provide a satisfactory explanation for the Mott gap collapse with doping
observed in photoemission experiments on electron-doped cuprates. Here this
approach is extended in a number of ways. RPA, mode coupling (via
self-consistent renormalization), and (to a limited extent) self-consistent
Born approximation calculations are compared to assess the roles of hot-spot
fluctuations and interaction with spin waves. When fluctuations are included,
the calculation satisfies the Mermin-Wagner theorem, and the mean-field gap and
transition temperature are replaced by pseudogap and onset temperature. The
model is in excellent agreement with experiments on the doping dependence of
both photoemission dispersion and magnetic properties. The magnetic phase
terminates in a quantum critical point (QCP), with a natural phase boundary for
this QCP arising from hot-spot physics.
Since the resulting T=0 antiferromagnetic transition is controlled by a
generalized Stoner factor, an ansatz is made of dividing the Stoner factor up
into a material-dependent part, the bare susceptibility and a
correlation-dependent part, the Hubbard U, which depends only weakly on doping.
From the material dependent part of the interaction, it is possible to
explain the striking differences between electron- and hole-doping, despite an
approximate symmetry in the doping of the QCP. The slower divergence of the
magnetic correlation length in hole doped cuprates may be an indication of more
Mott-like physics.Comment: This replaces cond-mat/0308469. 50 eps figures, revtex [Version 1 had
included old file
Feed-back on the development of a small scale Contact Erosion Test in the laboratory (characteristic size ~ 30 cm)
To determine the hydraulic load requested to initiate contact erosion process, tests are performed with an apparatus called the “Contact Erosion Test”. This device originally results from research carried out by Grenoble University, Électricité de France and Compagnie Nationale du Rhône, at the scale of ~60 cm. It has been adapted to a smaller scale in geophyConsult laboratory to conduct tests on samples extracted from core drilling. The instrumentation was improved to enable a better control of the hydraulic loading and avoid biases. The test protocol was modified, especially to better constrain the soil density at the interface. From the first series of test, we drew conclusions on the test repeatability and on the influence of parameters of the soil state. Discrepancies with previous results obtained at the scale of ~60 cm were identified. Therefore, a new erosion test campaign was planned to confirm and determine the reasons for these differences
- …