Biomedical subjects

C Pépin

Publications and source records attributed to C Pépin.

11 recordsLinked to original sources

PubMed2006

Quantum critical end point for the Kondo volume collapse model.

The Kondo volume collapse describes valence transitions in f-electron metals and is characterized by a line of first order transitions in the pressure-temperature phase plane terminated at critical end points. We analyze the quantum critical end point, when the lower end point is tuned to T=0, and determine the specific heat, thermal expansion, and compressibility. We find that the inclusion of quantum critical fluctuations leads to a novel bifurcation of the first order phase line. Finally, we show that critical strain fluctuations can cause both, superconductivity and non-Fermi liquid behavior near the critical point.

Journal Article↗

PubMed2005

Quantum correction to conductivity close to a 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 crossover between diffusive and ballistic regimes of quantum interference effects occurs at a temperature T*=1/taugamma(E(F)tau)2, where gamma is the parameter associated with the Landau damping of the spin fluctuations, tau is the impurity scattering time, and E(F) is the Fermi energy. For a generic choice of parameters, T* is smaller than the nominal crossover scale 1/tau. In the ballistic quantum critical regime, the conductivity behaves as T1/3.

Journal Article↗

PubMed2003

The break-up of heavy electrons at a quantum critical point.

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on the heavy-electron material YbRh2Si2 found that near a field-induced QCP electrons move ever more slowly and scatter off one another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and scattering cross-section. But these studies could not shed light on whether these properties were an artefact of the applied field, or a more general feature of field-free QCPs. Here we report that, when germanium-doped YbRh2Si2 is tuned away from a chemically induced QCP by magnetic fields, there is a universal behaviour in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current-carrying motions of the electron fluid.

Journal Article↗

PubMed1996

[Value and limits of exercise radionuclide ventriculography in chronic aortic insufficiency. Apropos of 47 operated cases].

The role of exercise radionuclide angiography in the therapeutic strategy of chronic aortic insufficiency remains controversial. In order to assess the value of this technique, 47 patients with pure chronic aortic insufficiency were evaluated before and one year after valve replacement. The preoperative EF decreases or does not increase on exercise (51.9% vs 48 +/- 10%; p < 0.001) in the majority of patients (78%), whereas, after surgery, the EF tends to increase on exercise (55 +/- 11% vs 57 +/- 11%; NS). Despite optimal surgical correction, 16 of the surviving 46 patients still had left ventricular dysfunction at one year after surgery. The preoperative parameters correlating with this dysfunction were, in order, resting radionuclide EF (r = 0.65; p = 0.0001) and the echocardiographic parameters: left ventricular endosystolic dimension, fractional shortening, and the radius/thickness ratio. The preoperative exercise radionuclide parameters did not correlate with postoperative left ventricular dysfunction. In patients without postoperative left ventricular dysfunction, the EF increased on exercise, contrary to the other group. The exercise EF remains valuable for retrospective evaluation of surgical benefit. The exercise radionuclide EF does not reflect myocardial contractile reserve alone but also the conditions of left ventricular load. This study confirms the superiority of the resting radionuclide EF and echocardiographic parameters over exercise radionuclide EF for the prediction of postoperative left ventricular dysfunction.

Aged↗

PubMed1994