Fermi surface instabilities in CeRh2Si2 at high magnetic field and pressure

We present thermoelectric power (TEP) studies under pressure and high magnetic field in the antiferromagnet CeRh2Si2 at low temperature. Under magnetic field, large quantum oscillations are observed in the TEP, S(H), in the antiferromagnetic phase. They suddenly disappear when entering in the polarized paramagnetic (PPM) state at Hc pointing out an important reconstruction of the Fermi surface (FS). Under pressure, S/T increases strongly of at low temperature near the critical pressure Pc, where the AF order is suppressed, implying the interplay of a FS change and low energy excitations driven by spin and valence fluctuations. The difference between the TEP signal in the PPM state above Hc and in the paramagnetic state (PM) above Pc can be explained by different FS. Band structure calculations at P = 0 stress that in the AF phase the 4f contribution at the Fermi level (EF) is weak while it is the main contribution in the PM domain. By analogy to previous work on CeRu2Si2, in the PPM phase of CeRh2Si2 the 4f contribution at EF will drop.

Quantum criticality and Lifshitz transition in the Ising system CeRu2Si2: Comparison with YbRh2Si2

New thermoelectric power (TEP) measurements on prototype heavy-fermion compounds close to magnetic quantum criticality are presented. The highly sensitive technique of TEP is an unique tool to reveal Fermi surface instabilities, referred here as Lifshitz transitions. The first focus is on the Ising CeRu2Si2 series. Doping CeRu2Si2 with Rh produces a decoupling between the first order metamagnetic transition and the pseudo-metamagnetism observed in the pure compound. Comparison is made with the case of YbRh2Si2 which is often considered as the archetype of local quantum criticality by contrast to CeRu2Si2, taken as an example of spin-density wave criticality. Up to now for ferromagnetic materials showing ferromagnetic wings, no simple case appears where the Fermi surface is preserved between the ferromagnetic and paramagnetic phases. An open issue is the consequence of Lifshitz transitions on superconductivity in these multiband systems.

Magnetic Polarization and Fermi Surface Instability: Case of YbRh2Si2

We report thermoelectric and resitivity measurements of antiferromagnetic heavy fermion compound YRh2Si2 at low temperatures down and under high magnetic field. At low temperature, the thermoelectric power and the resistivity present several distinct anomalies as a function of field around H_0 ~ 9.5 T when the magnetic polarization reaches a critical value. The anomalies are accompanied with a change of sign from negative at low magnetic field to positive at high field (H>H_0) and are resulting from a Lifshitz-type topological transition of the Fermi surface. A logarithmic divergence of S/T at T to 0 K just above H_0 (H=11.5 T) is quite comparable to the well known divergence of S/T in the temperature range above the antiferromagnetic order at H=0 T referred to as non Fermi liquid behavior. The transition will be compared to the well characterized Fermi surface change in CeRu2Si2 at its pseudo-metamagnetic transition.

Thermoelectric response of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$: evidence for strong correlation and low carrier density

We present a study of the Seebeck and Nernst coefficients of Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ extended up to 28 T. The large magnitude of the Seebeck coefficient in the optimally doped sample tracks a remarkably low normalized Fermi temperature, which, like other correlated superconductors, is only one order of magnitude larger than T$_c$. We combine our data with other experimentally measured coefficients of the system to extract a set of self-consistent parameters, which identify Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ as a low-density correlated superconductor barely in the clean limit. The system is subject to strong superconducting fluctuations with a sizeable vortex Nernst signal in a wide temperature window.

Thickness-tuned Superconductor-to-Insulator Transitions under magnetic field in a-NbSi

We have studied the thickness-induced superconductor-to-insulator transition in the presence of a magnetic field for a-NbSi thin films. Analyzing the critical behavior of this system within the dirty boson model, we have found a critical exponents product of $ u_d z$ > 0.4. The corresponding phase diagram in the (H,d) plane is inferred. This small exponent product as well as the non-universal value of the critical resistance found at the transition call for further investigations in order to thoroughly understand these transitions.

Nernst effect in the phase-fluctuating superconductor InO$_x$

We present a study of the Nernst effect in amorphous 2D superconductor InO$_x$, whose low carrier density implies low phase rigidity and strong superconducting phase fluctuations. Instead of presenting the abrupt jump expected at a BCS transition, the Nernst signal evolves continuously through the superconducting transition as previously observed in underdoped cuprates. This contrasts with the case of Nb$_{0.15}$Si$_{0.85}$, where the Nernst signal due to vortices below T$_{c}$ and by Gaussian fluctuations above are clearly distinct. The behavior of the ghost critical field in InO$_x$ points to a correlation length which does not diverge at $T_c$, a temperature below which the amplitude fluctuations freeze, but phase fluctuations survive.