S. Raymond

Transport evidence for pressure-induced superconductivity in CePd2Si2

Resistivity measurements performed under pressure on high quality single crystals of CePd2Si2 are reported. Pressure-induced superconductivity is observed in the range 2–7 GPa with an optimal Tc of 520 mK at 5.1 GPa. The superconducting properties of the sample as well as its peculiar zero pressure resistivity are tentatively linked to its preparation.

Phase diagram of heavy fermion systems

The Meccano of heavy fermion systems is shown on different cases going from anomalous monochalcogenides to cerium intermetallic compounds with special focus on the ideal case of the CeRu2Si2 series. Discussion is made in the frame of the interplay between valence, electronic structure (Fermi surface), and magnetism. The nice tools given by the temperature, the pressure, and the magnetic field allow to explore different ground states as well as the slow downhill race before reaching a Fermi liquid finish line at very low temperature. Experimentally, the Gruneisen parameter i.e. the ratio of the thermal expansion by the specific heat is a coloured magic number; its temperature, pressure, and magnetic field dependence is a deep disclosure of competing hierarchies and the conversion of this adaptive matter to external responses.

From heavy fermion antiferromagnetism to localized ferromagnetism: competition of two ground states in CeRu2Ge2 on cooling

Resistivity, specific heat and neutron scattering measurements performed on single crystals of CeRu2Ge2 are reported. The intrinsic behaviour of this compound is to exhibit a double transition from a paramagnetic to an antiferromagnetic and then a ferromagnetic state on lowering the temperature with TN = 8.3 K and respectively TC = 7.5 K. For the first time the antiferromagnetic order is characterized by neutron scattering. The propagation vector of the sine-wave modulated structure is k = (0.31,0,0) as in the Si doped compounds. This phase is found to be easily weakened by disorder, but an antiferromagnetic moment as large as 1.5 µB develops between TN and TC for the best crystal.

Two-dimensional spin fluctuations in Sr2RuO4

Abstract We report inelastic neutron scattering measurements carried out on single crystals of Sr2RuO4 in the normal state. The results are in agreement with band structure calculations that predict a sizable magnetic response at the incommensurate wave vector q o =(±2 π /3a,±2 π /3a,0), as expected from the nesting properties of the Fermi surface. The incommensurate spin fluctuations have been investigated mainly within the (a,b)-plane over a wide range of energies. Additional measurements along the c-axis shows that the magnetic correlations are two-dimensional.

Magnetic instabilities in CeRu2Si2 compounds

Abstract Neutron scattering experiments were recently performed in compounds of the CeRu2Si2 family. The topic addressed is the proximity of magnetic instabilities. The first one is the proximity of a T=0xa0K quantum phase transition experimentally achieved with lanthanum doping. The second kind of instability is linked to the competition between antiferromagnetic and ferromagnetic interactions in these materials. The pseudo-metamagnetic transition of CeRu2Si2 and the occurrence of an antiferromagnetic phase in the ferromagnetic compound CeRu2Ge2 were studied. Comparisons with macroscopic measurements are made using a simple analysis of the data.

Quantum and classical dynamics in mixed-spin one-dimensional antiferromagnets

Mixed-spin systems composed of quantum spin chains with gapped magnetic excitations interacting through `auxiliary magnetic ions show an effective separation between low-frequency classical and high-frequency quantum spin correlations. This phenomenon is realized in a family of rare-earth nickelates. Studies of these materials enable experimental measurements of some previously inaccessible fundamental properties of quantum spin chains to be made. The non-trivial intrinsic dynamics of the auxiliary spins gives rise to peculiar excitations of a mixed nature, and in certain cases may play a key role in long-range magnetic ordering.

High Pressure Resistivity of the Heavy Fermion Compound CeCu6

High pressure resistivity measurements up to 8.5 GPa performed on the archetypal heavy fermion compound CeCu6 are reported down to temperatures of 30 mK. The pressure variation of the different characteristic temperatures and resistivity exponents are analyzed in the framework of recent theories aiming to understand quantum phase transitions.

Magnetic structure and physical properties of the heavy fermion UIr2Si2

Abstract The CaBe 2 Ge 2 -type structure compounds (space group P4/nmm), especially UIr 2 Si 2 , are characterised by very high residual resistivity values at low temperature. In order to correlate physical and crystallographic behaviours, the physical properties of UIr 2 Si 2 have been measured on a single crystal already structurally characterised. Resistivity, specific heat and magnetic measurements as well as a neutron scattering study of the ordered magnetic moment are presented. These measurements categorise UIr 2 Si 2 as an antiferromagnet with T N = 6 K. A metamagnetic transition at a field of 1.5 T and heavy-fermion features with a linear specific heat coefficient of γ = 300 mJ/mol K 2 have been measured. For the first time, the antiferromagnetic structure of UIr 2 Si 2 has been determined by neutron diffraction to be (+, −, +, −) type with a low-temperature ( T = 1.9 K) magnetic moment of 0.10(1) μ B /U atom.

Pressure-induced residual resistivity anomaly in CeCu5Au

The electrical resistivity of the magnetically ordered CeCu5Au has been investigated under pressure up to 8.5 GPa. In the magnetically ordered region (p<3.4 GPa) the residual resistivity ρ0 shows a pronounced maximum as a function of pressure. Even in the non-magnetic region ρ0 decreases monotonically by more than a factor of three. These two effects can be qualitatively explained in terms of the interplay of pressure, magnetism and disorder in a strongly correlated electron system with weak disorder.

Magnetic and superconducting quantum critical points of heavy-fermion systems

Abstract Two examples of heavy-fermion systems are presented : CePd2Si2, an antiferromagnet with a quantum critical point at PC=28xa0kbar and UGe2 an itinerant ferromagnet which transits in a paramagnetic phase above PC=16xa0kbar. In CePd2Si2 the superconductivity domain is centered on PC. Special attention was given to the superconducting and magnetic anomalies at their superconducting and Neel temperatures. In UGe2 superconductivity appears in 9xa0kbar at a temperature TS, more than two orders of magnitude lower than the Curie temperature; furthermore, it occurs only on the magnetic border (P