J. Sierro

Pressure induced heavy fermion superconductivity of CeCu2Ge2

Systematic trends observed in the thermopower of heavy fermion compounds have led us to study CeCu2Ge2 under high pressure. As expected, the transport properties of this compound above 70 kbar were found to be quite similar to the normal pressure ones of CeCu2Si2. Around this pressure, magnetic ordering vanishes and superconductivity appears. At 101 kbar, the transition temperature Tc≈0.64 K and the critical field Bc2(0)≈2 T.

High-pressure transport coefficients of the heavy-fermion superconductor CeCu2Si2

Abstract The electrical resistivity and the absolute thermoelectric power of the dense Kondo compound CeCu2Si2 were measured under very high pressures. At low temperatures, the results indicate a transition in the normal-state electronic properties around 20–30 kbar, from a strongly correlated Fermi-liquid regime (the so-called heavy-fermion state) into an IV phase. The crossover is associated with a steep increase of the critical temperature from ∼ 0.9 to ∼ 2 K. It is argued that this behavior may be due to a change from heavy-fermion to normal sd-band superconductivity.

Transport properties under magnetic fields of the heavy fermion system CeRu2Si2 and related compounds (Ce, La)Ru2Si2

Measurements of thermoelectric power (TEP) and thermal conductivity of single crystals CeRu2Si2 and Ce1−xLaxRu2Si2 are reported with special emphasis on the effect of the magnetic field. The roles of the crystal electric field splitting and the Kondo effect on TEP curves are debated. The TEP results at low temperature are analyzed with a two-band model, and the parameters are compared with those extracted from the thermodynamic measurements. In CeRu2Si2 the occurrence of a coherent regime at very low temperature discussed. The influence of the magnetic field on the TEP and the thermal conductivity is investigated and related with the occurrence of the metamagnetic-like transition already reported in magnetization measurements.

Thermopower and magneto-thermopower of CeRu2Si2 single crystals

Abstract Thermopower ( S ) measurements under magnetic field on CeRu 2 Si 2 single crystals are reported. With B = 0, S exhibits three positive contributions correlated with crystal field splitting, Kondo-like temperature and the appearance of coherence effect at low temperature. Special attention is given to the influence of the metamagnetic-like transition on magneto-thermopower

Resistivity and susceptibility measurements on single crystalline CeAl3

Abstract The resistivity ϱ and the susceptibility χ of very small single crystals of CeAl 3 have been measured. The anisotropy of χ reflects the magnetic property of the crystal field doublet ground state | J z = ± 3/2〉. At low temperature, both the resistivity and the magnetoresistance provide evidence that some kind of magnetic order starts to develop at 1.6 K.

Extended transport measurements on high-purity CeB6

Measurements of the electrical resistivity ρ, the thermal conductivityK and the thermoelectric powerQ on a high-purity, single crystal of CeB6 at low temperature (80 mK-4 K) in a magnetic field (0–7 T) are reported. Special attention is given to the high sensitivity of the transport properties to the magnetic phase diagram. The existence of different ordered phases at moderate applied magnetic fields allows us to demonstrate that transport properties depend strongly on the ground state of heavy fermion compounds. We observe two distinct regions in the power laws describing the transport properties, above and below about 250 mK in a zero applied field. An interesting possibility of explaining the broad maximum observed inQ(T) atB=0 is to ascribe it to a magnon drag effect. Properties of CeB6 are compared to those of otherf instability compounds and to current theories.

μ+SR study in CeCu6 single crystal

Abstract μ + SR measurements have been carried out down to 40 mK on a high-quality single crystal. The zero-field data show no sign of magnetic ordering down to the lowest temperature investigated. Transverse-field data indicate a large orientational dependence of the muon Knight-shift and point to a breakdown of the relation between atomic and bulk susceptibility.

Crystal field anisotropy in Yb heavy electron compounds

Using 170Yb Mossbauer spectroscopy, the anisotropic single ion magnetization and the thermal variation of the electric field gradient at the Yb site have been measured in polycrystalline samples of the Yb heavy electron compounds YbCuAl, YbPd2Si2 and YbSi. Comparison with existing theories is discussed.

MAGNETIC ORDERING AND PARAMAGNETIC RELAXATION OF YB3+ IN YBNI2SI2

Abstract The magnetically ordered phase and the Yb 3+ paramagnetic relaxation in the compound YbNi 2 Si 2 have been investigated by 170 Yb Mossbauer spectroscopy between 0.05 and 80 K. The magnetic ordering transition is found to occur at T N = 2.1 K, the Yb 3+ magnetic moments being perpendicular to the fourfold symmetry axis ( c -axis). Above T N and upto 30 K, the thermal variation of the Yb 3+ paramagnetic relaxation rate is a superposition of an interionic exchange term (spin-spin relaxation) and of a T -linear conduction electron-4f spin exchange term (Korringa relaxation).

Resistivity and thermoelectric power of YbCu4.5 under very high pressure

Transport properties of YbCU4.5 at zero pressure and under high pressure are reported. The resistivity is strongly perturbed by the disorder of the crystallographic structure, showing notably high residual resistivity and anisotropy at low temperature. Several samples have been measured at zero pressure and results are dominated by the Kondo effect: resistivity anomaly and giant thermoelectric power. Crystal field effect is taken into consideration to account for resistivity data. Under pressure, the decrease of the Kondo temperature have been observed with two experiments, up to 17 and 93 kbar. The possible appearance of a magnetic ground state under pressure is discussed.