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We present transport and specific heat measurements on high quality single crystals of UPt3before and after irradiation by high energy electrons. The induced changes of the superconducting critical temperature are found to depend strongly on the distance from the irradiated surface. The dramatic effects of the irradiation on the specific heat in the superconducting state can then be simply explained in terms of an inhomogeneous distribution of superconducting transition temperatures. The question of the failure of the “universal limit” in the heat transport of UPt3is reexamined, and the conditions for a clean experimental test are established.

The superconducting critical current along the c-axis of the unconventional superconductor UPt3has been determined as a function of magnetic field direction in the basal (a-a*) plane in the three (A, B and C) phases of the mixed state. The critical current has maxima for field directions parallel to both the a- and a*-axis in the superconducting C phase, but has maxima only parallel to the a-axis in the A and B phases. Our experiments thus show that the anisotropy of the superconducting critical current in UPt3changes at the same field at which the symmetry of the order parameter changes. This demonstrates for the first time that the critical current is sensitive to the symmetry of the superconducting order parameter.

We report transport and calorimetric measurements at high pressure on CeIn 3 crystals grown from a flux of excess indium. This technique allows the growth of large crystals which are suitable for techniques which require large sample volumes. Measurements were carried out in a Bridgman cell which allowed pressures of up to 100 kbars to be reached. The results obtained for the antiferromagnetic and superconducting phase diagram are very similar to previous results obtained on small crystals and in hydrostatic cells. We also find a well defined Quantum Critical Point and Non Fermi-Liquid behaviour of the normal state resistivity. We report the first measurements of the upper critical field and of the specific heat at high pressure on this compound.

We report resistivity measurements performed on a high quality single crystal of CePd2Si2under hydrostatic pressure. At ambient pressure the de Haas–van Alphen effect has been also studied. Two different frequencies with weak angular dependence were detected with magnetic field lying in the basal plane, while another frequency was found with magnetic field parallel to the tetragonal c axis. Near the critical pressure, Pc∼27 kbar, where the antiferromagnetic transition vanishes, the normal state resistivity does not follow the usual Fermi-liquid (FL) behavior and is described by a T1.3law, while just below Pc, the resistivity shows clearly separated spin wave and electron–electron contributions. At Pc, the FL form of ρ(T) is not restored even at magnetic field up to 6T. The first appearance of superconductivity is observed at P=19 kbar, and the critical temperature increases with pressure up to 27 kbar. The analysis of the upper critical field at Pcshows that the superconducting state is well described by a weak coupling, clean limit model with a slightly anisotropic orbital limit and a strongly anisotropic paramagnetic one.

We have implemented AC calorimetric measurements in diamond anvil pressure cells. The range of pressure (0–20 GPa), the excellent hydrostatic conditions of solid 4He used as the pressure transmitting medium associated with the possibility of continuously tuning the pressure at low temperature makes this experiment ideally suited for investigating phase diagrams under pressure and in particular for studying the thermodynamics of correlated metals around a Quantum Critical Point (QCP) where magnetic order is suppressed at absolute zero temperature. We present here results on the heavy fermion compound CeRu2Ge2up to 8 GPa and down to 2 K. The specific heat anomalies associated with the different magnetic phase transitions are clearly resolved, allowing a precise determination of the magnetic phase diagram. On approaching the QCP close to 7 GPa, we observe a striking broadening of the antiferromagnetic transition associated with an anomalous behavior of the signal phase which could correspond to a change in the nature of this transition.

We have determined the magnetic structures of CePd2Ge2and Ce(Pd0.35Ni0.65)2Ge2by single-crystal neutron diffraction measurements. The structure of CePd2Ge2consists of ferromagnetically ordered sheets that are stacked antiferromagnetically along the propagation vector k=(1/2,1/2, 0), with the moment parallel to k. The magnetic moment and the Néel temperature are m=0.84(9)μBand TN=5.0(1)K,respectively. The magnetic structure of Ce(Pd0.35Ni0.65)2Ge2is similar but with a modulation of the moment in the cdirection and k=(1/2,1/2,±δ),with δ=0.16(1). The moment and the Néel temperature are reduced to m=0.51(3)μBand TN=3.1(1)K.

Superconducting and normal state properties of a single crystal of CeCu2Si2have been investigated by resistivity measurements under hydrostatic pressure and magnetic field. The low temperature resistivity behaviour (ρ= ρ0+ AT2, which is observed up to a temperature TA) is considered within the self-consistent renormalized (SCR) spin fluctuation model. Contrary to the model predictions, the product AT2Aincreases on approaching the quantum critical point (QCP). The temperature dependence of the upper critical field is analyzed assuming strong coupling and an intermediate regime between the clean and dirty limits. Both the low temperature resistivity behaviour at high pressure and the parameters deduced from the fits of Hc2point to a pressure induced decrease of the quasi-particle effective mass m*. The anisotropy of the initial slope of Hc2(T) and therefore that of the effective mass was found to change under pressure.

The superconducting properties of the heavy fermion UPt3have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE1gandE2u) of the superconducting order parameter in UPt3.

Thermal conductivity has been measured from 0.03 K to 0.8 K in single crystal CeNi2Ge2which is quite near the quantum critical point (QCP). The Wiedemann–Franz law is observed at low temperatures (T<0.1 K) for both directions (j⊥c andj ∥ c), indicating that quasiparticles are scattered elastically asT→0 K. With increasing temperature, a deviation from the Wiedemann–Franz law due to inelastic spin-fluctuation contributions is observed. The effect of proximity to the QCP in CeNi2Ge2appears in the spin-fluctuation contributions to the thermal and electrical conductivities.

Abstract: We present new measurements of the thermal conductivity (κ) of UPt3 down to very low temperatures (16 mK) and under magnetic fields (up to 4 T) which cover all the superconducting phases of UPt3. The measurements in zero field are compared with recent theoretical predictions for the thermal conductivity, which is dominated by impurity states at the lowest temperatures studied. The measurements under magnetic field at low temperatures are surprising since they don’t show the expected low field square root dependence, $$\kappa \propto \sqrt {\rm B} $$ . The discontinuity ofdκ/dT atT c changes drastically when passing from the high field low temperatureC phase to the low field high temperatureA phase: this is related to the change of the symmetry of the superconducting order parameter when crossing theA→C phase transition.

Neutron diffraction experiments under pressure on the cubic heavy-fermion compounds CeIn3 and CePb3 are reported. Resistivity measurements were performed on CeIn3 in order to study the approach to the magnetic-nonmagnetic transition. Comparisons are made with previous results obtained on CeAl2. In CePb3 and CeAl2, the pressure-induced change in the magnetic structure from modulated to simple-antiferromagnetic may correspond to a reduction of the magnetic anisotropy. The strong pressure decrease of the Néel temperature of CeIn3 for P>15 kbar indicates the proximity of the transition from a magnetic to a nonmagnetic ground state. In cubic heavy-fermion compounds, the appearance of a nonmagnetic ground state seems to be directly connected to the recovery of the full multiplet degeneracy of the rare-earth ion.

Low-temperature magnetostriction experiments have been performed on CeRu2Si2 single crystals in magnetic fields up to 15T. WhenH is applied parallel to the tetragonalc-axis, a very large expansion is observed in all lattice directions. A pronounced anomaly occurs at a field close to the “metamagnetic” threshold. The results are discussed with use of thermodynamic relations and support a very simple scaling behavior for the pressure dependence of the magnetization.

Extensive magnetization and magnetoresistivity experiments on CeRu2Si2 are described, with special emphasis on the metamagnetic-like transition that occurs atH*=80 kOe. By contrast to the low-field susceptibility, the differential susceptibility atH* almost diverges atT?0. It is suggested that high magnetic fields (H>H*) restore a situation where the interactions between particles have collapsed. Comparison is made with other well-known examples: TiBe2, UPt3, TmSe, and3He.

The thermal conductivity ? of the heavy-fermion superconductor UPt3 has been measured on two single crystals of different quality as a function of temperature and applied magnetic field for different orientations of the heat current relative to both the field and the crystalline axes. The temperature dependence of ? is far from exponential and nearly the same for both crystals, in which the heat current is, respectively, parallel and perpendicular to the hexagonalc axis, suggesting a gap structure with nodes in the basal plane and normal to it. The field dependence of ? is strongly anisotropic. In the best sample at low fields, where the scattering of heat carriers by vortices is thought to be important, ?(H) depends on the relative orientation of field and current. On the other hand, at high fields nearHc2 (in both samples), ?(H) depends on the relative orientation of field and crystalline axes, reflecting an anisotropy in the gap structure and in the Fermi velocities.

The magnetization and the specific heat of Ce1-xLaxRu2Si2 with x=0.13 are reported with special attention to the effect of magnetic field and the role of lanthanum doping. Evidence is given of differences between the undoped (x=0) and the solid solution (x?0) cases. A common feature is the occurrence of well-defined anomalies in the magnetization at the “metamagnetic” fieldHM independently of whether the ground state is one of long-range order or Pauli paramagnetism. Forx=0, the ground state appears to be a Pauli paramagnet for any strength of the magnetic field; quantum fluctuations or deviations from an ideal lattice may prevent the occurrence of a true static magnetic transition.

Magnetization measurements performed below 1 K on a single-crystalline sample of the heavy fermion compound CeRu2Si2 are reported. The field variation of the linear term of the specific heat ? derived from these data (via a Maxwell relation) exhibits a large peak at the metamagnetic-like transition at B* (=7.7 atT ? 0). Thermal-expansion measurements, show a drastic drop of the temperature where a Fermi liquid behavior is reached whenB ?B*. Similar experiments performed on a single-crystalline Ce0.95La0.05Ru2Si2 sample show a broadening of all the anomalies atB* and a reduction of their intensities. These results strongly suggest that pure CeRu2Si2 approaches a magnetic instability atB* forT ? 0.

The specific heat and thermal expansion of UPt3 were measured in the vicinity of the superconducting transition for magnetic fields up to 10 kOe, applied along the hexagonalc axis. Both thermodynamic techniques show clear evidence of two transitions in zero field, which merge at a critical field of approximately 8 kOe. The upper critical field curve determined by the resistivity for the upper transition exhibits a small but abrupt change of slope at a comparable value of the field. The phase diagram that emerges forH?c is therefore qualitatively similar to that previously found forH?c, this time with a critical point (or region) aroundH*=8 kOe,T*=0.36 K. The implications of a nearly isotropic phase diagram are discussed in connection with recent theories.

We present a review of recent magneto-volume experiments, such as thermal expansion and magnetostriction, performed on good-quality single crystals of heavy-fermion systems, like CeCu6, CeRu2Si2, URu2Si2, UBe13and UPt3. All investigated compounds exhibit large magneto-volume effects at low temperatures, as is reflected by their huge Grüneisen parameters (at least two orders of magnitude larger than in ordinary metals). A combination of specific-heat and thermal-expansion data yields furthermore information on the pressure dependence of the characteristic temperatures, and allows, in particular cases, for a detailed separation of the Kondo, crystal-field, phonon or other contributions to the thermal properties. We also discuss the magneto-volume effect connected with the metamagnetic-like transitions observed at 20 and 8 T (at 4.2K) for UPt3and CeRu2Si2, respectively.

Inelastic neutron scattering experiments were performed to study the spin dynamics of the heavy fermion compound Ce0.925La0.075Ru2Si2. This composition corresponds to the borderline of the magnetic-non magnetic transition in the family of alloys Ce1 − xLaxRu2Si2. Short range magnetic correlations of an inelastic nature characterize this system in addition to a small ordered moment of the order of 0.02µBwhich appears below 2 K. Comparison with the magnetic excitation spectrum of the Pauli paramagnet CeRu2Si2is given from the viewpoint of recent work on phase transitions near the magnetic instability point.

The magnetic scattering of neutrons by single crystals of the heavy fermion compound CeCu6has been measured with polarised neutrons and polarisation analysis. For higher temperatures the magnetic scattering due to the transverse component follows the cerium form factor. Structure appears when the temperature is lowered, providing evidence for magnetic correlations. Comparison with calculations indicates primarily ferromagnetic correlations for nearest neighbour cerium pairs, and also antiferromagnetic coupling for next nearest neighbour pairs. The static local cerium transverse components of the moments are found to be reduced by about 30% in passing from room temperatures to the mK range.

CeCu6is a typical heavy fermion system (λ= 1500 mJ/mol K2) without magnetic or superconducting ordering. Inelastic neutron scattering experiments have been performed with a very high energy resolution (0.1 meV) at very low temperatures (T= 0.1, 0.7 and 7 K). The recorded spectra show a broadening of the quasi-elastic peak at T= 0.7 and 7 K, well explained by a Lorentzian shape. However, at T= 0.1 K, the spectra exhibit an energy gap of 0.25 meV. Magnetic correlations exist in the Fermi liquid ground state (T= 0.1 K) and correspond to an antiferromagnetic coupling between (001) neighbouring planes.

Measurements of the specific heat, magnetization, and transport properties (resistivity, thermoelectric power, and thermal conductivity) of single-crystal CeCu6 are reported with special emphasis on the effect of magnetic field. The relative field variations of the differential susceptibility, the coefficients of the linear temperature term of the specific heat, and theT2 resistivity term are presented. They are directly related to the field curvature of the magnetization, indicating the itinerant nature of thef electrons. The Wilson ratio reaches a maximum atH=H*, which may correspond to a crossover from a low-field phase in which antiferromagnetic correlations dominate, to a highly polarized phase. Intersite coupling seems to play an important role in heavy-fermion compounds. Another interesting result is the occurrence of residual positive magnetoresistivity, which also appears to be a general feature of heavy-fermion compounds. The properties of CeCu6 are compared to those of otherf instability compounds and to present theories.

The unusual field dependence of the upper critical field in UBe13 can be explained by a field variation of the condensation energy which is estimated by the magnetoresistivity experiments. Another interesting feature is the occurrence of large critical fluctuations.

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.

Magnetic fluctuations in CeCu2Si2 were directly observed, both above the superconducting transition temperatureTC as well as below. Spatial correlations of the magnetisation density belowTC are weak, resulting in an essentially wavevector independent magnetic response. The intensity observed within an energy window of ±5 THz centred on ?=0 THz corresponds to 1µß per Ce atom. Higher energy excitations peaking between 7–10 THz were observed and are probably due to higher crystal field levels.

We discuss some consequences of the interplay between magnetism and superconductivity in the two heavy fermion systems URu2Si2 and UPt3, notably on the temperature dependence of the specific heat, on possible observation of Larkin-OvchinnikovFulde-Ferrel phase, and on the anisotropy of the upper critical field. We demonstrate that in UPt3, a clear double steep superconducting transition can be obtained reversibly.

Measurements of the electrical resistivity ?, 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.

Crystal growth of3He in restricted and bulk geometries is described. A striking feature is the growth of single crystals inside a metallic power of 0.3 µm mean grain size. Comparison of the intensity of the (1, 1, 0) and (2, 0, 0) Bragg reflections gives a direct estimate of the zero-point motion. No temperature dependence of the Debye- Waller factor is observed.

In the normal phase of UPt3, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT2 dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromTc to 146 mK (the lowest experimental temperature), a largeT2 contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.

Field dependence of the Hall resistivity has been measured below and above a metamagnetic-like transition (MT) at 50 mK-2 K in a high quality single crystal CeRu2Si2. The MT in the Fermi-liquid region is regarded as a continuous modification of resonance quasi-particle band near the Fermi level and no drastic change of carrier density is observed. In contrast, the MT in the incoherent region exhibits no clear change of effective mass or inelastic scattering time. The strong modification of the MT by the coherent effect is discussed.

The upper critical fieldHc2(T) of a high quality single crystal of UBe13 is studied with very low noise resistivity measurements. It shows a large but finite slope of —45 T/K, an unusual temperature dependence with an inflexion point atT/TC~ 0.5 and a large saturated limit forT? 0 ofHc2(0) = 14 T. The complete temperature dependence ofHc2(T) can be described by a simple model of strong coupling superconductor, assuming a full Pauli limitation and the occurence of a non-uniform superconducting state (FFLO state) at low temperature.

This note reviews magnetotransport data on the magnetic semiconductor Gd−xvxS4(v= vacancy). New data on the metallic side of the transition are presented. The results for two samples may be summarized as follows: 1) At low temperatures (T∼ 10 mK) the conductivity, σ(0) ∝ (H- Hc), where Hcis a critical field below which the material is an insulator; 2) for the sample with the larger critical field, the data are not consistent with variable range hopping: 3) for H/Hc≳ 1, the temperature dependence deviates only little from a √Tlaw.

Inelastic neutron scattering experiments were performed to study the spin dynamics of the heavy fermion compound Ce0,925La0.075Ru2Si2. This composition corresponds to the borderline of the magnetic-non magnetic transition in the family of alloys Ce1−xLaxRu2Si2. Short range magnetic correlations of an inelastic nature characterize this system in addition to a small ordered moment of the order of 0.02μBwhich appears below 2 K. Comparison with the magnetic excitation spectrum of the Pauli paramagnet CeRu2Si2is given from the viewpoint of recent work on phase transitions near the magnetic instability point.

Polarised neutrons and polarisation analysis were used in a neutron scattering study of UPt3 both above the superconducting transition temperatureTc as well as below. Thus the existence of magnetic fluctuations in UPt3 was established unambiguously. For varyingQ vectors the magnetic scattering intensity was integrated over energy transfers ±20 meV to obtain a measure of the corresponding magnetic correlation function. No indication for strong spatial correlations of magnetisation could be detected. Furthermore inelastic transitions up to energy transfers of 80 meV were observed.

The effect of spin fluctuations on the thermal properties of strongly correlated itinerant fermion systems is discussed based on the self-consistent renormalized (SCR) spin fluctuation model. Specific heat and magnetic susceptibility in the normal phase of liquid-3He and heavy electron compounds at finite temperatures are well described by the unified SCR picture including the mode-mode coupling of spin fluctuations beyond the random phase approximation (RPA).