L. Taillefer

Direct observation of heavy quasiparticles in UPt3 via the dHvA effect

We present the results of an investigation of the de Haas-van Alphen (dHvA) effect in the heavy fermion superconductor UPt3. Oscillations composed of up to 8 frequency components, corresponding to cyclotron orbits in a plane normal to the a-axis, have been detected in a high purity single crystal and a study of their amplitude as a function of temperature and magnetic field has been performed in the intervals 20–150 mK and 40–115 kG, respectively. From this study we obtain estimates of the cyclotron masses, found to range approximately from 25 to 90 times the bare electron mass, and of the mean free path, found to be in excess of 1000 A. The relationship between these findings and the results of conventional energy band calculations is discussed.

Normal and superconducting phases of heavy fermions

Abstract A review of recent experimental findings in the field of heavy fermions is given, with an emphasis on the three types of transition encountered: antiferromagnetic, metamagnetic and superconducting. The appearance of long-range antiferromagnetic ordering of small moments in compounds such as URu2Si2 and UPt3 is described in terms of its structure and observable consequences. A comparison is made between the behaviour of CeRu2Si2 and that of UPt3 below and above their magnetic field-induced transitions to a polarised state, drawing upon the detailed information now available from Fermi-surface studies, and from neutron scattering, specific heat and magnetoresistance measurements. Finally, the rapidly-growing evidence for a multiplicity of superconducting phases in UPt3 is discussed, in parallel with the related case of Th-doped UBe13. The issue of a possible coupling between superconducting and antiferromagnetic order parameters is addressed.

Superconducting phase diagram of UPt3 studied by thermal expansion and specific heat

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.

Anisotropy and the superconducting phases of UPt3

Abstract Recent results of several experimental studies on the superconducting behaviour of UPt 3 are reviewed. Special attention is given to the qualitative features: 1) the existence of two transitions at H = 0 and the phase diagram under a magnetic field applied both parallel ( H ‖ c ) and perpendicular ( H ⊥ c ) to the hexagonal axis, established by specific heat measurements, 2) the suppression of the high-temperature phase by a small hydrostatic pressure, 3) the reversal of the hitherto apparent correlation between the strength of the electron-electron interactions in the normal state and the onset of superconductivity, when investigated for each crystallographic axis separately by means of uniaxial stress and 4) the sharp break in the slopes of both the critical fields H c1 ( T ) and H c2 ( T ) for H ⊥ c .

Effect of pressure on the superconductive phase transition of UPt3

Abstract Measurements of the specific heat of the heavy-fermion compound UPt3 under hydrostatic pressure p up to 6 kbar show that the two superconductive transitions at temperatures Tc+ and Tc− (observed previously under zero pressure) decrease at a rate of dTc+/dp = −24 mK/kbar and dTc−/dp = −5 mK/kbar. The transitions merge at p ∗ = 3.7 kbar and T ∗ = 419 mK . These results suggest that the weak antiferromagnetic order which is suppressed on the same pressure scale acts as a symmetry-breaking field in splitting the superconductive transition.

The superconducting phases of UPt3 under pressure

The upper critical field Hc2(T) of the heavy fermion superconductor UPt3 for a magnetic field applied in the basal plane has been studied down to 150 mK and under hydrostatic pressure up to 10 kbar. The kink already observed in Hc2(T) at ambient pressure disappears by applying a relatively low pressure. This suggests that the high‐temperature low‐field superconducting phase reported recently is rapidly suppressed by pressure. A possible interpretation of these results is discussed in connection with recent theories.

On the superconducting phase transitions of UPt3

The possible role of antiferromagnetic order in causing a multiplicity of superconducting phase transitions in UPt 3 is investigated. Two results are presented which shed light on this question. First, a small hydrostatic pressure is found to have a significant effect on both 1) the neutron scattering intensity of the antiferromagnetic Bragg peak, which is more than halved by 2 kbar, and 2) the H c 2 ( T ) curve, from which the kink — regarded as a signature of phase multiplicity — disappears above 1.5 kbar. Secondly, the kink in H c 2 ( T ) is observed for all field directions within the basal plane. This evidence is discussed in connection with current theories.

High-field magnetoresistance of UPt3

Abstract The transverse magnetoresistance of the heavy-fermion superconductor UPt3 has been measured in a high quality single crystal as a function of magnetic field up to 28 T and of temperature down to 20 mK. At low temperatures, the ωcτ > 1 limit is achieved and the Fermi surface topology plays a dominant role. New features of the magnetoresistance are observed: an extraordinary anisotropy in the basal plane, a saturation at high fields up to the metamagnetic transition at 20 T, and a sudden and rapid increase above the transition, showing no sign of saturation at the highest fields. The effect of raising the temperature is also studied.

Low-temperature anomalies in the specific heat of heavy-fermion UPt3

High-precision specific-heat measurements (1.3 K < T < 20 K) on UPt3 yield two small, but distinct, anomalies at 6.2 K, where the c/T curves shows a kink, and at 7.3 K, where a small peak is observed. Samples of different origin gave identical results. The possible connection with antiferromagnetic order yielding small moments (TN = 5 K), as recently probed by neutron experiments, is discussed.

Thermal conductivity of superconducting UPt3 versus magnetic field: probing the gap structure

Abstract The thermal conductivity (κ) of the heavy-fermion superconductor UPt3 has been measured as a function of magnetic field ( H ) on a single crystal with the heat current in the basal plane. At low fields, where the scattering of heat carriers by vortices is thought to be important, κ( H ) depends on the relative orientation of field and current. On the other hand, in the vicinity of the upper critical field, the dependence of κ( H ) on the orientation of the field throws new light on the structure of the superconducting gap.