S.-W. Lin

Ultrasonic studies of the heavy fermion compounds

A review of ultrasonic measurements on the three heavy fermion compounds UPt3, URu2Si2 and CeCu6 is given. These compounds show one or more of the three types of co-operative behavior; Kondo-like effect, antiferromagnetism and superconductivity; all these features are observed as ultrasonic anomalies. Both UPt3 and CeCu6 show a strong coherence effect which is akin to a Kondo-like transition in the electronic system. Velocity anomalies are found at the antiferromagnetic transition for URu2Si2. UPt3 also has an antiferromagnetic transition which is believed to influence the subsequent superconducting ordering. For UPt3, the superconductivity is of an unconventional nature; multiple superconducting phases have been observed and a complete phase diagram has been obtained from high resolution velocity measurements.

High resolution ultrasound studies of UPt3 at very low temperatures

We have performed longitudinal sound velocity and attenuation measurements on a single crystal of UPt3 between 5 mK and 700 mK. The very low temperatures reached in this experiment allow a better comparison to the theoretical predictions of the longitudinal sound attenuation for different gap states. We have also observed for the first time a significant dispersion of sound associated with the superconducting transition. Within the resolution of our ultrasonic measurements on this sample, we did not observe any manifestation of the recently reported specific heat anomaly around 18 mK1.

Superconducting phase diagrams of UPt3 from ultrasonic velocity measurements

Abstract Superconducting phase diagrams of the heavy fermion compound UPt3 in the H-T plane for the magnetic field at 90°, 45° and 0° to the c-axis were determined by high-resolution ultrasonic velocity measurements. Rich features observed in the complex phase diagrams provide many constraints on the identification of the symmetry class for the superconducting order parameter in this compound.

Ultrasonic studies of superconducting phase diagram of UPt3

We present a superconducting phase diagram of UPt3 for a field at 45° to the c-axis constructed from high-resolution ultrasonic velocity measurements. It is unambiguously determined that the HFL(T) line intersects the Hc2(T) line where a kink is observed. The available data favors the interpretation that the H(Tc∗) line terminates on the HFL(T) line at a different point.

Superconducting phase diagram of UPt3 for field along non-symmetric orientation from ultrasonic measurements

Abstract A new HT superconducting phase diagram for UPt3 with the field at 45° to the c-axis from ultrasonic velocity measurements is presented. The HFL(T) transition line is found to merge with the Hc2(T) line and the T c ∗ (H>) line appears to terminate in the HFL(T>) line at a different point.

Ultrasonic velocity determination of the phase diagram of UPt3

Abstract We report ultrasonic velocity measurements on two UPt3 samples with three different field orientations (H⊥c, H//c and H∧c=45°). Velocity changes are observed at constant temperatures, as the field is swept, at the upper critical field Hc2 and at the flux lattice transition Hfl. Velocity changes are also observed at Tc + the upper transition temperature and Tc - the lower transition temperature as the temperature is swept at constant fields. Complete phase diagrams for UPt3 obtained by a single measurement technique on the same samples in the H-T plane are presented.

High‐resolution velocity measurements in the superconducting state of the heavy fermion compound UPt3

Ultrasonic measurements have been used very successfully in investigating multi‐phase superconductivity in the heavy fermion compound UPt3. Further probing of the superconducting properties in UPt3 demands even higher resolution in both ultrasonic velocity and attenuation measurements. In order to achieve this, a new approach was used in measuring ultrasonic velocity and attenuation, in which a digital dynamic control technique was utilized in the conventional pulse‐echo method. With this approach, longitudinal ultrasonic velocity and attenuation of frequencies up to 450 MHz were measured on a single crystal UPt3 sample in the superconducting state. The overall resolution in the velocity measurement is in the order of 0.1 ppm. This enables us to determine a complex superconducting phase diagram for UPt3 with unprecedented accuracy. Superconducting phase boundary lines were traced even within the transition width. The critical property of converging phase transition lines is thus determined unambiguously. ...

Ultrasonic measurements in superconducting UPt3

Ultrasonic measurements on the heavy fermion superconductors have proved a useful tool in elucidating the nature of the superconducting state. Earlier measurements of sound attenuation in superconducting UPt3 showed power‐law temperature dependences (rather than an exponential temperature dependence as expected for a BCS type superconductor), indicating the presence of nodes in the gap. Subsequent ultrasonic measurements have shown indications of phase transitions within the superconducting state. In particular, these ultrasonic velocity measurements show the presence of three superconducting phases in the mixed state. The size of the velocity jump can be related to the heat capacity jump. Further, using a thermodynamic analysis it is possible to deduce the order of the phase transition lines. [Work at NU supported by NSF, and the work at UWM supported by ONR.]

Superconducting phase diagram of UPt3

Abstract We present ultrasonic velocity data for the heavy fermion superconductor UPt 3 . UPt 3 is a strong candidate for unconventional superconductivity. Conclusive evidence for this comes from the observation of multiple superconducting phases. Earlier measurements of the phase diagram relied on a variety of different measurements on different samples. The phase boundaries obtained agreed qualitatively, but the important region near the H c 2 curve remained an open question. Our velocity measurements present the first complete phase diagram of UPt 3 based on a single measurement on a single sample. The sharp velocity signatures allow us to trace these signatures to within the width of the normal-superconducting transition. Our results indicate that there is a tetracritical point on the H c 2 curve.

Sound velocity and magnetization measurements near the antiferronagnetic transition in the heavy fernion system URu2Si2

Abstract The heavy fermion superconductor URu2Si2 has a superconducting transition at 1.4 K and an antiferromagnetic transition at 17.5K. We present measurements in the vicinity of the antiferromagnetic transition of both sound velocity and magnetization.