B. S. Shivaram

Constraints on the Strain-Dependence of the UPt3 Superconducting Transition From Longitudinal-Ultrasound Measurements

We present measurements of longitudinal ultrasound in UPt3 along both the a- and c-axis in the region of the two superconducting transitions, Tc+ and Tc*. Using expressions from a model for the double superconducting transition, our measurements constrain the strain-dependences of a number of parameters. In particular, these constraints imply that the observed anisotropy in the stress-dependence of Tc* results from anisotropy in the elastic compliance tensor, rather than anisotropy of the superconducting order parameter. We also report an anomalous lattice softening below 100 mK.

Universality in the magnetic response of metamagnetic metals.

We report measurements of the nonlinear susceptibility chi(3)(T) in the metamagnetic heavy fermion (HF) compound UPt3. At high temperatures, chi(3)(T) < 0 and is small. It turns positive for T <= 35 K, forms a peak at T congruent to 10 K, and then decreases to zero with further decreasing temperature. The peak in chi(3) occurs at a temperature T-3, which is roughly half of T-1, the temperature of the maximum in the linear susceptibility. We present results on URu2Si2 and UPd2Al3 to show that this feature is common to other HF materials. A two-level model to describe the metamagnetic transition, with separation between the levels being the only energy scale, captures all experimentally observed features.

Uniaxial pressure dependence of the critical field anisotropy in UPt3

Abstract UPt3 is known to possess a unique phase diagram. Close to T c, three superconducting phases A, B and C coexist with the normal phase N giving rise to a tetracritical point. Furthermore, the anisotropy in the upper critical field H c2 near T c is reversed at low temperatures. In this letter we show that the behaviour under uniaxial pressure is unusual as well. For pressure, P c parallel to the c axis, H c2 is enhanced at low temperatures for H parallel to the a axis but is depressed for H parallel to c. In contrast the B[sbnd]C transition is always depressed with increasing P c for both the field orientations. Order-parameter candidates selected to explain the tetracritical point in the phase diagram of UPt3 near T, must also yield results consistent with the behaviour of the phase lines observed by us at low temperatures.

Longitudinal ultrasound in the heavy electron superconductor URu2Si2

Abstract We report measurements of the attenuation and velocity of longitudinal sound waves with the wave-vector parallel to the a -axis in the heavy electron superconductor URu 2 Si 2 . In the superconducting state the attenuation shows an absorption peak at 0.8 K considerably below T c =1.05 K. Associated with it an anomalously large decrease in the sound velocity is also observed. We also report sound velocity measurements in the normal state and compare it to results expected from the theory of a pinned density wave.

High-field ultrasound measurements inUPt3and the single-energy-scale model of metamagnetism

We report longitudinal ultrasound velocity measurements for magnetic fields up to 33 T applied parallel to the a-axis of the heavy electron compound UPt

Ultrasound study of the superconducting phases of UPt3 under uniaxial stress

_{3}

Nonlinear Pauli susceptibilities in Sr3Ru2O7 and universal features of itinerant metamagnetism

. A characteristic dip in the sound velocity at the metamagnetic critical field,

The linear and non-linear magnetic response of a tri-uranium single molecule magnet

H_{c}=20

The superconducting phases of UPt3 under uniaxial stress

T, reported in earlier work is reproduced and shown to be independent of temperature at very low temperatures. We show that the single energy scale model (B.S. Shivaram et al., Phys. Rev. B89, 241107(R), 2014) captures the observed key features of the field dependence in the sound velocity shift,

The electromagnetic response of the heavy electron superconductor URu2Si2

\delta v_{s}