Bimal K. Sarma

Critical-field measurements in Nb-Ti composition-modulated alloys

We present measurements of the transition temperature and upper parallel critical field, to 15 kG, of layered Nb-Ti alloys for layer wavelengths between 6 and 6250 Å.

Collective modes and sound propagation in a magnetic field in superfluid3He-B

A high-resolution, ultrasonic (12–89 MHz) acoustic impedance technique has been used to investigate the order parameter collective modes in superfluid3He-B over a pressure range of 0–15 bar and in magnetic fields up to 180 mT. In agreement with earlier experiments, theJ=2 real squashing mode has been observed to split into five components in small magnetic fields. However, contrary to earlier theoretical estimates, the Zeeman shifts have been found to become extremely nonlinear as the magnetic field is increased. The extent of this nonlinearity is largest at low pressures and at temperatures close toTc. In comparison with recent theoretical work, the nonlinear Zeeman shifts may be explained as a result of two effects. First, there is a significant distortion of the B-phase energy gap in large magnetic fields. Second, there is an important coupling between the sameJzsubstates of the differentJ modes. In this sense the nonlinear evolution of the real squashing mode constitutes the observation of the Paschen-Back effect in3He-B. A comparison of the observed Zeeman shifts with theoretical expressions has yielded information about particle-particle and particle-hole interaction effects in the superfluid. In the limitT → 0 and above a threshold field, the real squashing mode has been found to possess additional structure. TheJz=0 substate has been observed to split into a doublet. The separation between the two components of the doublet is of the order of 100–200 kHz and remains independent of the magnetic field. The origin of the doublet may be understood in terms of a recent theory which postulates a texture-dependent collective mode frequency. Further, at extremely small fields the effects due to dispersion of the real squashing modes have been found to be important. The magnitude of the dispersion-induced mode splitting in zero field is found to be consistent with theoretical predictions. TheJ=2 squashing mode has also been studied in the presence of a magnetic field. TheJz=0 state of the squashing mode is observed to shift to lower temperatures in a magnetic field. An additional field dependence of the observed acoustic impedance is interpreted as the evolution of theJz=−1, −2 states, but appears to be inconsistent with theoretical predictions.

Zero-sound measurements near the pair-breaking edge in low pressure 3He-B

Zero-sound measurements have been performed in 3He-B for P < 5 bar. An attenuation peak near Tc has been completely resolved for P < 2 bar and is greater than the theoretical predictions for the pair-breaking contribution. In addition, while cφ remains constant until hv ∼ 2δ (T), vg is observed to decrease by ≈ 35%.

Measurements of the acoustic impedance on superfluid 3HeB

Abstract The locations of the squashing and real squashing (new) modes in superfluid 3Heue5f8B have been found using an acoustic impedance technique for frequencies of 12.1, 36.3 and 60.5 MHz at pressures between 1.74 and 4.88 bar.

Magnetic field investigation of the acoustic impedance resonance near 2Δ(T) in 3He-A

Abstract The acoustic impedance resonance near 2Δ( T ) in 3 He-A has been studied in magnetic fields up to H = 0.72 T with Hq . It now appears that the resonance reported by Meisel et al. is independent of Ĥ , q orientation and satisfies ω ( T ) = a ξ Δ ( T ), where Δ( T ) is the maximum in the weak coupling axial state gap and a ξ = 1.8.

PROBING COLLECTIVE MODES OF SUPERFLUID **3He-B WITH ZERO SOUND.

We review what is currently known experimentally about the collisionless collective mode spectrum of the order parameter of the B phase of superfluid 3He. The most powerful probe of this spectrum has proved to be longitudinal zero sound and our knowledge of the remaining modes comes about through mode coupling effects. A magnetic field splits some states into multiplets (the Zeeman effect) with the number of states given by 2J+1 where J is the total angular momentum assigned to the state; an alteration in the spectrum, analogous to the Paschen‐Back effect, is observed at high fields. The mode coupling leads to high attenuations over much of the range of interest and negates the use of the standard (phase sensitive) pulse transmission technique; this necessitates the use of acoustic impedance methods. Our acoustic and cryogenic techniques are discussed in some detail.

The angular dependence of acoustic shear wave propagation in a smectic B liquid crystal

Abstract The angular dependence of the two propagating shear waves in the smectic B material p -butoxybenzylidene- p ′-octylaniline

Ultrasonic velocity and attenuation measurements at the metamagnetic transition in UPt3

The longitudinal ultrasonic attenuation and velocity were measured on a single crystal of UPt3 in a high magnetic field of up to 23T. Earlier ultrasonic measurements had seen a softening of the lattice as evidenced by a huge drop in the velocity at a field of 20T. Both the attenuation and velocity have been measured in field sweeps. Accompanying the large dip in the velocity is a large attenuation peak at this metamagnetic transition. Measurements were also done at several frequencies.

Ultrasonic attenuation measurements of the flux lattice phase transition in the heavy fermion superconductor UPt/sub 3/

Attenuation measurements of longitudinal ultrasound in superconducting UPt/sub 3/ show a peak, below H/sub c2/, that depends strongly on the orientation of the field relative to the c-axis. This peak is seen only with longitudinal and not with transverse sound. It is show that, as the field is tipped away from the c axis into the basal plane. H/sub FL/ decreases from 1.3 T to 0.58 T (at the lowest temperature). The temperature dependence of H/sub FL/ for all orientations of the field is the same-a flat region, followed by a slow decrease in H/sub FL/ with increasing temperature.<<ETX>>

Acoustic Impedance Investigations of the Collective Modes of 3 He-B

The acoustic properties of 3He-R are governed by the effects of pair breaking and the interaction of various condensate collective modes with zero sound. The latter leads to sharp resonances and a large dispersion in the temperature dependence of the attenuation and velocity of sound respectively. The very large attenuations encountered require very short path lengths if the usual pulse t ransmission technique is applied. However, large changes in the propagation characteristics produce readily detectable changes in the acoustic impedance. We review a phenomonological model for the interaction of zero sound with a collective mode. This model allows a quantitative description of the acoustic impedance which, through the equivalent circuit of a transducer, yields the corresponding temperature dependant changes in the electrical impedance. We present the r esults of calculations based on this model and compare them with experimental data. The electronics for performing such experiments is also disclhsed.