John R. Beamish

Granular Nix (SiO2)100−x thin films as low temperature thermometers

A series of granular Nix (SiO2)100−x thin films have been prepared by rf sputtering with nickel concentrations between 60 and 90 at. %. The temperature dependence of the resistance is strongly composition dependent. In films with Ni concentrations below 70%, the resistance increases as the temperature decreases, but shows a crossover to a weaker temperature dependence below about 1 K. The magnetoresistance of these materials is small and negative in applied magnetic fields up to 6 T. The temperature dependence of the resistance and the small magnetoresistance indicate that films with Ni concentrations near 66% may be useful for thermometry over a wide range of temperatures.

The effect of 4He on the relaxation rates of the two-level systems in porous vycor glass

Abstract The sound speed in porous Vycor glass is strongly dependent on the amount of 4He adsorbed in the pores. This is due to the interaction between the Two-Level Systems (TLS) in the glass and excitations in the helium film. As helium is added, the TLS relaxation rates gradually increase, to reach a maximum at approximately 90% of ncrit, the critical coverage for the onset of superfluidity. Very close to ncrit a local minimum in the relaxation rates is found. A further increase in the coverage causes again an increase in the relaxation rates. For thicker films and filled pores, the relaxation rates become essentially independent of the amount of helium adsorbed. We then find a new behavior in which the relaxation contribution to the sound speed scales with temperature and frequency as T/ω l 4 . This indicates that an efficient relaxation channel becomes available for the TLS when there is superfluid in the pores. The rapid change in relaxation rates seen close to the completion of the “inert layer” indicates a change in excitation spectrum in the helium film at the onset of superfluidity.

Attenuation mechanisms in porous media containing helium

Results of the measurement of attenuation of transverse ultrasonic waves in three different porous materials containing liquid helium are presented.(AIP)

Granular nickel films as cryogenic thermometers

Abstract We have used RF sputtering to prepare a series of granular Ni x (SiO 2 ) 100− x films with nickel concentrations, x, between 60 and 90 atomic percent. The electrical resistivity of such films depends strongly on composition. We find that films with about 65 percent Ni have a temperature dependence suitable for use as cryogenic temperature sensors down to at least 100 mK. In addition, these films are relatively insensitive to magnetic fields. The largest resistance change we measured was about 1.5% in a field of 6 T at 200 mK, corresponding to a 4% temperature shift.

Critical behavior of superfluid 4He in porous alumina

Abstract We have made ultrasonic measurements of the superfluid density and the dissipation in several porous alumina samples filled with 4 He. In one sample (formed by a sol-gel process) with high porosity (92%) and large pores (R=1600 A), we found that the superfluid density disappeared at the transition with essentially the bulk exponent of 0.67, as expected. However, in a second material (slip-cast from fine powder) with lower porosity (52%) and much smaller pores (R=110 A), the superfluid critical exponent was 0.83±.02. The difference in critical behavior in the two media was also reflected in the dissipation. The sound attenuation in the first sample was due to viscous motion of the normal fluid and did not show any unusual behavior at the superfluid transition. In the small pore sample, on the other hand, there was a clear critical attenuation peak centered at the transition temperature.