Search

The construction and performance of a single stage nuclear demagnetization apparatus utilizing 0.267 mole of praseodymium nickel material is described. An optimization procedure to minimize entropy production during the demagnetization of the hyperfine enhanced material has been developed which enabled the cryostat to cool a liquid-3He sample to a temperature of 350 μK. The effects of measured heat leaks and inferred eddy-current heating are accounted for satisfactorily in a model of this procedure. [ABSTRACT FROM AUTHOR]

A method to integrate micromechanical frequency-determining elements along with the corresponding electro-mechanical transducers into a poly or single-crystal silicon film layer is demonstrated. A resistor dissipating several microwatt of power induces high-frequency resonant mechanical motion in a shallow-shell membrane. Transduction from the mechanical to the electrical domain is performed using implanted piezoresistors, which are sensitive to strain produced by resonant motion. Self-sustained oscillations at 10 MHz are demonstrated when the device is directly coupled to a high-impedance operational amplifier and a positive feedback loop. Finally, this letter discuss how the resonator and transducers may be incorporated into standard integrated-circuit technology. [ABSTRACT FROM AUTHOR]

Magnetotransport measurements have been performed in thin cobalt disilicide films epitaxially grown on Si(111) wafers. Films of thickness between 4.0 and 20.0 nm were studied in order to ascertain the important electron scattering rates. A temperature independent contribution to the phase breaking scattering rate was determined and attributed to spin–spin scattering of the conduction electrons which increases as the film thickness is decreased. The origin of this scattering and its importance to the low-temperature electron transport are discussed. [ABSTRACT FROM AUTHOR]

We present data for the energy dissipation factor Q-1 over a broad temperature range at various pressures of a torsion pendulum setup used to study 3He confined in a 98% open silica aerogel. Values for Q-1 above Tc are temperature independent and have weak pressure dependence. Below Tc, a deliberate axial compression of the aerogel by 10% widens the range of metastability for a superfluid equal spin pairing (ESP) state; we observe this ESP phase on cooling and the B phase on warming over an extended temperature region. While the dissipation for the B phase tends to zero as T → 0, Q-1 exhibits a peak value greater than that at Tc at intermediate temperatures. Values for Q-1 in the ESP phase are consistently higher than in the B phase and are proportional to ps/p until the ESP to B phase transition is attained. We apply a viscoelastic collision-drag model, which couples the motion of the helium and the aerogel through a frictional relaxation time τf. We conclude that unless τf is an order of magnitude larger than expected, an additional mechanism to dissipate energy not captured in the collision-drag model and related to the emergence of the superfluid order must exist. The extra dissipation below Tc is possibly associated with mutual friction between the superfluid phases and the clamped normal fluid. The pressure dependence of the measured dissipation in both superfluid phases is likely related to the pressure dependence of the gap structure of the "dirty" superfluid. The large dissipation in the ESP state is consistent with the phase being the A or the Polar with the order parameter nodes oriented in the plane of the cell and perpendicular to the aerogel anisotropy axis. [ABSTRACT FROM AUTHOR]

In bulk superfluid 3 He at zero magnetic field, two phases emerge with the B-phase stable everywhere except at high pressures and temperatures, where the A-phase is favoured. Aerogels with nanostructure smaller than the superfluid coherence length are the only means to introduce disorder into the superfluid. Here we use a torsion pendulum to study 3 He confined in an extremely anisotropic, nematically ordered aerogel consisting of ∼10 nm-thick alumina strands, spaced by ∼100 nm, and aligned parallel to the pendulum axis. Kinks in the development of the superfluid fraction (at various pressures) as the temperature is varied correspond to phase transitions. Two such transitions are seen in the superfluid state, and we identify the superfluid phase closest to T c at low pressure as the polar state, a phase that is not seen in bulk 3 He.

Mechanical dissipation poses a ubiquitous challenge to the performance of nanomechanical devices. Here we analyze the support-induced dissipation of high-stress nanomechanical resonators. We develop a model for this loss mechanism and test it on Si(3)N(4) membranes with circular and square geometries. The measured Q values of different harmonics present a nonmonotonic behavior which is successfully explained. For azimuthal harmonics of the circular geometry we predict that destructive interference of the radiated waves leads to an exponential suppression of the clamping loss in the harmonic index. Our model can also be applied to graphene drums under high tension.

We have investigated the dissipation (Q-1) using the torsion pendulum technique for pure 3He and 3He-4He mixtures in silica aerogel near the 3He superfluid transition (T(c)) in aerogel. With pure 3He the Q-1 decreases at the onset of superfluidity. When phase separated 3He-4He mixtures are introduced into the aerogel, the Q-1 does not decrease as rapidly and eventually increases for the highest 4He content. We provide a model for the related attenuation of transverse sound alpha that takes into account elastic and inelastic scattering processes and exhibits a decrease in alpha at T(c).