Search

A major achievement of the past decade has been the realization of macroscopic quantum systems by exploiting the interactions between optical cavities and mechanical resonators. In these systems, phonons are coherently annihilated or created in exchange for photons. Similar phenomena have recently been observed through phonon-cavity coupling—energy exchange between the modes of a single system mediated by intrinsic material nonlinearity. This has so far been demonstrated primarily for bulk crystalline, high-quality-factor (Q > 105) mechanical systems operated at cryogenic temperatures. Here, we propose graphene as an ideal candidate for the study of such nonlinear mechanics. The large elastic modulus of this material and capability for spatial symmetry breaking via electrostatic forces is expected to generate a wealth of nonlinear phenomena, including tunable intermodal coupling. We have fabricated circular graphene membranes and report strong phonon-cavity effects at room temperature, despite the modest Q factor (∼100) of this system. We observe both amplification into parametric instability (mechanical lasing) and the cooling of Brownian motion in the fundamental mode through excitation of cavity sidebands. Furthermore, we characterize the quenching of these parametric effects at large vibrational amplitudes, offering a window on the all-mechanical analogue of cavity optomechanics, where the observation of such effects has proven elusive.

No Heading We report on the acoustic and thermal behavior of a Stycast 1266 - quartz composite torsional oscillator below 100 mK. The epoxy exhibits heat release manifested by a strong time dependence of the sound velocity below 10 mK. Also, the Stycast sample is sensitive to both ? and µ radiation, displaying transients superposed on the resonant response of the composite oscillator, as well as heating when exposed to a 22Na source. The long thermal relaxation time of the tunneling states together with the high thermal resistance limits the lowest achievable temperature in this insulating amorphous material.

We present the results of measurements on a single crystal sample of scandium metal at temperatures down to 100 ?K using nuclear quadrupole resonance (NQR). We find two regimes in the relaxation curves: an initial fast relaxation, followed by a slower relaxation consistent with the three exponential recovery expected for an I = 7/2 system in zero external magnetic field. The Korringa constant for this longer time relaxation in our sample is 90 +- 9 msec K-1. By observing deviations in the ratio of the intensities of adjacent nuclear spin transitions at the lowest attainable temperatures, we have been able to make a determination of the sign of the total electric field gradient present in the crystal. We find that the lowest energy state of the nuclear spin system corresponds to mI = +-7/2. A combination of these deviations and pulse NQR allows us to use this system as an absolute thermometer in the ?Kelvin regime.

We compare the inelastic scattering rate in 220-Å-thick free-standing films with nearly identical films on substrates. The scattering rate was determined by fitting the magnetoresistance to theories of quantum transport. Although the films are sufficiently thin so as to modify the three-dimensional spectrum of thermal phonons, we find no significant difference between the scattering rates in free-standing and supported films. While experiments on other materials down to lower temperatures are necessary, we conclude that the cubic temperature dependence of the inelastic scattering rate in these thin aluminum films is not strongly affected by the phonon dimensionality.

We report electron heating measurements below the resistance maximum associated with the spin-glass freezing transition. The differential resistance showed features under the application of a dc electric field which cannot be readily interpreted in a simple electron heating picture. We demonstrate that these anomalies are resolved after computation of the resistance by integration of the differential results at various currents. An experiment was designed and carried out to test this procedure through the use of an applied magnetic field. The integration process is applicable to transport measurements carried out under combinations of ac and dc electric fields.

We have measured the superfluid fraction of3He confined within the pores of silver sinter. Four sintered substrates were prepared with packing fractions between 23% and 68% of solid density. Measurements were carried out at six pressures between 0 bar and 29 bar. These results show that the superfluid fraction is suppressed to a much greater extent than expected.

Measurements have been made of the normal fluid fraction of superfluid3He-B at eight different pressures between 0 and 29.2 bar. The minimum temperatures were such that ?n/? was reduced to less than 0.5%. The experiments, carried out independently at Cornell and Texas A&M Universities, used similar Andronikashvilli-type torsional oscillators. The results are qualitatively similar, but show significant differences due to the use of different temperature standards. Both sets of results are presented in tabular form. The inconsistencies in the temperature scales preclude the examination of the results for possible strong coupling effects.

Interferometric imaging of normal mode dynamics in electromechanical resonators, oscillating in the rf regime, is demonstrated by synchronous imaging with a pulsed nanosecond laser. Profiles of mechanical modes in suspended thin film structures and their equilibrium profiles are measured through all-optical Fabry–Perot reflectance fits to the temporal traces. As a proof of principle, the mode patterns of a microdrum silicon resonator are visualized, and the extracted vibration modes and equilibrium profile show good agreement with numerical estimations.

The article informs that Researchers at Cornell University's Low Temperature Physics Lab have published an paper investigating the operation of a Cryomech LHeP22 helium recovery and liquefaction system in their low temperature lab alongside a nuclear demagnetization refrigerator.

We present preliminary results from a torsional oscillator experiment designed to measure the superfluid fraction and transition temperature of3He in aerogel in magnetic fields up to 0.4 T.

We present results from a torsional oscillator experiment designed to measure the superfluid fraction and transition temperature of3He in aerogel when the surface3He is replaced with4He. The cell has an LCMN susceptibility thermometer mounted directly in the liquid outside of the aerogel to establish thermometry in the presence of4He on the heat exchanger surface. A capacitor is embedded within the aerogel which is sensitive to the presence of4He, and allows us to determine that the4He goes into the aerogel cell. Preliminary results indicate that at 8 bar, the addition of4He does not significantly alter the superfluid behavior.

We have measured the superfluid density of 3He liquid confined within the pores of a silver sinter plug. The silver was of nominal 70 nm size, and was packed to 54% of solid density. The mean pore size as measured was ?200 nm. The plug was mounted on a torsional oscillator, and the superfluid density can be measured by studying the decrease of the resonant period. We have measured the pressure and temperature dependence of the period shift and find that the superfluid density is suppressed to ?14% at 0 bar and is ?53% of its bulk value at 29 bar. There is substantial temperature dependence even at the lowest values of T/Tc(?0.25) achieved in these experiments.

We present new torsional oscillator data on the shear viscosity of liquid 3He at low temperatures. They are analyzed with a theory of Poiseuille flow, which is also applicable in regimes of temperature and pressure where the Knudsen number, characterizing the flow of the thermal excitations, may become large.

We have measured the suppresssion of the superfluid transition temperature due to confinement of the fluid by 350 nm diameter pores in Nuclepore filter. The experiments were carried out at several pressures between 0 bar and 29 bar, where the pressure variation allows us to vary the ratio of the coherence length to pore diameter by approximately a factor of five. The Nuclepore filter was mounted across an annular flow channel which formed part of the inertial bob of a torsional oscillator. Flow was induced by operating the device at its resonant frequency of 850 Hz at amplitudes between 50 pm and 10 nm. The measured suppression of the superfluid transition is less than that predicted by theory.