Miguel Gonzalez

Signal analysis and characterization of a micro-electro-mechanical oscillator for the study of quantum fluids

We present the full characterization of a micro-electro-mechanical system (MEMS) based probe developed for the study of quantum fluids. The device consists of a pair of parallel plates (200 × 200 μm2) with a well-defined gap (1.25 μm) in which a fluid film forms when immersed in liquid. The mobile plate is suspended above the fixed plate (substrate) by four serpentine springs. This geometry allows for the study of the properties of the surrounding liquid through the resonant behavior of the mobile plate. This device demonstrated its potential as a high precision probe suitable for the study of films of quantum fluids and also of quantum turbulence. In this work we present our detailed analysis of the device signal in our current measurement scheme. Based on our analysis, we determined the transduction factor of a device at room temperature and 4 K, which allows us to convert the measured quantities to physical quantities such as displacement, velocity, and force.

Ultrasound attenuation and aP−B−Tphase diagram of superfluidH3ein 98% aerogel

Longitudinal sound attenuation measurements in superfluid 3He in 98% aerogel were conducted at pressures between 14 and 33 bar and in magnetic fields up to 4.44 kG. The temperature dependence of the ultrasound attenuation in the A-like phase was determined for the entire superfluid region exploiting the field induced meta-stable A-like phase at the highest field. In the lower field, the A-B transition in aerogel was identified by a smooth jump in attenuation on both cooling and warming. Based on the transitions observed on warming, a phase diagram as a function of pressure (P), temperature (T) and magnetic field (B) is constructed. We find that the A-B phase boundary in aerogel recedes in a drastically different manner than in bulk in response to an increasing magnetic field. The implications of the observed phase diagram are discussed.

Ultrasound attenuation measurements in the A-like phase of superfluid 3He in aerogel

Liquid 3He impregnated in high porosity aerogel shows many interesting features: significant suppression of superfluid transition and substantial modification of the the A-like to B-like transition. Ultrasound has been found to be a sensitive tool to measure these changes. Recently, the ultrasound attenuation measurements done in the B-like phase provided strong evidence for gapless superfluidity in liquid 3He in aerogel. We conducted high frequency sound propagation measurements at 6.22 MHz in the A-like phase of superfluid 3He induced by magnetic fields up to 4.44 kG. The magnetic field is applied perpendicular to the direction of sound propagation to see the noticeable change in attenuation between the A-like and the B-like phases. We measured the sound attenuation in the A-like phase to be larger than in the B-like phase, and this observation enabled us to identify the A-like to B-like phase transition as a function of magnetic field and pressure.