|2||Speaker: Moses Chan, Penn State University|
Abstract: Torsional oscillator (TO) measurements of solid 4He confined in porous Vycor and in bulk form carried out at Penn State in 2004 found the resonant period of the TO showed an abrupt drop below 0.2K. This finding has since been confirmed in 30 experiments carried out in eleven laboratories. The drop in the resonant period was interpreted as a consequence of the onset of superfluidity or nonclassical rotational inertia (NCRI). This interpretation was buttressed by other experiments including the finding of a heat capacity peak near the onset temperature of NCRI and indirect evidence of quantum vortices. However, measurements at Alberta in 2008 and later at ENS (Paris) found the shear modulus of solid 4He, µ, shows a substantial increase when it is cooled below 0.2K with a temperature dependence that mimics the non-classical rotational inertia (NCRI) behavior. This increase in µ stiffens the entire torsion cell and can also lead to a drop in the resonant period of a TO. I will report on recent experiments specifically designed to delineate NCRI from the effect of increase in µ.