Upper limit of supersolidity in solid helium

The resonant period drop observed at low temperatures in torsional oscillators containing solid helium had been interpreted as a signature of a supersolid. However, it was found that the shear modulus increase found in solid helium at the same low temperature could also decrease the resonant period of the torsional oscillator. We report the results of a study in two different torsional oscillators that were specially designed to minimize the shear modulus effect and maximize any possible supersolid response. We were able to place an upper limit on the nonclassical rotational inertia or supersolid fraction of 4×10-6. Moreover, we have repeated an earlier experiment on the hcp He3 solid, which shows similar low-temperature stiffening to that in hcp He4. We found that the small drop of the resonant period measured in the hcp He3 sample is comparable in size to that found in the hcp He4 samples. These results strongly suggest that the resonant period drop reported in most torsional oscillator studies in the last decade is primarily a consequence of the shear modulus stiffening effect.