Absence of Supersolidity in Solid Helium in Porous Vycor Glass

Abstract

In 2004, Kim and Chan (KC) carried out torsional oscillator (TO) measurements of solid helium confined in porous Vycor glass and found an abrupt drop in the resonant period below 200 mK. The period drop was interpreted as probable experimental evidence of nonclassical rotational inertia (NCRI). This experiment sparked considerable activities in the studies of superfluidity in solid helium. More recent ultrasound and TO studies, however, found evidence that shear modulus stiffening is responsible for at least a fraction of the period drop found in bulk solid helium samples. The experimental configuration of KC makes it unavoidable to have a small amount of bulk solid inside the torsion cell containing the Vycor disc. We report here the results of a new helium in Vycor experiment with a design that is completely free from any bulk solid shear modulus stiffening effect. We found no measureable period drop that can be attributed to NCRI.

Low Frequency Elastic Measurements on Solid $$^{4}$$ 4 He in Vycor Using a Torsional Oscillator

“…The discrepancy between the apparent and expected changes in the shear modulus in [11] is even larger at higher temperatures. Equation (9) implies that the period shift exceeding 5 nsec observed during freezing and melting of the helium in the Vycor in [11] corresponds to an apparent shear modulus change of 770 MPa. The contribution of the helium to the shear modulus of the Vycor+helium system should be of order the stiffness of the helium [1], but 770 MPa is almost ten times greater than c 33 , the largest component of helium's stiffness tensor [27], at the 65 bar maximum pressure of the measurement in [11].…”

“…6) corresponds to a period shift of dp = 130 psec, but a period shift of 900 ps relative to the empty cell was observed between 30 and 900 mK in [11]. The discrepancy between the apparent and expected changes in the shear modulus in [11] is even larger at higher temperatures. Equation (9) implies that the period shift exceeding 5 nsec observed during freezing and melting of the helium in the Vycor in [11] corresponds to an apparent shear modulus change of 770 MPa.…”

“…In Kim and Chan [11], the TO frequency shifts that occurred as the helium in the Vycor froze and with solid helium in the Vycor were attributed to the stiffening of the Vycor/helium composite. However, it is not clear how such large frequency shifts could be produced by this stiffening.…”

“…However, it is not clear how such large frequency shifts could be produced by this stiffening. We can calculate the expected frequency shift of the TO in [11] using the technique from [29]: We take the thick part of the torsion rod, the Invar plate and the wings to be a perfectly rigid bob. The wings might not be perfectly rigid, but that should be a small, temperature independent perturbation to the resonant frequency.…”

“…An experiment in which a compound TO was used to study 4 He in Vycor [10] at two frequencies showed that the behavior was consistent with elastic effects but not with mass decoupling. When the original Vycor experiments [5] were recently repeated [11] with the Vycor coated by epoxy to eliminate any gaps, the TO anomaly disappeared. This strongly suggests that the anomaly originated in the bulk helium in gaps between the Vycor and walls.…”