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Researchers
Introduction
In 1967 Reatto and Chester [1], working at Cornell, made the seemingly paradoxical suggestion that superfluidity might be possible in solid helium. A few years later, Leggett [2] predicted that this superfluid state would decrease the rotational inertia of an annulus containing solid helium, a phenomenon he called "Non-classical Rotational Inertia (NCRI)".
For nearly forty years thereafter, no evidence was found for such a state. Finally, at Penn State in 2004, Kim and Chan used a torsional oscillator (TO) technique to observe what appeared to be precisely the NCRI described by Leggett: the resonant frequency of the oscillator chassis + solid helium system increased suddenly below about 200 mK, interpreted as an inertial decoupling of about 1% of helium atoms from the solid [3]. They found several other striking features of this "supersolid": the state can be destroyed by the introduction of 3He impurities or by driving the oscillator chassis above a particular velocity (10 microns per second), and that the NCRI exists both in the bulk solid and when it is confined to a porous material, which makes it highly disordered.
Since that time, a great deal of effort has been dedicated to investigating the nature of the helium solid at very low temperatures (~100 mK and below). Many new measurements have deepened the mystery of the new state, observing features which were not expected for a Reatto/Chester/Leggett supersolid. Is this really a true transition to a new phase of matter? Is the NCRI evidence of superfluid behavior, or does it admit a simpler explanation in non-superfluid terms?
Our group has been using a new instrument - a TO with an ultrasensitive SQUID-based displacement sensor - to study the complicated dynamics of solid 4He. We have performed a number of experiments designed to elucidate the relationship, if any, between possible superfluidity and other dynamical excitations of solid 4He. These measurements reveal that the description of the "supersolid" state is much more complex than a simple superfluid coexisting with a crystalline solid and may constitute an observation of the dynamics of the proposed "superglass" state [4,5].
[1] L. Reatto and G. V. Chester, Phys. Rev. 155, 88 (1967)
[2] A. J. Leggett, Phys. Rev. Lett. 25, 1543 (1970)
[3] E. Kim and M. H. W. Chan, Nature 427, 225 (2004)
[4] M. Boninsegni, N. Prokof'ev and B. Svistunov, Phys. Rev. Lett. 96, 105301 (2006)
[5] G. Biroli, C. Chamon and F. Zamponi, Phys. Rev. B 78, 224306 (2008)
Instrument
Image presented here is the schematic (left) and the picture (right) of our TO with the SQUID-based displacement sensor. The operation of the experiment is the following: applying an AC voltage to the drive electrodes rotates the Stycast chassis (containing the solid 4He in a 100µm-wide annular cavity of radius 4.5mm) about the axis of the beryllium-copper (BeCu) torsion rod. The angular displacement of a samarium-cobalt (SmCo) magnet mounted on the TO generates a change in the magnetic flux through the stationary pickup coil (shown) and input coil (not shown) of a dc-SQUID circuit and thereby an output voltage proportional to displacement, which we can then read with a lock-in amplifier. We operate the experiment in a feedback loop, which allows us to drive the oscillator exactly at its resonant frequency and also to measure the change its resonant frequency as we change the temperature. The SQUID-TO is placed at around the bottom of the sub-kelvin dilution refrigerator shown at the top of this page, all of which stand on an RF noise-proof sound room with ultra-low vibration level (Schematic given in the Heavy Fermion Physics page).
Results
The figure to the right shows our measurement of the Kim & Chan signal - the increase in the resonant frequency f(T) from its high temperature value f∞ as the temperature is lowered. Accompanying the frequency increase is a peak in the dissipation D(T) of the TO-4He system, where D(T) = 1/Q(T). (Q is the quality factor). This dissipation peak is unexpected in the context of a 3D superfluid system and explanation of its existence is one of the main challenges of researchers in the field.
Collaborators
- Kyoto University
- Los Alamos National Lab