Dr. Ethan Kassner, Dr. Anna Eyal, Azar Eyvazov


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].

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?

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)


Image presented here is the schematic (left) and the picture (right) of our TO with the SQUID-based displacement sensor. 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).

Recent Research Achievements

Interplay of Rotational, Relaxational, and Shear Dynamics in Solid 4He

We used the extremely high sensitivity of our SQUID-based torsional oscillator to introduce a new FID mapping technique [6] that reveals - as a function of both temperature and velocity - the slowing of mobile defects within solid 4He. These FID maps are shown in the left figure. We found that injecting some heat into the sample produced exactly the same change to its inertia as did a corresponding increase in its applied angular acceleration. In other words, the microscopic excitations that control the torsional oscillator response are generated equally well by either thermal or mechanical stimulation. Such an equivalence is not typically expected for a superfluid transition.

Furthermore, we found that the relaxation times of microscopic defect motions diverge smoothly with no sharp changes at a "critical temperature" or "critical velocity" where the solid 4He has been proposed to undergo a phase transition to a supersolid - see the left figure. In fact, these results are more consistent with a gradual reduction in the mobility of crystal defects, a situation that should also cause characteristic changes in the shear modulus of the material [7].

We then compared the FID maps from our torsional oscillator observations to the data from solid 4He shearing experiments performed by John Beamish and colleagues at the University of Alberta [8] and found that the shear response to different strain amplitudes was quantitatively similar to that of the torsional oscillator response to different rim velocities. This demonstrates that the torsional oscillator responses previously attributed to supersolidity are actually due to generation of the same microscopic excitations as those produced by directly shearing the solid. The next steps are to identify the type of microscopic crystal defect in solid 4He whose motion is so altered by shearing, and to resolve the possible existence of an additional superfluid component controlled by these defects - a "superglass" state.

For more information, see:

published article - download PDF
(Science 332, 821 - May 2011)

See also -
Nature NewsBlog by E. S. Reich

[6] FID: Free-Inertial Decay
[7] J. Day, J. Beamish, Nature 450, 853-856 (2007)
[8] J. Day, O. Syshchenko, J. Beamish, Phys. Rev. Lett. 104, 075302(2010)

Evidence for a Superglass State in Solid 4He

In the figure at right, we introduced the time-dependent Davidson-Cole plot technique in order to study ultra-slow relaxation dynamics of solid 4He. The helium dissipation is plotted against the frequency shift of our SQUID-TO at various temperatures below 300 mK. The resultant surface reveals the delicate origin of glassy defect motions within samples in the proposed phase of supersolidity. If superfluidity is the correct interpretation of blocked-annulus experiments for these materials, then the dynamics illustrated here are evidence of a "superglass" phase.

For more information, see:

published article - download PDF
(Science 324, 632 - May 2009)

See also -
Perspectives by John Saunders
(Science 324, 601)
News by Jon Cartwright
(Physics World, Apr 30 2009)
Commentary by Tony Leggett
(Journal Club for Conensed matter Physics)
Article on Physics Today (8 Jun 2009)


Dr. Ben Hunt - MIT
Dr. Ethan Pratt - NIST @ Boulder
Dr. Minoru Yamashita - Kyoto University
Dr. Matthias Graf - Los Alamos National Lab
Dr. Alexander Balatsky - Los Alamos National Lab