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Superglass Dynamics Of Solid Helium
Although solid 4 He may be a supersolid, it also exhibits many phenomena unexpected in that context. In order to measure the ﬁnely resolved time-dependent inertial response of this exotic quantum material, we constructed a vibrationallyisolated millikelvin cryostat with automated data acquisition and temperature control, and used a high-dynamic range DC-SQUID-based displacement sensor to detect its rotational susceptibility. We observed ultra-slow evolution towards equilibrium of the relaxation dynamics in the resonance frequency f ( T ) and dissipation D ( T ) of the oscillator with the appearance of the ‘supersolid’ state at low shear velocity v. One possibility is that such amorphous solid 4 He represents a new form of supersolid in which glassy dynamical excitations within the solid control the superﬂuid phase stiffness, with the microscopic nature of those excitations yet to be conclusively observed. Within driven materials, high shear agitation (or velocity v) tends to inject energy into dissipative motion and reduces the amount of heat required to equivalently agitate the system, with exquisite dependence on the microscopic physics that remain unknown in 4 He . We therefore observed the effect of increased shear agitation as a concurrent decrease in the thermal activation necessary to disrupt the putative supersolid. To measure this disruption, we developed a new free-inertial-decay technique, mapping out the entire velocity-temperature “phase diagram” for rotat- ing solid 4 He, and precisely observed the connection between shear agitation and temperature as the critical contour on the surface f (v, T ). We ﬁnd that shear agitation acts indistinguishably from temperature within this material, an observation which strongly suggests that the microscopic excitations controlling the supersolid transition are in a jammed, glassy, “effective temperature” state. Furthermore, we observed power-law relaxation times in the material, which indicates the presence of a broad glassy distribution of microscopic excitations. The fundamental open question about this material is whether a true superﬂuid component - associated with the anomalies in f (v, T ) and D (v, T ) - exists within it or not. Since torsion oscillators are not able to probe the possible DC ﬂow of such a component, we have developed a new class of devices designed to directly generate and measure the ﬂow of nanoporous liquid supercurrents through solid 4 He microcrystals.
dissertation or thesis