Resonant Ultrasound Spectroscopy and Supersolid Research

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****NON-TECHNICAL ABSTRACT****Virtually all processes, most importantly those used in energy production and distribution, involve friction or resistance, with the consequence that the processes suffer significant losses of energy. However, a major result of modern physics (specifically quantum mechanics) is the existence of substances which have absolutely no friction or resistance. Notable examples include superconductors, which can carry electrical currents with no energy loss and are candidates for future electric energy applications, and superfluids, which can flow without viscosity. A very recent discovery is the supersolid, which despite being a solid, can flow like a superfluid. How this can happen is currently a mystery, and a thorough scientific understanding of the phenomenon is necessary before any applications may be considered. As has been found for superconductors, the understanding of a "super" phenomenon may be advanced through measurements of the substance's elastic properties, or its rebound response to an applied force. This award supports a project that will incorporate a new method for measuring elastic properties of a supersolid in an attempt to increase our understanding of this material. The research will train young scientists in state-of-the-art methods of the often overlooked, but nontheless important, field of acoustics.****TECHNICAL ABSTRACT****One of the most remarkable recent discoveries in condensed matter physics has been the experimental observation of a supersolid state for 4He. It is quite an astounding phenomenon, and its physical origin remains a mystery. Most experimental studies involve a torsional oscillator, with the conventional interpretation that an increase in the oscillator's natural frequency indicates a "lost mass" which occurs when part of the helium in the cell becomes supersolid. Complete decoupling of a solid requires that the solid's shear elastic modulus and its shear viscosity vanish completely. An alternate possibility is that the frequency of the torsional oscillator increases because the 4He shear elastic modulus increases, and recent experimental measurements provide evidence for this possibility. This individual investigator award supports a project to make precise measurements of the 4He elastic constants over a range of temperatures passing through the supersolid transition. Resonant ultrasound spectroscopy, in which measurements of the cell's natural frequencies of vibration, together with a numerical (finite element) model of the cell, will be used to determine the elastic constants of the 4He. The research will train young scientists in novel methods of acoustics.