Properties of Superfluid Three Helium in Novel Porous Media at Very Low Temperatures

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w:\awards\awards96\num.doc 9705295 Parpia This Focused Research Group grant supports a comprehensive experimental and theoretical study of the effects of nanoscale disorder on quantum ground states, including superfluid phases. The discovery that the phase diagram of mixtures of 3He and 4He is radically altered by a low density aerogel medium was rapidly followed by studies of pure 3He in aerogel. These studies reveal that the phase diagram of the superfluid phases is substantially modified, altering the stability of the A and B phases as well as creating a new normal-to-superfluid transition even at zero temperature. The role of aerogel as a collection of impurities introduced into the superfluid will be explored by altering the density and correlations of the aerogel. The PIs will also study effects of other disordered porous media on the quantum phase diagrams. Experimental tools include NMR, heat capacity and ultrasound in addition to sound and torsional oscillator measurements to characterize the nature of the superfluid. The PIs will design new aerogels specifically tailored for studies of superfluid 3He and mixtures of 3He and 4He. Very low temperatures will in many instances be critical for the exploration of the phase diagram of pure 3He. %%% This Focused Research Group grant supports a comprehensive experimental and theoretical study of the effects of atomic-scale disorder on the nature of quantum-mechanical states, including superfluid states, which are preferred by liquid helium (isotopes of mass three and four atomic units) at temperatures close to absolute zero. Superfluids flow with no resistance from the surrounding medium or container. In the case of electron superfluids, i.e., superconductors such as lead below its transition temperature, this means electrical current flow with no resistance. Superfluid states a re found in nature in helium isotopes and electron systems at low temperatures, and are believed to taken on by neutrons in neutron stars. Superfluids permit classes of electronic devices called Superconducting Quantum Interference Devices (SQUID's) which are the world's most sensitive detectors of magnetic field. SQUID's are used in advanced medical practice in commercial electrocardiographs and magneto- encephalographs, which permit, respectively, non- invasive detection of heart and mental disorders. Superfluid helium put into rotation acts like a gyroscope: it has recently been demonstrated that such a gyroscope will allow detection of rotation at an unprecedented level of sensitivity. The research carried out under the present grant explores the effects of atom scale disorder on such superfluid states, and may have beneficial effects on technology involving sensor devices as mentioned, and more broadly, even to include present and future versions of microelectronic computer technology. ***