*****NON-TECHNICAL ABSTRACT******* Condensed matter, at low temperatures, displays properties that contradict our intuition. The most peculiar of these are superconductivity and superfluidity. The former means that some metals can conduct electricity without loss of energy below a certain temperature (its critical temperature). The latter means that liquid helium below its critical temperature has zero viscosity and flows without loss of energy. For example, if a superfluid in a bucket is rotating with the bucket, when the bucket is stopped, the superfluid will continue to rotate without slowing down, as long as it is maintained at low enough temperature to remain a superfluid. Another peculiar property that has been predicted is that solid helium could also exhibit superfluidity even though it is a solid. Although predicted, it is only recently that researchers have possibly observed this property of solid helium. It is not yet certain whether the observed behavior is the anticipated "supersolid" however a new, previously unknown, state of matter has been discovered. This award will support a project with the objective identifying the nature of this new state. The researchers at the University of California, San Diego and colleagues at Johns Hopkins University will investigate this new state of matter using a wide variety of experimental techniques. Along exploring the new state of matter, this project will provide training for a graduate student, a post doc, and undergraduates.
Solid helium is a unique material in that the atoms are very weakly bound and have small mass. Due to unusually large overlap of the wave functions of the atoms, the material may exhibit unusual quantum properties. The most dramatic possibility is that the solid will exhibit at the same time both crystalline order of the atoms and a Bose-Einstein condensation and/or superfluid behavior. Recent experimental results have found evidence of a new state of matter in solid He at very low temperatures. Whether this new state is the theoretically predicted "supersolid" state is still under investigation. This award will support researchers at the University of California, San Diego and colleagues at Johns Hopkins University in their attempt to determine the nature of this new state. A variety of experiments including neutron scattering, thermodynamic measurements, and acoustic/ultrasonic measurements will be used to explore and identify this state of matter. The work will bring a graduate student, a post doc and undergraduates into the discovery of a new state of matter that will contribute importantly to our understanding of condensed matter.
The objective of this work was to measure effects of quantum physics in solid helium by using neutron scattering. The most important outcome of the work is that there are elementary excitations in the solid that are not phonons. Explanation of the fundamental significance of the results requires defining these terms and some description of the physics involved. Low temperature physics began in the early 20th century when the liquefaction of helium provided access to temperatures near absolute zero (-459? Fahrenheit). This was soon followed by the startling discoveries of superconducting metals, that have no electrical resistance, and of superfluid liquid helium which has zero viscosity. Both of these phenomena are now known to be a consequence of the quantum physics of many particle systems. Theoretical speculation that some similar quantum phenomenon might be found in solid helium began in mid 20th century. The most unusual possibility was that it could be both a solid and a superfluid at sufficiently low temperature. Although no unambiguous evidence of this peculiar state has yet been found, the possibility of finding it or some other uniquely quantum properties was the motivation for this work. Our work, funded by this grant, measured the elementary excitations of solid helium, using neutron scattering. On the macroscopic scale, excitations in ordinary solids and liquids are sound waves. On the microscopic, quantum scale, these waves are called phonons. The dependence of the energy of phonons on wavelength is an important characterization of all condensed mater and is measured directly by neutron scattering. However, other types of excitations also exist in some systems. . In the superfluid state of liquid helium there is such an excitation, called a roton. It plays a critical role in the existence of the superfluid property. Our objective was to find such excitations in solid helium and we have done so. One that was anticipated by previous work is the displacement of single atoms from their equilibrium position. At sufficiently high energies these atoms become unbound and free to move through the solid (in a manner similar to electrons in metals). A second has some properties which we can calculate from phonon theory but others that are inconsistent with the calculations. A third very weak excitation could be a phonon if the second one is not a phonon but, otherwise, we do not yet know what it is. The data is still being explored and tested against phonon theory to resolve these issues and for this reason has not yet been published in full detail. Ultimately we hope to develop a theoretical description of the excitations that will explain what they are and predict other properties that may be observed because of them. Two other types of neutron scattering measurements have also been made with interesting outcomes. One used neutrons scattered at small angles(SANS). This technique measures features in the solid that are as much as 1000 times the size of a single helium atom. It reveals imperfections in the regular, periodic array of atoms in a crystal. These can be sharp shifts in alignment of the atoms, called dislocations, or they can be boundaries between small crystallites in a highly strained crystal. Previous experiments using other techniques had found sudden changes as a function of temperature that were attributed to changes of these imperfections. Of special interest is our use of the frequently observed, but not analyzed, phenomenon of streaks of intensity in the scattering pattern along radii of the otherwise circular pattern. They appear in strained crystals due to multiple elastic reflection of neutrons from small crystallites. Our calculations of the consequences of multiple reflections show that detailed study of the streaks provide information about the number of crystallites and the angle through which they have rotated relative to each other. Our data was taken in highly strained samples as a function of time while it was cooled. They provide details of the annealing process as a function of time and temperature. In a third experiment we studied reflections of the neutrons from a single crystal in order to measure the mean displacement of the atoms from their equilibrium positions as a function of temperature. At temperatures below about 1 Kelvin, this is entirely due to the quantum uncertainty of the positions of the atoms. If there were a supersolid transition at some temperature, this would increase. We did not observe any sudden change of the zero point motion down to about 0.1 Kelvin so that if a transition to the supersolid state occurs in that temperature range it must involve fraction of the atoms that is too small to observe by this method.
