****NON-TECHNICAL ABSTRACT**** This collaborative award supports a program primarily directed towards issues that stem from recent experimental observations of a supersolid state. A supersolid is a new state of matter in which part of the atoms in the solid show frictionless motion with respect to the rest of the solid, or the container that holds it. This resembles such phenomena as superconductivity and superfluidity. The precise nature of the supersolid state is currently a matter of lively debate. A combination of mechanical and x-ray experiments aims at correlating phenomena observed at the macroscopic level with the microscopic structure of the solid. This will provide critical information for a more general understanding of supersolidity. The project is a vehicle for intensive and sustained collaboration between research groups at the University of Delaware, Western Michigan University, Northern Illinois University, and Pennsylvania State University focused on experiments at the Advanced Photon Source at Argonne National Laboratory. This setting gives both the undergraduate and graduate students involved a much wider view of the physics enterprise, and their potential role in it, than is usually the case. It also provides a mechanism for the students to build up a network of contacts among peers early on and forms an excellent preparation for a scientific career.
The central goal of this collaborative project is to provide critical information necessary to interpret recent observations of a possible supersolid state in 4He. A series of combined x-ray diffraction, torsional oscillator (TO) and dielectric constant experiments will be used to investigate several questions: 1. Are there features in the x-ray diffraction directly indicative of a supersolid transition? E.g., a careful study of the Debye-Waller factor may provide limits on the Bose condensate fraction. 2. What aspects of crystal quality are important for the formation of a supersolid state? X-ray measurements will be used to determine the crystal quality, and will be combined with TO measurements in a simultaneous experiment. In addition, disorder will be introduced in a controlled way by growing the solid in porous media such as aerogels. 3. Does 4He form a commensurate solid at T = 0, or is the zero-point vacancy concentration > 0? High precision measurements of the temperature dependence of the lattice constants at constant volume, can distinguish between thermal and non-thermal vacancy populations. The training of graduate as well as undergraduate students forms an important part of this project. Its collaborative nature, as well as the fact that part of the work is done at the APS/ANL, gives students a much broader exposure to modes of research than is normally the case, and will form an excellent preparation for a scientific career.
The effect of quantum mechanics on macroscopic matter is greatly enhanced when it is cooled down to low temperature. Many ordinary metals, when cooled down to low temperature, become superconductor and allow the flow of electric current with no resistance or loss. Similarly, liquid helium-4 and liquid helium-3 becomes superfluid when they are cooled below 2.176K and 0.003 K above absolute zero. In the superfluid state these liquids can flow with no friction. In other words, when they are set in motion, they will never stop. Remarkably, Eunseong Kim and Moses Chan, the P.I. of this project reported results that suggest that even solid helium-4 can enter this superfluid state below 0.15K. This report stimulated a large number of other experiments. This project aims to uncover evidence of structural change by means of x-ray scattering studies, in solid helium when the samples are brought across the supersolid-normal solid transition. This is a collaborative project of Chan with Professor Norbert Mulders of the University of Delaware, Professor Clement Burns of Western Michigan University and Professor Laurence Lurio of Northern Illinois University. It was carried out at the synchrotron x-ray source at the Advanced Photon Source of the Argonne National Laboratory. We were given x-ray time twice every year and we brought our own dilution refrigerator to one of the beam lines to do the measurements. We did not see any structural signatures from our x-ray experiments that can be interpreted as a result of the supersolid-normal solid transition. Indeed very recently Chan found strong evidence that suggests that the period drop found in torsional oscillator experiments in multiple laboratories may very well be a consequence of a change in the shear modulus of solid helium rather than a signature of supersolidity. We did however find other interesting results in our studies. Specifically, we carried out an experiment to determine the structure of rapidly quenched solid helium-4. We developed a technique to quench an approximately 0.1 cc sample from the liquid phase at 3K to a solid at 0.3K in less than 1 second. A significant amount of diffuse scattering was observed, with a structure factor similar to the liquid. The diffuse scattering could in part originate from a proposed amorphous component, or entirely due to the large quantum mechanical zero point motion. It is quite interesting to see the effect of quantum mechanics show up in rapidly cooled solid helium. We also carried out experiment to study the crystal structure of solid helium in porous Vycor glass. It is interesting that while bulk solid helium assumes a hexagonal close packed (HCP) structure, inside the pores of Vycor of 7nm diameter, it assumes the body centered cubic (BCC) structure. At pressure above 100 atmospheres, the HCP structure appears. It will be interesting to see if computer simulation studies can reproduce this confinement effects. This project provides opportunities for graduate students to work in a national laboratory with a team of other scientists. Joshua West and Zhigang Cheng are beneficiaries in this respect. Josh is currently working as a staff physicist for High Precision Devices in Boulder, specializing in the design of cryostats. Zhigang will graduate at the end of 2012. The graduate students and the P.I. are very active in the outreach activities for K-12 students. We participate in a number of the outreach program of the Penn State Materials Research Science and Engineering Center (MRSEC). The activities include the mentoring of Research Experience for Undergraduates (REU) students and hands on shows for students on properties of materials at cryogenic temperatures.
