****NON-TECHNICAL ABSTRACT**** An important theme in physics is the study macroscopic quantum phenomena, or understanding the effect quantum mechanics on a collection of atoms or molecules of macroscopic dimensions. A familiar macroscopic quantum phenomenon is superconductivity where electricity can flow in some metals and oxides with absolutely no resistance below some specific temperature called the transition temperature. A related phenomenon is superfluidity where liquid helium, when cooled below 2K or 2 degrees above absolute zero, can flow forever when it is set in motion. During the previous funding cycle a remarkable discovery was made; such a superflow also occurs in solid helium below 0.2K. While the experiments have been replicated by other groups, this supersolid behavior is not yet understood. A number of theoretical papers suggest the supersolid phenomenon is a consequence of imperfections in the helium crystal. This individual investigator project will support an effort to grow and characterize the highest quality helium crystals and to look for supersolid behavior in them. If successful this project will lead to a better understanding of the phenomenon. Graduate students involved in this research will receive rigorous training in experimental physics and will be well prepared for careers in academia and industries. They will also participate in a range of K-12 outreach activities spearheaded by the P.I..
This individual investigator award supports experimental investigations on the nature of superfluidity in solid helium, which was discovered in a series of torsional oscillator experiments during the prior funding period. This behavior has been replicated by other laboratories, however many questions concerning the phenomenon of supersolidity remain. The crystal quality of the solid samples studied to date has not been well characterized and a number of theoretical papers suggest the observed supersolid behavior is a consequence of defects, dislocations and grain boundaries in the crystals. High quality helium crystals will be grown and investigated using ultrasound resonance, x-ray, and thermodynamic (heat capacity) techniques in an effort to further our understanding of the supersolid behavior. Graduate students involved in this research will receive rigorous training in cryogenics and precision measurements techniques and thus will be well prepared for future careers in academia and industries. They will also participate in a range of K-12 outreach activities spearheaded by the P.I..
When liquid helium is cooled to 2.176K, or just two degrees Celsius above absolute zero, it enters a superfluid state and flows with no friction. That means a superfluid stirred into rotation inside a container will never slow down. Conversely, if a container filled with superfluid helium is subjected to oscillatory or torsional motion, the superfluid will de-couple from the container and remain stationary. This phenomenon is analogous to superconductivity, where pairs of electrons can flow without any resistance. Both phenomena are manifestation of quantum mechanics at the macroscopic scale. In 2004, Eunseong Kim and Moses Chan, the Principal Investigator (P.I.) of this grant found evidence of superfluidity even in solid helium. In their experiments, solid helium confined in a container was subjected to torsional oscillation (see Fig. 1). The resonant frequency of the torsion cell (known as a torsional oscillator or TO) was found to increase abruptly when the temperature was cooled below 200 mK. The increase in frequency is consistent with the idea of a fraction of the solid helium inside the TO turning superfluid and decouples from the torsion cell and the rest of the solid. This discovery of a new state of matter or the supersolid has sparked the interest of the physics community and the experiment of Kim and Chan has now been replicated in 12 laboratories around the world. The interpretation of these TO results however is complicated by the subsequent observations in Canada and France of a stiffening of the solid at the same temperature. The stiffening of the solid can also increase the resonant frequency of a TO with solid helium inside. There is also evidence that disorder in the solid such as dislocation lines plays an important role in the appearance of supersolidity. The theme of the research program of the last four years is to bring understanding to these questions. We found evidence that the onset of supersoldity and the stiffening of the solid are independent phenomena but superfluidity requires a stiffened solid. We carried out experiments on solid helium samples with high and low crystallinity and found supersolid response in both. In a collaborative experiment with colleagues at the Ecole Normale Superieure in Paris, we confirmed the finding of supersolidity in a single crystal of solid helium in an optically transparent TO. In addition, we found a heat capacity peak at the same temperature where TOs show an increase in frequency. This supports the notion that solid helium is undergoing a phase transition from the normal solid to the supersolid. The existence of a supersolid phase recently gained support from an experiment at the Korea Advanced Institute of Science and Technology (KAIST) where quantum vortices and an experiment at the University of Massachusetts where direct flow of helium through a solid helium sample were reported. However, there are still a number of outstanding and puzzling questions. For example, the supersolid fraction found in different TOs ranges from 0.1 to 1%. In TOs that are particularly rigid, the superfluid fraction was found to be as small as 0.005%. More work are being planned to understand this puzzle. It is also important to find out whether the mechanism of supersolid in bulk solid helium is different for solid embedded in a porous matrix. During the last four years, the duration of the grant, a total of 13 papers were published in refereed journals, including 1 each in Nature and Science, 1 in Nature Physics, 6 in Physical Review Letters and 2 in Physical Review B. A total of 19 invited talks were given at national and international meetings on the subject, 11 by the P.I. and 8 by students supported by the grant. In addition, the P.I. gave a total of 15 and the students gave 8 invited lectures in universities and research institutions around the world. The P.I. considers the training of young scientists to be one of the primary raisons d’etre and in the long run the most important duty and privilege of any academic scientist. This project provided the opportunity for post-doctoral, graduate and undergraduate students to receive the most rigorous training in experimental physics. Three graduate students, Anthony Clark (Ph.D. 2007, post-doc at the University of Basel), Xi Lin (Ph.D. 2008, post-doc at MIT) and Josh West (Ph.D. 2010, Engineer, High Precision Devices) completed their Ph.D. with the support of this grant. Together with the P.I., the graduate students supported by this grant participated in a number of outreach activities, including the supervision of the research projects of undergraduate students and served as docents of science museum shows for primary and middle school students designed by Chan and his colleagues of the Penn State Materials Research Science and Engineering Center (Fig. 2).
