In the muon spin rotation (muSR) technique spin-polarized muons are used as magnetic resonance probes of electronic structure and magnetism in condensed matter. This award supports a study of magnetism and superconductivity in highly correlated electron metals and alloys, where the correlations between electrons dominate the behavior. The sensitivity of muSR to atomic-scale magnetism makes this technique an ideal tool to probe local effects of these strong electron correlations. The unique Pr-based heavy-fermion superconductor PrOs4Sb12 and Pr(Os,Ru)4Sb12 alloys will be studied to elucidate the mechanism for broken time reversal symmetry in the end compound. The interplay between magnetism and superconductivity will be investigated in (Pr,Nd)Os4Sb12 alloys. The unusual spin freezing and fluctuations recently found in the highly frustrated antiferromagnets NiGa2S4 and Pr2Ir2O7 will be studied in detail. Graduate students in this program will be well prepared for research/teaching careers in both basic and applied areas.
In simple metals the electrons that conduct electrical currents do not affect each others' behavior very much, but correlations between electrons are important and even crucial in understanding the behavior of more complex materials, the so-called strongly-correlated electron systems. Our research uses the muSR technique, in which a short-lived subatomic particle, the muon, probes its local magnetic environment on the atomic distance scale. This work will lead to better understanding of the behavior of (1) the so-called heavy-electron metals, in which electrons act as if they were hundreds or even thousands of times more massive than free electrons, and (2) "frustrated" magnets, in which competing magnetic interactions lead to new quantum phenomena. Graduate students in this program will gain valuable insight into facility-based research at the national and international level, and will be well prepared for research/teaching careers in both basic and applied condensed-matter physics and materials research.
Our research uses the muon spin rotation (µSR) technique as a local (atomic-scale) probe of magnetic phenomena. In this technique spin-polarized muons from a "meson factory" accelerator are implanted in the material of interest. The muon is a short-lived elementary particle that produces an energetic electron at the time of decay. Muons are sensitive to magnetic fields; their response to local fields can be determined by observing the direction of the decay electron at the time of decay. µSR is a sensitive probe of static (i.e., magnetically frozen) and dynamic (i.e., fluctuating due to thermal excitation) magnetic behavior, and yields important information on the consequences of strong electron correlations. Under NSF grant no. 0801407 we have carried out µSR studies of a number of phenomena in unconventional superconducting and magnetic materials. Our experimental results (1) clarify the nature of the superconducting state in the filled-skutterudite PrOs4Sb12 and its alloys, (2) reveal a number of properties of novel antiferromagnets related to geometrical frustration of the interactions between their magnetic atoms, (3) characterize an unexpected strong tendency to ferromagnetic correlation among conduction electrons in an antiferromagnet, (4) show that a so-called "non-Fermi liquid" system has no magnetic phase transition but exhibits a unique form of very slow magnetic fluctuations at low temperatures, (5) distinguish between two sources of paramagnetism in so-called "heavy-fermion" metals via studies of diluted alloys, and (6) reveal a hidden quantum critical point in an antiferromagnetic superconductor. Most of this research has been carried out in collaboration with O. O. Bernal, California State University, Los Angeles (CSULA). This collaboration, which has been active for several years, offers considerable advantages to its members, and is very efficient at data acquisition and analysis. Professor Bernal, who is at a predominantly teaching institution, continues to carry out research at a nationally and internationally visible level. Undergraduate and masters students at CSULA are directly involved in the research, and have presented papers at national conferences. We have also collaborated with P.-C. Ho, CSU Fresno, who has prepared samples and participated in µSR experiments at the TRIUMF meson facility in Vancouver, Canada. Both collaborations benefit a wide range of mostly minority students, from central Los Angeles to the San Joaquin valley, in addition to students at UC Riverside.
