Douglas E. MacLaughlin
Professor of Physics
Ph.D., UC, Berkeley, 1966.
Experimental solid state physics: nuclear magnetic resonance and muon spin rotation techniques. Office: 3030 Physics Building
Voice: (909) 787-5344
Fax: (909) 787-4529
E-mail: macl@citrus.ucr.edu

Magnetic Resonance in Novel Electronic Systems
D. E. MacLaughlin

My research interests center on understanding the behavior of certain exotic metals, alloys, and superconductors, in which the unifying thread is the importance of strong correlations between the conducting electrons. The so-called "heavy fermion" f-electron compounds comprise one class of strongly correlated electron materials. Electrons in the well-known high temperature superconductors are also strongly correlated. Viable explanations for much of the behavior of these materials continue to be elusive. 

Our research uses magnetic resonance, principally nuclear magnetic resonance (NMR) and the related muon spin rotation (µSR) technique, as a local probe of magnetic phenomena. Both techniques sense static and dynamic magnetic behavior on the atomic scale, and yield important information on the effect of strong electron correlations and their consequences. In the following I describe some of our magnetic resonance studies of correlated-electron physics. 

Non-Fermi liquids. There is widespread interest in a recently discovered class of heavy fermion alloys which exhibit unconventional properties at low temperatures. Thermodynamic and transport measurements in these materials indicate that the standard "Fermi liquid" description appropriate to a conventional metal does not apply. (The "liquid" in this designation refers to the conducting electrons, which are free to move through the metal.) Now Fermi liquid theory occupies a position somewhat similar to that of the Standard Model in particle physics: it is remarkably successful, so successful that it exercises considerable intellectual tyranny over our ways of thinking about metals! It is therefore a good thing that cases have been found where Fermi liquid theory manifestly breaks down. 

The origin of this unconventional "non-Fermi liquid" (NFL) behavior is highly controversial. One school of thought interprets NFL properties as arising from a novel form of band electron scattering from the f ions in the metal. An opposing view invokes quantum critical behavior at zero temperature, controlled by interactions between f-ion spins. Our NMR and µSR studies of NFL alloys suggest that a third mechanism, related to structural disorder in the alloy, can also give rise to NFL behavior, a result which has also proved to be controversial. Our work and that of several theoreticians has shown, however, that disorder-driven NFL behavior is present and in at least some cases dominant in random alloys. These exciting developments are all direct consequences of our experimental results. 

Colossal magnetoresistance. We have also begun a program of µSR studies of so-called "colossal magnetoresistance" (CMR) oxides. CMR is of considerable technological interest, as it may be applicable to data reading devices for magnetic media (computer disks) of importance in digital technology. Our most resent research combines µSR and "neutron spin echo" studies in CMR materials, and suggests that previously-studied charge and structural inhomogeneities can be grouped into two spatially separated regions, in close proximity, which possess very different dynamical magnetic properties. These appear to be intimately related to the field-dependent charge transport involved in CMR. 

Collaborations. Working arrangements and relations with the research community are quite different for practitioners of µSR compared to NMR. µSR studies must be performed at large "meson factories", which are particle accelerators capable of producing copious numbers of muons. These are maintained at laboratories such as the Paul Sherrer Institute PSI (Switzerland) and TRIUMF (Canada). Relatively large teams of collaborators are necessary to run the around-the-clock experiments which are necessary to take advantage of limited "beam time". Although NMR equipment is quite sophisticated, the experiments require only small laboratory setups. We have a state-of-the-art NMR laboratory at Riverside, and we also carry out experiments at Los Alamos, where a ³He/⁴He dilution refrigerator is available for experiments at temperatures below 1 K. Both our NMR and µSR research programs involve collaboration with researchers at universities (U.C. San Diego, U. of Florida), and government laboratories ( Los Alamos, National High Magnetic Field Laboratory, Iowa State Ames Laboratory ). We also have ongoing collaborations with research groups abroad ( Leiden University , ETH Zurich , and PSI).