The f electrons in rare earth and uranium intermetallic compounds usually possess magnetic and/or orbital degrees of freedom, and they often play important roles in low-temperature magnetic properties through the couplings between the f electrons and their surroundings. In particular, an exchange interaction between the f and conduction electrons gives rise to many-body effects on both single-ionic site (known as the Kondo effect) and inter-ionic sites (the Ruderman-Kittel-Kasuya-Yosida interaction: RKKY interaction). Although the former favors a paramagnetic ground state, the latter stabilizes a long-range magnetic order. They may compete with each other, yielding an unusual many-body state of strongly correlated electrons. This electronic state, the so-called "heavy-fermion state", is recognized to be evolution of Landau's Fermi-liquid quasiparticles with extremely enhanced effective mass. Thus far, various new phenomena related to the heavy-fermion state have been discovered, including multipole order, unconventional superconductivity, and quantum critical behavior.
To investigate these phenomena, not only macroscopic measurements but also microscopic measurements, such as NMR, neutron scattering, muon spin relaxation and x-ray diffraction, are indispensable. Our group has been investigating the nature of quantum critical phenomena such as unconventional superconductivity, quantum spin fluctuation and non-Fermi-liquid behavior by using both macroscopic and microscopic probes.
