College Park, Maryland      June 6 - 10 , 2004

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T2-D1 (1:30 PM): Recent progress on heavy fermion f-electron systems (Invited)

M. B. Maple (Dept. of Physics and Inst. for Pure and Applied Physical Sciences; University of California, San Diego)

Heavy fermion compounds are metallic compounds containing a sublattice of rare earth (R) or actinide (A) ions, which have electronic specific heat C_e coefficients γ = C_e/T with enormous values as large as several J/mol K², corresponding to quasiparticle masses of several hundred times the free electron mass. For most heavy fermion compounds, which are based on Ce, Yb, or U, the nonmagnetic heavy Fermi liquid ground state is believed to be associated with the Kondo effect arising from an antiferromagnetic exchange interaction between conduction electron spins and magnetic moments of the R or A ions with partially-filled f-electron shells (Kondo lattice). The nonmagnetic heavy Fermi liquid ground state is unstable with respect to the formation of other novel strongly correlated electron states, including unusual magnetically ordered states, unconventional (non s-wave) superconductivity with an energy gap that has line or point nodes on the Fermi surface, which sometimes coexists with magnetic order, and non-Fermi liquid (NFL) behavior in the normal state physical properties in the vicinity of a quantum critical point (QCP). NFL behavior and/or superconductivity have been observed under pressure in both antiferromagnetic (e.g., CeIn₃, CePd₂Si₂ ) and ferromagnetic (e.g., UGe₂) heavy fermion compounds in the vicinity of the critical pressure where the magnetic ordering temperature is suppressed to 0 K (magnetic QCP). This suggests that magnetic dipole moment fluctuations are responsible for the NFL behavior and mediate the pairing of superconducting electrons in at least some of these materials. Heavy fermion behavior has been observed in several Pr compounds with nonmagnetic Pr³⁺ ionic ground states in the crystalline electric field that appears to be due to the interaction between the charges of the conduction electrons and Pr³⁺ electric quadrupole moments. One of these heavy fermion Pr compounds, PrOs₄Sb₁₂, was found to exhibit unconventional superconductivity. In this compound, the superconducting state appears to consist of several distinct superconducting phases and to break time reversal symmetry, suggesting that the superconductivity may involve triplet spin pairing of electrons. The basic properties of heavy fermion systems with emphasis on recent developments are reviewed in this talk. This research was supported by the U. S. Department of Energy under Grant No. DE-FG02-04ER46105 and the National Science Foundation under Grant No. DMR-0335173.

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