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Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for

We report inelastic neutron scattering measurements of the phonon density of states in Mg 11B2, which has a superconducting transition at 39.2 K. The acoustic phonons extend in energy to 36 meV, and there are highly dispersive optic branches peaking at 54, 78, 89, and 97 meV. A simple Born-von Kàrmàn model reproduces the mode energies, and provides an estimate of the electron-phonon coupling of lambda approximately 0.9. Furthermore, the estimated boron and magnesium contributions to the isotope effect are in qualitative agreement with experiment. The data confirm that a conventional phonon mechanism, with moderately strong electron-phonon coupling, can explain the observed superconductivity.

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12, in which the ground state in the cubic crystal field potential of T h symmetry is a Γ1 singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair-breaking and increase Tc.PACS numbers: 74.70. Tx,71.27.+a,75.10.Dg,78.70.Nx Praseodymium filled skutterudite compounds, with general formula PrT 4 M 12 , where T is one of the transition metals Fe, Ru, or Os, and M is a pnictogen (P, As, or Sb), show a remarkable variety of interesting physical phenomena, including metal-insulator transitions [1], quadrupolar heavy fermion behavior [2,3], and superconductivity [4]. In particular, PrOs 4 Sb 12 has attracted attention as a heavy fermion superconductor, in which quadrupolar fluctuations may play an important role in the pairing mechanism [5]. This proposal is based on two key observations; (i) there is a significant f-electroninduced mass enhancement of the conduction electrons, observed in specific heat, upper critical field [5,6], and de Haas-van Alphen measurements [7], and (ii) the magnetic susceptibility indicates that the crystal field ground state is non-magnetic [5].A knowledge of the crystal field ground state is essential to understanding the role of the f-electrons in the superconductivity. In analyzing the magnetic susceptibility and specific heat, Bauer et al considered two possible crystal field models [5]. In cubic symmetry, the Pr 3+ ion splits into a singlet (Γ 1 ), a non-magnetic doublet (Γ 3 ), and two magnetic triplets (Γ 4 and Γ 5 ). Crystal field models with either the Γ 1 singlet or Γ 3 doublet as ground state were both broadly consistent with the data; in both cases, the Γ 5 triplet was the lowest excited level estimated to be at less than 1 meV in energy. A Γ 3 non-Kramers doublet ground state is of particular interest as it provides the necessary conditions for quadrupolar Kondo fluctuations to be responsible for the heavy fermion behavior [8], and was favored by analyses of the entropy [9,10]. However, the alternative Γ 1 singlet ground state has also been proposed following experiments that have explored the crossover to a field-induced ordered phase [11,12,13], so this important question remains unresolved.Inelastic neutron scattering is the most direct method of determining the crystal field potential and level scheme of metallic rare earth systems. In this report, we present the results of a comprehensive set of measurements of crystal field transitions in PrOs 4 Sb 12 as a function of temperature. From a simultaneous profile refinement of all the spectra, normalized on an absolute intensity scale, we have concluded that the Γ 1 singlet is the ground state level. Discr...

Time-of-flight spectroscopy using neutrons produced by a spallation source is used to measure the one-magnon scattering throughout the Brillouin zone for La2Cu04. The zone-boundary magnons have an energy ^WZB~0.3 12 ±0.005 eV and are good eigenstates of the quantum Heisenberg Hamiltonian in that they possess lifetimes > \Q/(o. A multiplicative renormalization of the overall frequency scale of classical spin-wave theory accounts for the quantum effects in the one-magnon spectrum.PACS numbers: 75.30.Ds, 74.70.Vy, 75.10.Jm, 75.50.Ee The unusual features of Heisenberg antiferromagnets when quantum mechanics is taken into account have been investigated for much of this century. For the onedimensional case, quantum fluctuations destabilize the Neel state entirely. Even in higher dimensions, quantum fluctuations lead to significant corrections to the groundstate energy [1,2] and the spin-w^ave excitations [3] which are strongest for low spin. The discovery of hightemperature superconductivity has focused attention on La2Cu04, apparently an excellent realization of a twodimensional (2D) spin-y quantum antiferromagnet [4]. The magnetic correlations in La2Cu04 have been investigated by several microscopic techniques, most notably light scattering [5], muon spin relaxation [6][7][8], and neutron scattering [9][10][11][12][13]. Unfortunately, because of experimental limitations and restrictions in momentum and energy transfer, no technique has been able to probe the excitations over the whole Brillouin zone. We describe here a neutron-scattering investigation performed on a spallation source, in which we were able to overcome previous difficulties and observe magnetic excitations over a wide range of wave vector and energy. Well-defined zoneboundary spin waves with an energy of /ift)zB=0.312 ±0.005 eV are found. Also, the dispersion relation shows the conventional spin-wave form.Following previous practice, we use the orthorhombic nomenclature to label reciprocal space so that the basal planes are parallel to the (AO/) zone. Even though a^c, in this experiment we do not distinguish between them, for convenience, we define a* and c* to lie in the horizontal and vertical planes of the instrument, respectively. The sample was an assembly of sixteen single crystals [14] aligned such that their in-plane axes coincided to better than 1.5°. The total mass of the crystals was 0.1 kg. As described elsewhere [12], crystals were grown from CuO-rich melts contained in a large Pt crucible. The room-temperature lattice parameters were a=5.375(2) A, * = 13.156(4) A, and c=5.409(2) A. The Neel temperatures of the individual crystals ranged between 260 and 290 K.Experiments were performed on the High-Energy Transfer Spectrometer (HET) [15] at the United Kingdom spallation neutron source ISIS of the Rutherford Appleton Laboratory. HET is a "direct geometry chopper" spectrometer. High-energy (spallation) neutrons are produced when an 800-MeV pulsed proton beam with a current of 100 juA hits a uranium target. A beam of monochromatic neutrons...