Joe David Thompson

Zero-field In-115-nuclear magnetic resonance (NMR) has been used to study single crystals of CePt2In7, which is a heavy-fermion antiferromagnet with a Neel temperature (T-N) of 5.2 K at ambient pressure. The NMR spectra under zero field near the 3 nu(Q) line of the orthorhombic In(3) sites are taken at 1.6 K under hydrostatic conditions at ambient pressure and 2.4 GPa. These data reveal the coexistence of commensurate and incommensurate antiferromagnetic (AFM) orders at ambient pressure and that the commensurate ordering is stabilized by increasing pressures. The nuclear spin-lattice relaxation rates (1/T-1) for In(3) sites indicate the localized nature of f electrons far above T-N. The values of 1/T-1 in the paramagnetic state decrease by applying pressure. In contrast, the residual values of 1/T-1 much below T-N increase by pressure.

We report measurements of the specific heat, electrical resistivity, and magnetic susceptibility for CePt1-xNixSi2 from which we develop a T - x phase diagram that includes a quantum critical point near x(cr) approximate to 0.125 and accompanying non-Fermi-liquid behavior in a "v"-shaped region. This phase diagram is strikingly similar to that of CePtSi2 under applied pressure P, suggesting that CePt1-xNixSi2 provides a model system in which a T - P - x phase diagram can be smoothly generated, there by allowing a systematic study of the influence of disorder on quantum criticality.

We report specific heat (C) and magnetization (M) of single crystalline Ce4Pt12Sn25 at temperature down to approximate to 50 mK and in fields up to 3 T. C/T exhibits a sharp anomaly at 180 mK, with a large jump in a Sommerfeld coefficient gamma = C/T of Delta gamma = 30 J/molK(2)-Ce, which, together with corresponding cusp-like magnetization anomaly, indicate antiferromagnetic (AFM) ground state with Nel temperature T-N = 0.18 K. Numerical calculations based on Heisenberg model reproduce well zero field specific heat data, and point to a very small Kondo scale T-K clearly placing Ce4Pt12Sn25 in the weak exchange coupling J < J(c) limit of the Doniac diagram. Magnetic field suppresses AFM state at H* approximate to 0.7 T, much more rapidly than indicated by theoretical calculations.

We have performed low-temperature specific heat C and thermal conductivity kappa measurements on the Ni-pnictide superconductors BaNi2As2 (T-c = 0.7 K) and SrNi2P2 (T-c = 1.4 K). The temperature dependences C(T) and kappa(T) of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual kappa for BaNi2As2 rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba0.6La0.4Ni2As2 shows a mild suppression of T-c and H-c2 relative to BaNi2As2.

We report the synthesis, crystal structure, and physical properties on the new compound UIr4Al15 which crystallizes with the NdRh4Al15.4 structure type in the tetragonal space group P4(2)/nmc. From single crystal X-ray diffraction, the compound was found to have unit cell parameters a = b = 9.0239(6) angstrom and c = 15.513(2) angstrom. The compound has been synthesized from the flux growth method using an excess of Al metal. UIr4Al15 undergoes antiferromagnetic order at approximately 20 K which is consistent with kink-like features in both electrical resistivity and specific heat measurements. Electrical resistivity under hydrostatic pressure shows that the antiferromagnetic ordering temperature is slightly shifted to higher temperatures which is suggestive of the very little pressure dependence and the localized nature of the U 5f electrons. Electronic structure calculations indicated that the Fermi surface of UIr4Al15 was quasi two-dimensional.

beta-UB2C exhibits itinerant ferromagnetism below T-e = 75 K; whereas, UNiSi2 exhibits ferromagnetism of localized uranium moments below T-e = 95 K and Kondo lattice behaviour at higher temperatures. We have found that ferromagnetism in both compounds is quenched at high pressure. In beta-UB2C the Curie temperature continuously approaches zero at 2.95 GPa, where resistivity and specific heat reveal the behaviour similar to that observed in Ce-based antiferromagnets near quantum critical points. In UNiSi2 the Curie temperature decreases gradually to 25 K at 5.34 GPa. Above this pressure hysteretic phenomena appear in the temperature dependences of resistivity between 5 and 17 K, signalling a change in the magnetic order. No signature of magnetism was found above 5.5 GPa down to 1.15 K. Kondo lattice behaviour of resistivity with the enhanced residual resistivity is observed in UNiSi2 above 5.5 GPa.

We report the temperature dependence of the low-temperature specific heat down to 400 mK of the electron-doped Ba(Fe0.92Co (0.0 8))(2)As-2 superconductors. We have measured two samples extracted from the same batch: first sample has been measured just after preparation with no additional heat treatment. The sample shows T-c=20 K, residual specific heat gamma(0)=3.6 mJ/mol K-2 and a Schottky-like contribution at low temperatures. A second sample has been annealed at 800 degrees C for two weeks and shows T-c=25 K and gamma(0)=1.4 mJ/mol K-2. By subtracting the lattice specific heat, from pure BaFe2As2, the temperature dependence of the electronic specific heat has been obtained and studied. For both samples the temperature dependence of C-el(T) clearly indicate the presence of low-energy excitations in the system. Their specific heat data cannot be described by single clean s- or d-wave models and the data requires an anisotropic gap scenario which may or may not have nodes.

The effect of geometrical frustration on the development of the heavy-fermion state and quantum criticality is studied in UAuCu4, UAuPt4, UAu3Ni2 samples through measurements of their magnetic susceptibility, heat capacity, and electrical resistivity. In addition, since lattice disorder can play a large role in defining magnetic properties in frustrated systems, extended X-ray absorption fine structure (EXAFS) data have also been obtained. The local structure results show a strong correlation with the magnetic properties in these samples.

We have performed magnetic susceptibility, specific heat, resistivity, and inelastic neutron scattering measurements on a single crystal of the heavy Fermion compound Ce(Ni0.935Pd0.065)(2)Ge-2, which is believed to be close to a quantum critical point (QCP) at T = 0. At lowest temperature(1.8-3.5 K), the magnetic susceptibility behaves as chi(T) - chi(0) proportional to T-1/6 with chi(0) = 0.032 x 10(-6) m(3)/mole (0.0025 emu/mole). For T < 1 K, the specific heat can be fit to the formula Delta C/T = -gamma(0) - T-1/2 with gamma(0) of order 700 mJ/mole-K-2. The resistivity behaves as rho = rho(0) + AT(3/2) for temperatures below 2 K. This low temperature behavior for gamma(T) and rho(T) is in accord with the SCR theory of Moriya and Takimoto[1]. The inelastic neutron scattering spectra show a broad peak near 1.5 meV that appears to be independent of Q; we interpret this as Kondo scattering with T-K = 17 K. In addition, the scattering is enhanced near Q=(1/2, 1/2, 0) with maximum scattering at Delta E = 0.45 meV; we interpret this as scattering from antiferromagnetic fluctuations near the antiferromagnetic QCP.

We report on low-temperature thermal conductivity of the noncentrosymmetric superconductor LaRhSi3 (T-c = 2.3 K), which is a paramagnetic analog of the pressure-induced superconductor CeRhSi3. In the normal state (either in zero field above T-c, or in magnetic field above H-c2), the thermal conductivity kappa is mostly due to electrons, and shows a nearly T-linear dependence. In the superconducting state, kappa decreases exponentially below T-c, and crosses over to kappa proportional to T-2 behavior at T << T-c. The temperature dependence of thermal conductivity of LaRhSi3 is similar to that of a number of conventional s-wave superconductors. The field dependence of the residual linear term kappa(0)/T as a function of magnetic field suggests that LaRhSi3 has no nodes in the superconducting gap.