I. K. Lum

Nodal to nodeless superconducting energy-gap structure change concomitant with fermi-surface reconstruction in the heavy-fermion compound CeCoIn(5).

The London penetration depth λ(T) was measured in single crystals of Ce_{1-x}R_{x}CoIn_{5}, R=La, Nd, and Yb down to T_{min}≈50  mK (T_{c}/T_{min}∼50) using a tunnel-diode resonator. In the cleanest samples Δλ(T) is best described by the power law Δλ(T)∝T^{n}, with n∼1, consistent with the existence of line nodes in the superconducting gap. Substitutions of Ce with La, Nd, and Yb lead to similar monotonic suppressions of T_{c}; however, the effects on Δλ(T) differ. While La and Nd substitution leads to an increase in the exponent n and saturation at n∼2, as expected for a dirty nodal superconductor, Yb substitution leads to n>3, suggesting a change from nodal to nodeless superconductivity. This superconducting gap structure change happens in the same doping range where changes of the Fermi-surface topology were reported, implying that the nodal structure and Fermi-surface topology are closely linked.

Probing the superconductivity of PrPt4Ge12 through Ce substitution

We report measurements of electrical resistivity, magnetic susceptibility, specific heat, and thermoelectric power on the system Pr1-xCexPt4Ge12. Superconductivity is suppressed with increasing Ce concentration up to x = 0.5, above which there is no evidence for superconductivity down to 1.1 K. The Sommerfeld coefficient {\gamma} increases with increasing x from 48 mJ/mol K^2 up to 120 mJ/mol K^2 at x = 0.5, indicating an increase in strength of electronic correlations. The temperature dependence of the specific heat at low temperatures evolves from roughly T^3 for x = 0 to e^(-\Delta /T) behavior for x = 0.05 and above, suggesting a crossover from a nodal to a nodeless superconducting energy gap or a transition from multiband to single-band superconductivity. Fermi-liquid behavior is observed throughout the series in low-temperature magnetization, specific heat, and electrical resistivity measurements.

Robust upward dispersion of the neutron spin resonance in the heavy fermion superconductor Ce1−xYbxCoIn5

The neutron spin resonance is a collective magnetic excitation that appears in the unconventional copper oxide, iron pnictide and heavy fermion superconductors. Although the resonance is commonly associated with a spin-exciton due to the d(s±)-wave symmetry of the superconducting order parameter, it has also been proposed to be a magnon-like excitation appearing in the superconducting state. Here we use inelastic neutron scattering to demonstrate that the resonance in the heavy fermion superconductor Ce1−xYbxCoIn5 with x=0, 0.05 and 0.3 has a ring-like upward dispersion that is robust against Yb-doping. By comparing our experimental data with a random phase approximation calculation using the electronic structure and the momentum dependence of the -wave superconducting gap determined from scanning tunnelling microscopy (STM) for CeCoIn5, we conclude that the robust upward-dispersing resonance mode in Ce1−xYbxCoIn5 is inconsistent with the downward dispersion predicted within the spin-exciton scenario.

Universal heat conduction in Ce1-xYbxCoIn5: Evidence for robust nodal d-wave superconducting gap

In the heavy-fermion superconductor Ce1-xYbxCoIn5, Yb doping was reported to cause a possible change from nodal d-wave superconductivity to a fully gapped d-wave molecular superfluid of composite pairs near x ≈ 0.07 (nominal value xnom = 0.2). Here we present systematic thermal conductivity measurements on Ce1-xYbxCoIn5 (x = 0.013, 0.084, and 0.163) single crystals. The observed finite residual linear term κ0/T is insensitive to Yb doping, verifying the universal heat conduction of the nodal d-wave superconducting gap in Ce1-xYbxCoIn5. Similar universal heat conduction is also observed in the CeCo(In1–yCdy)5 system. Furthermore, these results reveal a robust nodal d-wave gap in CeCoIn5 upon Yb or Cd doping.

The non-centrosymmetric heavy fermion ferromagnet Sm2Fe12P7

We report measurements of the electrical resistivity, magnetization and specific heat on single crystals of the non-centrosymmetric compound Sm2Fe12P7. The magnetization measurements demonstrate that Sm2Fe12P7 exhibits ferromagnetic order below TM, 1 = 6.3 K. The ratio of the effective magnetic moment obtained from a Curie-Weiss fit to the magnetic susceptibility in the paramagnetic state, to the saturation magnetic moment in the ordered state indicates that the ordered state is associated with itinerant electrons. The specific heat measurements reveal an enhanced value for the coefficient of the electronic specific heat γ ∼ 450 mJ mol (-1) K (-2) that is accompanied by a large coefficient A of the T(2) term in the electrical resistivity at low temperatures, suggesting a heavy fermion ground state. Several consecutive magnetic phase transitions indicative of competing magnetic energy scales and the observation of a metamagnetic transition in the magnetization data additionally suggest proximity to a quantum critical point.

Magnetic, thermal, and transport properties of the actinide based noncentrosymmetric compounds Th2Fe12P7 and U2Fe12P7

Magnetization, specific heat, and electrical resistivity measurements on single crystals of the noncentrosymmetric actinide based compounds U2Fe12P7 and Th2Fe12P7 are reported. The measurements reveal that U2Fe12P7 displays antiferromagnetic order at a Néel temperature T(N) ≈ 14 K, while Th2Fe12P7 is a metal which exhibits Pauli paramagnetism with no evidence for superconductivity for T ≥ 1.1 K. Magnetization measurements on U2Fe12P7 show complicated magnetic behavior involving the U and, possibly, Fe ions, as well; e.g., hysteretic temperature and field dependences and metamagnetism. Electrical resistivity measurements on U2Fe12P7 also indicate large spin disorder scattering of conduction electrons for T ≥ T(N).

From local moment to mixed-valence regime inCe1−xYbxCoIn5alloys

We investigated the onset of the many-body coherence in the f-orbital single crystalline alloys Ce(1-x)Yb(x)CoIn5 through thermodynamic and magneto-transport measurements. Our study shows the evolution of the many-body electronic state as the Kondo lattice of Ce moments is transformed into an array of Ce impurities. Specifically, we observe a smooth crossover from the predominantly localized Ce moment regime to the predominantly itinerant Yb f-electronic states regime for about 50% of Yb doping. Our analysis of the residual resistivity data unveils the presence of correlations between Yb ions, while from our analysis of specific heat data we conclude that for 0.65<x<0.775, ytterbium f-electrons strongly interact with the conduction electrons while the Ce moments remain completely decoupled. The sub-linear temperature dependence of resistivity across the whole range of Yb concentrations suggest the presence of a nontrivial scattering mechanism for the conduction electrons.

Quantum criticality and superconducting pairing in Ce1-xYbxCoIn5 alloys

Charge transport measurements under magnetic field and pressure on Ce1-xYbxCoIn5 single crystalline alloys revealed that: (i) relatively small Yb substitution suppresses the field induced quantum critical point, with a complete suppression for Yb doping x > 0.07; (ii) the superconducting transition temperature (Tc) and Kondo lattice coherence temperature (Tcoh) decrease with x, yet they remain finite over the wide range of Yb concentrations; (iii) both Tc and Tcoh increase with pressure; (iv) there are two contributions to resistivity, which show different temperature and pressure dependences, implying that both heavy and light quasiparticles contribute to inelastic scattering. We also analyzed the pressure dependence of both Tcoh and Tc within the composite pairing theory. In the purely static limit, we find that the composite pairing mechanism necessarily causes opposite behaviors of Tcoh and Tc with pressure: if Tcoh grows with pressure, Tc must decrease with pressure and vice versa.

Thermoelectric Properties of Correlated Electron Systems Ln3Pt4Ge6and LnPt4Ge12(Ln = Ce, Pr) and Non-centrosymmetric X2T12P7(X = Yb, Hf and T = Fe, Co)

We report measurements of the thermoelectric power Sand electrical resistivity ρ for correlated electron systems Ln 3Pt4Ge6and LnPt4Ge12(Ln= Ce, Pr) and X 2 T 12P7(X= Yb, Hf and T= Fe, Co). The thermoelectric power factor S 2∕ ρ is utilized as a means to assess the potential viability of these materials for thermoelectric applications. Sis observed to be sensitive to Lnin Ln 3Pt4Ge6and LnPt4Ge12with Ce-based compounds providing a much larger Sand S 2∕ ρ than Pr-based materials. The character of Sfor the Ce-based compounds is consistent with an intermediate Ce valence. In the case of X 2 T 12P7compounds reported herein, it appears that it is possible to tune the magnitude of Smore effectively by varying Trather than X; the magnitude of Sis significantly larger with T= Fe than when T= Co.