Heavy fermion superconductivity in the filled skutterudite compound PrOs4_4Sb12_{12}

The filled skutterudite compound \PrOsSb{} has been found to exhibit superconductivity with a critical temperature $T_\mathrm{c} = 1.85$ K that develops out of a heavy Fermi liquid with an effective mass $m^{*} \approx 50 m_\mathrm{e}$. The current experimental situation regarding the heavy fermion state, the superconducting state, and a high field, low temperature phase that is apparently associated with magnetic or quadrupolar order in \PrOsSb{} is briefly reviewed herein.Comment: 4 pages, 2 figures, presented at the LT23 23rd International Conference on Low Temperature Physics, Hiroshima, Japa

The symmetry of the superconducting order parameter in PuCoGa5_5

The symmetry of the superconducting order parameter in single-crystalline PuCoGa$_5$ ($T_{\rm c} = 18.5$ K) is investigated via zero- and transverse- field muon spin relaxation ($\mu$SR) measurements, probing the possible existence of orbital and/or spin moments (time reversal-symmetry violation TRV) associated with the superconducting phase and the in-plane magnetic-field penetration depth $\lambda(T)$ in the mixed state, respectively. We find no evidence for TRV, and show that the superfluid density, or alternatively, $\Delta\lambda(T) = \lambda(T) - \lambda(0)$, are $\propto T$ for $T/T_{\rm c} \leq 0.5$. Taken together these measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.Comment: 4 pages, 5 figure

Color Transparency Effects in Electron Deuteron Interactions at Intermediate Q^2

High momentum transfer electrodisintegration of polarized and unpolarized deuterium targets, $d(e,e'p)n$ is studied. We show that the importance of final state interactions-FSI, occuring when a knocked out nucleon interacts with the other nucleon, depends strongly on the momentum of the spectator nucleon. In particular, these FSI occur when the essential contributions to the scattering amplitude arise from internucleon distances $\sim 1.5~fm$. But the absorption of the high momentum $\gamma^*$ may produce a point like configuration, which evolves with time. In this case, the final state interactions probe the point like configuration at the early stage of its evolution. The result is that significant color transparency effects, which can either enhance or suppress computed cross sections, are predicted to occur for $\sim 4 GeV^2 \ge Q^2\leq~10~(GeV/c)^2$.Comment: 37 pages LaTex, 12 uuencoded PostScript Figures as separate file, to be published in Z.Phys.

The Magnetic Phase Diagram and the Pressure and Field Dependence of the Fermi Surface in UGe2_2

The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe$_2$ are measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B} parallel to the {\it a} axis, which is the easy axis of magnetization. Two anomalies are observed at {\it B$_x$}({\it P}) and {\it B}$_m$({\it P}) ({\it B$_x$} $>$ {\it B}$_m$ at any {\it P}), and the {\it P}-{\it B} phase diagram is presented. The Fermi surface and quasiparticle mass are found to vary smoothly with pressure up to 17.7 kbar unless the phase boundary {\it B$_x$}({\it P}) is crossed. The observed dHvA frequencies may be grouped into three according to their pressure dependences, which are largely positive, nearly constant or negative. It is suggested that the quasiparticle mass moderately increases as the boundary {\it B$_x$}({\it P}) is approached. DHvA effect measurements are also performed across the boundary at 16.8 kbar.Comment: to be published in Phys. Rev.

Very Low Temperature Tunnelling Spectroscopy in the heavy fermion superconductor PrOs4_4Sb12_{12}

We present scanning tunnelling spectroscopy measurements on the heavy fermion superconductor PrOs$_4$Sb$_{12}$. Our results show that the superconducting gap opens over a large part of the Fermi surface. The deviations from isotropic BCS s-wave behavior are discussed in terms of a finite distribution of values of the superconducting gap.Comment: 4 pages, 4 figure

Mixed-parity superconductivity in centrosymmetric crystals

A weak-coupling formalism for superconducting states possessing both singlet (even parity) and triplet (odd parity) components of the order parameter in centrosymmetric crystals is developed. It is shown that the quasiparticle energy spectrum may be non-degenerate even if the triplet component is unitary. The superconducting gap of a mixed-parity state may have line nodes in the strong spin-orbit coupling limit. The pseudospin carried by the superconducting electrons is calculated, from which follows a prediction of a kink anomaly in the temperature dependence of muon spin relaxation rate. The anomaly occurs at the phase boundary between the bare triplet and mixed-parity states. The stability of mixed-parity states is discussed within Ginzburg-Landau theory. The results may have immediate application to the superconducting series Pr(Os,Ru)4Sb12.Comment: 5 pages, 2 figures. Final version accepted to PR

Quasi particle interference of heavy fermions in resonant x ray scattering

Resonant x ray scattering RXS has recently become an increasingly important tool for the study of ordering phenomena in correlated electron systems. Yet, the interpretation of RXS experiments remains theoretically challenging because of the complexity of the RXS cross section. Central to this debate is the recent proposal that impurity induced Friedel oscillations, akin to quasi particle interference signals observed with a scanning tunneling microscope STM , can lead to scattering peaks in RXS experiments. The possibility that quasi particle properties can be probed in RXSmeasurements opens up a new avenue to study the bulk band structure ofmaterials with the orbital and element selectivity provided by RXS. We test these ideas by combining RXS and STM measurements of the heavy fermion compound CeMIn5 M Co, Rh . Temperature and doping dependent RXSmeasurements at the Ce M4 edge show abroad scattering enhancement that correlateswith the appearance of heavy f electron bands in these compounds. The scattering enhancement is consistentwith themeasured quasi particle interference signal in the STMmeasurements, indicating that the quasi particle interference can be probed through the momentum distribution of RXS signals. Overall, our experiments demonstrate new opportunities for studies of correlated electronic systems using the RXS techniqu

Probing the superconducting gap symmetry of PrRu4_{4}Sb12_{12}: A comparison with PrOs4_{4}Sb12_{12}

We report measurements of the magnetic penetration depth $\lambda$ in single crystals of PrRu$_{4}$Sb$_{12}$ down to 0.1 K. Both $\lambda$ and superfluid density $\rho_{s}$ exhibit an exponential behavior for $T$ $<$ 0.5$T_{c}$, with parameters $\Delta$(0)/\textit{k}$_{B}$\textit{T}$_{c}$ = 1.9 and $\lambda(0)$ = 2900 \AA. The value of $\Delta$(0) is consistent with the specific-heat jump value of $\Delta C/\gamma T_{c}$ = 1.87 measured elsewhere, while the value of $\lambda(0)$ is consistent with the measured value of the electronic heat-capacity coefficient $\gamma$. Our data are consistent with PrRu$_{4}$Sb$_{12}$ being a moderate-coupling, fully-gapped superconductor. We suggest experiments to study how the nature of the superconducting state evolves with increasing Ru substitution for Os

Superconductivity and crystalline electric field effects in the filled skutterudite series Pr(Os1−x_{1-x}Rux_x)4_4Sb12_{12}

X-ray powder diffraction, magnetic susceptibility $\chi(T)$, and electrical resistivity $\rho(T)$ measurements were made on single crystals of the filled skutterudite series Pr(Os$_{1-x}$Ru$_x$)$_4$Sb$_{12}$. One end of the series ($x = 0$) is a heavy fermion superconductor with a superconducting critical temperature $T_{c} = 1.85$ K, while the other end ($x = 1$) is a conventional superconductor with $T_{c} \approx 1$ K. The lattice constant $a$ decreases approximately linearly with increasing Ru concentration $x$. As Ru (Os) is substituted for Os (Ru), $T_{c}$ decreases nearly linearly with substituent concentration and exhibits a minimum with a value of $T_{c} = 0.75$ K at $x = 0.6$, suggesting that the two types of superconductivity compete with one another. Crystalline electric field (CEF) effects in $\chi_\mathrm{dc}(T)$ and $\rho(T)$ due to the splitting of the Pr$^{3+}$ nine-fold degenerate Hund's rule $J = 4$ multiplet are observed throughout the series, with the splitting between the ground state and the first excited state increasing monotonically as $x$ increases. The fits to the $\chi_\mathrm{dc}(T)$ and $\rho(T)$ data are consistent with a $\Gamma_{3}$ doublet ground state for all values of x, although reasonable fits can be obtained for a $\Gamma_{1}$ ground state for $x$ values near the end member compounds ($x = 0$ or $x = 1$).Comment: 10 pages, 8 figures, submitted to Phys. Rev.

Quadrupolar effect and rattling motion in heavy fermion superconductor PrOs_4Sb_{12}

The elastic properties of a filled skutterudite PrOs_4Sb_{12} with a heavy Fermion superconductivity at T_C=1.85 K have been investigated. The elastic softening of (C_{11}-C_{12})/2 and C_{44} with lowering temperature down to T_C indicates that the quadrupolar fluctuation due to the CEF state plays a role for the Cooper paring in superconducting phase of PrOs_4Sb_{12}. A Debye-type dispersion in the elastic constants around 30 K revealed a thermally activated Gamma_{23} rattling due to the off-center Pr-atom motion obeying tau=tau_{0}exp(E/k_{B}T) with an attempt time tau_0=8.8*10^{-11} sec and an activation energy E=168 K. It is remarkable that the charge fluctuation of the off-center motion with Gamma_{23} symmetry may mix with the quadrupolar fluctuation and enhance the elastic softening of (C_{11}-C_{12})/2 just above T_C.Comment: 5 pages, 4 figures, to be published to Phys. Rev.