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Register a new userAndreev reflection spectroscopy on the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$: Microscopic evidence for multiple superconducting order parameters
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Point-contact Andreev reflection spectroscopy was performed on single crystals of the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$, down to 90mK and up to 3Tesla. The conductance spectra showed multiple structures, including zero-bias peaks and spectral dips, which are interpreted as signatures of multiple pairing gaps, one clearly having nodes. Samples with 2% Ru replacing Os were also measured, showing pronounced spectral humps consistent with the emergence of a non-nodal gap. Detailed analysis of the spectral evolution revealed a magnetic field-emph{vs.}-temperature phase diagram for PrOs$_{4}$Sb$_{12}$ characterized by two distinct superconducting order parameters.
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We have investigated the de Haas-van Alphen effect in the Pr-based heavy fermion superconductor PrOs$_4$Sb$_{12}$.The topology of Fermi surface is close to the reference compound LaOs$_4$Sb$_{12}$ and well explained by the band structure calculation based on the FLAPW-LDA+U method, where the 4{it f} electrons are localized. We have confirmed a highly enhanced cyclotron effective mass 2.4$sim7.6m_{rm 0}$ which is apparently large compared to the usual Pr-based compounds.
We have measured the electrical resistivity, thermoelectric power, Hall coefficient, and magnetoresistance (MR) on single crystals of PrOs$_{4}$Sb$_{12}$, LaOs$_{4}$Sb$_{12}$ and NdOs$_{4}$Sb$_{12}$. All the transport properties in PrOs$_{4}$Sb$_{12}$ are similar to those in LaOs$_{4}$Sb$_{12}$ and NdOs$_{4}$Sb$_{12}$ at high temperatures, indicating the localized character of 4$f$-electrons. The transverse MR both in LaOs$_{4}$Sb$_{12}$ and PrOs$_{4}$Sb$_{12}$ tends to saturate for wide field directions, indicating these compounds to be uncompensated metals with no open orbit. We have determined the phase diagram of the field induced ordered phase by the MR measurement for all the principle field directions, which indicates an unambiguous evidence for the $Gamma_{rm 1}$ singlet crystalline electric field ground state.
The filled skutterudite compound PrOsSb{} exhibits superconductivity below a critical temperature $T_mathrm{c} = 1.85$ K that develops out of a nonmagnetic heavy Fermi liquid with an effective mass $m^{*} approx 50 m_mathrm{e}$, where $m_mathrm{e}$ is the free electron mass. Analysis of magnetic susceptibility, specific heat, electrical resistivity and inelastic neutron scattering measurements within the context of a cubic crystalline electric field yields a Pr$^{3+}$ energy level scheme that consists of a $Gamma_{3}$ nonmagnetic doublet ground state that carries an electric quadrupole moment, a low lying $Gamma_{5}$ triplet excited state at $sim 10$ K, and $Gamma_{4}$ triplet and $Gamma_{1}$ singlet excited states at much higher temperatures. The superconducting state appears to be unconventional and to consist of two distinct superconducting phases. An ordered phase of magnetic or quadrupolar origin occurs at high fields and low temperatures, suggesting that the superconductivity may occur in the vicinity of a magnetic or electric quadrupolar quantum critical point.
We report inelastic neutron scattering experiments performed to investigate the low energy magnetic excitations on single crystals of the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$. The observed excitation clearly softens at a wave vector Q = (1,0,0), which is the same as the modulation vector of the field-induced antiferro-quadrupolar ordering, and its intensity at Q = (1,0,0) is smaller than that around the zone center. This result directly evidences that this excitonic behavior is derived mainly from nonmagnetic quadrupolar interactions. Furthermore, the narrowing of the linewidths of the excitations below the superconducting transition temperature indicates the close connection between the superconductivity and the excitons.
Our previous point-contact Andreev reflection studies of the heavy-fermion superconductor CeCoIn$_5$ using Au tips have shown two clear features: reduced Andreev signal and asymmetric background conductance [1]. To explore their physical origins, we have extended our measurements to point-contact junctions between single crystalline heavy-fermion metals and superconducting Nb tips. Differential conductance spectra are taken on junctions with three heavy-fermion metals, CeCoIn$_5$, CeRhIn$_5$, and YbAl$_3$, each with different electron mass. In contrast with Au/CeCoIn$_5$ junctions, Andreev signal is not reduced and no dependence on effective mass is observed. A possible explanation based on a two-fluid picture for heavy fermions is proposed. [1] W. K. Park et al., Phys. Rev. B 72 052509 (2005); W. K. Park et al., Proc. SPIE-Int. Soc. Opt. Eng. 5932 59321Q (2005); W. K. Park et al., Physica C (in press) (cond-mat/0606535).
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