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Register a new userMultiple Superconducting Phases in New Heavy Fermion Superconductor PrOs4Sb12
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The superconducting gap structure of recently discovered heavy fermion superconductor PrOs4Sb12 was investigated by using thermal transport measurements in magnetic field rotated relative to the crystal axes. We demonstrate that a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state, giving a clear indication of the presence of two distinct superconducting phases with twofold and fourfold symmetries. We infer that the gap functions in both phases have a point node singularity, in contrast to the familiar line node singularity observed in almost all unconventional superconductors.
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We investigated the magnetic phase diagram of the first Pr-based heavy fermion superconductor PrOs4Sb12 by means of high-resolution dc magnetization measurements in low temperatures down to 0.06K. The temperature dependence of the magnetization M(T) at 0.1kOe exhibits two distinct anomalies at Tc1=1.83K and Tc2=1.65K, in agreement with the specific heat measurements at zero field. Increasing magnetic field H, both Tc1(H) and Tc2(H) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10kOe, the transition at Tc2(H) appears to merge into a line of the peak effect which is observed near the upper critical field Hc2 in the isothermal M(H) curves, suggesting a common origin for these two phenomena. The presence of the field-induced ordered phase (called phase A here) is confirmed for three principal directions above 40kOe, with the anisotropic A-phase transition temperature TA: TA[100] > TA[111] >TA[110]. The present results are discussed on the basis of crystalline-electrical-field level schemes with a non-magnetic ground state, with emphasis on a Gamma1 singlet as the possible ground state of Pr3+ in PrOs4Sb12.
Point-contact spectroscopy was performed on single crystals of the heavy-fermion superconductor CeCoIn_5 between 150 mK and 2.5 K. A pulsed measurement technique ensured minimal Joule heating over a wide voltage range. The spectra show Andreev-reflection characteristics with multiple structures which depend on junction impedance. Spectral analysis using the generalized Blonder-Tinkham-Klapwijk formalism for d-wave pairing revealed two coexisting order parameter components, with amplitudes Delta_1 = 0.95 +/- 0.15 meV and Delta_2 = 2.4 +/- 0.3 meV, which evolve differently with temperature. Our observations indicate a highly unconventional pairing mechanism, possibly involving multiple bands.
The thermal conductivity of the heavy-fermion superconductor CeCoIn_5 has been studied in a magnetic field rotating within the 2D planes. A clear fourfold symmetry of the thermal conductivity which is characteristic of a superconducting gap with nodes along the (+-pi,+-pi)-directions is resolved. The thermal conductivity measurement also reveals a first order transition at H_c2, indicating a Pauli limited superconducting state. These results indicate that the symmetry most likely belongs to d_{x^2-y^2}, implying that the anisotropic antiferromagnetic fluctuation is relevant to the superconductivity.
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.
Thermal conductivity and specific heat were measured in the superconducting state of the heavy fermion material Ce_{1-x}La_{x}CoIn_{5}. With increasing impurity concentration x, the suppression of T_{c} is accompanied by the increase in the residual electronic specific heat expected of a d-wave superconductor, but it occurs in parallel with a decrease in residual electronic thermal conductivity. This contrasting behavior reveals the presence of uncondensed electrons coexisting with nodal quasiparticles. An extreme multiband scenario is proposed, with a d-wave superconducting gap on the heavy-electron sheets of the Fermi surface and a negligible gap on the light, three-dimensional pockets.
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