Crystal field potential of PrOs4Sb12: consequences for superconductivity

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12, in which the ground state in the cubic crystal field potential of T(h) symmetry is a Gamma(1) singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair breaking and increase T(c).     

Anisotropic superconductivity in PrOs4Sb12

Recently two anisotropic superconducting gap functions have been observed in the skutterudite PrOs₄Sb₁₂. These order parameters are spin-triplet. There are at least 2 distinct phases in a magnetic field, bearing some resemblance to superfluid ³He. Here we present an analysis of the thermodynamic properties in these two superconducting states within the weak-coupling BCS theory.     

Flux-line lattice distortion in PrOs4Sb12

We report that the flux-line lattice in the cubic superconductor Pr(Os4Sb12 is strongly distorted from an ideal hexagonal lattice at very low temperatures in a small applied field. We attribute this to the presence of gap nodes in the superconducting state on at least some Fermi-surface sheets.     

Superconducting PrOs4Sb12: a thermal conductivity study

The superconducting state of the heavy fermion PrOs4Sb12 is studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The field and temperature dependence of the thermal conductivity confirm multiband superconductivity and point to fully open gaps on the whole Fermi surface.     

Low-temperature specific heat of the heavy-fermion superconductor PrOs4Sb12

We report the magnetic field dependence of the specific-heat C of single crystals of the first Pr-based heavy-fermion superconductor Pr(Os4Sb12. The variation of C at low temperature and the magnetic phase diagram inferred from C, the resistivity and magnetization provide compelling evidence of a doublet ground state. Two distinct superconducting anomalies in C indicate an unconventional superconducting state, where the splitting may arise from a weak lifting of the ground state degeneracy. In combination this identifies Pr(Os4Sb12 as a strong contender for quadrupolar pairing, i.e., superconductivity that is neither electron-phonon nor magnetically mediated.     

Multiband superconductivity in the heavy fermion compound PrOs4Sb12

The thermal conductivity of the heavy fermion superconductor Pr(Os(4)Sb(12) was measured down to T(c)/40 throughout the vortex state. At lowest temperatures and for magnetic fields H approximately 0.07H(c2), already 40% of the normal state thermal conductivity is restored. This behavior (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.     

Multiple superconducting phases in new heavy fermion superconductor PrOs4Sb12

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.     

Direct observation of quadrupolar excitons in the heavy-fermion superconductor PrOs4Sb12

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.     

Muon spin relaxation and isotropic pairing in superconducting PrOs4Sb12

Transverse-field muon-spin rotation measurements in the vortex-lattice of the heavy-fermion (HF) superconductor PrOs4Sb12 yield a temperature dependence of the magnetic penetration depth lambda indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic "quadrupolar Kondo" HF mechanism in PrOs4Sb12. The T=0 relaxation rate sigma(s)(0)=0.91(1) micros(-1) yields an estimated magnetic penetration depth lambda(0)=3440(20) A, which is considerably shorter than in other HF superconductors.     

Time-reversal symmetry-breaking superconductivity in heavy-fermion PrOs4Sb12 detected by muon-spin relaxation

We report on muon-spin relaxation measurements of the 4f(2)-based heavy-fermion superconductor filled-skutterudite Pr(Os4Sb12. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.     

Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: an Sb nuclear quadrupole resonance study

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with T(c)=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate, 1/T(1), and NQR frequency unravel a low-lying crystal-electric-field splitting below T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.     

Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs4Sb12

We have observed an unexpected enhancement of the lower critical field H(c1)(T) and the critical current I(c)(T) deep in the superconducting state below T approximately 0.6 K (T/T(c) approximately 0.3) in the filled skutterudite heavy fermion superconductor PrOs(4)Sb(12). From a comparison of the behavior of H(c1)(T) with that of the heavy fermion superconductors U(1-x)Th(x)Be(13) and UPt(3), we speculate that the enhancement of H(c1)(T) and I(c)(T) in PrOs(4)Sb(12) reflects a transition into another superconducting phase that occurs below T/T(c) approximately 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs(4)Sb(12) near T/T(c) approximately 0.3 that correlate with the features we have observed in H(c1)(T) and I(c)(T).     

A theoretical study of tunnelling conductance in ferromagnet/ PrOs4Sb12 junctions

The tunnelling conductance spectra of ferromagnet/PrOs4Sb12 junctions are theoretically investigated by using the Blonder-Tinkham-Klapwijk theory. Three pairs of possible candidate for the pairing symmetry of superconducting energy gap of the recently discovered heavy-fermion unconventional superconductor PrOs4Sb12 are chosen for calculation. We have studied the spin-polarization effect on the conductance spectra, with respect to different strength of ferromagnetism of the ferromagnet and different strength of the interface barrier. Moreover, we have discussed the influence of nodal structures of the superconducting energy gap on the conductance spectra. Different features of the tunnelling conductance spectra were got, which may serve as useful theoretical comparisons for future experiments.     

Multiple superconducting phases in heavy fermion compounds PrOs4Sb12 and CeCoIn5

In recently discovered heavy fermion compounds, quasi-two-dimensional CeCoIn₅ and skutterudite PrOs₄Sb₁₂, multiple superconducting phases with different symmetries manifest themselves belowT _c. The angle-resolved magnetothermal transport measurements revealed that in PrOs₄Sb₁₂ a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state. The ultrasound velocity measurements revealed that in CeCoIn5 the Fulde-Ferrel-Larkin-Ovchinikov (FFLO) phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices, appears at low temperature and high field. These results open up a new realm for the study of the superconductivity with multiple phases.     

Probing the superconducting gap symmetry of PrOs4Sb12: a penetration depth study

We report measurements of the magnetic penetration depth lambda in single crystals of PrOs4Sb12 down to 0.1 K, with the ac field applied along the a, b, and c directions. In all three field orientations, lambda approximately T2 and superfluid density rho(s) approximately T2 for T<0.3T(c). Data are best fit by the 3He A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter Delta(0)(0)/k(B)T(c)=2.6, suggesting that PrOs4Sb12 is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the double transitions at 1.75 and 1.85 K seen in other measurements.     

Time-Reversal symmetry violation and the structure of the superconducting order parameter of PrOs4Sb12

The antisymmetric two-electron wave functions corresponding to the symmetry of Cooper pairs are constructed for the symmetry group T _h of the PrOs₄Sb₁₂ superconductor and its subgroup D _2h . The dependence of the structure of these functions (the presence of point nodes) on the group of the wave vector and the multiplicity is investigated. The theoretical nodal structure obtained for the two-electron wave functions makes it possible to explain the experimentally observed structure of the superconducting order parameter of the PrOs₄Sb₁₂ compound (the presence of two phases with different nodal structures) by the time-reversal symmetry violation.     

Magnetic phase diagram of MnAs: Effect of magnetic field on structural and magnetic transitions

The boundary, in the T?H₀ plane, separating the MnP-type and NiAs-type crystallographic phases of MnAs was determined from magnetization and volume measurements. Transitions on one part of the boundary are of first order, but are of second order on the remaining part. The two parts are separated by a tricitical point.     

CeSb phase diagram and magnetic structures

The phase diagram (H, T) of CeSb has been investigated by neutron experiments. The numerous phases correspond to square wave structures. They can be generated by a periodic stacking of ferro and/or paramagnetic (001) planes which give a quite original type of order. The phase diagram feature is explained by a simple model.     

Temperature-pressure phase diagram of CsHSO4

Abstract

On the basis of group theoretical analysis of phase transitions between the phases with known space symmetries the space symmetries of high pressure phases are theoretically predicted. A part of temperaturepressure phase diagram of CsHSO₄ in the region of high temperatures and pressures is theoretically constructed. It is shown that the new pahse of the C¹ _2h space symmetry can be observed at high pressures.