CeOs4Sb12, a member of the skutterudite family, has an unusual semimetallic low-temperature L-phase that inhabits a wedge-like area of the field H—temperature T phase diagram. We have conducted ...measurements of electrical transport and megahertz conductivity on CeOs4Sb12 single crystals under pressures of up to 3 GPa and in high magnetic fields of up to 41 T to investigate the influence of pressure on the different H–T phase boundaries. While the high-temperature valence transition between the metallic H-phase and the L-phase is shifted to higher T by pressures of the order of 1 GPa, we observed only a marginal suppression of the S-phase that is found below 1 K for pressures of up to 1.91 GPa. High-field quantum oscillations have been observed for pressures up to 3.0 GPa and the Fermi surface of the high-field side of the H-phase is found to show a surprising decrease in size with increasing pressure, implying a change in electronic structure rather than a mere contraction of lattice parameters. We evaluate the field-dependence of the effective masses for different pressures and also reflect on the sample dependence of some of the properties of CeOs4Sb12 which appears to be limited to the low-field region.
The London penetration depth lambda(T) was measured in single crystals of Ce1-xRxCoIn5, R=La, Nd, and Yb down to Tminapproximate50 mK (Tc/Tmin~50) using a tunnel-diode resonator. In the cleanest ...samples Delta lambda(T) is best described by the power law Delta lambda(T)is proportional toTn, 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 Tc; however, the effects on Delta lambda(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.
Filled skutterudite antimonides, are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ...ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.
Magnetic critical scaling in URu(2-x)Re(x)Si(2) single crystals continuously evolves as the ferromagnetic critical temperature is tuned towards zero via chemical substitution. As the quantum phase ...transition is approached, the critical exponents gamma and (delta-1) decrease to zero in tandem with the critical temperature and ordered moment, while the exponent beta remains constant. This novel trend distinguishes URu(2-x)Re(x)Si(2) from stoichiometric quantum critical ferromagnets and appears to reflect an underlying competition between Kondo and ferromagnetic interactions.
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).
A quantum critical point (QCP) occurs upon chemical doping of the weak itinerant ferromagnet Sc3.1In . Remarkable for a system with no local moments, the QCP is accompanied by non-Fermi liquid ...behavior, manifested in the logarithmic divergence of the specific heat both in the ferro-and the paramagnetic states, as well as linear temperature dependence of the low-temperature resistivity. With doping, critical scaling is observed close to the QCP, as the critical exponents δ , γ , and β have weak composition dependence, with δ nearly twice and β almost half of their respective mean-field values. The unusually large paramagnetic moment μPM∼1.3μB/F.U. is nearly composition independent. Evidence for strong spin fluctuations, accompanying the QCP at xc=0.035±0.005 , may be ascribed to the reduced dimensionality of Sc3.1In , associated with the nearly one-dimensional Sc-In chains.
Spin-fluctuation-mediated unconventional superconductivity can emerge at the border of magnetism, featuring a superconducting order parameter that changes sign in momentum space. Detection of such a ...sign-change is experimentally challenging, since most probes are not phase-sensitive. The observation of a spin resonance mode (SRM) from inelastic neutron scattering is often seen as strong phase-sensitive evidence for a sign-changing superconducting order parameter, by assuming the SRM is a spin-excitonic bound state. Here we show that for the heavy fermion superconductor CeCoIn5, its SRM defies expectations for a spin-excitonic bound state, and is not a manifestation of sign-changing superconductivity. Instead, the SRM in CeCoIn5 likely arises from a reduction of damping to a magnon-like mode in the superconducting state, due to its proximity to magnetic quantum criticality. Our findings emphasize the need for more stringent tests of whether SRMs are spin-excitonic, when using their presence to evidence sign-changing superconductivity.Spin mediated unconventional superconductivity exhibits a sign changing order parameter which is often inferred from the observation of a spin resonance mode. Here, the authors demonstrate that the origin of the spin resonance mode for a heavy fermion superconductor may not lie, as previously thought, in the spin-excitonic bound state and therefore its relation to the ordering parameter needs to be reconsidered.
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