Cooper pairing in most of the known fermionic superfluids occurs via spin-1/2 quasiparticle interactions that lead to spin-singlet or spin-triplet pairing. In the topological semimetal YPtBi, strong spin-orbit coupling results in a band inversion between highly symmetric s- and p-like electronic bands and a degeneracy at the Γ point that ensures the manifold of j=3/2 quasiparticle states thrive near the Fermi level, where superconducting pairing occurs. Here we study the effects of magnetic and nonmagnetic disorder and carrier density on this exotic superconducting pairing state. By varying levels of disorder and carrier densities by nearly two and three orders of magnitude, respectively, we show that the superconducting critical temperature of YPtBi has a remarkable robustness, with little variation across this span. Our results suggest that superconductivity in YPtBi may reside in a regime where phase stiffness, rather than pair formation, governs the transition temperature. The insensitivity of Cooper pairing to dramatic changes in quasiparticle environment in a j=3/2 superconductor highlights a new form of protection realized in topological high-spin superconductors.

Daniel J. Campbell, Zachary E. Brubaker, 3 Connor Roncaioli, Prathum Saraf, Yuming Xiao, Paul Chow, Curtis Kenney-Benson, Dmitry Popov, Rena J. Zieve, Jason R. Jeffries, and Johnpierre Paglione Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742, USA Lawrence Livermore National Laboratory, Livermore, California 94550, USA Department of Physics, University of California, Davis, California 95616, USA HPCAT, X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA (Dated: July 23, 2019)

We report the results of high pressure x-ray diffraction, x-ray absorption, and electrical transport measurements of Kondo insulator Ce_3Bi_4Pt_3 up to 42 GPa, the highest pressure reached in the study of any Ce-based KI. We observe a smooth decrease in volume and movement toward intermediate Ce valence with pressure, both of which point to increased electron correlations. Despite this, temperature-dependent resistance data show the suppression of the interaction-driven ambient pressure insulating ground state. We also discuss potential ramifications of these results for the predicted topological KI state.

The effects of anneal-tuning of single-crystalline samples of CaFe_2As_2 synthesized via a molten Sn flux method are investigated using x-ray diffraction, chemical composition, electrical transport and magnetic susceptibility measurements in order to understand the role of growth conditions on the resultant phase diagram. Previous studies of CaFe_2As_2 crystals synthesized using a self-flux (FeAs) method revealed an ability to tune the structural and magnetic properties of this system by control of post-synthesis annealing conditions, resulting in an ambient pressure phase diagram that spans from tetragonal/orthorhombic antiferromagnetism to the collapsed tetragonal phase of this system. In this work, we compare previous results to those obtained on crystals synthesized via Sn flux, finding similar tunability in both self- and Sn-flux cases, but less sensitivity to annealing temperatures in the latter case, resulting in a temperature-shifted phase diagram.

We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr- doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered at energy scales about one order of magnitude larger than the Fermi energy scale in these semimetals. The phenomena noted above have been observed previously in other correlated metals in which the dominant interactions are electronic in origin. Further evidence of significant electron-electron interactions is obtained from the presence of quadratic temperature and frequency-dependent terms in the scattering rate at low temperatures and frequencies in the uncollapsed tetragonal structures of La-doped and Pr-doped CaFe2As2. For temperatures below the structure collapse transition in Pr-doped CaFe2As2 at 70 K, the scattering rate decreases due to weakening of electronic correlations, and the Drude spectral weight decreases due to modification of the low-energy electronic structure.

Single crystals of A(2)Cr(3)As(3) (A = K, Rb) were successfully grown using a self-flux method and studied via structural, transport, and thermodynamic measurement techniques. The superconducting state properties between the two species are similar, with critical temperatures of 6.1 and 4.8 K in K2Cr3As3 and Rb2Cr3As3, respectively. However, the emergence of a strong normal state electronic anisotropy in Rb2Cr3As3 suggests a unique electronic tuning parameter is coupled to the interchain spacing in the A(2)Cr(3)As(3) structure, which increases with alkali-metal ionic size while the one-dimensional [(Cr3As3)(2-)](infinity) chain structure itself remains essentially unchanged. Together with dramatic enhancements in both conductivity and magnetoresistance (MR), the appearance of a strong anisotropy in the MR of Rb2Cr3As3 is consistent with the proposed quasi-one-dimensional character of band structure and its evolution with alkali-metal species in this new family of superconductors.