Superconductivity in the topological non-trivial Dirac semimetal PdTe 2 was recently shown to be type-I. We here report measurements of the relative magnetic penetration depth, ∆λ, on several single crystals using a high precision tunnel diode oscillator technique. The temperature variation ∆λ(T ) follows an exponential function for T /T c < 0.4, consistent with a fully-gapped superconducting state and weak or moderately coupling superconductivity. By fitting the data we extract a λ(0)-value of ∼ 500 nm. The normalized superfluid density is in good agreement with the computed curve for a type-I superconductor with nonlocal electrodynamics. Small steps are observed in ∆λ(T ), which possibly relates to a locally lower T c due to defects in the single crystalline sample.

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The transition metal dichalcogenide PdTe 2 has attractive features based on its classification as a type-II Dirac semimetal and the occurrence of type-I superconductivity, providing a platform for discussion of a topological superconductor. Our recent work revealed that type-I superconductivity persists up to pressures of ∼2.5 GPa and the superconducting transition temperature T c reaches a maximum at around 1 GPa, which is inconsistent with the theoretical prediction. To understand its nonmonotonic variation and investigate superconductivity at higher pressures, we performed structural analysis by x-ray diffraction at room temperature below 8 GPa and electrical resistivity measurements at low temperatures from 1 to 8 GPa. With regard to the superconductivity beyond 1 GPa, the monotonic decrease in T c is reproduced without any noticeable anomalies; T c changes from 1.8 K at 1 GPa to 0.82 K at 5.5 GPa with dT c /dP ∼ −0.22 K/GPa. The crystal structure with space group P 3m1 is stable in the pressure range we examined. On the other hand, the normalized pressure-strain analysis (finite strain analysis) indicates that the compressibility changes around 1 GPa, suggesting that a Lifshitz transition occurs. We here discuss the effect of pressure on the superconducting and structural properties based on the comparison of these experimental results.

The compound Sr0.5Ce0.5FBiS2 belongs to the intensively studied family of layered BiS2 superconductors. It attracts special attention because superconductivity at T sc = 2.8 K was found to coexist with local-moment ferromagnetic order with a Curie temperature T C = 7.5 K. Recently it was reported that upon replacing S by Se T C drops and ferromagnetism becomes of an itinerant nature. At the same time T sc increases and it was argued superconductivity coexists with itinerant ferromagnetism. Here we report a muon spin rotation and relaxation study (μSR) conducted to investigate the coexistence of superconductivity and ferromagnetic order in Sr0.5Ce0.5FBiS2−xSex with x = 0.5 and 1.0. By inspecting the muon asymmetry function we find that both phases do not coexist on the microscopic scale, but occupy different sample volumes. For x = 0.5 and x = 1.0 we find a ferromagnetic volume fraction of ~8 % and ~30 % at T = 0.25 K, well below T C = 3.4 K and T C = 3.3 K, respectively. For x = 1.0 (T sc = 2.9 K) the superconducting phase occupies most (~64 %) of the remaining sample volume, as shown by transverse field experiments that probe the Gaussian damping due to the vortex lattice. We conclude ferromagnetism and superconductivity are macroscopically phase separated.

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