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We report a detailed calorimetric study on single crystals of the 5d-transition metal pnictide SrPt2As2 with a superconducting critical temperature Tc ∼5K. The peculiar field dependence of the electronic specific heat coefficient γ can be decomposed into two linear components. Moreover, the temperature evolution of the electronic specific heat below Tc is best described by a two-gap model. These findings suggest that two energy gaps are associated with the superconductivity. In parallel, we show that the spin-lattice relaxation time T1, through nuclear magnetic resonance measurement, obeys the so-called Korringa relation well. This, along with the T 2 dependence of resistivity at low temperatures, points to a Fermi liquid ground state in this material.A central issue in the field of superconductivity is to elucidate the origin of the pairing interaction, which in turn is intimately related to the pairing symmetry and the gap structure ∆(k). Notably, nodal d-wave superconductivity with d x 2 −y 2 pairing symmetry in cuprates is generally believed to originate from the generic spin fluctuations in CuO 2 planes [1]. While in iron-based pnictides, the role played by antiferromagnetic spin fluctuations is largely dependent on the strength of the iron 3d electron correlation and remains controversial albeit a sign-reversing s ± gap structure and multiple energy gaps have been reported [2,3]. In this regard, it is of fundamental importance to identify the gap structure in understanding the underlying mechanism for the superconducting pairing glue.Recently, motivated by the discovery of high T c superconductivity in ThCr 2 Si 2 -type pnictides AFe 2 As 2 (where A represents alkaline-earth metals)[4], a 5d-transition metal platinum-based 122 arsenide SrPt 2 As 2 was found to be superconducting below T c ∼ 5K [5][6][7]. In contrast to other 122 Fe-based superconductors, this ironfree SrPt 2 As 2 adopts a different CaBe 2 Ge 2 -type structure. Its structure can be viewed as consisting of Pt 2 As 2 tetrahedral layers alternating with As 2 Pt 2 layers stacked along the c-axis, the former layers with the Pt ion located in the center of each As tetrahedron and the lat- * Electronic address: xiaofeng.xu@hznu.edu.cn † Electronic address: ghcao@zju.edu.cn ter layer the opposite [8]. Remarkably, in analogy to Febased pnictides, the SrPt 2 As 2 compound also shows a structural phase transition at ∼470K, which is associated with charge-density-wave (CDW) formation [7]. In spite of these interesting discoveries, the nature of the low-lying quasiparticle excitations and the pairing symmetry have yet to be addressed, in particular the role of the electron-phonon interaction.In this context, we investigate the superconductivity of single crystals SrPt 2 As 2 via detailed heat capacity measurement, a bulk probe of the low-lying quasiparticle excitations. Its calorimetric responses, including the field evolution and the temperature dependence of the quasiparticle specific heat, are overall consistent with a scenario of two s-wave supercon...

By systematic chemical substitution of Pt and Ni in the newly-discovered superconductor Nb2PdS5 (Tc ∼6 K), we study the evolution of its superconducting properties with doping, focussing on the behavior of the upper critical field Hc2. In contrast to the previous results of Se doping on S sites, superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains. Most strikingly, the reduced Hc2, i.e., the ratio of Hc2/Tc, is seen to be significantly enhanced by the heavier Pt doping but suppressed in the Ni-doped counterparts, distinct from the nearly constant value in the Se doped samples. Our findings therefore suggest that the upper critical field of this system can be modified in a tunable fashion by chemical doping on the Pd chains with elements of varying mass numbers. The spin-orbit coupling on the Pd sites, by inference, should play an important role in the observed superconductivity and on the large upper critical field beyond the Pauli pair-breaking field.

We investigated the negative-chemical-pressure effect of Eu3Bi2S4−xSexF4 (0 ≤ x ≤ 2.0) by the partial substitution of S with Se. The crystalline lattice substantially expands as Se is doped, suggesting an effective negative chemical pressure. With Se/S doping, the charge-density-wave–like anomaly is suppressed, and meanwhile the superconducting transition temperature is enhanced. For , Tc reaches 3.35 K and bulk superconductivity is confirmed by the strong diamagnetic signal, with shielding volume fraction over 90%. Magnetic-susceptibility, specific-heat and Hall-effect measurements reveal that the Se/S doping increases the carrier density, corresponding to the increase of the average Eu valence. Our work provides a rare paradigm of negative-chemical-pressure effect.

Electronic nematics, an electron orientational order which breaks the underlying rotational symmetry, have been observed in iron pnictide superconductors several years after their discovery. However, the universality of the doping dependence of this phase and its relation to other symmetrybreaking orders (such as superconductivity) in distinct families of iron pnictides, remain outstanding questions. Here we use torque magnetometry as a probe to study the rotational symmetry breaking in EuFe2(As1−xPx)2 without introducing external pressure. The nematic phase is found to proliferate well above the structural transition and to persist into the superconducting regime at optimal doping, after which it becomes absent or very weak, in sharp contrast to the behaviour observed in BaFe2(As1−xPx)2. These measurements suggest a putative quantum nematic transition near optimal doping under the superconducting dome.