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Register a new userSuperconductivity at 2.5 K in new transition-metal chalcogenide Ta2PdSe5
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We report the synthesis and superconducting properties of a new transition-metal chalcogenide Ta$_2$PdSe$_5$. The measurements of resistivity, magnetization, and specific heat reveal that Ta$_2$PdSe$_5$ is a bulk superconductor with $T_c$ $simeq$ 2.5 K. The zero-field electronic specific heat in the superconducting state can be fitted with a two-gap BCS model. The upper critical field $H_{c2}$ shows a linear temperature dependence, and the value of $H_{c2}$(0) is much higher than the estimated Pauli limiting field $H_{c2}^{P}$ and orbital limiting field $H_{c2}^{orb}$. All these results of specific heat and upper critical field suggest that Ta$_2$PdSe$_5$ is a multi-band superconductor.
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The metal-metal bond in metal-rich chalcogenide is known to exhibit various structures and dominate interesting physical properties. Ta2Se can be obtained by both arc-melting and solid-state pellet methods. Ta2Se crystallizes a layered tetragonal structure with space group P4/nmm (S.G.129, Pearson symbol tP6). Each unit cell consists of four layers of body-centered closed packing Ta atoms sandwiched between two square nets of Se atoms, forming the Se-Ta-Ta-Ta-Ta-Se networks. A combined result of magnetic susceptibility, resistivity, and heat capacity measurements on Ta2Se indicate the bulk superconductivity with Tc = 3.8 (1) K. According to the first-principal calculations, the d orbitals in Ta atoms dominate the Fermi level in Ta2Se. The flat bands at gamma-point in the Brillouin zone (BZ) yield to the van Hove singularities in density of states (DOS) around the Fermi level, which is intensified by introducing spin-orbit coupling (SOC) effect, thus, could be critical for the superconductivity in Ta2Se. The physical properties especially superconductivity is completely different from Ta-rich alloys or transition metal dichalcogenide TaSe2.
We report theoretical and experimental results on transition metal pnictide WP. The theoretical outcomes based on tight-binding calculations and density functional theory indicate that WP exhibits the nonsymmorphic symmetries and is an anisotropic three-dimensional superconductor. This conclusion is supported by magnetoresistance experimental data as well as by the investigation of the superconducting fluctuations of the conductivity in the presence of a magnetic external field, both underlining a three dimensional behavior.
We explore the physical properties of a unified microscopic theory for the coexistence of superconductivity and charge density waves in two-dimensional transition metal dichalcogenides. In the case of particle-hole symmetry the elementary particles are Dirac fermions at the nodes of the charge density wave gap. When particle-hole symmetry is broken electron (hole) pockets are formed around the Fermi surface. The superconducting ground state emerges from the pairing of nodal quasi-particles mediated by acoustic phonons via a piezoelectric coupling. We calculate several properties in the s-wave superconducting phase, including specific heat, ultra-sound absorption, nuclear magnetic relaxation, thermal, and optical conductivities. In the case with particle-hole symmetry, the specific heat jump at the transition deviates strongly from ordinary superconductors. The nuclear magnetic response shows an anomalous anisotropy due to the broken time-reversal symmetry of the superconducting gap, induced by the triple charge density wave state. The loss of lattice inversion symmetry in the charge density wave phase leads to anomalous coherence factors in the optical conductivity and to the appearance of an absorption edge at the optical gap energy. Furthermore, optical and thermal conductivities display anomalous peaks in the infrared when particle-hole symmetry is broken.
We consider possible high temperature superconductivity (high-T$_c$) in transition metal compounds with a cubic zinc-blende lattice structure. When the electron filling configuration in the d-shell is close to d$^7$, all three t$_{2g}$ orbitals are near half filling with strong nearest neighbor antiferromagnetic (AFM) superexchange interactions. We argue that upon doping, this electronic environment can be one of genes to host unconventional high T$_c$ with a time reversal symmetry broken $d_{2z^2-x^2-y^2} pm i d_{x^2-y^2}$ pairing symmetry. With gappless nodal points along the diagonal directions, this state is a direct three dimensional analogue to the two dimensional $B_{1g}$ d-wave state in cuprates. We suggest that such a case may be realized in electron doped CoN, such as CoN$_{1-x}$O$_x$ and (H, Li)$_{1-x}$CoN.
Following a recent proposal by Burrard-Lucas et al. [unpublished, arXiv: 1203.5046] we intercalated FeSe by Li in liquid ammonia. We report on the synthesis of new LixFe2Se2(NH3)y phases as well as on their magnetic and superconducting properties. We suggest that the superconducting properties of these new hybride materials appear not to be influenced by the presence of electronically-innocent Li(NH2) salt moieties. Indeed, high onset temperatures of 44 K and shielding fractions of almost 80% were only obtained in samples containing exclusively Lix(NH3)y moieties acting simultaneously as electron donors and spacer units. The c-axis of the new intercalated phases is strongly enhanced when compared to the alkali-metal intercalated iron selenides A1-xFe2-ySe2 with A = K, Rb, Cs, Tl with T c = 32 K.
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