A huge enhancement of the superconducting transition temperature Tc was observed in a tetragonal FeSe superconductor under high pressure. The onset temperature was as high as 27K at 1.48GPa and the pressure coefficient showed an extremely high value of 9.1K∕GPa. The upper critical field Hc2 was estimated to be ∼72T at 1.48GPa. Because of the high Hc2, the FeSe system can be applied to superconducting wire rods.

$\alpha \text{-FeSe}$ with the PbO structure is a key member of the family of high-$T_c$ iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted ${\text{FeSe}}_4$ tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that the application of hydrostatic pressure first rapidly increases $T_c$ which attains a broad maximum of 37 K at $\sim 7\text{GPa}$ before decreasing to 6 K upon further compression to $\sim 14\text{GPa}$. Complementary synchrotron x-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of $\alpha \text{-FeSe}$, revealing an extremely soft solid with a bulk modulus, $K_0=30.7\left(1.1\right)\text{GPa}$ and strong bonding anisotropy between interlayer and intralayer directions that transforms to the more densely packed $\beta$ polymorph above $\sim 9\text{GPa}$. The nonmonotonic $T_c\left(P\right)$ behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties.

Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.Comment: 23 pages, 5 figures, 1 table (table caption has been revised), to appear in J. Phys. Soc. Jp

Exotic superconductivity has often been discovered in materials with a layered (two-dimensional) crystal structure. The low dimensionality can affect the electronic structure and can realize high transition temperatures (Tc) and/or unconventional superconductivity mechanisms. We show superconductivity in a new bismuth-oxysulfide compound Bi4O4S3. Crystal structure analysis indicates that this superconductor has a layered structure composed of a stacking of spacer layers and BiS2 layers. Band calculation suggests that the Fermi level for Bi4O4S3 is just on the peak position of the partial density of states of the Bi 6p orbital within the BiS2 layer. The BiS2 layer will be a basic structure which provides another universality class for a layered superconducting family, and this opens up a new field in the physics and chemistry of low-dimensional superconductors.

We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature Tc was observed with the substitution of S or Te for Se; the Tc increased with S substitution by up to 20 %, and also increased with Te substitution up to 75 %. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine Tc in the FeSe system.Comment: 17 pages, 15 figures, 1 tabl