FeSe with the PbO structure is a key member of the family of new high-Tc iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe4 tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases Tc which attains a broad maximum of 37 K at ∼7 GPa (this is one of the highest Tc ever reported for a binary solid) before decreasing to 6 K upon further compression to ∼14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of α-FeSe, revealing an extremely soft solid with a bulk modulus, K0 = 30.7(1.1) GPa and strong bonding anisotropy between inter-and intra-layer directions that transforms to the more densely packed β-polymorph above ∼9 GPa. The non-monotonic Tc(P ) 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. PACS numbers: 74.70.Dd, 74.25.Ha, 61.05.C-The α-polymorph of the simple binary FeSe phase has recently emerged as a superconductor with an ambient P T c of ∼8-13 K. 1,2 Its structure comprises stacks of edgesharing FeSe 4 tetrahedra with a packing motif essentially identical to that of the FeAs layers in the families of the FeAs-based high-T c superconductors 3,4,5,6,7 but lacking any interleaved ion spacers or insulating layers. The structural analogy is reinforced by the observation that below 70 K the high-temperature crystal structure becomes metrically orthorhombic (space group Cmma), 8 displaying an identical distortion of the FeSe layers to that observed in the iron oxyarsenide family. 9,10 Theoretical calculations also find a very similar 2D electronic structure to that of the FeAs-based superconductors with cylindrical electron sections at the zone corner and cylindrical hole surface sections. 11 Moreover, superconductivity in FeSe is very sensitive to defects and disorder and occurs over a limited range of FeSe 1−δ nonstoichiometry. 12The effect of applied pressure on T c provides crucial information in differentiating between competing models of superconductivity and in the FeSe binary, T c is initially extremely sensitive to P and rises rapidly to 27 K at 1.48 GPa. 2 At the same time, antiferromagnetic spin fluctuations present above T c are strongly enhanced by pressure. 13 In the FeAs-based superconductors, the response of T c to pressurization is complex and sensitively depends on the composition of the materials and their doping level. Both positive and negative initial pressure coefficients, dT c /dP have been measured. Typically for the REFeAsO 1−x F x families, dT c /dP is positive at low doping levels and switches over to a negative value as x increases. 14,15,16,17,18 Moreover, for systems where the initial dT c /dP is positive, there is a critical value of P above which the trend is reversed and T c then decreases...

A huge enhancement of the superconducting transition temperature T c was observed in a tetragonal FeSe superconductor under high pressure. The onset temperature was as high as 27 K at 1.48 GPa and the pressure coefficient showed an extremely high value of 9.1 K/GPa. The upper critical field H c2 was estimated to be ~ 72 T at 1.48 GPa. Because of the high H c2 , the FeSe system can be applied to superconducting wire rods.Since the great discovery of cuprate high-T c superconductors, over 20 years have passed. Recently many researchers have made much effort focused on non-cuprate, new superconductors such as MgB 2 with a T c of 39 K. 1,2,3

Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-T c (high transition temperature) cuprates and the Febased superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO 2 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 BiS 2 -based layered compound Bi 4 O 4 S 3 . It was found that superconductivity of Bi 4 O 4 S 3 originates from the BiS 2 layers. The crystal structure is composed of a stacking of BiS 2 superconducting layers and the spacer layers, which resembles those of high-T c cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS 2 -based layered superconductor LaO 1Àx F x BiS 2 , with a T c as high as 10.6 K.

We have established a plot of the anion height dependence of superconducting transition temperature T c for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38 Å. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and search for the new Fe-based superconductors with higher T c .2