In this study, we performed high-pressure electrical resistivity measurements of polycrystalline FeSe in the pressure range of 1-16.0 GPa at temperatures of 4-300 K. A precise evaluation of Tc from zero-resistivity temperatures revealed that Tc shows a slightly distorted dome-shaped curve, with maximum Tc (30 K) at 6 GPa, which is lower than a previously reported Tc value (∼37 K). With the application of pressure, the temperature dependence of resistivity above Tc changes dramatically to a linear dependence; a non-Fermi-liquid-like "high-Tc" phase appears above 3 GPa. We found a striking correlation between Tc and the Se height: the lower the Se height, the more enhanced is Tc. Moreover, this relation is broadly applicable to other iron pnictides, strongly indicating that hightemperature superconductivity can appear only around the optimum anion height (∼1.38Å). On the basis of these results, we suggest that the anion height should be considered as a key determining factor of Tc of iron-based superconductors containing various anions.

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High-pressure resistivity experiments were performed on FeSe 0:5 Te 0:5 . The onset temperature of T c (T onset c ) increases rapidly from 13.5 to 26.2 K upon applying pressure up to 2 GPa. Above 2 GPa, T c decreases linearly and the metallic phase was observed at P ¼ 14 GPa. The same relationship between normalized T c and pressure in both FeSe 0:5 Te 0:5 and FeSe suggests a universal pressure dependence in this system. This report presents the first observation of a phase transition from the superconducting to the metallic phase on the P-T c phase diagram.

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Neutron scattering experiments were performed on single crystals of layered cobalt-oxides La 2−x Ca x CoO 4 (LCCO) to characterize the charge and spin orders in a wide hole-doping range of 0.3x0.8. For a commensurate value of x=0.5 in (H,0,L) plane, two types of superlattice reflections concomitantly appear at low temperature; one corresponds to a checkerboard charge ordered pattern of Co 2+ /Co 3+ ions and the other is magnetic in origin. Further, the latter magnetic-superlattice peaks show two types of symmetry in the reflections, suggesting antiferromagnetic-stacking (AF-S) and ferromagnetic-stacking (F-S) patterns of spins along the c direction. From the hole-doping dependence, the in-plane correlation lengths of both charge and spin orders are found to give a maximum at x=0.5. These features are the same with those of x=0.5 in La 1−x Sr 1+x MnO 4 (LSMO), a typical checkerboard and spin ordered compound. However, in (H,H,L) plane, we found a magnetic scattering peak at Q=(1/4,1/4,1/2) position below T N . This magnetic peak can not be understood by considering the Co 2+ spin configuration, suggesting that this peak is originated from Co 3+ spin order. By analyzing these superlattice reflections, we found that they are originated from high-spin state of Co 3+ spin order.KEYWORDS: Cobalt oxide, Spin and charge order, Checkerboard charge order, Spin state of Co 3+ , Neutron diffraction IntroductionLayered transition-metal oxides have attracted much attention due to their wide variety of magnetic, electrical and structural properties 1) . In some of the doped transition metal compounds, a real space ordering exists due to the doped charge carriers in a certain carrier concentration, resulting in a charge ordering (CO) and sometimes an orbital ordering (OO). The case of half-doped manganite La 0.5 Sr 1.5 MnO 4 (LSMO) has interesting aspects for several reasons. Sternlieb et al. 2) reported the superlattice reflections corresponding to an alternating Mn 3+ /Mn 4+ checkerboard charge ordered pattern below T co =210K. Below Néel temperature, T N =110K, magnetic ordering was found to have a unit cell with dimensions 2a tet × 2a tet ×c relative to the chemical unit cell. The checkerboard order is thought to arise from orbital ordering on the Mn sites, and subsequently a zig-zag type orbital order, concomitant with CO, was directly observed in a single-layered manganite 3) by resonant x-ray scattering. This 1/25 J. Phys. Soc. Jpn. Full Paper CO-OO transition was found in many half-doped manganates. Half doped manganites with the small bandwidth and disorder which corresponds to the small and similar size of A site ions such as Pr 0.5 Ca 1.5 MnO 4 exhibit a long-range CO-OO 4) . This indicates that A-site ions are important to mediate the long-range CO-OO because of controlling the band-width and disorder by A-site ions. Recently, Zaliznyak et al. 5) investigated a checkerboard charge order of Co 2+ /Co 3+ and Co 2+ magnetic order in the half-doped cobaltate La 1.5 Sr 0.5 CoO 4 (LSCO) by elastic neutron scattering. From ...

The electronic structure of the lightly electron-doped correlated spin-orbit insulator Sr 2 IrO 4 has been studied by angle-resolved photoelectron spectroscopy. We have observed the coexistence of a lower Hubbard band and an in-gap band; the momentum dependence of the latter traces that of the band calculations without on-site Coulomb repulsion. The in-gap state remained anisotropically gapped in all observed momentum areas, forming a remnant Fermi-surface state, evolving towards the Fermi energy by carrier doping. These experimental results show a striking similarity with those observed in deeply underdoped cuprates, suggesting the common nature of the nodal liquid states observed in both compounds. DOI: 10.1103/PhysRevB.96.041106 Unconventional physics of superconductivity near the metal-insulator transition in strongly correlated Mott insulators has been one of the major themes in a variety of systems, such as cuprates, iron-based compounds, heavyelectron systems, and organic materials [1]. Recently, much attention has been given to 5d-electron systems in which the magnitude of spin-orbit coupling is comparable to the transfer-integral and Coulomb repulsion energies, and this interplay may produce possible novel phases. Sr 2 IrO 4 is a good example of such a system for which the electronic states can be well described by considering spin-orbit coupling as well as Coulomb repulsion energy U [2,3].Sr 2 IrO 4 is an antiferromagnetic insulator with T N = 240 K, and is isostructural to one of the parent compounds of cuprate superconductors, namely, La 2 CuO 4 [4]. Similar to the cuprates, the electronic structure is highly two dimensional, as revealed by angle-resolved photoelectron spectroscopy (ARPES) [5,6]. Unlike cuprates, to date, Sr 2 IrO 4 has not shown superconductivity, although a possible emergence of superconductivity in this system has been theoretically predicted by carrier doping [7][8][9][10]. On the other hand, a d-wave gapped state and Fermi arc behavior have been observed in both the bulk [11] and surface [12][13][14] electronic structures of doped Sr 2 IrO 4 , similar to the cuprates. Such a similarity is puzzling and raises several questions, but this is merely due to the lack of momentum-resolved data in a wide range of doping, especially in the deeply underdoped regime. This is indeed crucial to explore if this anisotropic gap has the same origin as the pseudogap in cuprate superconductors and if the gap is related to superconductivity.To address these unsettled issues, we have studied how this d-wave gapped state evolves by doping in lightly doped Sr 2−x La x IrO 4 (x = 0, 0.04, 0.08) using ARPES. We have observed a dispersive in-gap state that evolves by carrier doping and coexists with the lower Hubbard band (LHB) seen (Fig. S1).ARPES experiments were performed at the 1-squared beamline of BESSY II, using a Scienta-Omicron R8000 analyzer. Circularly polarized light with hν = 100 eV was used to excite the photoelectrons. Clean surfaces for measurements were obtained by in situ cleaving ...

Understanding the overall features of magnetic excitation is essential for clarifying the mechanism of Cooper pair formation in iron-based superconductors. In particular, clarifying the relationship between magnetism and superconductivity is a central challenge because magnetism may play a key role in their exotic superconductivity. BaFe2As2 is one of ideal systems for such investigation because its superconductivity can be induced in several ways, allowing a comparative examination. Here we report a study on the spin fluctuations of the hole-overdoped iron-based superconductors Ba1-xKxFe2As2 (x = 0.5 and 1.0; Tc = 36 K and 3.4 K, respectively) over the entire Brillouin zone using inelastic neutron scattering. We find that their spin spectra consist of spin wave and chimney-like dispersions. The chimney-like dispersion can be attributed to the itinerant character of magnetism. The band width of the spin wave-like dispersion is almost constant from the non-doped to optimum-doped region, which is followed by a large reduction in the overdoped region. This suggests that the superconductivity is suppressed by the reduction of magnetic exchange couplings, indicating a strong relationship between magnetism and superconductivity in iron-based superconductors.