One of the main challenges in understanding high T C superconductivity is to disentangle the rich variety of states of matter that may coexist, cooperate, or compete with d-wave superconductivity. At center stage is the pseudogap phase, which occupies a large portion of the cuprate phase diagram surrounding the superconducting dome 1 . Using scanning tunneling microscopy, we find that a static, non-dispersive, "checkerboard"-like electronic modulation exists in a broad regime of the cuprate phase diagram and exhibits strong doping dependence. The continuous increase of checkerboard periodicity with hole density strongly suggests that the checkerboard originates from charge density wave formation in the anti-nodal region of the cuprate Fermi surface. These results reveal a coherent picture for static electronic orderings in the cuprates and shed important new light on the nature of the pseudogap phase. Author ContributionsWDW, MCB and KC shared equal responsibility for all aspects of this project from instrument construction through data collection and analysis. TK grew the samples and helped refine the STM. TT and HI contributed to sample growth. YW contributed to analysis and writing of the manuscript. EWH advised.

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An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co–O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 µV·K-1 at 100 K and increases with temperature up to 210 µV·K-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (Z T) of the Co-225 whiskers is estimated 1.2–2.7 at T≥873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

The unclear relationship between cuprate superconductivity and the pseudogap state remains an impediment to understanding the high transition temperature (T(c)) superconducting mechanism. Here, we used magnetic field-dependent scanning tunneling microscopy to provide phase-sensitive proof that d-wave superconductivity coexists with the pseudogap on the antinodal Fermi surface of an overdoped cuprate. Furthermore, by tracking the hole-doping (p) dependence of the quasi-particle interference pattern within a single bismuth-based cuprate family, we observed a Fermi surface reconstruction slightly below optimal doping, indicating a zero-field quantum phase transition in notable proximity to the maximum superconducting T(c). Surprisingly, this major reorganization of the system's underlying electronic structure has no effect on the smoothly evolving pseudogap.

We have investigated the crystal structure and superconducting properties of thin films of FeSe 0.5 Te 0.5 grown on eight different substrates. Superconductivity is not correlated with the lattice mismatch, but rather it is correlated with the degree of in-plane orientation and with the lattice parameter ratio c/a. The best superconducting properties are observed in films on MgO and LaAlO 3 (T c zero of 9.5 K). TEM observation shows that the presence or absence of the amorphous-like layer at the substrate surface plays a key role in determining the structural and superconducting properties of the grown films.* E-mail address: imai@maeda1.c.u-tokyo.ac.jpAfter the discovery of superconductivity in F-doped LaFeAsO, 1) numerous studies on iron-based superconductors have been carried out. One common iron-based superconductor is FeSe with a superconducting transition temperature T c of 8 K, 2) and the partial substitution of Te for Se raises T c to a maximum of 14 K.3)This material has the tetragonal PbO-type structure, which is the simplest structure of all the iron-based superconductors. Thus, FeSe and related materials are considered the most suitable systems to investigate how superconductivity correlates to the crystal structures.Many studies on the film growth of FeSe 1-x Te x have already been reported. 4-11)However, the question of what substrates are suitable for the growth of thin FeSe 1-x Te x films remains controversial. For example, Kumary et al. 6) reported that the T c value of the film on SrTiO 3 (STO) was higher than on LaAlO 3 (LAO). In contrast, Han et al. 7)reported an opposite result; the T c value on LAO was higher than that on STO. In addition, Bellingeri et al. thickness were fixed at 573 K and at approximately 50 nm, respectively. The crystal structure and the orientation of the films were characterized by a θ-2θ and a 4-circle X-ray diffraction (XRD) using Cu Kα radiation at room temperature. We also performed a transmission electron microscopy (TEM) observation. The electrical resistivity (ρ) was measured by a four-terminal method from 2 to 300 K. Figure 1 shows the XRD patterns of the eight films. Except for a few unidentified peaks, only the 00l reflections of a tetragonal PbO-type structure are observed, which shows that the out-of-plane alignment is excellent. It should be noted that the c-axis orientation is observed even in the film prepared on the (0001) plane in hexagonal Al 2 O 3 , as shown in Fig. 1 (h). This indicates that the FeSe 1-x Te x films intrinsically favor two-dimensional growth. The temperature dependence of ρ is summarized in Fig. 2. As can be easily seen, the eight films exhibit a variety of ρ(T)behavior. Except for the film on Al 2 O 3 , the ρ value of these films at T = 300 K is

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca 3 Co 4 O 9 , Na 0.6 CoO 2 , and Bi 2 Sr 2 Co 2 O 9 , can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5-2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E F in these compounds. The density of states at E F is finite but negligibly small at room temperature, because E F is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near E F determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.

In-situ epitaxial growth of FeSe 0.5 Te 0.5 thin films is demonstrated on a nonoxide substrate CaF 2 .Structural analysis reveals that compressive stress is moderately added to 36-nm-thick FeSe 0.5 Te 0.5 , which pushes up the critical temperature to above 15 K, showing higher values than that of bulk crystals. The critical current density at T = 4.5 K reaches 5.9 × 10 4 Acm −2 at µ 0 H = 10 T, and 4.2 × 10 4 Acm −2 at µ 0 H = 14 T. These results indicate that fluoride substrates have high potential for the growth of iron-based superconductors in comparison with popular oxide substrates.Since the discovery of iron-based superconductors, 1) much effort has been devoted to establish a thin-film growth technique of these compounds. 2-8) At an early stage, iron-chalcogenide superconductors 9) had been considered rather inappropriate for practical applications simply because of their low critical temperature (T c ). However, pressure-effect studies demonstrate that the potential T c of iron-chalcogenide superconductors is as high as 37 K, 10, 11) which motivated us to begin the study of thin-film growth of FeSe 1−x Te x . Through many reports, [12][13][14][15][16] one problem has gradually emerged; There is a close correlation between T c and the structure of the films grown epitaxially, but their lattice parameters are influenced by too many growth parameters, and the lattice parameters of the substrate material are not a dominant factor. [17][18][19] In other words, FeSe 1−x Te x can be grown epitaxially on a single-crystalline substrate, but the lattice parameters cannot be designed in accordance with those of the substrates. One of the possible reasons that we have previously proposed for this is oxygen contamination from oxide substrates, 18,19) and we actually confirmed the presence of oxygen at the interface of FeSe 1−x Te x grown on YSZ and LaSrAlO 4 . In order to avoid this problem and further improve superconducting properties, it is important to investigate the film growth *

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