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Even at low Ca concentrations, the binary-element intercalated graphite K1–xCaxC8exhibits high superconducting transition temperatures Tc, closer to that of CaC6(11.5 K) than KC8(0.169 K). To investigate the behaviors of K and Ca within the graphite matrix, their local structures have been investigated by using X-ray fluorescence holography and extended X-ray absorption fine structure (EXAFS) spectroscopy. The atomic images reconstructed from the K–Kα and Ca–Kβ holograms showed that K0.7Ca0.3C8did not take the solid-solution type random distribution of Ca and K atoms; instead, a nanoscale phase separation of CaC6and KC8was observed, which was also supported by the EXAFS results. While the lattice constant of K0.7Ca0.3C8was close to that of KC8, we found a nanoscale Ca layer dispersed within the sample. The Ca nanolayer was offset from the center between the C sublayers. The superconducting behavior found in K1–xCaxC8was discussed with two scenarios of percolation of CaC6and deformation of graphene based on such a specific inhomogeneous binary element monatomic layers. This study represents an important step for understanding the superconducting properties in a nanoscale phase-separated system.

Fe5–xGeTe2is a two-dimensional van der Waals material that exhibits ferromagnetic order with a high Curie temperature (TC) of around room temperature. In addition to TC, two magnetic transitions occur with decreasing temperature, and a charge-ordered state is observed at low temperatures. We employed Ge Kα X-ray fluorescence holography (XFH) for Fe5–xGeTe2to directly investigate the local structure in the charge-ordered state, i.e., the 3×3superstructure. The Ge Kα XFH results revealed local atomic structures around the Ge atom, thus clarifying the simultaneous locations and arrangements of the Te, Fe, and Ge atoms. The atomic positions relative to the Ge atom are useful for understanding the coexistence of the ideal 1 × 1 structure and 3×3superstructure found in the charge-ordered state.

A graphite-like material boron carbide (BCx) was synthesized under various heat treatment conditions and extensively characterized. First, we synthesized the BCxprecursor phase by a single-step reaction using a mixed solution of BBr3and C6H6. We confirmed that the precursor phase had a graphite-like structure with B–C chemical bonds, but its crystallinity was poor. To improve their crystallinity, we annealed the precursor sample at high temperature using a high-frequency furnace and determined the annealing condition. We also investigated the magnetic properties of BCx. The high-temperature annealing for the precursor phase yields the highest Pauli paramagnetic susceptibility χPauli, indicating the highest density of states at the Fermi level. Accordingly, the high-temperature treatment for the precursor phase is significant to improve its crystallinity and physical properties. In addition, we synthesized a Ti-intercalated material TiBC by using the same procedure as that for making the BCxprecursor phase. The crystal structure can be indexed by the AlB2structure, indicating that Ti atoms are intercalated between the BC layers. The χPaulivalue of TiBC is obtained to be 1 order of magnitude smaller than that of BCx, suggesting the compensation of hole carriers by electron doping through Ti intercalation into the BCxsystem.

The pressure dependence of superconductivity in alkaline earth metal-doped FeSe, (NH3)yAExFeSe (AE: Ca, Sr, and Ba), prepared by the liquid ammonia (NH3) technique was investigated over a wide pressure range, in order to clarify the correlation between superconducting transition temperature Tcand FeSe layer distance. Doping with an alkaline earth metal atom implies the preparation of FeSe doped with a divalent metal atom. The correlation between the Tcand layer distance has been investigated thus far for FeSe doped with a monovalent metal atom, i.e., alkaline metal-doped FeSe ((NH3)yAxFeSe (A: Na, K, Rb, and Cs)). The Tcvalue of (NH3)yAxFeSe increased with an increase in the lattice constant c, which corresponds to twice of the FeSe layer distance. In (NH3)yAExFeSe (AE: Ca, Sr, and Ba), Tcvalues decreased with pressures up to 10 GPa, and superconductivity was not clearly observed above 10 GPa. Pressure-dependent X-ray diffraction of (NH3)yAExFeSe indicated no structural phase transitions up to ∼25 GPa, with a rapid decrease in cwith pressure and saturation above 10 GPa. Tc–cplots of (NH3)yAExFeSe were drawn from Tc–pand c–pplots obtained in this study. Moreover, we analyze Tc–cplots of (NH3)yAExFeSe based on those depicted for both monovalent and divalent atom-doped FeSe, demonstrating a clear linear relationship in the Tc–cplot with similar slopes. Simply put, the different valence between monovalent and divalent atoms does not remarkably affect the cdependence of Tc, and a universal rule that Tcincreases with an increase in cis unambiguously established for both compounds.

The local structures around the Bi and Pd atoms in β-PdBi2were determined from Bi Lα and Pd Kα X-ray fluorescence holography (XFH) images measured with synchrotron radiation at room temperature, demonstrating clear atomically resolved images along the c-axis (z-axis). More precisely, the location of atoms in the atomic layer at z= 0 Å was unambiguously determined from each hologram; that is, the location of Bi atoms at z= 0 Å was determined from Bi Lα XFH, whereas the location of Pd atoms at z= 0 Å was unambiguously determined from Pd Kα XFH. Moreover, the Bi atoms in the atomic plane at z= 3.6 Å were clearly reconstructed from Bi Lα XFH with Barton’s algorithm. However, clear atomic images of the planes at z= 1.8 Å and z= 2.9 Å were not obtained from the Bi Lα holograms, nor were they observed at z= 1.8 Å in the case of the Pd Kα holograms; some weak atomic images of Bi at z= 4.7 Å were observed from Pd Kα holograms. Thus, two different patterns can be distinguished for the atomic images in the plane at z≠ 0 Å: a clear atomic image and an ambiguous atomic image, owing to the order or disorder between the atomic layers. The small fluctuation or the small disorder between the atomic layers formed through the attractive interaction of Bi––Pd2+–Bi–provides clear atomic images, whereas the atomic layers formed via weak van der Waals interactions lead to ambiguous atomic images. Thus, the atomic images of β-PdBi2obtained by XFH clearly reproduce not only the crystal structure determined by single-crystal X-ray diffraction but also provide information on thermal fluctuation and static disorder between the atomic layers.

A high carrier transport capability with strong π–π interactions is expected to result from the extension of π conjugation in small molecules. This is an important factor to create promising molecules, leading to the development of high-performance and new applications in organic electronic devices. We systematically demonstrate the charge transport capability and trap density of states in dibenzo[n]phenacenes (DBnPs) (n= 5–7) with a hybrid π structure between the phenacene and acene cores. A DB6P field-effect transistor (FET) achieves a relatively high-field-effect mobility beyond 2 cm2V–1s–1, which indicates excellent FET performance in comparison with DB5P and DB7P. The calculated charge-transfer integrals based on the crystal structure and the distribution of the trap density of states confirm the superior transport capability and FET properties in DB6P. We demonstrate that molecules with C2hsymmetry result in high crystallinity and close packing in crystals, leading to high-performance FETs.