This article reviews new superconducting phases of carbon-based materials. During the past decade, new carbon-based superconductors have been extensively developed through the use of intercalation chemistry, electrostatic carrier doping, and surface-proving techniques. The superconducting transition temperature T c of these materials has been rapidly elevated, and the variety of superconductors has been increased. This review fully introduces graphite, graphene, and hydrocarbon superconductors and future perspectives of high-T c superconductors based on these materials, including present problems. Carbon-based superconductors show various types of interesting behavior, such as a positive pressure dependence of T c. At present, experimental information on superconductors is still insufficient, and theoretical treatment is also incomplete. In particular, experimental results are still lacking for graphene and hydrocarbon superconductors. Therefore, it is very important to review experimental results in detail and introduce theoretical approaches, for the sake of advances in condensed matter physics. Furthermore, the recent experimental results on hydrocarbon superconductors obtained by our group are also included in this article. Consequently, this review article may provide a hint to designing new carbon-based superconductors exhibiting higher T c and interesting physical features.
Na-intercalated FeSe 0.5 Te 0.5 was prepared using the liquid NH 3 technique, and a superconducting phase exhibiting a superconducting transition temperature (T c ) as high as 27 K was discovered. This can be called the high-T c phase since a 21 K superconducting phase was previously obtained in (NH 3 ) y Na x FeSe 0.5 Te 0.5 . The chemical composition of the high-T c phase was determined to be (NH 3 ) 0.61(4) Na 0.63(5) Fe 0.85 Se 0.55(3) Te 0.44(2) . The x-ray diffraction patterns of both phases show that a larger lattice constant c (i.e., FeSe 0.5 Te 0.5 plane spacing) produces a higher T c . This behavior is the same as that of metal-doped FeSe, suggesting that improved Fermi-surface nesting produces the higher T c . The high-T c phase converted to the low-T c phase within several days, indicating that it is a metastable phase. The temperature dependence of resistance for both phases was recorded at different magnetic fields, and the critical fields were determined for both phases. Finally, the T c versus c phase diagram was prepared for the metal-doped FeSe 0.5 Te 0.5 , which is similar to that of metal-doped FeSe, although the T c is lower.
We investigated the pressure-dependence of electric transport and crystal structure of Ag-doped Bi 2 Se 3 . In the sample prepared by Ag-doping of Bi 2 Se 3 , the Bi atom was partially replaced by Ag, i.e., Ag 0.05 Bi 1.95 Se 3 . X-ray diffraction (XRD) patterns of Ag 0.05 Bi 1.95 Se 3 measured at 0 -30 GPa showed three different structural phases, with rhombohedral, monoclinic and tetragonal structures forming in turn as pressure increased, and structural phase transitions at 8.8 and 24 GPa. Ag 0.05 Bi 1.95 Se 3 showed no superconductivity down to 2.0 K at 0 GPa, but under pressure, superconductivity suddenly appeared at 11 GPa. The magnetic field (H) dependence of the superconducting transition temperature, T c , was measured at 11 and 20.5 GPa, in order to investigate whether the pressure-induced superconducting phase is explained by either p-wave polar model or s-wave model.
We prepared two superconducting phases, which are called "low-T c phase" and "high-T c phase" of (NH 3) y Na x FeSe showing T c 's of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The T c of the 35 K at ambient pressure rapidly decreases with increasing pressure up to 10 GPa, and it remains unchanged up to 22 GPa. Finally, superconductivity was not observed down to 1.4 K at 29 GPa, i.e., T c < 1.4 K. The T c of the 44 K phase also shows a monotonic decrease up to 15 GPa and it weakly decreases up to 25 GPa. These behaviors suggest no pressure-driven high-T c phase (called "SC-II") between 0 and 25 GPa for the low-T c and high-T c phases of (NH 3) y Na x FeSe, differing from the behavior of (NH 3) y Cs x FeSe, which has a pressure-driven high-T c phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The T cc phase diagram for both low-T c and high-T c phases shows that the T c can be linearly scaled with c (or FeSe plane spacing), where c is a lattice constant. The reason why a pressure-driven high-T c phase (SC-II) was found for neither low-T c nor high-T c phases of (NH 3) y Na x FeSe is fully discussed, suggesting a critical c value as the key to forming the pressure-driven high-T c phase (SC-II). Finally, the precise T cc phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and NH 3 or amine, indicating two distinct T cc lines below c = 17.5Å.
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