Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance Cg. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance CEDL between the ionic liquid and graphene involves the series connection of Cg and the quantum capacitance Cq, which is proportional to the density of states. We investigated the variables that determine CEDL at the molecular level by varying the number of graphene layers n and thereby optimising Cq. The CEDL value is governed by Cq at n < 4, and by Cg at n > 4. This transition with n indicates a composite nature for CEDL. Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor.

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The superconducting transition temperature T c of ammoniated metal-doped FeSe (NH 3 ) y M x FeSe (M: metal atom) has been scaled with the FeSe plane spacing, and it has been suggested that the FeSe plane spacing depends on the location of metal atoms in (NH 3 ) y M x FeSe crystals. Although the crystal structure of (NH 3 ) y Li x FeSe exhibiting a high T c (∼44 K) was determined from neutron diffraction, the structure of (NH 3 ) y M x FeSe exhibiting a low T c (∼32 K) has not been determined thus far. Here, we determined the crystal structure of (NH 3 ) y Cs 0.4 FeSe (T c = 33 K) through the Rietveld refinement of the x-ray diffraction (XRD) pattern measured with synchrotron radiation at 30 K. The XRD pattern was analyzed based on two different models, on-center and off-center, under a space group of I 4/mmm. In the on-center structure, the Cs occupies the 2a site and the N of NH 3 may occupy either the 4c or 2b site, or both. In the off-center structure, the Cs may occupy either the 4c or 2b site, or both, while the N occupies the 2a site. Only an on-center structure model in which the Cs occupies the 2a and the N of NH 3 occupies the 4c site provided reasonable results in the Rietveld analysis. Consequently, we concluded that (NH 3 ) y Cs 0.4 FeSe can be assigned to the on-center structure, which produces a smaller FeSe plane spacing leading to the lower T c .

We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)yNaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of Tc was fully investigated as a function of x in (NH3)yNaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x  =  1.1, with Tc =  45 K, showing that the Tc depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)yNaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-Tc phase (Tc = 32.5 K) are 3.9120(9) and 14.145(8) Å, respectively, while a = 3.8266(7) Å and c = 17.565(9) Å for the high-Tc phase (~46 K). The c increases in the high Tc phase, implying that the Tc is directly related to c. In (NH3)yLixFeSe material, the Tc varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of Tc is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.

The temperature dependence of the resistivity ( ρ ) of Ag-doped Bi 2 Se 3 (Ag x Bi 2−x Se 3 ) shows insulating behavior above 35 K, but below 35 K, ρ suddenly decreases with decreasing temperature, in contrast to the metallic behavior for non-doped Bi 2 Se 3 at 1.5–300 K. This significant change in transport properties from metallic behavior clearly shows that the Ag doping of Bi 2 Se 3 can effectively tune the Fermi level downward. The Hall effect measurement shows that carrier is still electron in Ag x Bi 2−x Se 3 and the electron density changes with temperature to reasonably explain the transport properties. Furthermore, the positive gating of Ag x Bi 2−x Se 3 provides metallic behavior that is similar to that of non-doped Bi 2 Se 3 , indicating a successful upward tuning of the Fermi level.