Rigorous test of electron-hole symmetry for high- Tc superconductivity using epitaxial thin films

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The playground for high-T_c superconductivity is the CuO_2 plane common to both p- and n-type high-T_c superconductivity, and the electronic phase diagram of high-T_c cuprates is roughly symmetric between p- and n-type doping. Hence, it has been claimed that "electron-hole" symmetry holds for high-T_c superconductivity. Based on this claim, the doped Mott insulator scenario has been widely accepted, in which the parent material is a Mott insulator (anti-ferromagnetic insulator) and high-T_c superconductivity develops when the insulator is exposed to either p- or n-type doping. However, it should be borne in mind that electron-hole symmetry is far from obvious and even surprising. Since the mother compounds of high- T_c superconductors can be regarded as charge-transfer insulators, doped holes go on the oxygen sites but doped electrons go on the Cu sites, which should result in doped carriers with quite different natures. Furthermore it must be emphasized that the argument for the above … More "electron-hole" symmetry is based on a comparison of p- and n-type doping in different structures, namely, hole doping in the K_2NiF_4 (T) structure and electron doping in the Nd_2CuO_4 (T) structure.In principle, it is desirable to compare hole and electron doping in the same crystal structure. However, such a comparison has not yet been undertaken because it is empirically known in bulk synthesis that hole doping is possible only in octahedral (CuO_6) or pyramidal (CuO_5) cuprates whereas electron doping is possible only in square-planar (CuO_4) cuprates. For example, electron doping in the T structure or hole doping in the T' structure has never been achieved in bulk synthesis. However, in this project we report that Ce can be incorporated into the K_2NiF_4 lattice [T-La_2CuO_4 (LCO)] by employing a low-temperature synthetic route with molecular beam epitaxy (MBE) and that Sr/Ca can be incorporated into the Nd_2CuO_4 lattice by employing a low-temperature synthetic route with metal organic deposition (MOD). The former results revealed that Ce doping makes T-LCO more insulating, which is in sharp contrast to Sr (or Ba) doping in T-LCO, which makes the compound metallic and superconducting. The observed smooth increase in resistivity from the hole-doped side to the electron-doped side indicates that the electron-hole symmetry is broken in the T-phase materials. The latter result revealed that T'-RE_2CuO_4 can be superconducting even with no doping and that hole doping increases T whereas electron doping decreases T_c. Electron-hole symmetry is also broken in the T'-phase materials. Both of the results throw strong skepticism on the currently accepted "doped Mott-insulator scenario" on high- T_c superconductivity. Less