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

使用外延薄膜严格测试高温超导的电子-空穴对称性

基本信息

批准号：
18340098
负责人：
NAITO Michio
金额：
$ 11.09万
依托单位：
Tokyo University of Agriculture and Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2006
资助国家：
日本
起止时间：
2006 至 2007
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-18340098/
关键词：
high- T_c superconductors electronic phase diagram electron-hole symmetry Mott-Hubbard insulators epitaxial thin films molecular beam epitaxy metal organic decomposition 酸化物高温超伝導体

项目摘要

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

高T_c超导的主要场所是p型和n型高T_c超导所共有的CuO_2平面，高T_c铜氧化物的电子相图在p型和n型掺杂之间大致对称。因此，有人认为高T_c超导体具有“电子-空穴”对称性。基于这一观点，掺杂的Mott绝缘体方案已被广泛接受，其中母体材料是Mott绝缘体（反铁磁绝缘体），当绝缘体暴露于p型或n型掺杂时，高T_c超导性发展。然而，应该记住，电子-空穴对称性远不明显，甚至令人惊讶。由于高温超导体的母体化合物可以看作是电荷转移绝缘体，掺杂的空穴在氧位上，而掺杂的电子在铜位上，这就导致了不同性质的掺杂载流子。此外，必须强调的是，上述论点 ...更多信息 “电子-空穴”对称性是基于不同结构中p型和n型掺杂的比较，即K_2NiF_4（T）结构中的空穴掺杂和Nd_2CuO_4（T）结构中的电子掺杂。然而，这样的比较尚未进行，因为它是经验已知的，在块体合成，空穴掺杂是可能的，只有在八面体（CuO_6）或金字塔（CuO_5）的铜氧化物，而电子掺杂是可能的，只有在正方形平面（CuO_4）的铜氧化物。例如，在体合成中从未实现T结构中的电子掺杂或T'结构中的空穴掺杂。在本项目中，我们采用分子束外延（MBE）低温合成法将Ce引入到K_2NiF_4晶格[T-La_2CuO_4（LCO）]中，采用金属有机物沉积（MOD）低温合成法将Sr/Ca引入到Nd_2CuO_4晶格中。结果表明，Ce的掺杂使T-LCO具有更好的绝缘性，而Sr（或Ba）的掺杂使T-LCO具有更好的金属性和超导性。所观察到的电阻率从空穴掺杂侧到电子掺杂侧的平滑增加表明在T相材料中电子-空穴对称性被破坏。后者的结果表明，T '-RE_2CuO_4在不掺杂的情况下也能超导，空穴掺杂使T增大，而电子掺杂使T_c减小。电子-空穴对称性在T '相材料中也被破坏。这两个结果都对目前公认的高T_c超导的“掺杂莫特绝缘体方案”提出了强烈的质疑。少