AI 中文总结
本研究通过结构实验发现强过掺杂YBa$_2$Cu$_3$O$_y$的$T_c$随掺杂量恒定，提出空穴占据$a_1$对称态的两带模型以解释超出单带穹顶情景的超导现象。
AI 中文摘要
为了解释多个过掺杂铜氧化物中存在的扩展超导区域这一与穹顶情景相悖的观测结果，本研究通过中子及同步辐射X射线粉末衍射，对YBa$_2$Cu$_3$O$_y$的晶体结构展开研究，其中在高压下实现了高达$y = 7.4$的强氧过掺杂。键价和分析表明，过量氧产生的额外空穴中，有1/5转移至CuO$_2$面，使空穴密度提升至$p=0.27$空穴/Cu，而根据穹顶情景，此时超导性应已消失。相反，本研究的数据证实了此前的观测结果[Okai、Ono与Mitsuhashi，《Physica C：超导电性》第366卷，164页（2002年）]，即超导临界温度$T_c$随$y$保持恒定。本研究的数据分析从顶端氧与平面Cu离子间的键短得多这一角度解释了上述差异，这表明额外空穴占据了由$d_{3z^2-r^2}$轨道形成的$a_1$对称态，而非通常由$d_{x^2-y^2}$轨道形成的$b_1$对称Zhang-Rice单重态。对单晶开展合适的光谱测量或能支持这类两带情景，而这将需要完全不同的理论方法来解释铜氧化物的超导性。
英文摘要
In order to explain the observation of an extended superconducting region in several overdoped cuprates, which contrasts the dome scenario, by means of neutron and synchrotron x-ray powder diffraction we study the crystal structure of YBa$_2$Cu$_3$O$_{y}$, where strong oxygen overdoping up to $y = 7.4$ is achieved under high-pressure. A bond valence sum analysis indicates that 1/5 of the extra holes created by the excess oxygen are transferred to the CuO$_2$ planes, thus increasing the hole density up to $p=0.27$ hole/Cu, where superconductivity is expected to vanish according to the dome scenario. Instead, our data confirm a previous observation [Okai, Ono and Mitsuhashi, Physica C: Superconductivity {\bf 366}, 164 (2002)] that the superconducting critical temperature, $T_c$, remains constant with $y$. Our data analysis accounts for this discrepancy in terms of the much shorter bond between the apical oxygen and the planar Cu ion, which suggests that the extra holes occupy the $a_1$-symmetry states formed by $d_{3z^2-r^2}$ orbitals, instead of the usual $b_1$-symmetry Zhang-Rice singlet states formed by $d_{x^2-y^2}$ orbitals. Suitable spectroscopic measurements on single crystals may support such a two-band scenario, which would require a totally different theoretical approach to explain superconductivity in cuprates.
Comments12 pages, 7 figures
