Rønnow H M, Renner Ch, Aeppli G, Kimura T, Tokura Y
Laboratory for Neutron Scattering, ETH-Zürich and Paul Scherrer Institut, 5232 Villigen, Switzerland.
Nature. 2006 Apr 20;440(7087):1025-8. doi: 10.1038/nature04650.
A remarkable feature of layered transition--metal oxides-most famously, the high-temperature superconductors--is that they can display hugely anisotropic electrical and optical properties (for example, seeming to be insulating perpendicular to the layers and metallic within them), even when prepared as bulk three-dimensional single crystals. This is the phenomenon of 'confinement', a concept at odds with the conventional theory of solids, and recognized as due to magnetic and electron-lattice interactions within the layers that must be overcome at a substantial energy cost if electrons are to be transferred between layers. The associated energy gap, or 'pseudogap', is particularly obvious in experiments where charge is moved perpendicular to the planes, most notably scanning tunnelling microscopy and polarized infrared spectroscopy. Here, using the same experimental tools, we show that there is a second family of transition-metal oxides--the layered manganites La(2-2x)Sr(1+2x)Mn2O7--with even more extreme confinement and pseudogap effects. The data demonstrate quantitatively that because the charge carriers are attached to polarons (lattice- and spin-textures within the planes), it is as difficult to remove them from the planes through vacuum-tunnelling into a conventional metallic tip, as it is for them to move between Mn-rich layers within the material itself.
层状过渡金属氧化物（最著名的是高温超导体）的一个显著特征是，即使制备成块状三维单晶，它们也能表现出极大的各向异性电学和光学性质（例如，垂直于层时似乎是绝缘的，而在层内则是金属性的）。这就是“限制”现象，这一概念与传统固体理论相悖，并且被认为是由于层内的磁相互作用和电子-晶格相互作用导致的，如果电子要在层间转移，必须付出相当大的能量代价才能克服这些相互作用。相关的能隙，即“赝能隙”，在电荷垂直于平面移动的实验中尤为明显，最显著的是扫描隧道显微镜和偏振红外光谱实验。在这里，我们使用相同的实验工具表明，存在第二类过渡金属氧化物——层状锰氧化物La(2 - 2x)Sr(1 + 2x)Mn2O7——具有更极端的限制和赝能隙效应。数据定量地表明，由于电荷载流子附着于极化子（平面内的晶格和自旋结构），通过真空隧穿进入传统金属尖端将它们从平面中移除，与它们在材料本身富含锰的层间移动一样困难。
