Kohsaka Y, Taylor C, Wahl P, Schmidt A, Lee Jhinhwan, Fujita K, Alldredge J W, McElroy K, Lee Jinho, Eisaki H, Uchida S, Lee D-H, Davis J C
LASSP, Department of Physics, Cornell University, Ithaca, New York 14853, USA.
Nature. 2008 Aug 28;454(7208):1072-8. doi: 10.1038/nature07243.
The antiferromagnetic ground state of copper oxide Mott insulators is achieved by localizing an electron at each copper atom in real space (r-space). Removing a small fraction of these electrons (hole doping) transforms this system into a superconducting fluid of delocalized Cooper pairs in momentum space (k-space). During this transformation, two distinctive classes of electronic excitations appear. At high energies, the mysterious 'pseudogap' excitations are found, whereas, at lower energies, Bogoliubov quasi-particles-the excitations resulting from the breaking of Cooper pairs-should exist. To explore this transformation, and to identify the two excitation types, we have imaged the electronic structure of Bi(2)Sr(2)CaCu(2)O(8+delta) in r-space and k-space simultaneously. We find that although the low-energy excitations are indeed Bogoliubov quasi-particles, they occupy only a restricted region of k-space that shrinks rapidly with diminishing hole density. Concomitantly, spectral weight is transferred to higher energy r-space states that lack the characteristics of excitations from delocalized Cooper pairs. Instead, these states break translational and rotational symmetries locally at the atomic scale in an energy-independent way. We demonstrate that these unusual r-space excitations are, in fact, the pseudogap states. Thus, as the Mott insulating state is approached by decreasing the hole density, the delocalized Cooper pairs vanish from k-space, to be replaced by locally translational- and rotational-symmetry-breaking pseudogap states in r-space.
氧化铜莫特绝缘体的反铁磁基态是通过在实空间（r 空间）中每个铜原子处局域一个电子来实现的。移除一小部分这些电子（空穴掺杂）会将这个系统转变为动量空间（k 空间）中离域库珀对的超导流体。在这个转变过程中，出现了两类独特的电子激发。在高能区，发现了神秘的“赝能隙”激发，而在低能区，应该存在博戈留波夫准粒子——由库珀对破裂产生的激发。为了探索这个转变，并识别这两种激发类型，我们同时对 Bi(2)Sr(2)CaCu(2)O(8 + δ)在 r 空间和 k 空间中的电子结构进行了成像。我们发现，尽管低能激发确实是博戈留波夫准粒子，但它们只占据 k 空间的一个受限区域，该区域随着空穴密度的减小而迅速收缩。与此同时，光谱权重转移到了具有与离域库珀对激发特征不同的更高能量的 r 空间态上。相反，这些态在原子尺度上以与能量无关的方式局部破坏平移和旋转对称性。我们证明这些不寻常的 r 空间激发实际上就是赝能隙态。因此，随着通过降低空穴密度接近莫特绝缘态，离域库珀对从 k 空间消失，取而代之的是 r 空间中局部破坏平移和旋转对称性的赝能隙态。
