Microscopic electronic heterogeneity and its effect on the optical spectrum of high-temperature superconductors

我们用各种光谱方法，特别是STM/STS，研究了微观不均匀对高T_c氧化铜超导体光谱的影响，并应用于控制良好的单晶样品。本研究的主要结果是：(1)阐明了两个不同的准粒子(QP)在d波超导体中所起的作用。结点QP具有较长的寿命/相干性，这导致了量子力学干涉引起的局域电子态密度(LDOS)的周期性调制。另一方面，反节点QP是非相干的，并且受到晶格无序的强烈影响，这是造成高T_c铜酸盐微观不均匀的主要原因。(2)在铜酸盐各种晶格位置的无序中，CuO_2平面附近的顶端氧块中的无序对CuO_2平面的电子态影响最大。这种无序稳定了一种竞争秩序，在各种光谱中表现为假缝隙。通过对STM/STS结果的特殊分析，我们揭示了竞争有序/伪隙相是一种电子玻璃。(3)由于铜酸盐被认为是沿c轴的一系列约瑟夫森结，所以反节点QP与c轴光谱密切相关，在c轴光谱中，超导响应表现为约瑟夫森等离子体模。我们证明了平行于CuO_2平面的磁场使约瑟夫森结不均匀，结果在太赫兹频率区出现了新的约瑟夫森等离子体模。我们还发现，可以通过改变外加磁场的强度来调节新约瑟夫森等离子体模的频率。

We investigated the effect of microscopic heterogeneity on the optical spectrum of high-Tc copper oxide superconductors using various spectroscopic methods, particularly STM/STS, applied to well-controlled single crystalline samples. The major results of the present research are:(1)Elucidation of the roles played by two distinct quasiparticles (QP), nodal and antinodal quasiparticles, in d-wave superconductors. Nodal QP has longer lifetime/coherence which gives rise to the periodic modulations of the local-electronic-density-of-states (LDOS) due to quantum-mechanical interference. On the other hand, antinodal QP is incoherent, and strongly influenced by lattice disorder, playing a major role in the microscopic heterogeneity in high-Tc cuprates.(2)Among disorder in various lattice sites of cuprates, the disorder in the apical oxygen block next to the CuO2 plane is found to most strongly affects the electronic state of the CuO2 plane. This disorder stabilizes a competing order which shows up as a pseudogap in the various spectroscopies. Using a special analysis on the STM/STS result, we revealed that the competing order/the pseudogap phase is a sort of electromic glass.(3)The antinodal QP is intimately related to the c-axis optical spectrum in which the superconducting response appears as a Josephson plasma mode because cuprates are regarded as a series of Josephson junctions along the c-axis. We demonstrated that magnetic fields applied parallel to the CuO2 planes make the Josephson junctions heterogeneous, and, as a consequence, a new Josephson plasma mode appears in the terahertz frequency region. We also showed that the frequency of the new Josephson plasma mode can be tuned by changing the strength of applied magnetic field.