Far-infrared plasma and electromagnetic waves in anisotropic superconductors

在高温超导体的c轴偏振远红外反射谱中出现的等离子体边是我们小组在铜酸镧超导体中首次发现的，现在被认为是铜酸盐超导体的普遍现象。对于铋铜酸盐的等离子体频率位于微波区，因为它们非常大的各向异性。我们主要研究了La_2-xSr_xCuO_4和<a2>YBa_Cu_3O_4<7-y1>超导体的约瑟夫森等离子体频率随掺杂、Zn取代和外加磁场的变化。此外，利用薄LSCO晶体，等离子体波的传播和耗散进行了研究。主要结果和发现是：1.约瑟夫森等离子体频率（ω_p）主要由T_c以上的c轴正常态电阻率决定。随着掺杂的进行，高温超导体的各向异性变弱，从而c轴电阻率变低。结果表明，各向异性的减小导致ω_p的增大。2.高T_c超导体对频率高于ω_p的c轴偏振光变得透明，这是由光与约瑟夫森等离子体之间的耦合电磁波极化激元的传播所解释的。发现极化激元具有弱阻尼，这是由于高T_c超导性的无隙性，是库珀对与d波耦合的固有特性。3.铜酸铋中的Ω_p对外加磁场非常敏感，而铜酸镧中的Ω_p对外加磁场不敏感。这种差异是由于在涡旋钉扎强度的差异，它证明了钉扎变得更强的掺杂在任何铜酸盐系统。

A plasma edge appearing in the c-axis polarized far-infrared reflectivity spectrum of high-T_C superconductors, which was first discovered for La-cuprates by our group, is now recognized as a general phenomenon of cuprate superconductors. For Bi cuprates the plasma frequency is located in the microwave region because of their extremely large anisotropy. This plasma is now called as Josephson plasma, since its appearance is connected to Josephson coupling between superconducting CuO_<> layrs.We have pursued, mainly on La_2-_XSr_XCuO_4 and YBa_<a2>Cu_3O_<7-y1>, how the Josephson plasma frequency varies with doping, Zn-substitution, and applied magnetic fields. In addition, using thin LSCO crystals, the propagation and dissipation of the plasma wave have been investigated. Major results and findings are :1.The Josephson plasma frequency (omega_p) is determined predominantly by the value of the c-axis normal-state resistivity just above T_c. The anisotropy of the high-T_c superconductors becomes weaker and thus the c-axis resistivity becomes lower as doping proceeds. It is demonstrated that the decrease in anisotropy leads to an increase in omega_p.2.It is shown that the high-T_c superconductors become transparent for c-axis polarized light with frequency higher than omega_p. This is explained by the propagation of a polariton which is a coupled electromagnetic wave between light and Josephson plasma. The polariton is found to suffur weak damping which arises from the gaplessness of high-T_c superconductivity, intrinsic of cooper pairing with d-wave symmetry.3.While omega_p in Bi cuprates is very sensitive to externally applied magnetic fields, that in La cuprates is found to be insensitive. This difference is ascribable to the difference in the vortex pinning strength, and it turns out that the pinning becomes stronger with increase of doping in any cuprate system.