Longitudinal Josephson-plasma excitation in Bi2Sr2CaCu2O8+δ: Direct observation of the Nambu-Goldstone mode in a superconductor
Longitudinal Josephson-plasma excitation in Bi2Sr2CaCu2O8+δ: Direct observation of the Nambu-Goldstone mode in a superconductor
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Bi2Sr2CaCu2O8+δ 中的纵向约瑟夫森等离子体激发：超导体中南部-戈德斯通模式的直接观察
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
M. Tachiki
中科院分区：
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作者：
K. Kadowaki;I. Kakeya;M. Gaifullin;T. Mochiku;S. Takahashi;T. Koyama;M. Tachiki
最近，人们研究了单晶高温超导体${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}}$在磁场中的微波频率范围内的电磁谐振吸收现象。使用具有 ${mathrm{TE}}_{102}$ 模式的矩形微波腔谐振器技术，这种吸收被明确识别为集体纵向约瑟夫森等离子体激发。由于超导态是相位对称性破缺的状态，因此有序态应该伴随着集体激发（南部-戈德斯通模式）。长期以来，这种激发一直被认为是不可观测的，因为大库仑间隙（安德森-希格斯-基布尔机制）的形成，在库仑间隙上方可能存在激发等离子体的强阻尼机制。然而，在各向异性层状系统（例如 ${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}},$）的情况下，库仑间隙可能非常小，并且等离子体模式可能位于微波频率区域。利用特征色散关系（使我们能够将纵向模式与横向模式分开），在 ${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}}$ 中观察到的微波吸收明确归因于纵向激励。我们相信，这是超导体中南部-戈德斯通模式的第一个也是直接的实验证据，并直接证明了自发破缺对称性概念中的安德森-希格斯-基布尔（AHK）机制在超导相变的情况下确实有效。由于这里观察到的有限等离子体频率意味着由于相对论意义上的 AHK 机制而形成了有限质量的等离子体（相子），因此超导体中的上述情况对应于 Weinberg 和 Salam 所示的弱相互作用和电磁相互作用的统一规范场理论的直接映射。
Electromagnetic resonant absorption phenomena in a microwave frequency range have recently been studied in the single-crystalline high-temperature superconductor ${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}}$ in magnetic fields. Using a rectangular microwave cavity resonator technique with ${mathrm{TE}}_{102}$ mode this absorption is unambiguously identified to be the collective longitudinal Josephson plasma excitation. Since the superconducting state is a state with a broken phase symmetry, the ordered state should be accompanied by the collective excitation (Nambu-Goldstone mode). This excitation has long been thought not to be observable, because of the formation of the large Coulomb gap (Anderson-Higgs-Kibble mechanism), above which strong damping mechanisms of excited plasma are presumably present. However, this Coulomb gap can be very small in the case of anisotropic layered system such as ${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}},$ and the plasma mode may lie in a microwave frequency region. Using characteristic dispersion relations, which enables us to separate out the longitudinal mode from the transverse one, the microwave absorption observed in ${mathrm{Bi}}_{2}{mathrm{Sr}}_{2}{mathrm{CaCu}}_{2}{mathrm{O}}_{8+ensuremath{delta}}$ is unambiguously attributed to the longitudinal excitations. We believe that this is the first and the direct experimental evidence of the Nambu-Goldstone mode in a superconductor and provides a direct proof that the Anderson-Higgs-Kibble (AHK) mechanism within the concept of spontaneously breaking symmetry is indeed valid in the case of superconducting phase transition. Since the finite plasma frequency observed here signifies formation of the finite mass of the plasma (phason) due to the AHK mechanism in the relativistic sense, the above-mentioned scenario in a superconductor corresponds to a direct mapping from the unified gauge-field theory of weak interaction and electromagnetic interaction shown by Weinberg and Salam.