Longitudinal Josephson-plasma excitation in Bi2Sr2CaCu2O8+delta: Direct observation of the Nambu-Goldstone mode in a superconductor

Electromagnetic resonant absorption phenomena in a microwave frequency range have recently been studied in the single-crystalline high-temperature superconductor Bi2Sr2CaCu2O8+delta in magnetic fields. Using a rectangular microwave cavity resonator technique with TE102 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 he 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 Bi2Sr2CaCu2O8+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 Bi2Sr2CaCu2O8+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.