Superconducting plasma excitation at microwave frequencies in parallel magnetic fields in Bi 2Sr 2CaCu 2O 8+ δ

Abstract

Josephson plasma resonance has been studied in a wide microwave frequency range between 10 and 52 GHz in a magnetic field parallel to the ab-plane in underdoped Bi 2Sr 2CaCu 2O 8+ δ . Above about 30 GHz two resonance modes were observed: one (LT mode) appears at low temperatures and another (HT mode) at higher temperatures, leaving a temperature gap between two regions. These two resonance modes exhibit a sharp contrast to each other both on temperature and magnetic field dependences and show distinct characters entirely different from the c-axis Josephson plasma resonance. From temperature and field scan experiments at various frequencies it is suggested that the LT mode can be attributed to the coupled Josephson plasma mode with Josephson vortices, while the HT mode is a new plasma mode associated possibly with the periodic array of Josephson vortices.