Excitation Gap in the Normal State of Underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}

• Published in: Science (American Association for the Advancement of Science) Vol. 273; no. 5273; pp. 325 - 329
• Main Authors: Loeser, A. G., Z.-X. Shen, Dessau, D. S., Marshall, D. S., Park, C. H., Fournier, P., Kapitulnik, A.
• Format: Journal Article
• Language: English
• Published: American Society for the Advancement of Science 19-07-1996
• Subjects: Binding energy; Crystals; Doping; Electrons; Fermi surfaces; Momentum; Phase diagrams; Photoelectric emission; Physics; Research Article; Superconductors
• ISSN: 0036-8075; 1095-9203

Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature T$_c$ is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the d$_{x^2-y^2}$ gap observed in the superconducting state. This observation is consistent with results from several other experimental techniques, which also indicate the presence of a gap in the normal state. Some possible theoretical explanations for this effect are reviewed.