We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi2Sr2Ca1-xDyxCu2O 8+δ. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap Δ does not fall proportionally, but instead stays constant or increases slightly, in violation of the BCS mean-field theory result. The different doping dependences of Δ and kTc indicate that they represent different energy scales. The measurements also show that Δ is highly anisotropic and consistent with a dx2-y2 order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above Tc. The existence of a normal state gap is related to the failure of Δ to scale with Tc in theoretical models that predict pairing without phase coherence above Tc.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 1996|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics