D. S. Marshall

Excitation Gap in the Normal State of Underdoped Bi2Sr2CaCu2O8+δ

Angle-resolved photoemission experiments reveal evidence of an energy gap in the normal state excitation spectrum of the cuprate superconductor Bi2Sr2CaCu2O8+δ. This gap exists only in underdoped samples and closes around the doping level at which the superconducting transition temperature Tc is a maximum. The momentum dependence and magnitude of the gap closely resemble those of the dx2−y2 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.

Evolution of the electronic structure with doping in Bi2Sr2CaCu2O8+δ - Angle resolved photoemission results

Abstract We present experimental results showing that the electronic structure of the high Tc cuprate superconductors evolves from having large pieces of Fermi surface (FS) at high doping levels to small pieces of FS centered at (π/2,π/2) at low doping levels. The dramatic change in the FS occurs with only subtle changes in the electronic structure, which occur mostly near the (π,0) point of the Brillouin zone. We suggest that this evolution is intimately related to the evolution of the antiferromagnetic (AF) order with doping.

Anomalous superconducting state gap size versus behavior in underdoped

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap \Delta 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 \Delta and kT_c indicate that they represent different energy scales. The measurements also show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above T_c. The existence of a normal state gap is related to the failure of \Delta to scale with T_c in theoretical models that predict pairing without phase coherence above T_c.

Anomalous superconducting state gap size versus {ital T}{sub {ital c}} behavior in underdoped Bi{sub 2}Sr{sub 2}Ca{sub 1{minus}{ital x}}Dy{sub {ital x}}Cu{sub 2}O{sub 8+{delta}}

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap \Delta 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 \Delta and kT_c indicate that they represent different energy scales. The measurements also show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above T_c. The existence of a normal state gap is related to the failure of \Delta to scale with T_c in theoretical models that predict pairing without phase coherence above T_c.

ANOMALOUS SUPERCONDUCTING STATE GAP SIZE VERSUS TC BEHAVIOR IN UNDERDOPED BI2SR2CA1-XDYXCU2O8+DELTA

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap \Delta 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 \Delta and kT_c indicate that they represent different energy scales. The measurements also show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above T_c. The existence of a normal state gap is related to the failure of \Delta to scale with T_c in theoretical models that predict pairing without phase coherence above T_c.