Absence of a pseudogap in the in-plane infrared response of Bi2Sr2CaCu2O(8+d)

The ab-plane reflectance of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) thin films was measured in the 30–25000 cm − 1 range for one underdoped ( T c = 70 K), and one overdoped sample ( T c = 63 K) as a function of temperature (10–300 K). We find qualitatively similar behaviors in the temperature dependence of the normal-state infrared response of both samples. Above T c , the effective spectral weight, obtained from the integrated conductivity, does not decrease when T decreases, so that no opening of an optical pseudogap is seen. We suggest that these are consequences of the pseudogap opening first in the k = (0 , π ) direction, according to ARPES, and of the in-plane infrared conductivity being mostly sensitive to the k = ( π, π ) direction.