We report topology of the Fermi surface of Bi 2 Sr 2 CaCu 2 O 81d superconducting system at the optimum doping determined by sequential angle-scanning photoemission combined with high intensity of synchrotron radiation. The Fermi surface at the optimum doping has a pseudogap around ͑p, 0͒ as in underdoped samples, showing missing segments near the M points, and shadow bands around ͑0.5p, 0.5p͒ and equivalent locations. The k dependence of the pseudogap shows a particular asymmetric topology

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... metric topology that imposes a strong constraint on its theoretical explanation. [S0031-9007 (97) 04603-6] PACS numbers: 71.18. + y, 74.72.Hs, 79.60. -i There is a gold rush to understand the normal state electronic properties of doped cuprates targeted to a proper model that can explain the high T c superconductivity. Angle resolved photoemission spectroscopy (ARPES) has been widely used to investigate the Fermi surface [1,2], however, the measurements by ARPES are limited by several factors including quality of the samples, their surface morphology and stability. Among the variety of cuprate superconductors, most of the ARPES studies have been made on Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) superconductor due to its suitability for the ARPES. A majority of the work has been addressed to the overdoped samples with a T c of 83-85 K [3-11] and underdoped samples with T c of 10-83 K [11] [12] [13] [14] where optimal doping corresponds to a T c of 91 K. The general agreement on the presence of a large hole-like Fermi surface centered around the X and Y points with an extended Van Hove singularity near M ͑p, 0͒ in the overdoped case has been reached [3] [4] [5] [6] [7] [8] [9] [10] [11] . Recently, it has been found that the underdoped samples [12] [13] [14] show an anomalous Fermi surface with partial gap (called pseudogap) near the ͑p, 0͒ locations. The gap disappears in the overdoped regime. However, the key question, whether the system at the optimum charge density giving the highest T c shows the pseudogap or not, remains open. The presence of the pseudogap is a relevant experimental point for models for the pairing interaction in high T c superconductors predicting (1) preformed pairs at T . T c , and (2) a strong coupling of electrons to the collective excitations. The constant initial energy angle scanned photoemission spectroscopy [15, 16] has been used to overcome a possible overlooking of important features on the Fermi surface in the standard ARPES method based on energy distribution curves (EDCs) [1] . This method has already been applied to determine the Fermi surface of an overdoped Bi2212 system using a laboratory ultraviolet radiation source [9,10], while the anomalous Fermi surface in the underdoped case has been investigated only by the EDC method. The present Letter is intended to report a global view of the Fermi surface features of the Bi2212 system at the optimum doping ͑T c 91 K͒ by the constant initial energy angle scanning photoemission method combined with the high intensity of the synchrotron radiation. The results show a Fermi surface having hot spots of high spectral intensity falling on the lines of k y 0.23n Å 21 along the ͑p, p͒ direction. The measurements were made on a single crystal of size 6 3 6 3 1 mm 3 , grown by the floating zone method showing negligible dislocations and deviation from the 2212 stoichiometry [17] . The crystal was well characterized for its transport, structural, and other characteristics showing a sharp superconducting transition (width 1 K) at the T c of 91 K and characteristic structural features of superconducting samples with superstructure due to the incommensurate modulations of both BiO and CuO 2 plane [18] . The ARPES measurements were carried out at the Laboratoire pour l'Utilisation du Rayonnement Electromagnetique (LURE) (Orsay-France) on the SU6 undulator 0031-9007͞97͞79(18)͞3467(4)$10.00