Improved experimental conditions enabled us to increase the signal-to-noise ratio of the photoemission spectra for the superconducting state of Bi2Ca2SrCu20s, taken with high angular and energy resolution. This also enabled us to reveal a pronounced minimum that separates the two basic features of the spectrum, the narrow quasiparticle excitation peak and the still controversial broad band at lower kinetic energies. The minimum is approximately 3A below the Fermi level.PACS numbers: 74.70.Vy, 79.60.Cn High-resolution photoemission spectroscopy has recently emerged as a fundamental probe of the superconducting state for high-temperature superconductivity [1], Early experiments clearly established the presence of a gap in the electronic spectrum [2][3][4][5]. The spectrum in the superconducting state is dominated, of course, by the narrow peak corresponding to quasiparticle excitations [2,6]. This peak, however, is accompanied by a broad excitation band at lower kinetic energies, whose nature has not been clarified and whose relevance to the mechanism of high-temperature superconductivity has been emphasized by several authors [7][8][9].We carefully analyzed the spectrum with angular and energy resolution similar to or better than those of previous high-resolution studies-but with enhanced signalto-noise ratio. This enabled us to clearly reveal an additional feature that appears important for the general interpretation of the spectra and of the corresponding superconducting state: a pronounced minimum that separates the broad band from the narrow peak. The minimum could not be easily seen in previous studies because of the relatively large noise, and therefore its position could not be determined. We found that the position is close to 3A below the Fermi level, where A is the gap parameter (i.e., 2A is the gap). A broad-band threshold at 3A is predicted, for example, by Littlewood and Varma [7,8] based on the "marginal-Fermi-liquid" model, by Miiller, Arnold, and Swithart [10], and by other theorists [11].Our experimental procedure was a standard synchrotron-radiation photoemission approach, enhanced in order to achieve high angular and energy resolution with a high signal-to-noise level; the results are, to the best of our knowledge, state of the art in solid-state photoemission spectroscopy. The main factors for simultaneously reaching high resolution and high signal-to-noise levels were as follows: the use of very high-quality single crystals of Bi2Ca2SrCu2C>8 cleaved in situ under ultrahigh vacuum, whose structural and transport properties were carefully characterized; the use of a 4-m normal-incidence photon monochromator with resolving power up to 4.3 x 10 3 ; the use of a carefully magnetically shielded high-resolution VSW electron analyzer; and the use of synchrotron radiation.In order to test reproducibility, the experiments were performed on twenty different samples from different batches. In each case, many different points of the Brillouin zone were explored by varying the photoelectron collection ge...

Articles you may be interested inEvidence for field enhanced electron capture by EL2 centers in semiinsulating GaAs and the effect on GaAs radiation detectors A. S. EPSTEIN with the rate of cooling in the above region. Silver selenide powders 14 were compacted using 0.05 g/addition and a pressure of 70000 Ib/in. 2 • The disks were heated at a rate of ",3 deg/min and sintered at 200°C for 15 min. Sample temperatures were monitored using an Land N recorder which recorded the voltages of the thermocouples inserted in the sample. The samples were cooled at various rates. It was noted that the most important region to control in the cooling curve in order to directly influence the figure of merit was where (3 phase grain growth occurred, i.e., the temperature 14 The silver/selenium ratio was > 2.0 and between 2.0 and 2.28. Different batches of silver selenide may have slightly differen t ratios. range mentioned above. The results of our investigation are shown in Fig. 5. The abscissa in Fig. 5 is actually plotted in terms of the time to cool from the supercooled phase transition temperature (117°C) to 93°C. This is the cooling in the (32 phase and is the region of (32 phase grain growth. It is noted from Fig. 5 that a maximum in figure of merit appears to occur around 7 min. Extremely slow or fast cooling rates give figures of merit at room temperature of "'2XlO~3 per degree. ACKNOWLEDGMENT

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