Photoemission spectromicroscopy study of aBi2Sr2CaCu2

Abstract

We performed a photoemission microscopy study on cleaved surfaces of a Bi 2 Sr 2 CaCu 2 O 8ϩ␦ single crystal. Our findings show that pronounced inhomogeneity effects exist even on good quality single crystals. The length scale of the observed features ranges from a few microns to a few hundred microns. We discuss the importance of our results for the interpretation of conventional photoemission experiments on Bi 2 Sr 2 CaCu 2 O 8ϩ␦ which typically probe an area of the order of 1 mm 2 .Shortly after the discovery of high-T c superconductors, 1 it was recognized that the surface of these compounds is inhomogeneous. A high-resolution electron-energy-loss spectroscopy investigation, with a spatial resolution of the order of 50 m, demonstrated that both superconducting and nonsuperconducting areas coexist on the surface of cleaved YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 single crystals. 2 Inhomogeneities of the chemical composition were reported by scanning Auger microscopy studies of YBa 2 Cu 3 O 7Ϫy surfaces 3 and by photoemission electron microscope investigations of HgBa 2 Ca 2 Cu 3 O 8ϩ␦ and HgBa 2 CaCu 2 O 6ϩ␦ ͑Ref. 4͒ and single crystals of Bi 2 Sr 2 CaCu 2 O 8ϩ␦ . 5 More recently, the importance of the availability of spatially resolving techniques has been demonstrated by a scanning photoemission microscopy study of Ba 1Ϫx K x BiO 3Ϫy single crystals. 6 At the Spectromicroscopy beamline of ELETTRA, 7 we performed a photoemission microscopy study on the cleaved surface of a Bi 2 Sr 2 CaCu 2 O 8ϩ␦ single crystal. This technique uses the strength of the photoemission spectroscopic analysis, which probes the single-particle density of states. Here we put particular attention to the region close to the Fermi edge which is the important one for the superconductivity. This allows one to detect, with a submicron spatial resolution, variations of the electronic structure which are more subtle than topographical or chemical composition inhomogeneities and which would therefore be undetectable with other microscopy techniques.The oxygen doping of the Bi 2 Sr 2 CaCu 2 O 8ϩ␦ sample was close to optimum (T c ϭ91 K). More details about the sample and its characterization by low-energy electron diffraction ͑LEED͒ and angle-resolved photoelectron spectroscopy ͑ARPES͒ can be found in Ref. 8. The sample was cleaved and measured at room temperature. The photon energy used was 95 eV. The energy of the photoelectrons was analyzed by a hemispherical analyzer with a 16-channel detector ͑the difference in energy between adjacent channels is 8% of the analyzer's pass energy͒. The images we present here are obtained by taking the counts of a single channel and are therefore associated to a given energy, or the sum of a few channels and are therefore associated to a given energy range. The photoelectrons were detected in the horizontal plane at 30°grazing angle from the sample surface with an acceptance of Ϯ5°. At the kinetic energy of the valence band, this gives k ʈ ϭ4.2Ϯ0.2 Å Ϫ1 . By comparison with the size of the Brillouin zone (⌫X...

Characterization of Surface Chemical States of a Thick Insulator: Chemical State Imaging on MgO Surface

“…Normally, a spectromicroscopy technique is used to map the distribution of the composition or chemical bonding on the sample surface under investigation. [3][4][5] Even with its limitations in accuracy and the amount of obtainable information, the spectromicroscopy technique is still useful for examining a thin insulating layer if it is prepared on a conducting layer. For a thick insulator, the peak shift reaches 100-300 electron volts and no meaningful surface information can be extracted from the obtained spectrum.…”