Chapter 15
Persistent Photoconductivity in High-T Superconductors
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Axel Hoffmann , Z . F. Ren , J . Y . Lao , J . H . Wang , D. Girata , W. Lopera , P. Prieto , and Ivan K. Schuller 4
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Downloaded by PENNSYLVANIA STATE UNIV on September 13, 2012 | http://pubs.acs.org Publication Date: September 2, 1999 | doi: 10.1021/bk-1999-0730.ch015
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Department of Physics, University of California at San Diego, 9500 Gilman Drive, La Jolla, C A 92093-0319 State University of New York at Buffalo, Buffalo, NY 14260-3000 Universidad de Antioquia, Medellin, Colombia Universidad del Valle, Cali, Colombia 2
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Persistent photoconductivity is an interesting and unusual property of high T superconductors. Underdoped YBa Cu O shows upon illu mination a decrease of resistivity and an increase of T . These photoinduced changes are persistent at low temperatures and relax at room temperatures. A possible model for persistent photoconductivity in these materials is that electrons of photoinduced electron-hole pairs are trapped at localized states spatially separated from the conduction layer. We recently measured persistent photoconductivity in Tl Ba CuO and Y-doped Bi Sr CaCu , which indicates that persistent photocon ductivity might be a common phenomena in high-T superconductors. c
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One of the interesting features of the high-T cuprates is, that their superconducting and normal state properties can be changed by varying their carrier concentration. Typically this is accomplished by chemical doping, where either one cation is substi tuted by another one (e.g. Sr for La in La2-xSr Cu04), or the oxygen-stoichiometry is varied (e.g. YBa2Cu3 0), they were annealed at 500° C following pressure-temperature curves for a specific stoichiometry of the phase diagram (5). After annealing the oxygen content was determined by the c-axis pa rameter (6) measured by high resolution X-ray diffraction and by T (7) for the super conducting films. The resistivity measurements were done with a four probe technique, with the sample either directly immersed in liquid nitrogen, in a close cycle refrigerator, or in a He-flow cryostat, each equipped with optical quartz windows. For the illumination several different light sources were used. Most of the excitation measurements were done using a halogen lamp or an Ar laser (λ = 514 nm), with a typical power density at the sample surface of 1 Wcm" . For measuring spectral dependencies a 1000 W Hg-Xe arc lamp was used with an infrared water filter to protect the sample and the optics from excessive heating. Specific wavelengths in the range 250-900 nm were chosen with interference band-pass-filters having a bandwidth of 10 nm. Using this setup the power density of monochromatic light ranged between 0.04 and 5 mWcm" . After each light excitation the sample was relaxed at room temperature while moni toring the resistivity to determine when the sample was fully relaxed. c
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Basic Effect The basic effect of illumination on the resistivity and T is shown in Figure 2 for an underdoped GdBa2Cu3
