Persistent Photoconductivity in High-Tc Superconductors - ACS

Sep 2, 1999 - 1 Department of Physics, University of California at San Diego, 9500 Gilman Drive, La Jolla, ... 3 Universidad de Antioquia, Medellin, C...
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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