EXTRINSIC PHOTOCONDUCTIVITY IN FERROELECTRICS DUE TO SURFACE LAYERS V. M. FRIDKIN, A. A. GREKOV, E. A. SAVTCHENKO et T. R. VOLK J. Phys. Colloques, 33 C2 (1972) C2-127-C2-129 DOI: 10.1051/jphyscol:1972241