THE BEHAVIOR OF THERMOPOWER IN THE YBA2-XLAXCU3OY SYSTEM - CORRELATION BETWEEN THE BAND PARAMETERS IN NORMAL-STATE AND CRITICAL-TEMPERATURE

The thermopower in the YBa2-xLaxCu3Oy system (x = 0 - 0.5) has been st udied. An analysis made in terms of the narrow-band model revealed tre nds in the variation of the main band parameters with increasing lanth anum concentration. The dependence of the conduction band width on lan thanum concentration was found to pass through a weak minimum at x = 0 .05 - 0.1, which correlates in position with the maximum in the concen tration dependences of the critical temperature and degree of sample o rthorhombicity. The results obtained and their comparison with Fe, Co- ->Cu substitution data permit a conclusion that the main factor affect ing the properties of the YBa2Cu3Oy system under nonisovalent substitu tion of various cations is the increasing disorder in the chain-oxygen subsystem caused by increasing impurity content. (C) 1998 American In stitute of Physics.