HOLE FILLING IN A NARROW CONDUCTION-BAND IN YBA2CU3-XCOXO7-DELTA(X-LESS-THAN-OR-EQUAL-TO-0.3)

Measurements of the temperature dependence of the absolute thermopower (S) on samples of YBa2CU3-xCox07-delta with 0 less-than-or-equal-to x less-than-or-equal-to 0.3 and delta<0.05 show that, between 100-200 K , S has a maximum that depends on x, according to the relation S(m)(x) almost-equal-to (k(B)/\e\)ln[(1+x)/(1-x)]. This is the expected satur ation value of S, for noninteracting carriers in a narrow conduction b and with x doped electrons above half-filling. The temperature depende nce of the resistivity (p) exhibits a crossover from wide-band to narr ow-band behavior, in a conductor with a mean free path limited by impu rities. The drop of S at higher temperatures, accompanied by a drop in drho/dT, can be attributed to excitation of electrons from the conduc tion band to higher-energy levels associated with the dopant, or to th e contribution of activated hopping to the transport coefficients.