Variation inTcand carrier concentration in Tl-based superconductors

Abstract

$T_c$ variations observed in some Tl-based superconductors were investigated. Clear correlations were found between $T_c$, carrier concentration, and c-axis length. In particular, for ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CuO}}_6$, a decrease in oxygen content of about 0.1 per formula unit, which corresponded to a decrease in hole concentration of 0.2, increased $T_c$ up to about 80 K, and elongated the c axis by about 0.4%. The carrier concentration in that system could be decreased by substitution of La for Ba, which resulted in an increased $T_c$ value. $T_c$ variations caused by a change in oxygen content were also observed in ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$ and ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{10}$. It was demonstrated that superconductivity appears in a certain appropriate range of carrier concentration.