Upper critical field and irreversibility field of over-doped Tl2Ba2CuO6+delta superconductors

We have carried out magnetic measurements and transmission electron microsc opy (TEM) study on over-doped Tl2Ba2CuO6+delta (Tl-2201) single crystal sam ples with a range of different oxygen content. Magnetic measurements show t he existence of an irreversibility line that is considerably lower than the upper critical field B-c2 and displays a positive upward curvature. We pro pose that the critical field determined from magnetoresistive transitions f or strongly over-doped sample is not the true upper critical field but the irreversibility field instead. TEM study has shown that phase separation du e to oxygen inhomogeneity is presented in the strongly overdoped samples th at have very low T-c value and exhibit unusual temperature dependence of th e upper critical field.