Thallium barium calcium copper oxide
Thallium barium calcium copper oxide, or TBCCO (pronounced "tibco"), is a family of high-temperature superconductors having the generalized chemical formula TlBaCaCuO. TlBaCaCuO (TBCCO-2223) was discovered in Prof. Allen M. Hermann's laboratory in the physics department of the University of Arkansas in October 1987 by the post-doctoral researcher Zhengzhi Sheng and Prof. Hermann. The bulk superconductivity in this material was confirmed by observations of magnetic flux expulsion and flux trapping signals (under zero field cooled and field cooled conditions) with a SQUID magnetometer in the superconductor laboratory of Timir Datta in the University  of South Carolina. Allen Hermann announced his discovery and the critical temperature of 127 K, in Houston, Texas at the World Congress on Superconductivity organized by Paul Chu in February 1988. The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBaCaCuO, whereas the second series containing two Tl–O layers has a formula of TlBaCaCuO with "n" =1, 2 and 3. In the structure of TlBaCuO (Tl-2201), there is one CuO layer with the stacking sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In  TlBaCaCuO (Tl-2212), there are two Cu–O layers with a Ca layer in between. Similar to the TlBaCuO structure, Tl–O layers are present outside the Ba–O layers. In TlBaCaCuO (Tl-2223), there are three CuO layers enclosing Ca layers between each of these. In Tl-based superconductors, "T" is found to increase with the increase in CuO layers. However, the value of "T" decreases after four CuO layers in TlBaCaCuO, and in the TlBaCaCuO compound, it decreases after three CuO layers.