High-pressure phase relations of   Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals
  We have investigated the pressure dependence of the superconducting transition temperature $T_{c}$ up to 18 GPa of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals ranging from the highly underdoped through the nearly optimally doped to the highly overdoped level. For all three samples studied, $T_{c}$ is found to increase initially and then saturate at a critical pressure $P_{c}$ but decrease modestly with further compression. Oxygen doping tends to reduce the increase in $T_{c}$ and $P_{c}$. A new high-pressure phase diagram between the saturated $T_{c}$ and $P_{c}$ is then obtained. Theoretical interpretation is given by using the competition between the hole carrier density and pairing interaction strength based on the high-pressure transport data of the resistivity and Hall coefficient in this system. 
Xiao-Jia Chen, Viktor V. Struzhkin, Russell J. Hemley, Ho-kwang Mao,   and Chris Kendziora