The superconducting properties of YBa2Cu4O8 single crystals under hydrostatic pressure (up to 9.5 GPa)

Abstract d T c /d P of YBa 2 Cu 4 O 8 single crystals has been measured up to 9 GPa under hydrostatic pressure. The large and positive initial value (d T c /d P ) 0 =5.5 K/Gpa is confirmed. T c ( P ) saturates above 5 GPa. Dramatic changes, in the T c ( P ) behavior are observed when the pressure is non-hydrostatic. They are caused by the twinning of the crystals. Opposite to the case of YBa 2 Cu 3 O 7 , the results for hydrostatic pressure are well reproducible. This is probably due to the well defined oxygen stoichiometry of YBa 2 Cu 4 O 8 single crystals.