PRESSURE-DEPENDENCE OF CHARGE-TRANSFER-TYPE GAPS IN CUO2 LAYERED COMPOUNDS

Pressure dependence of the electronic gap has been investigated for insulating analogs of high-T(c) superconductors, the so-called T (La2CuO4), T* (LaTbCuO4), and T' (Eu2CuO4) -phase compounds with single-CuO2-layered structures. With application of hydrostatic pressure, the optical gap energy was observed to increase for the T'- and T*-phase compounds and decrease for the T-phase compound. The results can be explained in terms of two competing contributions to the charge-transfer-type gap, i.e., Madelung site potentials and electron hopping energies which respectively tend to increase and decrease the gap energy under high pressures.