Electronic compressibility and charge imbalance relaxation in cuprate superconductors

In the material SmLa _{1- x}Sr _xCuO _{4- δ} with alternating intrinsic Josephson junctions we explain theoretically the relative amplitude of the two plasma peaks in transmission by taking into account the spatial dispersion of the Josephson Plasma Resonance in c-direction due to charge coupling. From this and the magnetic field dependence of the plasma peaks in the vortex solid and liquid states it is shown that the electronic compressibility of the CuO ₂ layers is consistent with a free electron value. Also the London penetration depth λ_ab≈1100 Å near T_c can be determined. The voltage response in the IV-curve of a Bi ₂Sr ₂CaCu ₂O ₈ mesa due to microwave irradiation or current injection in a second mesa is related to the nonequilibrium charge imbalance of quasiparticles and Cooper pairs and from our experimental data the relaxation time ∼100 ps is obtained.