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We study the relaxation of the exciton spin (longitudinal relaxation time $T_{1}$) in single asymmetrical quantum dots due to an interplay of the short--range exchange interaction and acoustic phonon deformation. The calculated relaxation rates are found to depend strongly on the dot size, magnetic field and temperature. For typical quantum dots and temperatures below 100 K, the zero--magnetic field relaxation times are long compared to the exciton lifetime, yet they are strongly reduced in high magnetic fields. We discuss explicitly quantum dots based on (In,Ga)As and (Cd,Zn)Se semiconductor compounds.Comment: accepted for Phys. Rev.

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The photovoltaic current density in crystals without a center of symmetry depends bilinearly on the components of the light polarization vector via a third-rank tensor. For LiNb0,:Fe and LiNbO,: Cu the tensor properties are confirmed and three of the four non-vanishing components determined. The dependence on impurity concentration and on photon energy is also investigated.