Effects of uniaxial stress on excitons inCu2O

Abstract

We have investigated the effects of uniaxial stress up to 2.5 kbar on the first four dipole-forbidden states of the yellow exciton series in cuprous oxide. The linear and nonlinear splittings and shifts as well as the symmetries of the stress-split components were determined using the quadrupole-dipole Raman-scattering technique involving odd-parity phonons. We have observed the triply degenerate $1S$ quadrupole state split into a singlet and a doublet with the singlet energy increasing and the doublet energy decreasing with stress as first seen by Gross and Kaplyanskii. For the higher quadrupole $S$ states, however, the sense of the splitting was reversed from that of the $1S$, while the magnitude of the $3S$ splitting exceeded that of the $1S$ as previously noted by Agekyan and Stepanov. The quadratic stress coefficients for all components of the $1S$, $3D$, and $4S$ states were found to be equal while those for the $3S$ state were markedly different. A theoretical analysis was carried out for [001] stress using the effective-Hamiltonian formalism including both deformation of the bands and electron-hole exchange. The theory accounts satisfactorily for the general features of the $1S$, $3D$, and $4S$ states only if both the magnitude and sign of the exchange constant are considered to be dependent on the exciton state. For the $3S$, however, the nonlinear stress dependence cannot be reconciled with the theory for yellow excitons. These results suggest that the "$3S$ yellow exciton" may actually belong to the green-exciton series, or that interactions between yellow and green excitons with different principal quantum numbers may be significant although not included in the theory.