It is shown that the ordinary semiclassical theory of the absorption of light by exciton states is not completely satisfactory (in contrast to the case of absorption due to interband transitions). A more complete theory is developed. It is shown that excitons are approximate bosons, and, in interaction with the electromagnetic field, the exciton field plays the role of the classical polarization field. The eigenstates of the system of crystal and radiation field are mixtures of photons and excitons. The ordinary one-quantum optical lifetime of an excitation is infinite. Absorption occurs only when "three-body" processes are introduced. The theory includes "local field" effects, leading to the Lorentz local field correction when it is applicable. A Smakula equation for the oscillator strength in terms of the integrated absorption constant is derived.

a Quantum-Mechanical Theory of the Contribution of Excitons to the Complex Dielectric Constant of Crystals.

We review the methods which have been developed over the past several years to determine the behavior of solids whose properties vary randomly at the microscopic level, with principal attention to systems having composition variation on a well-defined structure (random "alloys"). We begin with a survey of the various elementary excitations and put the dynamics of electrons, phonons, magnons, and excitons into one common descriptive Hamiltonian; we then review the use of double-time thermodynamic Green's functions to determine the experimental properties of systems. Next we discuss these aspects of the problem which derive from the statistical specification of the microscopic parameters; we examine what information can and cannot be obtained from averaged Green's functions. The central portion of the review concerns methods for calculating the averaged Green's function to successively better approximation, including various self-consistent methods, and higher-order cluster effects. The last part of the review presents a comparison of theory with the experimental results of a variety of properties---optical, electronic, magnetic, and neutron scattering. An epilogue calls attention to the similarity between these problems and those of other fields where random material heterogeneity has played an essential role.

The theory and properties of randomly disordered crystals and related physical systems

Optical properties of highly excited direct gap semiconductors

The many theoretical studies of the various magnetic, thermal and magnetoelastic properties for paramagnetic and ferromagnetic transition metals and alloys in the simple itinerant electron model are reviewed. The important amendments to the simple itinerant electron model or Stoner model of magnetism due to the spin wave excitations and spin fluctuations are explained.

https://iopscience.iop.org/article/10.1088/0034-4885/44/4/001/pdf

Itinerant electron magnetism

Condensation effects of excitons

Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width, \(\varDelta\varOmega_{1}\), of excitation light. In the case of \(\varDelta\varOmega_{1}{=}0.25\) meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of \(\varDelta\varOmega_{1}{=}2.3\) meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.

Radiative and Two-Photon Resonance Raman Processes Associated with Excitonic Molecules in CuCl

Through the studies of excitation spectra of emissions in CuCl crystals by a frequency tunable dye laser, excitonic molecules have been found to be generated directly by the giant two-photon absorption at the photon energy of 3.187 eV, as suggested by Hanamura. Two-photon excitation of crystals at 1.6 K enhanced very narrow emission bands at 3.1647 eV and at 3.1709 eV. These photon energies coincide with those of the high energy edges of the M L and M T bands due to the radiative annihilation of excitonic molecules having K =0. From these facts the Bose condensation of excitonic molecules at K ∼0 has been suggested.

Generation of Excitonic Molecules by Giant Two-Photon Absorption in CuCl and Their Bose Condensation

The Bose Condensation of Excitonic Molecules in CuCl Crystals

Giant two-photon absorption for the direct generation of excitonic molecue is measured. Three absorption peaks are found at 2.9540, 2.9561 and 2.9576 eV, which are responsible for the two-photon transitions to the \(\varGamma_{1}(J{=}0)\), Γ 5 ( J =2) and \(\varGamma_{3}(J{=}2)\) states, respectively, of the excitonic molecule.

Nobukata Nagasawa

Papers

Radiative and Two-Photon Resonance Raman Processes Associated with Excitonic Molecules in CuCl

Generation of Excitonic Molecules by Giant Two-Photon Absorption in CuCl and Their Bose Condensation

The Bose Condensation of Excitonic Molecules in CuCl Crystals

Energy Level Scheme of Excitonic Molecule and Exciton in CuBr

Light mixing induced by the giant two-photon absorption of excitonic molecules in CuCl