Electron-optical Phonon Coupling Research Articles

Electron-phonon coupling and superconductivity of hydrogen implanted aluminium

We present a theoretical investigation of the observed increase in the superconducting transition temperature T c in hydrogen implanted aluminium samples over its value in pure aluminium metal. Using the recent structural data obtained by electron microscopy, which did not show (within a 1% error) any increase in the lattice parameter for hydrogen loaded samples, we have evaluated the electron-phonon coupling constant λ within the McMillan approximation. Our theoretical prediction is based on an ab initio calculation of the electronic term η using our augmented plane wave (APW) determination of the energy bands, density of states and Fermi energy properties of aluminium dihydride. For aluminium dihydride, although the Fermi level does not cut the low lying H-H antibonding bands, the hydrogen s and p states have an important contribution which leads to a large value of the electronic term at the hydrogen sites η H . In the present calculation we find η H = 1.78 eV A ̊ −2 compared with 0.64 eV Å −2 in PdH; η H is even larger than the value previously obtained by one of us for AlH 2 using a large lattice constant. The electronic contribution at the aluminium site η Al decreases by 20% from its value in pure aluminium. Our results indicate that provided the optical phonon frequencies are not too high, the electron-optical phonon coupling mechanism should contribute to the increase in T c observed in aluminium hydrides.

Study of electron-phonon interaction and magneto-optical anomalies in two-dimensionally confined systems

This paper presents a study of the nature of electron-optical phonon coupling in two-dimensionally confined systems (e.g., inversion or accumulation layers in metal-oxide semiconductors, heterojunctions, and superlattices of polar semiconductors) and shows that magneto-optical anomalies in two dimensions provide a powerful tool for their investigation. Several models of electron-optical phonon coupling are studied. These include coupling with (i) three-dimensional optical phonon with fixed ${k}_{z}$, the wave vector normal to the two-dimensional plane of confinement, (ii) three-dimensional optical phonons with arbitrary ${k}_{z}$ due to the loss of wave-vector selection rules in finite structures, (iii) interface optical-phonon modes, and (iv) purely two-dimensional optical-phonon modes. Where appropriate, zone-folding effects are explicitly taken into account. Resonant magneto-optical absorption studies are shown to be a means of unambiguously probing the electron-phonon coupling in these novel materials. Within the framework of the Fr\ohlich model and including only the resonant self-energy diagram in the theory, it is shown that at resonance the relevant cyclotron peak splits into a doublet. The resonance effect is found to be much sharper in two dimensions compared to the bulk case, a consequence of the lack of ${k}_{z}$ continuum of electronic energy. For a given magnetic field of strength $B$, the resonant splitting is proportional to ${B}^{\frac{3}{4}}$ for two-dimensionally confined carriers, in contrast to the ${B}^{\frac{1}{2}}$ behavior in the bulk. The proportionality factor depends on the specific phonon mechanism considered. The spectral weights of the split peaks are evaluated. Allowing, phenomenologically, finite width of the resonant Landau levels, as well as for the Landau level from which optical transition originates, the expressions for optical absorption and Raman scattering are obtained. The significance of electron-light vertex correction is investigated and shown to be important under certain conditions. The possibility of a resonant splitting in two dimensions in direct intersubband transition without any magnetic field is also investigated.

Electron-phonon coupling and resonant magneto-phonon effect in optical behaviour of two-dimensionally confined charge carriers

Very recently enhanced electron-optical phonon coupling effects have been reported in the two-dimensionally confined carriers in GaAs/Al 1− x Ga x As superlattices and quantum wells. We report here the results of a study of several electron-optical phonon couplings possible in such systems. Where appropriate, zone-folding effects are explicitly taken into account. The relevance of some of these interactions to III–V compound semiconductor inversion/accumulation layers is noted. Certain special features of two-dimensional confinement in these systems make it possible to use resonant magneto—absorption studies as an unambiguous probe of these enhanced electron-phonon couplings. Results for the resonant splitting of the confined Landau level—optical phonon states are obtained within an approximation that retains only the resonant self-energy term of the upper Landau level. The resonant splitting is found to be proportional to B 3 4 for two-dimensionally confined carriers, in contrast to the B 2 3 behavior found in three-dimensional bulk semiconductors. Influence of damping and electron-light vertex correction is also discussed.