Sang Don Choi

Theory of cyclotron resonance lineshape for electron-phonon systems in two coupling schemes

Abstract A theory of cyclotron resonance lineshape due to electron-phonon interactions is presented on the basis of the projection operator method introduced by Argyres and Sigel. In addition to the extremely weak coupling scheme dealt with earlier, the moderately weak coupling scheme is included in the theory. The lineshape functions derived in the two schemes are similar to Lodder-Fujitas formula, and are reduced to Kawabatas formula in the elastic scattering approximation.

Derivation of nonlinear optical conductivity by using a reduction identity and a state-dependent projection method

A new nonlinear optical conductivity formula for a system of electrons interacting with phonons was derived using a reduction identity and a state-dependent projection technique introduced by the authors. The results include a general formula for the nonlinear optical conductivity of the general rank and the linear, first-order nonlinear and second-order nonlinear conductivity are calculated in terms of the linewidth. The linewidth term includes the electron and phonon distribution functions properly. Therefore, it is possible to explain the phonon emission and absorption in all electron transition processes in an organized manner.

Critique of the theories of cyclotron resonance lineshape

Abstract Argyres-Sigels critique of the theories of cyclotron resonance lineshape obtained by Kawabata and by Lodder and Fujita is reviewed. All the higher order terms in the expansion of the lineshape functions are proved to be finite at the resonance peaks as well as in the wings of the absorption lines, implying that the critique is unreasonable.

Derivation of linewidths for optical transitions in quantum wells due to longitudinal optical phonon scattering

Utilizing the quantum statistical operator algebra technique, we derive a new absorption coefficient formula for the optical transition in a system of electrons interacting with longitudinal optical phonons in a quantum well. The electron and phonon distribution functions are included in the lineshape function. So we can explain the phonon absorptions and emissions in an organized way for all the electron transition processes. The numerical results show that the width decreases with the well width and increases with the temperature.

Derivation of second-order nonlinear optical conductivity by the projection-diagram method

A projection-diagram method is introduced for optical conductivity with lineshape functions, which takes into account the population criterion that the electron and phonon distribution functions are multiplicatively combined along with the energy conservation factors for proper interpretation of emission and absorption of phonons and photons in all the processes of electron transitions. It is further shown that the second order nonlinear optical conductivity of the system of electrons interacting with phonons, obtained using this method, is identical with that derived by using the state dependent projectors and the KC reduction identities [J. Phys. A: Math. Theor. 43, 165203 (2010)]. We expect that this method can reduce the amount of many-body calculation and can be of help in providing physical intuition into solid state quantum dynamics and representing perturbation expressions for such systems.

Optical Transition Linewidths due to Piezoelectric Phonon Scattering in Two-Dimensional Electron Systems

Utilizing the optical transition formula derived by a projection method on the linear response scheme, we obtain the optical transition linewidths for the electron system in square well due to piezoelectric phonon scattering in CdS and ZnS. We find that the widths increase with the temperature, but decrease as the well width increases for both materials. We also find that the widths decrease as the electron density increases, and the widths in CdS are larger than those in ZnS since the average electromechanical coupling constant is larger.

Scattering effects of phonons in two polymorphic structures of gallium nitride

Effects of piezoelectric and longitudinal optical (LO) phonon scatterings on transport of electrons confined in quasi-two-dimensional square wells of wurtzite and zinc-blende structures are compared by using a theory of absorption power derived in the linear response scheme. We find for GaN that the absorption power for both wurtzite and zinc-blende structures is keenly affected by the screening in such a way that the power increases, but the half width decreases as the electron density increases, and the piezoelectric phonon scattering is affected by the screening more than the optical phonon scattering. We also find that the piezoelectric phonon scattering (LO phonon scattering) is dominant at high (low) density and low (high) temperature in the wurtzite structure, whereas the tendency is reverse for the zinc-blende structure.

Calculation of Dilational and Uniaxial Shear Potentials of Ge by a State-Independent Many-Body Projection Technique

Utilizing state-independent projection operators, we present a new optical conductivity formula for cyclotron transition in the system of electrons interacting anisotropically with phonons. The line-shape factor appearing in the conductivity tensor contains the many body effects for electrons and phonons. Applying this formula, we determine the two deformation potentials (dilation potential Ξ d and uniaxial shear potential Ξ u ) of Ge in the quantum limit. By fitting the present theoretical values with the experimental data of Murase, Enjouji and Otsuka [J. Phys. Soc. Jpn. 29 (1970) 1248] and Kobori, Ohyama and Otsuka [J. Phys. Soc. Jpn. 59 (1990) 2141], we obtain Ξ u =17.0±0.6 eV and Ξ d =-10.88±0.47 eV.

Magnetic field dependence of cyclotron resonance linewidths in Ge and Si by a projection technique

The magnetic field dependence of cyclotron resonance linewidths is examined theoretically on the basis of the projection technique. It is shown that in the quantum limit of pure Ge and Si the widths increase with the magnetic field, and the result for Ge agrees quite well with the existing experimental data.

Theory of phonon-modulated electron spin relaxation time based on the projection-reduction method

This paper introduces a new method for a formula for electron spin relaxation time of a system of electrons interacting with phonons through phonon-modulated spin—orbit coupling using the projection-reduction method. The phonon absorption and emission processes as well as the photon absorption and emission processes in all electron transition processes can be explained in an organized manner, and the result can be represented in a diagram that can provide intuition for the quantum dynamics of electrons in a solid. The temperature (T) dependence of electron spin relaxation times (T1) in silicon is T1 ∝ T−1.07 at low temperatures and T1 ∝ T−3.3 at high temperatures for acoustic deformation constant Pad = 1.4 × 107 eV and optical deformation constant Pod = 4.0 × 1017 eV/m. This means that electrons are scattered by the acoustic deformation phonons at low temperatures and optical deformation phonons at high temperatures, respectively. The magnetic field (B) dependence of the relaxation times is T1 ∝ B−2.7 at 100 K and T1 ∝ B−2.3 at 150 K, which nearly agree with the result of Yafet, T1 ∝ B−3.0 ~ B−2.5.