David Ko - Academia.edu (original) (raw)
Papers by David Ko
Social Science Research Network, 2023
Physics and Chemistry of Materials with Low-Dimensional Structures, 1992
ABSTRACT Following the recent interest in path integral approaches to studying the properties of ... more ABSTRACT Following the recent interest in path integral approaches to studying the properties of low-dimensional quantum magnets we present an examination of the mean field properties of a two-dimensional quantum magnet, together with a detailed analysis of the validity of mean field theory in determining the phase diagram. We find that there are serious restrictions to the validity of the mean field approximation in these systems, the most important of which is an uncontrolled shifting of the zero of the free energy with respect to local spin rotation. We present a formalism which includes fluctuations beyond the mean field limit so that the problems may, in principle, be dealt with, and the effects of fluctuations on the mean field solutions may also be analysed.
International Conference on Science and Technology of Synthetic Metals, 1994
ABSTRACT form only given. Electron-electron interaction has often been considered to play an impo... more ABSTRACT form only given. Electron-electron interaction has often been considered to play an important role in the electronic properties of the BEDT-TTF salts, and the large difference between the transport and the cyclotron effective masses has often been used as an indication of a Hubbard model type enhancement. Using a Gutzwiller approximation for the Hubbard model in two dimensions, we have examined the behaviour of the effective mass enhancement as a function of electron density n and the effective electron-electron interaction U/t. Our results show that qualitatively, the effective mass has initially a strong decrease with decreasing effective electron-electron repulsion which levels off to a slower rate of change, qualitatively consistent with observations of the mass dependence with pressure in (BEDT-TTF)/sub 2/Cu(NCS)/sub 2/. The effects of further correlations in the approximations may give better quantitative behaviour, and will be discussed.
Physical Review B, 1994
Effects of vacancies in the triangular spin-1/2 antiferromagnet. CL O'Donnell and David ... more Effects of vacancies in the triangular spin-1/2 antiferromagnet. CL O'Donnell and David YK Ko Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, England. Received 21 March 1994 We examine the effects ...
Physical Review B, 1994
Deposition growth modes from numerical simulations. David YK Ko and Flavio Seno Department of Phy... more Deposition growth modes from numerical simulations. David YK Ko and Flavio Seno Department of Physics, University of Oxford 1 Keble Road, Oxford, OX1 3NP, United Kingdom. Received 15 August 1994 We present results ...
Synthetic Metals, 1993
We present results from an ab initio density functional calculation of the electron-vibronic coup... more We present results from an ab initio density functional calculation of the electron-vibronic coupling strengths in neutral and charged bis(ethylenedithio)tetrathiafulvalene (BEDT-TI'F) molecules. Results for tetrathiafulvalene (T'IT) are also presented and compared with known experimental values as a measure of the accuracy of our calculation. We find that the highest occupied molecular orbital (HOMO) in ET couples strongly to modes involving the double-bonded carbon atoms as well as the sulfur atoms and that the contribution of the vibration modes involving the hydrogen atoms to the vibronic coupling is negligible. The implication of this result for the mechanism of superconductivity is discussed in the light of recent isotope effect experimental results.
Semiconductor Science and Technology, 1987
ABSTRACT The authors have used an empirical pseudopotential method to study the pressure dependen... more ABSTRACT The authors have used an empirical pseudopotential method to study the pressure dependence of the electronic states at the GaAs/AlGaAs heterojunction. Application to tunnelling current studies, with particular reference to experiments by Eaves and co-workers (1986), is made. They find that the conduction band Gamma -X crossover produces no significant effect. Non-parabolicity effects are found to be important. They also find that the calculated results differed significantly from the experimental data, indicating the presence of some other factors they have not included.
Semiconductor Science and Technology, 1988
ABSTRACT The authors present the results on the band-structure effects in single-barrier tunnelli... more ABSTRACT The authors present the results on the band-structure effects in single-barrier tunnelling, calculated using a new scattering matrix method. Their results show that the transmission amplitude may be divided into two regions. The first is for electron energies in the band gap of the barrier material, where the transmission is found to be a single state property, with the Gamma state dominant. The appropriate barrier height, regardless of the direct/indirect nature of the barrier band gap, is the Gamma - Gamma band offset. The tunnelling behaviour is therefore single state like. The other regime is for electrons with energies above the barrier material's conduction band edge where, for indirect band-gap barriers, a sharp transition to an X dominated behaviour is observed. Coupling between the barrier Gamma and the X states are found in thin barriers and X well resonances are obtained. The single barrier transmission in this regime is therefore a multistate property.
Journal of the Optical Society of America A, 1988
ABSTRACT We present a new scattering matrix formalism for the modeling of electromagnetic wave pr... more ABSTRACT We present a new scattering matrix formalism for the modeling of electromagnetic wave propagation in stratified media. It is computationally efficient and stable and is well suited to the layer geometry that is characteristic of stratified materials. It is applied successfully to the modeling of total attenuated reflection in nematic liquid crystals with beyond-critical-angle incidence when the conventional transfer matrix methods normally fail.
Journal of Physics: Condensed Matter, 1993
ABSTRACT Rare earth metals exhibit a rich spectrum of bulk magnetic order. When they are multilay... more ABSTRACT Rare earth metals exhibit a rich spectrum of bulk magnetic order. When they are multilayered, these metals offer the possibility of producing a wide variety of tunable magnetic properties. The authors present an analysis of the magnetic excitations appropriate to the ferromagnetic, spiral and antiferromagnetic phases of such multilayers. They find that the interfaces play a very important role in determining the ground state of the system, and systems with identical spin equilibrium configurations can also have significantly different spin excitations, suggesting corresponding differences in their thermodynamic properties.
Journal of Physics: Condensed Matter, 1992
ABSTRACT The authors study the electronic structure of the GaAs/AlAs Fibonacci superlattice using... more ABSTRACT The authors study the electronic structure of the GaAs/AlAs Fibonacci superlattice using a semi-empirical sp3s* tight-binding method. They find that a self-similar energy spectrum can be seen in the band structure, although the energy spectrum depends strongly on the wavevector parallel to the layers. Furthermore, they find that a localization character is enhanced due to the band hybridization, producing a spiky density of states and a localization-like effect of the wavefunctions in the hybridized energy region. The reasons for the localization-like behaviour are discussed briefly.
Journal of Physics A: Mathematical and General, 1992
ABSTRACT The authors consider one-dimensional Ising systems of length L with an interaction cut-o... more ABSTRACT The authors consider one-dimensional Ising systems of length L with an interaction cut-off R(L) in the limit R to infinity . They present free energy arguments for order at low temperatures provided R(L))(A In L)1 kappa /, where kappa is a new exponent; this opens the possibility of one-dimensional pseudo-order in macroscopic systems.
Journal of Applied Physics, 1989
The trends in the pressure dependence of the electronic transmission in single- and multiple-barr... more The trends in the pressure dependence of the electronic transmission in single- and multiple-barrier GaAs/AlGaAs heterostructures are calculated using a microscopic pseudopotential method. Electric effects are also included in our model. We find that in single-barrier systems the pressure induced direct to indirect band-gap transition alone does not affect the electron tunneling properties. It is only when the barrier’s X minimum is lowered below the electron energy that significant changes are found in the electron transmission. In a multiple-barrier system the lowering of the barrier X minimum due to the applied pressure and bias effectively pushes up the Γ resonant levels in the GaAs such that a clear negative differential resistance signal is lost.
ABSTRACT Available from UMI in association with The British Library. This thesis presents the res... more ABSTRACT Available from UMI in association with The British Library. This thesis presents the results of a microscopic calculation of the electron tunnelling properties in finite semiconductor heterostructure systems. Empirical pseudopotentials are used to incorporate the microscopic crystal potential and a scattering matrix method is developed to solve the electron wave function through the system. The effects of the inherent bandstructure on the tunnelling characteristics are examined in some detail. Non-parabolicity in the Gamma state bandstructure in the bandgap region alone is found to produce significant deviations in the tunnelling current and resonant energies from the simplistic effective mass approximation. Many-band effects are found to be important for indirect bandgap barriers where the electron energy lies above the barriers' X minima. The situation in the purely tunnelling regime is dominated by the Gamma state, regardless of the nature of the bandgap. Finite electric fields in large systems are considered. Gamma-X real space intervalley transfers are found when the electron energy traverses the conduction band edge of a material. The total wave function is in general complicated due to the highly complex mixing of the different states' contributions. The effects of the bandstructure changes on the tunnelling properties in single and multiple barrier systems due to the application of hydrostatic pressure are also examined. The results show that in single barriers the direct to indirect bandgap transition alone does not modify significantly the tunnelling property but deviations from the single -band behaviour are found when the conduction band edge is lowered below the electron energy. In multiple barrier systems the lowering of the barrier's X minima effectively pushes up the Gamma resonant level in wells such that a clear negative differential resistance signal would be lost.
Journal de Physique I, 1993
Physical Review B, 1993
The results of a fully ab initio determination of the vibronic coupling strengths in C6o and C6o ... more The results of a fully ab initio determination of the vibronic coupling strengths in C6o and C6o where the vibrational and the electronic structure are treated on an equal footing are presented. We obtain accurate vibrational frequencies and the corresponding coupling constants are obtained without further parameters. We find that the coupling to both the lowand high-frequency molecular modes are important, and their relevance to the Jahn-Teller effect and superconductivity are brieAy discussed.
Social Science Research Network, 2023
Physics and Chemistry of Materials with Low-Dimensional Structures, 1992
ABSTRACT Following the recent interest in path integral approaches to studying the properties of ... more ABSTRACT Following the recent interest in path integral approaches to studying the properties of low-dimensional quantum magnets we present an examination of the mean field properties of a two-dimensional quantum magnet, together with a detailed analysis of the validity of mean field theory in determining the phase diagram. We find that there are serious restrictions to the validity of the mean field approximation in these systems, the most important of which is an uncontrolled shifting of the zero of the free energy with respect to local spin rotation. We present a formalism which includes fluctuations beyond the mean field limit so that the problems may, in principle, be dealt with, and the effects of fluctuations on the mean field solutions may also be analysed.
International Conference on Science and Technology of Synthetic Metals, 1994
ABSTRACT form only given. Electron-electron interaction has often been considered to play an impo... more ABSTRACT form only given. Electron-electron interaction has often been considered to play an important role in the electronic properties of the BEDT-TTF salts, and the large difference between the transport and the cyclotron effective masses has often been used as an indication of a Hubbard model type enhancement. Using a Gutzwiller approximation for the Hubbard model in two dimensions, we have examined the behaviour of the effective mass enhancement as a function of electron density n and the effective electron-electron interaction U/t. Our results show that qualitatively, the effective mass has initially a strong decrease with decreasing effective electron-electron repulsion which levels off to a slower rate of change, qualitatively consistent with observations of the mass dependence with pressure in (BEDT-TTF)/sub 2/Cu(NCS)/sub 2/. The effects of further correlations in the approximations may give better quantitative behaviour, and will be discussed.
Physical Review B, 1994
Effects of vacancies in the triangular spin-1/2 antiferromagnet. CL O'Donnell and David ... more Effects of vacancies in the triangular spin-1/2 antiferromagnet. CL O'Donnell and David YK Ko Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, England. Received 21 March 1994 We examine the effects ...
Physical Review B, 1994
Deposition growth modes from numerical simulations. David YK Ko and Flavio Seno Department of Phy... more Deposition growth modes from numerical simulations. David YK Ko and Flavio Seno Department of Physics, University of Oxford 1 Keble Road, Oxford, OX1 3NP, United Kingdom. Received 15 August 1994 We present results ...
Synthetic Metals, 1993
We present results from an ab initio density functional calculation of the electron-vibronic coup... more We present results from an ab initio density functional calculation of the electron-vibronic coupling strengths in neutral and charged bis(ethylenedithio)tetrathiafulvalene (BEDT-TI'F) molecules. Results for tetrathiafulvalene (T'IT) are also presented and compared with known experimental values as a measure of the accuracy of our calculation. We find that the highest occupied molecular orbital (HOMO) in ET couples strongly to modes involving the double-bonded carbon atoms as well as the sulfur atoms and that the contribution of the vibration modes involving the hydrogen atoms to the vibronic coupling is negligible. The implication of this result for the mechanism of superconductivity is discussed in the light of recent isotope effect experimental results.
Semiconductor Science and Technology, 1987
ABSTRACT The authors have used an empirical pseudopotential method to study the pressure dependen... more ABSTRACT The authors have used an empirical pseudopotential method to study the pressure dependence of the electronic states at the GaAs/AlGaAs heterojunction. Application to tunnelling current studies, with particular reference to experiments by Eaves and co-workers (1986), is made. They find that the conduction band Gamma -X crossover produces no significant effect. Non-parabolicity effects are found to be important. They also find that the calculated results differed significantly from the experimental data, indicating the presence of some other factors they have not included.
Semiconductor Science and Technology, 1988
ABSTRACT The authors present the results on the band-structure effects in single-barrier tunnelli... more ABSTRACT The authors present the results on the band-structure effects in single-barrier tunnelling, calculated using a new scattering matrix method. Their results show that the transmission amplitude may be divided into two regions. The first is for electron energies in the band gap of the barrier material, where the transmission is found to be a single state property, with the Gamma state dominant. The appropriate barrier height, regardless of the direct/indirect nature of the barrier band gap, is the Gamma - Gamma band offset. The tunnelling behaviour is therefore single state like. The other regime is for electrons with energies above the barrier material's conduction band edge where, for indirect band-gap barriers, a sharp transition to an X dominated behaviour is observed. Coupling between the barrier Gamma and the X states are found in thin barriers and X well resonances are obtained. The single barrier transmission in this regime is therefore a multistate property.
Journal of the Optical Society of America A, 1988
ABSTRACT We present a new scattering matrix formalism for the modeling of electromagnetic wave pr... more ABSTRACT We present a new scattering matrix formalism for the modeling of electromagnetic wave propagation in stratified media. It is computationally efficient and stable and is well suited to the layer geometry that is characteristic of stratified materials. It is applied successfully to the modeling of total attenuated reflection in nematic liquid crystals with beyond-critical-angle incidence when the conventional transfer matrix methods normally fail.
Journal of Physics: Condensed Matter, 1993
ABSTRACT Rare earth metals exhibit a rich spectrum of bulk magnetic order. When they are multilay... more ABSTRACT Rare earth metals exhibit a rich spectrum of bulk magnetic order. When they are multilayered, these metals offer the possibility of producing a wide variety of tunable magnetic properties. The authors present an analysis of the magnetic excitations appropriate to the ferromagnetic, spiral and antiferromagnetic phases of such multilayers. They find that the interfaces play a very important role in determining the ground state of the system, and systems with identical spin equilibrium configurations can also have significantly different spin excitations, suggesting corresponding differences in their thermodynamic properties.
Journal of Physics: Condensed Matter, 1992
ABSTRACT The authors study the electronic structure of the GaAs/AlAs Fibonacci superlattice using... more ABSTRACT The authors study the electronic structure of the GaAs/AlAs Fibonacci superlattice using a semi-empirical sp3s* tight-binding method. They find that a self-similar energy spectrum can be seen in the band structure, although the energy spectrum depends strongly on the wavevector parallel to the layers. Furthermore, they find that a localization character is enhanced due to the band hybridization, producing a spiky density of states and a localization-like effect of the wavefunctions in the hybridized energy region. The reasons for the localization-like behaviour are discussed briefly.
Journal of Physics A: Mathematical and General, 1992
ABSTRACT The authors consider one-dimensional Ising systems of length L with an interaction cut-o... more ABSTRACT The authors consider one-dimensional Ising systems of length L with an interaction cut-off R(L) in the limit R to infinity . They present free energy arguments for order at low temperatures provided R(L))(A In L)1 kappa /, where kappa is a new exponent; this opens the possibility of one-dimensional pseudo-order in macroscopic systems.
Journal of Applied Physics, 1989
The trends in the pressure dependence of the electronic transmission in single- and multiple-barr... more The trends in the pressure dependence of the electronic transmission in single- and multiple-barrier GaAs/AlGaAs heterostructures are calculated using a microscopic pseudopotential method. Electric effects are also included in our model. We find that in single-barrier systems the pressure induced direct to indirect band-gap transition alone does not affect the electron tunneling properties. It is only when the barrier’s X minimum is lowered below the electron energy that significant changes are found in the electron transmission. In a multiple-barrier system the lowering of the barrier X minimum due to the applied pressure and bias effectively pushes up the Γ resonant levels in the GaAs such that a clear negative differential resistance signal is lost.
ABSTRACT Available from UMI in association with The British Library. This thesis presents the res... more ABSTRACT Available from UMI in association with The British Library. This thesis presents the results of a microscopic calculation of the electron tunnelling properties in finite semiconductor heterostructure systems. Empirical pseudopotentials are used to incorporate the microscopic crystal potential and a scattering matrix method is developed to solve the electron wave function through the system. The effects of the inherent bandstructure on the tunnelling characteristics are examined in some detail. Non-parabolicity in the Gamma state bandstructure in the bandgap region alone is found to produce significant deviations in the tunnelling current and resonant energies from the simplistic effective mass approximation. Many-band effects are found to be important for indirect bandgap barriers where the electron energy lies above the barriers' X minima. The situation in the purely tunnelling regime is dominated by the Gamma state, regardless of the nature of the bandgap. Finite electric fields in large systems are considered. Gamma-X real space intervalley transfers are found when the electron energy traverses the conduction band edge of a material. The total wave function is in general complicated due to the highly complex mixing of the different states' contributions. The effects of the bandstructure changes on the tunnelling properties in single and multiple barrier systems due to the application of hydrostatic pressure are also examined. The results show that in single barriers the direct to indirect bandgap transition alone does not modify significantly the tunnelling property but deviations from the single -band behaviour are found when the conduction band edge is lowered below the electron energy. In multiple barrier systems the lowering of the barrier's X minima effectively pushes up the Gamma resonant level in wells such that a clear negative differential resistance signal would be lost.
Journal de Physique I, 1993
Physical Review B, 1993
The results of a fully ab initio determination of the vibronic coupling strengths in C6o and C6o ... more The results of a fully ab initio determination of the vibronic coupling strengths in C6o and C6o where the vibrational and the electronic structure are treated on an equal footing are presented. We obtain accurate vibrational frequencies and the corresponding coupling constants are obtained without further parameters. We find that the coupling to both the lowand high-frequency molecular modes are important, and their relevance to the Jahn-Teller effect and superconductivity are brieAy discussed.