Jung Hyun Oh

Phonon thermal conductivity in silicon nanowires: The effects of surface roughness at low temperatures

Using a Green’s function method based on an elastic wave equation, the effects of surface roughness and the nanowire-contact interface scattering on phonon thermal conductivity are studied at low temperatures. It is found that the interface geometry between a nanowire and its contacts affects the transmission function at small energies related to the gapless modes and it gives rise to deviated results from the universal conductance. It is also shown that the surface roughness is crucial in the suppression of phonon thermal conductivity with reducing the nanowire size by averaging the transmission function over the rough-surface configurations. Furthermore, the phonon mean free path is proportional to the ratio of the correlation length and roughness heights quadratically as well as the cross-section area of the nanowire.

Transport theory of coupled quantum dots based on the auxiliary-operator method

We formulate the theory of electron transport through coupled-quantum dots by extending the auxiliary operator representation. By using the generating functional technique, we derive the exact expressions for currents, dot-occupation numbers and spin correlations, and examine them based on the non-equilibrium Greens function method under the non-crossing approximation (NCA). Our formulation generalizes the previous NCA approaches by allowing full occupation numbers with a finite Coulomb repulsion.

Phonon transport in Si nanowires with elastically dissimilar barriers

As one of the efforts to enhance the thermoelectric conversion efficiency, phonon transport through elastically dissimilar barriers embedded in Si nanowires is investigated. Using a Green’s function method based on an elastic wave equation, the transmission function is calculated for various barrier materials with different acoustic impedance. It is found that the insertion of silicide (especially PtSi) layers into the Si nanowire substantially suppresses the phonon transmission function and, thus, is promising alternative to enhance the efficiency of thermoelectric devices.

Enhanced ionized impurity scattering in nanowires

The electronic resistivity in silicon nanowires is investigated by taking into account scattering as well as the donor deactivation from the dielectric mismatch. The effects of poorly screened dopant atoms from the dielectric mismatch and variable carrier density in nanowires are found to play a crucial role in determining the nanowire resistivity. Using Greens function method within the self-consistent Born approximation, it is shown that donor deactivation and ionized impurity scattering combined with the charged interface traps successfully to explain the increase in the resistivity of Si nanowires while reducing the radius, measured by Bjork et al. [Nature Nanotech. 4, 103 (2009)].

First-principles study of the equilibrium structures of clusters

We investigate the atomic structure of (n = 9-14) clusters using the first-principles pseudopotential method within the local-density-functional approximation (LDA). The equilibrium geometries of small clusters with tend to be capped prismatic structures. For n = 13, we find a surface-like metallic compact structure which is derived from a capped icosahedron and competes with a stable trigonal prism, while this structure is the most stable for n = 14. These results are compatible with the observed stability of and , as compared to clusters with nearby values of n, against chemical reactions with simple molecules. The effect of electron-electron correlations on the energetics of isomers with n = 13 is examined through variational quantum Monte Carlo calculations, and the LDA energy ordering remains unchanged, consistently with previous diffusion quantum Monte Carlo calculations.

Suppression of phonon transport in multiple Si/PtSi heterostructures

Using a Green function method based on an atomic vibration model, herein we report the results from our investigation of phonon transport through multiple Si/PtSi layered structures. In contrast with values predicted using elastic wave theory and an impedance mismatch method, we find that a detailed atomic-vibration approach exhibits significantly suppressed phonon transport and leads to a 30-times reduction of the thermal conductance, compared to that of Si bulk. We attribute the origin of the suppression to the lack of PtSi phonon modes in the energy range of 20–30 meV, and to the effects of interface scattering between Si and PtSi layers.

Effects of Rashba and Dresselhaus spin–orbit interactions on the ground state of two-dimensional localized spins

Starting with the indirect exchange model influenced by the Rashba and the Dresselhaus spin-orbit interactions, we derive the Dzyaloshinskii-Moriya interaction of localized spins. The strength of the Dzyaloshinskii-Moriya interaction is compared with that of the Heisenberg exchange term as a function of atomic distance. Using the calculated interaction strengths, we discuss the formation of various atomic ground states as a function of temperature and external magnetic field. By plotting the magnetic field-temperature phase diagram, we present approximate phase boundaries between the spiral, Skyrmion and ferromagnetic states of the two-dimensional weak ferromagnetic system.

Correlation effects in a quantum dot at high magnetic fields

We investigate the effects of electron correlations on the ground-state energy and the chemical potential of a droplet confined by a parabolic potential at high magnetic fields. We demonstrate the importance of correlations in estimating the transition field at which the first edge reconstruction of the maximum density droplet occurs in the spin-polarized regime.

Dimensional crossover and temperature dependence of the heat capacity in two-direction double-barrier resonant-tunnelling structures

We calculate the heat capacity of electrons as a function of the electron density and temperature in two-direction double-barrier resonant-tunnelling structures. The strength of the barrier potential increases in one direction, so that the system becomes 2D; the heat capacity as a function of the electron density goes to a step-like shape, which is similar to the one in the DOS. From to to 1D, the heat capacity reflects the peaks in the DOS with increasing electron density, and becomes a sawtooth-like shape in 1D. However, an asymmetric peak in the DOS makes two peaks in the heat capacity because the available DOS in thermal excitations becomes smaller as the chemical potential approaches the peak in the DOS. The heat capacity shows a linear dependence on the temperature in 3D and 2D, but not in 1D even at low temperatures. It exhibits a square root T dependence when the chemical potential is located near a pole in the DOS.

The influence of ionized impurity scattering on the thermopower of Si?nanowires

The thermopower of Si nanowires was investigated on the basis of electronic transport theory, taking into account ionized impurity scattering as well as electron-phonon scattering. It was found that the enhancement of the Seebeck coefficient in nanowires arising from quantum confinement is unimportant due to the ionized impurity scattering associated with donor deactivation. Furthermore, because the electrical conductivity is degraded significantly as the nanowire size becomes smaller, despite the accompanying slightly enhanced Seebeck coefficient, the reduction of the nanowire size is not beneficial, at least for the thermopower of devices.