Year

2019

Degree Name

Doctor of Philosophy

Department

School of Physics

Abstract

In this thesis, we study thermionic emission in two kinds of Dirac materials, namely Dirac semimetals and nodal-ring semimetals. Starting with the linear energy-momentum dispersion, we develop a modified Richardson-Dushman (RD) law to describe the thermionic emission current in 3D Dirac semimetals. The modified RD law has no mass dependence, which is significantly different from the RD law. We found the average energy carried by a degree of freedom in Dirac semimetals is twice of that in conventional materials. As a result, 3D Dirac semimetals have the best thermal efficiency and coefficient of performance when compared to conventional semiconductors and graphene.

The density of states of 3D Dirac semimetals is smaller than that of 3D conventional materials, which results in a relatively smaller thermionic current density. A new type of 3D Dirac material, nodal-ring has a larger density of states near to the Dirac cones due to it having more Dirac cones compared to Dirac semimetals. We developed a modified RD law to calculate its thermionic emission current. The results show the thermionic emission current can be enhanced by the nodal-ring. Additionally, it has different thermionic emission in the x- and y-directions due to the anisotropic energymomentum dispersion.

Thermionic emission has many potential applications in harvesting thermal energy and cooling. We calculate the heat transfer from electronic devices without and with thermionic cooling. Without thermionic cooling, the internal temperature of the devices is at best equal to and usually higher than the temperature of the surrounding environment. However, when thermionic cooling is employed to transport heat, the internal temperature can be considerably lower than the environmental temperature.

Additionally, hot carrier relaxation is studied in gapped Dirac semimetals. A finite gap relaxes the selection rule and gives rise to a nonvanishing internode coupling via phonon scattering. The gap also enhances the intra-node scattering. By using the Boltzmann transport equation, we find that the relaxation rate increases with the square of the gap and the electron temperature.

Finally, we investigate the strong tunable photo-mixing in semi-Dirac semimetals in the terahertz regime. The third-order photoresponses along the linear and parabolic directions have been analyzed and determined quantitatively. We have found a remarkable tunability of the mixing efficiency along the parabolic direction by a small electric field in the linear direction, up to two orders of magnitude.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.