Thermionic emission and electron transport in Dirac materials

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

Thermionic enhanced heat transfer in electronic devices based on 3D Dirac materials

We calculate the heat transfer from electronic devices based on three-dimensional Dirac materials 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. In the proposed thermionic cooling process, the energy efficiency can be as high as 75% of the Carnot efficiency

Superconducting pair-breaking under intense sub-gap terahertz radiation

We study the effect of a strong and low frequency (ω \u3c Δ, the superconducting gap) electrical field on a superconducting state. It is found that the superconducting gap decreases with the field intensity and wavelength. The physical mechanism for this dependence is the multiphoton absorption by a superconducting electron. By constructing the state of a superconducting electron dressed by photons, we determined the dependence of the superconducting gap on E / ω and temperature. We show that the critical temperature is determined by the parameter E / ω which is distinct from that induced by the heating effect. The result is consistent with experimental findings. This result can be applied to study terahertz nonlinear superconducting metamaterials

Optical conductivity of a commensurate graphene-topological insulator heterostructure

The optical conductivity of a heterostructure formed by a commensurate stacking of graphene and a topological insulator (TI) is investigated using the Kubo formalism. Both the intra- and interband AC conductivities are found to be sensitive to the graphene-TI coupling. The direct interband transition in graphene which is the origin of the universal conductance is forbidden due to the topological nature is the coupling. Furthermore, the graphene-TI coupling gives rise to additional broken symmetries, resulting in both the inter- and intraband conductivity to be reduced in the graphene-TI heterostructure. By varying the Fermi energy of the heterostructure, the band that gives the largest contribution changes, which in turn affects the overall electronic transport

High efficiency and non-Richardson thermionics in three dimensional Dirac materials

Three dimensional (3D) topological materials have a linear energy dispersion and exhibit many electronic properties superior to conventional materials such as fast response times, high mobility, and chiral transport. In this work, we demonstrate that 3D Dirac materials also have advantages over conventional semiconductors and graphene in thermionic applications. The low emission current suffered in graphene due to the vanishing density of states is enhanced by an increased group velocity in 3D Dirac materials. Furthermore, the thermal energy carried by electrons in 3D Dirac materials is twice of that in conventional materials with a parabolic electron energy dispersion. As a result, 3D Dirac materials have the best thermal efficiency or coefficient of performance when compared to conventional semiconductors and graphene. The generalized Richardson-Dushman law in 3D Dirac materials is derived. The law exhibits the interplay of the reduced density of states and enhanced emission velocity

Thermionic emission in nodal-ring semimetals

© 2020 Author(s). We theoretically investigate the thermionic emission from nodal-ring semimetals. The thermionic emission is found to be anisotropic in the x- and y-directions. The anisotropic emission can be enhanced by increasing the radius of nodal-ring b. The main feature of nodal-ring semimetals not only results in anisotropic thermionic emission but also affects the value of thermionic emission current density (TECD). The TECD of the lower branch of the energy-momentum dispersion increases with b, while the TECD of the upper branch decreases with b. Unlike in conventional materials, the TECD in nodal-ring semimetals depends on Fermi energy that is similar to the situation in Dirac semimetals. The underlined reason is that Dirac semimetals and nodal-ring semimetals have a linear or a linear-like energy-momentum dispersion while conventional materials have a parabolic energy-momentum dispersion. The TECD of nodal-ring semimetals depends strongly on work function and temperature

Dynamical polarization in a graphene-topological-insulator Heterostructure

The frequency, chemical potential, hopping energy and temperature dependence of polarization in a graphene-topological-insulator heterostructure is investigated. The polarization is found to be sensitive to the graphene-topological-insulator hopping energy and chemical potential. Compared to hopping energy and chemical potential, temperature has a relatively small effect on the polarization which only slightly changes the peak value. The unique band structures of graphene-topologicalinsulator heterostructures give rise to dual polarization peaks. Furthermore, the position of the polarization peak that originates from the graphene bands is robust while the position of the other peak can be tuned by varying the hopping energy and chemical potential. From the polarization function two branches of plasma dispersion are observed due to the coupling of graphene and the surface states of topological insulators

Strong tunable photomixing in semi-Dirac materials in the terahertz regime

We demonstrate a strong and anisotropic photomixing effect in an electronic system whose energy–momentum dispersion is parabolic in the direction and linear in the direction, such as a TiO2/VO2 multilayered structure. 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. In the terahertz (THz) regime, the third-order response is comparable to the linear response under an applied field of 103–104  V/cm. Additionally, the nonlinear response persists at room temperature. The results may have applications where different current responses are required along different directions in the THz regime. We demonstrate a strong and anisotropic photomixing effect in an electronic system whose energy–momentum dispersion is parabolic in the direction and linear in the direction, such as a TiO2/VO2 multilayered structure. 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. In the terahertz (THz) regime, the third-order response is comparable to the linear response under an applied field of 103–104  V/cm. Additionally, the nonlinear response persists at room temperature. The results may have applications where different current responses are required along different directions in the THz regime