Van der Waals (vdW) materials have attracted much interest for their myriad unique electronic, mechanical and thermal properties. In particular, they are promising candidates for monochromatic, table-top X-ray sources. This work reveals that the versatility of the table-top vdW X-ray source goes beyond what has been demonstrated so far. By introducing a tilt angle between the vdW structure and the incident electron beam, it is theoretically and experimentally shown that the accessible photon energy range is more than doubled. This allows for greater versatility in real-time tuning of the vdW X-ray source. Furthermore, this work shows that the accessible photon energy range is maximized by simultaneously controlling both the electron energy and the vdW structure tilt. These results should pave the way for highly tunable, compact X-ray sources, with potential applications including hyperspectral X-ray fluoroscopy and X-ray quantum optics.
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.
The intra- and interband optical conductivities of a semi-Dirac system are determined. It was found that the conductivity in the linear direction is considerably stronger than the conductivity in the parabolic direction. For an electrical field applied along a non-principal axis, both the the longitudinal and transverse current are nonzero. Due to the anisotropy of the system, the transverse conductivity for an oblique applied field can exceed the longitudinal conductivity.
We experimentally measure quantum recoil in Smith-Purcell radiation, achieved by scattering free electrons off the periodic lattice of van der Waals crystals to generate multimode coherent X-rays.
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