S4 Full measured spectra, background subtraction and peak deconvolution procedure . .

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.

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.

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 calculate the relaxation rate of hot carriers in a Cd3As2 semi-metal with a finite gap. The quantization of the transverse momentum gives rise to a minimum gap at the Dirac point. Additional chemical doping further increases the gap. 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. It is shown that the relaxation rate is proportional to the square of the gap. By considering the decay of the electron distribution function, we find that the relaxation rate increases with the square of the gap and the electron temperature.

We study the effect of a strong and low frequency (ω < 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. Abstract:We study the effect of a strong and low frequency (ω < 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 multi-photon 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 / and temperature. We show that the critical temperature is determined by the parameter / which is distinct from that induced by the heating effect. The result is consistent with experimental findings.This result can be applied to study the terahertz nonlinear superconducting metamaterials. *czhang@uow.edu.au

We present a detailed investigation on mechanical properties of Zr-X (X = Ti, Hf and Sc) alloy systems using the first-principles calculations in conjunction with special quasi-random structures (SQSs). It is found that the strength of mechanical coefficients such as elastic constants and elastic modulus depend linearly on the composition in Zr-Hf and Zr-Sc systems, whereas they depend parabolically on the composition in Zr-Ti system. Such a phenomenon is mainly induced by the presence of Zr-X metallic bond in the alloy systems. In addition, the strength of mechanical coefficients can be well described as a function of alloying composition by a special quadratic function that only has one coefficient F : y = M Zr (1−x)+M X x+x(1−x)F. With this function, mechanical coefficients of Zr-X alloy systems in whole composition range can be quickly estimated. Moreover Ti can considerably enhance the ductility of Zr when Ti concentration is 0.43.