2019). Nonlinear optical response of the alpha-T-3 model due to the nontrivial topology of the band dispersion. Physical Review B, 100 (3), 035440-1-035440-16. Nonlinear optical response of the alpha-T-3 model due to the nontrivial topology of the band dispersion AbstractWe study the electronic contribution to the nonlinear optical response of the α-T3 model. This model is an interpolation between a graphene (α = 0) and dice (α = 1) lattice. Using a second-quantized formalism, we calculate the first-and third-order responses for a range of α and chemical potential values as well as considering a band gap in the first-order case. Conductivity quantization is observed for the first-order, while higher-order harmonic generation is observed in the third-order response with the chemical potential determining which applied field frequencies both quantization and harmonic generation occur at. We observe a range of experimentally accessible critical fields between 102-106 V/m with dynamics depending on α, μ, and the applied field frequency. Our results suggest an α-T3-like lattice could be an ideal candidate for use in terahertz devices.We study the electronic contribution to the nonlinear optical response of the α-T 3 model. This model is an interpolation between a graphene (α = 0) and dice (α = 1) lattice. Using a second-quantized formalism, we calculate the first-and third-order responses for a range of α and chemical potential values as well as considering a band gap in the first-order case. Conductivity quantization is observed for the first-order, while higher-order harmonic generation is observed in the third-order response with the chemical potential determining which applied field frequencies both quantization and harmonic generation occur at. We observe a range of experimentally accessible critical fields between 10 2 -10 6 V/m with dynamics depending on α, μ, and the applied field frequency. Our results suggest an α-T 3 -like lattice could be an ideal candidate for use in terahertz devices.

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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 TiO 2 /VO 2 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 10 3-10 4 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.

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

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A novel mathematical approach has been developed to complete the inversion of the Biot-Savart law in one-and two-dimensional cases from measurements of the perpendicular component of the magnetic field using the well-developed Magneto-Optical Imaging technique. Our approach, especially in the 2D case, is provided in great detail to allow a straightforward implementation as opposed to those found in the literature. Our new approach also refines our previous results for the 1D case [Johansen et al., Phys. Rev. B 54, 16264 (1996)], and streamlines the method developed by Jooss et al. [Physica C 299, 215 (1998)] deemed as the most accurate if compared to that of Roth et al. [J. Appl. Phys. 65, 361 (1989)]. We also verify and streamline the iterative technique, which was developed following Laviano et al. [Supercond. Sci. Technol. 16, 71 (2002)] to account for in-plane magnetic fields caused by the bending of the applied magnetic field due to the demagnetising effect. After testing on magneto-optical images of a high quality YBa 2 Cu 3 O 7 superconducting thin film, we show that the procedure employed is effective.

By employing a linearised Boltzmann equation, we calculate the magneto-optical properties of twisted bilayer graphene using non-magnetic wave functions. Both transverse and longitudinal responses are calculated up to the second order in applied magnetic field with their twist angle and Fermi level dependence examined. We find that increasing the twist angle increases the transverse metallic response so long as the Fermi level remains below the upper conduction band. Interlayer transitions provide an appreciable enhancement when the Fermi level traverses the gap between the two conduction bands. Interlayer transitions are also responsible for a nonzero anomalous Hall conductivity in this model. As the Fermi level moves towards zero, the longitudinal response begins to dominate and a highly anisotropic negative magneto-resistance is observed.