Terahertz magnetospectroscopy of HgTe-based heterostructure materials

This work is devoted to the study of mercury cadmium telluride based heterostructures employing techniques such as terahertz spectroscopy and magnetophotoconductivity. An important goal of this work is to obtain a better understanding of the topological boundary states in these materials and their influence on electro-optical effects. The investigations focus on two-dimensional HgTe quantum wells as well as three-dimensional CdₓHg₁₋ₓTe films in both topologically trivial and non-trivial regimes. Using continuous and pulsed terahertz excitation, fascinating optoelectronic phenomena were discovered in the course of this study. A detailed investigation of the terahertz photoconductivity in HgTe QWs with inverted and non-inverted parabolic as well as linear band dispersion revealed that these structures manifest a distinct sign-alternating photoresponse in magnetic field. Upon increase of the external, out-of-plane magnetic field, the photoconductivity systematically changes its sign. This remarkable effect was observed in QWs corresponding to both topologically trivial and non-trivial regimes. Additionally, it was studied in samples with different geometries including conventional Hall bar and Corbino disk design. Notably, the analysis of the photoconductivity in Corbino disk samples brought up an elegant optoelectronic method to probe the carrier mobility in such systems. During the research on the sign-alternating photoconductivity, a qualitatively different behavior of the photoresponse was observed in a high-mobility 20 nm QW. Instead of a single or double sign inversion in magnetic field, pronounced ω/ωc -periodic oscillations were observed in photoresistivity for the Fermi energy lying in the conduction band. Further analysis demonstrated that these oscillations have the same origin as microwave-induced resistance oscillations (MIRO), which have previously been observed only in systems with ultra-high mobility, such as ultra-clean high-mobility GaAs quantum wells, under illumination with microwave radiation. Similar to Shubnikov-de Haas oscillations, which result from an interplay of Fermi energy and cyclotron energy, MIRO express the commensurability between the photon energy and the cyclotron energy. Recently, their terahertz analogue, the terahertz-induced resistance oscillations (TIRO), were observed in GaAs quantum wells as well as in three-dimensional HgTe topological insulators. The results presented in this work represent the first observation of such MIRO-like oscillations in HgTe quantum wells. In addition to two-dimensional systems, the optoelectronic properties of three-dimensional CdₓHg₁₋ₓTe films were studied in the last part of this thesis. The investigation of topologically non-trivial CdₓHg₁₋ₓTe films with an inverted band ordering revealed cyclotron resonance involving the two-dimensional surface states. The surface state resonance was detected in terahertz radiation transmission as well as photogalvanic experiments. However, it was absent in the investigated CdₓHg₁₋ₓTe films with higher cadmium contents corresponding to the topologically trivial regime, demonstrating the origin of the resonance in the surface state carriers. Moreover, experiments with different film designs showed the importance of sharp interfaces between topologically trivial and non-trivial layers for the formation of fully two-dimensional topological surface states

Highly superlinear giant terahertz photoconductance in GaAs quantum point contacts in the deep tunneling regime

A highly superlinear in radiation intensity photoconductance induced by continuous wave terahertz laser radiation with low intensities has been observed in quantum point contacts made of GaAs quantum wells operating in the deep tunneling regime. For very low values of the normalized dark conductance Gdark/G0≈10−6, with the conductance quantum G0=2e2/h, the photoconductance scales exponentially with the radiation intensity, so that already at 100mW/cm2, it increases by almost four orders of magnitude. This effect is observed for a radiation electric field oriented along the source drain direction. We provide model considerations of the effect and attribute it to the variation of the tunneling barrier height by the radiation field made possible by local diffraction effects. We also demonstrate that cyclotron resonance due to an external magnetic field manifests itself in the photoconductance, completely suppressing the photoresponse

Terahertz Magnetospectroscopy of Cyclotron Resonances from Topological Surface States in Thick Films of CdxHg1 – xTe

Herein, studies of the cyclotron resonance (CR) in thick Cd????Hg1−????Te films with different cadmium contents corresponding to materials with and without band inversion, as well as critical content corresponding to an almost linear energy dispersion are presented. The results demonstrate that the formation of 2D topological surface states requires sharp interfaces between layers with and without band inversion, in which case the corresponding CR is clearly observed for the out-of-plane orientation of magnetic field but does not show up for an in-plane orientation. In contrast, all samples having more conventional technological design with smooth interfaces (i.e., containing regions of Cd????Hg1−????Te with gradually changing Cd content x) show equally pronounced CR in both in-plane and out-of-plane magnetic field revealing that CR is excited in effectively 3D states. Modeling of the surface states for different film designs supports main observations. In all samples, additional broad helicity-independent resonances are observed, which are attributed to photoionization and magnetic freeze-out of impurity states

Terahertz Photogalvanics in Twisted Bilayer Graphene Close to the Second Magic Angle

We report on the observation of photogalvanic effects in tBLG with a twist angle of 0.6 degrees. We show that excitation of the tBLG bulk causes a photocurrent, whose sign and magnitude are controlled by the orientation of the radiation electric field and the photon helicity. The observed photocurrent provides evidence for the reduction of the point group symmetry in low twist-angle tBLG to the lowest possible one. The developed theory shows that the current is formed by asymmetric scattering in gyrotropic tBLG. We also detected the photogalvanic current formed in the vicinity of the edges. For both bulk and edge photocurrents, we demonstrate the emergence of pronounced oscillations upon variation of the gate voltage. The gate voltages associated with the oscillations correlate with peaks in resistance measurements. These are well explained by interband transitions between a multitude of isolated bands in tBLG

Terahertz spin ratchet effect in magnetic metamaterials

We report on spin ratchet currents driven by terahertz radiation electric fields in a Co/Pt magnetic metamaterial formed by triangle-shaped holes forming an antidots lattice and subjected to an external magnetic field applied perpendicularly to the metal film plane. We show that for a radiation wavelength substantially larger than the period of the antidots array the radiation causes a polarization-independent spin-polarized ratchet current. The current is generated by the periodic asymmetric radiation intensity distribution caused by the near-field diffraction at the edges of the antidots, which induces spatially inhomogeneous periodic electron gas heating, and a phase-shifted periodic asymmetric electrostatic force. The developed microscopic theory shows that the magnetization of the Co/Pt film results in a spin ratchet current caused by both the anomalous Hall and the anomalous Nernst effects. Additionally, we observed a polarization-dependent trigonal spin photocurrent, which is caused by the scattering of electrons at the antidot boundaries resulting in a spin-polarized current due to the magnetization. Microscopic theory of these effects reveals that the trigonal photocurrent is generated at the boundaries of the triangle antidots, whereas the spin ratchet is generated due to the spatially periodic temperature gradient over the whole film. This difference causes substantially different hysteresis widths of these two currents