Experimental setups for transmission and photovoltage measurements: a) Faraday configuration with both magnetic field and the incident THz beam oriented along the normal to the sample surface; b) Voigt configuration with an in-plane magnetic field; c) Faraday configuration with the sample being tilted by an angle θ with respect to the common direction of the laser beam and magnetic field.

Experimental setups for transmission and photovoltage measurements: a) Faraday configuration with both magnetic field and the incident THz beam oriented along the normal to the sample surface; b) Voigt configuration with an in-plane magnetic field; c) Faraday configuration with the sample being tilted by an angle θ with respect to the common direction of the laser beam and magnetic field.

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We present studies of the cyclotron resonance (CR) in thick CdxHg1−xTe 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. Our results demonstrate that formation of two‐dimensional topological surface states requires sharp...

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Context 1
... of the experiments were performed with normally incident THz radiation and magnetic field oriented either perpendicular to the film surface (Faraday geometry) or in-plane (Voigt geometry) (Figure 3a,b). In addition, experiments in Faraday geometry with the sample tilted by an angle θ were conducted ( Figure 3c). ...
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... experiments on radiation transmission, a pyroelectric detector was placed behind the sample (Figure 3). Photovoltage and photoconductivity measurements were conducted using a standard lock-in technique. ...
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... in some photoconductivity measurements, we applied a low-frequency AC bias and used a double-modulation technique described in previous studies. [39,47] Because main conclusions of this work are based on the comparison of results obtained using the three setups in Figure 3, it is important to recall the basic distinctions between them in application to 3D and 2D electron systems. In a 3D system, CR is expected to emerge in all three setups; the orientation of both the laser beam and static magnetic field with respect to the sample surface is immaterial for a 3D isotropic system; the dynamics of carriers is different for parallel (Faraday) and perpendicular (Voigt) relative orientation of the magnetic field and THz radiation propagation directions. ...
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... the case of normal orientation of magnetic field (as in Faraday geometry in Figure 3a), one thus expects a usual CR dip in transmission for both 2D and 3D electrons. On the contrary, for in-plane magnetic field (as in Voigt geometry in Figure 3b) 2D electrons should manifest no CR, whereas 3D electrons can still experience CR. ...
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... that, in the photovoltage response, two CR peaks are clearly resolved even for the lowest frequency f ¼ 0.69 THz, in which case the CR dips in magnetotransmission get merged (Figure 4c). Strikingly, applying a tilted magnetic field (see the experimental setup in Figure 3c), we obtain that the CR positions are independent of the tilt angle θ (Figure 6b). This observation provides a first clear evidence that the resonances are excited in a 3D electron gas. ...
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... of the magnetotransmission in Voigt geometry (Figure 3b) confirm the aforementioned conclusions showing that even for a magnetic field oriented in-plane both CRs are still present (Figure 7). This observation rules out the possibility that any of the observed resonances originates from 2D electrons. ...

Citations

... www.nature.com/scientificreports/ As it has been shown in 19 for analogous structures based on thick topological Hg 1−x Cd x Te films, the electronic states located at smooth and sharp interfaces between the topological and trivial phases have different structures. While the sharp interface forms 2D electron states, the smooth interface produces 3D-like states. ...
... While the sharp interface forms 2D electron states, the smooth interface produces 3D-like states. It was suggested that the sharp interface forms true 2D electron states, whereas for the smooth interface, a series of Volkov-Pankratov interface states are formed 19 . In the latter case, these electronic states are effectively 3D, in an analogy to conventional quasi-2D electron states in a wide quantum well occupying multiple subbands. ...
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In this paper, we show that electron states formed in topological insulators at the interfaces topological phase–trivial phase and topological phase–vacuum may possess different properties. This is demonstrated on an example of heterostructures based on thick topological Hg 1− x Cd x Te films, in which the PT -symmetric terahertz photoconductivity is observed. It is shown that the effect originates from features of the interface topological film–trivial buffer/cap layer. The PT -symmetric terahertz photoconductivity is not provided by electron states formed at the interface topological film–vacuum.
... In their subsequent studies, the DFT method for a dilute Mn concentration range 0.05 < x < 0.2 was used [24]. Nevertheless, the last report regarding TI states in 3D and 2D systems has always been analysed based on the kp method [25,26]. ...
Article
On the basis of the eight-band kp model, the theoretical engineering of HgTe/HgCd(Mn)Te double quantum well (DQW) structures is presented in this study. An external magnetic field and structural inversion asymmetry are taken into account. The advantages of HgTe two-dimensional topological insulator (2D TI) layers with x-Cd or x-Mn over pure 2D TI HgTe films are theoretically and experimentally proven. All advantages, such as the larger Fermi velocity, very high mobility of the surfaces states, the larger g factor for such mixed layers together with the sharpens on the edge layers in the 2D TI structures and the structural inversion asymmetry (which can be easy realized in the 2D DQW case), mean that the presented structures are encouraging as the basis of, for example, high resolution terahertz detectors. Considering this possible application, the optical transition for the cyclotron resonance and the excitation transfer in the terahertz region is analyzed. The results of the presented investigation are very promising for the application of HgTe/HgCd(Mn)Te DQW structures in terahertz detector.
Article
Close to the semimetal-to-semiconductor topological phase transition, the band structure of HgCdTe is represented by massive Kane fermions that, in a semirelativistic approach, are characterized by two parameters: rest mass and velocity. Using terahertz magneto-optical spectroscopy, we explore the band structure evolution of HgCdTe films with hydrostatic pressure in the vicinity of the band-gap collapse. By analyzing the energies of interband optical transitions as a function of magnetic field, we have determined the rest mass of Kane fermions at different hydrostatic pressures. The pressure dependence of the rest mass allows us to obtain the hydrostatic deformation potential ac−av at low temperature, where ac and av are the deformation potentials of the conduction and valence bands, respectively.
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Long-wavelength stimulated emission (SE) is studied in optically pumped HgCdTe quantum well heterostructures with dielectric waveguides. Continuous temperature tuning of the wavelength from 27 µm to 18 µm is achieved in structures with optimized waveguides. Above 27 µm, SE clamps at 31 µm wavelength, where mode leaking is reduced due to Reststrahlen effect in GaAs substrate. The operating temperature is mainly limited by the activation of Auger recombination in quasi-equilibrium conditions, while at low temperatures we expect that lowering initial carrier heating would enhance the gain considerably. We conclude that exploiting Reststrahlen effect should allow one to achieve continuous wavelength tuning around 30 µm and operating wavelengths up to 40 µm with technologically attainable epistructure thickness. rumyantsev@ipmras.ru This is the author's peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS Appl. Phys. Lett. 121, 182103 (2022); https://doi.org/10.1063/5.0128783
Article
We report on the observation of symmetry breaking and the circular photogalvanic effect in CdxHg1−xTe alloys. We demonstrate that irradiation of bulk epitaxial films with circularly polarized terahertz radiation leads to the circular photogalvanic effect (CPGE) yielding a photocurrent whose direction reverses upon switching the photon helicity. This effect is forbidden in bulk zinc-blende crystals by symmetry arguments; therefore, its observation indicates either the symmetry reduction of bulk material or that the photocurrent is excited in the topological surface states formed in a material with low cadmium concentration. We show that the bulk states play a crucial role because the CPGE was also clearly detected in samples with noninverted band structure. We suggest that strain is a reason for the symmetry reduction. We develop a theory of the CPGE showing that the photocurrent results from the quantum interference of different pathways contributing to the free-carrier absorption (Drude-like) of monochromatic radiation.