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Register a new userWeak antilocalization in partially relaxed 200-nm HgTe films
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The anomalous magnetoresistance caused by the weak antilocalization (WAL) effects in 200-nm HgTe films is experimentally studied. The film is a high quality 3D topological insulator with much stronger spatial separation of surface states than in previously studied thinner HgTe structures. However, in contrast to that films, the system under study is characterized by a reduced partial strain resulting in an almost zero bulk energy gap. It has been shown that at all positions of the Fermi level the system exhibits a WAL conductivity correction superimposed on classical parabolic magnetoresistance. Since high mobility of carriers, the analysis of the obtained results was performed using a ballistic WAL theory. The maximum of the WAL conductivity correction amplitude was found at a Fermi level position near the bulk energy gap indicating to full decoupling of the surface carriers in these conditions. The WAL amplitude monotonously decreases when the density of either bulk electrons or holes increases that results from the increasing coupling between surface and bulk carriers.
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Surface states of topological insulators (TIs) have been playing the central role in the majority of outstanding investigations in low-dimensional electron systems for more than 10 years. TIs based on high-quality strained HgTe films demonstrate a variety of subtle physical effects. The strain leads to a bulk band gap but limits a maximum HgTe strained film thickness, and therefore, the majority of experiments were performed on films with a thickness of less than 100 nm. Since a spatial separation of topological states is crucial for the study of a single-surface response, it is essential to increase the HgTe thickness further. In this work, by combining transport measurements together with capacitance spectroscopy, we perform an analysis of a 200-nm partially relaxed HgTe film. The Drude fit of the classical magnetotransport reveals the ambipolar electron-hole transport with a high electron mobility. A detailed analysis of Shubnikov-de Haas oscillations in both conductivity and capacitance allows us to distinguish three groups of electrons, identified as electrons on top and bottom surfaces and bulk electrons. The indirect bulk energy gap value is found to be close to zero. It is established that the significant gap decrease does not affect the surface states, which are found to be well resolved and spin nondegenerate. The presented techniques allow investigations of other three-dimensional TIs, regardless of the presence of bulk conductivity.
Recently, it has been theoretically predicted that Cd3As2 is a three dimensional Dirac material, a new topological phase discovered after topological insulators, which exhibits a linear energy dispersion in the bulk with massless Dirac fermions. Here, we report on the low-temperature magnetoresistance measurements on a ~50nm-thick Cd3As2 film. The weak antilocalization under perpendicular magnetic field is discussed based on the two-dimensional Hikami-Larkin-Nagaoka (HLN) theory. The electron-electron interaction is addressed as the source of the dephasing based on the temperature-dependent scaling behavior. The weak antilocalization can be also observed while the magnetic field is parallel to the electric field due to the strong interaction between the different conductance channels in this quasi-two-dimensional film.
We report on the study of magneto-photogalvanic and magnetotransport phenomena in 200 nm partially strained HgTe films. This thickness is slightly larger than the estimated critical thickness of lattice relaxation leaving the film partially relaxed with the value of the energy gap close to zero. We show that illumination of HgTe films with monochromatic terahertz laser radiation results in a giant resonant photocurrent caused by the cyclotron resonance in the surface states. The resonant photocurrent is also detected in the reference fully strained 80 nm HgTe films previously shown to be fully gapped 3D topological insulators. We show that the resonance positions in both types of films almost coincide demonstrating the existence of topologically protected surface states in thick HgTe films. The conclusion is supported by magnetotransport experiments.
We study the weak antilocalization (WAL) effect in the magnetoresistance of narrow HgTe wires fabricated in quantum wells (QWs) with normal and inverted band ordering. Measurements at different gate voltages indicate that the WAL is only weakly affected by Rashba spin-orbit splitting and persists when the Rashba splitting is about zero. The WAL signal in wires with normal band ordering is an order of magnitude smaller than for inverted ones. These observations are attributed to a Dirac-like topology of the energy bands in HgTe QWs. From the magnetic-field and temperature dependencies we extract the dephasing lengths and band Berry phases. The weaker WAL for samples with a normal band structure can be explained by a non-universal Berry phase which always exceeds pi, the characteristic value for gapless Dirac fermions.
The results of experimental study of interference induced magnetoconductivity in narrow HgTe quantum wells of hole-type conductivity with a normal energy spectrum are presented. Interpretation of the data is performed with taking into account the strong spin-orbit splitting of the energy spectrum of the two-dimensional hole subband. It is shown that the phase relaxation time found from the analysis of the shape of magnetoconductivity curves for the relatively low conductivity when the Fermi level lies in the monotonic part of the energy spectrum of the valence band behaves itself analogously to that observed in narrow HgTe quantum wells of electron-type conductivity. It increases in magnitude with the increasing conductivity and decreasing temperature following the $1/T$ law. Such a behavior corresponds to the inelasticity of electron-electron interaction as the main mechanism of the phase relaxation and agrees well with the theoretical predictions. For the higher conductivity, despite the fact that the dephasing time remains inversely proportional to the temperature, it strongly decreases with the increasing conductivity. It is presumed that a nonmonotonic character of the hole energy spectrum could be the reason for such a peculiarity. An additional channel of the inelastic interaction between the carriers in the main and secondary maxima occurs when the Fermi level arrives the secondary maxima in the depth of the valence.
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