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Register a new userTwo-dimensional semimetal in a wide HgTe quantum well: magnetotransport and energy spectrum
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Added by
Alexander Germanenko
Publication date
2012
fields
Physics
and research's language is
English
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The results of experimental study of the magnetoresistivity, the Hall and Shubnikov-de Haas effects for the heterostructure with HgTe quantum well of 20.2 nm width are reported. The measurements were performed on the gated samples over the wide range of electron and hole densities including vicinity of a charge neutrality point. Analyzing the data we conclude that the energy spectrum is drastically different from that calculated in framework of $kP$-model. So, the hole effective mass is equal to approximately $0.2 m_0$ and practically independent of the quasimomentum ($k$) up to $k^2gtrsim 0.7times 10^{12}$ cm$^{-2}$, while the theory predicts negative (electron-like) effective mass up to $k^2=6times 10^{12}$ cm$^{-2}$. The experimental effective mass near k=0, where the hole energy spectrum is electron-like, is close to $-0.005 m_0$, whereas the theoretical value is about $-0.1 m_0$.
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Black phosphorus (bP) is the second known elemental allotrope with a layered crystal structure that can be mechanically exfoliated down to atomic layer thickness. We have fabricated bP naked quantum wells in a back-gated field effect transistor geometry with bP thicknesses ranging from $6pm1$ nm to $47pm1$ nm. Using an encapsulating polymer superstrate, we have suppressed bP oxidation and have observed field effect mobilities up to 600 cm$^2$/Vs and on/off current ratios exceeding $10^5$. Importantly, Shubnikov-de Haas (SdH) oscillations observed in magnetotransport measurements up to 35 T reveal the presence of a 2-D hole gas with Schrodinger fermion character in an accumulation layer at the bP/oxide interface. Our work demonstrates that 2-D electronic structure and 2-D atomic structure are independent. 2-D carrier confinement can be achieved in layered semiconducting materials without necessarily approaching atomic layer thickness, advantageous for materials that become increasingly reactive in the few-layer limit such as bP.
Quantum wells (QWs) based on mercury telluride (HgTe) thin films provide a large scale of unusual physical properties starting from an insulator via a two-dimensional Dirac semimetal to a three-dimensional topological insulator. These properties result from the dramatic change of the QW band structure with the HgTe film thickness. Although being a key property, these energy dispersion relations cannot be reflected in experiments due to the lack of appropriate tools. Here we report an experimental and theoretical study of two HgTe quantum wells with inverted energy spectrum in which two-dimensional semimetallic states are realized. Using magneto-optical spectroscopy at sub-THz frequencies we were able to obtain information about electron and hole cyclotron masses at all relevant Fermi level positions and different charge densities. The outcome is also supported by a Shubnikov-de Haas analysis of capacitance measurements, which allows obtaining information about the degeneracy of the active modes. From these data, it is possible to reconstruct electron and hole dispersion relations. Detailed comparative analysis of the energy dispersion relations with theoretical calculations demonstrates a good agreement, reflecting even several subtle features like band splitting, the second conduction band, and the overlaps between the first conduction and first valence band. Our study demonstrates that the cyclotron resonance experiments can be efficiently used to directly obtain the band structures of semimetallic 2D materials.
The results of experimental study of interference induced magnetoconductivity in narrow quantum well HgTe with the normal energy spectrum are presented. Analysis is performed with taking into account the conductivity anisotropy. It is shown that the fitting parameter tau_phi corresponding to the phase relaxation time increases in magnitude with the increasing conductivity (sigma) and decreasing temperature following the 1/T law. Such a behavior is analogous to that observed in usual two-dimensional systems with simple energy spectrum and corresponds to the inelasticity of electron-electron interaction as the main mechanism of the phase relaxation. However, it drastically differs from that observed in the wide HgTe quantum wells with the inverted spectrum, in which tau_phi being obtained by the same way is practically independent of sigma. It is presumed that a different structure of the electron multicomponent wave function for the inverted and normal quantum wells could be reason for such a discrepancy.
The two-dimensional topological insulator phase has been observed previously in single HgTe-based quantum wells with inverted subband ordering. In double quantum wells (DQWs), coupling between the layers introduces additional degrees of freedom leading to a rich phase picture. By studying local and nonlocal resistance in HgTe-based DQWs, we observe both the gapless semimetal phase and the topological insulator phase, depending on parameters of the samples and according to theoretical predictions. Our work establishes the DQWs as a promising platform for realization of multilayer topological insulators.
The microwave photoresistance of a two-dimensional topological insulator in a HgTe quantum well with an inverted spectrum has been experimentally studied under irradiation at frequencies of 110-169 GHz. Two mechanisms of formation of this photoresistance have been revealed. The first mechanism is due to transitions between the dispersion branches of edge current states, whereas the second mechanism is caused by the action of radiation on the bulk of the quantum well.
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