Cyclotron resonance photoconductivity of a two-dimensional electron gas in HgTe quantum wells

Far-infrared cyclotron resonance photoconductivity (CRP) is investigated in HgTe quantum wells (QWs) of various widths grown on (013) oriented GaAs substrates. It is shown that CRP is caused by the heating of two-dimensional electron gas (2DEG). From the resonance magnetic field strength effective masses and their dependence on the carrier concentration is obtained. We found that the effective mass in each sample slightly increases from the value (0.0260 \pm 0.0005)m_0 at N_s = 2.2x10^11 cm^(-2) to (0.0335 \pm 0.0005)m_0 at N_s = 9.6x10^11 cm^(-2). Compared to determination of effective masses by the temperature dependence of magnitudes of the Shubnikov-de Haas (SdH) oscillations used so far in this material our measurements demonstrate that the CRP provides a more accurate (about few percents) tool. Combining optical methods with transport measurements we found that the transport time substantially exceeds the cyclotron resonance lifetime as well as the quantum lifetime which is the shortest.Comment: 3 pages, 2 figure

Band structure of semimagnetic Hg1-yMnyTe quantum wells

The band structure of semimagnetic Hg_1-yMn_yTe/Hg_1-xCd_xTe type-III quantum wells has been calculated using eight-band kp model in an envelope function approach. Details of the band structure calculations are given for the Mn free case (y=0). A mean field approach is used to take the influence of the sp-d exchange interaction on the band structure of QW's with low Mn concentrations into account. The calculated Landau level fan diagram and the density of states of a Hg_0.98Mn_0.02Te/Hg_0.3Cd_0.7Te QW are in good agreement with recent experimental transport observations. The model can be used to interpret the mutual influence of the two-dimensional confinement and the sp-d exchange interaction on the transport properties of Hg_1-yMn_yTe/Hg_1-xCd_xTe QW's.Comment: 12 pages, 4 figure

The Quantum Spin Hall Effect: Theory and Experiment

The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells. By varying the thickness of the quantum well, the band structure changes from a normal to an "inverted" type at a critical thickness $d_c$. We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te quantum wells. We review both the fabrication of the sample and the experimental setup. For thin quantum wells with well width $d_{QW}< 6.3$ nm, the insulating regime shows the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells ($d_{QW}> 6.3$ nm), the nominally insulating regime shows a plateau of residual conductance close to $2e^2/h$. The residual conductance is independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance is destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, $d_c= 6.3$ nm, is also independently determined from the occurrence of a magnetic field induced insulator to metal transition.Comment: Invited review article for special issue of JPSJ, 32 pages. For higher resolution figures see official online version when publishe

Half-Parabolic Quantum Wells of Diluted Magnetic Semiconductor Cd1−x\text{}_{1-x}Mnx\text{}_{x}Te

We report on magnetooptical studies of MBE-grown half-parabolic CdTe/Cd$\text{}_{x}$Mn$\text{}_{1-x}$Te quantum well structures. The value of the valence band offset Q$\text{}_{v}$=0.4 ± 0.05 was determined by comparing energies of optical transitions in the absence of a magnetic field with model calculations. This value was verified by fitting the observed spin splitting of the lowest heavy hole (hh) state. We discuss also the temperature dependence of Q$\text{}_{v}$