12 research outputs found
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 . 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
nm, the insulating regime shows the conventional behavior of vanishingly small
conductance at low temperature. However, for thicker quantum wells ( nm), the nominally insulating regime shows a plateau of residual
conductance close to . 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, 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 CdMnTe
We report on magnetooptical studies of MBE-grown half-parabolic CdTe/CdMnTe quantum well structures. The value of the valence band offset Q=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
Fingerprint of different spin-orbit terms for spin transport in HgTe quantum wells
Using k · p theory, we derive an effective four-band model describing the physics of the typical two-dimensional topological insulator (HgTe/CdTe quantum well (QW)) in the presence of an out-of-plane (in the z-direction) inversion breaking potential and an in-plane potential. We find that up to third order in perturbation theory, only the inversion breaking potential generates new elements to the four-band Hamiltonian that are off-diagonal in spin space. When this new effective Hamiltonian is folded into an effective twoband model for the conduction (electron) or valence (heavy hole) bands, two competing terms appear: (i) a Rashba spin–orbit interaction originating from inversion breaking potential in the z-direction and (ii) an in-plane Pauli term as a consequence of the in-plane potential. Spin transport in the conduction band is further analysed within the Landauer–Büttiker formalism. We find that for asymmetrically doped HgTe QWs, the behaviour of the spin-Hall conductance is dominated by the Rashba term