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ABSTRACT: Quantum wells of HgTe doped with Mn display the quantum anomalous Hall effect
due to the magnetic moments of the Mn ions. In the presence of a magnetic
field, these magnetic moments induce an effective nonlinear Zeeman effect,
causing a nonmonotonic bending of the Landau levels. As a consequence, the
quantized (spin) Hall conductivity exhibits a reentrant behavior as one
increases the magnetic field. Here, we will discuss the appearance of different
types of reentrant behavior as a function of Mn concentration, well thickness,
and temperature, based on the qualitative form of the Landau-level spectrum in
an effective four-band model.
03/2012;
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ABSTRACT: We report transport studies on a three-dimensional, 70-nm-thick HgTe layer, which is strained by epitaxial growth on a CdTe substrate. The strain induces a band gap in the otherwise semimetallic HgTe, which thus becomes a three-dimensional topological insulator. Contributions from residual bulk carriers to the transport properties of the gapped HgTe layer are negligible at mK temperatures. As a result, the sample exhibits a quantized Hall effect that results from the 2D single cone Dirac-like topological surface states.
Physical Review Letters 03/2011; 106(12):126803. · 7.37 Impact Factor
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ABSTRACT: HgTe/HgCdTe quantum wells with the inverted band structure have been probed
using far infrared magneto-spectroscopy. Realistic calculations of Landau level
diagrams have been performed to identify the observed transitions.
Investigations have been greatly focused on the magnetic field dependence of
the peculiar pair of "zero-mode" Landau levels which characteristically split
from the upper conduction and bottom valence bands, and merge under the applied
magnetic field. The observed avoided crossing of these levels is tentatively
attributed to the bulk inversion asymmetry of zinc blend compounds.
11/2010;
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B. Büttner,
C. X. Liu,
G. Tkachov, E. G. Novik,
C. Brüne,
H. Buhmann,
E. M. Hankiewicz,
P. Recher,
B. Trauzettel,
S C Zhang,
L. W. Molenkamp
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ABSTRACT: Dirac fermions have been studied intensively in condensed matter physics in
recent years. Many theoretical predictions critically depend on the number of
valleys where the Dirac fermions are realized. In this work, we report the
discovery of a two dimensional system with a single valley Dirac cone. We study
the transport properties of HgTe quantum wells grown at the critical thickness
separating between the topologically trivial and the quantum spin Hall phases.
At high magnetic fields, the quantized Hall plateaus demonstrate the presence
of a single valley Dirac point in this system. In addition, we clearly observe
the linear dispersion of the zero mode spin levels. Also the conductivity at
the Dirac point and its temperature dependence can be understood from single
valley Dirac fermion physics.
09/2010;
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ABSTRACT: We calculate transport properties of HgTe quantum wells that exhibit the quantum spin Hall effect. We concentrate on the ballistic bulk contribution as a function of aspect ratio and Fermi energy. We show that the conductance and the shot noise are distinctively different for the so-called normal regime (the topologically trivial case) and the so-called inverted regime (the topologically non-trivial case). Thus, it is possible to verify the topological order of two-dimensional topological insulators such as HgTe quantum wells not only via observable edge properties but also via observable bulk properties. In addition, we show that the bulk contribution can even exceed the edge contribution for certain parameter regimes (and in all regimes for the case of the shot noise). We test the validity of our analytical approach against a tight-binding model that allows us to include random disorder numerically which is shown to have only a minor effect as long as its strength does not exceed the bulk gap.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
08/2010;
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Nature Physics 05/2010; 6(6):448-454. · 18.97 Impact Factor
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B. Wittmann,
S. N. Danilov,
V. V. Bel'kov,
S. A. Tarasenko, E. G. Novik,
H. Buhmann,
C. Brüne,
L. W. Molenkamp,
Z. D. Kvon,
N. N. Mikhailov,
S. A. Dvoretsky,
N. Q. Vinh,
A. F. G. van der Meer,
B. Murdin,
S. D. Ganichev
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ABSTRACT: We describe the observation of the circular and linear photogalvanic effects in HgTe/CdHgTe quantum wells. The interband absorption of mid-infrared radiation as well as the intrasubband absorption of terahertz (THz) radiation in the QWs structures is shown to cause a dc electric current due to these effects. The photocurrent magnitude and direction varies with the radiation polarization state and crystallographic orientation of the substrate in a simple way that can be understood from a phenomenological theory. The observed dependences of the photocurrent on the radiation wavelength and temperature are discussed.
02/2010;
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ABSTRACT: We calculate bulk transport properties of two-dimensional topological insulators based on HgTe quantum wells in the ballistic regime. Interestingly, we find that the conductance and the shot noise are distinctively different for the so-called normal regime (the topologically trivial case) and the so-called inverted regime (the topologically non-trivial case). Thus, it is possible to verify the topological order of a two-dimensional topological insulator not only via observable edge properties but also via observable bulk properties. This is important because we show that under certain conditions the bulk contribution can dominate the edge contribution which makes it essential to fully understand the former for the interpretation of future experiments in clean samples. Comment: 5 pages, 4 figures
12/2009;
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H. Diehl,
V. A. Shalygin,
L. E. Golub,
S. A. Tarasenko,
S. N. Danilov,
V. V. Bel'kov, E. G. Novik,
H. Buhmann,
C. Brüne,
E. L. Ivchenko,
S. D. Ganichev
[show abstract]
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ABSTRACT: We report on the observation of nonlinear magneto-gyrotropic photogalvanic effect in HgTe/HgCdTe quantum wells. The interband absorption of mid-infrared radiation as well as the intrasubband absorption of terahertz radiation in the heterostructures is shown to cause a dc electric current in the presence of an in-plane magnetic field. A cubic in magnetic field component of the photocurrent is observed in quantum wells with the inverted band structure only. The experimental data are discussed in terms of both the phenomenological theory and microscopic models. Comment: 10 pages, 9 figures
05/2009;
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ABSTRACT: We report the first electrical manipulation and detection of the mesoscopic intrinsic spin-Hall effect (ISHE) in semiconductors through non-local electrical measurement in nano-scale H-shaped structures built on high mobility HgTe/HgCdTe quantum wells. By controlling the strength of the spin-orbit splittings and the n-type to p-type transition by a top-gate, we observe a large non-local resistance signal due to the ISHE in the p-regime, of the order of kOhms, which is several orders of magnitude larger than in metals. In the n-regime, as predicted by theory, the signal is at least an order of magnitude smaller. We verify our experimental observation by quantum transport calculations which show quantitative agreement with the experiments.
01/2009;
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ABSTRACT: We investigate an effective low energy theory of HgTe quantum wells near their mass inversion thickness in a perpendicular magnetic field. By comparison of the effective band structure with a more elaborated and well-established model, the parameter regime and the validity of the effective model is scrutinized. Optical transitions in HgTe quantum wells are analyzed. We find selection rules which we functionalize to optically manipulate edge state transport. Qualitatively, our findings equally apply to optical edge current manipulation in graphene.
01/2009;
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ABSTRACT: We have studied the temperature dependent thermopower of gate-defined, lateral quantum dots in the Coulomb blockade regime using an electron heating technique. The line shape of the thermopower oscillations depends strongly on the contributing tunneling processes. Between 1.5 K and 40 mK a crossover from a pure sawtooth- to an intermitted sawtooth-like line shape is observed. The latter is attributed to the increasing dominance of cotunneling processes in the Coulomb blockade regime at low temperatures.
09/2006;
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ABSTRACT: The magnetotransport properties of a series of Hg1−yMnyTe quantum wells (y = 2%) reveal anomalous sequences of quantum Hall plateaus and corresponding minima in the longitudinal magnetoresistance which correspond solely to odd filling factors. This anomaly originates from the very large spin splitting in this material, which exceeds the cyclotron energy even at moderate magnetic fields (B>2T). An 8×8 band k∙p approach has been used to calculate the magnetic field dependence of the Landau levels. Finite temperature and Landau level broadening considerations confirm that subband crossings in the vicinity of the Fermi energy are responsible for the suppression of even filling factor quantum Hall states.
Applied Physics Letters 05/2005; 86(21):212104-212104-3. · 3.84 Impact Factor
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ABSTRACT: The band structure of semimagnetic Hg1−yMnyTe∕Hg1−xCdxTe type-III quantum wells (QW’s) has been calculated using an eight-band k∙p 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 Hg0.98Mn0.02Te∕Hg0.3Cd0.7Te 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 Hg1−yMnyTe∕Hg1−xCdxTe QW’s.
Phys. Rev. B. 09/2004; 72(3).
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Y. S. Gui,
C. R. Becker,
N. Dai,
J. Liu,
Z. J. Qiu, E. G. Novik,
M. Schaefer,
X. Z. Shu,
J. -H. Chu,
H. Buhmann,
L. W. Molenkamp
[show abstract]
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ABSTRACT: We have investigated beating patterns in Shubnikov-de Haas oscillations for HgTe/Hg_{0.3}Cd_{0.7}Te(001) quantum wells with electron densities of 2 to 3 X 10^{12} cm^{-2}. Up to 12 beating nodes have been observed at magnetic fields between 0.9 and 6 T. Zero magnetic field spin-orbit splitting energies up to 30 meV have been directly determined from the node positions as well as from the intersection of self-consistently calculated Landau levels. These values, which exceed the thermal broadening of Landau levels, k_B T, at room temperature, are in good agreement with Rashba spin-orbit splitting energies calculated by means of an 8 X 8 kp Kane model. The experimental Shubnikov-de Haas oscillations are also in good agreement with numerical simulations based on this model. Comment: 5 pages, 6 figures
06/2004;
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ABSTRACT: The transport properties of micrometer scale structures fabricated from high-mobility HgTe quantum wells have been investigated. A special photoresist and Ti masks were used, which allow for the fabrication of devices with characteristic dimensions down to 0.45 μm. Evidence that the transport properties are dominated by ballistic effects in these structures is presented. Monte Carlo simulations of semiclassical electron trajectories show good agreement with the experiment. © 2003 American Institute of Physics.
Applied Physics Letters 08/2003; 83(7):1376-1378. · 3.84 Impact Factor
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ABSTRACT: The transport properties of micrometer scale structures fabricated from high-mobility HgTe quantum-wells have been investigated. A special photoresist and Ti masks were used, which allow for the fabrication of devices with characteristic dimensions down to 0.45 $\mu$m. Evidence that the transport properties are dominated by ballistic effects in these structures is presented. Monte Carlo simulations of semi-classical electron trajectories show good agreement with the experiment. Comment: 3 pages, 3 figures; minor revisions: replaced "inelastic mean free path" with "transport mean free path"; corrected typing errors; restructered most paragraphs for easier reading; accepted for publication in APL
05/2003;
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ABSTRACT: A quantum mechanical model based on a Green's function approach has been used to calculate the transmission probability of electrons traversing a two-dimensional electron gas injected and detected via mode-selective quantum point contacts. Two-dimensional scattering potentials, back-scattering, and temperature effects were included in order to compare the calculated results with experimentally observed interference patterns. The results yield detailed information about the distribution, size, and the energetic height of the scattering potentials. Comment: 7 pages, 6 figures
10/2002;
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Helgi Diehl,
Vadim Shalygin,
Leonid Golub,
Sergey Tarasenko,
Sergey Danilov,
Vassilij Belkov, E. G. Novik,
H. Buhmann,
C. Brüne,
L. W. Molenkamp,
Eougenious Ivchenko,
Sergey Ganichev
[show abstract]
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ABSTRACT: We report on the observation of magnetic-field-induced photocurrent in HgTe/HgCdTe quantum wells of different widths. Both the intrasubband and interband absorption of infrared/terahertz radiation in the heterostructures is shown to cause a dc electric current in the presence of an in-plane magnetic field. The photocurrent behavior upon variation in the radiation polarization, magnetic-field strength, and temperature is studied. At a moderate magnetic field the current exhibits a linear field dependence. At high magnetic fields, however, it becomes nonlinear and is dominated by a cubic in magnetic-field contribution. The latter effect is observed in quantum wells with the inverted band structure only. The experimental results are analyzed in terms of the phenomenological theory and microscopic models of magnetogyrotropic photogalvanic effect based on asymmetry of optical transitions and/or asymmetric relaxation of carriers in the momentum space. The effect is shown to be related to the gyrotropic properties of the structures. The developed theory of magnetogyrotropic photocurrent describes well all experimental results. It is shown that both intrasubband and interband optical transitions may lead to spin-related as well as to spin-independent magnetic-field-induced photocurrents.
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ABSTRACT: HgTe/HgxCd1-xTe quantum wells with the inverted band structure have been probed using far-infrared magnetospectroscopy. Realistic calculations of Landau level diagrams have been performed to identify the observed transitions. Investigations have been greatly focused on the magnetic field dependence of the peculiar pair of “zero-mode” Landau levels, which characteristically split from the upper conduction and bottom valence bands, and merge under the applied magnetic field. The observed avoided crossing of these levels is tentatively attributed to the bulk inversion asymmetry of zinc-blende compounds.
Phys. Rev. B. 83(11).
