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We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg0.3Cd0.7Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a twodimensional topological insulator. However, high and non-quantized values of channel resistances show that the topological protection length (i.e. the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is ∼ 100 µm. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasi-periodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.

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Terahertz spectroscopy experiments at magnetic fields and low temperatures were carried out on samples of different gate shapes processed on a high electron mobility GaAs/AlGaAs heterostructure. For a given radiation frequency, multiple magnetoplasmon resonances were observed with a dispersion relation described within a local approximation of the magnetoconductivity tensor. The second harmonic of the cyclotron resonance was observed and its appearance was interpreted as resulting from a high frequency, inhomogeneous electromagnetic field on the border of a two-dimensional electron gas with a metallic gate and/or an ohmic contact. In a two-dimensional electron gas (2DEG) subject to a perpendicular magnetic field (B), electrical dipole transitions between Landau levels (LL) are restricted by the selection rule: N → N ± 1, where N = 0, 1, ... is the LL number. However, this strict selection rule was shown to be relaxed in some cases. Observations of harmonics of the cyclotron resonance (CR) were reported in a Si metal-oxide-semiconductor field-effect transistor at a sufficiently high gate polarization.1 They were explained as a result of a simultaneous action of a short-range scattering by charged traps at the Si/SiO 2 interface and the electron-electron scattering.2 Recently, Dai et al. reported on a strong response of a 2DEG at the second harmonic of the cyclotron resonance (2CR).3 The experiments were carried out at temperature (T ) between 0.3 K and 1.4 K on a GaAs/AlGaAs sample with a 2DEG mobility of 3 × 10 7 cm 2 /Vs and at radiation frequency (f ) from 60 GHz to 120 GHz. The 2CR peak was shown to be distinct from microwave-induced resistance oscillations (MIRO) 4 and disappeared at about 1.4 K. A similar observation was reported by Hatke et al. on a 2DEG in a few GaAs/AlGaAs wafers with the electron mobility of the order of 10 7 cm 2 /Vs in experiments carried out at T = 0.5 K or 1.5 K. In a series of papers 5-7 properties of a peak appearing in the photoresponse in a proximity to the 2CR were analyzed in a great detail. It was shown that the peak did not result from the MIRO and that its properties could not be explained within existing theoretical models.In the present paper we report on the observation of a photoresponse of a 2DEG at magnetic fields corresponding to the excitation of the 2CR. There are essential differences between our experiments and the experiments described in Refs. 3, 5-7. First, we investigated a 2DEG which resides in a single GaAs/AlGaAs heterostructure in comparison to modulation doped AlGaAs/GaAs/AlGaAs single quantum wells. Second, our experiments were carried out at 4.2 K and not in a mK regime. For this reason, the electron mobility in our samples was of the order of 6 × 10 5 cm 2 /Vs and not 10 7 cm 2 /Vs. This clearly indicates that the conditions of our measurements did not correspond to that of the well developed MIRO. Third, we explored both the low (120-660 GHz) and high (1.5-2.5 THz) frequency bands, which essentially enlarges the frequency range used previously, and...

Low temperature, high magnetic field experiments were carried out with monochromatic terahertz (THz) sources to reveal multimode spectra of magnetoplasmons excited in gated and ungated samples processed on a high electron mobility GaAs/AlGaAs heterostructure. We show that playing with the geometry and thickness of the gate one can control both the plasmon dispersion relation and selection rules for plasmon excitation, giving a tool to a better control of plasmon resonances in THz detectors.

In order to characterize magnetic-field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the cyclotron resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with the THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.

The unexpected "0.7" plateau of conductance quantisation is usually observed for ballistic onedimensional devices. In this work we study a quasi-ballistic quantum wire, for which the disorder induced backscattering reduces the conductance quantisation steps. We find that the transmission probability resonances coexist with the anomalous plateau. The studies of these resonances as a function of the in-plane magnetic field and electron density point to the presence of spin polarisation at low carrier concentrations and constitute a method for the determination of the effective g-factor suitable for disordered quantum wires.PACS numbers: 73.63. Nm, 73.23.Ad, 72.25.Dc It is expected that quantum point contacts (QPC) and quantum wires (QW) will act as active components of future nano-electronic devices and circuits. Therefore, the renewed interest in transport and spin properties of one-dimensional (1D) systems recently takes place in the mesoscopic physics community. In those studies, special attention is directed towards the long standing problem of quantum transport -the so called "0.7 anomaly" [1] most often, but not exclusively, observed for devices fabricated on modulation doped GaAs/AlGaAs heterostructures. Usually, anomalous behavior is observed in transport data as a "kink" on the conductance G vs. the device width curve, occurring for the low carrier densities, when G ∼ 0.7 × 2e 2 /h, here −e is the electron charge and h is the Planck constant. The origin of this effect is currently under active debate since this anomaly seems to be an universal, but still unexplained feature of one-dimensional mesoscopic transport. Experimentally, the magnetic field dependence of the additional plateau is common for all studied systems -by applying a parallel in-plane field the 0.7 feature evolves gradually towards 0.5 × 2e 2 /h conductance step, when only one spinpolarised level is occupied [1,2,3,4,5,6]. Therefore, it has been suggested that such an anomalous plateau is due to spontaneous spin polarisation of one-dimensional electron liquid, caused by exchange interactions among carriers in the constricted geometry of the device [2,7]. If it is so, the 1D systems may be used as an efficient spin filter with possible practical applications. This point of * Also at Instytut Fizyki Teoretycznej, Uniwersytet Warszawski, Hoża 69, PL 00-681 Warszawa, Poland; ERATO Semiconductor Spintronics Project, Japan Science Technology Agency, 1-18 Kitamemachi, Aoba-ku Sendai, 980-0023, Japan † Also at ERATO Semiconductor Spintronics Project, Japan Science and Technology Agency, 1-18 Kitamemachi, Aoba-ku Sendai, 980-0023, Japan view is supported by magnetic focusing data obtained for the p-type device, which reveal the static spin polarisation of holes transmitted through the constriction [8].Furthermore, recent shot-noise measurements carried out for n-type QPC [9] show that distinct transport channels exist at G < 2e 2 /h = G 0 , presumably related to spin, exhibiting quite different transmission probabilities.Many experiments, howeve...

To understand a terahertz (THz) response of a point contact device, a number of samples based on CdTe/CdMgTe quantum wells grown by a molecular beam epitaxy were investigated at low temperatures and high magnetic fields. The experiments involved magnetotransport, photocurrent, and transmission measurements carried out with monochromatic THz sources or a Fourier spectrometer. Samples of different geometry with and without gate metallization were used. We observed excitations of a two-dimensional electron plasma in the form of optically induced Shubnikov-de Haas oscillations, cyclotron resonance transitions, and magneto-plasmon resonances. A polaron effect was observed at magnetic fields higher than 10 T. A point contact device processed with an electron beam lithography was investigated as a detector of THz radiation. It was shown that the main mechanism responsible for a THz performance of the point contact was excitation of magneto-plasmons with a wavevector defined by geometrical constrictions of the device mesa.