F. Teppe

Observation of three-dimensional massless Kane fermions in a zinc-blende crystal

Graphene and topological-insulator surfaces are well known for their two-dimensional conic electronic dispersion relation. Now three-dimensional hyperconic dispersion is shown for electrons in a HgCdTe crystal—once again bridging solid-state physics and quantum electrodynamics.M. Orlita, 2, ∗ D. M. Basko, M. S. Zholudev, 5 F. Teppe, W. Knap, V. I. Gavrilenko, N. N. Mikhailov, S. A. Dvoretskii, P. Neugebauer, C. Faugeras, A.-L. Barra, G. Martinez, and M. Potemski Laboratoire National des Champs Magnétiques Intenses, CNRS-UJF-UPS-INSA, Grenoble, France Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Praha 2, Czech Republic Université Grenoble 1/CNRS, LPMMC UMR 5493, B.P. 166, 38042 Grenoble, France Laboratoire Charles Coulomb (L2C), UMR CNRS 5221, GIS-TERALAB, Université Montpellier II, 34095 Montpellier, France Institute for Physics of Microstructures, RAS, Nizhny Novgorod, Russia A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090, Russia Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany

Magnetic field effect on the terahertz emission from nanometer InGaAs/ AlInAs high electron mobility transistors

The influence of the magnetic field on the excitation of plasma waves in InGaAs/AlInAs lattice matched high electron mobility transistors is reported. The threshold source-drain voltage of the excitation of the terahertz emission shifts to higher values under a magnetic field increasing from 0 to 6 T. We show that the main change of the emission threshold in relatively low magnetic fields (smaller than approximately 4 T) is due to the magnetoresistance of the ungated parts of the channel. In higher magnetic fields, the effect of the magnetic field on the gated region of the device becomes important.

Room temperature terahertz detectors based on semiconductor nanowire field effect transistors

Self-assembled nanowires represent a new interesting technology to be explored in order to increase the cut-off frequency of electronic THz detectors. They can be developed in field effect transistor (FET) and diode geometries exploiting non-linearities of either the transconductance or the current-voltage characteristic as detection mechanism. In this work we demonstrate that semiconductor nanowires can be used as building blocks for the realization of highsensitivity terahertz one-dimensional FET detectors. In order to take advantage of the low effective mass and high mobilities achievable in III-V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly doped with selenium to control the charge density and to optimize source-drain and contact resistance. The detection mechanism exploits the non-linearity of the transconductance: the THz radiation field is fed at the gate-source electrodes with wide band antennas, and the rectified signal is then read at the drain output in the form of a DC voltage. Responsivity values as large as 1 V/W at 0.3 THz have been obtained, with noise equivalent powers (NEP) < 2 × 10-9 W/√Hz at room temperature. The large existing margins for technology improvements, the scalability to higher frequencies, and the possibility of realizing multi-pixel arrays, make these devices highly competitive as a future solution for THz detection.

Oblique modes effect on terahertz plasma wave resonant detection in InGaAs/InAlAs multichannel transistors

We report on the demonstration of narrow terahertz plasma wave resonant detection at low temperature in 200nm gate length InGaAs∕InAlAs multichannel high electron mobility transistors. We observe that the resonant detection linewidth is smaller than in full channel high electron mobility transistors. We interpret this shrinking by the effect of multichannel geometry that does not allow oblique plasma mode propagation along the channel.

Room temperature imaging at 1.63 and 2.54 THz with field effect transistor detectors

GaAs nanometric field effect-transistors are used for room temperature single-pixel imaging using radiation frequencies above 1 THz. Images obtained in transmission mode at 1.63 THz are recorded using transistors operating in a photovoltaic mode with spatial resolution of 300 μm and voltage sensitivity of about 8 mV/W. A reduction in response with increasing frequency was observed and mitigated by the application of a source-drain current, leading to the demonstration of imaging at up to 2.54 THz.

Terahetz detection by heterostructed InAs/InSb nanowire based field effect transistors

Heterostructured InAs/InSb nanowire (Nw) based field effect transistors (FET) have been fabricated and tested as Terahetz radiation detectors. While responsivity and noise equivalent power compare with the ones of InAs nanowire detectors, the presence of small-gap InSb semiconductor gives rise to interesting physical effects such an increase of the detected signal with charge injection through the wire, at odds with standard FET-detectors. Additionally, the photodetected signal voltage changes its sign after a threshold gate bias, which we explain considering surface-related transport and field asymmetries imposed by the use of a lateral gate electrode.

Room temperature coherent and voltage tunable terahertz emission from nanometer-sized field effect transistors

We report on reflective electro-optic sampling measurements of terahertz emission from nanometer-gate-length InGaAs-based high electron mobility transistors. The room temperature coherent gate-voltage tunable emission is demonstrated. We establish that the physical mechanism of the coherent terahertz emission is related to the plasma waves driven by simultaneous current and optical excitation. A significant shift of the plasma frequency and the narrowing of the emission with increasing channel’s current are observed and explained as due to the increase in the carriers’ density and drift velocity.

Terahertz emission induced by optical beating in nanometer-length field-effect transistors

We report on photo-induced terahertz radiation with a high spectral purity generated by a submicron sized InGaAs-based high-electron-mobility transistor. The emission peak is due to the electron-hole pairs photocreated in the transistor channel at the frequency of the beating of two cw-laser sources. The radiation frequency corresponds to the lowest fundamental plasma mode in the gated region of the transistor channel. The observed high emission quality factor at 200 K is interpreted as a result of stream-plasma instability in the two-dimensional electron gas whose appearance is emphasized by the reduction of the velocity relaxation rate with the temperature.

Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond.

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the “single-valley” analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this “bilayer graphene” phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.

Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells

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 m. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasi-periodic resistance uctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.