Spin texture of Bi2Se3 thin films in the quantum tunneling limit.

By means of spin- and angle-resolved photoelectron spectroscopy we studied the spin structure of thin films of the topological insulator Bi2Se3 grown on InP(111). For thicknesses below six quintuple layers the spin-polarized metallic topological surface states interact with each other via quantum tunneling and a gap opens. Our measurements show that the resulting surface states can be described by massive Dirac cones which are split in a Rashba-like manner due to the substrate induced inversion asymmetry. The inner and the outer Rashba branches have distinct localization in the top and the bottom part of the film, whereas the band apices are delocalized throughout the entire film. Supported by calculations, our observations help in the understanding of the evolution of the surface states at the topological phase transition and provide the groundwork for the realization of two-dimensional spintronic devices based on topological semiconductors.

Macroscopic Quantum Tunneling of a Topological Ferromagnet.

The recent advent of topological states of matter spawned many significant discoveries. The quantum anomalous Hall (QAH) effect is a prime example due to its potential for applications in quantum metrology, as well as its influence on fundamental research into the underlying topological and magnetic states and into axion electrodynamics. Here, electronic transport studies on a (V,Bi,Sb)2 Te3 ferromagnetic topological insulator nanostructure in the QAH regime are presented. This allows access to the dynamics of an individual ferromagnetic domain. The domain size is estimated to be in the 50-100 nm range. Telegraph noise resulting from the magnetization fluctuations of this domain is observed in the Hall signal. Careful analysis of the influence of temperature and external magnetic field on the domain switching statistics provides evidence for quantum tunneling (QT) of magnetization in a macrospin state. This ferromagnetic macrospin is not only the largest magnetic object in which QT is observed, but also the first observation of the effect in a topological state of matter.

Effect of Manganese Alloying on Infrared Detectors Made of Pb1-xMnxTe/CdTe Multilayer Composite.

The properties of Pb1-xMnxTe/CdTe multilayer composite grown by molecular beam epitaxy on a GaAs substrate were studied. The study included morphological characterization by X-ray diffraction, scanning electron microscopy, secondary ion mass spectroscopy, as well as electron transport and optical spectroscopy measurements. The main focus of the study was on the sensing properties of photoresistors made of Pb1-xMnxTe/CdTe in the infrared spectral region. It was shown that the presence of Mn in the Pb1-xMnxTe conductive layers shifted the cut-off wavelength toward blue and weakened the spectral sensitivity of the photoresistors. The first effect was due to an increase in the energy gap of Pb1-xMnxTe with an increase in Mn concentration, and the second was due to a pronounced deterioration in the crystal quality of the multilayers owing to the presence of Mn atoms, as shown by the morphological analysis.

Milliwatt terahertz harmonic generation from topological insulator metamaterials.

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example, in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi2Se3 topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW-an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.

Quantum anomalous Hall edge channels survive up to the Curie temperature.

Achieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials.