Berezinskii-Kosterlitz-Thouless Transition in the Type-I Weyl Semimetal PtBi.

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with ∼ 275-400 mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal.

Determination of Cleavage Energy and Efficient Nanostructuring of Layered Materials by Atomic Force Microscopy.

A method is presented to use atomic force microscopy to measure the cleavage energy of van der Waals materials and similar quasi-two-dimensional materials. The cleavage energy of graphite is measured to be 0.36 J/m, in good agreement with literature data.

Metamagnetism of Weakly Coupled Antiferromagnetic Topological Insulators.

The magnetic properties of the van der Waals magnetic topological insulators MnBi_{2}Te_{4} and MnBi_{4}Te_{7} are investigated by magnetotransport measurements. We evidence that the relative strength of the interlayer exchange coupling J to the uniaxial anisotropy K controls a transition from an A-type antiferromagnetic order to a ferromagneticlike metamagnetic state. A bilayer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching field of the metamagnet.

Enhanced Mobility of Spin-Helical Dirac Fermions in Disordered 3D Topological Insulators.

The transport length l and the mean free path l are determined for bulk and surface states in a BiSe nanoribbon by quantum transport and transconductance measurements. We show that the anisotropic scattering of spin-helical Dirac fermions results in a strong enhancement of l (≈ 200 nm) and of the related mobility μ (≈ 4000 cm V s), which confirms theoretical predictions.1 Despite strong disorder, the long-range nature of the scattering potential gives a large ratio l/l ≈ 8, likely limited by bulk/surface coupling.

Band Bending Inversion in Bi2Se3 Nanostructures.

Shubnikov-de Haas oscillations were studied under high magnetic field in Bi2Se3 nanostructures grown by chemical vapor transport, for different bulk carrier densities ranging from 3 × 10(19) cm(-3) to 6 × 10(17) cm(-3). The contribution of topological surface states to electrical transport can be identified and separated from bulk carriers and massive two-dimensional electron gas. Band bending is investigated, and a crossover from upward to downward band bending is found at low bulk density as a result of a competition between bulk and interface doping.

P-doping mechanisms in catalyst-free gallium arsenide nanowires.

Doped catalyst-free GaAs nanowires have been grown by molecular beam epitaxy with the gallium-assisted method. The spatial dependence of the dopant concentration and resistivity have been measured by Raman spectroscopy and four point electrical measurements. Along with theoretical considerations, the doping mechanisms have been revealed.