Bias-Free Access to Orbital Angular Momentum in Two-Dimensional Quantum Materials.

The demonstration of a topological band inversion constitutes the most elementary proof of a quantum spin Hall insulator (QSHI). On a fundamental level, such an inverted band gap is intrinsically related to the bulk Berry curvature, a gauge-invariant fingerprint of the wave function's quantum geometric properties in Hilbert space. Intimately tied to orbital angular momentum (OAM), the Berry curvature can be, in principle, extracted from circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES), were it not for interfering final state photoelectron emission channels that obscure the initial state OAM signature.

Absence of edge reconstruction for quantum Hall edge channels in graphene devices.

Quantum Hall (QH) edge channels propagating along the periphery of two-dimensional (2D) electron gases under perpendicular magnetic field are a major paradigm in physics. However, groundbreaking experiments that could use them in graphene are hampered by the conjecture that QH edge channels undergo a reconstruction with additional nontopological upstream modes. By performing scanning tunneling spectroscopy up to the edge of a graphene flake on hexagonal boron nitride, we show that QH edge channels are confined to a few magnetic lengths at the crystal edges.

Helical quantum Hall phase in graphene on SrTiO.

The ground state of charge-neutral graphene under perpendicular magnetic field was predicted to be a quantum Hall topological insulator with a ferromagnetic order and spin-filtered, helical edge channels. In most experiments, however, an insulating state is observed that is accounted for by lattice-scale interactions that promote a broken-symmetry state with gapped bulk and edge excitations. We tuned the ground state of the graphene zeroth Landau level to the topological phase through a suitable screening of the Coulomb interaction with the high dielectric constant of a strontium titanate (SrTiO) substrate.

Low-Magnetic-Field Regime of a Gate-Defined Constriction in High-Mobility Graphene.

We report on the evolution of the coherent electronic transport through a gate-defined constriction in a high-mobility graphene device from ballistic transport to quantum Hall regime upon increasing the magnetic field. At a low field, the conductance exhibits Fabry-Pérot resonances resulting from the npn cavities formed beneath the top-gated regions. Above a critical field B* corresponding to the cyclotron radius equal to the npn cavity length, Fabry-Pérot resonances vanish, and snake trajectories are guided through the constriction with a characteristic set of conductance oscillations.

Weakly-coupled quasi-1D helical modes in disordered 3D topological insulator quantum wires.

Disorder remains a key limitation in the search for robust signatures of topological superconductivity in condensed matter. Whereas clean semiconducting quantum wires gave promising results discussed in terms of Majorana bound states, disorder makes the interpretation more complex. Quantum wires of 3D topological insulators offer a serious alternative due to their perfectly-transmitted mode.

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.

Quasiballistic transport of Dirac fermions in a Bi2Se3 nanowire.

Quantum coherent transport of surface states in a mesoscopic nanowire of the three-dimensional topological insulator Bi(2}Se(3) is studied in the weak-disorder limit. At very low temperatures, many harmonics are evidenced in the Fourier transform of Aharonov-Bohm oscillations, revealing the long phase coherence length of spin-chiral Dirac fermions. Remarkably, from their exponential temperature dependence, we infer an unusual 1/T power law for the phase coherence length L(φ)(T).