Origin of the quasi-quantized Hall effect in ZrTe5.

The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the Dirac semimetal ZrTe5. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe5 samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe5 electronic structure and its Dirac-type semi-metallic character.

Systematics of molecular self-assembled networks at topological insulators surfaces.

The success of topological insulators (TI) in creating devices with unique functionalities is directly connected to the ability of coupling their helical spin states to well-defined perturbations. However, up to now, TI-based heterostructures always resulted in very disordered interfaces, characterized by strong mesoscopic fluctuations of the chemical potential that make the spin-momentum locking ill-defined over length scales of few nanometers or even completely destroy topological states. These limitations call for the ability to control topological interfaces with atomic precision. Here, we demonstrate that molecular self-assembly processes driven by inherent interactions among the constituents offer the opportunity to create well-defined networks at TIs surfaces. Even more remarkably, we show that the symmetry of the overlayer can be finely controlled by appropriate chemical modifications. By analyzing the influence of the molecules on the TI electronic properties, we rationalize our results in terms of the charge redistribution taking place at the interface. Overall, our approach offers a precise and fast way to produce tailor-made nanoscale surface landscapes. In particular, our findings make organic materials ideal TIs counterparts, because they offer the possibility to chemically tune both electronic and magnetic properties within the same family of molecules, thereby bringing us a significant step closer toward an application of this fascinating class of materials.

Probing the electronic properties of individual MnPc molecules coupled to topological states.

Hybrid organic/inorganic interfaces have been widely reported to host emergent properties that go beyond those of their single constituents. Coupling molecules to the recently discovered topological insulators, which possess linearly dispersing and spin-momentum-locked Dirac fermions, may offer a promising platform toward new functionalities. Here, we report a scanning tunneling microscopy and spectroscopy study of the prototypical interface between MnPc molecules and a Bi2Te3 surface. MnPc is found to bind stably to the substrate through its central Mn atom. The adsorption process is only accompanied by a minor charge transfer across the interface, resulting in a moderately n-doped Bi2Te3 surface. More remarkably, topological states remain completely unaffected by the presence of the molecules, as evidenced by the absence of scattering patterns around adsorption sites. Interestingly, we show that, while the HOMO and LUMO orbitals closely resemble those of MnPc in the gas phase, a new hybrid state emerges through interaction with the substrate. Our results pave the way toward hybrid organic-topological insulator heterostructures, which may unveil a broad range of exciting and unknown phenomena.

Quantum interference mapping of Rashba-split Bloch states in Bi/Ag(111).

We report on low-temperature scanning tunneling spectroscopy investigations of the (sqrt[3]×sqrt[3]) Bi/Ag(111)R30° surface alloy which provides a giant Rashba-type spin splitting. We observed spectroscopic features that are assigned to two Rashba-split bands. Quantum interference mapping shows that backscattering is not only allowed below but also above the Rashba energy. We argue that the observed behavior can be understood within the Bloch picture where k refers to the crystal momentum and the velocity of an electronic state is defined as v(n)(E) = 1/ℏ ∇(k)E(n)(k). The analysis of the energy dispersion of scattering channels reveals a conventional Rashba splitting for the unoccupied Rashba bands, while hybridization is observed in the occupied states.

Temperature and size dependence of antiferromagnetism in mn nanostructures.

We report on variable-temperature STM investigations of the spontaneous long-range magnetic order of Mn monolayer nanostructures epitaxially grown on stepped W(110). The measurements reveal that the onset of the antiferromagnetic order is closely related to the Mn nanostructure width along the [001] direction, with a decreasing Néel temperature as we move from a 2D toward a quasi-1D system. In contrast, lateral confinement along the [110] direction seems to play a less important role. The results are discussed in terms of anisotropic exchange coupling and of boundary effects, both potentially stabilizing long-range magnetic order in nanostructures confined in the [110] direction.