M. Leandersson

Observation of quantum-tunnelling-modulated spin texture in ultrathin topological insulator Bi2Se3 films

Understanding the spin-texture behaviour of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nanodevices. Here, by using spin-resolved photoemission spectroscopy with p-polarized light in topological insulator Bi2Se3 thin films, we report tunnelling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We report a systematic binding energy- and wavevector-dependent spin polarization for the topological surface electrons in the ultrathin gapped-Dirac-cone limit. The polarization decreases significantly with enhanced tunnelling realized systematically in thin insulating films, whereas magnitude of the polarization saturates to the bulk limit faster at larger wavevectors in thicker metallic films. We present a theoretical model that captures this delicate relationship between quantum tunnelling and Fermi surface spin polarization. Our high-resolution spin-based spectroscopic results suggest that the polarization current can be tuned to zero in thin insulating films forming the basis for a future spin-switch nanodevice.

Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO2

We present a spectroscopic study that reveals that the metal-insulator transition of strained VO2 thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions.

Spin-valley locking in the normal state of a transition-metal dichalcogenide superconductor

Metallic transition-metal dichalcogenides (TMDCs) are benchmark systems for studying and controlling intertwined electronic orders in solids, with superconductivity developing from a charge-density wave state. The interplay between such phases is thought to play a critical role in the unconventional superconductivity of cuprates, Fe-based and heavy-fermion systems, yet even for the more moderately-correlated TMDCs, their nature and origins have proved controversial. Here, we study a prototypical example, 2H-NbSe2, by spin- and angle-resolved photoemission and first-principles theory. We find that the normal state, from which its hallmark collective phases emerge, is characterized by quasiparticles whose spin is locked to their valley pseudospin. This results from a combination of strong spin–orbit interactions and local inversion symmetry breaking, while interlayer coupling further drives a rich three-dimensional momentum dependence of the underlying Fermi-surface spin texture. These findings necessitate a re-investigation of the nature of charge order and superconducting pairing in NbSe2 and related TMDCs.

One-dimensional spin texture of Bi(441): Quantum spin Hall properties without a topological insulator

The high index (441) surface of bismuth has been studied using scanning tunneling microscopy (STM), angle resolved photoemission spectroscopy (APRES), and spin-resolved ARPES. The surface is strongly corrugated, exposing a regular array of (110)-like terraces. Two surface localized states are observed, both of which are linearly dispersing in one in-plane direction (k(x)), and dispersionless in the orthogonal in-plane direction (k(y)), and both of which have a Dirac-like crossing at k(x) = 0. Spin ARPES reveals a strong in-plane polarization, consistent with Rashba-like spin-orbit coupling. One state has a strong out-of-plane spin component, which matches with the miscut angle, suggesting its possible origin as an edge state. The electronic structure of Bi(441) has significant similarities with topological insulator surface states and is expected to support one-dimensional quantum spin Hall-like coupled spin-charge transport properties with inhibited backscattering, without requiring a topological insulator bulk.

Adsorbate-Induced Modification of the Confining Barriers in a Quantum Box Array

Quantum devices depend on addressable elements, which can be modified separately and in their mutual interaction. Self-assembly at surfaces, for example, formation of a porous (metal-) organic network, provides an ideal way to manufacture arrays of identical quantum boxes, arising in this case from the confinement of the electronic (Shockley) surface state within the pores. We show that the electronic quantum box state as well as the interbox coupling can be modified locally to a varying extent by a selective choice of adsorbates, here C60, interacting with the barrier. In view of the wealth of differently acting adsorbates, this approach allows for engineering quantum states in on-surface network architectures.

Resonant photoemission study of La1-xSrxMnO3 single crystals

A feature at a binding energy of similar to 2.5eV which has not previously been discussed in the literature is observed in the photoemission spectra of the valence band of La1-xSrxMnO3 single crystals. To reveal the origin of this feature measurements of resonant photoemission on La1-xSrxMnO3 were performed as well as absorption spectra at the La N and Mn L-2,L-3 edges at liquid nitrogen and room temperatures. Only for Mn L edge we observed a clear resonance for a binding energy corresponding to the feature of interest. Comparison with the calculated density of states led to the conclusion that the feature corresponds to Mn t(2g) states.

Angle-resolved photoemission spectroscopy study of Bi2Sr2CaCu2−xNixO8+δ: states near the zone center

Abstract We have studied the influence of Ni doping on the electronic band structure of Bi 2 Sr 2 CaCu 2− x Ni x O 8+δ (x single crystals by angle-resolved photoemission spectroscopy. The study of electronic states close to the zone center Γ (0,0) was the primary objective of these experiments. In numerous studies of doping-free Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) the states at the zone center were assigned to the “ghost” image of the electronic structure due to diffraction of the outgoing electron through the Bi–O superlattice (SL) at e k ± Q , where Q is the SL vector. Here we demonstrate that in Ni doped Bi2212 an extra scattering for photoelectrons exists along the Γ–Y direction and that the main band and “diffraction replica” have different photon energy and time dependence. This difference casts doubt on the straightforward interpretation that the states close to the zone center arise from the diffraction effect.