Felix Baumberger

Recent transport experiments reported a quantum spin Hall insulator phase in monolayer 1T-TaIrTe4 gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative Van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe4 using microfocus laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which linear transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of Van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe4.

The fate of the Fermi surface in bulk electron-doped Sr₂IrO₄ remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr_2−x La _x IrO₄ up to x = 0.2, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to ≃ 1 + x recently observed above x* ≃ 0.16¹. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is T* ≃ 200 K for x = 0.2, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.

We study the electronic structure of bulk 1T-TaSe_{2} in the charge-density wave phase at low temperature. Our spatially and angle-resolved photoemission data show insulating areas coexisting with metallic regions characterized by a chiral Fermi surface and moderately correlated quasiparticle bands. Additionally, high-resolution laser angle-resolved photoemission reveals variations in the metallic regions, with series of low-energy states, whose energy, number, and dispersion can be explained by the formation of quantum well states of different thicknesses. Dynamical mean field theory calculations show that the observed rich behavior can be rationalized by assuming occasional stacking faults of the charge density wave. Our results indicate that the diverse electronic phenomena reported previously in 1T-TaSe_{2} are dictated by the stacking arrangement and the resulting quantum size effects while correlation effects play a secondary role.

Ordinary metals contain electron liquids within well-defined ‘Fermi’ surfaces at which the electrons behave as if they were non-interacting. In the absence of transitions to entirely new phases such as insulators or superconductors, interactions between electrons induce scattering that is quadratic in the deviation of the binding energy from the Fermi level. A long-standing puzzle is that certain materials do not fit this ‘Fermi liquid’ description. A common feature is strong interactions between electrons relative to their kinetic energies. One route to this regime is special lattices to reduce the electron kinetic energies. Twisted bilayer graphene^1–4 is an example, and trihexagonal tiling lattices (triangular ‘kagome’), with all corner sites removed on a 2 × 2 superlattice, can also host narrow electron bands⁵ for which interaction effects would be enhanced. Here we describe spectroscopy revealing non-Fermi-liquid behaviour for the ferromagnetic kagome metal Fe₃Sn₂ (ref. ⁶). We discover three C₃-symmetric electron pockets at the Brillouin zone centre, two of which are expected from density functional theory. The third and most sharply defined band emerges at low temperatures and binding energies by means of fractionalization of one of the other two, most likely on the account of enhanced electron–electron interactions owing to a flat band predicted to lie just above the Fermi level. Our discovery opens the topic of how such many-body physics involving flat bands^7,8 could differ depending on whether they arise from lattice geometry or from strongly localized atomic orbitals^9,10.

Our understanding of quantum materials is commonly based on precise determinations of their electronic spectrum by spectroscopic means, most notably angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. Both require atomically clean and flat crystal surfaces, which are traditionally prepared by in situ mechanical cleaving in ultrahigh vacuum chambers. We present a new approach that addresses three main issues of the current state-of-the-art methods: (1) Cleaving is a highly stochastic and, thus, inefficient process; (2) fracture processes are governed by the bonds in a bulk crystal, and many materials and surfaces simply do not cleave; and (3) the location of the cleave is random, preventing data collection at specified regions of interest. Our new workflow is based on focused ion beam machining of micro-strain lenses, in which shape (rather than crystalline) anisotropy dictates the plane of cleavage, which can be placed at a specific target layer. As proof-of-principle, we show ARPES results from micro-cleaves of Sr2RuO4 along the ac plane and from two surface orientations of SrTiO3, a notoriously difficult to cleave cubic perovskite.

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based microfocus angle resolved photoemission (μ-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters, which provide an accurate description of the electronic structure for all thicknesses.

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($μ$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.