arXiv subjects

M. Ünzelmann

Publications and source records attributed to M. Ünzelmann.

8 recordsLinked to original sources

arXiv2026

Acoustic Wave-Function Imaging of Sublattice Physics in a Kagome Lattice

The canonical kagome band structure hosts a Dirac cone, saddle-point van Hove singularities (vHS), and a flat band. Characteristically, these features exhibit distinct sublattice localization within the three-site basis of the kagome lattice. However, experimental proof of this sublattice character is challenging and requires direct access to both the real-space wave function and its associated momentum-space texture. Here, we implement an acoustic kagome lattice on the cm scale and measure the full excitation spectrum, thereby imaging the wave function at each lattice site, including both amplitude and phase. This enables investigation of the momentum-dependent sublattice texture across the complete band structure, particularly at the flat band and at the mixed- and pure-sublattice van Hove singularities. Our results are in excellent agreement with minimal tight-binding model calculations and constitutes the first direct experimental observation of the sublattice-resolved band structure in a kagome lattice.

cond-mat.str-el↗

arXiv2026

Growth of Altermagnetic α-MnTe Films: Substrate Variation and Surface Modification

Manganese telluride (MnTe) in its hexagonal α-MnTe crystal structure has evolved as one of the altermagnet workhorse materials. The synthesis of MnTe thin films is highly relevant for both fundamental science and device applications. Here, we report on the epitaxial growth of MnTe thin films and heterostructures. The films are studied by X-ray and electron diffraction as well as soft X-ray angle-resolved photoemission spectroscopy. We demonstrate the ability to grow high-quality α-MnTe on various substrates, ranging from transparent band insulators, over topological insulators, metallic transition metal chalcogenides, to the van der Waals ferromagnet Fe$_3$GeTe$_2$. While insulating substrates are useful for transport experiments or optical spectroscopy, metallic topological surface states may trigger spintronic interface effects, such as spin-orbit torques. Metallic substrates, in general, are highly relevant to avoid charging at the insulating MnTe films in electron spectroscopy or microscopy methods. Lastly, ferromagnetic substrates will be of interest to control magnetization across the interface. In addition, we discuss the formation of superstructures on the MnTe(0001) surfaces, that emerge directly after growth, upon subsequent tellurium evaporation and after thermal treatment. This will be relevant in further studying the surface magnetic and electronic structure in α-MnTe.

cond-mat.mtrl-sci↗

arXiv2026

Observation and Control of Moiré-Tailored Topological Dirac States

Moiré heterostructures provide a powerful framework for tailoring electronic band structures via controlled long-range periodic superlattice potentials. Beyond widely studied moiré-tailored flat bands, folded band structures can host emergent Dirac states, which have recently attracted considerable interest. Direct momentum-resolved observation of gapless moiré-Dirac quasiparticles, however, is challenging and has so far remained elusive. By performing angle-resolved photoemission spectroscopy measurements on an epitaxial surface-moiré structure, we here provide direct spectroscopic evidence of moiré-dressed Dirac states with topological character. Driven by the one-dimensional superlattice potential, electrons propagate anisotropically with a weak but massless Dirac dispersion along the confinement direction. The observed band crossings belong to topological nodal lines pinned to the mini-Brillouin zone boundaries. As such, they are enforced and robustly protected by the non-symmorphic symmetry of the superlattice. Finally, we demonstrate that the topological excitations can be almost continuously controlled by tuning the moiré lattice periodicity, directly unveiling moiré heterostructures as a promising platform for creating and controlling topological moiré-Dirac states.

cond-mat.str-el↗

arXiv2025

Prediction and observation of the first antiferromagnetic topological insulator

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables a number of exotic phenomena having potential applications in spintronics. So far, MTIs have only been created by means of doping nonmagnetic TIs with 3d transition metal elements, however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered magnetic compound, could be an ideal solution to these problems, but no such material was observed to date. Here, using density functional theory we predict and further confirm by means of structural, transport, magnetic, angle- and spin-resolved photoemission spectroscopy measurements the realization of the antiferromagnetic (AFM) TI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($Θ$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S = ΘT_{1/2}$, giving rise to the $Z_2$ topological classification of AFM insulators. We find $Z_2 = 1$ for MnBi$_2$Te$_4$, which confirms its topologically nontrivial nature. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ exhibits a giant bandgap in the topological surface state as evidenced by ab initio calculations and photoemission measurements. These results culminate almost a decade-long search of an AFMTI, predicted in 2010. Furthermore, MnBi$_2$Te$_4$ is the first intrinsic magnetic TI realized experimentally.

cond-mat.mtrl-sci↗

arXiv2024

Tomographic Imaging of Orbital Vortex Lines in Three-Dimensional Momentum Space

We report the experimental discovery of orbital vortex lines in the three-dimensional (3D) band structure of a topological semimetal. Combining linear and circular dichroism in soft x-ray angle-resolved photoemission (SX-ARPES) with first-principles theory, we image the winding of atomic orbital angular momentum, thereby revealing - and determining the location of - lines of vorticity in full 3D momentum space. Our observation of momentum-space vortex lines with quantized winding number establishes an analogue to real-space quantum vortices, for instance, in type-II superconductors and certain non-collinear magnets. These results establish multimodal dichroism in SX-ARPES as an approach to trace 3D orbital textures. Our present findings particularly constitute the first imaging of non-trivial quantum-phase winding at line nodes and may pave the way to new orbitronic phenomena in quantum materials

cond-mat.str-el↗

arXiv2023

Strongly Anisotropic Spin and Orbital Rashba Effect at a Tellurium - Noble Metal Interface

We study the interplay of lattice, spin and orbital degrees of freedom in a two-dimensional model system: a flat square lattice of Te atoms on a Au(100) surface. The atomic structure of the Te monolayer is determined by scanning tunneling microscopy (STM) and quantitative low-energy electron diffraction (LEED-IV). Using spin- and angle-resolved photoelectron spectroscopy (ARPES) and density functional theory (DFT), we observe a Te-Au interface state with highly anisotropic Rashba-type spin-orbit splitting at the X point of the Brillouin zone. Based on a profound symmetry and tight-binding analysis, we show how in-plane square lattice symmetry and broken inversion symmetry at the Te-Au interface together enforce a remarkably anisotropic orbital Rashba effect which strongly modulates the spin splitting.

cond-mat.mtrl-sci↗

arXiv2023

Momentum-space signatures of Berry flux monopoles in a Weyl semimetal

Since the early days of Dirac flux quantization, magnetic monopoles have been sought after as a potential corollary of quantized electric charge. As opposed to magnetic monopoles embedded into the theory of electromagnetism, Weyl crystals exhibit Berry flux monopoles in reciprocal parameter space. As a function of crystal momentum, such monopoles locate at the degeneracy point of the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of Berry flux monopoles in TaAs as a paradigmatic Weyl semimetal. We have probed the orbital and spin angular momentum (OAM and SAM) of the Weyl-fermion states by angle-resolved photoemission spectroscopy at bulk-sensitive soft X-ray energies (SX-ARPES) combined with photoelectron spin detection and circular dichroism. Supported by first-principles calculations, our measurements image characteristics of a topologically non-trivial winding of the OAM at the Weyl nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results experimentally visualize the non-trivial momentum-space topology in a Weyl semimetal, promising to have profound implications for the study of quantum-geometric effects in solids.

cond-mat.mtrl-sci↗

arXiv2019

Surface states and Rashba-type spin polarization in antiferromagnetic MnBi$_2$Te$_4$

The layered van der Waals antiferromagnet MnBi$_2$Te$_4$ has been predicted to combine the band ordering of archetypical topological insulators such as Bi$_2$Te$_3$ with the magnetism of Mn, making this material a viable candidate for the realization of various magnetic topological states. We have systematically investigated the surface electronic structure of MnBi$_2$Te$_4$(0001) single crystals by use of spin- and angle-resolved photoelectron spectroscopy experiments. In line with theoretical predictions, the results reveal a surface state in the bulk band gap and they provide evidence for the influence of exchange interaction and spin-orbit coupling on the surface electronic structure.

cond-mat.str-el↗