arXiv subjects

Søren Ulstrup

Publications and source records attributed to Søren Ulstrup.

At least 19 recordsLinked to original sources

arXiv2026

Observation of dodecagonal replica bands in 30$^{\circ}$-twisted bilayer WSe$_2$

Twisted bilayers of two-dimensional (2D) transition metal dichalcogenides are promising systems for achieving tunable quasicrystalline orders with emergent properties. The underpinning electronic structure and scattering processes in 2D quasicrystals with multiorbital dodecagonal replica bands have so far not been determined. Here, we utilize angle-resolved photoemission spectroscopy (ARPES) with micrometer spatial resolution to directly observe replica bands in 30$^{\circ}$-twisted bilayer WSe$_2$. The symmetry and intensity distribution of the observed replicas are explained by interlayer Umklapp scattering from bottom to top WSe$_2$ layers. Our spectral function measurements are consistent with the presence of a van Hove singularity adjacent to the $\mathrm{K}$-valleys, which underlines the possibility of inducing electronic reconstructions in bilayers with a large interlayer twist angle.

cond-mat.mes-hall↗

arXiv2026

Micro- and nanoscale focusing across the XUV range of the ASTRID2 light source with a capillary optic

Focusing of synchrotron light across extreme ultraviolet (XUV) and soft X-ray regimes is increasingly desired for photoemission-based techniques where reduced beam width gives access to smaller samples such as microscopic single crystals and functioning two-dimensional (2D) heterostructures and devices. Many existing focusing methods, however, are not able to take full advantage of the synchrotron beam due to limited photon energy range or low transmission. Modern capillary optics have enabled achromatic, high transmission focusing of XUV and X-ray light. Here, we present a detailed characterisation of such an achromatic capillary optic installed at the AU-SGM4 beamline for spatial- and angle-resolved photoemission spectroscopy (ARPES) experiments at the ASTRID2 light source. The transmission of the capillary as a function of photon energy is given, and the dependence of the beam width, position, and transmission are measured against the source size. Analysis of the far-field image of the beam allows for slope errors on the inner surface of the capillary to be overcome by selectively aperturing the beam, resulting in a minimum beam width of 900 nm measured in a photoemission geometry.

physics.optics↗

arXiv2026

Direct visualization of gate-tunable flat bands in twisted double bilayer graphene

The symmetry-broken correlated states in twisted double bilayer graphene (TDBG) can be tuned via several external knobs, including twist angle, displacement field, and carrier density. However, a direct, momentum-resolved characterization of how these parameters reshape the flat-band structure remains limited. In this study, we employ micro focused angle-resolved photoemission spectroscopy to investigate the flat-band dispersion of TDBG at a twist angle of 1.6, systematically varying the displacement field and carrier density via electrostatic gating. We directly observe multiple flat moir'e minibands near charge neutrality, including a flat remote valence band residing below the low-energy flat-band manifold. Furthermore, the dominant Coulomb repulsive energy over the flat- band bandwidth suggests favorable conditions for the emergence of interaction-driven correlated phenomena in TDBG. These findings establish that the formation and evolution of flat bands in TDBG arises from the interplay between the electron filling and the displacement field.

cond-mat.mes-hall↗

arXiv2026

Band offsets and stability of WSe$_2$/RuCl$_3$ van der Waals charge-transfer contacts

The layered Mott insulator $α$-RuCl$_3$ induces degenerate hole-doping in two-dimensional semiconductors due to its large electron affinity, making it a promising charge-transfer material for establishing ohmic contacts in electronic devices. In order to assess the applicability and guide the design of devices incorporating RuCl$_3$ it is critical to determine the electronic structure and robustness of the band offsets that underpin the transport properties of semiconductors in contact with RuCl$_3$. Here, we apply micro-focused angle-resolved photoemission spectroscopy to determine the electronic structure of single-layer WSe$_2$ contacted to RuCl$_3$ on hexagonal boron nitride substrates. We find that formation of a functioning WSe$_2$/RuCl$_3$ contact leads to a valence band shift of $(0.68 \pm 0.05)$ eV towards the Fermi energy in WSe$_2$. The charge transfer effect is challenging to observe as it depends sensitively on fabrication conditions such as solvent exposure, quality of interface encapsulation and heating of RuCl$_3$, imposing strict requirements on device design to attain high-quality contacts.

cond-mat.mes-hall↗

arXiv2026

Large-scale Integration of Experimental and Computational Data for 2D Materials

The past decade has seen rapid growth in the number of experimentally realized two-dimensional (2D) materials with diverse chemical and physical properties. However, information on their crystal structure, synthesis routes, and measured or predicted properties, remains scattered across thousands of publications. Here we consolidate this fragmented knowledge by establishing X2DB - an open infrastructure that integrates experimental and computational data on 2D materials. Using extensive literature mining and direct community uploads, we identify 370 unique 2D materials that have been realized in monolayer or few-layer form, and link them to their digital counterparts in computational databases, enabling consistent ab initio characterization of their properties across monolayer, bilayer and bulk forms. We describe the structure and content of the database highlighting its support for community uploads, illustrate how it can be used to generate new scientific insight and introduce a hierarchical classification of the known set of 2D materials. Our work provides a foundation for the integration and cross-fertilization of experimental and theoretical knowledge, opening new avenues for data-driven, predictive synthesis of novel 2D materials.

cond-mat.mtrl-sci↗

arXiv2026

Direct nanoscale mapping of band alignment in single-layer semiconducting lateral heterojunctions

Atomic-scale control over band alignment in single-layer lateral heterostructures (LHSs) of dissimilar transition metal dichalcogenides (TMDCs) is critical for nextgeneration electronic, optoelectronic, and quantum technologies. However, direct experimental access to interfacial electronic states with nanometer precision remains a significant challenge. Here, we employ angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) to directly map the epitaxial alignment and valence band evolution across MoSe2-WSe2 LHSs. By combining nanoARPES with spatially resolved photoluminescence, we correlate the evolution of the valence band maximum and exciton features across both atomically sharp and compositionally graded diffusive interfaces. We identified type-II band alignments governed by both material composition and interstitial-induced modifications of band offsets, in close agreement with density functional theory calculations. These results reveal fundamental mechanisms of electronic structure modulation at 1D TMDC heterointerfaces and provide a robust platform for tailored band engineering in van der Waals materials.

cond-mat.mes-hall↗

arXiv2025

Resonantly enhanced photoemission from topological surface states in MnBi$_6$Te$_{10}$

The dispersion of topological surface bands in MnBi$_2$Te$_4$-based magnetic topological insulator heterostructures is strongly affected by band hybridization and is spatially inhomogeneous due to varying surface layer terminations on microscopic length scales. Here, we apply micro-focused angle-resolved photoemission spectroscopy with tunable photon energy from 18 to 30 eV to distinguish bulk valence and conduction bands from surface bands on the three surface terminations of MnBi$_6$Te$_10$. We observe a strong enhancement of photoemission intensity from the topological surface bands at the Bi O4 absorption edge, which is exploited to visualize a gapless Dirac cone on the MnBi$_2$Te$_4$-terminated surface and varying degrees of hybridization effects in the surface bands on the two distinct Bi$_2$Te$_3$-terminated surfaces.

cond-mat.mtrl-sci↗

arXiv2025

Quasiparticle gap renormalization driven by internal and external screening in a WS$_2$ device

The electronic band gap of a two-dimensional semiconductor within a device architecture is sensitive to variations in screening properties of adjacent materials in the device and to gate-controlled doping. Here, we employ micro-focused angle resolved photoemission spectroscopy to separate band gap renormalization effects stemming from environmental screening and electron-doping during \textit{in situ} gating of a single-layer WS$_{2}$ device. The WS$_{2}$ is supported on hBN and contains a section that is exposed to vacuum and another section that is encapsulated by a graphene contact. We directly observe the doping-induced semiconductor-metal transition and band gap renormalization in the two sections of WS$_2$. Surprisingly, a larger band gap renormalization is observed in the vacuum-exposed section than in the graphene-encapsulated - and thus ostensibly better screened - section of the WS$_2$. Using $GW$ calculations, we determine that intrinsic screening due to stronger doping in vacuum exposed WS$_2$ exceeds the external environmental screening in graphene-encapsulated WS$_2$.

cond-mat.mes-hall↗

arXiv2025

Multi Moire Networks in Engineered Lateral Hetero-Bilayers: Programmable Phononic Reconfiguration and Second Harmonic Generation

Moire engineering in two-dimensional transition metal dichalcogenides enables access to correlated quantum phenomena. Realizing such effects demands simultaneous control over twist angle and material composition to modulate phonons, excitons, and their interactions. However, most studies rely on exfoliated flakes, limiting scalability and systematic exploration. Here, we demonstrate a scalable multi-moire network by vertically stacking CVD-grown monolayer lateral heterostructures. Signatures of moire non-rigidity, including phonon frequency softening, linewidth broadening, and strain localization, are attributed to two lattice relaxation modes; rotational reconstruction and volumetric dilation. Micro-angle-resolved photoemission spectroscopy reveals that interfacial orbital interactions modulate interlayer coupling. At aligned angles, molybdenum diselenides exhibit reduced valley polarization and Davydov splitting, indicating strain-induced symmetry breaking and chiral phonon effects. Notably, SHG modulation was obderved with variation in twist angle due to lower coherence and band-offset-driven phase delay. First-principles calculations support these findings. This work provides a route to programmable, scalable multi-moire platforms for opto-straintronics, quantum sensing, and on-chip photonics.

cond-mat.mes-hall↗

arXiv2024

Van der Waals engineering of ultrafast carrier dynamics in magnetic heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ and its non-magnetic counterpart Bi$_2$Te$_3$ host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$ following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy, and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi$_2$Te$_4$ and Bi$_2$Te$_3$ surface terminations of MnBi$_4$Te$_7$, revealing an instantaneous occupation of states associated with the Bi$_2$Te$_3$ surface layer followed by carrier extraction into the adjacent MnBi$_2$Te$_4$ layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi$_2$Te$_4$ septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi$_2$Te$_4$-based van der Waals compounds.

cond-mat.mtrl-sci↗

arXiv2024

Direct view of gate-tunable miniband dispersion in graphene superlattices near the magic twist angle

Superlattices from twisted graphene mono- and bi-layer systems give rise to on-demand many-body states such as Mott insulators and unconventional superconductors. These phenomena are ascribed to a combination of flat bands and strong Coulomb interactions. However, a comprehensive understanding is lacking because the low-energy band structure strongly changes when the electron filling is varied. Here, we gain direct access to the filling-dependent low energy bands of twisted bilayer graphene (TBG) and twisted double bilayer graphene (TDBG) by applying micro-focused angle-resolved photoemission spectroscopy to in situ gated devices. Our findings for the two systems are in stark contrast: The doping dependent dispersion for TBG can be described in a simple model, combining a filling-dependent rigid band shift with a many-body related bandwidth change. In TDBG, on the other hand, we find a complex behaviour of the low-energy bands, combining non-monotonous bandwidth changes and tuneable gap openings. Our work establishes the extent of electric field tunability of the low energy electronic states in twisted graphene superlattices and can serve to underpin the theoretical understanding of the resulting phenomena.

cond-mat.mes-hall↗

arXiv2024

Impact of a $MoS_2$ monolayer on the nanoscale thermoelastic response of silicon heterostructures

Understanding the thermoelastic response of a nanostructure is crucial for the choice of materials and interfaces in electronic devices with improved and tailored transport properties, at the length scales of the present technology. Here we show how the deposition of a $MoS_2$ monolayer can strongly modify the nanoscale thermoelastic dynamics of silicon substrates close to their interface. We achieve this result by creating a transient grating with extreme ultraviolet light, using ultrashort free-electron laser pulses, whose $\approx$84 nm period is comparable to the size of elements typically used in nanodevices, such as electric contacts and nanowires. The thermoelastic response, featured by coherent acoustic waves and an incoherent relaxation, is tangibly modified by the presence of monolayer $MoS_2$. Namely, we observed a major reduction of the amplitude of the surface mode, which is almost suppressed, while the longitudinal mode is basically unperturbed, aside from a faster decay of the acoustic modulations. We interpret this behavior as a selective modification of the surface elasticity and we discuss the conditions to observe such effect, which might be of immediate relevance for the design of Si-based nanoscale devices.

cond-mat.mtrl-sci↗

arXiv2024

Autonomous microARPES

Angle-resolved photoemission spectroscopy (ARPES) is a technique used to map the occupied electronic structure of solids. Recent progress in X-ray focusing optics has led to the development of ARPES into a microscopic tool, permitting the electronic structure to be spatially mapped across the surface of a sample. This comes at the expense of a time-consuming scanning process to cover not only a three-dimensional energy-momentum ($E, k_z, k_y$) space but also the two-dimensional surface area. Here, we implement a protocol to autonomously search both $\mathbf{k}$- and real space in order to find positions of particular interest, either because of their high photoemission intensity or because of sharp spectral features. The search is based on the use of Gaussian process regression and can easily be expanded to include additional parameters or optimisation criteria. This autonomous experimental control is implemented on the SGM4 micro-focus beamline of the synchrotron radiation source ASTRID2.

cond-mat.mtrl-sci↗

arXiv2024

Revealing flat bands and hybridization gaps in a twisted bilayer graphene device with microARPES

Controlling the electronic structure of two-dimensional materials using the combination of twist angle and electrostatic doping is an effective means to induce emergent phenomena. In bilayer graphene with an interlayer twist angle near the magic angle, the electronic dispersion is strongly modified by a manifold of hybridizing moiré Dirac cones leading to flat band segments with strong electronic correlations. Numerous technical challenges arising from spatial inhomogeneity of interlayer interactions, twist angle and device functionality have so far limited momentum-resolved electronic structure measurements of these systems to static conditions. Here, we present a detailed characterization of the electronic structure exhibiting miniband dispersions for twisted bilayer graphene, near the magic angle, integrated in a functional device architecture using micro-focused angle-resolved photoemission spectroscopy. The optimum conditions for visualizing the miniband dispersion are determined by exploiting the spatial resolution and photon energy tunability of the light source and applied to extract a hybridization gap size of $(0.14 \pm 0.03)$~eV and flat band segments extending across a moiré mini Brillouin zone. \textit{In situ} electrostatic gating of the sample enables significant electron-doping, causing the conduction band states to shift below the Fermi energy. Our work emphasizes key challenges in probing the electronic structure of magic angle bilayer graphene devices and outlines conditions for exploring the doping-dependent evolution of the dispersion that underpins the ability to control many-body interactions in the material.

cond-mat.mes-hall↗

arXiv2023

Discovery of interlayer plasmon polaron in graphene/WS$_2$ heterostructures

Harnessing electronic excitations involving coherent coupling to bosonic modes is essential for the design and control of emergent phenomena in quantum materials [1]. In situations where charge carriers induce a lattice distortion due to the electron-phonon interaction, the conducting states get "dressed". This leads to the formation of polaronic quasiparticles that dramatically impact charge transport, surface reactivity, thermoelectric and optical properties, as observed in a variety of crystals and interfaces composed of polar materials [2-6]. Similarly, when oscillations of the charge density couple to conduction electrons the more elusive plasmon polaron emerges [7], which has been detected in electron-doped semiconductors [8-10]. However, the exploration of polaronic effects on low energy excitations is still in its infancy in two-dimensional (2D) materials. Here, we present the discovery of an interlayer plasmon polaron in heterostructures composed of graphene on top of SL WS$_2$. By using micro-focused angle-resolved photoemission spectroscopy (microARPES) during in situ doping of the top graphene layer, we observe a strong quasiparticle peak accompanied by several carrier density-dependent shake-off replicas around the SL WS$_2$ conduction band minimum (CBM). Our results are explained by an effective many-body model in terms of a coupling between SL WS$_2$ conduction electrons and graphene plasmon modes. It is important to take into account the presence of such interlayer collective modes, as they have profound consequences for the electronic and optical properties of heterostructures that are routinely explored in many device architectures involving 2D transition metal dichalcogenides (TMDs) [11-15].

cond-mat.mes-hall↗

arXiv2023

Ultrafast X-ray imaging of the light-induced phase transition in VO2

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

cond-mat.str-el↗

arXiv2023

External screening and lifetime of exciton population in single-layer ReSe$_2$ probed by time- and angle-resolved photoemission spectroscopy

The semiconductor ReSe$_2$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ReSe$_2$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53 \pm 0.02)$ eV, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by $τ_1 = (110 \pm 10)$ fs and $τ_2 = (650 \pm 70)$ fs, facilitated by recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than 17.1 Å, implying significant exciton delocalization due to screening from the bilayer graphene substrate.

cond-mat.mes-hall↗

arXiv2022

Nanoscale view of engineered massive Dirac quasiparticles in lithographic superstructures

Massive Dirac fermions are low-energy electronic excitations characterized by a hyperbolic band dispersion. They play a central role in several emerging physical phenomena such as topological phase transitions, anomalous Hall effects and superconductivity. This work demonstrates that massive Dirac fermions can be controllably induced by lithographically patterning superstructures of nanoscale holes in a graphene device. Their band dispersion is systematically visualized using angle-resolved photoemission spectroscopy with nanoscale spatial resolution. A linear scaling of effective mass with feature sizes is discovered, underlining the Dirac nature of the superstructures. In situ electrostatic doping dramatically enhances the effective hole mass and leads to the direct observation of an electronic band gap that results in a peak-to-peak band separation of (0.64 $\pm$ 0.03) eV, which is shown via first-principles calculations to be strongly renormalized by carrier-induced screening. The presented methodology outlines a new approach for band structure engineering guided by directly viewing structurally- and electrically-tunable massive Dirac quasiparticles in lithographic superstructures at the nanoscale.

cond-mat.mes-hall↗