arXiv subjects

Philip Hofmann

Publications and source records attributed to Philip Hofmann.

At least 37 records · Page 2Linked to original sources

arXiv2024

Kramers nodal line in the charge density wave state of YTe$_3$ and the influence of twin domains

Recent studies have focused on the relationship between charge density wave (CDW) collective electronic ground states and nontrivial topological states. Using angle-resolved photoemission and density functional theory, we establish that YTe$_3$ is a CDW-induced Kramers nodal line (KNL) metal, a newly proposed topological state of matter. YTe$_3$ is a non-magnetic quasi-2D chalcogenide with a CDW wave vector ($q_{\rm cdw}$) of 0.2907c$^*$. Scanning tunneling microscopy and low energy electron diffraction revealed two orthogonal CDW domains, each with a unidirectional CDW and similar YTe$_3$. The effective band structure (EBS) computations, using DFT-calculated folded bands, show excellent agreement with ARPES because a realistic x-ray crystal structure and twin domains are considered in the calculations. The Fermi surface and ARPES intensity plots show weak shadow bands displaced by $q_{\rm cdw}$ from the main bands. These are linked to CDW modulation, as the EBS calculation confirms. Bilayer split main and shadow bands suggest the existence of crossings, according to theory and experiment. DFT bands, including spin-orbit coupling, indicate a nodal line along the $Σ$ line from multiple band crossings perpendicular to the KNL. Additionally, doubly degenerate bands are only found along the KNL at all energies, with some bands dispersing through the Fermi level.

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↗

arXiv2023

Access to the full 3D Brillouin zone with time resolution, using a new tool for pump-probe ARPES

Here we report the first time- and angle-resolved photoemission spectroscopy (TR-ARPES) with the new Fermiologics "FeSuMa" analyzer. The new experimental setup has been commissioned at the Artemis laboratory of the UK Central Laser Facility. We explain here some of the advantages of the FeSuMa for TR-ARPES and discuss how its capabilities relate to those of hemispherical analyzers and momentum microscopes. We have integrated the FeSuMa into an optimized pump-probe beamline that permits photon-energy- (i.e., kz-) dependent scanning, using probe energies generated from high harmonics in a gas jet. The advantages of using the FeSuMa in this situation include the possibility of taking advantage of its "fisheye" mode of operation.

cond-mat.mtrl-sci↗

arXiv2023

Charge transfer-induced Lifshitz transition and magnetic symmetry breaking in ultrathin CrSBr crystals

Ultrathin CrSBr flakes are exfoliated \emph{in situ} on Au(111) and Ag(111) and their electronic structure is studied by angle-resolved photoemission spectroscopy. The thin flakes' electronic properties are drastically different from those of the bulk material and also substrate-dependent. For both substrates, a strong charge transfer to the flakes is observed, partly populating the conduction band and giving rise to a highly anisotropic Fermi contour with an Ohmic contact to the substrate. The fundamental CrSBr band gap is strongly renormalized compared to the bulk. The charge transfer to the CrSBr flake is substantially larger for Ag(111) than for Au(111), but a rigid energy shift of the chemical potential is insufficient to describe the observed band structure modifications. In particular, the Fermi contour shows a Lifshitz transition, the fundamental band gap undergoes a transition from direct on Au(111) to indirect on Ag(111) and a doping-induced symmetry breaking between the intra-layer Cr magnetic moments further modifies the band structure. Electronic structure calculations can account for non-rigid Lifshitz-type band structure changes in thin CrSBr as a function of doping and strain. In contrast to undoped bulk band structure calculations that require self-consistent $GW$ theory, the doped thin film properties are well-approximated by density functional theory if local Coulomb interactions are taken into account on the mean-field level and the charge transfer is considered.

cond-mat.str-el↗

arXiv2023

Paramagnetic Electronic Structure of CrSBr: Comparison between Ab Initio GW Theory and Angle-Resolved Photoemission Spectroscopy

We explore the electronic structure of paramagnetic CrSBr by comparative first principles calculations and angle-resolved photoemission spectroscopy. We theoretically approximate the paramagnetic phase using a supercell hosting spin configurations with broken long-range order and applying quasiparticle self-consistent $GW$ theory, without and with the inclusion of excitonic vertex corrections to the screened Coulomb interaction (QS$GW$ and QS$G\hat{W}$, respectively). Comparing the quasi-particle band structure calculations to angle-resolved photoemission data collected at 200 K results in excellent agreement. This allows us to qualitatively explain the significant broadening of some bands as arising from the broken magnetic long-range order and/or electronic dispersion perpendicular to the quasi two-dimensional layers of the crystal structure. The experimental band gap at 200 K is found to be at least 1.51 eV at 200 K. At lower temperature, no photoemission data can be collected as a result of charging effects, pointing towards a significantly larger gap, which is consistent with the calculated band gap of $\approx$ 2.1 eV.

cond-mat.str-el↗

arXiv2023

Current driven insulator-to-metal transition without Mott breakdown in Ca$_2$RuO$_4$

The electrical control of a material's conductivity is at the heart of modern electronics. Conventionally, this control is achieved by tuning the density of mobile charge carriers. A completely different approach is possible in Mott insulators such as Ca$_2$RuO$_4$, where an insulator-to-metal transition (IMT) can be induced by a weak electric field or current. This phenomenon has numerous potential applications in, e.g., neuromorphic computing. While the driving force of the IMT is poorly understood, it has been thought to be a breakdown of the Mott state. Using in operando angle-resolved photoemission spectroscopy, we show that this is not the case: The current-driven conductive phase arises with only a minor reorganisation of the Mott state. This can be explained by the co-existence of structurally different domains that emerge during the IMT. Electronic structure calculations show that the boundaries between domains of slightly different structure lead to a drastic reduction of the overall gap. This permits an increased conductivity, despite the persistent presence of the Mott state. This mechanism represents a paradigm shift in the understanding of IMTs, because it does not rely on the simultaneous presence of a metallic and an insulating phase, but rather on the combined effect of structurally inhomogeneous Mott phases.

cond-mat.str-el↗

arXiv2022

Charge density wave induced nodal lines in LaTe$_3$

LaTe$_3$ is a noncentrosymmetric (NC) material with time reversal (TR) symmetry in which the charge density wave (CDW) is hosted by the Te bilayers. Here, we show that LaTe$_3$ hosts a Kramers nodal line (KNL), a twofold degenerate nodal line that connects the TR invariant momenta in NC achiral systems, using angle resolved photoemission spectroscopy (ARPES), density functional theory (DFT), effective band structure (EBS) calculated by band unfolding, and symmetry arguments. DFT incorporating spin-orbit coupling (SOC) reveals that the KNL -- protected by the TR and lattice symmetries -- imposes gapless crossings between the bilayer-split CDW-induced shadow bands and the main bands. In excellent agreement with the EBS, ARPES data corroborate the presence of the KNL and show that the crossings traverse the Fermi level. Furthermore, spinless nodal lines - entirely gapped out by the SOC - are formed by the linear crossings of the shadow and main bands with a high Fermi velocity.

cond-mat.mtrl-sci↗

arXiv2022

Charge density wave-generated Fermi surfaces in NdTe$_3$

The electronic structure of NdTe$_3$ in the charge density wave phase (CDW) is investigated by angle-resolved photoemission spectroscopy. The combination of high-quality crystals and careful surface preparation reveals subtle and previously unobserved details in the Fermi surface topology, allowing an interpretation of the rich and unexplained quantum oscillations in the rare earth tritellurides RTe$_3$. In particular, several closed Fermi surface elements can be observed that are related to CDW-induced replicas of the original bands, leading to the curious situation in which a CDW does not only remove Fermi surface elements but creates new ones that are observable in transport experiments. Moreover, a large residual Fermi surface is found in the CDW gap, very close to the position of the gapped normal-state Fermi surface. Its area agrees very well with high-frequency quantum oscillations in NdTe$_3$ and its presence is explained by strong electron-phonon coupling combined with the quasi one-dimensional character of the CDW. Finally, we identify the origin of the low-frequency $α$ quantum oscillations ubiquitous for the lighter R elements in the RTe$_3$ family and responsible for the high mobility in these compounds.

cond-mat.str-el↗

arXiv2022

Epitaxial growth of a two-dimensional topological insulator candidate: monolayer Si2Te2

Hexagonal Si2Te2 monolayers (ML-Si2Te2) were predicted to show strain-dependent band-crossover between semiconducting and room-temperature quantum spin Hall phases. However, investigations on this artificial two-dimensional (2D) material have mainly been restricted to theoretical calculations because its bulk counterpart does not exist naturally. Here, we report on the successful epitaxial growth of ML-Si2Te2 films on Sb2Te3 thin film substrates. High-quality (1*1) ML-Si2Te2 films with a coverage as high as 95% were obtained as revealed by scanning tunneling microscopy. X-ray photoelectron spectroscopy confirms the absence of intermixing between Si2Te2 and Sb2Te3 at the interface. By combining scanning tunneling spectroscopy with density functional theory calculations, we demonstrate the semiconducting band structure of ML-Si2Te2 on Sb2Te3. Furthermore, it is theoretically predicted that the system can be driven into the nontrivial phase via reducing the strain by 4.4% using strain engineering. Our results pave the way for in-depth investigations on this 2D topological insulator candidate.

cond-mat.mtrl-sci↗

arXiv2022

Tracking the surface atomic motion in a coherent phonon oscillation

X-ray photoelectron diffraction is a powerful tool for determining the structure of clean and adsorbate-covered surfaces. Extending the technique into the ultrafast time domain will open the door to studies as diverse as the direct determination of the electron-phonon coupling strength in solids and the mapping of atomic motion in surface chemical reactions. Here we demonstrate time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from the free electron laser FLASH. We collect Se 3d photoelectron diffraction patterns over a wide angular range from optically excited Bi$_2$Se$_3$ with a time resolution of 140 fs. Combining these with multiple scattering simulations allows us to track the motion of near-surface atoms within the first 3 ps after triggering a coherent vibration of the A$_{1g}$ optical phonons. Using a fluence of 4.2 mJ/cm$^2$ from a 1.55 eV pump laser, we find the resulting coherent vibrational amplitude in the first two interlayer spacings to be on the order of 1 pm.

cond-mat.mtrl-sci↗

arXiv2022

One-dimensional electronic states in a natural misfit structure

Misfit compounds are thermodynamically stable stacks of two-dimensional materials, forming a three-dimensional structure that remains incommensurate in one direction parallel to the layers. As a consequence, no true bonding is expected between the layers, with their interaction being dominated by charge transfer. In contrast to this well-established picture, we show that interlayer coupling can strongly influence the electronic properties of one type of layer in a misfit structure, in a similar way to the creation of modified band structures in an artificial moiré structure between two-dimensional materials. Using angle-resolved photoemission spectroscopy with a micron-scale light focus, we selectively probe the electronic properties of hexagonal NbSe$_2$ and square BiSe layers that terminate the surface of the (BiSe)$_{1+δ}$NbSe$_2$ misfit compound. We show that the band structure in the BiSe layers is strongly affected by the presence of the hexagonal NbSe$_2$ layers, leading to quasi one-dimensional electronic features. The electronic structure of the NbSe$_2$ layers, on the other hand, is hardly influenced by the presence of the BiSe. Using density functional theory calculations of the unfolded band structures, we argue that the preferred modification of one type of bands is mainly due to the atomic and orbital character of the states involved, opening a promising way to design novel electronic states that exploit the partially incommensurate character of the misfit compounds.

cond-mat.mtrl-sci↗

arXiv2021

Single-crystal graphene on Ir(110)

A single-crystal sheet of graphene is synthesized on the low-symmetry substrate Ir(110) by thermal decomposition of C$_2$H$_4$ at 1500 K. Using scanning tunneling microscopy, low-energy electron diffraction, angle-resolved photoemission spectroscopy, and ab initio density functional theory the structure and electronic properties of the adsorbed graphene sheet and its moiré with the substrate are uncovered. The adsorbed graphene layer forms a wave pattern of nm wave length with a corresponding modulation of its electronic properties. This wave pattern is demonstrated to enable the templated adsorption of aromatic molecules and the uniaxial growth of organometallic wires. Not limited to this, graphene on Ir(110) is also a versatile substrate for 2D-layer growth and makes it possible to grow epitaxial layers on ureconstructed Ir(110).

cond-mat.mtrl-sci↗

arXiv2021

Bulk band structure of Sb$_2$Te$_3$ determined by angle-resolved photoemission spectroscopy

The bulk band structure of the topological insulator \sbte~ is investigated by angle-resolved photoemission spectroscopy. Of particular interest is the dispersion of the uppermost valence band with respect to the topological surface state Dirac point. The valence band maximum has been calculated to be either near the Brillouin zone centre or in a low-symmetry position in the $\barΓ-\bar{M}$ azimuthal direction. In order to observe the full energy range of the valence band, the strongly p-doped crystals are counter-doped by surface alkali adsorption. The data show that that the absolute valence band maximum is likely to be found at the bulk $Γ$ point and predictions of a low-symmetry position with an energy higher than the surface Dirac point can be ruled out.

cond-mat.mtrl-sci↗

arXiv2021

Visualizing band structure hybridization and superlattice effects in twisted MoS$_2$/WS$_2$ heterobilayers

A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two dimensional van der Waals heterostructure produces a moiré superlattice with a periodic potential, which can be fine-tuned by introducing a twist angle between the materials. This approach is promising both for controlling the interactions between the TMDs and for engineering their electronic band structures, yet direct observation of the changes to the electronic structure introduced with varying twist angle has so far been missing. Here, we probe heterobilayers comprised of single-layer MoS$_2$ and WS$_2$ with twist angles of $(2.0 \pm 0.5)^{\circ}$, $(13.0 \pm 0.5)^{\circ}$, and $(20.0 \pm 0.5)^{\circ}$ and investigate the differences in their electronic band structure using micro-focused angle-resolved photoemission spectroscopy. We find strong interlayer hybridization between MoS$_2$ and WS$_2$ electronic states at the $\bar{\mathrmΓ}$-point of the Brillouin zone, leading to a transition from a direct bandgap in the single-layer to an indirect gap in the heterostructure. Replicas of the hybridized states are observed at the centre of twist angle-dependent moiré mini Brillouin zones. We confirm that these replica features arise from the inherent moiré potential by comparing our experimental observations with density functional theory calculations of the superlattice dispersion. Our direct visualization of these features underscores the potential of using twisted heterobilayer semiconductors to engineer hybrid electronic states and superlattices that alter the electronic and optical properties of 2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2021

Fermi surface tomography

Fermi surfaces, three-dimensional (3D) abstract interfaces that define the occupied energies of electrons in a solid, are important for characterizing and predicting the thermal, electrical, magnetic, and optical properties of crystalline metals and semiconductors [1]. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (kF) from energy and angular distribution of photoelectrons dislodged by monochromatic light [2]. Existing electron analyzers are able to determine a number of kF-vectors simultaneously, but current technical limitations prohibit a direct high-resolution 3D Fermi surface mapping. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces and restricting a detailed comparison with the widely available nowadays calculated 3D Fermi surfaces. Here we show that using a simpler instrumentation, based on the Fourier electron optics combined with a retardation field of the detector, it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface recorded in the full Brillouin zone along the kz-direction as well as other experimental results featuring multiple advantages of our technique. In combination with various light sources, including synchrotron radiation, our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast electronic line width broadening in the C 1s core level of graphene

Core level binding energies and absorption edges are at the heart of many experimental techniques concerned with element-specific structure, electronic structure, chemical reactivity, elementary excitations and magnetism. X-ray photoemission spectroscopy (XPS) in particular, can provide information about the electronic and vibrational many-body interactions in a solid as these are reflected in the detailed energy distribution of the photoelectrons. Ultrafast pump-probe techniques add a new dimension to such studies, introducing the ability to probe a transient state of the many-body system. Here we use a free electron laser to investigate the effect of a transiently excited electron gas on the core level spectrum of graphene, showing that it leads to a large broadening of the C 1s peak. Confirming a decade-old prediction, the broadening is found to be caused by an exchange of energy and momentum between the photoemitted core electron and the hot electron system, rather than by vibrational excitations. This interpretation is supported by a line shape analysis that accounts for the presence of the excited electrons. Fitting the spectra to this model directly yields the electronic temperature of the system, in agreement with electronic temperature values obtained from valence band data. Furthermore, making use of time- and momentum-resolved C 1s spectra, we illustrate how the momentum change of the outgoing core electrons leads to a small but detectable change in the time-resolved photoelectron diffraction pattern and to a nearly complete elimination of the core level binding energy variation associated with the narrow $σ$-band in the C 1s state. The results demonstrate that the XPS line shape can be used as an element-specific and local probe of the excited electron system and that X-ray photoelectron diffraction investigations remain feasible at very high electronic temperatures.

cond-mat.str-el↗

arXiv2021

Electronic Properties of Single-Layer CoO$_2$/Au(111)

We report direct measurements via angle-resolved photoemission spectroscopy (ARPES) of the electronic dispersion of single-layer CoO$_2$. The Fermi contour consists of a large hole pocket centered at the $\overlineΓ$ point. To interpret the ARPES results, we use density functional theory (DFT) in combination with the multi-orbital Gutzwiller Approximation (DFT+GA), basing our calculations on crystalline structure parameters derived from x-ray photoelectron diffraction and low-energy electron diffraction. Our calculations are in good agreement with the measured dispersion. We conclude that the material is a moderately correlated metal. We also discuss substrate effects, and the influence of hydroxylation on the CoO$_2$ single-layer electronic structure.

cond-mat.mtrl-sci↗

arXiv2021

Accessing the spectral function of in operando devices by angle-resolved photoemission spectroscopy

Progress in performing angle-resolved photoemission spectroscopy (ARPES) with high spatial resolution in the order of 1~$μ$m or less (nanoARPES) has opened the possibility to map the spectral function of solids on this tiny scale and thereby obtain detailed information on the materials' \emph{local} electronic band structure and many-body interactions. Recently, nanoARPES has been used to study simple electronic devices, based on two-dimensional materials, with the possibility of tuning the carrier type and density by field effect-gating, and while passing a current through the device. It was demonstrated that nanoARPES can detect possible changes in the materials' electronic structure in these situations and that it can map the local doping, conductance and mobility. This article reviews these first \emph{in operando} ARPES results on devices, discusses the resulting new insights, as well as the perspectives for future developments of the technique.

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