The preparation of extensive, atomically flat surfaces remains a central challenge in modern quantum materials research, as many crystals lack natural cleavage planes suitable for advanced surface-sensitive investigations. Here, we demonstrate that laser scribing guided by an ultrafast laser can be applied to facilitate easy cleavage along a desired crystallographic plane under ultra-high vacuum. The method is validated on two brittle materials, and Si. The technique allows precise spatial localization of the cleaving site and produces extensive, uniformly oriented, and atomically flat surfaces, as verified by scanning electron microscopy (SEM) and atomic force microscopy (AFM). When applied to, the technique enables angle-resolved photoemission spectroscopy (ARPES) measurements of surface electronic states characteristic of the two-dimensional electron liquid (2DEL) hosted at its bare (100) surface. Moreover, ultrafast laser scribing is significantly faster than focused ion beam (FIB) techniques for preparing cleavable planes, offering a more accessible and efficient approach. Owing to its broad applicability, this method establishes a powerful and general framework to prepare high-quality surfaces for advanced photoemission and microscopic investigations of quantum phenomena.

We investigated the element-specific electronic structure and charge-carrier dynamics of a single-crystal ferromagnet CoS2 with complementary x-ray spectroscopy techniques. Hard x-ray photoemission (HAXPES) is used to provide crucial information on the bulk electronic structure and chemical bonding in CoS2 that is compared against the isoelectronic paramagnet CoSe2. The Co 1⁢𝑠 core-level line shows several satellite features for CoS2, showing explicit charge-transfer processes and local screening of the core hole by S ligands, whereas no such features are observed in CoSe2. The satellite structures indicate the electronic configuration of divalent Co2+ as a combination of 𝑑8⁢Ḻ and 𝑑9⁢Ḻ2 in addition to the nominal ionic 𝑑7 state, where Ḻ represents an S 3⁢𝑝 hole. We employ resonant Auger spectroscopy across the S 𝐾-edge for CoS2 to obtain electron delocalization times to adjacent Co atomic sites. The fast carrier dynamics are attributed to strongly screened Coulomb interactions and hence a facile carrier delocalization. The strong hybridization formed between the Co 3⁢𝑑 and S 3⁢𝑝 states with pronounced charge-transfer character reflects a self-doped system with a finite density 𝑛 of holes at the sulfur site (Ḻ𝑛), in line with recent models that indicate a negative charge-transfer energy for CoS2. In addition to HAXPES data, we also report on experimental and theoretical 𝐿-edge x-ray absorption and x-ray magnetic circular dichroism data for CoS2 that demonstrate multiconfiguration effects in the excitation process. To enable a direct comparison of the experimental spectra, we used density functional theory calculations to obtain the projected density of states to describe the ground-state electronic structure. The existence of fast carrier dynamics and strong charge-transfer properties, demonstrated in this study, highlights the unique nature of CoS2 with a wide potential in topological spintronics applications and integration in energy-related device platforms.

The discovery of ferroelectric phases in HfO₂-based films has reignited interest in ferroelectrics and their application in resistive switching (RS) devices. This study investigates the pivotal role of electrodes in facilitating the Schottky-to-Ohmic transition (SOT) observed in devices consisting of ultrathin epitaxial ferroelectric Hf_0.93Y_0.07O₂ (YHO) films deposited on La_0.67Sr_0.33MnO₃-buffered Nb-doped SrTiO₃ (NbSTO|LSMO) with Ti|Au top electrodes. These findings indicate combined filamentary RS and ferroelectric switching occurs in devices with designed electrodes, having an ON/OFF ratio of over 100 during about 10⁵ cycles. Transport measurements of modified device stacks show no change in SOT when the ferroelectric YHO layer is replaced with an equivalent hafnia-based layer, Hf_0.5Zr_0.5O₂ (HZO). However, incomplete SOT is observed for variations in the top electrode thickness or material, as well as LSMO electrode thickness. This underscores the importance of employing oxygen-reactive electrodes and a bottom electrode with reduced conductivity to stabilize SOT. These findings provide valuable insights for enhancing the performance of ferroelectric RS devices through integration with filamentary RS mechanism.

The discovery of ferroelectricity in nanoscale hafnia-based oxide films has spurred interest in understanding their emergent properties. Investigation focuses on the size-dependent polarization behavior, which is sensitive to content and movement of oxygen vacancies. Though polarization switching and electrochemical reactions is shown to co-occur, their relationship remains unclear. This study employs X-ray photoelectron spectroscopy with depth sensitivity to examine changes in electrochemical states occurring during polarization switching. Contrasting Hf0.5Zr0.5O2 (HZO) with Hf0.88La0.04Ta0.08O2 (HLTO), a composition with an equivalent structure and comparable average ionic radius, electrochemical states are directly observed for specific polarization directions. Lower-polarization films exhibit more significant electrochemical changes upon switching, suggesting an indirect relationship between polarization and electrochemical state. This research illuminates the complex interplay between polarization and electrochemical dynamics, providing evidence for intrinsic polar states in HfO2-based ferroelectrics.

Condensed matter physics has often provided a platform for investigating the interplay between particles and fields in cases that have not been observed in high-energy physics. Here, using angle-resolved photoemission spectroscopy, we provide an example of this by visualizing the electronic structure of a noncentrosymmetric magnetic Weyl semimetal candidate NdAlSi in both the paramagnetic and ferrimagnetic states. We observe surface Fermi arcs and bulk Weyl fermion dispersion as well as the emergence of new Weyl fermions in the ferrimagnetic state. Our results establish NdAlSi as a magnetic Weyl semimetal and provide an experimental observation of ferrimagnetic regulation of Weyl fermions in condensed matter.

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.

The prototypical chalcogenide perovskite BaZrS3, characterized by its direct band gap, exceptionally strong light-harvesting ability, and good carrier transport properties, provides fundamental prerequisites for a promising photovoltaic material. This inspired the synthesis of BaZrS3 in the form of thin films, using sputtering and rapid thermal processing, aimed at device fabrication for future optoelectronic applications. Using a combination of short- and long-range structural information from X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we have elucidated how, starting from a random network of Ba, Zr, and S atoms, thermal treatment induces crystallization and growth of BaZrS3 and explained its impact on the observed photoluminescence (PL) properties. We also provide a description of the electronic structure and substantiate the surface material chemistry using a combination of depth-dependent photoelectron spectroscopy (PES) using hard X-ray (HAXPES) and traditional Al K alpha radiation. From the knowledge of the optical band gap of BaZrS3 thin films, synthesized at an optimal temperature of 900 C-degrees, and our estimation of the valence band edge position with respect to the Fermi level, one may conclude that these semiconductor films are intrinsic in nature with a slight n-type character. A detailed understanding of the growth mechanism and electronic structure of BaZrS3 thin films helps pave the way toward their utilization in photovoltaic applications.

We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The dd excitation energies are similar on the two sides of the ferromagnetic transition temperature, T-C similar to 61 K, although MD in RIXS is predominant at 0.4 T magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intraorbital dd excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS2 by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d5/2 core peak at 230.1 eV emerges in the annealed MoS2 associated with nonstoichiometric MoSx (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 1014 cm-2, we observe a defect peak at ∼1.72 eV (referred to as LXD) at room temperature in the photoluminescence (PL) spectrum. The LXD peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LXD emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LXD peak can be suppressed by annealing the defective MoS2 in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS2 are influenced by sulfur vacancies at room and low temperatures.

Hot carrier solar cells hold promise for exceeding the Shockley-Queisser limit. Slow hot carrier cooling is one of the most intriguing properties of lead halide perovskites and distinguishes this class of materials from competing materials used in solar cells. Here we use the element selectivity of high-resolution X-ray spectroscopy and density functional theory to uncover a previously hidden feature in the conduction band states, the sigma-pi energy splitting, and find that it is strongly influenced by the strength of electronic coupling between the A-cation and bromide-lead sublattice. Our finding provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms. Our work emphasizes the optoelectronic role of the A-cation, provides a comprehensive view of A-cation effects in the crystal and electronic structures, and outlines a broadly applicable spectroscopic approach for assessing the impact of chemical alterations of the A-cation on perovskite electronic structure. The A-cation influence on the mechanism of slow hot carrier cooling in perovskites is controversial. Here, Man et al. resolve a debated issue regarding A-cation influence on the electronic structure of lead halide perovskites.