Littérature scientifique sur le sujet « Quantum materials. ARPES. RIXS »

The recently discovered topological quantum materials (TQMs) have electronic structures that can be characterized by certain topological invariants. In these novel materials, the unusual bulk and surface electrons not only give rise to many exotic physical phenomena but also foster potential new technological applications. To characterize the unusual electronic structures of these new materials, investigators have used angle-resolved photoemission spectroscopy (ARPES) as an effective experimental tool to directly visualize the unique bulk and surface electronic structures of TQMs. In this review, we first give a brief introduction of TQMs and ARPES, which is followed by examples of the application of ARPES to different TQMs ranging from topological insulators to Dirac and Weyl semimetals. We conclude with a brief perspective of the current development of ARPES and its potential application in the study of TQMs.

Novel battery materials offer the promise of greatly increased energy densities for automative and other applications. However, their practical adoption is often limited by problems in electrochemical performance, such as poor rate capability, voltaic efficiency, and cyclability. Resonant Inelastic X-ray Spectroscopy (RIXS) has become a valuable tool for understanding the redox mechanisms in such materials, representing a first step towards solving problems in electrochemical performance. In many cases, however, the underlying origins of RIXS features are unclear. For example, redox mechanisms in Li-rich cathodes are associated with several absorption and emission features in oxygen RIXS measurements, but a clear picture of what redox mechanisms create these features remains elusive. As a step towards identifying redox mechanisms in Li-rich and related cathode materials, this presentation reviews existing theories of reaction mechanisms in these compounds, revisits experimental RIXS data on these materials and related oxides, and presents ab initio calculations evaluating possible origins of commonly observed features. Results suggest that some features could arise from molecular oxygen residing in low-symmetry environments. Additionally, prospects for using error-corrected quantum computation to more accurately model such electronic structure problems will be discussed.

Angle-resolved photoemission spectroscopy with sub-micrometer spatial resolution (μ-ARPES), has become a powerful tool for studying quantum materials. To achieve sub-micrometer or even nanometer-scale spatial resolution, it is important to focus the incident light beam (usually from synchrotron radiation) using x-ray optics, such as the zone plate or ellipsoidal capillary mirrors. Recently, we developed a laser-based μ-ARPES with spin-resolution (LMS-ARPES). The 177 nm laser beam is achieved by frequency-doubling a 355 nm beam using a KBBF crystal and subsequently focused using an optical lens with a focal length of about 16 mm. By characterizing the focused spot size using different methods and performing spatial-scanning photoemission measurement, we confirm the sub-micron spatial resolution of the system. Compared with the μ-ARPES facilities based on the synchrotron radiation, our LMS-ARPES system is not only more economical and convenient, but also with higher photon flux (>5 × 1013 photons/s), thus enabling the high-resolution and high-statistics measurements. Moreover, the system is equipped with a two-dimensional spin detector based on exchange scattering at a surface-passivated iron film grown on a W(100) substrate. We investigate the spin structure of the prototype topological insulator Bi2Se3 and reveal a high spin-polarization rate, confirming its spin-momentum locking property. This lab-based LMS-ARPES will be a powerful research tool for studying the local fine electronic structures of different condensed matter systems, including topological quantum materials, mesoscopic materials and structures, and phase-separated materials.

Bismuth has been the key element in the discovery and development of topological insulator materials. Previous theoretical studies indicated that Bi is topologically trivial and it can transform into the topological phase by alloying with Sb. However, recent high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements strongly suggested a topological band structure in pure Bi, conflicting with the theoretical results. To address this issue, we studied the band structure of Bi and Sb films by ARPES and first-principles calculations. The quantum confinement effectively enlarges the energy gap in the band structure of Bi films and enables a direct visualization of the Z 2 topological invariant of Bi. We find that Bi quantum films in topologically trivial and nontrivial phases respond differently to surface perturbations. This way, we establish experimental criteria for detecting the band topology of Bi by spectroscopic methods.

Many remarkable properties of quantum materials emerge from states with intricate coupling between the charge, spin and orbital degrees of freedom. Ultrafast photo-excitation of these materials holds great promise for understanding and controlling the properties of these states. Here, we introduce time-resolved resonant inelastic X-ray scattering (tr-RIXS) as a means of measuring the charge, spin and orbital excitations out of equilibrium. These excitations encode the correlations and interactions that determine the detailed properties of the states generated. After outlining the basic principles and instrumentations of tr-RIXS, we review our first observations of transient antiferromagnetic correlations in quasi two dimensions in a photo-excited Mott insulator and present possible future routes of this fast-developing technique. The increasing number of X-ray free electron laser facilities not only enables tackling long-standing fundamental scientific problems, but also promises to unleash novel inelastic X-ray scattering spectroscopies. This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays’.

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

Angle-resolved photoemission spectroscopy using a micro-focused beam spot [micro-angle-resolved photoemission spectroscopy (ARPES)] is becoming a powerful tool to elucidate key electronic states of exotic quantum materials. We have developed a versatile micro-ARPES system based on the synchrotron radiation beam focused with a Kirkpatrick–Baez mirror optics. The mirrors are monolithically installed on a stage, which is driven with five-axis motion, and are vibrationally separated from the ARPES measurement system. Spatial mapping of the Au photolithography pattern on Si signifies the beam spot size of 10 µm (horizontal) × 12 µm (vertical) at the sample position, which is well suited to resolve the fine structure in local electronic states. Utilization of the micro-beam and the high precision sample motion system enables the accurate spatially resolved band-structure mapping, as demonstrated by the observation of a small band anomaly associated with tiny sample bending near the edge of a cleaved topological insulator single crystal.

We present STM/STS, ARPES and magnetotransport studies of the surface topography and electronic structure of pristine Bi2Se3 in comparison to Bi1.96Mg0.04Se3 and Bi1.98Fe0.02Se3. The topography images reveal a large number of complex, triangle-shaped defects at the surface. The local electronic structure of both the defected and non-defected regions is examined by STS. The defect-related states shift together with the Dirac point observed in the undefected area, suggesting that the local electronic structure at the defects is influenced by doping in the same way as the electronic structure of the undefected surface. Additional information about the electronic structure of the samples is provided by ARPES, which reveals the dependence of the bulk and surface electronic bands on doping, including such parameters as the Fermi wave vector. The subtle changes of the surface electronic structure by doping are verified with magneto-transport measurements at low temperatures (200 mK) allowing the detection of Shubnikov-de Haas (SdH) quantum oscillations.

Abstract Electronic structure of LaAlO3/SrTiO3 (LAO/STO) samples, grown at low oxygen pressure and post-annealed ex situ, was investigated by soft-x-ray ARPES focussing on the Fermi momentum (k F) of the mobile electron system (MES). X-ray irradiation of these samples at temperatures below 100 K creates oxygen vacancies (VOs) injecting Ti t 2g-electrons into the MES. At this temperature the oxygen out-diffusion is suppressed, and the VOs should appear mostly in the top STO layer. The x-ray generated MES demonstrates, however, a pronounced three-dimensional (3D) behavior as evidenced by variations of its experimental k F over different Brillouin zones. Identical to bare STO, this behavior indicates an unexpectedly large extension of the x-ray generated MES into the STO depth. The intrinsic MES in the standard LAO/STO samples annealed in situ, in contrast, demonstrates purely two-dimensional (2D) behaviour. The relevance of our ARPES data analysis is supported by model calculations to compare the intensity vs gradient methods of the k F determination as a function of the energy resolution ratio to the bandwidth. Based on self-interaction-corrected DFT calculations of the MES induced by VOs at the interface and in STO bulk, we discuss possible scenarios of the puzzling 3D-ity. It may involve either a dense ladder of quantum-well states formed in a long-range interfacial potential or, more likely, x-ray-induced bulk metallicity in STO accessed in the ARPES experiment through a short-range interfacial barrier. The mechanism of this metallicity may involve remnant VOs and photoconductivity-induced metallic states in the STO bulk, and even more exotic mechanisms such as x-ray induced formation of Frenkel pairs.

Quantum materials exhibit distinctive electronic, magnetic, and optical properties, including topologically protected surface states, strong spin-orbit coupling, and quantum confinement effects. These properties make them promising for next-generation technologies like quantum computing and spintronics.Two different experimental techniques, such as Angle-resolved photoemission spectroscopy (ARPES) and Resonant in-elastic X-ray scattering (RIXS), have been utilized to study the electronic structure of three quantum materials: Hafnium (0001), Monolayer WSe2 on Au, and (Ge(0.87)Mn(0.13)Te.For Hafnium (0001), Our theoretical predictions indicated the presence of Rashba-split surface states with significant spin-momentum locking, characterized by a strong d-character. Experimental ARPES measurements, conducted with both He lamp sources and synchrotron radiation, are in excellent agreement with the theoretical band structures. However, deviations such as additional flat bands due to surface contaminants and the absence of certain predicted surface states were observed. Furthermore, linear and circular dichroism experiments highlighted the sensitivity of electronic states to light polarization, elucidating the orbital character of surface bands.In the case of monolayer WSe2, large-area monolayers of WSe2 were exfoliated using a well-established template-stripped gold method using two different templates, Mica and Silicon. Both experimental (microARPES) and theoretical spectra of WSe2/Au showed a strong hybridization influencing the electronic states around the Brillouin zone centre at Gamma and of charge transfer between Au and WSe2, however unaffected at valence band maximum (VBM) K point. Further corroborated by core level spectra taken on different regions of the thickness of WSe2 on Au, a substrate interaction of the nature covalent like quasi bonding (CLQB) was revealed.RIXS measurements were carried out on multiferroicmaterial Ge(0.87)Mn(0.13)Te to fathom the effect of external electric and magnetic fields on dd excitations. By applying a 3.1 V electric field, RIXS measurements showed a clear change in dd excitations, indicating a response related to the ferroelectric order of the material. Theoretical simulations were performed using the Anderson Impurity Model (AIM) within the Quanty software framework. The theoretical spectra of XAS and RIXS are in good agreement with the experimental spectra for oh symmetry with a crystal field parameter (10Dq) of 0.4 eV
Les matériaux quantiques présentent des propriétés électroniques, magnétiques et optiques distinctives, telles que des états de surface protégés topologiquement, un couplage spin-orbite fort et des effets de confinement quantique. Ces propriétés en font des candidats prometteurs pour les technologies de prochaine génération, comme l'informatique quantique et la spintronique.Deux techniques expérimentales différentes, telles que la spectroscopie de photoémission résolue en angle (ARPES) et la diffusion résonante inélastique des rayons X (RIXS), ont été utilisées pour étudier la structure électronique de trois matériaux quantiques : Hafnium (0001), Monocouche de WSe2 sur Au, et Ge(0.87)Mn(0.13)Te.Pour le Hafnium (0001), nos prédictions théoriques ont indiqué la présence d’états de surface de type Rashba avec un verrouillage spin-moment significatif, caractérisés par un fort caractère d. Les mesures expérimentales ARPES, réalisées à l'aide de sources de lampe He et de rayonnement synchrotron, sont en excellent accord avec les structures de bandes théoriques. Cependant, des écarts ont été observés, tels que des bandes plates supplémentaires dues à des contaminants de surface et l'absence de certains états de surface prédits. De plus, les expériences de dichroïsme linéaire et circulaire ont mis en évidence la sensibilité des états électroniques à la polarisation de la lumière, élucidant ainsi le caractère orbital des bandes de surface.Dans le cas de la monocouche de WSe2, de grandes monocouches de WSe2 ont été exfoliées en utilisant une méthode bien établie d'or pelable avec deux différents gabarits, le mica et le silicium. Les spectres expérimentaux (microARPES) et théoriques de WSe2/Au ont montré une forte hybridation influençant les états électroniques autour du centre de la zone de Brillouin à Gamma et un transfert de charge entre Au et WSe2, cependant, non affectés au point K du maximum de la bande de valence (VBM). Confirmé davantage par les spectres de niveaux fondamentaux pris sur différentes régions d’épaisseur de WSe2 sur Au, une interaction avec le substrat de type liaison quasi-covalente (CLQB) a été révélée.Des mesures RIXS ont été effectuées sur le matériau multiferroïque Ge(0.87)Mn(0.13)Te pour approfondir l'effet des champs électriques et magnétiques externes sur les excitations dd. En appliquant un champ électrique de 3,1 V, les mesures RIXS ont montré un changement clair dans les excitations dd, indiquant une réponse liée à l'ordre ferroélectrique du matériau. Des simulations théoriques ont été réalisées en utilisant le modèle d'impureté d'Anderson (AIM) dans le cadre du logiciel Quanty. Les spectres théoriques de l'absorption des rayons X (XAS) et du RIXS sont en bon accord avec les spectres expérimentaux pour une symétrie oh avec un paramètre de champ cristallin (10Dq) de 0,4 eV

Physics
Ph.D.
In this dissertation, we used a combination of synchrotron-based x-ray spectroscopic techniques such as angle-resolved photoelectron spectroscopy (ARPES), soft x-ray ARPES, hard x-ray photoelectron spectroscopy (HAXPES), and soft x-ray absorption spectroscopy (XAS) to investigate momentum-resolved and angle-integrated electronic structure of advanced three- and two-dimensional materials and interfaces. The results from the experiments were compared to several types of state-of-the-art first-principles theoretical calculations. In the first part of this dissertation we investigated the effects of spin excitons on the surface states of samarium hexaboride (SmB6), which has gained a lot of interest since it was proposed to be a candidate topological Kondo insulator. Here, we utilized high-resolution (overall resolution of approximately 3 meV) angle-resolved and angle-integrated valence-band photoemission measurements at cryogenic temperatures (1.2 K and 20 K) to show evidence for a V-shap
Temple University--Theses

This thesis covers different problems that arise due to crystal and pseudospin anisotropy present in 3d and 5d transition metal oxides. We demonstrate that the methods of computational quantum chemistry can be fruitfully used for quantitative studies of such problems. In Chapter 2, Chapter 3, and Chapter 7 we show that it is possible to reliably calculate local multiplet splittings fully ab initio, and therefore help to assign peaks in experimental spectra to corresponding electronic states. In a situation of large number of peaks due to low local symmetry such assignment using semi-empirical methods can be very tedious and non-unique. Moreover, in Chapter 4 we present a computational scheme for calculating intensities as observed in the resonant inelastic X-ray scattering and X-ray absorption experiments. In our scheme highly-excited core-hole states are calculated explicitly taking into account corresponding orbital relaxation and electron polarization. Computed Cu L-edge spectra for the Li2CuO2 compound reproduce all features present in experiment. Unbiased ab initio calculations allow us to unravel a delicate interplay between the distortion of the local ligand cage around the transition metal ions and the anisotropic electrostatic interactions due to second and farther coordination shells. As shown in Chapter 5 and Chapter 6 this interplay can lead to the counter intuitive multiplet structure, single-ion anisotropy, and magnetic g factors. The effect is quite general and may occur in compounds with large difference between charges of metal ions that form anisotropic environment around the transition metal, like Ir 4+ in plane versus Sr 2+ out of plane in the case of Sr2IrO4. An important aspect of the presented study is the mapping of the quantum chemistry results onto simpler physical models, namely extended Heisenberg model, providing an ab initio parametrization. In Chapter 5 we employ the effective Hamiltonian technique for extracting parameters of the anisotropic Heisenberg model with single-ion anisotropy in the case of quenched orbital moment and second-order spin-orbit coupling. Calculated strong easy-axis anisotropy of the same order of magnitude as the symmetric exchange is consistent with experimentally-observer all-in/all-out magnetic order. In Chapter 6 we introduce new flavour of the mapping procedure applicable to systems with first-order spin-orbit coupling, such as 5d 5 iridates based on analysis of the wavefunction and interaction with magnetic field. In Chapter 6 and Chapter 7 we use this new procedure to obtain parameters of the pseudospin anisotropic Heisenberg model. We find large antisymmetric exchange leading to the canted antiferromagnetic state in Sr2IrO4 and nearly ideal one-dimensional Heisenberg behaviour of the CaIrO3, both agree very well with experimental findings.

Electron energy bands in solids are introduced. Free electron theory for metals is presented: the Fermi gas, Fermi energy and temperature. Electrical and thermal conductivity are interpreted, including the Wiedermann–Franz law. The Hall effect and information it brings about charge carriers is discussed. Plasma oscillations of conduction electrons and the optical properties of metals are examined. Formation of quasi-particles of an electron and its screening cloud are discussed. Electron-electron and electron-phonon scattering and how they affect the mean free path are treated. Then the analysis of crystalline materials using electron Bloch waves is presented. Tight and weak binding cases are examined. Electron band structure is explained including Brillouin zones, electron kinematics and effective mass. Fermi surfaces in crystals are treated. The ARPES technique for exploring dispersion relations is explained.

The Advanced Laser Light source (ALLS) laboratory provides high-repetition-rate ultrashort light pulses based on ytterbium laser technology. Recently, we have developed a novel time-and-angle-resolved photoemission (TR-ARPES) end-station to explore rapid electron dynamics in quantum materials under intense optical excitation in the near- and mir-infrared range. These intense pulses, generated using our in-house-built optical parametric amplifier (OPA) ranging from 1.6 to 8 µm with a duration of around 100 fs, are fully characterized using the FROSt technique.