Towards Time-Resolved Atomic Structure Determination by X-Ray Standing Waves at a Free-Electron Laser
Giuseppe Mercurio, Igor A. Makhotkin, Igor Milov, Young Yong Kim, Ivan A. Zaluzhnyy, Siarhei Dziarzhytski, Lukas Wenthaus, Ivan A. Vartanyants, Wilfried Wurth
We demonstrate the structural sensitivity and accuracy of the standing wave technique at a high repetition rate free-electron laser, FLASH at DESY in Hamburg, by measuring the photoelectron yield from the surface SiO2 of Mo/Si multilayers. These experiments open up the possibility to obtain unprecedented structural information of adsorbate and surface atoms with picometer spatial accuracy and femtosecond temporal resolution. This technique will substantially contribute to a fundamental understanding of chemical reactions at catalytic surfaces and the structural dynamics of superconductors.
Momentum-resolved electronic structure at a buried interface from soft x-ray standing-wave angle-resolved photoemission
角分辨光电子能谱（ARPES）是研究电子结构的一种强有力的技术，但它缺乏直接研究两种材料之间掩埋界面的能力。我们解决这个问题，结合ARPES与软X射线驻波（SW）激发（SWARPES），其中SW配置文件扫描通过样品的深度。研究了典型磁性隧道结La_（0.7）Sr_（0.3）MnO_3/SrTiO_3中的埋层界面。深度和动量分辨地图的Mn 3D EG和T2G状态从中心，散装和界面样区域的La0.7Sr0.3MnO3表现出明显不同的行为与在界面处的Mn键合的变化一致。我们比较的实验结果，国家的最先进的密度泛函和一步光电发射理论，令人鼓舞的协议，这表明广泛的未来应用这种技术。
Nanostructures of Titanium oxide (TiO2) are being studied for many promising applications, e.g., solar photovoltaics, solar water splitting for H2 fuel generation etc., due to their excellent photo-catalytic properties. We have synthesized low-dimensional TiO2 nanoparticles by gas phase CW CO2 laser pyrolysis. The laser synthesis process has been optimized for the deposition of highly pure, nearly mono-dispersed TiO2 nanoparticles on silicon substrates. Hard x-ray standing wave-field (XSW) measurements in total reflection geometry were carried out on the BL-16 beamline of Indus-2 synchrotron r…
Control of structural couplings at the complex-oxide interfaces is a powerful platform for creating new ultrathin layers with electronic and magnetic properties unattainable in the bulk. However, with the capability to design and control the electronic structure of such buried layers and interfaces at a unit-cell level, a new challenge emerges to be able to probe these engineered emergent phenomena with depth-dependent atomic resolution as well as element- and orbital selectivity. Here, we utilize a combination of core-level and valence-band soft x-ray standing-wave photoemission spectroscopy,…
Determination of the Spacing Between Hydrogen-Intercalated Quasi-Free-Standing Monolayer Graphene and 6H-SiC(0001) Using Total-Reflection High-Energy Positron Diffraction
Imaging of van der Waals Materials via Standing-Wave Photoemission Microscopy: Depth-Resolved Electronic Structure of WS2
Temple University（天普大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Carnegie Mellon University（卡内基梅隆大学）；Northeastern University（东北大学）；Quantum Materials and Sensing Institute, Northeastern University（东北大学量子材料与传感研究所）；Center for X-ray Optics, Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室X射线光学中心）；U.S. Naval Research Laboratory（美国海军研究实验室）；Tampere University（坦佩雷大学）；Forschungszentrum Jülich GmbH（于利希研究中心有限公司）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林赫尔霍兹材料能源中心）
When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the …
Here, we analyze in a non-contact fashion charge carrier mobility as a function of injection level and temperature in silicon by time resolved THz spectroscopy (TRTS) and parametrize our data by the classical semi-empirical models of Klaassen and Dorkel & Leturcq. Our experimental results are in very good agreement with the pioneering works of Krausse and Dännhauser analyzing this phenomena by employing contact-based methods. This agreement, that validates our methodology, can only be achieved by considering charge carrier diffusion effects following above bandgap near-surface pump photo-excit…
A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO$_2$ layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-atomic layer depth resolution of these properties. Applying SW photoemission to Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the Sr…
We demonstrate that combining standing-wave (SW) excitation with resonant inelastic x-ray scattering (RIXS) can lead to depth resolution and interface sensitivity for studying orbital and magnetic excitations in correlated oxide heterostructures. SW-RIXS has been applied to multilayer heterostructures consisting of a superconductor La$_{1.85}$Sr$_{0.15}$CuO$_{4}$(LSCO) and a half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO). Easily observable SW effects on the RIXS excitations were found in these LSCO/LSMO multilayers. In addition, we observe different depth distribution of the …
Complete Raman Tensor Determination in Birefringent $β$-Ga$_2$O$_3$ by Single-Stage Hyperspectral Analysis of Polarization Angle-Resolved Raman Spectra
Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V.（柏林莱布尼茨固体电子学保罗·德鲁德研究所）；Technische Universität Berlin（柏林工业大学）；Leibniz-Institut für Kristallzüchtung（莱布尼茨晶体生长研究所）；Institut de Ciència de Materials de Barcelona, ICMAB-CSIC（巴塞罗那材料科学研究所）
Photo-induced charge separation in transition metal dichalcogenide heterobilayers is being explored for moiré excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum spaces may be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the WS2/MoS2 heterobilayer using time and angle resolved photoemission spectroscopy (TR-ARPES). Upon photoexcitation in the K valleys, we detect electrons in M/2, M, and Q valleys/points on time scales as short as ~70 fs, …
We study conceptional problems of a photo-electron emission (PEE) process from a free exciton in insulating crystals. In this PEE process, only the electron constituting the exciton is suddenly emitted out of the crystal, while the hole constituting the exciton is still left inside and forced to be recoiled back to its original valence band. This recoil on the hole is surely reflected in the spectrum of the PEE with a statistical distribution along the momentum-energy curve of the valence band. This distribution is nothing but the square of the exciton wave function amplitude, since it shows h…
Hybrid multiferroics such as BiFeO$_3$ (BFO) and La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) heterostructures are highly interesting functional systems due to their complex electronic and magnetic properties. One of the key parameters influencing the emergent properties is the quality of interfaces, where varying interdiffusion lengths can give rise to different chemistry and distinctive electronic states. Here we report high-resolution depth resolved chemical and electronic investigation of BFO/LSMO superlattice using standing-wave hard X-ray photoemission spectroscopy in the first-order Bragg as well…
In this work, we studied the bulk band structure of a topological insulator (TI) Bi2Se3 and determined the contributions of the Bi and Se orbital states to the valence bands using standing wave-excited hard x-ray photoemission spectroscopy (SW-HAXPES). This SW technique can provide the element-resolved information and extract individual Bi and Se contributions to the Bi2Se3 valence band. Comparisons with density functional theory (DFT) calculations (LDA and GW) reveal that the Bi 6s, Bi 6p, and Se 4p states are dominant in the Bi2Se3 HAXPES valence band. These findings pave a way for studying …
Momentum resolved electron energy-loss spectroscopy was used to measure the dispersions of excitons in a free-standing monolayer of WSe2. Besides the parabolically dispersed valley excitons, a sub-gap dispersive exciton was observed at specific q values for the first time. The simultaneous STEM-ADF imaging of the local probed areas suggests that this sub-gap exciton is attributed to the prolific Se vacancies.
We report results of simultaneous x-ray reflectivity and grazing incidence x-ray fluorescence measurements in combination with x-ray standing wave assisted depth resolved near edge x-ray absorption measurements to reveal new insights on chemical speciation of W in a W-B4C superlattice structure. Interestingly, our results show existence of various unusual electronic states for the W atoms especially those sitting at the surface and interface boundary of a thin film medium as compared to that of the bulk. These observations are found to be consistent with the results obtained using first princi…
The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved …
We report on the precise determination of both the band gap E$_\text{g}$, and the characteristic energy $U$ of the band tail of localized defect states, for monocrystalline Ag$_2$ZnSnSe$_4$. Both photoluminescence excitation and time-resolved photoluminescence studies lead to E$_\text{g} = 1223\pm3$ meV, and $U = 20\pm3$ meV, at 6 K. The interest of the methodology developed here is to account quantitatively for the time-resolved photoluminescence and photoluminescence excitation spectra by only considering standard textbook density of states, and state filling effects. Such an approach is dif…
We propose a table-top method to obtain bilayer quasi-free-standing epitaxial-graphene (QFSEG) on SiC(0001). By applying a microwave annealing in air to a monolayer epitaxial graphene (EG) grown on SiC(0001), the buffer layer is decoupled from the SiC substrate and becomes the second EG layer as confirmed by the low energy electron diffraction, high-resolution transmission electron microscopy, Raman scattering spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. The most likely mechanism of the decoupling is given by the oxidation of the SiC surface, w…
The electron band structure of manganese-adsorbed graphene on an SiC(0001) substrate has been studied using angle-resolved photoemission spectroscopy. Upon introducing manganese atoms, the conduction band of graphene completely disappears and the valence band maximum is observed at 0.4 eV below Fermi energy. At the same time, the slope of the valence band decreases, approaching the electron band structure calculated using the local density approximation method. While the former provides experimental evidence of the formation of nearly free-standing graphene on an SiC substrate, concomitant wit…
Co/BaTiO$_3$(001) is one of the most interesting multiferroic heterostructures as it combines different ferroic phases, setting this way the fundamentals for innovative technical applications. Various theoretical approaches have been applied to investigate the electronic and magnetic properties of Co/BaTiO$_3$(001). Here we determine the magnetic properties of 3 ML Co/BaTiO$_3$ by calculating spin-polarized electron diffraction as well as angle-resolved photoemission spectra, with both methods being well established as surface sensitive techniques. Furthermore, we discuss the impact of alterin…
We present angle resolved photoemission spectra for Ag(111) and Au(111) single crystals in normal emission geometry, taken at closely spaced intervals for photon energies between 8 eV and 160 eV. The most dominant transitions observed are attributed to f-derived final states located about 16 eV to 17 eV above EF for both materials. These transitions exhibit very distinct resonance phenomena and selection rules and are reminiscent of the angular momentum characteristics of the states involved. The excited electron lifetime is in the attosecond (as) range as determined from the energy width of t…
We present a terahertz (THz) frequency-domain spectroscopic (FDS) ellipsometer design which suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements \textit{geometry}, \textit{coating} and \textit{modulation}. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, \textit{in-situ} gas cells or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit op…
Paramagnetic point defects in solids may exhibit a rich set of interesting and not yet fully resolved physics. In particular, character of wavefunctions and electron-phonon coupling in these defects may highly influence their interaction with external magnetic fields. Complex interplay between the electronic orbitals, phonons and electron spin determines the effective pseudospin of the system that we demonstrate on vanadium and molybdenum defects in hexagonal silicon carbide (SiC) by means of \textit{ab initio} calculations. In this Rapid Communication, we find a giant anisotropy in the $g$-te…
