Search

Kagome materials have emerged as a setting for emergent electronic phenomena that encompass different aspects of symmetry and topology. It is debated whether the XV$_6$Sn$_6$ kagome family (where X is a rare earth element), a recently discovered family of bilayer kagome metals, hosts a topologically non-trivial ground state resulting from the opening of spin-orbit coupling gaps. These states would carry a finite spin-Berry curvature, and topological surface states. Here, we investigate the spin and electronic structure of the XV$_6$Sn$_6$ kagome family. We obtain evidence for a finite spin-Berry curvature contribution at the center of the Brillouin zone, where the nearly flat band detaches from the dispersing Dirac band because of spin-orbit coupling. In addition, the spin-Berry curvature is further investigated in the charge density wave regime of ScV$_6$Sn$_6$, and it is found to be robust against the onset of the temperature-driven ordered phase. Utilizing the sensitivity of angle resolved photoemission spectroscopy to the spin and orbital angular momentum, our work unveils the spin-Berry curvature of topological kagome metals, and helps to define its spectroscopic fingerprint., Comment: 21 pages, 4 figures

Two-dimensional van der Waals magnetic semiconductors display emergent chemical and physical properties and hold promise for novel optical, electronic and magnetic "few-layers" functionalities. Transition-metal iodides such as CrI

We explored the properties of the quasi-binary Bi2Se3–Bi2S3 system over a wide compositional range. X-ray diffraction analysis demonstrates that rhombohedral crystals can be synthesized within the solid solution interval 0–22 mol% Bi2S3, while at 33 mol% Bi2S3 only orthorhombic crystals are obtained. Core level photoemission spectroscopy reveals the presence of Bi3+, Se2− and S2− species and the absence of metallic species, thus indicating that S incorporation into Bi2Se3 proceeds prevalently through the substitution of Se with S. Spin- and angle-resolved photoemission spectroscopy shows that topological surface states develop on the surfaces of the Bi2Se3−ySy (y ≤0.66) rhombohedral crystals, in close analogy with the prototypical case of Bi2Se3, while the orthorhombic crystals with higher S content turn out to be trivial semiconductors. Our results connect unambiguously the phase diagram and electronic properties of the Bi2Se3–Bi2S3 system.

The occurrence of oxygen-driven metal-insulator-transition (MIT) in SrNbO3 (SNO) thin films epitaxially grown on (110)-oriented DyScO3 has been reported. SNO films are fabricated by the pulsed laser deposition technique at different partial O-2 pressure to vary the oxygen content and their structural, optical, and transport properties are probed. SNO unit cell has been found to shrink vertically as the oxygen content increases but keeping the epitaxial matching with the substrate. The results of Fourier-transform infra-red spectroscopy show that highly oxygenated SNO samples (i.e., grown at high oxygen pressure) show distinct optical conductivity behavior with respect to oxygen deficient films, hence demonstrating the insulating character of the formers with respect to those fabricated with lower pressure conditions. Tailoring the optical absorption and conductivity of strontium niobate epitaxial films across the MIT will favor novel applications of this material.

Funding: Deutsche Forschungsgemeinschaft (DFG) through the SFB 1143 and the Wurzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter–ct.qmat (EXC 2147, Project No. 390858490), as well as the support of the HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL). We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Leverhulme Trust, and the Royal Society. We thank the Elettra synchrotron for access to the APE-HE beamline under proposal number 20195300. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. Part of this work has been performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy Progetti Internazionali) project (www.trieste.NFFA.eu).

We report on the reproducible surface topological electron states in Bi2Se3 topological insulator thin films when epitaxially grown by Pulsed Laser Deposition (PLD) on (0 0 1)-oriented SrTiO3 (STO) perovskite substrates. Bi2Se3 has been reproducibly grown with single (0 0 1)-orientation and low surface roughness as controlled by ex-situ X-ray diffraction and in situ scanning tunnel microscopy and low-energy electron diffraction. Finally, in situ synchrotron radiation angle-resolved photo-emission spectroscopy measurements show a single Dirac cone and Dirac point at E B ∼ 0.38 eV located in the center of the Brillouin zone likewise found from exfoliated single-crystals. These results demonstrate that the topological surface electron properties of PLD-grown Bi2Se3 thin films grown on (0 0 1)-oriented STO substrates open new perspectives for applications of multi-layered materials based on oxide perovskites.

Probing the energy and spin electron properties of materials by means of photoemission spectroscopy gives insights into the low-energy phenomena of matter driven by spin orbit coupling or exchange interaction. The information that can be derived from complete photoelectron spectroscopy experiments, beyond $E$(k), is contained in the photoemission transition matrix elements that determine peak intensities. We present here a complete photoemission study of the spin-polarized bands of $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$, a material that presents a surface spin-texture. Circular dichroism in angular-resolved photoemission spectroscopy (CD-ARPES) data are compared with spin-polarized angular-resolved spectra (SARPES) as measured with linearly polarized radiation in a well-characterized experimental chirality, at selected photon energy values. CD-ARPES is due to a matrix element effect that depends strongly on photon energy and experimental geometry: we show that it cannot be used to infer intrinsic spin properties in $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$. On the other hand, SARPES data provide reliable direct information on the spin properties of the electron states. The results on $2H\text{\ensuremath{-}}{\mathrm{NbSe}}_{2}$ are discussed, and general methodological conclusions are drawn on the best experimental approach to the determination of the spin texture of quantum materials.

The electronic properties of strontium ruthenate SrRuO3 perovskite oxide thin filmsare modified by epitaxial strain, as determined by growing on different substrates by pulsedlaser deposition. Temperature dependence of the transport properties indicates that tensilestrain deformation of the SrRuO3 unit cell reduces the metallicity of the material as well as itsmetal-insulator-transition (MIT) temperature. On the contrary, the shrinkage of the Ru–O–Rubuckling angle due to compressive strain is counterweighted by the increased overlap of theconduction Ru-4d orbitals with the O-2p ones due to the smaller interatomic distances resulting intoan increased MIT temperature, i.e., a more conducting material. In particular, in the more metallicsamples, the core level X-ray photoemission spectroscopy lineshapes show the occurrence of anextra-peak at the lower binding energies of the main Ru-3d peak that is attributed to screening,as observed in volume sensitive photoemission of the unstrained material.

The electronic properties of anatase titanium dioxide (${\mathrm{Ti}\mathrm{O}}_{2}$) thin films epitaxially grown on ${\mathrm{La}\mathrm{Al}\mathrm{O}}_{3}$ substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] and infrared spectroscopy. The ${\mathrm{Ti}}^{3+}$ fraction in ${\mathrm{Ti}\mathrm{O}}_{2\ensuremath{-}x}$ is varied either by changing the oxygen pressure during deposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the ${\mathrm{Ti}\mathrm{O}}_{2}$ thin films provides evidence of highly uniform crystallographic order in both as-grown and in situ UHV-annealed samples. The increased amount of ${\mathrm{Ti}}^{3+}$ as a consequence of UHV annealing is calibrated by in situ XPS and XAS analysis. The as-grown ${\mathrm{Ti}\mathrm{O}}_{2}$ samples, with a low $\mathrm{Ti}$${}^{3+}$ concentration, show distinct electronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR) region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with the presence of localized defect states (DSs). With the increasing level of ${\mathrm{Ti}}^{3+}$ induced by the postannealing process, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0 eV. These results indicate the possibility of tailoring the optical absorption of anatase ${\mathrm{Ti}\mathrm{O}}_{2}$ films from the visible to the near-infrared region.

We report the study of anatase TiO2(001)-oriented thin films grown by pulsed laser deposition on LaAlO3(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti(3+)-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti(3+) electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti(3+) only in a well-defined range of deposition pressure; outside this range, Ti(3+) and the strength of the in-gap states are reduced.

Two-dimensional (2D) metallic states induced by oxygen vacancies (${V}_{O}\mathrm{s}$) at oxide surfaces and interfaces provide opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used angle resolved photoelectron spectroscopy combined with density-functional theory (DFT) to study the reactivity of ${V}_{O}$-induced states at the (001) surface of anatase ${\mathrm{TiO}}_{2}$, where both 2D metallic and deeper lying in-gap states (IGs) are observed. The 2D and IG states exhibit remarkably different evolutions when the surface is exposed to molecular ${\mathrm{O}}_{2}$: while IGs are almost completely quenched, the metallic states are only weakly affected. DFT calculations indeed show that the IGs originate from surface ${V}_{O}\mathrm{s}$ and remain localized at the surface, where they can promptly react with ${\mathrm{O}}_{2}$. In contrast, the metallic states originate from subsurface vacancies whose migration to the surface for recombination with ${\mathrm{O}}_{2}$ is kinetically hindered on anatase ${\mathrm{TiO}}_{2}$ (001), thus making them much less sensitive to oxygen dosing.

The electronic properties of hole- and electron-doped manganites were probed by a combination of x-ray absorption and photoemission spectroscopies. Hole-doped La0.7Ba0.3MnO3 and electron-doped La0.7Ce0.3MnO3 thin films were epitaxially grown on SrTiO3 substrates by means of pulsed laser deposition. Ex-situ x-ray diffraction demonstrated the substrate/film epitaxy relation and in-situ low energy electron diffraction provided evidence of high structural order of film surfaces. By combining synchrotron x-ray absorption and x-ray photoemission spectroscopy, evidence of Mn ions into a 2+ state as a result of the Ce 4 + substitution in the electron-doped manganites was provided. Angular resolved photo-emission spectroscopy (ARPES) results showed a predominance of z2-orbitals at the surface of both hole- and, unexpectedly, electron-doped manganites thus questioning the validity of the commonly accepted scenario describing the electron filling in manganites’ 3d orbitals in oxide manganites. The precise determination of the electronic and orbital properties of the terminating layers of oxide manganites paves the way for engineering multi-layered heterostructures thus leading to novel opportunities in the field of quantum electronics.

The electronic structure of Si(110)"16 x 2" double-domain, single-domain and 1 x 1 surfaces have been investigated using spin- and angle-resolved photoemission at sample temperatures of 77 K and 300 K. Angle-resolved photoemission was conducted using horizontally- and vertically-polarised 60 eV and 80 eV photons. Band-dispersion maps revealed four surface states ($S_1$ to $S_4$) which were assigned to silicon dangling bonds on the basis of measured binding energies and photoemission intensity changes between horizontal and vertical light polarisations. Three surface states ($S_1$, $S_2$ and $S_4$), observed in the Si(110)"16 x 2" reconstruction, were assigned to Si adatoms and Si atoms present at the edges of the corrugated terrace structure. Only one of the four surface states, $S_3$, was observed in both the Si(110)"16 x 2" and 1 x 1 band maps and consequently attributed to the pervasive Si zigzag chains that are components of both the Si(110)"16 x 2" and 1 x 1 surfaces. A state in the bulk-band region was attributed to an in-plane bond. All data were consistent with the adatom-buckling model of the Si(110)"16 x 2" surface. Whilst room temperature measurements of $P_y$ and $P_z$ were statistically compatible with zero, $P_x$ measurements of the enantiomorphic A-type and B-type Si(110)"16 x 2" surfaces gave small average polarisations of around 1.5\% that were opposite in sign. Further measurements at 77 K on A-type Si(110)"16 x 2" surface gave a smaller value of +0.3\%. An upper limit of $\sim1\%$ may thus be taken for the longitudinal polarisation., Main paper: 12 pages and 11 figures. Supplemental information: 5 pages and 2 figures

Funding: Framework of the nanoscience foundry and fine analysis (NFFA-MIUR Italy Progetti Internazionali) project; Leverhulme Trust (Grant No. RL-2016-006); Royal Society. Among Transition-Metal Dichalcogenides, mono and few-layers thick VSe2 has gained much recent attention following claims of intrinsic room-temperature ferromagnetism in this system, which have nonetheless proved controversial. Here, we address the magnetic and chemical properties of Fe/VSe2 heterostructure by combining element sensitive absorption spectroscopy and photoemission spectroscopy. Our x-ray magnetic circular dichroism results confirm recent findings that both native mono/few-layer and bulk VSe2 do not show any signature of an intrinsic ferromagnetic ordering. Nonetheless, we find that ferromagnetism can be induced, even at room temperature, after coupling with a Fe thin film layer, with antiparallel alignment of the moment on the V with respect to Fe. We further consider the chemical reactivity at the Fe/VSe2 interface and its relation with interfacial magnetic coupling. Postprint

By combining bulk sensitive soft-x-ray angular-resolved photoemission spectroscopy and first-principles calculations we explored the bulk electron states of ${\mathrm{WTe}}_{2}$, a candidate type-II Weyl semimetal featuring a large nonsaturating magnetoresistance. Despite the layered geometry suggesting a two-dimensional electronic structure, we directly observe a three-dimensional electronic dispersion. We report a band dispersion in the reciprocal direction perpendicular to the layers, implying that electrons can also travel coherently when crossing from one layer to the other. The measured Fermi surface is characterized by two well-separated electron and hole pockets at either side of the $\mathrm{\ensuremath{\Gamma}}$ point, differently from previous more surface sensitive angle-resolved photoemission spectroscopy experiments that additionally found a pronounced quasiparticle weight at the zone center. Moreover, we observe a significant sensitivity of the bulk electronic structure of ${\mathrm{WTe}}_{2}$ around the Fermi level to electronic correlations and renormalizations due to self-energy effects, previously neglected in first-principles descriptions.

We report the study of anatase TiO

Currently, there is a flurry of research interest on materials with an unconventional electronic structure, and we have already seen significant progress in their understanding and engineering towards real-life applications. The interest erupted with the discovery of graphene and topological insulators in the previous decade. The electrons in graphene simulate massless Dirac Fermions with a linearly dispersing Dirac cone in their band structure, while in topological insulators, the electronic bands wind non-trivially in momentum space giving rise to gapless surface states and bulk bandgap. Weyl semimetals in condensed matter systems are the latest addition to this growing family of topological materials. Weyl Fermions are known in the context of high energy physics since almost the beginning of quantum mechanics. They apparently violate charge conservation rules, displaying the ‘chiral anomaly’, with such remarkable properties recently theoretically predicted and experimentally verified to exist as low energy quasiparticle states in certain condensed matter systems. Not only are these new materials extremely important for our fundamental understanding of quantum phenomena, but also they exhibit completely different transport phenomena. For example, massless Fermions are susceptible to scattering from non-magnetic impurities. Dirac semimetals exhibit non-saturating extremely large magnetoresistance as a consequence of their robust electronic bands being protected by time reversal symmetry. These open up whole new possibilities for materials engineering and applications including quantum computing. In this review, we recapitulate some of the outstanding properties of WTe2, namely, its non-saturating titanic magnetoresistance due to perfect electron and hole carrier balance up to a very high magnetic field observed for the very first time. It also indicative of hosting Lorentz violating type-II Weyl Fermions in its bandstructure, again first predicted candidate material to host such a remarkable phase. We primarily focus on the findings of our ARPES, spin-ARPES, and time-resolved ARPES studies complemented by first-principles calculations.