Band unfolding with a general transformation matrix: from code implementation to interpretation of photoemission spectra

Unfolding of a supercell band structure into a primitive Brillouin zone is important for understanding implications of structural distortions, disorder, defects, solid solutions on materials electronic structure. Necessity of the band unfolding is also recognised in interpretation of angle-resolved photoemission spectroscopy (ARPES) measurements. We describe an extension of the fold2Bloch package by implementing an arbitrary transformation matrix used to establish a relation between primitive cell and supercell. This development allows us to overcome limitations of supercells constructed exclusively by scaling of primitive cell lattice vectors. It becomes possible to transform between primitive and conventional cells as well as include rotations. The fold2Bloch is publicaly available from a GitHub repository as a FORTRAN code. It interfaces with the all-electron full-potential WIEN2k and the pseudopotential VASP density functional theory packages. The fold2Bloch is supplemented by additional pre- and post-processing utilities that aid in generating k points in the supercell (such that they later fall onto a desired path in the primitive Brillouin zone after unfolding) and plotting the unfolded band structure. We selected Sr$_2$IrO$_4$ as an illustrative example and, for the first time, present its properly unfolded band structure in direct comparison with ARPES measurements. In addition, critical importance of the band unfolding for interpretation of SrIrO$_3$ ARPES data is illustrated and discussed as a perspective.Comment: 30 pages, 7 figures, 2 tables, supporting information is not include

Widely tunable two-colour seeded free-electron laser source for resonant-pump resonant-probe magnetic scattering

International audienceThe advent of free-electron laser (FEL) sources delivering two synchronized pulses of different wavelengths (or colours) has made available a whole range of novel pump–probe experiments. This communication describes a major step forward using a new configuration of the FERMI FEL-seeded source to deliver two pulses with different wavelengths, each tunable independently over a broad spectral range with adjustable time delay. The FEL scheme makes use of two seed laser beams of different wavelengths and of a split radiator section to generate two extreme ultraviolet pulses from distinct portions of the same electron bunch. The tunability range of this new two-colour source meets the requirements of double-resonant FEL pump/FEL probe time-resolved studies. We demonstrate its performance in a proof-of-principle magnetic scattering experiment in Fe–Ni compounds, by tuning the FEL wavelengths to the Fe and Ni 3p resonances

From epitaxial growth of ferrite thin films to spin-polarized tunnelling

International audienceThis paper presents a review of the research which is focused on ferrite thin films for spintronics. First, I will describe the potential of ferrite layers for the generation of spin-polarized currents. In the second step, the structural and chemical properties of epitaxial thin films and ferrite-based tunnel junctions will be presented. Particular attention will be given to ferrite systems grown by oxygen-assisted molecular beam epitaxy. The analysis of the structure and chemistry close to the interfaces, a key-point for understanding the spin-polarized tunnelling measurements, will be detailed. In the third part, the magnetic and magneto-transport properties of magnetite (Fe3O4) thin films as a function of structural defects such as the antiphase boundaries will be explained. The spin-polarization measurements (spin-resolved photoemission, tunnel magnetoresistance) on this oxide predicted to be half-metallic will be discussed. Fourth, the potential of magnetic tunnel barriers, such as CoFe2O4, NiFe2O4 or MnFe2O4, whose insulating behaviour and the high Curie temperatures make it exciting candidates for spin filtering at room temperature will be described. Spin-polarized tunnelling experiments, involving either Meservey–Tedrow or tunnel magnetoresistance measurements, will reveal significant spin-polarizations of the tunnelling current at low temperatures but also at room temperatures. Finally, I will mention a few perspectives with ferrite-based heterostructures

Crystal field effects on the photoemission spectra in Cr2O3 thin films: from multiplet splitting features to the local structure

International audienceChanges in the shape of X-ray photoemission (XPS) spectra can be related to changes in the local structure of a transi-tion metal. By combining Crystal Field Multiplet calculations and well-controlled molecular beam epitaxy growth of alpha-Cr2O3(0001) thin films on alpha-Al2O3(0001) substrates, we prove that it possible to link the features of Cr 2p XPS spectra with local distortions of CrO6 octahedra and d-orbitals reorganization. Hence, we show that the splitting of the Cr 2p3/2 envelope is related to the degeneracy of the t2g orbital triplet, which corresponds to a fully relaxed structure. Conversely, the broad unstructured Cr 2p3/2 envelope relies on splitting of t2g orbitals and it is the fingerprint of large trigonal distor-tions of CrO6 octahedra. Then, using the Cr 2p XPS as a structural tool for -Cr2O3, we show that the Cr2O3 protective layer formed by oxidation of polycrystalline Ni30Cr alloy exhibits in-plane strains at early oxidation stages and grows preferentially along the c-axis

Plasmon / interband transitions coupling in the UV from large scale nanostructured Ni films

International audienceWe report on the synthesis of Ni nanoparticles via thin film thermal annealing. The as prepared particles exhibit a tunable average diameter ranging from 13 nm to 44 nm depending on the initial deposited film thickness and are covered with a stable NiO x shell. This technique is suitable for large scale fabrication of Ni nanoparticles onto substrates. The study of the magnetic and optical properties of these nanostructures revealed a ferromagnetic behaviour at room temperature and a localized surface plasmon resonance in the UV-range, promoting Ni nanoparticles as a suitable material for UV-plasmonic applications. The coupling between plasmon and interband transitions have also been studied

X-ray diffraction imaging of metal–oxide epitaxial tunnel junctions made by optical lithography: use of focused and unfocused X-ray beams

International audienceX-ray diffraction techniques are used in imaging mode in order to characterize micrometre-sized objects. The samples used as models are metal–oxide tunnel junctions made by optical lithography, with lateral sizes ranging from 150 µm down to 10 µm and various shapes: discs, squares and rectangles. Two approaches are described and compared, both using diffraction contrast: full-field imaging (topography) and raster imaging (scanning probe) using a micrometre-sized focused X-ray beam. It is shown that the full-field image gives access to macroscopic distortions (e.g. sample bending), while the local distortions, at the micrometre scale (e.g. tilts of the crystalline planes in the vicinity of the junction edges), can be accurately characterized only using focused X-ray beams. These local defects are dependent on the junction shape and larger by one order of magnitude than the macroscopic curvature of the sample

Impact of epitaxial strain on crystal field splitting of α\alpha-Cr2_2O3_3(0001) thin films quantified by X-ray photoemission spectroscopy

International audienceThe influence of epitaxial strain on the electronic structure of $\alpha$-Cr$_2$O$_3$(0001) thin films is probed by combining X-ray photoemission spectroscopy and crystal field multiplet calculations. In-plane lattice strain introduces distortions in the CrO$_6$ octahedron and splits the 3d orbital triplet t$_2g$ into a$_1$ + e orbitals. For relaxed thin films, the lines-shape of the Cr 2p core levels are well reproduced when the t$_2g$ subset is fully degenerated. In-plane tensile strain stabilizes a$_1$ with respect to e orbitals, whereas compressive strain destabilizes a$_1$ orbitals. Understanding these crystal field variations is essential for tuning the physical properties of $\alpha$-Cr$_2$O$_3$ thin films

Controlling the magnetic exchange coupling in hybrid heterojunctions via spacer layers of π -conjugated molecules

International audienceMastering and understanding the magnetic couplings between magnetic electrodes separated by organic layers are crucial for developing new hybrid spintronic devices. We study the magnetic exchange interactions in organic-inorganic heterojunctions and unveil the possibility of controlling the strength of the magnetic exchange coupling between two ferromagnetic electrodes across π-conjugated molecules’ (α-sexithiophene or para-sexiphenyl) ultrathin film. In Fe3O4/π-conjugated molecules/Co magnetic tunnel junctions, an antiferromagnetic interlayer exchange coupling with variable strength is observed according to the nature of the aromatic rings (thiophene or phenyl groups). The underlying physical mechanism is revealed by ab initio calculations relating the strength of magnetic coupling to the spin moment penetration into a molecular layer at the molecule/Co interface. The prospect that magnetic coupling between two ferromagnetic electrodes can be mediated and tuned by organic molecules opens different perspectives in the way magnetization of organic tunnel junctions or spin valves can be driven

Electrostriction, Electroresistance and Electromigration in Epitaxial BaTiO3 - based Heterostructures: Role of Interfaces and Electric Poling

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