31 research outputs found
Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet
Polarization dependent x-ray absorption spectroscopy was used to study the
magnetic ground state and the orbital occupation in bulk-phase VI van der
Waals crystals below and above the ferromagnetic and structural transitions.
X-ray natural linear dichroism and X-ray magnetic circular dichroism spectra
acquired at the V edges are compared against multiplet cluster
calculations within the frame of the ligand field theory to quantify the
intra-atomic electronic interactions at play and evaluate the effects of
symmetry reduction occurring in a trigonally distorted VI unit. We observed
a non zero linear dichroism proving the presence of an anisotropic charge
density distribution around the V ion due to the unbalanced
hybridization between the Vanadium and the ligand states. Such hybridization
acts as an effective trigonal crystal field, slightly lifting the degeneracy of
the ground state. However, the energy splitting associated to the
distortion underestimates the experimental band gap, suggesting that the
insulating ground state is stabilized by Mott correlation effects rather than
via a Jahn-Teller mechanism. Our results clarify the role of the distortion in
VI and establish a benchmark for the study of the spectroscopic properties
of other van der Waals halides, including emerging 2D materials with mono and
few-layers thickness, whose fundamental properties might be altered by reduced
dimensions and interface proximity
Spectroscopic studies of GTA welding plasmas. Temperature calculation and dilution measurement
International audienc
An integrated ultra-high vacuum apparatus for growth and in situ characterization of complex materials
Here we present an integrated ultra-high vacuum apparatus \u2013 named MBE-Cluster \u2013 dedicated to the growth
and in situ structural, spectroscopic and magnetic characterization of complex materials. Molecular Beam
Epitaxy (MBE) growth of metal oxides, e.g. manganites, and deposition of patterned metallic layers can be
fabricated and in situ characterized by reflection high-energy electron diffraction (RHEED), low-energy
electron diffraction (LEED) - Auger Electron Spectroscopy, X-ray photoemission spectroscopy (PES) and
azimuthal longitudinal magneto-optic Kerr effect (MOKE). The temperature can be controlled in the range
from 5 to 580 K, with the possibility of application of magnetic fields H up to \ub17 kOe and electric fields E for
voltages up to \ub1500 V. The MBE-Cluster operates for in-house research as well as user facility in combination
with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator (HHG) facility for timeresolved
spectroscopy
Stabilization of an Enantiopure Sub-monolayer of Helicene Radical Cations on a Au(111) Surface through Noncovalent Interactions
Unveiling the electronic structure of pseudo-tetragonal WO thin films
WO is a binary 5d compound which has attracted remarkable attention due
to the vast array of structural transitions that it undergoes in its bulk form.
In the bulk, a wide range of electronic properties has been demonstrated,
including metal-insulator transitions and superconductivity upon doping. In
this context, the synthesis of WO thin films holds considerable promise for
stabilizing targeted electronic phase diagrams and embedding them in
technological applications. However, to date, the electronic structure of
WO thin films is experimentally unexplored, and only characterized by
numerical calculations. Underpinning such properties experimentally would be
important to understand not only the collective behavior of electrons in this
transition metal oxide, but also to explain and engineer both the observed
optical responses to carriers' concentration and its prized catalytic activity.
Here, by means of tensile strain, we stabilize WO thin films into a stable
phase, which we call pseudo-tetragonal, and we unveil its electronic structure
by combining photoelectron spectroscopy and density functional theory
calculations. This study constitutes the experimental demonstration of the
electronic structure of WO thin-films and allows us to pin down the first
experimental benchmarks of the fermiology of this system
Zno thin films growth optimization for piezoelectric application
The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O2 is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode
The planar triangular S = 3/2 magnet AgCrSe2 : magnetic frustration, short range correlations, and field tuned anisotropic cycloidal magnetic order
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).Our studies evidence an anisotropic magnetic order below TN = 32~K. Susceptibility data in small fields of about 1~T reveal an antiferromagnetic (AFM) order for H ⊥ c, whereas for H || c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H ⊥ c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H+ = 5~T, which is absent for H || c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fan-like arrangement above H+. A fully FM polarized state is obtained above the saturation field of H⊥S = 23.7~T at 2~K with a magnetization of Ms = 2.8~μB/Cr. For H || c, M(H) monotonously increases and saturates at the same Ms value at HIIS = 25.1~T at 4.2~K. Above TN, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T* = 45~K, which is a clear fingerprint of short-range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possible related to the non-centrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen based delafossites, can be tuned by relatively small, experimentally accessible, magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.PostprintPeer reviewe