11 research outputs found
Cubic symmetry and magnetic frustration on the spin lattice in KIrCl
Cubic crystal structure and regular octahedral environment of Ir
render antifluorite-type KIrCl a model fcc antiferromagnet with a
combination of Heisenberg and Kitaev exchange interactions. High-resolution
synchrotron powder diffraction confirms cubic symmetry down to at least 20 K,
with a low-energy rotary mode gradually suppressed upon cooling. Using
thermodynamic and transport measurements, we estimate the activation energy of
eV for charge transport, the antiferromagnetic Curie-Weiss
temperature of K, and the extrapolated saturation
field of T. All these parameters are well reproduced \textit{ab
initio} using eV as the effective Coulomb repulsion
parameter. The antiferromagnetic Kitaev exchange term of K is about
one half of the Heisenberg term K. While this combination removes
a large part of the classical ground-state degeneracy, the selection of the
unique magnetic ground state additionally requires a weak second-neighbor
exchange coupling K. Our results suggest that KIrCl may
offer the best possible cubic conditions for Ir and demonstrates the
interplay of geometrical and exchange frustration in a high-symmetry setting.Comment: 9 page
Toward cubic symmetry for Ir: structure and magnetism of antifluorite KIrBr
Crystal structure, electronic state of Ir, and magnetic properties of
the antifluorite compound KIrBr are studied using high-resolution
synchrotron x-ray diffraction, resonant inelastic x-ray scattering (RIXS),
thermodynamic and transport measurements, and ab initio calculations. The
crystal symmetry is reduced from cubic at room temperature to tetragonal below
170 K and eventually to monoclinic below 122 K. These changes are tracked by
the evolution of the non-cubic crystal-field splitting measured by
RIXS. Non-monotonic changes in are ascribed to the competing effects
of the tilt, rotation, and deformation of the IrBr octahedra as well as
tetragonal strain on the electronic levels of Ir. The N\'eel temperature
of K exceeds that of the isostructural KIrCl, and the
magnitude of frustration on the fcc spin lattice decreases. We argue that the
replacement of Cl by Br weakens electronic correlations and enhances magnetic
couplings.Comment: published version: 13 pages + Supplemental Materia
Antiferroelectric instability in the kagome francisites Cu3Bi(SeO3)2O2X (X=Cl,Br)
Density-functional calculations of lattice dynamics and high-resolution
synchrotron powder diffraction uncover antiferroelectric distortion in the
kagome francisite CuBi(SeO)OCl below 115K. Its Br-containing
analogue is stable in the room-temperature crystal structure down to at least
10K, although the Br compound is on the verge of a similar antiferroelectric
instability and reveals local displacements of Cu and Br atoms. The
I-containing compound is stable in its room-temperature structure according to
density-functional calculations. We show that the distortion involves
cooperative displacements of Cu and Cl atoms, and originates from the
optimization of interatomic distances for weakly bonded halogen atoms. The
distortion introduces a tangible deformation of the kagome spin lattice and may
be responsible for the reduced net magnetization of the Cl compound compared to
the Br one. The polar structure of CuBi(SeO)OCl is only
slightly higher in energy than the non-polar antiferroelectric structure, but
no convincing evidence of its formation could be obtained.Comment: 11 pages, 7 figure
Phonon mode calculations and Raman spectroscopy of the bulk-insulating topological insulator BiSbTeSe2
The tetradymite compound BiSbTeSe2 is one of the most bulk-insulating three-dimensional topological insulators, which makes it important in topological insulator research. It is a member of the solid-solution system Bi2-xSbxTe3-ySey, for which the local crystal structure, such as the occupation probabilities of each atomic site, is not well understood. We have investigated the temperature- and polarization-dependent spontaneous Raman scattering in BiSbTeSe2, revealing a much higher number of lattice vibrational modes than predicted by group-theoretical considerations for the space group R (3) over barm corresponding to an ideally random solid-solution situation. The density-functional calculations of phonon frequencies show a very good agreement with experimental data for parent material Bi2Te3, where no disorder effects were found. In comparison to Bi2Te3 the stacking disorder in BiSbTeSe2 causes a discrepancy between theory and experiment. Combined analysis of experimental Raman spectra and density-functional-theory-calculated phonon spectra for different types of atomic orders showed coexistence of different sequences of layers in the material and that those with Se in the center and a local order of Se-Bi-Se-Sb-Te are the most favored
Untargeted Plasma Metabolomic Profiling in Patients with Depressive Disorders: A Preliminary Study
Depressive disorder is a multifactorial disease that is based on dysfunctions in mental and biological processes. The search for biomarkers can improve its diagnosis, personalize therapy, and lead to a deep understanding of the biochemical processes underlying depression. The purpose of this work was a metabolomic analysis of blood serum to classify patients with depressive disorders and healthy individuals using Compound Discoverer software. Using high-resolution mass spectrometry, blood plasma samples from 60 people were analyzed, of which 30 were included in a comparison group (healthy donors), and 30 were patients with a depressive episode (F32.11) and recurrent depressive disorder (F33.11). Differences between patient and control groups were identified using the built-in utilities in Compound Discoverer software. Compounds were identified by their accurate mass and fragment patterns using the mzCloud database and tentatively identified by their exact mass using the ChemSpider search engine and the KEGG, ChEBI, FDA UNII-NLM, Human Metabolome and LipidMAPS databases. We identified 18 metabolites that could divide patients with depressive disorders from healthy donors. Of these, only two compounds were tentatively identified using the mzCloud database (betaine and piperine) based on their fragmentation spectra. For three compounds ((4S,5S,8S,10R)-4,5,8-trihydroxy-10-methyl-3,4,5,8,9,10-hexahydro-2H-oxecin-2-one, (2E,4E)-N-(2-hydroxy-2-methylpropyl)-2,4-tetradecadienamide and 17α-methyl-androstan-3-hydroxyimine-17β-ol), matches were found in the mzCloud database but with low score, which could not serve as reliable evidence of their structure. Another 13 compounds were identified by their exact mass in the ChemSpider database, 9 (g-butyrobetaine, 6-diazonio-5-oxo-L-norleucine, 11-aminoundecanoic acid, methyl N-acetyl-2-diazonionorleucinate, glycyl-glycyl-argininal, dilaurylmethylamine, 12-ketodeoxycholic acid, dicetylamine, 1-linoleoyl-2-hydroxy-sn-glycero-3-PC) had only molecular formulas proposed, and 4 were unidentified. Thus, the use of Compound Discoverer software alone was not sufficient to identify all revealed metabolites. Nevertheless, the combination of the found metabolites made it possible to divide patients with depressive disorders from healthy donors