Magneto-Optická a THz Spektroskopie

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    Pyrolysis of Colombian spent coffee grounds (SCGs), characterization of bio-oil, and study of its antioxidant properties
    (TAYLOR & FRANCIS LTD, 2023-12-14) Malagón-Romero, Dionisio; Torres-Velasquez, Andres C.; Tinoco Navarro, Lizeth Katherine; Arrubla-Velez, Juan P.
    Coffee is the second most consumed beverage in the world. Spent coffee grounds (SCGs) are promising raw material for pyrolysis. The Kissinger-Akahira-Sunose model (KAS) and Ozawa-Flynn Wall (OFW), determined the pyrolysis activation energy to be 142.40 and 66.08 kJ/mol, respectively. The pyrolysis products were bio-oil with a 22.5% yield and a calorific power of 28.91 MJ/kg. The bio-oil composition was 50.86% fatty acids, 31.65% linear hydrocarbons, and 11.31% aromatics, as determined by GC-MS. The total phenolic content (TPC) was 362 & PLUSMN; 44 mg GA g(-1), with DPPH activity of 24.8 & PLUSMN; 2% inhibition and antioxidant activity of 108.5 & PLUSMN; 13.6 mg L-1 of Trolox equivalents. The results showed that bio-oil could be used as a biofuel and a source of chemical compounds.
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    High harmonic generation in monolayer MoS2 controlled by resonant and near-resonant pulses on ultrashort time scales
    (AIP Publishing, 2023-08-01) Peterka, Pavel; Slobodeniuk, Artur; Novotný, Tomáš; Suthar, Pawan; Bartoš, Miroslav; Trojánek, František; Malý, Petr; Kozák, Martin
    We report on experimental investigation of nonperturbative high harmonic generation (HHG) in monolayer MoS2 in the ultraviolet spectral region driven by mid-infrared light. We study how the HHG is influenced by pre-excitation of the monolayer using resonant and near-resonant pulses in a pump-probe-like scheme. The resonant light creates high density exciton population. Due to ultrafast dephasing caused by electron-electron scattering, the HHG is suppressed in the presence of pre-excited carriers. In the case of near-resonant excitation with photon energy below the exciton transition, the dynamics of the observed suppression of the HHG yield contains a fast component, which is a consequence of momentum scattering at carriers, which are excited by two-photon transition when the two pulses temporally overlap in the sample. This interpretation is supported by comparing the experimental data with theoretical calculations of the two-photon absorption spectrum of the MoS2 monolayer. This work demonstrates a possibility to control HHG in low-dimensional materials on ultrashort timescales by combining the driving strong-field pulse with a weak near-resonant light.
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    Variation of Spin-Transition Temperature in the Iron(III) Complex Induced by Different Compositions of the Crystallization Solvent
    (American Chemical Society, 2023-03-01) Nemec, Ivan; Kotásková, Lucie; Herchel, Radovan
    We crystallized the Schiff-base iron(III) spin-crossover complex [Fe(3,5Cl-L5)(NCSe)] from different two-component solvent mixtures containing methanol and chloroform (phi = V(CH3OH)/V(solvent) = 0.05, 0.25, 0.50, 0.83, and 1.00). The obtained crystalline products were characterized by X-ray diffraction, and it was confirmed that they are all composed of the same crystalline phase, and they do not contain any crystal solvent. However, significant differences in magnetic properties were observed, and thermal hysteresis changed from (in K) 121T down arrow and 134T up arrow for phi = 0.05 and 0.25, down to 72T down arrow and 96T up arrow for phi = 1.00. The crystal structures of the low-spin and high-spin phases were studied theoretically and experimentally.
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    Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration
    (Elsevier, 2023-07-01) Pouchlý, Václav; Talimian, Ali; Kaštyl, Jaroslav; Chvíla, Martin; Ščasnovič, Erik; Beltran, Ana Maria; Lozano , Juan Gabriel; Galusek, Dušan
    The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 degrees C/min) and fast (100 degrees C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. mu-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.
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    Quantum phase transitions probed by EPR spectra in dimeric spin arrays with supramolecular couplings
    (Elsevier, 2023-04-01) Calvo, Rafael; Sartoris, Rosana P.; Nascimento, Otaciro R.; Šedivý, Matúš; Sojka, Antonín; Neugebauer, Petr; Santana, Vinicius Tadeu
    Dimeric compounds with nearly isolated molecular units (d-units) having pairs of spins s1 and s2 coupled by antiferromagnetic (AFM) exchange H-0 = -J(0)s(1) center dot s(2) are non-trivial quantum spin systems having pri-mary roles in magnetism. Weakly coupled infinite arrays of AFM d-units in crystal structures have an appealing spin dynamic arising from their energy-gapped level structure and display magnetic properties with important roles in materials science. They received great additional attention when it was discov-ered that the spin entanglement introduced by interdimeric couplings with magnitude J1 (with IJ(1) I << IJ(0)I) gives rise to bosonic systems with novel properties and quantum phase transitions at high temperature (T). In this work, we collect recent advances in the interpretation of EPR spectral changes in terms of quantum phase transitions of arrays of d-units in the presence of weak interdimeric couplings. We review previous investigations of the problem and focused new experiments on the paradigmatic compound copper acetate monohydrate (CAH), collecting a detailed set of spectra in single-crystal and powder samples. The spectral features arising from this coupling are merging and narrowing of the peaks of the spectra of single crystals for specific magnetic field (B-0) orientations, and an extraordinary "U -peak" in the powder samples associated with the quantum phase transition of the dimeric spin array. Our historical overview collects studies of similar compounds with the same dimeric feature in which the U-peaks were generally misinterpreted as a double-quantum transition, or ignored. We describe pro-cedures to identify and quantify the U-peak and the merging and narrowing phenomena, with a protocol to extract the interdimeric coupling magnitude. As a novel contribution, we explain the experimental results by proposing a spin model with a microscopic flip-flop mechanism involving the absorption and emission of two simultaneous spin-one excitations having energy IJ(0)I, connecting singlet and triplet levels of neighbor d-units and giving rise to a quantum phase displaying spin entanglement. This phase is tuned with the orientation of B-0 applied along directions within "magic rings", the positions where the EPR peaks of the dimeric units intersect, that we propose as a phase diagram. Our model considers explic-itly the role of energy conservation in the process and allows analyzing and simulating the features of the EPR spectra arising from the couplings, describing their roles in the spectral behavior and the magnetic phases. In conclusion, we review the history of dimeric compounds and the possibility of detecting inter-dimeric couplings in the EPR spectra experimentally, and we introduce a novel spin model for the dimeric array appropriate to analyze EPR data which allows us to understand the spin dynamics and the phase transitions arising from these couplings. (c) 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).