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3. ULTRAFAST ELECTRON DYNAMICS IN CORRELATED SYSTEMS PROBED BY TIME-RESOLVED PHOTOEMISSION SPECTROSCOPYTanusree Saha, 2023, doktorska disertacija Opis: Complex systems in condensed matter are characterized by strong coupling
between different degrees of freedom constituting a solid. In materials
described by many-body physics, these interactions may lead to
the formation of new ground states such as excitonic insulators, Mott
insulators, and charge and spin density waves. However, the inherent
complexity in such materials poses a challenge to identifying the
dominant interactions governing these phases using equilibrium studies.
Owing to the distinct timescales associated with the elementary interactions,
such complexities can be readily addressed in the non-equilibrium
regime. Additionally, these materials might also show the emergence
of new, metastable “hidden“ phases under non-equilibrium. The thesis
investigates the ultrafast timescales of fundamental interactions in candidate
systems by employing time-and angle-resolved photoemission spectroscopy
in the femtosecond time domain. In the (supposed) excitonic
insulator model system Ta2NiSe5, the timescale of band gap closure
and the dependence of rise time (of the photoemission signal) on the
photoexcitation strength point to a predominantly electronic origin of
the band gap at the Fermi level. The charge density wave (CDW) -
Mott insulator 1T-TaS2 undergoes photoinduced phase transition to two
different phases. The initial one is a transient phase which resembles
the systems’s high temperature equilibrium phase, followed by a long-lived
“hidden“ phase with a different CDW amplitude and is primarily
driven by the CDW lattice order. For the spin density wave system
CaFe2As2 where multiple bands contribute in the formation of Fermi surfaces,
selective photoexcitation was used to disentangle the role played
by different electron orbitals. By varying the polarization of photoexcitation
pulses, it is observed that dxz/dyz orbitals primarily contribute to
the magnetic ordering while the dxy orbitals have dominant role in the
structural order. The findings of the present study provide deeper perspectives
on the underlying interactions in complex ground phases of
matter, therefore, initiating further experimental and theoretical studies
on such materials. Ključne besede: complex systems, charge density wave, excitonic insulator, metastable phase, Mott insulator, non-equilibrium, spin density wave, timescales, time- and angle-resolved photoemission, ultrafast dynamics Objavljeno v RUNG: 01.06.2023; Ogledov: 1413; Prenosov: 28 Celotno besedilo (13,34 MB) |
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7. Correlation between FeCl2 electrolyte conductivity and electrolysis efficiencyUroš Luin, Matjaž Valant, Iztok Arčon, 2022, objavljeni povzetek znanstvenega prispevka na konferenci Opis: The electrolysis efficiency is an important aspect of the Power-to-Solid energy storage technology (EST) based
on the iron chloride electrochemical cycle [1]. This cycle employs an aqueous FeCl2 catholyte solution for the
electro-reduction of iron. The metal iron deposits on the cathode. The energy is stored as a difference in the
redox potential of iron species. Hydrogen, as an energy carrier, is released on demand over a fully controlled
hydrogen evolution reaction between metallic Fe0 and HCl (aq) [1]. Due to these characteristics, the cycle is
suitable for long-term high-capacity and high-power energy storage. In a previous work [2] we revealed that
the electrolyte conductivity linearly increases with temperature. Contrary, the correlation between the
electrolyte concentration and efficiency is not so straightforward. Unexpectedly small efficiency variations were
found between 1 and 2.5 mol dm-3 FeCl2 (aq) followed by an abrupt efficiency drop at higher concentrations.
To explain the behavior of the observed trends and elucidate the role of FeCl2 (aq) complex ionic species we
performed in situ X-ray absorption studies. We made a dedicated experimental setup, consisting of a tubular
oven and PMMA liquid absorption cell, and performed the measurements at the DESY synchrotron P65
beamline. The XAS investigation covered XANES and EXAFS analyses of FeCl2 (aq) at different
concentrations (1 - 4 molL-1) and temperatures (25 - 80 °C). We found that at low temperature and low FeCl2
concentration the octahedral first coordination sphere around Fe is occupied by one Cl ion at a distance of 2.33
(±0.02) Å and five water molecules at a distance of 2.095 (±0.005) Å [3]. The structure of the ionic complex
gradually changes with an increase in temperature and/or concentration. The apical water molecule is
substituted by a chlorine ion to yield a neutral Fe[Cl2(H2O)4]0. The transition from the single charged
Fe[Cl(H2O)5]+ to the neutral Fe[Cl2(H2O)4]0 causes a significant drop in the solution conductivity, which well
correlates with the existing conductivity models [3].
[1] M. Valant, “Procedure for electric energy storage in solid matter. United States Patent and
Trademark Office. Patent No. US20200308715,” Patent No. US20200308715, 2021.
[2] U. Luin and M. Valant, “Electrolysis energy efficiency of highly concentrated FeCl2 solutions
for power-to-solid energy storage technology,” J. Solid State Electrochem., vol. 26, no. 4, pp.
929–938, Apr. 2022, doi: 10.1007/S10008-022-05132-Y.
[3] U. Luin, I. Arčon, and M. Valant, “Structure and Population of Complex Ionic Species in
FeCl2 Aqueous Solution by X-ray Absorption Spectroscopy,” Molecules, vol. 27, no. 3, 2022,
doi: 10.3390/molecules27030642. Ključne besede: Iron chloride electrochemical cycle, Power-to-Solid energy storage, XANES, EXAFS, electrical
conductivity, electrolyte complex ionic species structure and population Objavljeno v RUNG: 26.09.2022; Ogledov: 1599; Prenosov: 0 (1 glas) Gradivo ima več datotek! Več... |
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9. Graph based feature extraction and classification of wet and dry cough signals: A machine learning approachMohanachandran Nair Sindhu Swapna, 2021, izvirni znanstveni članek Opis: This article proposes a unique approach to bring out the potential of graph-based features to reveal the
hidden signatures of wet (WE) and dry (DE) cough signals, which are the suggestive symptoms of various
respiratory ailments like COVID 19. The spectral and complex network analyses of 115 cough signals are
employed for perceiving the airflow dynamics through the infected respiratory tract while coughing. The
different phases of WE and DE are observed from their time-domain signals, indicating the operation of the
glottis. The wavelet analysis of WE shows a frequency spread due to the turbulence in the respiratory tract.
The complex network features namely degree centrality, eigenvector centrality, transitivity, graph density
and graph entropy not only distinguish WE and DE but also reveal the associated airflow dynamics. A better
distinguishability between WE and DE is obtained through the supervised machine learning techniques
(MLTs)—quadratic support vector machine and neural net pattern recognition (NN), when compared to
the unsupervised MLT, principal component analysis. The 93.90% classification accuracy with a precision
of 97.00% suggests NN as a better classifier using complex network features. The study opens up the
possibility of complex network analysis in remote auscultation. Ključne besede: wet cough, dry cough, complex network, quadratic SVM, neural net Objavljeno v RUNG: 30.06.2022; Ogledov: 1115; Prenosov: 0 Gradivo ima več datotek! Več... |
10. Complex network-based cough signal analysis for digital auscultation: a machine learning approachMohanachandran Nair Sindhu Swapna, 2022, izvirni znanstveni članek Opis: The paper proposes a novel approach to bring out the potential of complex networks based on graph theory to unwrap the hidden characteristics of cough signals, croup (BC), and pertussis (PS). The spectral and complex network analyses of 48 cough sounds are utilized for understanding the airflow through the infected respiratory tract. Among the different phases of the cough sound time-domain signals of BC and PS – expulsive (X), intermediate (I), and voiced (V) - the phase ‘I’ is noisy in BC due to improper glottal functioning. The spectral analyses reveal high-frequency components in both cough signals with an additional high-intense low-frequency spread in BC. The complex network features created by the correlation mapping approach, like number of edges (E), graph density (G), transitivity (), degree centrality (D), average path length (L), and number of components () distinguishes BC and PS. The higher values of E, G, and for BC indicate its musical nature through the strong correlation between the signal segments and the presence of high-intense low-frequency components in BC, unlike that in PS. The values of D, L, and discriminate BC and PS in terms of the strength of the correlation between the nodes within them. The linear discriminant analysis (LDA) and quadratic support vector machine (QSVM) classifies BC and PS, with greater accuracy of 94.11% for LDA. The proposed work opens up the potentiality of employing complex networks for cough sound analysis, which is vital in the current scenario of COVID-19. Ključne besede: Complex network analysis, Auscultation, Croup cough, Pertussis
Spectral analysis, Machine learning techniques Objavljeno v RUNG: 30.06.2022; Ogledov: 1388; Prenosov: 0 Gradivo ima več datotek! Več... |