24378 research outputs found
Sort by
Nitrile Groups as Build In Molecular Sensors for Interfacial Effects at Electrocatalytically Active Carbon Nitrogen Materials
Full text linkElectrocatalytic reactions are influenced by various interfacial phenomena including nonspecific interaction forces. For many examples, their contributions to the catalytic cycle have yet to be identified. Noncovalent interactions between the electrode and the electrolyte can be described by the local electric field environment at the interface and are experimentally accessible based on the Vibrational Stark Effect. We herein present a carbon based C2N type electrocatalyst that is active for the hydrogen evolution reaction and that contains nitrile functions as Stark reporter groups. With this system, we expand the range of electrocatalytically active systems suitable for electrochemical Stark spectroscopy while taking a step away from pure model systems. The stretching mode nu C N was analyzed via experimental and calculated Raman spectroscopy, revealing a defect character of the inherent CN groups. The nu C N peak position was furthermore studied via in situ electrochemical Raman spectroscopy. At noncatalytic conditions, a linear dependence between an applied electric potential and nu C N peak shift is observed, resulting in a red shift at a more negative potential. At catalytic conditions, deviations from the linearity occur, and a semipermanent blue shift of the CN peak is observed after electrocatalysis, implying a restructuring of the electrochemical double layer and therefore a change in the local electric field environment due to the catalytic turnover and the associated interfacial processe
Phase diagram of the XXZ pyrochlore model from pseudo Majorana functional renormalization group
Full text linkWe calculate the magnetic phase diagram of the spin 1 2 nearest neighbor XXZ pyrochlore model using the pseudo Majorana functional renormalization group in the temperature flow formalism. Our phase diagram as a function of temperature and coupling ratio, allowing both longitudinal and transverse couplings to be ferromagnetic and antiferromagnetic, reveals a large nonmagnetic regime at low temperatures, which includes the quantum spin ice phase near the antiferromagnetic Ising model, as well as the antiferromagnetic Heisenberg and XY models. We are able to detect magnetic phase transitions via critical finite size scaling down to temperatures two orders of magnitude smaller than the spin interactions, demonstrating the remarkably good performance of our method upon approaching the ground state. Specifically, the low temperature transition from the zero flux quantum spin ice phase into the transverse ferromagnetic phase shows very good agreement with previous quantum Monte Carlo results. Comparing our findings with classical results, we identify a quantum order by disorder effect near the antiferromagnetic XY model. In magnetically disordered regimes, we find characteristic patterns of broadened pinch points in the spin structure factor and investigate their evolution when approaching magnetically ordered phases. We also compute linear responses to lattice symmetry breaking perturbations and identify a possible lattice nematic ground state of the antiferromagnetic XY mode
Frustrated Classical Spin Models Equilibrium and Dynamical Properties in Connection with Real Materials
No full textFrustrated magnetic systems are characterized by competing interactions that in some cases prevent conventional magnetic ordering, leading to unconventional ground states such as quantum and classical spin liquids. These states exhibit strong entanglement, fractional ized excitations, and emergent topological phenomena. Such systems are of interest not only from a theoretical perspective, due to their rich and unconventional physics, but also from a materials standpoint, as they offer promising platforms for realizing exotic phases in real compounds. The theoretical study of magnetic models using numerical methods plays a crucial role in advancing our understanding of the magnetic properties of materials. It helps interpret experimental results, predicts properties not yet explored experimentally, and enhances the interpretation of data through the development of new numerical tools. In this thesis, we address these aspects through three main investigations. First, we study classical spin models on the distorted windmill lattice, which is relevant to the spin liquid candidate PbCuTe2O6. Through this study, we determine the origin of frustration in this compound and examine the thermodynamic behavior and the magnetic excitations of the associated classical model. Additionally, by mapping out the classical phase diagram, we identify a novel type of classical spin liquid. Next, we investigate the dynamical signatures of soft modes, focusing on quartic spin oscillations in isotropic systems with spiral magnetic order. We show that these modes exhibit a gap that decreases with temperature, a feature observable in real materials through inelastic neutron scattering experiments. Finally, we introduce a machine learning based approach to infer the underlying magnetic Hamilto nian. Specifically, we train a neural network on synthetic spectra generated using linear spin wave theory and apply the trained model to analyze experimental inelastic neutron scattering spectr
Reviving Nitrogen Vacancy Centers in Diamond via Local Surface Modification
Full text linkSurface termination of semiconductors is important for their applications in electronics because it governs electrical properties at interfaces. For quantum sensors and qubits based on spins in solids, this is crucial when they are located a few nanometers below the crystal surface. In the case of diamond, oxygen termination is preferential for quantum sensing with nitrogen vacancy NV centers. Here, we present local surface modification of a nonconductive diamond surface utilizing conductive atomic force microscopy under laser illumination. By applying this method, we demonstrate not only a removal of fluorescence background but also control over the charge state of single NV centers. The latter show an improvement of the optically detected magnetic resonance contrast from 1 up to 29 after the treatment. We assume that local surface oxidation is happening on the diamond, which has already been demonstrated for conducting hydrogenated surfaces, but its implementation to nonconductive surfaces remains challengin
High Crystallinity Quasi Spherical Single phase Vanadium Dioxide Nanoparticles for Thermochromic Applications
No full textVanadium dioxide VO2 has attracted significant attention as a promising material for energy saving smart windows because of its reversible metal insulator transition, which is accompanied by pronounced changes in optical properties, particularly in near infrared NIR light transmittance. This thermochromic property of VO2 is broadly influenced by various factors, such as the phase, morphology, and crystallinity of the film. Despite numerous attempts to synthesize high quality VO2 nanoparticles NPs , there remains a dearth of literature reporting the successful formation of spherical VO2 NPs amp; 8764;25 nm that exhibit both high monoclinic P21 c M1 phase purity and crystallinity. In this study, we present the synthesis of highly crystalline VO2 M1 NPs with a size of amp; 8764;25 nm achieved through precise control of reaction temperature and pH using a hydrothermal method. We identified specific temperature pH conditions under which the synthesis exclusively yielded single phase VO2 M1 NPs. Within these temperature pH conditions, a higher temperature and lower pH resulted in a larger particle size, higher crystallinity, and improved thermochromic properties. The synthesis condition of 240 C pH 6.5 resulted in optimal thermochromic performance with a single layered VO2 film achieving an integrated luminous transmittance Tlum of 53.8 and a solar modulation ability amp; 9651;Tsol of 12.2 . Furthermore, when forming a composite film using the optimized VO2 NPs and a UV curable adhesive, amp; 9651;Tsol was improved while a similar Tlum was maintained. We also demonstrated well performing sandwich type thermochromic windows comprising glass VO2 composite glass or plastic VO2 composite plastic for practical application
THz EPR Based Direct Spectroscopic Determination of the Anisotropy Barrier of a Heteroleptic Bis Chelate Cobalt II Complex with Bis sulfonamido benzene Ligand and its Dynamic Magnetic Properties
No full textA new mononuclear, four coordinate heteroleptic cobalt II complex 1 was synthesised utilising N,N amp; 8242; 1,2 Phenylenebis 4 methylbenzenesulfonamide and 6,6 amp; 8242; Dimethyl 2,2 amp; 8242; bipyridine as ligands and structurally characterised. Magnetic susceptibility and magnetisation measurements, along with ab initio calculations, revealed a large easy axis type magnetic anisotropy of the cobalt II centre. Frequency dependent ac susceptibility measurements revealed out of phase signals, indicating a slow relaxation of magnetisation, even in the absence of an additional static magnetic field. The corresponding relaxation behaviour can be attributed to a combination of Orbach and phonon assisted processes within the observable temperature range. Using FD FT THz EPR spectroscopy, the magnetic anisotropy barrier was determined to be unexpectedly large with a value of 146.4 amp; 8197;mathematical equation . The deviation from the expected barrier, based on correlations with the elongation parameter mathematical equation , is attributed to the charge distribution in the coordination environmen
Sustainable Upgrading of Biomass A Thermodynamic Approach to Fine Tuning Product Selectivity for Glycerol Oxidation
Full text linkCalculated thermodynamic properties for the electrochemical glycerol oxidation at different temperatures and potentials indicate that external applied bias has a more significant influence on reaction selectivity than temperatur
Processing of AlSi13Mg5 foams using Mg and AlMg50 blowing agents
No full textAluminium closed cell foams with superior structure and properties can be produced using Mg based blowing agents. In this study, AlSi13Mg5 foams were produced via the powder metallurgy route using two different Mg based blowing agents pure Mg powders and pre alloyed AlMg50 powders. Mass spectrometry was performed to determine their hydrogen desorption characteristics. In Mg powder, the peak gas release occurred at 364 C compared to the delayed peak gas release at 420 C from AlMg50 powder. The structure and properties of the foams produced using these two blowing agents were compared by performing image analysis and compression tests, respectively. On average, the foams produced using AlMg50 blowing agents exhibited slightly better porous structure and strength. Peak strength of the foams produced using Mg and AlMg50 powder is 7.3 0.6 MPa and 8 0.4 MPa, respectivel
Non thermal electrons open the non equilibrium pathway of the phase transition in FeRh
Full text linkThe optical excitation of metals initially creates short lived non Fermi distributions of the electrons. The electrons and holes excited far above and below the Fermi level quickly relax to hot Fermi distributions that subsequently cool via electron phonon scattering. Here, we show that such non thermal charge carriers beyond the Fermi distribution speed up the prototypical first order antiferromagnetic to ferromagnetic phase transition in FeRh. In ultrafast x ray diffraction experiments, we vary the maximum electron temperature by increasing the pump pulse duration up to 10 amp; 8201;ps. For direct optical excitation of FeRh, ferromagnetic domains nucleate within 8 amp; 8201;ps as soon as the successively deposited energy surpasses the site specific threshold energy. In contrast, suppressing the direct optical excitation by an optically opaque Pt layer leads to a nucleation on a 50 amp; 8201;ps timescale driven by the near equilibrium heat transport. These findings unambiguously identify the photo excitation of non thermal electrons and not electron phonon non equilibria to enable the rapid phase transition in FeR
Engineering strategies to minimize bubble induced optical losses in photoelectrochemical water splitting
Full text linkGas bubble evolution presents a significant challenge to achieving efficient solar water splitting by blocking catalytic active sites, increasing electrolyte ohmic resistance, and scattering incident solar photons. The bubble induced optical loss is especially important in photoelectrochemical PEC cells, as specific cell configurations require light to travel through the electrolyte before reaching the photoelectrodes. Understanding bubble characteristics under various operating conditions and the associated bubble induced optical losses is essential to optimize the PEC water splitting efficiency. In this study, we use optical measurements and bubble shadowgraphy to demonstrate that operating PEC water splitting systems at elevated pressure up to 4 bar reduces bubble induced optical losses by a factor of four, with only a minimal ?1 increase in thermodynamic cell voltage. In addition, lowering the electrolyte buffer concentration further mitigates the bubble induced optical losses, albeit at the cost of increased overpotential due to higher ohmic resistance and increased adhesion of the larger bubbles. These quantitative results provide valuable insights into the design and practical implementation of PEC water splitting cells, and the approach can be extended to other gas evolving photoelectrochemical conversion system