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Studying all optical switching in GdFe using both single and double laser pulses
No full textThis thesis investigates all optical switching and domain wall DW dynamics in GdFe ferrimagnetic alloys following femtosecond laser excitation. Advanced magnetic imaging techniques, including photoemission electron microscopy PEEM and Kerr microscopy, are used to capture the magnetic behavior of the sample. The study examines single pulse all optical toggle switching, where the magnetization of the films is reversed by a single p polarized laser pulse, independent of the initial magnetization direction. Specifically, we investigate the mechanisms behind deviations from deterministic toggle switching, especially near magnetic domain walls. The study reveals that non deterministic switching arises from intrinsic factors, such as domain wall elasticity and laser induced motion related to the sample itself, as well as extrinsic factors, like laser pulse variations, which depend on the setup. Further, double pulse excitation experiments probe the fastest toggle switching frequency, where two laser pulses with an adjustable time delay between them excite the sample. By using different fluence ratios of the pulses as a function of the time delay, the experiments explore the role of fluence in re switching. It was found that re switching occurs within a time delay between 4 and 40 ps after the first pulse, with optimal conditions when the first pulse is just above the single pulse switching threshold and the fluence of the second pulse is 0.5 0.7 times that of the first pulse. Atomistic spin dynamic simulations support these findings, offering insights into cooling dynamics where re switching requires that electron and lattice temperatures equilibrate. We also investigated the behavior of the sample at time delays of 2 ps or less between pulses, following double pulse switching at two temperatures, one below 70K and one above room temperature the magnetic compensation temperature. Findings indicate that the threshold fluence required to induce multi domain formation or demagnetization varies with the sample s initial temperature, with more deterministic toggle switching observed at the lower temperature of 70K. Notably, the switching probability rises drastically at these short time delays, where the second pulse significantly amplifies the effect of the first, even if one or both pulses are below the threshold fluence of single pulse switching. Additionally, we examine DW motion in GdFe thin films with an in plane easy axis under laser excitation. Depending on the DW s position relative to the laser spot, either entropic torque or magnon spin transfer torque may dominate the motion. Although we did not identify a single, consistent direction of DW movement, it is evident that for DW motion, the DWmust be located within the amp; 963;x region of the laser spot. Together, these findings advance our understanding of high speed, energy efficient magnetic switching and DW control, with promising implications for ultrafast magnetic storage technologie
Odd Even Effects in the Structure and Thermal Stability of Carboxylic Acid Anchored Monolayers on Naturally Oxidized Aluminum Surface
No full textSelf assembled monolayers SAMs are broadly used for molecular engineering of surfaces and interfaces, which demands control over their structure and properties. An important tool in this context is the so called odd even effects exploiting the dependence of the SAM structure on the parity of the number of building blocks forming the backbone of SAM building molecules. Even though these effects influence parameters crucial for SAM applications, they have been mainly studied on coinage metals Au and Ag until now. Here, using the series of biphenyl substituted carboxylic acids BPnCOO, n 0 4 , we show that structural odd even behavior occurs as well on technologically relevant surface of naturally oxidized aluminum representative of other oxide surfaces , with the even numbered monolayers exhibiting higher packing density and lower molecular inclination than the odd numbered analogs. Despite these structural changes, the SAM desorption energy remains nearly constant at a high value amp; 8764;1.5 eV making BPnCOO AlOx a promising system for organic electronics application
Minimizing Inter Lattice Strain to Stabilize Li Rich Cathodeby Order Disorder Control
No full textLi rich Mn based layered LMR cathodes with anionic redox chemistry show great potential for next generation sustainable Li ion battery LIB applications due to the low cost and high energy density. However, the asynchronous structural evolutions with cycling in the heterogeneous composite structure of LMR lead to serious lattice strain and thus fast electrochemical decay, which hinders the commercialization of LMR cathodes. Here, an order disorder coherent LMR cathode is demonstrated that exhibits a higher average voltage by 0.25 V , negligible voltage decay 97.6 voltage retention after 100 cycles at 100 mA g amp; 8722;1 , and enhanced cycling stability 98 capacity retention after 200 cycles at 100 mA g amp; 8722;1 compared to its layered oxide counterparts. It is proposed that this order disorder coherent structure design can promote a more synchronous and homogeneous structure evolution during charge and discharge, thus minimizing lattice strain, which significantly prevents layer collapse and collective degradation at high voltage, improving the electrochemical stability. The study displays the feasibility of optimizing the performance of Li rich cathode materials through a dedicated order disorder structure control for sustainable energy storag
Morphological and Chemical State Effects in Pulsed CO2 Electroreduction on Cu 100 unveiled by Quasi in situ Correlated Spectro Microscopy Measurements
No full textSubjecting copper to short anodic pulses during the electrocatalytic reduction of carbon dioxide CO2RR has been shown to improve the activity and selectivity towards hydrocarbons and alcohols. Nonetheless, the nature of the active sites is still unclear. Here the evolution of the morphology, chemical state and crystal structure of Cu 100 exposed to potential pulses during the CO2RR was resolved by a combination of spectroscopy, microscopy and diffraction methods applied concurrently. Under anodic potential pulses, n10 facets were formed. Moreover, alternating anodic to cathodic potential pulses during the CO2RR also lead to the stabilization of copper oxide species located either at the surface or directly underneath ultrathin metallic copper layers, depending on the specific pulse potential treatment applied. Both findings are key for the enhanced ethylene and ethanol production reported during pulsed CO2R
Observation of distorted tilted conical phase at the surface of a bulk chiral magnet with resonant elastic x ray scattering
Get PDFWe report on various magnetic configurations including spirals and skyrmions at the surface of the magnetic insulator Cu2OSeO3 at low temperatures with a magnetic field applied along 100 using resonant elastic X ray scattering REXS . We observe a well ordered surface state referred to as a distorted tilted conical spiral dTC phase over a wide range of magnetic fields. The dTC phase shows characteristic higher harmonic magnetic satellites in the REXS reciprocal space maps. Skyrmions emerge following static magnetic field cycling and appear to coexist with the dTC phase. Our results indicate that this phase represents a distinct and stable surface state that does not disappear with field cycling and persists until the field strength is increased sufficiently to create the field polarized stat
Redirecting the Peptide Cleavage Causes Protease Inactivation
No full textCysteine and serine proteases cleave peptides through covalent catalysis by generating a transient adduct with the N terminal part of the substrate after releasing its C terminal part. We demonstrate the unique redirection of this event leading to strong enzyme inactivation. For targeting human cathepsin B, a cysteine protease of significant therapeutic importance, we designed tailored peptidomimetics with a variety of dipeptide fragments directed toward the occluding loop and equipped with numerous N terminal carbamate warheads. The carbamate deprotonation catalyzed by the active site thiolate initiates the redirected cleavage. The C terminal part of the inhibitors remains covalently attached to the protease. Hydrolysis of such carbamoyl enzyme complexes is catalytically unsupported rendering inhibition irreversible. This novel mechanism of action comprises a significant extension of the covalent drug spac
Surface Deposition of Dome Shaped Metal Organic Complexes A New Approach to the Generation of Single Site Catalysts
No full textA novel approach combining the advantages of heterogeneous with those of homogeneous catalysis is the deposition of metal organic complexes on a metallic surface to create well defined single site catalysts. Dome shaped organometallic complexes with weakly binding coligands are well suited for this purpose. With this in mind, a new dithia [2.1.1] 2,6 pyridinophane ligand has been synthesized. The corresponding molybdenum 0 tricarbonyl complex is structurally and spectroscopically characterized in the bulk and in homogeneous solution. Monolayers of this complex are deposited on Au 111 and investigated with the help of surface spectroscopy infrared reflection absorption spectroscopy, X ray photoelectron spectroscopy, and near edge X ray absorption fine structure . These methods indicate a slightly tilted orientation of the complex on the gold surface, which is confirmed by density functional theory DFT calculations. The reactivity of the complex toward dioxygen is evaluated and compared to analogous complexes supported by aza and thiacalix[3] 2,6 pyridine ligand
Seeing the invisible XRF reveals lead distributions in coral organisms grown in the Red Sea Gulf of Aqaba
Get PDFLead Pb contamination in marine ecosystems poses a significant ecological threat. Not much is known about its effects on coral reefs, which serve as vital biodiversity hotspots and climate refuges. This study revealed bioaccumulation of Pb in the organism of two stony coral species, Stylophora pistillata and Pocillopora verrucosa, widely studied in the Gulf of Aqaba. Microfocus X ray fluorescence XRF imaging revealed widespread accumulation of Pb within the coral tissues but not in the skeletons. The finding that Pb predominantly accumulates in the soft tissues with no evidence of Pb in the mineral suggests that exposure was short or of low concentration. Both highlight the great sensitivity of coral organisms to Pb uptake, with likely negative impacts on the organism. We suggest that anthropogenic Pb contamination, intensified by factors such as urban runoff or industrial discharges, still poses a serious risk to coral health and resilience despite years of efforts to curb exposure. Future research should focus on the kinetics of Pb bioaccumulation, effects of short term versus long term exposure and the combined effects of heavy metals and temperature on coral physiolog
Modulation of spin reorientation and exchange bias in SmMn0.25Fe0.75O3 orthoferrite single crystal
No full textFuturistic multi state storage devices and energy efficient data storage sensors depend on advanced functional materials that manifest both spin and charge ordering. Here, we report the growth of SmMn0.25Fe0.75O3 SMFO single crystals, which crystallize in an orthorhombic structure space group Pbnm , as determined through measurements of temperature dependent X ray diffraction XRD . The SMFO crystals display varying physical properties due to modulation of the spin reorientation transition SRT amp; 915;4 amp; 8594; amp; 915;2 at 382 K, which is relatively lower than the 450 K transition reported in pristine SmFeO3. Moreover, the addition of Mn 25 induces a new spin reorientation TSR1 amp; 915;2 amp; 8594; amp; 915;1 occurring at 175 K. The observed exchange bias effect in the SMFO crystal shows a divergence at the onset of spin reorientation temperature. The spin canting angle in SMFO is characterized within the context of mixed ferromagnetic and antiferromagnetic domains. We further elaborate direct correlation between anisotropic lattice compression and the spin canting angles. The mutual competing 3d 3d and 3d 4f spin exchange interactions between the magnetic moments of Sm3 ions and Fe3 Mn3 ions, which results in minimal lattice distortions that break inversion symmetry. This leads to the Dzyaloshinskii Moriya D M interaction that further induces a net electric dipole moment. These experimental findings indicate that competing exchange interactions are crucial in controlling the exchange bias, spin anisotropy, lattice distortion, and ferroelectricity, which are beneficial for the development of functional devices and their application
Improving Perovskite CIGS Tandem Solar Cells for Higher Power Conversion Efficiency through Light Management and Bandgap Engineering
Get PDFPerovskite and chalcopyrite materials are excellent absorbers for highly efficient, all thin film tandem solar cells. This work presents a certified world record for such a device, achieving a power conversion efficiency of 24.6 1.1 under steady state conditions. The best IV parameters extracted from certified current amp; 8722;voltage measurements presented a short circuit current density of around 19.3 mA cm2, an open circuit voltage of 1.765 V, and a fill factor of 71.8 . In comparison to our previous record, the current density improved considerably, mainly due to the lowering of the bandgap of the bottom subcell and the improved optics of the top perovskite cel