Optical tools are promising for spin-wave generation because of the possibilities of ultrafast manipulation and local excitation. However, a single laser pulse can inject spin waves (SWs) only with a ...broad frequency spectrum, resulting in short propagation distances and low wave amplitudes. Here, we excite a magnetic garnet film by a train of fs-laser pulses with a 1-GHz repetition rate so that the pulse separation is shorter than the decay time of magnetic modes, which allows us to achieve a collective impact on the magnetization and establish a quasistationary source of spin waves, namely, a coherent accumulation of magnons (“magnon cloud”). This approach has several appealing features: (i) The magnon source is tunable, (ii) the SW amplitude can be significantly enhanced, (iii) the SW spectrum is quite narrow, providing long-distance propagation, (iv) the periodic pumping results in an almost constant-in-time SW amplitude for the distances larger than 20μm away from the source, and (v) the SW emission shows pronounced directionality. These results expand the capabilities of ultrafast coherent optical control of magnetization and pave the way for applications in data processing, including the quantum regime. The quasistationary magnon accumulation might also be of interest for applications in magnon Bose-Einstein condensates.
The Landé or g-factors of charge carriers are decisive for the spin-dependent phenomena in solids and provide also information about the underlying electronic band structure. We present a ...comprehensive set of experimental data for values and anisotropies of the electron and hole Landé factors in hybrid organic-inorganic (MAPbI
, MAPb(Br
Cl
)
, MAPb(Br
Cl
)
, FAPbBr
, FA
Cs
PbI
Br
, MA=methylammonium and FA=formamidinium) and all-inorganic (CsPbBr
) lead halide perovskites, determined by pump-probe Kerr rotation and spin-flip Raman scattering in magnetic fields up to 10 T at cryogenic temperatures. Further, we use first-principles density functional theory (DFT) calculations in combination with tight-binding and k ⋅ p approaches to calculate microscopically the Landé factors. The results demonstrate their universal dependence on the band gap energy across the different perovskite material classes, which can be summarized in a universal semi-phenomenological expression, in good agreement with experiment.
Plasmonics allows light to be localized on length scales much shorter than its wavelength, which makes it possible to integrate photonics and electronics on the nanoscale. Magneto-optical materials ...are appealing for applications in plasmonics because they open up the possibility of using external magnetic fields in plasmonic devices. Here, we fabricate a new magneto-optical material, a magnetoplasmonic crystal, that consists of a nanostructured noble-metal film on top of a ferromagnetic dielectric, and we demonstrate an enhanced Kerr effect with this material. Such magnetoplasmonic crystals could have applications in telecommunications, magnetic field sensing and all-optical magnetic data storage.
Currently spin waves are considered for computation and data processing as an alternative to charge currents. Generation of spin waves by ultrashort laser pulses provides several important advances ...with respect to conventional approaches using microwaves. In particular, focused laser spot works as a point source for spin waves and allows for directional control of spin waves and switching between their different types. For further progress in this direction it is important to manipulate with the spectrum of the optically generated spin waves. Here we tackle this problem by launching spin waves by a sequence of femtosecond laser pulses with pulse interval much shorter than the relaxation time of the magnetization oscillations. This leads to the cumulative phenomenon and allows us to generate magnons in a specific narrow range of wavenumbers. The wavelength of spin waves can be tuned from 15 μm to hundreds of microns by sweeping the external magnetic field by only 10 Oe or by slight variation of the pulse repetition rate. Our findings expand the capabilities of the optical spin pump-probe technique and provide a new method for the spin wave generation and control.
A novel method of creating new lightweight, aluminum-metallic, composite materials under halides melt at temperatures 973–1073 К under air atmosphere is proposed. The method for synthesizing ...aluminum-based metallic composite materials, containing up to 2 wt. % graphene sheets uniformly distributed in a metal matrix, is entirely new, having no analogies in current science and practice. The synthesis of graphene nanosheets in a metal matrix is one-step, simultaneous process, taking place directly in molten aluminum under alkali halides melt without the necessity of a separate stage of synthesis and introduction of graphene. This has the potential to facilitate the inexpensive synthesis of aluminum-graphene composites with a high concentration of graphene.
The aluminum-graphene composites formed according to this method are characterized by a high uniformity of graphene films with linear dimensions from 100 nm to 50 μm and a thickness from one to three nm in the metal bulk.
No aluminum carbide forms under synthesis; the aluminum-graphene and aluminum-graphite composites are resistant to corrosion in NaCl solution.
The hardness, strength and ductility of aluminum-graphene composites are at least 2–3 times higher than the initial aluminum material, proportional to the concentration of graphene.
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•Aluminum-carbon metallic composite materials firstly synthesized by chemical interaction of carbide with molten aluminum.•Aluminum-graphene metallic composite with uniformly distributed graphene films inside aluminum matrix.•Hardness of aluminum-graphene composites is 2–3 times higher than initial aluminum.•Composites without aluminum carbide formation have high corrosion resistance.
Hybrid structures synthesized from different materials have attracted considerable attention because they may allow not only combination of the functionalities of the individual constituents but also ...mutual control of their properties. To obtain such a control an interaction between the components needs to be established. For coupling the magnetic properties, an exchange interaction has to be implemented which typically depends on wavefunction overlap and is therefore short-ranged, so that it may be compromised across the hybrid interface. Here we study a hybrid structure consisting of a ferromagnetic Co layer and a semiconducting CdTe quantum well, separated by a thin (Cd, Mg)Te barrier. In contrast to the expected p-d exchange that decreases exponentially with the wavefunction overlap of quantum well holes and magnetic atoms, we find a long-ranged, robust coupling that does not vary with barrier width up to more than 30 nm. We suggest that the resulting spin polarization of acceptor-bound holes is induced by an effective p-d exchange that is mediated by elliptically polarized phonons.
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•Catalysts Ru/C, Pt/ZrO2, NiCuMo/SiO2 increase the yield of methoxyphenols in hydrogenolysis of aspen wood.•Optimal process conditions, providing the high yield of methoxyphenols were ...established.•Yield of methoxyphenols is higher at catalytic hydrogenolysis of native lignin compared to isolated ethanol lignin.•Catalytic hydrogenolysis in supercritical ethanol allows to fractionate the aspen wood on cellulose and methoxyphenols.
Catalytic hydrogenolysis in the medium of supercritical organic solvents is a promising way of wood lignins depolymerization into liquid products.
In this study, for the first time, the catalytic properties of bifunctional catalysts Ru/C, Pt/ZrO2, NiCuMo/SiO2, containing nanosized metal particles on acidic groups are compared in the processes of aspen wood and ethanol lignin hydrogenolysis in supercritical ethanol.
The most active catalysts are Ru/C and Pt/ZrO2 which provide the high conversion of wood (to 78.0 wt%), significant yield of liquid products (to 50.6 wt%) and low yield of solid rest (to 22.0 wt%) at temperature 250 °C and H2 pressure 9.0 MPa. These catalysts increase the yield of monomeric compounds in liquid products from 10.5 % to 50.4 % on mass of lignin.
GC–MS analysis shows that alkyl derivatives of methoxyphenols (mainly propyl syringol and propyl guaiacol) are dominated in liquid products. Solid products of aspen wood catalytic hydrogenolysis contain mainly cellulose (to 82.2 wt%). Therefore, the catalytic hydrogenolysis in supercritical ethanol in the presence of by functional catalysts Ru/C and Pt/ZrO2 allows to fractionate the aspen wood biomass on cellulose and liquid products enriched with propyl syringol and propyl guaiacol.
In catalytic hydrogenolysis of ethanol lignin the yield of alkyl derivatives of methoxyphenols is lower compared to wood. This is probably due to the reduced content of reactive β-O-4 bonds in the structure of ethanol lignin compared to native lignin of aspen wood. As follows from the results obtained, native lignin of wood is easier depolymerized to monomeric compounds in the process of catalytic hydrogenolysis than ethanol lignin, isolated from wood.
According to GPC data, the catalysts shift to the region of lower molecular mass the molecular mass distribution of liquid products of aspen wood and ethanol lignin hydrogenolysis.
Abstract
A concept of the front-end system of the XCELS (eXawatt Center for Extreme Light Studies) facility is presented. Its design is aimed at achieving high stability of laser radiation parameters ...and possibility of their control in a wide range. Optically synchronised chirped signal (wavelength 910 nm, bandwidth more than 100 nm, and duration ∼3 ns) and pump (wavelength 1054 nm, bandwidth ∼1 nm, and duration ∼4 ns) pulses for XCELS parametric amplifiers will be implemented at the output of the front-end system. Chirped femtosecond pulses with energies above 100 mJ no more than 15 fs long after compression, with carrier-envelope phase (CEP) stabilisation will have a repetition rate up to 100 Hz, which will allow one to implement active energy stabilisation and to minimise the angular jitter of the emitted beam at the XCELS output. The application of picosecond pumping in the parametric amplifier of the front-end system should provide a high contrast of femtosecond pulses. The pump pulse will be linearly frequency-modulated; this approach will not affect the parametric amplification efficiency but make it possible to use spectral methods to control the pump pulse shape in order to form a pulse of specified shape at the output of power amplifiers, even under conditions of their strong saturation.
Context.
Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that ...a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population.
Aims.
The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure.
Methods.
A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ~15 GPa at a rim toward hundreds of gigapascals in the center.
Results.
Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ~50 GPa. In the region of ~50–60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ~60 GPa due to an onset of silicate melting. At pressures higher than ~150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared region while changes to the MIR spectrum are minimal.
Conclusions.
Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions.
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