Simulation of Hot-Carrier Dynamics and Terahertz Emission in Laser-Excited Metallic Bilayers

We present a multiscale model that simulates optically induced spin currents in metallic bilayer structures that emit terahertz radiation after optical pulse excitation. We describe hot-electron transport in a metallic bilayer by a Boltzmann transport equation, which is solved numerically by a particle-in-cell approach. Optical excitation and propagation effects are taken into account by our determining the emitted terahertz waves from the excited-carrier dynamics. We apply this approach to an Fe/Pt bilayer and show in detail how microscopic scattering effects and transport determine the emitted signal. The versatility of the approach presented here allows it to be readily adapted to a wide spectrum of spintronic-terahertz-emitter designs. As an example, we show how the terahertz generation efficiency, defined as the output-power-to-input-power ratio, can be increased and optimized with use of serially stacked layers in conjunction with terahertz antireflective coatings. We derive an analytical expression for the terahertz emission of a single layer that allows us to determine the relationship between the emitted field and the current profile that generates it.German Science Foundation [SFB/TRR 173 Spin+X]; Graduate School of Excellence MAINZ (Excellence Initiative) [DFG/GSC 266]; [SFB/TRR 173]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Modification of spintronic terahertz emitter performance through defect engineering

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons. We clarify the influence of the electron-defect scattering lifetime on the spectral shape and the interface transmission on the THz amplitude, and how this is linked to structural defects of bilayer emitters. The results of our study define a roadmap of the properties of emitted as well as detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.Comment: 33 pages, 13 figure

Suppression of axionic charge density wave and onset of superconductivity in the chiral Weyl semimetal Ta₂Se₈I

A Weyl semimetal with strong electron-phonon interaction can show axionic coupling in its insulator state at low temperatures, owing to the formation of a charge density wave (CDW). Such a CDW emerges in the linear-chain-compound Weyl semimetal Ta2Se8I below 263 K, resulting in the appearance of the dynamical condensed-matter axion quasiparticle. In this paper, we demonstrate that the interchain coupling in Ta2Se8I can be varied to suppress the CDW formation with pressure, while retaining the Weyl semimetal phase at high temperatures. Above 17 GPa, the Weyl semimetal phase does not survive, and we induce superconductivity, due to the amorphization of the iodine sublattice. Structurally, the quasi-one-dimensional Ta-Se chains remain intact and provide a channel for superconductivity. We highlight that our results show a near-complete suppression of the gap induced by the axionic charge density wave at pressures inaccessible to previous studies. Including this CDW phase, our experiments and theoretical predictions and analysis reveal the complete phase diagram of Ta2Se8I and its relationship to the nearby superconducting state. The results demonstrate Ta2Se8I to be a distinctively versatile platform for exploring correlated topological states. © 2021 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society

An Experimental Study on the Ordered Alloy Ni_2Cr

The ordered alloy Ni_2Cr has been investigated by means of electron diffraction, electron microscopy, calorimetry, resistometry and tensile tests. The formation of the Pt_2Mo type superstructure is revealed by an electron diffraction study on the single crystalline specimens. The size of ordered domains is of the order of magnitude of several hundred A even in the well annealed state. The so-called \u27K-state\u27 which is characterized by the initial increase in electrical resistivity upon isothermal annealing is attributed to the existence of very fine ordered domains. Superdislocations consisting of triple dislocations are sometimes observed in various stages of ordering, and the mechanical properties are briefly discussed in the light of electron microscopic observation. The energy and entropy of transformation are evaluated as about 1.0 kcal/g・atom and 1.2 cal/g・atom-deg on the alloy of 32.1 at% Cr annealed at 500℃ for about 3000 hr

SURFACE ENERGY AND EQUILIBRIUM SHAPE OF L12-TYPE A3B ORDERING ALLOYS

Calculation of surface energy of L12-type A3B ordering alloy was made on the basis of a broken bond model using Bragg-Williams approximation. Prior to the calculation of surface energy, calculation of broken bond density in the same alloy was made. Numerical results for Cu3Au alloy were presented as functions of surface orientation and degree of order. Applying Wulff's theorem to calculated γ-plots of the alloy, their external equilibrium shapes for the alloy with various degree of order (η) were predicted

ORDERING AND DISORDERING PHENOMENA AT/NEAR THE SURFACE OF D1a TYPE ORDERING ALLOYS

Ordering and disordering phenomena at/near the surface of D1a type ordering alloys (Ni4Mo and Ni4W) were studied by field-ion microscopy experimentally and by theoretical calculation based on a broken bond model and Bragg-Williams approximation. The main experimental results are as follows : (i) an ordered phase nucleates preferentially at surfaces with certain low index orientation based on the fcc matrix (immediately below Tc), (ii) surface layers of specimens become disordered (around 0.9Tc) and (iii) faceting based on the ordered structure occurs (around 0.8Tc). In the theoretical calculations, (i) surface energy, (ii) surface free energy, (iii) equilibrium degree of order and (iv) surface nucleation rate were obtained. The calculated results explain peculiar phenomena in the above experimental results