95 research outputs found
Dynamics of lattice disorder in perovskite materials, polarization nanoclusters and ferroelectric domain wall structures
The nexus between classic ferroelectricity and the structure of perovskite materials hinges on the concept of lattice disorder. Although the ordered perovskites display short-range displacements of the central cations around their equilibrium points, the lattice disorder dynamically unfolds to generate a myriad of distorted rhombohedral lattices characterized by the hopping of the central cations across directions. It is discovered that the lattice disorder correlates with the emergence of minimum configuration energy pathways for the central cations, resulting in spatially modulated ultrafast polarization nanocluster arrangements that are stabilized by the electric charge defects in the material. Through high-resolution phonon dispersion analyses encompassing molecular dynamics (MD) and density functional theory (DFT) simulations, we provide unequivocal evidence linking the hopping of central cations to the development of diffuse soft phonon modes observed throughout the phase transitions of the perovskite. Through massive MD simulations, we unveil the impact of lattice disorder on the structures of domain walls at finite-temperature vis-à-vis collective activation and deactivation of pathways. Furthermore, our simulations demonstrate the development of hierarchical morphotropic phase boundary (MPB) nanostructures under the combined influence of externally applied pressure and stress relaxation, characterized by sudden emergence of zig-zagged monoclinic arrangements that involve dual shifts of the central cations. These findings have implications for tailoring MPBs in thin-film structures and for the light-induced mobilization of DWs. Avenues are finally uncovered to the exploration of lattice disorder through gradual shear strain application.Peer ReviewedPostprint (published version
Geometry-induced spin-filtering in photoemission maps from WTe surface states
We demonstrate that an important quantum material WTe exhibits a new type
of geometry-induced spin-filtering effect in photoemission, stemming from low
symmetry that is responsible for its exotic transport properties. Through the
laser-driven spin-polarized angle-resolved photoemission Fermi surface mapping,
we showcase highly asymmetric spin textures of electrons photoemitted from the
surface states of WTe. Such asymmetries are not present in the initial
state spin textures, which are bound by the time-reversal and crystal lattice
mirror plane symmetries. The findings are reproduced qualitatively by
theoretical modeling within the one-step model photoemission formalism. The
effect could be understood within the free-electron final state model as an
interference due to emission from different atomic sites. The observed effect
is a manifestation of time-reversal symmetry breaking of the initial state in
the photoemission process, and as such it cannot be eliminated, but only its
magnitude influenced, by special experimental geometries.Comment: 5 pages, 3 figure
Control of ultrafast laser ablation efficiency by stress confinement due to strong electron localization in high-entropy alloys
In the context of current state of the art, understanding the laser ablation efficiency decrease for pulse durations High-entropy alloy; CrMnFeCoNi; Ultrafast laser ablation; Pulse duration; Ablation efficiency; Stress confinementexceeding the mechanical relaxation time of a few ps remains a pending research question. A heuristic approach may be used to reveal the role of effective penetration depth on ablation efficiency. Extending familiar contributions of this quantity by a term related to the mechanical surface expansion during pulse irradiation, the relation of ablation efficiency and pulse duration is deciphered. Thus, longer pulses are coupled into an expanded surface, revealing a direct link to the violation of stress confinement. To best demonstrate this hypothesis, a material with high electron–phonon coupling as well as low thermal conductivity, i.e., strong electron localization, is required. These properties are accomplished by high-entropy alloys, and the CrMnFeCoNi alloy serves as prime candidate. We report on single-pulse ablation efficiency experiments of the CrMnFeCoNi alloy which are support by our proposed model.In the context of current state of the art, understanding the laser ablation efficiency decrease for pulse durations High-entropy alloy; CrMnFeCoNi; Ultrafast laser ablation; Pulse duration; Ablation efficiency; Stress confinementexceeding the mechanical relaxation time of a few ps remains a pending research question. A heuristic approach may be used to reveal the role of effective penetration depth on ablation efficiency. Extending familiar contributions of this quantity by a term related to the mechanical surface expansion during pulse irradiation, the relation of ablation efficiency and pulse duration is deciphered. Thus, longer pulses are coupled into an expanded surface, revealing a direct link to the violation of stress confinement. To best demonstrate this hypothesis, a material with high electron–phonon coupling as well as low thermal conductivity, i.e., strong electron localization, is required. These properties are accomplished by high-entropy alloys, and the CrMnFeCoNi alloy serves as prime candidate. We report on single-pulse ablation efficiency experiments of the CrMnFeCoNi alloy which are support by our proposed model
Electron correlations in CoMnFeSi Heusler compounds
This study presents the effect of local electronic correlations on the
Heusler compounds CoMnFeSi as a function of the concentration
. The analysis has been performed by means of first-principles
band-structure calculations based on the local approximation to spin-density
functional theory (LSDA). Correlation effects are treated in terms of the
Dynamical Mean-Field Theory (DMFT) and the LSDA+U approach. The formalism is
implemented within the Korringa-Kohn-Rostoker (KKR) Green's function method.
In good agreement with the available experimental data the magnetic and
spectroscopic properties of the compound are explained in terms of strong
electronic correlations. In addition the correlation effects have been analysed
separately with respect to their static or dynamical origin. To achieve a
quantitative description of the electronic structure of
CoMnFeSi both static and dynamic correlations must be treated
on equal footing.Comment: 12 pages, 5 figure
Surface spin polarization of the non-stoichiometric Heusler compound Co2Mn(alpha)Si
Using a combined approach of spin-resolved photoemission spectroscopy, band
structure and photoemission calculations we investigate the influence of bulk
defects and surface states on the spin polarization of Co2Mn(alpha)Si thin
films with bulk L21 order. We find that for Mn-poor alloys the spin
polarization at EF is negative due to the presence of Co_Mn antisite and
minority surface state contributions. In Mn-rich alloys, the suppression of
Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and
the influence of minority surface states on the photoelectron spin polarization
is reduced
Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric -GeTe(111)
Rashba materials have appeared as an ideal playground for spin-to-charge
conversion in prototype spintronics devices. Among them, -GeTe(111) is
a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong
spin-orbit interaction gives rise to giant Rashba coupling. Its room
temperature ferroelectricity was recently demonstrated as a route towards a new
type of highly energy-efficient non-volatile memory device based on switchable
polarization. Currently based on the application of an electric field, the
writing and reading processes could be outperformed by the use of femtosecond
(fs) light pulses requiring exploration of the possible control of
ferroelectricity on this timescale. Here, we probe the room temperature
transient dynamics of the electronic band structure of -GeTe(111) using
time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments
reveal an ultrafast modulation of the Rashba coupling mediated on the fs
timescale by a surface photovoltage (SPV), namely an increase corresponding to
a 13 % enhancement of the lattice distortion. This opens the route for the
control of the FE polarization in -GeTe(111) and FE semiconducting
materials in quantum heterostructures.Comment: 31 pages, 12 figure
Collective topological spin dynamics in a correlated spin glass
The interplay between spin-orbit interaction (SOI) and magnetic order is
currently one of the most active research fields in condensed matter physics
and leading the search for materials with novel and tunable magnetic and spin
properties. Here we report on a variety of unexpected and unique observations
in thin multiferroic \GeMnTe films. The ferrimagnetic order in this
ferroelectric semiconductor is found to reverse with current pulses six orders
of magnitude lower as for typical spin-orbit torque systems. Upon a switching
event, the magnetic order spreads coherently and collectively over macroscopic
distances through a correlated spin-glass state. Lastly, we present a novel
methodology to controllably harness this stochastic magnetization dynamics,
allowing us to detect spatiotemporal nucleation of topological spin textures we
term ``skyrmiverres''.Comment: 26 pages, 10 figures, 2 table
Adoption of Instant Messaging Technologies by University Students
The main objective of this paper is to better understand the nature and patterns of students’ socialization patterns in relation to the adoption of Instant Messaging (IM) systems. A model based on the Extended Planned Behavior Theory (EPBT) was applied to a sample of 80 students of software engineering at the University of New South Wales, Australia. Based on the EPBT model, a questionnaire was administered to these students. A number of key concepts were identified in relation to the students’ adoption of IM. It was also found that students use IM to support a number of task-related purposes such as collaborating with their classmates about group work and assignments, as well as for scheduling and coordinating meetings and significant results were obtained
- …