arXiv subjects

Sangeeta Sharma

Publications and source records attributed to Sangeeta Sharma.

34 records · Page 2Linked to original sources

arXiv2023

Verification of ultrafast spin transfer effects in FeNi alloys

The optical intersite spin transfer (OISTR) effect was recently verified in Fe$_{50}$Ni$_{50}$ using magneto-optical Kerr measurements in the extreme ultraviolet range. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe$_{50}$Ni$_{50}$, Fe$_{19}$Ni$_{81}$ and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

cond-mat.mtrl-sci↗

arXiv2023

Unraveling Femtosecond Spin and Charge Dynamics with EUV T-MOKE Spectroscopy

The magneto-optical Kerr effect (MOKE) in the extreme ultraviolet (EUV) regime has helped to elucidate some of the key processes that lead to the manipulation of magnetism on ultrafast timescales. However, as we show in this paper, the recently introduced spectrally-resolved analysis of such data can lead to surprising experimental observations, which might cause misinterpretations. Therefore, an extended analysis of the EUV magneto-optics is necessary. Via experimental determination of the dielectric tensor, we find here that the non-equilibrium excitation in an ultrafast magnetization experiment can cause a rotation of the off-diagonal element of the dielectric tensor in the complex plane. In direct consequence, the commonly analyzed magneto-optic asymmetry may show time-dependent behaviour that is not directly connected to the magnetic properties of the sample. We showcase such critical observations for the case of ultrafast magnetization dynamics in Ni, and give guidelines for the future analysis of spectrally-resolved magneto-optical data and its comparison with theory.

cond-mat.mtrl-sci↗

arXiv2023

Partially deorbitalized meta-GGA

Mejia-Rodriguez and Trickey recently proposed a procedure for removing the explicit dependence of meta-GGA exchange-correlation energy functionals $E_{\rm xc}$ on the kinetic energy density $τ$. We present a simple modification to this approach in which the exact Kohn-Sham $τ$ is used as input for $E_{\rm xc}$ but the functional derivative of $τ$ with respect to the density $ρ$, required to calculate the potential term $\int d^3r'\,δE_{\rm xc}/δτ({\bf r}')|_ρ\cdot δτ({\bf r}')/δρ({\bf r})$, is evaluated using an approximate kinetic energy density functional. This ensures that the Kohn-Sham potential is a local multiplicative function as opposed to the non-local potential of a generalized Kohn-Sham approach. Electronic structure codes can be easily modified to use the new method. We validate it by quantifying the accuracy of the predicted lattice parameters, bulk moduli, magnetic moments and cohesive energies of a large set of periodic solids. An unanticipated benefit of this method is to gauge the quality of approximate kinetic energy functionals by checking if the self-consistent solution is indeed at the variational minimum.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

The ultrafast and element-specific response of magnetic systems containing ferromagnetic 3d transition metals and 4d/5d heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic 3d and induced 4d/5d magnetic moments upon femtosecond laser excitation exist. In this work, we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows to accurately disentangle the element-specific response in spite of overlapping Co and Fe M$_{2,3}$ as well as Pt O$_{2,3}$ and N$_7$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic phase diagram of the austenitic Mn-rich Ni-Mn-(In,Sn) Heusler alloys

Heusler compounds have been intensively studied owing to the important technological advancements that they provide in the field of shape memory, thermomagnetic energy conversion and spintronics. Many of their intriguing properties are ultimately governed by their magnetic states and understanding and possibly tuning them is evidently of utmost importance. In this work we examine the \alloys alloys with Density Functional Theory simulations and $^{55}$Mn Nuclear Magnetic Resonance and combine these two methods to carefully describe their ground state magnetic order. In addition, we compare the results obtained with the conventional generalized gradient approximation with the ones of strongly constrained and appropriately normed (SCAN) semilocal functionals for exchange and correlation. Experimental results eventually allow to discriminate between two different scenarios identified by ab initio simulations.

cond-mat.mtrl-sci↗

arXiv2022

Nanomechanical spectroscopy of 2D materials

We introduce a nanomechanical platform for fast and sensitive measurements of the spectrally-resolved optical dielectric function of 2D materials. At the heart of our approach is a suspended 2D material integrated into a nanomechanical resonator illuminated by a wavelength-tunable laser source. From the heating-related frequency shift of the resonator as well as its optical reflection measured as a function of photon energy, we obtain the real and imaginary parts of the dielectric function. Our measurements are unaffected by substrate-related screening and do not require any assumptions on the underling optical constants. This fast ($τ_{rise}$ $\sim$ 135 ns), sensitive (noise-equivalent power = 90 $\frac{pW}{\sqrt{Hz}}$ ), and broadband (1.2 $-$ 3.1 eV, extendable to UV-THz) method provides an attractive alternative to spectroscopic or ellipsometric characterisation techniques.

cond-mat.mes-hall↗

arXiv2022

The Giant Spin Hall Effect at Optical Frequencies

We generalize the spin Hall angle to laser pulses of finite frequencies in the linear response regime and predict a giant optical spin Hall effect. Namely, for certain transition metal elements, at particular frequencies, the spin current can be a significant fraction of the charge current, and even exceed it for XUV frequencies. By maximizing spin current while minimizing the charge current, we thus minimize a major source of heating in spintronic devices. We employ {\it ab-initio} time-dependent density functional theory (TDDFT), and with real-time simulations calculate the conductivity and transverse spin conductivity for all $3$d, $4$d, and $5$d transition metals for frequencies up to $50$ eV. In the XUV frequency range we find values greater than $1$ for the spin Hall angle, indicating spin currents larger than the charge current can be generated.

cond-mat.mes-hall↗

arXiv2021

Uncovering the role of the density of states in controlling ultrafast spin dynamics

At the ultrafast limit of optical spin manipulation is the theoretically predicted phenomena of optical intersite spin transfer (OISTR), in which laser induced charge transfer between the sites of a multi-component material leads to control over magnetic order. A key prediction of this theory is that the demagnetization efficiency is determined by the availability of unoccupied states for intersite charge transfer. Employing state-of-the-art magneto-optical Kerr effect measurements with femtosecond time resolution, we probe this prediction via a systematic comparison of the ultrafast magnetic response between the 3d ferromagnets, Fe, Co, and Ni, and their respective Pt-based alloys and multilayers. We find that (i) the demagnetization efficiency in the elemental magnets increases monotonically from Fe, via Co to Ni and, (ii), that the gain in demagnetization efficiency of the multi-component system over the pure element counterpart scales with the number of empty 3d minority states, exactly as predicted by the OISTR effect. We support these experimental findings with ab initio time-dependent density functional theory calculations that we show to capture the experimental trends very well.

cond-mat.mtrl-sci↗

arXiv2021

Spin/valley coupled dynamics of electrons and holes at the ${\text{MoS}_{2}-\text{MoSe}_{2}}$ interface

The coupled spin and valley degrees of freedom in transition metal dichalcogenides (TMDs) are considered a promising platform for information processing. Here, we use a TMD heterostructure ${\text{MoS}_{2}-\text{MoSe}_{2}}$ to study optical pumping of spin/valley polarized carriers across the interface and to elucidate the mechanisms governing their subsequent relaxation. By applying time-resolved Kerr and reflectivity spectroscopies, we find that the photoexcited carriers conserve their spin for both tunneling directions across the interface. Following this, we measure dramatically different spin/valley depolarization rates for electrons and holes, $\sim 30\,{\text{ns}}^{-1}$ and $< 1\,{\text{ns}}^{-1}$, respectively and show that this difference relates to the disparity in the spin-orbit splitting in conduction and valence bands of TMDs. Our work provides insights into the spin/valley dynamics of free carriers unaffected by complex excitonic processes and establishes TMD heterostructures as generators of spin currents in spin/valleytronic devices.

cond-mat.mes-hall↗

arXiv2020

Where does the spin angular momentum go in laser induced demagnetisation?

The dynamics of ultrafast demagnetisation in 3$d$ magnets is complicated by the presence of both spin ${\v S}$ and orbital ${\v L}$ angular momentum, with the microscopic mechanism by which the magnetic moment is redistributed to the lattice, and at what time scales, yet to be resolved. Employing state-of-the-art time dependent density function theory we disentangle the dynamics of these two momenta. Utilising ultra short (5~fs) pulses that separate spin-orbit (SO) and direct optical excitation time scales, we demonstrate a two-step microscopic mechanism: (i) an initial loss of ${\v L}$ due to laser excitation, followed post pulse by (ii) an increase of ${\v L}$ as ${\v S}$ transfers to ${\v L}$ during subsequent ($> 15$~fs) SO induced spin-flip demagnetisation. We also show that to see an unambiguous transfer of ${\v S}$ to ${\v L}$ a short pulse is required.

cond-mat.mtrl-sci↗

arXiv2020

Extending solid-state calculations to ultra long-range length scales

We present a method which enables solid-state density functional theory calculations to be applied to systems of almost unlimited size. Computations of physical effects up to the micron length scale but which nevertheless depend on the microscopic details of the electronic structure, are made possible. Our approach is based on a generalization of the Bloch state which involves an additional sum over a finer grid in reciprocal space around each ${\bf k}$-point. We show that this allows for modulations in the density and magnetization of arbitrary length on top of a lattice-periodic solution. Based on this, we derive a set of ultra long-range Kohn-Sham equations. We demonstrate our method with a sample calculation of bulk LiF subjected to an arbitrary external potential containing nearly 3500 atoms. We also confirm the accuracy of the method by comparing the spin density wave state of bcc Cr against a direct supercell calculation starting from a random magnetization density. Furthermore, the spin spiral state of $γ$-Fe is correctly reproduced and the screening by the density of a saw-tooth potential over 20 unit cells of silicon is verified.

cond-mat.mtrl-sci↗

arXiv2020

Distinct Ultrafast Electronic and Magnetic Response in M-edge Magnetic Circular Dichroism

Experimental investigations of ultrafast magnetization dynamics increasingly employ resonant magnetic spectroscopy in the ultraviolet spectral range. Besides allowing to disentangle the element-specific transient response of functional magnetic systems, these techniques also promise to access attosecond to few-femtosecond dynamics of spin excitations. Here, we report on a systematic study of transient magnetic circular dichroism (MCD) on the transition metals Fe, Co and Ni as well as on a FeNi and GdFe alloy and reveal a delayed onset between the electronic and magnetic response. Supported by \textit{ab-initio} calculations, we attribute our observation to a transient energy shift of the absorption and MCD spectra at the corresponding elemental resonances due to non-equilibrium changes of electron occupations.

physics.optics↗

arXiv2019

Perfect and controllable nesting in the small angle twist bilayer graphene

Parallel ("nested") regions of a Fermi surface (FS) drive instabilities of the electron fluid, for example the spin density wave in elemental chromium. In one-dimensional materials, the FS is trivially fully nested (a single nesting vector connects two "Fermi dots"), while in higher dimensions only a fraction of the FS consists of parallel sheets. We demonstrate that the tiny angle regime of twist bilayer graphene (TBLG) possess a phase, accessible by interlayer bias, in which the FS consists entirely of nestable "Fermi lines": the first example of a completely nested FS in a 2d material. This nested phase is found both in the ideal as well as relaxed structure of the twist bilayer. We demonstrate excellent agreement with recent STM images of topological states in this material and elucidate the connection between these and the underlying Fermiology. We show that the geometry of the "Fermi lines" network is controllable by the strength of the applied interlayer bias, and thus that TBLG offers unprecedented access to the physics of FS nesting in 2d materials.

cond-mat.mtrl-sci↗

arXiv2018

Petahertz Spintronics

The enigmatic coupling between electronic and magnetic phenomena was one of the riddles propelling the development of modern electromagnetism. Today, the fully controlled electric field evolution of ultrashort laser pulses permits the direct and ultrafast control of electronic properties of matter and is the cornerstone of light-wave electronics. In sharp contrast, because there is no first order interaction between light and spins, the magnetic properties of matter can only be affected indirectly on the much slower tens-of-femtosecond timescale in a sequence of optical excitation followed by the rearrangement of the spin structure. Here we record an orders of magnitude faster magnetic switching with sub-femtosecond response time by initiating optical excitations with near-single-cycle laser pulses in a ferromagnetic layer stack. The unfolding dynamics are tracked in real-time by a novel attosecond time-resolved magnetic circular dichroism (atto-MCD) detection scheme revealing optically induced spin and orbital momentum transfer (OISTR) in synchrony with light field driven charge relocation. In tandem with ab-initio quantum dynamical modelling, we show how this mechanism provides simultaneous control over electronic and magnetic properties that are at the heart of spintronic functionality. This first incarnation of attomagnetism observes light field coherent control of spin-dynamics in the initial non-dissipative temporal regime and paves the way towards coherent spintronic applications with Petahertz clock rates.

physics.app-ph↗

arXiv2018

Coupled Kohn-Sham equations for electrons and phonons

This work establishes the algebraic structure of the Kohn-Sham equations to be solved in a density formulation of electron and phonon dynamics, including the superconducting order parameter. A Bogoliubov transform is required to diagonalize both the fermionic and bosonic Kohn-Sham Hamiltonians since they both represent a non-interacting quantum field theory. The Bogoliubov transform for phonons is non-Hermitian in the general case, and the corresponding time-evolution is non-unitary. Several sufficient conditions for ensuring that the bosonic eigenvalues are real are provided and a practical method for solving the system is described. Finally, we produce a set of approximate mean-field potentials which are functionals of the electronic and phononic density matrices and depend on the electron-phonon vertex.

cond-mat.supr-con↗

arXiv2016

Anomalous Dirac point transport due to extended defects in bilayer graphene

Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the conductivity distribution $P(σ)$ is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.

cond-mat.mtrl-sci↗