arXiv subjects

Stefano Sanvito

Publications and source records attributed to Stefano Sanvito.

At least 73 records · Page 4Linked to original sources

arXiv2018

Non-trivial spatial dependence of the spin torques in L1$_0$ FePt-based tunnelling junctions

We present an {\it ab-initio} study of the spin-transfer torque in a Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider a FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultra-thin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and L$1_0$-FePt, the torque is sharply attenuated and it is transferred to FePt only for a Fe seed layer that is less than two-atomic-planes thick. We attribute these features to the different spatial profiles of the exchange and correlation field and the induced non-equilibrium spin accumulation. The calculated tunnelling magneto-resistance of the Fe/MgO/FePt/Fe junctions studied is enhanced with respect to the one of Fe/MgO/Fe, while it is reduced with Fe intercalation. Our work shows that L$1_0$-FePt junctions can be promising candidates for current-operated magnetic devices and that the magnetic texture at the atomic scale has an important effect on the spin transfer torque.

cond-mat.mtrl-sci↗

arXiv2018

Electron trapping by neutral pristine ferroelectric domain walls in BiFeO$_3$

First-principles calculations for pristine neutral ferroelectric domain walls in BiFeO$_3$ reveal that excess electrons are selectively trapped by the domain walls, while holes are only weakly attracted. Such trapped excess electrons may be responsible for the thermally activated electrical conductivity at domain walls observed in experiments. In the case of a periodic array of domain walls, the trapped excess electrons create a zigzag potential, whose amplitude depends on the electron concentration in the material and the domain-wall distance. The potential is asymmetric for 71° and 109° domain walls. This could modify the open-circuit voltage in a solar cell and hence influence the photoelectric effect in BiFeO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Spin-Phonon coupling parameters from maximally localized Wannier functions and first principles electronic structure: the case of durene single crystal

Spin-orbit interaction is an important vehicle for spin relaxation. At finite temperature lattice vibrations modulate the spin-orbit interaction and thus generate a mechanism for spin-phonon coupling, which needs to be incorporated in any quantitative analysis of spin transport. Starting from a density functional theory \textit{ab initio} electronic structure, we calculate spin-phonon matrix elements over the basis of maximally localized Wannier functions. Such coupling terms form an effective Hamiltonian to be used to extract thermodynamic quantities, within a multiscale approach particularly suitable for organic crystals. The symmetry of the various matrix elements are analyzed by using the $Γ$-point phonon modes of a one-dimensional chain of Pb atoms. Then the method is employed to extract the spin-phonon coupling of solid durene, a high-mobility crystal organic semiconducting. Owing to the small masses of carbon and hydrogen spin-orbit is weak in durene and so is the spin-phonon coupling. Most importantly we demonstrate that the largest contribution to the spin-phonon interaction originates from Holstein-like phonons, namely from internal molecular vibrations.

cond-mat.mtrl-sci↗

arXiv2018

AFLOW-CHULL: Cloud-oriented platform for autonomous phase stability analysis

$\textit{A priori}$ prediction of phase stability of materials is a challenging practice, requiring knowledge of all energetically-competing structures at formation conditions. Large materials repositories $\unicode{x2014}$ housing properties of both experimental and hypothetical compounds $\unicode{x2014}$ offer a path to prediction through the construction of informatics-based, $\textit{ab-initio}$ phase diagrams. However, limited access to relevant data and software infrastructure has rendered thermodynamic characterizations largely peripheral, despite their continued success in dictating synthesizability. Herein, a new module is presented for autonomous thermodynamic stability analysis implemented within the open-source, $\textit{ab-initio}$ framework AFLOW. Powered by the AFLUX Search-API, AFLOW-CHULL leverages data of more than 1.8 million compounds currently characterized in the AFLOW.org repository and can be employed locally from any UNIX-like computer. The module integrates a range of functionality: the identification of stable phases and equivalent structures, phase coexistence, measures for robust stability, and determination of decomposition reactions. As a proof-of-concept, thorough thermodynamic characterizations have been performed for more than 1,300 binary and ternary systems, enabling the identification of several candidate phases for synthesis based on their relative stability criterion $\unicode{x2014}$ including 18 promising $C15_{b}$-type structures and two half-Heuslers. In addition to a full report included herein, an interactive, online web application has been developed showcasing the results of the analysis, and is located at aflow.org/aflow-chull.

cond-mat.mtrl-sci↗

arXiv2018

Search for alternative magnetic tunnel junctions based on all-Heusler stacks

By imposing the constraints of structural compatibility, stability and a large tunneling magneto-resistance, we have identified the Fe$_3$Al/BiF$_3$/Fe$_3$Al stack as a possible alternative to the well-established FeCoB/MgO/FeCoB in the search for a novel materials platform for high-performance magnetic tunnel junctions. Various geometries of the Fe$_3$Al/BiF$_3$/Fe$_3$Al structure have been analyzed, demonstrating that a barrier of less than 2~nm yields a tunneling magneto-resistance in excess of 25,000~\% at low bias, without the need for the electrodes to be half-metallic. Importantly, the presence of a significant spin gap in Fe$_3$Al for states with $Δ_1$ symmetry along the stack direction makes the TMR very resilient to high voltages.

cond-mat.mtrl-sci↗

arXiv2018

Wannier-function-based constrained DFT with nonorthogonality-correcting Pulay forces in application to the reorganization effects in graphene-adsorbed pentacene

Pulay terms arise in the Hellman-Feynman forces in electronic structure calculations when one employs a basis set made of localized orbitals that move with their host atoms. If the total energy of the system depends on a subspace population defined in terms of the localized orbitals across multiple atoms, then unconventional Pulay terms will emerge due to the variation of the orbital nonorthogonality with ionic translation. Here, we derive the required exact expressions for such terms, which cannot be eliminated by orbital orthonormalization. We have implemented these corrected ionic forces within the linear-scaling density functional theory (DFT) package ONETEP, and have used constrained DFT to calculate the reorganization energy of a pentacene molecule adsorbed on a graphene flake. The calculations are performed by including ensemble DFT, corrections for periodic boundary conditions, and empirical Van der Waals interactions. For this system we find that tensorially invariant population analysis yields an adsorbate subspace population that is very close to integer-valued when based upon nonorthogonal Wannier functions, and also but less precisely when using pseudoatomic functions. Thus, orbitals can provide a very effective population analysis for constrained DFT. Our calculations show that the reorganization energy of the adsorbed pentacene is typically lower than that of pentacene in the gas phase. We attribute this effect to steric hindrance.

cond-mat.mtrl-sci↗

arXiv2018

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical, optical, spin and valley related properties. Here, we report on spin polarized tunneling through chemical vapor deposited (CVD) multilayer MoS2 (~7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5 - 2 % has been observed, corresponding to spin polarization of 5 - 10 % in the measured temperature range of 300 - 75 K. First principles calculations for ideal junctions results in a tunnel magnetoresistance up to 8 %, and a spin polarization of 26 %. The detailed measurements at different temperatures and bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomenon that control their performance.

cond-mat.mtrl-sci↗

arXiv2018

XDFT: an efficient first-principles method for neutral excitations in molecules

State-of-the-art methods for calculating neutral excitation energies are typically demanding and limited to single electron-hole pairs and their composite plasmons. Here we introduce excitonic density-functional theory (XDFT) a computationally light, generally applicable, first-principles technique for calculating neutral excitations based on generalized constrained DFT. In order to simulate an M-particle excited state of an N-electron system, XDFT automatically optimizes a constraining potential to confine N-M electrons within the ground-state Kohn-Sham valence subspace. We demonstrate the efficacy of XDFT by calculating the lowest single-particle singlet and triplet excitation energies of the well-known Thiel molecular test set, with results which are in excellent agreement with time-dependent DFT. Furthermore, going beyond the capability of adiabatic time-dependent DFT, we show that XDFT can successfully capture double excitations. Overall our method makes optical gaps, excition bindings and oscillator strengths readily accessible at a computational cost comparable to that of standard DFT. As such, XDFT appears as an ideal candidate to work within high-throughput discovery frameworks and within linear-scaling methods for large systems.

physics.chem-ph↗

arXiv2018

A simple descriptor for energetics at fcc-bcc metal interfaces

We have developed a new and user-friendly interface energy calculation method that avoids problems deriving from numerical differences between bulk and slab calculations, such as the number of k points along the direction perpendicular to the interface. We have applied this to 36 bcc-fcc metal interfaces in the (100) orientation and found a clear dependence of the interface energy on the difference between the work functions of the two metals, on the one hand, and the total number of d electrons on the other. Greater mechanical deformations were observed in fcc crystals than in their bcc counterparts. For each bcc metal, the interface energy was found to follow the position of its d band, whereas the same was not observed for fcc.

cond-mat.mtrl-sci↗

arXiv2018

Strain-induced Weyl and Dirac states and direct-indirect gap transitions in group-V materials

We perform comprehensive density-functional theory calculations on strained two-dimensional phosphorus (P), arsenic (As) and antimony (Sb) in the monolayer, bilayer, and bulk $α$-phase, from which we compute the key mechanical and electronic properties of these materials. Specifically, we compute their electronic band structures, band gaps, and charge-carrier effective masses, and identify the qualitative electronic and structural transitions that may occur. Moreover, we compute the elastic properties such as the Young's modulus $Y$; shear modulus $G$; bulk modulus $\mathcal{B}$; and Poisson ratio $ν$ and present their isotropic averages of as well as their dependence on the in-plane orientation, for which the relevant expressions are derived. We predict strain-induced Dirac states in the monolayers of As and Sb and the bilayers of P, As, and Sb, as well as the possible existence of Weyl states in the bulk phases of P and As. These phases are predicted to support charge velocities up to $10^6$~$\textrm{ms}^{-1}$ and, in some highly anisotropic cases, permit one-dimensional ballistic conductivity in the puckered direction. We also predict numerous band gap transitions for moderate in-plane stresses. Our results contribute to the mounting evidence for the utility of these materials, made possible by their broad range in tuneable properties, and facilitate the directed exploration of their potential application in next-generation electronics.

cond-mat.mtrl-sci↗

arXiv2017

Proposal for a dual spin filter based on [VO(C$_3$S$_4$O)$_2$]$^{2-}$

Polynuclear magnetic molecules often present dense transmission spectra with many overlapping conduction spin channels. Single-metal complexes display a sparser density of states, which in the presence of a fixed external magnetic field makes them interesting candidates for spin filtering. Here we perform a DFT study of a family of bis- and tris-dithiolate vanadium complexes sandwiched between Au(111) electrodes and demonstrate that [VO(C$_3$S$_4$O)$_2$]$^{2-}$ can behave as a dual spin filter. This means that an external stimulus can switch between the selective transmission of spin-up and spin-down carriers. By using an electrostatic gate as external stimulus we show that the onset for the spin-up conductance is at a voltage V$_g$ = -0.51 V but a small shift to V$_g$ = -0.63 V is capable of activating spin-down transport. For both cases, we estimate a large low-bias conductance (approx. 2 μS at Vbias < 50 mV) with excellent spin selectivity (> 99.5%). We conclude by commenting on the general molecular requirements for the chemical design of further such spintronics components.

cond-mat.mes-hall↗

arXiv2017

The AFLOW Fleet for Materials Discovery

The traditional paradigm for materials discovery has been recently expanded to incorporate substantial data driven research. With the intent to accelerate the development and the deployment of new technologies, the AFLOW Fleet for computational materials design automates high-throughput first principles calculations, and provides tools for data verification and dissemination for a broad community of users. AFLOW incorporates different computational modules to robustly determine thermodynamic stability, electronic band structures, vibrational dispersions, thermo-mechanical properties and more. The AFLOW data repository is publicly accessible online at aflow.org, with more than 1.7 million materials entries and a panoply of queryable computed properties. Tools to programmatically search and process the data, as well as to perform online machine learning predictions, are also available.

cond-mat.mtrl-sci↗

arXiv2017

Multi-scale modelling of current-induced switching in magnetic tunnel junctions using ab initio spin transfer torques

There exists a significant challenge in developing efficient magnetic tunnel junctions with low write currents for non-volatile memory devices. With the aim of analysing potential materials for efficient current-operated magnetic junctions we have developed a multi-scale methodology combining the ab initio calculations of spin-transfer torque with large-scale time-dependent simulations using atomistic spin dynamics. In this work we introduce our multi-scale approach including a discussion on a number of possible mapping schemes the ab initio spin torques into the spin dynamics. We demonstrate this methodology on a prototype Co/MgO/Co/Cu tunnel junction showing that the spin torques are primarily acting at the interface between the Co free layer and MgO. Using spin dynamics we then calculate the reversal switching times for the free layer and the critical voltages and currents required for such switching. Our work provides an efficient, accurate and versatile framework for designing novel current-operated magnetic devices, where all the materials details are take into account.

cond-mat.mes-hall↗

arXiv2017

Evolution of Weyl orbit and quantum Hall effect in Dirac semimetal Cd3As2

Owing to the coupling between open Fermi arcs on opposite surfaces, topological Dirac semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. Here, by lowering the carrier density in Cd3As2 nanoplates, we observe a crossover from multiple- to single-frequency Shubnikov-de Haas (SdH) oscillations when subjected to out-of-plane magnetic field, indicating the dominant role of surface transport. With the increase of magnetic field, the SdH oscillations further develop into quantum Hall state with non-vanishing longitudinal resistance. By tracking the oscillation frequency and Hall plateau, we observe a Zeeman-related splitting and extract the Landau level index as well as sub-band number. Different from conventional two-dimensional systems, this unique quantum Hall effect may be related to the quantized version of Weyl orbits. Our results call for further investigations into the exotic quantum Hall states in the low-dimensional structure of topological semimetals.

cond-mat.mtrl-sci↗

arXiv2017

Designing magnetism in Fe-based Heusler alloys: a machine learning approach

Combining material informatics and high-throughput electronic structure calculations offers the possibility of a rapid characterization of complex magnetic materials. Here we demonstrate that datasets of electronic properties calculated at the ab initio level can be effectively used to identify and understand physical trends in magnetic materials, thus opening new avenues for accelerated materials discovery. Following a data-centric approach, we utilize a database of Heusler alloys calculated at the density functional theory level to identify the ideal ions neighbouring Fe in the $X_2$Fe$Z$ Heusler prototype. The hybridization of Fe with the nearest neighbour $X$ ion is found to cause redistribution of the on-site Fe charge and a net increase of its magnetic moment proportional to the valence of $X$. Thus, late transition metals are ideal Fe neighbours for producing high-moment Fe-based Heusler magnets. At the same time a thermodynamic stability analysis is found to restrict $Z$ to main group elements. Machine learning regressors, trained to predict magnetic moment and volume of Heusler alloys, are used to determine the magnetization for all materials belonging to the proposed prototype. We find that Co$_2$Fe$Z$ alloys, and in particular Co$_2$FeSi, maximize the magnetization, which reaches values up to 1.2T. This is in good agreement with both ab initio and experimental data. Furthermore, we identify the Cu$_2$Fe$Z$ family to be a cost-effective materials class, offering a magnetization of approximately 0.65T.

cond-mat.mtrl-sci↗

arXiv2017

Theoretical evaluation of [V$^{IV}$(α-C$_3$S$_5$)]${^2-}$ as nuclear-spin sensitive single-molecule spin transistor

In a straightforward application of molecular nanospintronics to quantum computing, single-molecule spin transistors can be used to measure and control nuclear spin qubits. A jump in the conductance occurs when the electronic spin inverts its polarization, and this happens at a so-called anticrossing between energy levels, which in turn only takes place at a specific magnetic field determined by the nuclear spin state. So far, this procedure has only been implemented for the terbium(III) bis(phthalocyaninato) complex. Here we explore theoretically whether a similar behavior is expected for a highly stable molecular spin qubit, the vanadium tris-dithiolate complex [V$^{IV}$(α-C$_3$S$_5$)]${^2-}$. We consider such molecule sandwiched into a two-terminal device and determine the spin-dependent conductance. We verify that the transport channel at minimal bias voltage does not overlap with the occupied spin orbitals, indicating that the spin states may survive in the conduction regime. We estimate some physical parameters to guide the experiments, and verify the robustness of the theoretical methodology by applying it to two chemically related vanadium complexes.

cond-mat.mes-hall↗

arXiv2017

Spin-orbit Hamiltonian for organic crystals from first principles electronic structure and Wannier functions

Spin-orbit coupling in organic crystals is responsible for many spin-relaxation phenomena, going from spin diffusion to intersystem crossing. With the goal of constructing effective spin-orbit Hamiltonians to be used in multiscale approaches to the thermodynamical properties of organic crystals, we present a method that combines density functional theory with the construction of Wannier functions. In particular we show that the spin-orbit Hamiltonian constructed over maximally localised Wannier functions can be computed by direct evaluation of the spin-orbit matrix elements over the Wannier functions constructed in absence of spin-orbit interaction. This eliminates the prob- lem of computing the Wannier functions for almost degenerate bands, a problem always present with the spin-orbit-split bands of organic crystals. Examples of the method are presented for isolated molecules, for mono-dimensional chains of Pb and C atoms and for triarylamine-based one-dimansional single crystals.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio dynamical exchange interactions in frustrated anti-ferromagnets

The ultrafast response to an optical pulse excitation of the spin-spin exchange interaction in transition metal anti-ferromagnets is studied within the framework of the time-dependent spin-density functional theory. We propose a formulation for the purely dynamical exchange interaction, which is non-local in space, and it is derived starting from {\it ab initio} arguments. Then, we investigate the effect of the laser pulse on the onset of the dynamical process. It is found that we can distinguish two types of excitations, both activated immediately after the action of the laser pulse. While the first one can be associated to a Stoner-like excitation and involves the transfer of spin from one site to another, the second one is related to the ultrafast modification of a Heisenberg-like exchange interaction and can trigger the formation of spin waves in the first few hundred femtoseconds of the time evolution.

cond-mat.mes-hall↗