Materials Science (cond-mat.mtrl-sci)

Abstract: The Z method is a popular atomistic simulation method for determining the melting temperature of solids by using a sequence of molecular dynamics(MD) runs in the microcanonical(NVE) ensemble to target the lowest system energy where the solid always melts. Homogeneous melting at the superheating critical limit($T_h$), is accompanied by a temperature drop to the equilibrium melting temperature($T_m$). Implementation of the Z method interfaced with modern {\it ab initio} electronic structure packages use Hellman-Faynman dynamics to propagate the ions in the NVE ensemble with the Mermin free energy plus the ionic kinetic energy conserved. So the electronic temperature($T_{el}$) is kept fixed along the trajectory which may introduce some spurious ion-electron interactions in MD runs with large temperature changes. We estimate possible systematic errors in evaluating melting temperature with different choices of $T_{el}$. MD runs with the $T_{el}$ = $T_h$ and $T_{el}$ = $T_m$ shows that the difference in melting temperature can be 200-300 K (3-5\% of the melting temperature) for our two test systems. Our results are in good agreement with previous studies with different methods, suggesting the CaSiO$_3$ and SiO$_2$ melts at around 6500 at 100 GPa and 6000 K at 160 GPa. The melting temperature decreases with increasing $T_{el}$ due to the increasing entropic stabilisation of the liquid and the system melts about 3 times faster with $T_{el} = T_h$ than with $T_{el} = T_m$. A careful choice of $T_{el}$ in BOMD is essential for the critical evaluation of the Z method especially at very high temperatures. Inspection of the homogeneous melting process shows that melting occurs via a two-step mechanism: 1) melting of the anion sublattice is accompanied by a small drop in temperature and 2) the formation of small defects which trigger the formation of small liquid clusters and fully melted.

Abstract: Electronic structure calculations based on Density Functional Theory have successfully predicted numerous ground state properties of a variety of molecules and materials. However, exchange and correlation functionals currently used in the literature, including semi-local and hybrid functionals, are often inaccurate to describe the electronic properties of heterogeneous solids, especially systems composed of building blocks with large dielectric mismatch. Here, we present a dielectric-dependent range-separated hybrid functional, SE-RSH, for the investigation of heterogeneous materials. We define a spatially dependent fraction of exact exchange inspired by the static Coulomb-hole and screened-exchange (COHSEX) approximation used in many body perturbation theory, and we show that the proposed functional accurately predicts the electronic structure of several non-metallic interfaces, three- and two-dimensional, pristine and defective solids and nanoparticles.

Abstract: Electronic properties of spin polarized antiferromagnetic ACrO3 (A = La, Y) are explored with Hubbard Model using Density Functional Theory (DFT). These two isostructural systems are investigated using the different Hubbard energy and analyzed the hybridization of chromium 3d orbitals and oxygen 2p orbitals and the change in energy band gaps against the Hubbard energy. The bond length and bond angle affect significantly the orbital contributions of Cr-3d and O-2p electrons for both the system. We noticed that the Cr-O hybridization affects the orbital degeneracy and is substantiated with partial density of states. These results emphasize the contribution of Hubbard energy in correlated electron systems.

Abstract: Supramolecular chemistry protocols applied on surfaces offer compelling avenues for atomic scale control over organic-inorganic interface structures. In this approach, adsorbate-surface interactions and two-dimensional confinement can lead to morphologies and properties that differ dramatically from those achieved via conventional synthetic approaches. Here, we describe the bottom-up, on-surface synthesis of one-dimensional coordination nanostructures based on an iron (Fe)-terpyridine (tpy) interaction borrowed from functional metal-organic complexes used in photovoltaic and catalytic applications. Thermally activated diffusion of sequentially deposited ligands and metal atoms, and intra-ligand conformational changes, lead to Fe-tpy coordination and formation of these nanochains. Low-temperature Scanning Tunneling Microscopy and Density Functional Theory were used to elucidate the atomic-scale morphology of the system, providing evidence of a linear tri-Fe linkage between facing, coplanar tpy groups. Scanning Tunneling Spectroscopy reveals highest occupied orbitals with dominant contributions from states located at the Fe node, and ligand states that mostly contribute to the lowest unoccupied orbitals. This electronic structure yields potential for hosting photo-induced metal-to-ligand charge transfer in the visible/near-infrared. The formation of this unusual tpy/tri-Fe/tpy coordination motif has not been observed for wet chemistry synthesis methods, and is mediated by the bottom-up on-surface approach used here.

Abstract: Magnetic tunnel junctions (MTJs) with bcc(001)-type structures such as Fe(001)/MgO(001)/Fe(001), have been widely used as the core of various spintronic devices such as magnetoresistive memories; however, the limited material selection of (001)-type MTJs hinders the further development of spintronic devices. Here, as an alternative to the (001)-type MTJs, an fcc(111)-type MTJ using a fully epitaxial CoFe/rock-salt MgAlO (MAO)/CoFe is explored to introduce close-packed lattice systems into MTJs. Using an atomically flat Ru(0001) epitaxial buffer layer, fcc(111) epitaxial growth of the CoFe/MAO/CoFe trilayer is achieved. Sharp CoFe(111)/MAO(111) interfaces are confirmed due to the introduction of periodic dislocations by forming a 5:6 in-plane lattice matching structure. The fabricated (111) MTJ exhibits a tunnel magnetoresistance ratio of 37% at room temperature (47% at 10 K). Symmetric differential conductance curves with respect to bias polarity are observed, indicating the achievement of nearly identical upper and lower MAO interface qualities. Despite the charge-uncompensated (111) orientation for a rock-salt-like MAO barrier, the achievement of flat, stable, and spin-polarized barrier interfaces opens a promising avenue for expanding the design of MTJ structures.

Abstract: Nonlocal spin polarization phenomena are thoroughly investigated in the devices made of chiral metallic single crystals of CrNb$_3$S$_6$ and NbSi$_2$ as well as of polycrystalline NbSi$_2$. We demonstrate that simultaneous injection of charge currents in the opposite ends of the device with the nonlocal setup induces the switching behavior of spin polarization in a controllable manner. Such a nonlocal spin polarization appears regardless of the difference in the materials and device dimensions, implying that the current injection in the nonlocal configuration splits spin-dependent chemical potentials throughout the chiral crystal even though the current is injected into only a part of the crystal. We show that the proposed model of the spin dependent chemical potentials explains the experimental data successfully. The nonlocal double-injection device may offer significant potential to control the spin polarization to large areas because of the nature of long-range nonlocal spin polarization in chiral materials.

Abstract: Dynamic nucleation of dislocations caused by a stress front ('shock') of amplitude $\sigma_{\rm a}$ moving with speed $V$ is investigated by solving numerically the Dynamic Peierls Equation with an efficient method. Speed $V$ and amplitude $\sigma_{\rm a}$ are considered as independent variables, with $V$ possibly exceeding the longitudinal wavespeed $c_{\rm L}$. Various reactions between dislocations take place such as scattering, dislocation-pair nucleation, annihilation, and crossing. Pairs of edge dislocation are always nucleated with speed $v\gtrsim c_{\rm L}$ (and likewise for screws with $c_{\rm L}$ replaced by $c_{\rm S}$, the shear wavespeed). The plastic wave exhibits self-organization, forming distinct `bulk' and `front' zones. Nucleations occur either within the bulk or at the zone interface, depending on the value of $V$. The front zone accumulates dislocations that are expelled from the bulk or from the interface. In each zone, dislocation speeds and densities are measured as functions of simulation parameters. The densities exhibit a scaling behavior with stress, given by $((\sigma_a/\sigma_{\rm th})^2-1)^\beta$, where $\sigma_{\rm th}$ represents the nucleation threshold and $0<\beta<1$.

Abstract: Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn4N from the ${\epsilon}$-phase of the manganese nitride as an emergent rare-earth-free spintronic material due to its perpendicular magnetic anisotropy, small saturation magnetization, high thermal stability, and large domain wall velocity. We have achieved high-quality (001)-ordered Mn$_{4}$N thin film by sputtering Mn onto ${\eta}$-phase Mn$_{3}$N$_{2}$ seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn$_{3}$N$_{2}$/Mn layers leads to a continuous evolution of the layers to Mn$_{3}$N$_{2}$/Mn$_{2}$N/Mn$_{4}$N, Mn$_{2}$N/Mn$_{4}$N, and eventually Mn$_{4}$N alone. The ferrimagnetic Mn$_{4}$N indeed exhibits perpendicular magnetic anisotropy, and forms via a nucleation-and-growth mechanism. The nitrogen ion migration is also manifested in a significant exchange bias, up to 0.3 T at 5 K, due to the interactions between ferrimagnetic Mn$_{4}$N and antiferromagnetic Mn$_{3}$N$_{2}$ and Mn$_{2}$N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.

Abstract: Ni is the second most abundant element in the Earth's core. Yet, its effects on the inner core's structure and formation process are usually disregarded because of its similar atomic numbers with Fe. Using ab initio molecular dynamics simulations, we find that the bcc phase can spontaneously crystallize in liquid Ni at temperatures above Fe's melting point at inner core pressures. The melting temperature of Ni is shown to be 700-800 K higher than that of Fe at 323-360 GPa. Phase relations among hcp, bcc, and liquid differ between Fe and Ni. Ni can be a bcc stabilizer for Fe at high temperatures and inner core pressures. A small amount of Ni can accelerate Fe's crystallization under core pressure. These results suggest Ni may substantially impact the structure and formation process of the solid inner core.

Abstract: Infrared and Raman spectroscopies are ubiquitous techniques employed in many experimental laboratories, thanks to their fast and non-destructive nature able to capture materials' features as spectroscopic fingerprints. Nevertheless, these measurements frequently need theoretical support in order to unambiguously decipher and assign complex spectra. Linear-response theory provides an effective way to obtain the higher-order derivatives needed, but its applicability to modern exchange-correlation functionals remains limited. Here, we devise an automated, open-source, user-friendly approach based on ground-state density-functional theory and the electric enthalpy functional to allow seamless calculations of first-principles infrared and Raman spectra. By employing a finite-displacement and finite-field approach, we allow for the use of any functional, as well as an efficient treatment of large low-symmetry structures. Additionally, we propose a simple scheme for efficiently sampling the Brillouin zone with different electric fields. To demonstrate the capabilities of our approach, we provide illustrations using the ferroelectric LiNbO$_3$ crystal as a paradigmatic example. We predict infrared and Raman spectra using various (semi)local, Hubbard corrected, and hybrid functionals. Our results also show how PBE0 and extended Hubbard functionals yield in this case the best match in term of peak positions and intensities, respectively.

Abstract: Multiferroic materials have undergone extensive research in the past two decades in an effort to produce a sizable room-temperature magneto-electric (ME) effect in either exclusive or composite materials for use in a variety of electronic or spintronic devices. These studies have looked into the ME effect by switching the electric polarization by the magnetic field or switching the magnetism by the electric field. Here, an innovative way is developed to knot the functional properties based on the tremendous modulation of electronics and magnetization by the electric field of the topotactic phase transitions (TPT) in heterostructures composed of metallic-magnet/TPT-material. It is divulged that application of a nominal potential difference of 2-3 Volts induces gigantic changes in magnetization by 100-250% leading to colossal Voltomagnetic effect, which would be tremendously beneficial for low-power consumption applications in spintronics. Switching electronics and magnetism by inducing TPT through applying an electric field requires much less energy, making such TPT-based systems promising for energy-efficient memory and logic applications as well as opening a plethora of tremendous opportunities for applications in different domains.

Abstract: The prototypical delafossite metal PdCoO$_2$ has been the subject of intense interest for hosting exotic transport properties. Using first-principles transport calculations and theoretical modeling, we reveal that the high electrical conductivity of PdCoO$_2$ at room temperature originates from the contributions of both high Fermi velocities, enabled by Pd $4d_{z^2}-5s$ hybridization, and exceptionally weak electron-phonon coupling, which leads to a coupling strength ($\lambda=0.057$) that is nearly an order of magnitude smaller than those of common metals. The abnormally weak electron-phonon coupling in PdCoO$_2$ results from a low electronic density of states at the Fermi level, as well as the large and strongly facetted Fermi surface with suppressed Umklapp electron-phonon matrix elements. We anticipate that our work will inform the design of unconventional metals with superior transport properties.

Abstract: Spin-orbit coupling (SOC) in chiral materials can induce chirality-dependent spin splitting, enabling electrical manipulation of spin polarization. Here, we use first-principles calculations to investigate the electronic states of chiral one-dimensional (1D) InSeI, which has two enantiomorphic configurations with left- and right-handedness. We find that opposite spin states exist in the left- and right-handed 1D InSeI with significant spin splitting. Although the spin states at the conduction band minimum (CBM) and valence band maximum (VBM) are both degenerate, a direct-to-indirect bandgap transition occurs when a moderate tensile strain ($\sim$4%) is applied along the 1D chain direction, leading to chirality-dependent and collinear spin-momentum locking at the CBM. These findings indicate that 1D InSeI is a promising material for chiral spintronics.

Abstract: We formulate a Landau theory for altermagnets, a class of colinear compensated magnets with spin-split bands. Starting from the non-relativistic limit, this Landau theory goes beyond a conventional analysis by including spin-space symmetries, providing a simple framework for understanding the key features of this family of materials. We find a set of multipolar secondary order parameters connecting existing ideas about the spin symmetries of these systems, their order parameters and the effect of non-zero spin-orbit coupling. We account for several features of canonical altermagnets such as RuO$_2$, MnTe and CuF$_2$ that go beyond symmetry alone, relating the order parameter to key observables such as magnetization, anomalous Hall conductivity and magneto-elastic and magneto-optical probes. Finally, we comment on generalizations of our framework to a wider family of exotic magnetic systems deriving from the zero spin-orbit coupled limit.

Abstract: MnSiN$_2$ is a transition metal nitride with Mn and Si ions displaying an ordered distribution on the cation sites of a distorted wurtzite-derived structure. The Mn$^{2+}$ ions reside on a 3D diamond-like covalent network with strong superexchange pathways. We simulate its electronic structure and find that the N anions in MnSiN$_2$ act as $\sigma$- and $\pi$-donors, which serve to enhance the N-mediated superexchange, leading to the high N\'{e}el ordering temperature of $T_N$ = 443 K. Polycrystalline samples of MnSiN$_2$ were prepared to reexamine the magnetic structure and resolve previously reported discrepancies. An additional magnetic canting transition is observed at $T_\mathrm{cant}$ = 433 K and the precise canted ground state magnetic structure has been resolved using a combination of DFT calculations and powder neutron diffraction. The calculations favor a $G$-type antiferromagnetic spin order with lowering to $Pc^\prime$. Irreducible representation analysis of the magnetic Bragg peaks supports the lowering of the magnetic symmetry. The computed model includes a 10$^\circ$ rotation of the magnetic spins away from the crystallographic $c$-axis consistent with measured powder neutron diffraction data modeling and a small canting of 0.6$^\circ$.

Abstract: Attaining high reversibility of electrodes and electrolyte is essential for the longevity of secondary batteries. Rechargeable zinc-air batteries (RZABs), however, encounter drastic irreversible changes in the zinc anodes and air cathodes during cycling. To uncover the mechanisms of reversibility loss in RZABs, we investigate the evolution of zinc anode, alkaline electrolyte, and air electrode through experiments and first-principles calculations. Morphology diagrams of zinc anodes under versatile operating conditions reveal that the nano-sized mossy zinc dominates the later cycling stage. Such anodic change is induced by the increased zincate concentration due to hydrogen evolution, which is catalyzed by the mossy structure and results in oxide passivation on electrodes, and eventually leads to low true Coulombic efficiencies and short lifespans of batteries. Inspired by these findings, we finally present a novel overcharge-cycling protocol to compensate the Coulombic efficiency loss caused by hydrogen evolution and significantly extend the battery life.

Abstract: By performing density functional calculations, we find a giant magnetocrystalline anisotropy (MCA) constant in abundant element cerium (Ce) substituted M-type hexaferrite, in the energetically favorable strontium site, assisted by a quantum confined electron transfer from Ce to specific iron (2a) site. Remarkably, the calculated electronic structure shows that the electron transfer leads to the formation of Ce$^{3+}$ and Fe$^{2+}$ at the $2a$ site producing an occupied Ce($4f^1$) state below the Fermi level that adds a significant contribution to MCA and magnetic moment. A half Ce-substitution forms a metallic state, while a full substitution retains the semiconducting state of the strontium-hexaferrite (host). In the latter, the band gap is reduced due to the formation of charge transferred states in the gap region of the host. The optical absorption coefficient shows an enhanced anisotropy between light polarization in parallel and perpendicular directions. Calculated formation energies, including the analysis of probable competing phases, and elastic constants confirm that both compositions are chemically and mechanically stable. With successful synthesis, the Ce-hexaferrite can be a new high-performing critical-element-free permanent magnet material adapted for use in devices such as automotive traction drive motors.

Abstract: Using machine learning method, we investigate various domain walls for the recently discovered single-element ferroelectrics bismuth monolayer (Nature 617, 67 (2023)). We find the charged domain wall configuration has a lower energy than the uncharged domain wall structure due to its low electrostatic repusion potential. Two stable charged domain wall configurations exhibit topological interfacial states near their domain walls, which is caused by the change of the Z_2 number between ferroelectric and paraelectric states. Interestingly, different from the edge states of topological insulators, the energies of topological interfacial states for these two structure are splited due to the build-in electric fields in ferroelectrics. We also find a stable uncharged domain wall configutation can reduce band gap which is caused by the domain wall. Our works indicate that domain walls in two-dimensional bismuth may be a good platform for ferroelectric domain wall devices.