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AgF2 has been proposed as a cuprate analogue which requires strong correlation and marked covalence. On the other hand, fluorides are usually quite ionic and 4d transition metals tend to be less correlated than their 3d counterparts, which calls for further scrutiny. We combine valence band photoemission and Auger-Meitner spectroscopy of AgF and AgF2 together with computations in small clusters to estimate values of the Ag 4d Coulomb interaction U 4d and charge-transfer energy. Based on these values, AgF2 can be classified as a charge-transfer correlated insulator according to the Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material is a cuprate analogue from the point of view of correlations, suggesting that it should become a high-temperature superconductor if metallization is achieved by doping. We present also a computation of the Hubbard U in density functional "+U" methods and discuss its relation to the Hubbard U in spectroscopies., Comment: 12 pages, 10 figures

Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how noncollinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI_{3}, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models., We acknowledge financial support from the Swedish Research Council (VR—2018-06807). M. S. acknowledges the support of Ministerio de Ciencia e Innovación (MICINN-Spain) through Grant No. PID2019–108573GBC22, and Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S); of Generalitat de Catalunya (Grant No. 2017 SGR1506); and of the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). P. B. and S. P. acknowledge financial support from Italian MIUR under the PRIN project “Tuning and Understanding Quantum Phases in 2D Materials— Quantum2D,” Grant No. 2017Z8TS5B and “TWEET: Towards Ferroelectricity in Two Dimensions,” Grant No. 2017YCTB59, respectively. D. A. and S. P. acknowledge financial support by the Nanoscience Foundries and Fine Analysis (NFFA-MIUR Italy) project. The authors thankfully acknowledge the computer resources at Pirineus and the technical support provided by CSUC (RES-FI- 2021-1-0034). Additionally, computational work was done on resources at PDC, Stockholm, via the Swedish National Infrastructure for Computing (SNIC)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

We present a combined polarized neutron and x-ray scattering study on two enantiopure langasite single crystals aimed at the determination of their absolute structural and magnetic chiralities and the coupling between them. Our respective data sets unambiguously reveal two samples of opposite structural chirality, where the magnetic handedness is pinned by the structural one. Simple energy considerations of the magnetic exchange and single-ion anisotropy parameters reveal that it is not the Dzyaloshinskii-Moriya interaction but the local single-ion anisotropy on a triangular plaquette which plays a key role in stabilizing one of the two magnetic helices.

We put forward the concept of a bulk Rashba effect emerging in a multiferroic material, such as an antiferromagnetic system with a polar crystal structure. According to symmetry considerations, while time-reversal and space-inversion symmetries are both broken, there exist specific spin flipping operations that relate opposite spin sites in the magnetic crystal structure. As a consequence, at certain high-symmetry points in the momentum space, the magnetic point group allows the spin angular momentum to be locked to the linear momentum, a typical feature of the Rashba effect. In such a case, spin-splitting effects induced by spin-orbit coupling can arise, similar to what happens in non-magnetic Rashba systems. As a prototypical example, ab-initio calculations of antiferromagnetic BiCoO3 in the polar structure reveal that a large Rashba-like band- and spin- splitting occurs at the conduction band bottom, having a large weight from Bi-p orbital states. Moreover, we show that the spin texture of such a multiferroic can be modulated by applying a magnetic field. In particular, an external in-plane magnetic field is predicted not only to induce spin-canting, but also a distortion of the energy isocontours and a shift of the spin-vortex (centered on the high-symmetry point and characteristic of Rashba effect) along a direction perpendicular to the applied field., Comment: 6 pages, 5 figures, submitted for publication

Ferroelectric Rashba semiconductors (FERSC) are a novel class of multifunctional materials showing a giant Rashba spin splitting which can be reversed by switching the electric polarization. Although they are excellent candidates as channels in spin field effect transistors, the experimental research has been limited so far to semiconducting GeTe, in which ferroelectric switching is often prevented by heavy doping and/or large leakage currents. Here, we report that ${\mathrm{CsBiNb}}_{2}{\mathrm{O}}_{7}$, a layered perovskite of Dion-Jacobson type, is a robust ferroelectric with sufficiently strong spin-orbit coupling and spin texture reversible by electric field. Moreover, we reveal that its topmost valence band's spin texture is quasi-independent from the momentum, as a result of the low symmetry of its ferroelectric phase. The peculiar spin-polarization pattern in the momentum space may yield the so-called ``persistent spin helix,'' a specific spin-wave mode which protects the spin from decoherence in the diffusive transport regime, potentially ensuring a very long spin lifetime in this material.

Polar lacunar spinels, such as $\mathrm{Ga}{\mathrm{V}}_{4}{\mathrm{S}}_{8}$ and $\mathrm{Ga}{\mathrm{V}}_{4}\mathrm{S}{\mathrm{e}}_{8}$, were proposed to host skyrmion phases under magnetic field. In this work, we put forward, as a candidate for N\'eel-type skyrmion lattice, the isostructural $\mathrm{GaM}{\mathrm{o}}_{4}{\mathrm{S}}_{8}$, which is systematically studied via both first-principles calculations and Monte Carlo simulations of a model Hamiltonian. Electric polarization, driven by the Jahn-Teller distortion, is predicted to arise in $\mathrm{GaM}{\mathrm{o}}_{4}{\mathrm{S}}_{8}$, showing a comparable size but an opposite sign with respect to that evaluated in V-based counterparts and explained in terms of different electron counting arguments and resulting distortions. Interestingly, a larger spin-orbit coupling of $4d$ orbitals with respect to $3d$ orbitals in vanadium spinels leads to stronger Dzyaloshinskii-Moriya interactions, which are beneficial to stabilize a cycloidal spin texture, as well as smaller-sized skyrmions $(\mathrm{radius}l10\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. Furthermore, the possibly large exchange anisotropy of $\mathrm{GaM}{\mathrm{o}}_{4}{\mathrm{S}}_{8}$ may lead to a ferroelectric-ferromagnetic ground state as an alternative to the ferroelectric-skyrmionic one, thus calling for further experimental verification.

Dirac points in two-dimensional electronic structures are a source for topological electronic states due to the $\pm \pi$ Berry phase that they sustain. Here we show that two rutile multilayers (namely (WO$_2$)$_2$/(ZrO$_2$)$_n$ and (PtO$_2$)$_2$/(ZrO$_2$)$_n$, where an active bilayer is sandwiched by a thick enough (n=6 is sufficient) band insulating substrate, show semi-metallic Dirac dispersions with a total of four Dirac cones along the $\Gamma-M$ direction. These become gapped upon the introduction of spin-orbit coupling, giving rise to an insulating ground state comprising four edge states. We discuss the origin of the lack of topological protection in terms of the valley spin-Chern numbers and the multiplicity of Dirac points. We show with a model Hamiltonian that mirror-symmetry breaking would be capable of creating a quantum phase transition to a strong topological insulator, with a single Kramers pair per edge., Comment: 7 pages, 4 figures

By means of density functional theory based calculations, we study the role of spin-orbit coupling in the new family of ABC hyperferroelectrics [Phys. Rev. Lett. 112, 127601 (2014)]. We unveil an extremely rich physics strongly linked to ferroelectric properties, ranging from the electric control of bulk Rashba effect to the existence of a three dimensional topological insulator phase, with concomitant topological surface states even in the ultrathin film limit. Moreover, we predict that the topological transition, as induced by alloying, is followed by a Weyl semi-metal phase of finite concentration extension, which is robust against disorder, putting forward hyperferroelectrics as promising candidates for spin-orbitronic applications., 5 pages, 3 figures

It has been recently suggested that the coexistence of ferroelectricity and Rashba-like spin-splitting effects due to spin-orbit coupling in a single material may allow for a non-volatile electric control of spin degrees of freedom. In the present work, we compared the structural and ferroelectric properties of tetragonal and rhombohedral phases of ferroelectric BiAlO$_3$ by means of density-functional calculations. In both phases, we carefully investigated Rashba and Dresselhaus effects, giving rise to spin-splitting in their bulk electronic structure, particularly near the conduction band minimum, supplementing our first-principles results with an effective $ k\cdot p$ model analysis. The full reversal of the spin texture with ferroelectric polarization switching was also predicted. BiAlO$_3$ can therefore be considered as the first known oxide to exhibit a coexistence of ferroelectricity and Rashba-Dresselhaus effects., 8 pages, 5 figures

The properties of two molecular-based magnetic helices, composed of $3d$ metal Co and Mn ions bridged by nitronyl nitroxide radicals, are investigated by density-functional calculations. Their peculiar and distinctive magnetic behavior is here elucidated by a thorough description of their magnetic, electronic, and anisotropy properties. Metal ions are antiferromagnetically coupled with the radicals, leading to a ferrimagnetically ordered ground state. A strong metal-radical exchange coupling is found, about 44 and 48 meV for Co and Mn helices, respectively. The latter have also relevant next-nearest-neighbor Mn-Mn antiferromagnetic interactions (of $\ensuremath{\sim}6$ meV). Co sites are characterized by noncollinear uniaxial anisotropies, whereas Mn sites are rather isotropic. A key result pertains to the Co helix: The microscopic picture resulting from density-functional calculations allows us to propose a spin Hamiltonian of increased complexity with respect to the commonly employed Ising Hamiltonian, suitable for the study of finite-temperature behavior, and that seems to clarify the puzzling scenario of multiple characteristic energy scales observed in experiments.

By means of density functional theory calculations, we predict that several two dimensional AB binary monolayers, where A and B atoms belong to group IV or III-V, are ferroelectric. Dipoles arise from the buckled structure, where the A and B ions are located on the sites of a bipartite corrugated honeycomb lattice with trigonal symmetry. We discuss the emerging valley-dependent properties and the coupling of spin and valley physics, which arise from the loss of inversion symmetry, and explore the interplay between ferroelectricity and Rashba spin-spitting phenomena. We show that valley-related properties originate mainly from the binary nature of AB monolayers, while the Rashba spin-texture developing around valleys is fully controllable and switchable by reversing the ferroelectric polarization.

Recent interest in novel functionalities arising at domain walls of ferroic materials naturally calls for a microscopic understanding. To this end, first-principles calculations have been performed in order to provide solid evidence of polar distortions in the twin walls of nonpolar CaTiO${}_{3}$ and magnetic CaMnO${}_{3}$. We show that such polar displacements arise from rotation and/or tilting octahedral distortions---cooperatively acting at the twin wall in both considered systems---rather than from a proper secondary ferroelectric instability, as often believed. Our results are in excellent agreement with experimental observations of domain walls in CaTiO${}_{3}$. In addition, we show that magnetic properties at the twin wall in CaMnO${}_{3}$ are also modified, thus suggesting an unexplored route to achieve and detect multiferroic ordering in a single-phase material.

We explore how the ferroelectric polarization of antiferromagnetic E-type orthorhombic HoMnO${}_{3}$ can be increased, by investigating the effects of in-plane strain on both the magnetic properties and the ferroelectric polarization, using combined density functional theory calculations and a model Hamiltonian technique. Our results show that the net polarization is strongly enhanced under compressive strain, due to an increase of the elec-tronic contribution to the polarization. In contrast, the ionic contribution is found to decrease. We identify the electron-lattice coupling, due to Jahn-Teller (JT) distortions, and its response to strain, to be responsible for the observed behavior. The JT-induced orbital ordering of occupied Mn-${e}_{g}^{1}$ electrons in alternating $3{x}^{2}\ensuremath{-}{r}^{2}3{y}^{2}\ensuremath{-}{r}^{2}$ orbital states in the unstrained structure, changes under in-plane compressive strain to a mixture with ${x}^{2}\ensuremath{-}{z}^{2}{y}^{2}\ensuremath{-}{z}^{2}$ states. The asymmetric hopping of ${e}_{g}$ electrons between Mn ions along zigzag spin chains (typical of the AFM-E spin configuration) is therefore enhanced under strain, explaining the large value of the polarization. Using a degenerate double-exchange model including electron-phonon interaction, we reproduce the change in the orbital ordering pattern. In this picture, the orbital ordering change is related to a change of the Berry phase of the ${e}_{g}$ electrons. This causes an increase of the electronic contribution to the polarization.

A joint theoretical approach, combining macroscopic symmetry analysis with microscopic methods (density functional theory and a model cluster Hamiltonian), is employed to shed light on magnetoelectricity in Ba${}_{2}$CoGe${}_{2}$O${}_{7}$. The recently reported experimental behavior of polarization guided by a magnetic field [Murakawa et al., Phys. Rev. Lett. 105, 137202 (2010)] can be predicted on the basis of phenomenological Landau theory. On the microscopic side, two main ingredients are needed for the cross-coupling between magnetic and dipolar degrees of freedom: on-site spin-orbit coupling and the spin-dependent hybridization between O-$p$ and transition metal $d$ states. Structural constraints related to the noncentrosymmetric symmetry and the particular configuration of CoO${}_{4}$ tetrahedrons provide additional features for a peculiar magnetoelectricity to develop.

We study by the Gutzwiller approximation the melting of the valence bond crystal phase of a bilayer Hubbard model at sufficiently large inter-layer hopping. We find that a superconducting domain, with order parameter $d_{z^2-r^2}$, $z$ being the inter-layer direction and $r$ the intra-layer one, is stabilized variationally close to the half-filled non-magnetic Mott insulator. Superconductivity exists at half-filling just at the border of the Mott transition and extends away from half-filling into a whole region till a critical doping, beyond which it gives way to a normal metal phase. This result suggests that superconductivity should be unavoidably met by liquefying a valence bond crystal, at least when each layer is an infinite coordination lattice and the Gutzwiller approximation becomes exact. Remarkably, this same behavior is well established in the other extreme of two-leg Hubbard ladders, showing it might be of quite general validity., Comment: 9 pages, 5 figures