arXiv subjects

Luca Binci

Publications and source records attributed to Luca Binci.

13 recordsLinked to original sources

arXiv2026

Magnetic interactions and spin orders in Cr$_8$ and V$_8$ ring-shaped molecular magnets from non-collinear ab initio calculations

We employ density functional theory within a non-collinear framework to investigate the magnetic properties of the octanuclear molecular rings Cr$_8$ and V$_8$. Our aim is to generalize the evaluation of the effective magnetic interactions by explicitly including non-collinear spin configurations, thereby refining our understanding of their dependence upon the underlying electronic structure and molecular geometry. By analyzing the energetics of a variety of magnetic configurations, particularly non-collinear arrangements with neighboring spins oriented along different directions, we move beyond the exchange-only Heisenberg Hamiltonian describing the low-energy sector of the excitation spectrum. This approach enables us to distinguish between in-plane and out-of-plane exchange interactions, and to incorporate biquadratic coupling terms into the effective spin Hamiltonian. We reveal significant antisymmetric exchange interactions of the Dzyaloshinskii-Moriya (DM) type whose dependence on the curvature of the annular structure is clarified by a comparison with the results obtained from linear chains of equal composition. Our work demonstrates that interactions beyond conventional exchange, particularly biquadratic anisotropic terms, in the spin Hamiltonian are essential for accurately capturing the low-energy excitations of these systems. The closest quantitative agreement with experimental results (particularly for the case of Cr$_8$) is achieved when extended Hubbard functionals are used for the evaluation of the effective magnetic couplings.

cond-mat.mtrl-sci↗

arXiv2026

Mechanisms of alkali ionic transport in amorphous oxyhalides solid state conductors

Amorphous oxyhalides have attracted significant attention due to their relatively high ionic conductivity ($>$1 mS cm$^{-1}$), excellent chemical stability, mechanical softness, and facile synthesis routes via standard solid-state reactions. These materials exhibit an ionic conductivity that is almost independent of the underlying chemistry, in stark contrast to what occurs in crystalline conductors. In this work, we employ an accurately fine-tuned machine learning interatomic potential to construct large-scale molecular dynamics trajectories encompassing hundreds of nanoseconds to obtain statistically converged transport properties. We find that the amorphous state consists of chain fragments of metal-anion tetrahedra of various lenght. By analyzing the residence time of alkali cations migrating around tetrahedrally-coordinated trivalent metal ions, we find that oxygen anions on the metal-anion tetrahedra limit alkali diffusion. By computing the full Einstein expression of the ionic conductivity, we demonstrate that the alkali transference number of these materials is strongly influenced by distinct-particles correlations, while at the same time they are characterized by an alkali Haven ratio close to one, implying that ionic transport is largely dictated by uncorrelated self-diffusion. Finally, by extending this analysis to chemical compositions $AMX_{2.5}\textsf{O}_{0.75}$, spanning different alkaline ($A$ = Li, Na, K), metallic ($M$ = Al, Ga, In), and halogen ($X$ = Cl, Br, I) species, we clarify why the diffusion properties of these materials remain largely insensitive to variations in atomic chemistry.

cond-mat.mtrl-sci↗

arXiv2025

Comparative study of magnetic exchange parameters and magnon dispersions in NiO and MnO from first principles

Spin-wave excitations are fundamental to understanding the behavior of magnetic materials and hold promise for future information and communication technologies. Yet, modeling these accurately in transition-metal compounds remains challenging, starting from the self-interaction errors affecting localized and partially filled $d$-orbitals in density-functional theory (DFT) with (semi-)local functionals. In this work, we compare three advanced first-principles approaches for computing magnetic exchange parameters and magnon dispersions in NiO and MnO, all based on a common DFT+$U$ ground state with ab initio Hubbard $U$ values obtained from density-functional perturbation theory. Two methods extract exchange parameters directly: one via total-energy differences using the four-state mapping ($\Delta E$), and the other via the magnetic force theorem (MFT) using infinitesimal spin rotations. Magnon dispersions are then obtained from a Heisenberg Hamiltonian through linear spin-wave theory (LSWT). The third approach, time-dependent density-functional perturbation theory with $U$ (TDDFPT+$U$), yields magnon dispersions directly from the dynamical spin susceptibility, with exchange parameters fitted a posteriori, for comparison, via LSWT. Our results show that TDDFPT+$U$ and the Heisenberg model based on $\Delta E$-derived parameters align well with experimental neutron scattering data, whereas the MFT-based approach shows larger discrepancies, possibly due to some inherent approximations and limitations of the particular implementation used. This study benchmarks the accuracy of state-of-the-art first-principles techniques for spin-wave modeling and contributes to advancing reliable computational tools for the study and design of magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

The Interplay Between Electron Localization, Magnetic Order, and Jahn-Teller Distortion that Dictates LiMnO$_2$ Phase Stability

The development of Mn-rich cathodes for Li-ion batteries promises to alleviate supply chain bottlenecks in battery manufacturing. Challenges in Mn-rich cathodes arise from Jahn-Teller (JT) distortions of Mn$^{3+}$, Mn migration, and phase transformations to spinel-like order, which can affect the electrochemical performance. These phenomena motivate an ab initio re-examination of the thermodynamics of the LiMnO2 polymorphs. It is found that the generalized gradient approximation (GGA - PBEsol) and meta-GGA (r2SCAN) density functionals with empirical on-site Hubbard U corrections yield spurious stable phases for LiMnO2, such as predicting a phase with gamma-LiFeO2-like order (g-LiMnO2) to be the ground state instead of the orthorhombic (Pmmn) phase, which is the experimentally known ground state. Accounting for the antiferromagnetic (AFM) order in each structure has a substantial effect on the total energies and resulting phase stability. By using hybrid-GGA (HSE06) and GGA with self-consistent Hubbard parameters (on-site U and inter-site V), the experimentally observed LiMnO2 phase stability trends are recovered. The calculated Hubbard U in the experimentally observed orthorhombic, layered, and spinel phases are significantly smaller than U in g-LiMnO2 and disordered layered structures. The smaller values of U are correlated with a collinear ordering of JT distortions, in which all $e_g$ orbitals are oriented in the same direction. This cooperative JT effect leads to increased Mn-O covalency, which contributes to the greater electronic stability compared to the phases with noncollinear JT arrangements, and also generate greater vibrational entropy, which helps stabilize these phases at high temperature. These phases are shown to be strongly insulating with large calculated band gaps > 3 eV, computed using HSE06 and $G_0W_0$.

cond-mat.mtrl-sci↗

arXiv2024

Magnons from time-dependent density-functional perturbation theory and nonempirical Hubbard functionals

Spin excitations play a fundamental role in understanding magnetic properties of materials, and have significant technological implications for magnonic devices. However, accurately modeling these in transition-metal and rare-earth compounds remains a formidable challenge. Here, we present a fully first-principles approach for calculating spin-wave spectra based on time-dependent (TD) density-functional perturbation theory (DFPT), using nonempirical Hubbard functionals. This approach is implemented in a general noncollinear formulation, enabling the study of magnons in both collinear and noncollinear magnetic systems. Unlike methods that rely on empirical Hubbard $U$ parameters to describe the ground state, and Heisenberg Hamiltonians for describing magnetic excitations, the methodology developed here probes directly the dynamical spin susceptibility (efficiently evaluated with TDDFPT throught the Liouville-Lanczos approach), and treats the linear variation of the Hubbard augmentation (in itself calculated non-empirically) in full at a self-consistent level. Furthermore, the method satisfies the Goldstone condition without requiring empirical rescaling of the exchange-correlation kernel or explicit enforcement of sum rules, in contrast to existing state-of-the-art techniques. We benchmark the novel computational scheme on prototypical transition-metal monoxides NiO and MnO, showing remarkable agreement with experiments and highlighting the fundamental role of these newly implemented Hubbard corrections. The method holds great promise for describing collective spin excitations in complex materials containing localized electronic states.

cond-mat.mtrl-sci↗

arXiv2024

Machine learning Hubbard parameters with equivariant neural networks

Density-functional theory with extended Hubbard functionals (DFT+$U$+$V$) provides a robust framework to accurately describe complex materials containing transition-metal or rare-earth elements. It does so by mitigating self-interaction errors inherent to semi-local functionals which are particularly pronounced in systems with partially-filled d and f electronic states. However, achieving accuracy in this approach hinges upon the accurate determination of the on-site $U$ and inter-site $V$ Hubbard parameters. In practice, these are obtained either by semi-empirical tuning, requiring prior knowledge, or, more correctly, by using predictive but expensive first-principles calculations. Here, we present a machine learning model based on equivariant neural networks which uses atomic occupation matrices as descriptors, directly capturing the electronic structure, local chemical environment, and oxidation states of the system at hand. We target here the prediction of Hubbard parameters computed self-consistently with iterative linear-response calculations, as implemented in density-functional perturbation theory (DFPT), and structural relaxations. Remarkably, when trained on data from 12 materials spanning various crystal structures and compositions, our model achieves mean absolute relative errors of 3% and 5% for Hubbard $U$ and $V$ parameters, respectively. By circumventing computationally expensive DFT or DFPT self-consistent protocols, our model significantly expedites the prediction of Hubbard parameters with negligible computational overhead, while approaching the accuracy of DFPT. Moreover, owing to its robust transferability, the model facilitates accelerated materials discovery and design via high-throughput calculations, with relevance for various technological applications.

cond-mat.mtrl-sci↗

arXiv2023

The type-I antiferromagnetic Weyl semimetal InMnTi$_2$

Topological materials have been a main focus of studies in the past decade due to their protected properties that can be exploited for the fabrication of new devices. Among them, Weyl semimetals are a class of topological semimetals with non-trivial linear band crossing close to the Fermi level. The existence of such crossings requires the breaking of either time-reversal or inversion symmetry and is responsible for the exotic physical properties. In this work we identify the full-Heusler compound InMnTi$_2$, as a promising, easy to synthesize, $T$- and $I$-breaking Weyl semimetal. To correctly capture the nature of the magnetic state, we employed a novel $\mathrm{DFT}+U$ computational setup where all the Hubbard parameters are evaluated from first-principles; thus preserving a genuinely predictive \textit{ab initio} character of the theory. We demonstrate that this material exhibits several features that are comparatively more intriguing with respect to other known Weyl semimetals: the distance between two neighboring nodes is large enough to observe a wide range of linear dispersions in the bands, and only one kind of such node's pairs is present in the Brillouin zone. We also show the presence of Fermi arcs stable across a wide range of chemical potentials. Finally, the lack of contributions from trivial points to the low-energy properties makes the materials a promising candidate for practical devices.

cond-mat.mtrl-sci↗

arXiv2023

Noncollinear DFT+$U$ and Hubbard parameters with fully-relativistic ultrasoft pseudopotentials

The magnetic, noncollinear parametrization of Dudarev's DFT+$U$ method is generalized to fully-relativistic ultrasoft pseudopotentials. We present the definition of the DFT+$U$ total energy functional, and the calculation of forces and stresses in the case of orthogonalized atomic orbitals defining the localised Hubbard manifold, where additional contributions arising from the derivative of the inverse square root of the overlap matrix appear. We further extend the perturbative calculation of the Hubbard $U$ parameters within density-functional perturbation theory to the noncollinear relativistic case, by exploiting an existing and recently developed theoretical approach that takes advantage of the time-reversal operator to solve a second Sternheimer equation. We validate and apply the new scheme by studying the electronic structure and the thermodynamics of the binary compounds EuX (where X = O, S, Se, Te is a chalcogen atom), as representative simple crystals, and of the pyrochlore Cd$_2$Os$_2$O$_7$, representative of a more structurally complex oxide.

cond-mat.mtrl-sci↗

arXiv2023

Born effective charges and vibrational spectra in super and bad conducting metals

Interactions mediated by electron-phonon coupling are responsible for important cooperative phenomena in metals such as superconductivity and charge-density waves. The same interaction mechanisms produce strong collision rates in the normal phase of correlated metals, causing sizeable reductions of the dc conductivity and reflectivity. As a consequence, low-energy excitations like phonons, which are crucial for materials characterization, become visible in optical infrared spectra. A quantitative assessment of vibrational resonances requires the evaluation of dynamical Born effective charges, which quantify the coupling between macroscopic electric fields and lattice deformations. We show that the Born effective charges of metals crucially depend on the collision regime of conducting electrons. In particular, we describe, within a first principles framework, the impact of electron scattering on the infrared vibrational resonances, from the undamped, collisionless regime to the overdamped, collision-dominated limit. Our approach enables the interpretation of vibrational reflectance measurements of both super and bad conducting metals, as we illustrate for the case of strongly electron-phonon coupled superhydride H$_3$S.

cond-mat.mtrl-sci↗

arXiv2022

Hybridization driving distortions and multiferroicity in rare-earth nickelates

For decades transition-metal oxides have generated a huge interest due to the multitude of physical phenomena they exhibit. In this class of materials, the rare-earth nickelates, $R$NiO$_3$, stand out for their rich phase diagram stemming from complex couplings between the lattice, electronic and magnetic degrees of freedom. Here, we present a first-principles study of the low-temperature phase for two members of the $R$NiO$_3$ series, with $R=$ Pr, Y. We employ density-functional theory with Hubbard corrections accounting not only for the on-site localizing interactions among the Ni--$3d$ electrons ($U$), but also the inter-site hybridization effects between the transition-metals and the ligands ($V$). All the \textit{U} and \textit{V} parameters are calculated from first-principles using density-functional perturbation theory, resulting in a fully \emph{ab initio} methodology. Our simulations show that the inclusion of the inter-site interaction parameters $V$ is necessary to simultaneously capture the features well-established by experimental characterizations of the low-temperature state: insulating character, antiferromagnetism and bond disproportionation. On the contrary, for some magnetic orderings the inclusion of on-site interaction parameters $U$ alone completely suppresses the breathing distortion occurring in the low-temperature phase and produces an erroneous electronic state with a vanishing band gap. In addition -- only when both the \textit{U} and \textit{V} are considered -- we predict a polar phase with a magnetization-dependent electric polarization, supporting very recent experimental observations that suggest a possible occurrence of type-II multiferroicity for these materials.

cond-mat.str-el↗

arXiv2022

Pulay forces in density-functional theory with extended Hubbard functionals: From nonorthogonalized to orthogonalized manifolds

We present a derivation of the exact expression for Pulay forces in density-functional theory calculations augmented with extended Hubbard functionals, and arising from the use of orthogonalized atomic orbitals as projectors for the Hubbard manifold. The derivative of the inverse square root of the orbital overlap matrix is obtained as a closed-form solution of the associated Lyapunov (Sylvester) equation. The expression for the resulting contribution to the forces is presented in the framework of ultrasoft pseudopotentials and the projector-augmented-wave method, and using a plane wave basis set. We have benchmarked the present implementation with respect to finite differences of total energies for the case of NiO, finding excellent agreement. Owing to the accuracy of Hubbard-corrected density-functional theory calculations - provided the Hubbard parameters are computed for the manifold under consideration - the present work paves the way for systematic studies of solid-state and molecular transition-metal and rare-earth compounds.

cond-mat.mtrl-sci↗

arXiv2021

First-principles theory of infrared vibrational spectroscopy in metals and semimetals: application to graphite

We develop an ab initio method to simulate the infrared vibrational response of metallic systems in the framework of time-dependent density functional perturbation theory. By introducing a generalized frequency-dependent Born effective charge tensor, we show that phonon peaks in the reflectivity of metals can be always described by a Fano function, whose shape is determined by the complex nature of the frequency-dependent effective charges and electronic dielectric tensor. The IR vibrational properties of graphite, chosen as a representative test case to benchmark our method, are found to be accurately reproduced. Our approach offers a first-principle scheme for the prediction and understanding of IR reflectance spectra of metals, that may represent one of the few available tools of investigation of these materials when subjected to extremely high-pressure conditions.

cond-mat.mtrl-sci↗

arXiv2018

Rapid detection of coherent tunneling in an InAs nanowire quantum dot through dispersive gate sensing

Dispersive sensing is a powerful technique that enables scalable and high-fidelity readout of solid-state quantum bits. In particular, gate-based dispersive sensing has been proposed as the readout mechanism for future topological qubits, which can be measured by single electrons tunneling through zero-energy modes. The development of such a readout requires resolving the coherent charge tunneling amplitude from a quantum dot in a Majorana-zero-mode host system faithfully on short time scales. Here, we demonstrate rapid single-shot detection of a coherent single-electron tunneling amplitude between InAs nanowire quantum dots. We have realized a sensitive dispersive detection circuit by connecting a sub-GHz, lumped element microwave resonator to a high-lever arm gate on one of dots. The resulting large dot-resonator coupling leads to an observed dispersive shift that is of the order of the resonator linewidth at charge degeneracy. This shift enables us to differentiate between Coulomb blockade and resonance, corresponding to the scenarios expected for qubit state readout, with a signal to noise ratio exceeding 2 for an integration time of 1 microsecond. Our result paves the way for single shot measurements of fermion parity on microsecond timescales in topological qubits.

cond-mat.mes-hall↗