arXiv subjects

Cesare Franchini

Publications and source records attributed to Cesare Franchini.

At least 19 recordsLinked to original sources

arXiv2026

Machine-learned dynamics of surface polarons at reduced oxide surfaces

Reducible oxides exhibit a rich interplay of electronic, structural, and chemical properties that underpins applications in catalysis, photovoltaics, batteries, and energy storage. This interplay is strongly shaped by excess electrons, often introduced by oxygen vacancies, that localize as small polarons and influence charge transport and surface chemistry. At surfaces, these polarons play a central role in charge localization, mobility, and reactivity, yet their finite-temperature dynamics remain difficult to access from first principles due to the long time scales needed to adequately sample polaron's hopping. To overcome this limitation, we extend machine-learning-assisted polaron dynamics to redox-active oxide surfaces, using oxygen-deficient rutile TiO$_2$(110) as a paradigmatic case. By accessing several nanoseconds of dynamics over a range of temperatures, we show that small-polaron mobility at the reduced rutile TiO$_2$(110) surface is suppressed by several orders of magnitude relative to the corresponding bulk material, providing a microscopic interpretation of the lower electron mobilities observed in porous rutile TiO$_2$ compared with single-crystal samples. This suppressed mobility arises from the loss of favourable hopping pathways: surface polaron motion is largely confined to planar inter-row trajectories within the second topmost layers, with only rare interlayer hopping events. These results establish a transferable machine-learning strategy for investigating polaron dynamics in reducible oxides.

cond-mat.mtrl-sci↗

arXiv2026

Machine Learning the order-disorder Jahn-Teller transition in LaMnO$_3$

We investigate the Jahn-Teller structural phase transition in LaMnO$_3$ at $T_{JT} \simeq 750$ K using molecular dynamics simulations based on machine-learning force fields trained on ab initio data. Analysis of the site-site correlation function of the distortions reveals that the transition is driven by the ordering of the $Q_2$ Jahn-Teller distortion of the MnO$_6$ octahedra, which acts as the order parameter and establishes the order-disorder nature of the transition. Dynamical local distortions are found to persist above $T_{JT}$. Our results reproduce the experimental temperature dependence of both structural and phonon properties and highlight the presence of anharmonic effects at finite temperature. More broadly, the combined use of machine-learning molecular dynamics and velocity autocorrelation function analysis provides a robust framework for uncovering the microscopic mechanisms of structural phase transitions in correlated materials. In particular, this approach enables a clear distinction between order-disorder transitions and alternative mechanisms, such as displacive behavior, through the temperature evolution of vibrational properties.

cond-mat.stat-mech↗

arXiv2026

Photoexcited Hole States at the SrTiO3(001) Surface Imaged with Noncontact AFM

The behaviour of excess charges in ionic lattices, such as the formation of polarons and charge trapping at defect sites, influences the physical and chemical properties of materials and translates into applications in electronics, optics, photovoltaics, and catalysis. Here we show that the bulk-terminated SrTiO3(001) surface accumulates photoexcited charges and keeps the associated photovoltage for many days at cryogenic temperatures. A combination of scanning tunneling microscopy, atomic force microscopy (STM/AFM) and Kelvin probe force microscopy (KPFM) was used to measure this photovoltage and to localize the photoexcited charges with atomic precision down to the single-quasiparticle limit. Density functional theory (DFT) shows that holes favor localization at oxygen 2p orbitals adjacent to Sr vacancies, creating long-lived trapped states. The methodology presented here provides guidelines for imaging of charges trapped in the crystal lattice using noncontact AFM.

cond-mat.mtrl-sci↗

arXiv2026

Polaron-mediated anisotropic exchange in 2D magnets

Two-dimensional (2D) magnets offer a rich platform for exploring emergent spin phenomena due to their unique and diverse magnetic properties. Beyond intrinsic magnetism, external manipulation$\unicode{x2013}$such as defect engineering, molecular adsorption, or charge doping$\unicode{x2013}$offers powerful routes to control their magnetic behavior. In this work, we demonstrate that localized electron polarons provide an effective means to modulate magnetism in 2D magnets. Using first-principles calculations, we investigate polaron formation in monolayer MnPS$_3$ and compute the resulting changes in magnetic exchange interactions. Our results reveal that polarons can locally break magnetic symmetry and induce anisotropic exchange couplings, highlighting a novel mechanism for tuning magnetic textures. This insight opens promising pathways for designing atomic-scale control of magnetism, with potential impact on spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Polaron-driven switching of octupolar order in doped 5d$^2$ double perovskite

We investigate how doping-induced small polarons impact the low-temperature multipolar orders of the $5d^2$ double perovskite Ba$_2$CaOsO$_6$. By computing intersite exchange interactions between 5d$^1$ localized hole polarons and 5d$^2$ magnetic ions from first principles, we demonstrate the reversal of the dominant octupolar exchange from ferromagnetic to antiferromagnetic. Solving the corresponding effective Hamiltonian we find this reversal to account for the progressive suppression of the ferro-octupolar order and the reduction of the ordering temperature upon Na doping. These findings clarify previously ambiguous experimental observations and demonstrate that charge doping in the form of small polarons offers a viable route to tuning intersite exchange interactions in spin-orbit-entangled materials, enabling the emergence of novel quantum orders.

cond-mat.str-el↗

arXiv2026

Importance of nonlinear long-range electron-phonon interaction for the carrier mobility of anharmonic halide perovskites

The interaction between the electrons and the lattice vibrations in a solid is responsible for various important effects, such as formation of polarons, temperature dependent bandgaps, phonon-limited carrier transport, and conventional superconductivity. Most works assume a linear electron-phonon interaction, where the electron only interacts with one phonon at a time. However, the validity of this assumption has not been verified in polar anharmonic materials, where large ionic displacements may invalidate the assumption of linear interaction. Here, we show that nonlinear electron-phonon interactions contribute significantly to the finite-temperature electron mobility of the inorganic lead halide perovskite CsPbI$_3$. The effect of nonlinear interaction is taken into account using the recently derived expression for the long-range part of the one-electron-two-phonon matrix element. We calculate the electron mobility from first principles within the self-energy relaxation-time approximation, treating the electron-phonon coupling in the long-range approximation. Despite these approximations, the calculated mobilities are in good agreement with the available experimental data, while enabling us to isolate and quantify the contribution of nonlinear electron-phonon interactions relative to the conventional linear coupling. We find that the one-electron-two-phonon interaction modifies the temperature dependence of the mobility in CsPbI$_3$ and reduces its room-temperature value by about 10\%. This sizeable contribution results from the combined effects of strong lattice anharmonicity and large thermal phonon populations, the latter being enhanced by the low phonon frequencies associated with the heavy constituent atoms. These results show when nonlinear electron-phonon interactions become relevant, and indicate they should be considered for finite-temperature properties of halide perovskites.

cond-mat.mtrl-sci↗

arXiv2026

Hydrogen Activation via Dihydride Formation on a Rh1/Fe3O4(001) Single-Atom Catalyst

Hydrogen activation is a key elementary step in catalytic hydrogenation. In heterogeneous catalysis, it usually proceeds through dissociative adsorption on metal nanoparticles followed by surface diffusion or spillover, whereas homogeneous catalysts activate H2 through dihydride or dihydrogen intermediates at a single metal center. Here, we show that isolated Rh adatoms supported on Fe3O4(001) activate hydrogen through formation of a stable dihydride species without atomic H spillover. Temperature-programmed desorption, X-ray photoelectron spectroscopy, and scanning tunneling microscopy collectively reveal strong (approximately 1 eV) hydrogen adsorption exclusively at isolated Rh1 sites, while isotope-exchange experiments further demonstrate that hydrogen remains localized. Density-functional theory based calculations indicate a barrierless conversion from molecular H2 to the dihydride, and random-phase approximation calculations further confirm the relative stability of the dihydride. Together, these results show that single-atom Rh sites cleave and bind H2 through a dihydride pathway analogous to homogeneous complexes, establishing a mechanistic bridge between homogeneous and heterogeneous catalysis.

cond-mat.mtrl-sci↗

arXiv2025

CO on a Rh/Fe3O4 single-atom catalyst: high-resolution infrared spectroscopy and near-ambient-pressure scanning tunnelling microscopy

Infrared reflection absorption spectroscopy (IRAS) offers a powerful route to bridging the materials and pressure gaps between surface science and powder catalysis. Using a newly developed IRAS setup optimised for dielectric single crystals, we investigate CO adsorption on the model single-atom catalyst Rh/Fe3O4(001). IRAS resolves three species: monocarbonyls at isolated, twofold-coordinated Rh adatoms, monocarbonyls at fivefold-coordinated Rh atoms embedded in the surface, and gem-dicarbonyls at isolated, twofold-coordinated Rh adatoms. Under ultra-high vacuum (UHV) conditions, RhCO monocarbonyl species at adatom sites dominate. Rh(CO)2 gem-dicarbonyl formation is kinetically hindered and occurs predominantly through CO-induced dissociation of Rh dimers rather than sequential adsorption of two CO molecules at an isolated, twofold Rh adatom. The sequential-adsorption pathway to Rh(CO)2 becomes accessible at millibar CO pressures as evidenced by near-ambient-pressure scanning tunnelling microscopy (NAP-STM). These findings link the UHV behaviour to that expected under realistic reaction conditions. Assignments of the vibrational frequencies to individual species rely on isotopic labelling, thermal treatments, and a review of previous SPM, XPS, and TPD data, supported by density functional theory (DFT)-based calculations. While theory provides qualitative insight, such as the instability of dicarbonyls on fivefold-coordinated Rh atoms, it does not yet reproduce experimental frequencies quantitatively and is sensitive to the computational parameters, highlighting the need for robust experimental benchmarks. The spectroscopic fingerprints established here provide a reliable foundation for identifying Rh coordination environments in oxide-supported single-atom catalysts.

cond-mat.mtrl-sci↗

arXiv2025

Coupling between ferroelectric distortions and excitonic properties in PbTiO$_3$

PbTiO$_3$ is a ferroelectric perovskite semiconductor with favourable electronic and optical properties, making it suitable for a wide range of applications, including photo-catalysis and (opto)electronic devices. Despite its relevance, an accurate ab-initio description of the optical absorption spectrum and of the impact of ferroelectric distortion on the excitonic properties is still lacking. We combine $G_0W_0$ and Bethe-Salpeter Equation calculations to investigate the electronic and optical properties of PbTiO$_3$, tracking the evolution of its excitonic spectrum along the transition from the cubic paraelectric to the tetragonal ferroelectric phase. As the polar distortion increases, the first absorption peak of the cubic phase splits into two distinct features due to symmetry breaking, which partially lifts the degeneracy of the underlying excitonic state. Crucially, the distortion further introduces an in-plane/out-of-plane anisotropy in the spectra and controls the energy separation between the resulting excitonic branches. These findings highlight the potential for tuning the optical absorption properties of PbTiO$_3$ via the application of an external electric field.

cond-mat.mtrl-sci↗

arXiv2025

Metastable Multi-centered Polarons in BiVO$_{4}$

Polarons, quasiparticles formed through interactions between lattice and charge carriers (electrons and holes), strongly influence the electronic and optical properties of functional materials. In nanostructured BiVO$_{4}$, polaron formation and dynamics govern photocatalytic efficiency and charge transport, yet the microscopic nature remains not fully resolved. Here, using first-principles calculations, we report the formation of multi-centered polarons, in contrast to the more common single-centered states. Moreover, electron polarons exhibit pronounced anisotropy compared to the isotropic hole counterpart, reflecting a distinct character in charge-lattice coupling. These theoretical insights offer a direct interpretation of optical and spectroscopic experiments, providing strong evidence of anisotropic multi-centered polaronic behavior in BiVO$_{4}$. The presence of multiple in-gap states, especially from multi-centered polarons, introduces new channels for charge transport and recombination, possibly offering opportunities to control carrier dynamics in nanoscale photocatalytic and optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2025

Temperature dependent ferroelectricity in strained KTaO3 with machine learned force field

Ferroelectric materials are a class of dielectrics that exhibit spontaneous polarization which can be reversed under an external electric field. The emergence of ferroelectric order in incipient ferroelectrics is a topic of considerable interest from both fundamental and applied perspectives. Among the various strategies explored, strain engineering has been proven to be a powerful method for tuning ferroelectric polarization in materials. In the case of KTaO3, first principles calculations have suggested that strain can drive a ferroelectric phase transition. In this study, we investigate the impact of in-plane uniaxial and biaxial strain, ranging from 0% to 1%, on pristine KTaO3 to explore its potential for ferroelectricity induction via inversion symmetry breaking. By integrating density functional theory calculations with the stochastic self-consistent harmonic approximation assisted by on the fly machine learned force field, we obtain accurate structural information and dynamical properties under varying strain conditions while incorporating higher-order anharmonic effects. Employing the Berry phase method, we obtained the ferroelectric polarization of the strained structures over the entire temperature range up to 300 K. Our findings provide valuable insights into the role of strain in stabilizing ferroelectricity in KTaO3, offering guidance for future experimental and theoretical studies on strain-engineered ferroelectric materials.

cond-mat.mtrl-sci↗

arXiv2025

AMaRaNTA: Automated First-Principles Exchange Parameters In 2D Magnets

Two-dimensional (2D) magnets host a wide range of exotic magnetic textures, whose low-energy excitations and finite-temperature properties are typically described by effective spin models based on Heisenberg-like Hamiltonians. A key challenge in this framework is the reliable determination, from ab initio calculations, of exchange parameters and their anisotropic components, crucial for stabilising long-range order. Among the different strategies proposed for this task, the energy-mapping method -- based on total-energy calculations within Density Functional Theory (DFT) -- is the most widely adopted, but it typically requires laborious, multi-step procedures. To overcome this limitation, we introduce AMaRaNTA (Automating Magnetic paRAmeters iN a Tensorial Approach), a computational package that systematically automates the energy-mapping method, specifically through its ``four-state'' formulation, to extract exchange and anisotropy parameters in 2D magnets. In its current implementation, AMaRaNTA returns the nearest-neighbour exchange tensor, complemented by scalar parameters for second- and third-nearest-neighbour exchange interactions as well as single-ion anisotropy. Together, these provide a minimal yet sufficient set of parameters to capture magnetic frustration and anisotropies, essential for stabilising several observed magnetic states in 2D materials. Applied to a representative subset of the Materials Cloud 2D Structure database, AMaRaNTA demonstrates robust, automated and reproducible screening of magnetic interactions, with clear potential for high-throughput simulations.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic behavior of the $5d^1$ Re-based double perovskite Sr$_2$ZnReO$_6$

The subtle interplay between spin-orbit coupling, exchange interactions, and cation ordering can lead to exotic magnetic states in transition-metal ions. We report a comprehensive study of the Re-based (5$d^1$) ordered double perovskite oxide Sr$_2$ZnReO$_6$ combining synchrotron x-ray diffraction (XRD), magnetic susceptibility, muon spin relaxation ($\mu$SR) measurements, and density functional theory (DFT) calculations. XRD reveals that Sr$_2$ZnReO$_6$ crystallizes in the monoclinic structure (space group $P2_1/n$) at low temperature. Magnetic susceptibility data indicate a transition below $\sim$13 K, with $M$--$H$ loops showing ferromagnetic-like hysteresis and an unusually high coercive field of 23 kOe at 2 K. Zero-field $\mu$SR measurements detect static and spatially disordered internal fields below $T_M \simeq $ 12 K, consistent with a canted antiferromagnetic ground state determined by detailed DFT and force-theorem in Hubbard-I calculations. The reduced high-temperature effective moment ($\sim0.76~\mu_B$) and very small static moment ($\lesssim 0.2~\mu_B$) derived from $\mu$SR analysis and local-field simulations indicate a decisive role of spin-orbit coupling. Through a combined experimental and computational approach we unambiguously determine the canted antiferromagnetic order in Sr$_2$ZnReO$_6$, showing that a very small ordered moment coexists with an exceptionally large coercivity. These results underscore the crucial role of spin-orbit coupling and orbital ordering, providing new insights into magnetism in 5$d^1$ double perovskites.

cond-mat.str-el↗

arXiv2025

Hidden orders in spin-orbit entangled correlated insulators

In many materials, ordered phases and their order parameters are easily characterized by standard experimental methods. "Hidden order" refers to a phase transition in which an ordered state emerges without such an easily detectable order parameter, despite clear thermodynamic evidence of the transition. The underlying mechanisms for these unconventional states of matter stem from spin-orbit coupling, which intertwines inter-site exchange, classical electron-magnetic interactions, and electron-lattice effects. This physics is elusive to experimental probes and beyond traditional theories of insulating magnetism, requiring sophisticated methodologies for its exploration. In this Review, we survey exotic hidden-order phases in correlated insulators, particularly focusing on the latest progress in material-specific theories and numerical approaches. The relevant degrees of freedom in these phases are local high-rank multipole moments of magnetic and charge density that emerge from spin-orbit entangled correlated shells of heavy d and f electron ions and interact on the lattice via various mechanisms. We discuss approaches to modelling hidden orders in realistic systems via direct ab initio calculations or by constructing low-energy many-body effective Hamiltonian. We also describe how these new theoretical tools have helped to uncover driving mechanisms for recently discovered multipolar phases in double perovskites of heavy transition metals, and how they have proved instrumental in disentangling the role of various interactions in "traditional" f-electron multipolar materials like actinide dioxides. In both cases, material-specific theories have played a key role in interpreting and predicting experimental signatures of hidden orders.

cond-mat.str-el↗

arXiv2025

Multi-Technique Characterization of Rhodium Gem-Dicarbonyls on TiO$_2$(110)

Gem-dicarbonyls of transition metals supported on metal (oxide) surfaces are common intermediates in heterogeneous catalysis. While infrared (IR) spectroscopy is a standard tool for detecting these species on applied catalysts, the ill-defined crystallographic environment of species observed on powder catalysts renders data interpretation challenging. In this work, we apply a multi-technique surface science approach to investigate rhodium gem-dicarbonyls on a single-crystalline rutile TiO$_2$(110) surface. We combine spectroscopy, scanning probe microscopy, and Density Functional Theory (DFT) to determine their location and coordination on the surface. IR spectroscopy shows the successful creation of gem-dicarbonyls on a titania single crystal by exposing deposited Rh atoms to CO gas, followed by annealing to 200-250 K. Low-temperature scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) data reveal that these complexes are mostly aligned along the [001] crystallographic direction, corroborating theoretical predictions. Notably, x-ray photoelectron spectroscopy (XPS) data reveal multiple rhodium species on the surface, even when the IR spectra show only the signature of rhodium gem-dicarbonyls. As such, our results highlight the complex behavior of carbonyls on metal oxide surfaces, and demonstrate the necessity of multi-technique approaches for the adequate characterization of single-atom catalysts.

physics.chem-ph↗

arXiv2025

Polarons with arbitrary nonlinear electron-phonon interaction

We develop an exact computational method based on numerical X-propagators for solving polaron models with arbitrary nonlinear couplings of local vibration modes to the electron density and magnitude of the hopping amplitude. Our approach covers various polaron models, some of which were impossible to treat by any existing approximation-free techniques. Moreover, it remains efficient in the most relevant but computationally challenging regime of phonon frequencies much smaller than the electron bandwidth. As a case study, we consider the double-well type nonlinear model with quadratic ($g_2<0$) and quartic ($g_4>0$) interactions describing a broad class of technologically important materials, such as quantum paraelectric compounds and halide perovskites. We observe, depending on the model parameters, three qualitatively different regimes: (i) quantum interplay of quartic and quadratic interactions which suppresses effects of the quadratic coupling, (ii) intermediate-coupling regime with exponential $\propto \exp(\alpha g_2 \Omega^{-1/4})$ scaling of the quasiparticle weight and mass renormalization, and (iii) strong-coupling asymptotic behavior.

cond-mat.str-el↗

arXiv2025

Total Variation-Based Image Decomposition and Denoising for Microscopy Images

Experimentally acquired microscopy images are unavoidably affected by the presence of noise and other unwanted signals, which degrade their quality and might hide relevant features. With the recent increase in image acquisition rate, modern denoising and restoration solutions become necessary. This study focuses on image decomposition and denoising of microscopy images through a workflow based on total variation (TV), addressing images obtained from various microscopy techniques, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning electron microscopy (SEM). Our approach consists in restoring an image by extracting its unwanted signal components and subtracting them from the raw one, or by denoising it. We evaluate the performance of TV-$L^1$, Huber-ROF, and TGV-$L^1$ in achieving this goal in distinct study cases. Huber-ROF proved to be the most flexible one, while TGV-$L^1$ is the most suitable for denoising. Our results suggest a wider applicability of this method in microscopy, restricted not only to STM, AFM, and SEM images. The Python code used for this study is publicly available as part of AiSurf. It is designed to be integrated into experimental workflows for image acquisition or can be used to denoise previously acquired images.

eess.IV↗

arXiv2025

Coupling between small polarons and ferroelectricity in BaTiO3

In this study, we investigate the formation of electron and hole small polarons in the prototypical ferroelectric material BaTiO3, with a focus on their interaction with ferroelectric distortive fields. To accurately describe the ferroelectric phase in electronically correlated BaTiO3, we employ the HSE06 hybrid density functional, which addresses the limitations of conventional density functional theory (DFT) and Hubbard-corrected DFT+U models, providing a more precise depiction of both ferroelectric and polaronic behaviors. Our analysis spans three structural phases of BaTiO3: cubic, tetragonal, and rhombohedral. We uncover a unique phase-dependent trend in electron polaron stability, which progressively increases across the structural phases, peaking in the rhombohedral phase due to the constructive coupling between the polaron and ferroelectric phonon fields. In contrast, hole polarons exhibit a stability pattern largely unaffected by the phase transitions. Furthermore, we observe that polaron self-trapping significantly alters the local ferroelectric distortive pattern, which propagates to neighboring sites but has a minimal effect on the long-range macroscopic spontaneous polarization. Charge trapping is also associated with localized spin formation, opening new possibilities for enhanced functionalities in multiferroic materials.

cond-mat.mtrl-sci↗