arXiv subjects

Riccardo Mazzarello

Publications and source records attributed to Riccardo Mazzarello.

At least 19 recordsLinked to original sources

arXiv2026

Room-temperature antiferromagnetic resonance in NaMnAs

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs -- a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the Néel vector oriented along the tetragonal axis. At $ B = 0 $, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($ k = 0 $) magnon mode. Its energy softens considerably with increasing $ T $, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs in the range 0.1-0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Multiscale simulations guided advances for all-optical phase-change waveguides

Photonic computing using chalcogenide phase-change materials (PCMs) is under active development for energy-efficient artificial intelligence (AI) applications. A key requirement is to enable as many optically programmable levels per device as possible, while maintaining relatively low optical loss. In this work, we carry out multiscale simulations using density functional theory and finite-difference time-domain methods, proposing a "the shorter the better" strategy to optimize the performance of Sb2Te photonic waveguide devices. Our subsequent experimental characterizations of Sb2Te thin films and optical device measurements fully verify our theoretical predictions. In particular, we reveal the unconventional optical properties of metastable crystalline Sb2Te, and utilize these features for device design, yielding a simultaneous improvement in both the programming window and the optical loss. Overall, an optical programming precision exceeding 7-bit is achieved using a single waveguide cell, setting a new record for all-optical phase-change memory devices. Our work serves as a compelling example of computational material design, which demonstrates the predictive power of multiscale simulations in guiding the design of phase-change photonic devices for enhanced performance.

cond-mat.mtrl-sci↗

arXiv2026

Melting behavior and dynamical properties of Cr2Ge2Te6 phase-change material

Cr2Ge2Te6 (CrGT) is known as an intrinsic ferromagnetic semiconductor and a promising candidate for phase-change memory applications. In amorphous CrGT, Cr atoms form non-defective octahedral motifs with Te atoms, similar to those in the crystalline phase. The abundance of Cr[Te6] octahedra is regarded as the key structural factor in reducing the resistance drift coefficient of amorphous CrGT. However, the stage at which these octahedra emerge during melt-quench amorphization remains unclear. Here, we present ab initio molecular dynamics (AIMD) simulations to model the melting process of crystalline CrGT and to investigate the dynamical properties of liquid and supercooled liquid CrGT in detail. Upon heating, Ge atoms are observed to leave their lattice sites earlier than Cr and Te atoms, diffusing into the van der Waals gap and initiating the collapse of the layered structure. The Cr[Te6] octahedra are more robust, maintaining their structural pattern up to 1400 K despite continuous rupture and re-formation of Cr-Te bonds. At higher temperatures, Cr and Te atoms start to migrate independently. In supercooled liquid CrGT at 550 K, most Cr-centered octahedra remain intact, with only limited Cr-Te bond breaking. The collective motion of these octahedra in this temperature regime helps explain why crystallization in CrGT devices can be accomplished in tens of nanoseconds.

cond-mat.mtrl-sci↗

arXiv2026

Liquid anomalies and Fragility of Supercooled Antimony

Phase-change materials (PCMs) based on group IV, V, and VI elements, such as Ge, Sb, and Te, exhibit distinctive liquid-state features, including thermodynamic anomalies and unusual dynamical properties, which are believed to play a key role in their fast and reversible crystallization behavior. Antimony (Sb), a monoatomic PCM with ultrafast switching capabilities, stands out as the only elemental member of this group for which the properties of the liquid and supercooled states have so far remained unknown. In this work, we use large-scale molecular dynamics simulations with a neural network potential trained on first-principles data to investigate the liquid, supercooled, and amorphous phases of Sb across a broad pressure-temperature range. We uncover clear signatures of anomalous behavior, including a density maximum and non-monotonic thermodynamic response functions, and introduce a novel octahedral order parameter that captures the structural evolution of the liquid. Moreover, extrapolation of the viscosity to the glass transition, based on configurational and excess entropies, indicates that Sb is a highly fragile material. Our results present a compelling new case for the connection between the liquid-state properties of phase-change materials and their unique ability to combine high amorphous-phase stability with ultrafast crystallization.

cond-mat.mtrl-sci↗

arXiv2025

Atomistic understanding of two-dimensional monatomic phase-change material for non-volatile optical applications

Elemental antimony (Sb) is a promising material for phase-change memory, neuromorphic computing and nanophotonic applications, because its compositional simplicity can prevent phase segregation upon extensive programming. Scaling down the film thickness is a necessary step to prolong the lifetime of amorphous Sb, but the optical properties of Sb are also significantly altered as the thickness is reduced to a few nanometers, adding complexity to device optimization. In this work, we aim to provide atomistic understanding of the thickness-dependent optical responses in Sb thin films. As thickness decreases, both the extinction coefficient and optical contrast reduce in the near-infrared spectrum, consistent with previous optical measurements. Such thickness dependence gives rise to a bottom thickness limit of 2 nm in photonic applications, as predicted by coarse-grained device simulations. Further bonding analysis reveals a fundamentally different behavior for amorphous and crystalline Sb upon downscaling, resulting in the reduction of optical contrast. Thin film experiments are also carried out to validate our predictions. The thickness-dependent optical trend is fully demonstrated by our ellipsometric spectroscopy experiments, and the bottom thickness limit of 2 nm is confirmed by structural characterization experiments. Finally, we show that the greatly improved amorphous-phase stability of the 2 nm Sb thin film enables robust and reversible optical switching in a silicon-based waveguide device.

cond-mat.mtrl-sci↗

arXiv2025

Role of seed layer in growing atomically flat TiTe2/Sb2Te3 heterostructure thin films at the wafer scale

Chalcogenide phase-change materials (PCMs) are a leading candidate for advanced memory and computing applications. Epitaxial-like growth of chalcogenide thin films at the wafer scale is important to guarantee the homogeneity of the thin film but is challenging with magnetron sputtering, particularly for the growth of phase-change heterostructure (PCH), such as TiTe2/Sb2Te3. In this work, we report how to obtain highly textured TiTe2/Sb2Te3 heterostructure thin films with atomically sharp interfaces on standard silicon substrates. By combining atomic-scale characterization and ab initio simulations, we reveal the critical role of the Sb2Te3 seed layer in forming a continuous Si-Sb-Te mixed transition layer, which provides a wafer-scale flat surface for the subsequent epitaxial-like growth of TiTe2/Sb2Te3 thin film. By gradually reducing the thickness of the seed layer, we determine its critical limit to be ~2 nm. Non-negligible in-plane tensile strain was observed in the TiTe2 slabs due to the lattice mismatch with the adjacent Sb2Te3 ones, suggesting that the chemical interaction across the structural gaps in the heterostructure is stronger than a pure van der Waals interaction. Finally, we outline the potential choices of chalcogenides for atomically flat seed layers on standard silicon substrates, which can be used for wafer-scale synthesis of other high-quality PCM or PCH thin films.

cond-mat.mtrl-sci↗

arXiv2025

From Continuous to First-Order-Like: Amorphous-to-Amorphous Transition in Phase-Change Materials

Polymorphism is ubiquitous in crystalline solids. Amorphous solids, such as glassy water and silicon, may undergo amorphous-to-amorphous transitions (AATs). The nature of AATs remains ambiguous, due to diverse system-dependent behaviors and experimental challenges to characterize disordered structures. Here, we identify two ordered motifs in amorphous phase-change materials and monitor their interplay upon pressure-induced AATs. Tuning temperature, we find a crossover from continuous to first-order-like AATs. The crossover emerges at a special pressure-temperature combination, where the AAT encounters a maximum in crystallization rate. Analyzing the two ordered motifs in a two-state model, we draw a phenomenological parallel to the phase transition behavior of supercooled water near its second critical point. This analogy raises an intriguing question regarding the existence of a critical-like point within amorphous solids.

cond-mat.mtrl-sci↗

arXiv2025

Spin glass behavior in amorphous CrSiTe3 alloy

Owing to the intrinsically high crystallization temperatures, layered phase-change materials, such as CrGeTe3 and InGeTe3, are attracting attention for embedded memory applications, In addition to the electrical contrast, a major change in magnetic properties is observed in CrGeTe3 upon switching from the crystalline to the amorphous state. In this work, we report a combined ab initio modeling and magnetic characterization study on the isostructural silicon parent compound of CrGeTe3, namely, CrSiTe3. Amorphous CrSiTe3 has similar structural properties to amorphous CrGeTe3; however, it shows a smaller energy difference between the ferromagnetic configuration and the random magnetic configuration, indicating a high probability of spin glass formation. Indeed, direct-current and alternating-current magnetic measurements show that the coercive force of amorphous CrSiTe3 is higher than that of amorphous CrGeTe3. Therefore, the pinning effect of spins is enhanced in amorphous CrSiTe3, leading to a more robust spin glass state with a higher freezing temperature. The large magnetic contrast between the amorphous and crystalline phase could make CrSiTe3 a potential candidate for phase-change magnetic switching applications.

cond-mat.mtrl-sci↗

arXiv2024

Ab initio investigation of layered TMGeTe3 alloys for phase-change applications

Chalcogenide phase-change materials (PCMs) are one of the most mature candidates for next-generation memory technology. Recently, CrGeTe3 (CrGT) emerged as a promising PCM due to its enhanced amorphous stability and fast crystallization for embedded memory applications. The amorphous stability of CrGT was attributed to the complex layered structure of the crystalline motifs needed to initiate crystallization. A subsequent computational screening work identified several similar compounds with good thermal stability, such as InGeTe3, CrSiTe3 and BiSiTe3. Here, we explore substitution of Cr in CrGT with other 3d metals, and predict four additional layered alloys to be dynamically stable, namely, ScGeTe3, TiGeTe3, ZnGeTe3 and MnGeTe3. Thorough ab initio simulations performed on both crystalline and amorphous models of these materials indicate the former three alloys to be potential PCMs with sizable resistance contrast. Furthermore, we find that crystalline MnGeTe3 exhibits ferromagnetic behavior, whereas the amorphous state probably forms a spin-glass phase. This makes MnGeTe3 a promising candidate for magnetic phase-change applications.

cond-mat.mtrl-sci↗

arXiv2023

Multiscale simulations of growth-dominated Sb$_2$Te phase-change material for non-volatile photonic applications

Chalcogenide phase-change materials (PCMs) are widely applied in electronic and photonic applications, such as non-volatile memory and neuro-inspired computing. Doped Sb$_2$Te alloys are now gaining increasing attention for on-chip photonic applications, due to their growth-driven crystallization features. However, it remains unknown whether Sb$_2$Te also forms a metastable crystalline phase upon nanoseconds crystallization in devices, similar to the case of nucleation-driven Ge-Sb-Te alloys. Here, we carry out ab initio simulations to understand the changes in optical properties of amorphous Sb$_2$Te upon crystallization and post annealing. During the continuous transformation process, changes in the dielectric function are highly wavelength-dependent from the visible-light range towards the telecommunication band. Our finite-difference time-domain simulations based on the ab initio input reveal key differences in device output for color display and photonic memory applications upon tellurium ordering. Our work serves as an example of how multiscale simulations of materials can guide practical photonic phase-change applications.

cond-mat.mtrl-sci↗

arXiv2023

Spin glass behavior in amorphous Cr2Ge2Te6 phase-change alloy

The layered crystal structure of Cr2Ge2Te6 shows ferromagnetic ordering at the two-dimensional limit, which holds promise for spintronic applications. However, external voltage pulses can trigger amorphization of the material in nanoscale electronic devices, and it is unclear whether the loss of structural ordering leads to a change in magnetic properties. Here, we demonstrate that Cr2Ge2Te6 preserves the spin-polarized nature in the amorphous phase, but undergoes a magnetic transition to a spin glass state below 20 K. Quantum-mechanical computations reveal the microscopic origin of this transition in spin configuration: it is due to strong distortions of the Cr-Te-Cr bonds, connecting chromium-centered octahedra, and to the overall increase in disorder upon amorphization. The tunable magnetic properties of Cr2Ge2Te6 could be exploited for multifunctional, magnetic phase-change devices that switch between crystalline and amorphous states.

cond-mat.mtrl-sci↗

arXiv2022

In situ characterization of vacancy ordering in Ge-Sb-Te phase-change memory alloys

Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microscopy (HRTEM) experiments and ab initio density functional theory (DFT) calculations, providing a comprehensive real-time and real-space view of the vacancy ordering process. We also discuss the impact of vacancy ordering on altering the electronic and optical properties of GeSb2Te4, which is relevant to multilevel storage applications. The phase evolution paths in Ge-Sb-Te alloys are illustrated using a summary diagram, which serves as a guide for designing phase-change memory devices.

cond-mat.mtrl-sci↗

arXiv2021

Unraveling the optical contrast in Sb2Te and AgInSbTe phase-change materials

Chalcogenide phase-change materials (PCMs) show a significant contrast in optical reflectivity and electrical resistivity upon crystallization from the amorphous phase and are leading candidates for non-volatile photonic and electronic applications. In addition to the flagship Ge2Sb2Te5 phase-change alloy, doped Sb2Te alloys, in particular AgInSbTe used in rewritable optical discs, have been widely investigated for decades, and nevertheless the theoretical insights on the optical properties of this important family of PCMs are scarce. Here, we carry out thorough ab initio simulations to gain an atomistic understanding of the optical properties of Sb2Te and AgInSbTe. We show that the large optical contrast between the amorphous and crystalline phase stems from the change in bond type in the parent compound Sb2Te. Ag and In impurities serve mostly the purpose of stabilization of the amorphous phase, and have marginal impact on the large variation in the dielectric function upon the phase transitions.

cond-mat.mtrl-sci↗

arXiv2019

Monatomic phase change memory

Phase change memory has been developed into a mature technology capable of storing information in a fast and non-volatile way, with potential for neuromorphic computing applications. However, its future impact in electronics depends crucially on how the materials at the core of this technology adapt to the requirements arising from continued scaling towards higher device densities. A common strategy to finetune the properties of phase change memory materials, reaching reasonable thermal stability in optical data storage, relies on mixing precise amounts of different dopants, resulting often in quaternary or even more complicated compounds. Here we show how the simplest material imaginable, a single element (in this case, antimony), can become a valid alternative when confined in extremely small volumes. This compositional simplification eliminates problems related to unwanted deviations from the optimized stoichiometry in the switching volume, which become increasingly pressing when devices are aggressively miniaturized. Removing compositional optimization issues may allow one to capitalize on nanosize effects in information storage.

physics.app-ph↗

arXiv2013

Structural and electronic properties of hybrid graphene and boron nitride nanostructures on Cu

Recently, two-dimensional nanostructures consisting of alternating graphene and boron nitride (BN) domains have been synthesized. These systems possess interesting electronic and mechanical properties, with potential applications in electronics and optical devices. Here, we perform a first-principles investigation of models of BN-C hybrid monolayers and nanoribbons deposited on the Cu(111) surface, a substrate used for their growth in said experiments. For the sake of comparison, we also consider BN and BC2N nanostructures. We show that BN and BC2N monolayers bind weakly to Cu(111), whereas monolayers with alternating domains interact strongly with the substrate at the B-C interface, due to the presence of localized interface states. This binding leads to a deformation of the monolayers and sizable n-doping. Nanoribbons exhibit a similar behaviour. Furthermore, they also interact significantly with the substrate at the edge, even in the case of passivated edges. These findings suggest a route to tune the band gap and doping level of BN-C hybrid models based on the interplay between nanostructuring and substrate-induced effects.

cond-mat.mes-hall↗

arXiv2013

Electronic and magnetic properties of zigzag graphene nanoribbons on the (111) surface of Cu, Ag and Au

We have carried out an ab initio study of the structural, electronic and magnetic properties of zigzag graphene nanoribbons on Cu(111), Ag(111) and Au(111). Both, H-free and H-terminated nanoribbons are considered revealing that the nanoribbons invariably possess edge states when deposited on these surfaces. In spite of this, they do not exhibit a significant magnetization at the edge, with the exception of H-terminated nanoribbons on Au(111), whose zero-temperature magnetic properties are comparable to those of free-standing nanoribbons. These results are explained by the different hybridization between the graphene 2p orbitals and those of the substrates and, for some models, by the sizable charge transfer between the surface and the nanoribbon. Interestingly, H-free nanoribbons on Au(111) and Ag(111) exhibit two main peaks in the local density of states around the Fermi energy, which originate from different states and, thus, do not indicate edge magnetism.

cond-mat.mes-hall↗

arXiv2012

Weak antilocalization and disorder-enhanced electron interactions in crystalline GeSbTe

Phase change materials can be reversibly switched between amorphous and crystalline states and often show strong contrast in the optical and electrical properties of these two phases. They are now in widespread use for optical data storage, and their fast switching and a pronounced change of resistivity upon crystallization are also very attractive for nonvolatile electronic data storage. Nevertheless there are still several open questions regarding the electronic states and charge transport in these compounds. In this work we study electrical transport in thin metallic films of the disordered, crystalline phase change material Ge$_1$Sb$_2$Te$_4$. We observe weak antilocalization and disorder enhanced Coulomb interaction effects at low temperatures, and separate the contributions of these two phenomena to the temperature dependence of the resistivity, Hall effect, and magnetoresistance. Strong spin-orbit scattering causes positive magnetoresistance at all temperatures, and a careful analysis of the low-field magnetoresistance allows us to extract the temperature dependent electron dephasing rate and study other scattering phenomena. We find electron dephasing due to inelastic electron-phonon scattering at higher temperatures, electron-electron scattering dephasing at intermediate temperatures, and a crossover to weak temperature dependence below 1 K.

cond-mat.mes-hall↗

arXiv2009

Kondo Conductance in an Atomic Nanocontact from First Principles

The electrical conductance of atomic metal contacts represents a powerful tool to detect nanomagnetism. Conductance reflects magnetism through anomalies at zero bias -- generally with Fano lineshapes -- due to the Kondo screening of the magnetic impurity bridging the contact. A full atomic-level understanding of this nutshell many-body system is of the greatest importance, especially in view of our increasing need to control nanocurrents by means of magnetism. Disappointingly, zero bias conductance anomalies are not presently calculable from atomistic scratch. In this Letter we demonstrate a working route connecting approximately but quantitatively density functional theory (DFT) and numerical renormalization group (NRG) approaches and leading to a first-principles conductance calculation for a nanocontact, exemplified by a Ni impurity in a Au nanowire. A Fano-like conductance lineshape is obtained microscopically, and shown to be controlled by the impurity s-level position. We also find a relationship between conductance anomaly and geometry, and uncover the possibility of opposite antiferromagnetic and ferromagnetic Kondo screening -- the latter exhibiting a totally different and unexplored zero bias anomaly. The present matching method between DFT and NRG should permit the quantitative understanding and exploration of this larger variety of Kondo phenomena at more general magnetic nanocontacts.

cond-mat.mes-hall↗