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Giulio Monaco

Publications and source records attributed to Giulio Monaco.

18 recordsLinked to original sources

Recent Advances in Metallic Glasses

This paper reviews recent advances in the field of metallic glasses, focusing on the development of novel experimental techniques and in silico models. We discuss progress in experimental characterization, additive manufacturing, multiscale modeling approaches, and the growing role of machine learning in understanding and designing these complex materials. On the experimental side, we highlight measurements of thermophysical properties of supercooled liquids via fast chip calorimetry and enhancements in mechanical properties through rejuvenation treatments. This work underscores the crucial role of short-range order and medium-range order in controlling metallic glass mechanical properties. Recent progress in structural probes allows in situ observations of deformation mechanisms, positioning the field well to further advance our understanding of mechanical properties. Additive manufacturing of metallic glasses is discussed as one encouraging new manufacturing route for metallic glasses. We examine laser powder-bed fusion process physics and the central trade-off between amorphicity and densification, including heat affected zone devitrification and defects formation, together with emerging mitigation strategies and applications. On the theoretical and simulation side, we review advances in nanoscale, mesoscale, and continuum modeling of metallic glasses that have led to promising approaches by which multiscale schemes can incorporate data sourced from atomic-scale simulations. These efforts have helped to elucidate the connection between the glass structure and mechanical and rheological responses. We also cover the development of machine learning interatomic potentials for metallic glasses, along with machine learning driven prediction of glass forming ability and inverse design methods. Finally, challenges and directions for future research are presented and discussed.

cond-mat.mtrl-sci

A low-background setup for in-situ X-ray total scattering combined with fast scanning calorimetry

We demonstrate a setup combining fast scanning calorimetry with X-ray total scattering at a synchrotron beamline, allowing for \emph{in-situ} characterizations of the nano-scale structure of samples during and after temperature scans. The setup features a portable vacuum chamber giving high signal-to-background ratio even on amorphous samples, which enables the observation of detailed structural changes between different sample states. We show three use cases, including one which leverages the high cooling rate of 10$^4$\,K/s achievable by this setup. Our demonstration opens the door to various applications in materials science where it is important to understand the interplay between structure and thermodynamics.

physics.ins-det

Speed of sound in methane under conditions of planetary interiors

We present direct observations of acoustic waves in warm dense matter. We analyze wave-number- and energy-resolved x-ray spectra taken from warm dense methane created by laser heating a cryogenic liquid jet. X-ray diffraction and inelastic free-electron scattering yield sample conditions of 0.3$\pm$0.1 eV and 0.8$\pm$0.1 g/cm$^3$, corresponding to a pressure of $\sim$13 GPa. Inelastic x-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of $\sim$50 meV, we could clearly distinguish the Brillouin peaks from the quasielastic Rayleigh feature. Data at different wave numbers were utilized to derive a sound speed of 5.9$\pm$0.5 km/s, marking a high-temperature data point for methane and demonstrating consistency with Birch's law in this parameter regime.

physics.plasm-ph

Structural changes across thermodynamic maxima in supercooled liquid tellurium: a water-like scenario

Liquid polymorphism is an intriguing phenomenon which has been found in a few single-component systems, the most famous being water. By supercooling liquid Te to more than 130 K below its melting point and performing simultaneous small-angle and wide-angle X-ray scattering measurements, we observe clear maxima in its thermodynamic response functions around 615 K, suggesting the possible existence of liquid polymorphism. A close look at the underlying structural evolution shows the development of intermediate-range order upon cooling, most strongly around the thermodynamic maxima, which we attribute to bond-orientational ordering. The striking similarities between our results and those of water, despite the lack of hydrogen-bonding and tetrahedrality in tellurium, indicate that water-like anomalies may be a general phenomenon among liquid systems with competing bond- and density-ordering.

cond-mat.soft

Universal two-component dynamics in supercritical fluids

Despite the technological importance of supercritical fluids, controversy remains about the details of their microscopic dynamics. In this work, we study four supercritical fluid systems -- water, Si, Te, and Lennard-Jones fluid -- \emph{via} classical molecular dynamics simulations. A universal two-component behavior is observed in the intermolecular dynamics of these systems, and the changing ratio between the two components leads to a crossover from liquidlike to gaslike dynamics, most rapidly around the Widom line. We find evidence to connect the liquidlike component dominating at lower temperatures with intermolecular bonding, and the component prominent at higher temperatures with free-particle, gaslike dynamics. The ratio between the components can be used to describe important properties of the fluid, such as its self-diffusion coefficient, in the transition region. Our results provide insight into the fundamental mechanism controlling the dynamics of supercritical fluids, and highlight the role of spatiotemporally inhomogenous dynamics even in thermodynamic states where no large-scale fluctuations exist in the fluid.

cond-mat.soft

Vibrational dynamics of non-crystalline solids

The boson peak (BP) is an excess of vibrational states over the Debye law appearing at terahertz frequencies. It is found in all glasses and marks the crossover between the long-wavelength behavior, where the solid can be considered as an isotropic continuum, and the region where the wavelength of the sound wave starts to experience the microscopic details of the structure. This chapter is devoted to a review of the main experimental observations regarding the vibrational dynamics of amorphous solids, as detected by neutron and X-ray scattering techniques. A first part of the chapter is devoted to the measurements of the BP and its evolution as a function of external parameters, such as temperature, pressure, or density. The second part of the chapter reviews the wavevector evolution of the dynamic structure factor, which provides evidence of a pseudo-acoustic propagating mode up to frequencies comparable to those of the BP. This longitudinal mode has a sound attenuation which follows the Rayleigh scattering law and a negative dispersion that can partially explain the deviation from the Debye law. At higher frequencies, the inelastic spectrum presents a complex pattern of vibrations, with evidences of two peaks in various systems. To conclude, we will highlight the information that can be gained on the nature of the glass vibrational modes from a comparison with the dynamics of the corresponding polycrystal.

cond-mat.dis-nn

Two-component dynamics and the liquid-like to gas-like crossover in supercritical water

Molecular-scale dynamics in sub- to super-critical water is studied with inelastic X-ray scattering and molecular dynamics simulations. The obtained longitudinal current correlation spectra can be decomposed into two main components: a low-frequency (LF), gas-like component and a high-frequency (HF) component arising from the O--O stretching mode between hydrogen-bonded molecules, reminiscent of the longitudinal acoustic mode in ambient water. With increasing temperature, the hydrogen-bond network diminishes and the spectral weight shifts from HF to LF, leading to a transition from liquid-like to gas-like dynamics with rapid changes around the Widom line.

cond-mat.soft

A $j_\mathrm{eff} = 1/2$ pseudospinon continuum in CaIrO$_3$

In so-called $j_\mathrm{eff} = 1/2$ systems, including some iridates and ruthenates, the coherent superposition of $t_\mathrm{2g}$ orbitals in the ground state gives rise to hopping processes that strongly depend on the bond geometry. Resonant inelastic x-ray scattering (RIXS) measurements on CaIrO$_3$ reveal a prototypical $j_\mathrm{eff} = 1/2$ pseudospinon continuum, a hallmark of one-dimensional (1D) magnetic systems despite its three-dimensional crystal structure. The experimental spectra compare very well to the calculated magnetic dynamical structure factor of weakly coupled spin-1/2 chains. We attribute the onset of such quasi-1D magnetism to the fundamental difference in the magnetic interactions between the $j_\mathrm{eff} = 1/2$ pseudospins along the corner- and edge-sharing bonds in CaIrO$_3$.

cond-mat.str-el

Resonant inelastic x-ray scattering of magnetic excitations under pressure

Resonant inelastic x-ray scattering (RIXS) is an extremely valuable tool for the study of elementary, including magnetic, excitations in matter. Latest developments of this technique mostly aimed at improving the energy resolution and performing polarization analysis of the scattered radiation, with a great impact on the interpretation and applicability of RIXS. Instead, this article focuses on the sample environment and presents a setup for high-pressure low-temperature RIXS measurements of low-energy excitations. The feasibility of these experiments is proved by probing the magnetic excitations of the bilayer iridate Sr$_3$Ir$_2$O$_7$ at pressures up to 12 GPa.

cond-mat.str-el

Scientific Opportunities with an X-ray Free-Electron Laser Oscillator

An X-ray free-electron laser oscillator (XFELO) is a new type of hard X-ray source that would produce fully coherent pulses with meV bandwidth and stable intensity. The XFELO complements existing sources based on self-amplified spontaneous emission (SASE) from high-gain X-ray free-electron lasers (XFEL) that produce ultra-short pulses with broad-band chaotic spectra. This report is based on discussions of scientific opportunities enabled by an XFELO during a workshop held at SLAC on June 29 - July 1, 2016

physics.ins-det

Relaxation dynamics induced in glasses by the absorption of hard X-ray photons

X-ray photon correlation is used to probe the slow dynamics of the glass-former B2O3 across the glass transition. In the undercooled liquid phase the decay times of the measured correlation functions are consistent with visible light scattering results and independent of the incoming flux; in the glass they are instead temperature independent and show a definite dependence on the X-ray flux. This dependence can be exploited to obtain information on the volume occupied by the atoms that move in the glass following an absorption event. The length scale derived in this way, of the order of the nanometer, is consistent with that reported for the dynamical heterogeneities, suggesting the existence of a new scheme to get access to this fundamental quantity.

cond-mat.dis-nn

Resonant X-ray Emission Spectroscopy With a Standing Wave Excitation

The Borrmann effect is the anomalous transmission of x rays in perfect crystals under diffraction conditions. It arises from the interference of the incident and diffracted waves, which creates a standing wave with nodes at strongly absorbing atoms. Dipolar absorption of x rays is thus diminished, which makes the crystal nearly transparent for certain x-ray wave vectors. Indeed, a relative enhancement of electric quadrupole absorption via the Borrmann effect has been demonstrated recently. Here we show that the Borrmann effect has a significantly larger impact on resonant x-ray emission than is observable in x-ray absorption. Emission from a dipole forbidden intermediate state may even dominate the corresponding x-ray spectra. Our work extends the domain of x-ray standing wave methods to resonant x-ray emission spectroscopy and provides means for novel spectroscopic experiments in d- and f-electron systems.

cond-mat.mtrl-sci

A computationally efficient method to solve the Takagi-Taupin equations for a large deformed crystal

We present a treatise on solving the Takagi-Taupin equations in the case of a strain field with an additional, spatially slowly varying component (owing to \emph{e.g.}~heat expansion or angular compression). We show that the presence of such a component in a typical case merely shifts the reflectivity curve as a function of wavelength or incidence angle, while having a negligible effect on its shape. On the basis of the derived result, we develop a computationally efficient method to calculate the reflectivity curve of a large deformed crystal. The validity of the method is demonstrated by comparing computed reflectivity curves with experimental ones for bent silicon wafers. A good agreement is observed.

physics.optics

Temperature dependence of crystal field excitations in CuO

We report a study on the temperature dependence of the charge-neutral crystal field (dd) excitations in cupric oxide, using nonresonant inelastic x-ray scattering (IXS) spectroscopy. Thanks to a very high energy resolution (60 meV), we observe thermal effects on the dd excitation spectrum fine structure between temperatures of 10-320 K. With an increasing temperature, the spectra broaden considerably. We assign the temperature dependence of the dd excitations to the relatively large electron-phonon coupling.

cond-mat.str-el

Screening in YBCO at large wave vectors

We present experimental inelastic x-ray scattering (IXS) and ab initio time-dependent density-functional-theory (TDDFT) studies of YBa2Cu3O7-δ. The response of the low-lying Ba 5p and Y 4p core electrons is shown to interact strongly with the Cu 3d and O 2p excitations, with important consequences on screening. The agreement between IXS and TDDFT results is excellent, apart from a new type of excitations, mainly related to loosely bound Ba electrons and significantly affected by correlations. This points to correlation mechanisms not fully described by TDDFT that might have a role in giving rise to antiscreening.

cond-mat.supr-con

Anomalous properties of the acoustic excitations in glasses on the mesoscopic length-scale

The low-temperature thermal properties of dielectric crystals are governed by acoustic excitations with large wavelengths that are well described by plane waves. This is the Debye model, which rests on the assumption that the medium is an elastic continuum, holds true for acoustic wavelengths large on the microscopic scale fixed by the interatomic spacing, and gradually breaks down on approaching it. Glasses are characterized as well by universal low-temperature thermal properties, that are however anomalous with respect to those of the corresponding crystalline phases. Related universal anomalies also appear in the low-frequency vibrational density of states and, despite of a longstanding debate, still remain poorly understood. Using molecular dynamics simulations of a model monatomic glass of extremely large size, we show that in glasses the structural disorder undermines the Debye model in a subtle way: the elastic continuum approximation for the acoustic excitations breaks down abruptly on the mesoscopic, medium-range-order length-scale of about ten interatomic spacings, where it still works well for the corresponding crystalline systems. On this scale, the sound velocity shows a marked reduction with respect to the macroscopic value. This turns out to be closely related to the universal excess over the Debye model prediction found in glasses at frequencies of ~1 THz in the vibrational density of states or at temperatures of ~10 K in the specific heat.

cond-mat.stat-mech

Is the Fragility of a Liquid Embedded in the Properties of its Glass?

When a liquid is cooled below its melting temperature it usually crystallizes. However, if the quenching rate is fast enough, it is possible that the system remains in a disordered state, progressively losing its fluidity upon further cooling. When the time needed for the rearrangement of the local atomic structure reaches approximately 100 seconds, the system becomes "solid" for any practical purpose, and this defines the glass transition temperature $T_g$. Approaching this transition from the liquid side, different systems show qualitatively different temperature dependencies of the viscosity, and, accordingly, they have been classified introducing the concept of "fragility". We report experimental observations that relate the microscopic properties of the {\it glassy phase} to the fragility. We find that the vibrational properties of the glass {\it well below} $T_g$ are correlated with the fragility value. Consequently, we extend the fragility concept to the glassy state and indicate how to determine the fragility uniquely from glass properties well below $T_g$.

cond-mat.dis-nn

Density fluctuations in the intermediate glass-former glycerol: a Brillouin light scattering study

Brillouin scattering has been used to measure the dynamic structure factor of glycerol as a function of temperature from the high temperature liquid to the glassy state. Our investigation aims at understanding the number and the nature of the relaxation processes active in this prototype glass forming system in the high frequency region. The associated character of glycerol is reflected by a rather simple relaxations pattern, while the contributions coming from intra-molecular channels are negligible in the GHz frequency region. The temperature behavior of the characteristic frequency and lifetime of the longitudinal acoustic modes is analyzed, suggesting that a phenomenological model which only includes the structural ($α$) process and the unrelaxed viscosity is able to catch the leading contributions to the dynamics of the density fluctuations. This ansatz is also supported by a combined analysis of light and inelastic x-ray scattering spectra. The temperature dependence of the characteristic time of the $α$-process, $τ_α$, obtained by a full-spectrum analysis conforms to the $α$-scale universality, i.e. the values $τ_α$ revealed by different experimental techniques are proportional the ones to the others. The non-erogodicity parameter smoothly decreases on increasing the temperature, and no signature of the cusp-like behavior predicted by the idealized mode coupling theory and observed in other glass-formers is found in glycerol.

cond-mat.soft