arXiv subjects

Svemir Rudić

Publications and source records attributed to Svemir Rudić.

6 recordsLinked to original sources

arXiv2026

Molecular neutron spectroscopy techniques applied to ceramics $α$-SiC and $β$-Ga$_2$O$_3$

Neutron spectroscopy is a powerful technique for determining the vibrational states of matter. Instruments with fixed geometry may measure inelastic scattering at a limited set of angles, producing a 1-D spectrum $S(ω)$. Such measurements are usually simulated in a DOS-like semi-analytic incoherent approximation, well-established for study of bending/stretching modes in molecular crystals. In this work we empirically test the simulation method for two ceramics with industrial electronic applications that act as "worst-case" systems. The phonon scattering from $α$-SiC and $β$-Ga$_2$O$_3$ is coherent, depends on momentum transfer $Q$ and sits in frequencies below the typical "fingerprint" range of molecular spectroscopy. Inelastic neutron-scattering measurements of powders were performed with two contrasting spectrometers at cryogenic and elevated temperatures, and simulations performed using a variety of density-functional approximations. We find that for 1-D powder spectra from a compact instrument, the approximate simulations are easily comparable with experimental spectra and give similar results to a more computationally-intensive numerical sampling of the coherent spectrum. Given the success with these systems, the approximate method appears to be suitable for modelling inelastic neutron scattering by harmonic phonons of almost any powder sample with this technique. When a $Q$-resolved instrument is used to collect the 2-D dynamical structure factor $S(Q,ω)$, numerical averaging is still required to capture phonon features. Our simulations of inelastic scattering from $α$-SiC in the 6H polytype using the PBEsol functional gave good agreement with the experiments. By contrast, the RSCAN functional gave the best agreement with the measured spectra of $β$-Ga$_2$O$_3$ and is recommended for future work on the lattice dynamics of this material.

cond-mat.mtrl-sci↗

arXiv2025

Predicting neutron experiments from first principles: A workflow powered by machine learning

Machine learning has emerged as a powerful tool in materials discovery, enabling the rapid design of novel materials with tailored properties for countless applications, including in the context of energy and sustainability. To ensure the reliability of these methods, however, rigorous validation against experimental data is essential. Scattering techniques -- using neutrons, X-rays, or electrons -- offer a direct way to probe atomic-scale structure and dynamics, making them ideal for this purpose. In this work, we describe a computational workflow that bridges machine learning-based simulations with experimental validation. The workflow combines density functional theory, machine-learned interatomic potentials, molecular dynamics, and autocorrelation function analysis to simulate experimental signatures, with a focus on inelastic neutron scattering. We demonstrate the approach on three representative systems: crystalline silicon, crystalline benzene, and hydrogenated scandium-doped BaTiO3, comparing the simulated spectra to measurements from four different neutron spectrometers. While our primary focus is inelastic neutron scattering, the workflow is readily extendable to other modalities, including diffraction and quasi-elastic scattering of neutrons, X-rays, and electrons. The good agreement between simulated and experimental results highlights the potential of this approach for guiding and interpreting experiments, while also pointing out areas for further improvement.

cond-mat.mtrl-sci↗

arXiv2021

Green reconstruction of MIL-100 (Fe) in water for high crystallinity and enhanced guest encapsulation

MIL-100 (Fe) is a highly porous metal-organic framework (MOF), considered as a promising carrier for drug delivery, and for gas separation and capture applications. However, this functional material suffers from elaborated and toxic synthesis that may hinder its biomedical use and large-scale production to afford commercial applications. Herein, we report a green mechanochemical water immersion approach to yield highly crystalline MIL100 (Fe) material. Subsequently, we have harnessed this strategy for facile fabrication of drug@MOF composite systems, comprising (guests) 5-fluorouracil, caffeine, or aspirin encapsulated in the pores of (host) MIL-100 (Fe). Inelastic neutron scattering was uniquely used to probe the guest host interactions arising from pore confinement of the drug molecules, giving additional insights into the reconstruction mechanism. Our results pave the way to the green production of MIL type materials and bespoke guest-encapsulated composites by minimizing the use of toxic chemicals, whilst enhancing energy efficiency and material's life cycle central to biotechnological applications.

cond-mat.mtrl-sci↗

arXiv2020

Secondary relaxation in the terahertz range in 2-adamantanone from theory and experiments

We applied the recently developed Generalized Langevin Equation (GLE) approach for dielectric response of liquids and glasses to link the vibrational density of states (VDOS) to the dielectric response of a model orientational glass (OG). The dielectric functions calculated based on the GLE, with VDOS obtained in experiments and simulations as inputs, are compared with experimental data for the paradigmatic case of 2-adamantanone at various temperatures. The memory function is related to the integral of the VDOS times a spectral coupling function $γ(ω_p)$, which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. With respect to previous empirical fittings, the GLE-based fitting reveals a broader temperature range over which the secondary relaxation is active. Furthermore, the theoretical analysis provides a clear evidence of secondary relaxation being localized within the THz ($0.5-1$ THz) range of eigenfrequencies, and thus not too far from the low-energy modes involved in $α$-relaxation. In the same THz region, the same material displays a crowding of low-energy optical modes that may be related to the secondary relaxation.

cond-mat.soft↗

arXiv2019

Emergence of glassy features in halomethane crystals

Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational, and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we studied the halomethane family C-Br_n-Cl_4-n (n = 0,1,2) at low temperature where, despite being perfectly translationally ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For n = 1,2 dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of CCl4. Measurements of the low-temperature specific heat (Cp) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced Cp/T^3 and in the reduced density of states g(w)/w^2 determined by means of neutron spectroscopy, as well as a linear term in Cp usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully ordered crystal. Being CCl4 a fully ordered crystal, we also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the boson peak of glasses, finding it to essentially arise from a piling up (at around 3 - 4 meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits.

cond-mat.mtrl-sci↗

arXiv2017

Detecting Molecular Rotational Dynamics Complementing the Low-Frequency Terahertz Vibrations in a Zirconium-Based Metal-Organic Framework

We show clear experimental evidence of co-operative terahertz (THz) dynamics observed below 3 THz (~100 cm-1), for a low-symmetry Zr-based metal-organic framework (MOF) structure, termed MIL-140A [ZrO(O2C-C6H4-CO2)]. Utilizing a combination of high-resolution inelastic neutron scattering and synchrotron radiation far-infrared spectroscopy, we measured low-energy vibrations originating from the hindered rotations of organic linkers, whose energy barriers and detailed dynamics have been elucidated via ab initio density functional theory (DFT) calculations. For completeness, we obtained Raman spectra and characterized the alterations to the complex pore architecture caused by the THz rotations. We discovered an array of soft modes with trampoline-like motions, which could potentially be the source of anomalous mechanical phenomena, such as negative linear compressibility and negative thermal expansion. Our results also demonstrate coordinated shear dynamics (~2.5 THz), a mechanism which we have shown to destabilize MOF crystals, in the exact crystallographic direction of the minimum shear modulus (Gmin).

cond-mat.mtrl-sci↗