Materials Science (cond-mat.mtrl-sci)

Abstract: Predicting nanoscale thermal transport in dielectrics requires models, such as the Boltzmann transport equation (BTE), that account for phonon boundary scattering in structures with complex geometries. Although the BTE has been validated against several key experiments, its computational expense limits its applicability. Here, we demonstrate the use of an analytic reduced-order model for predicting the thermal conductivity in dimensionally confined materials, i.e., monolithic and porous thin films, and rectangular and cylindrical nanowires. The approach uses the recently developed "Ballistic Correction Model" (BCM) which accounts for materials' full distribution of phonon mean-free-paths. The model is validated against BTE simulations for a selection of base materials, obtaining excellent agreement. By furnishing a precise yet easy-to-use prediction of thermal transport in nanostructures, our work strives to accelerate the identification of materials for energy-conversion and thermal-management applications.

Abstract: We investigate the electronic structure of $2H$-$\mathrm{Nb}\mathrm{S}_2$ and $h$$\mathrm{BN}$ by angle-resolved photoemission spectroscopy (ARPES) and photoemission intensity calculations. Although in bulk form, these materials are expected to exhibit band degeneracy in the $k_z=\pi/c$ plane due to screw rotation and time-reversal symmetries, we observe gapped band dispersion near the surface. We extract from first-principles calculations the near-surface electronic structure probed by ARPES and find that the calculated photoemission spectra from the near-surface region reproduce the gapped ARPES spectra. Our results show that the near-surface electronic structure can be qualitatively different from the bulk one due to partially broken nonsymmorphic symmetries.

Abstract: Electron-phonon coupling is a fundamental process that governs the energy relaxation dynamics of solids excited by ultrafast laser pulses. It has been found to strongly depend on electron temperature as well as on nonequilibrium effects. Recently, the effect of occupational nonequilibrium in noble metals, which outlasts the fully kinetic stage, has come into increased focus. In this work, we investigate the influence of nonequilibrium density distributions in gold on the electron-phonon coupling. We find a large effect on the coupling parameter which describes the energy exchange between the two subsystems. Our results challenge the conventional view that electron temperature alone is a sufficient predictor of electron-phonon coupling.

Abstract: The presented work is focused on the optical and magneto-optical characterization of TeO2-ZnO-BaO (TZB) tellurite glasses. We investigated the refractive index and extinction coefficient dispersion by spectroscopic ellipsometry from ultraviolet, 0.193 um, up to mid infrared, 25 um spectral region. Studied glasses exhibited large values of linear (n632 = 1.91-2.09) and non-linear refractive index (n2 = 1.20-2.67x10-11 esu), Verdet constant (V632 = 22-33 radT-1m-1) and optical band gap energy (Eg = 3.7-4.1 eV). The materials characterization revealed that BaO substitution by ZnO leads (at constant content of TeO2) to an increase in linear and nonlinear refractive index as well as Verdet constant while the optical band gap energy decreases. Fiber drawing ability of TeO2-ZnO-BaO glassy system has been demonstrated on 60TeO2-20ZnO-20BaO glass with presented mid infrared attenuation coefficient. Specific parameters such as dispersion and single oscillator energy, Abbe number, and first-/ third-order optical susceptibility are enclosed together with the values of magneto-optic anomaly derived from the calculation of measured dispersion of the refractive index.

Abstract: Main desired features of biaxial tests are: uniformity of stresses and strains; high strain levels in gauge areas; reliable constitutive parameters identification. Despite cruciform specimen suitability to modern tensile devices, standard testing techniques are still debated because of difficulties in matching these demands. This work aims at providing rational performance objectives and efficient cruciform specimens shapes in view of constitutive parameter fitting. Objective performance is evaluated along particular lines lying on principal directions in equibiaxial tensile tests. A rich specimen profile geometry is purposely optimized in silico by varying cost function and material compressibility. Experimental tests, monitored via digital image correlation, are carried out for validation. New shapes are designed and tested in a biaxial tensile apparatus and show to perform better than existing ones. Parameter fitting is efficiently performed by only exploiting full field strain measurements along lines. Small gauge areas and small fillet radii cruciform specimens get closer to the ideal behavior. For constitutive parameters identification in two-dimensional tensile experiments, data analysis on gauge lines deformation suffices.

Abstract: Metal-organic frameworks (MOFs) are an incredibly diverse group of highly porous hybrid materials, which are interesting for a wide range of possible applications. For a reliable description of many of their properties accurate computationally highly efficient methods, like force-field potentials (FFPs), are required. With the advent of machine learning approaches, it is now possible to generate such potentials with relatively little human effort. Here, we present a recipe to parametrize two fundamentally different types of exceptionally accurate and computationally highly efficient machine learned potentials, which belong to the moment-tensor and kernel-based potential families. They are parametrized relying on reference configurations generated in the course of molecular dynamics based, active learning runs and their performance is benchmarked for a representative selection of commonly studied MOFs. For both potentials, comparison to a random set of validation structures reveals close to DFT precision in predicted forces and structural parameters of all MOFs. Essentially the same applies to elastic constants and phonon band structures. Additionally, for MOF-5 the thermal conductivity is obtained with full quantitative agreement to single-crystal experiments. All this is possible while maintaining a high degree of computational efficiency, with the obtained machine learned potentials being only moderately slower than the extremely simple UFF4MOF or Dreiding force fields. The exceptional accuracy of the presented FFPs combined with their computational efficiency has the potential of lifting the computational modelling of MOFs to the next level.