Thermal conductivity of glasses: first-principles theory and applications

Predicting the thermal conductivity of glasses from first principles has hitherto been a very complex problem.The established Allen-Feldman and Green-Kubo approaches employ approximations with limited validity—the former neglects anharmonicity,the latter misses the quantum Bose-Einstein statistics of vibrations—and require atomistic models that are very challenging for first-principles methods.Here,we present a protocol to determine from first principles the thermal conductivityκ(T)of glasses above the plateau(i.e.,above the temperature-independent region appearing almost without exceptions in theκ(T)of all glasses at cryogenic temperatures).The protocol combines the Wigner formulation of thermal transport with convergence-acceleration techniques,and accounts comprehensively for the effects of structural disorder,anharmonicity,and Bose-Einstein statistics.We validate this approach in vitreous silica,showing that models containing less than 200 atoms can already reproduceκ(T)in the macroscopic limit.We discuss the effects of anharmonicity and the mechanisms determining the trend ofκ(T)at high temperature,reproducing experiments at temperatures where radiative effects remain negligible.

Towards high-throughput many-body perturbation theory: efficient algorithms and automated workflows

The automation of ab initio simulations is essential in view of performing high-throughput(HT)computational screenings oriented to the discovery of novel materials with desired physical properties.In this work,we propose algorithms and implementations that are relevant to extend this approach beyond density functional theory(DFT),in order to automate many-body perturbation theory(MBPT)calculations.Notably,an algorithm pursuing the goal of an efficient and robust convergence procedure for GW and BSE simulations is provided,together with its implementation in a fully automated framework.This is accompanied by an automatic GW band interpolation scheme based on maximally localized Wannier functions,aiming at a reduction of the computational burden of quasiparticle band structures while preserving high accuracy.The proposed developments are validated on a set of representative semiconductor and metallic systems.

Thermodynamics and dielectric response of BaTiO_(3)by data-driven modeling

Modeling ferroelectric materials from first principles is one of the successes of density-functional theory and the driver of much development effort,requiring an accurate description of the electronic processes and the thermodynamic equilibrium that drive the spontaneous symmetry breaking and the emergence of macroscopic polarization.We demonstrate the development and application of an integrated machine learning model that describes on the same footing structural,energetic,and functional properties of barium titanate(BaTiO_(3)),a prototypical ferroelectric.The model uses ab initio calculations as a reference and achieves accurate yet inexpensive predictions of energy and polarization on time and length scales that are not accessible to direct ab initio modeling.These predictions allow us to assess the microscopic mechanism of the ferroelectric transition.The presence of an order-disorder transition for the Ti off-centered states is the main driver of the ferroelectric transition,even though the coupling between symmetry breaking and cell distortions determines the presence of intermediate,partly-ordered phases.Moreover,we thoroughly probe the static and dynamical behavior of BaTiO_(3)across its phase diagram without the need to introduce a coarse-grained description of the ferroelectric transition.Finally,we apply the polarization model to calculate the dielectric response properties of the material in a full ab initio manner,again reproducing the correct qualitative experimental behavior.

General invariance and equilibrium conditions for lattice dynamics in 1D,2D,and 3D materials

The long-wavelength behavior of vibrational modes plays a central role in carrier transport,phonon-assisted optical properties,superconductivity,and thermomechanical and thermoelectric properties of materials.Here,we present general invariance and equilibrium conditions of the lattice potential;these allow to recover the quadratic dispersions of flexural phonons in low-dimensional materials,in agreement with the phenomenological model for long-wavelength bending modes.We also prove that for any low-dimensional material the bending modes can have a purely out-of-plane polarization in the vacuum direction and a quadratic dispersion in the long-wavelength limit.In addition,we propose an effective approach to treat invariance conditions in crystals with non-vanishing Born effective charges where the long-range dipole-dipole interactions induce a contribution to the lattice potential and stress tensor.Our approach is successfully applied to the phonon dispersions of 158 two-dimensional materials,highlighting its critical relevance in the study of phonon-mediated properties of low-dimensional materials.

Connector theory for reusing model results to determine materials properties

The success of Density Functional Theory(DFT)is partly due to that of simple approximations,such as the Local Density Approximation(LDA),which uses results of a model,the homogeneous electron gas,to simulate exchange-correlation effects in real materials.We turn this intuitive approximation into a general and in principle exact theory by introducing the concept of a connector:a prescription how to use results of a model system in order to simulate a given quantity in a real system.In this framework,the LDA can be understood as one particular approximation for a connector that is designed to link the exchange-correlation potentials in the real material to that of the model.Formulating the in principle exact connector equations allows us to go beyond the LDA in a systematic way.Moreover,connector theory is not bound to DFT,and it suggests approximations also for other functionals and other observables.We explain why this very general approach is indeed a convenient starting point for approximations.We illustrate our purposes with simple but pertinent examples.

Million-scale data integrated deep neural network for phonon properties of heuslers spanning the periodic table

Existing machine learning potentials for predicting phonon properties of crystals are typically limited on a material-to-materialbasis, primarily due to the exponential scaling of model complexity with the number of atomic species. We address this bottleneckwith the developed Elemental Spatial Density Neural Network Force Field, namely Elemental-SDNNFF. The effectiveness andprecision of our Elemental-SDNNFF approach are demonstrated on 11,866 full, half, and quaternary Heusler structures spanning 55elements in the periodic table by prediction of complete phonon properties. Self-improvement schemes including active learningand data augmentation techniques provide an abundant 9.4 million atomic data for training. Deep insight into predicted ultralowlattice thermal conductivity (<1 Wm^(−1) K^(−1)) of 774 Heusler structures is gained by p–d orbital hybridization analysis. Additionally, aclass of two-band charge-2 Weyl points, referred to as “double Weyl points”, are found in 68% and 87% of 1662 half and 1550quaternary Heuslers, respectively.

Precision and efficiency in solid-state pseudopotential calculations

Despite the enormous success and popularity of density-functional theory,systematic verification and validation studies are still limited in number and scope.Here,we propose a protocol to test publicly available pseudopotential libraries,based on several independent criteria including verification against all-electron equations of state and plane-wave convergence tests for phonon frequencies,band structure,cohesive energy and pressure.Adopting these criteria we obtain curated pseudopotential libraries(named SSSP or standard solid-state pseudopotential libraries),that we target for high-throughput materials screening(“SSSP efficiency”)and high-precision materials modelling(“SSSP precision”).This latter scores highest among open-source pseudopotential libraries available in theΔ-factor test of equations of states of elemental solids.

Screw dislocation structure and mobility in body centered cubic Fe predicted by a Gaussian Approximation Potential

The plastic flow behavior of bcc transition metals up to moderate temperatures is dominated by the thermally activated glide of screw dislocations,which in turn is determined by the atomic-scale screw dislocation core structure and the associated kink-pair nucleation mechanism for glide.Modeling complex plasticity phenomena requires the simulation of many atoms and interacting dislocations and defects.These sizes are beyond the scope of first-principles methods and thus require empirical interatomic potentials.Especially for the technological important case of bcc Fe,existing empirical interatomic potentials yield spurious behavior.Here,the structure and motion of the screw dislocations in Fe are studied using a new Gaussian Approximation Potential(GAP)for bcc Fe,which has been shown to reproduce the potential energy surface predicted by density-functional theory(DFT)and many associated properties.The Fe GAP predicts a compact,non-degenerate core structure,a single-hump Peierls potential,and glide on{110},consistent with DFT results.The thermally activated motion at finite temperatures occurs by the expected kink-pair nucleation and propagation mechanism.The stress-dependent enthalpy barrier for screw motion,computed using the nudgedelastic-band method,follows closely a form predicted by standard theories with a zero-stress barrier of~1 eV,close to the experimental value of 0.84 eV,and a Peierls stress of~2 GPa consistent with DFT predictions of the Peierls potential.

Absolute band alignment at semiconductor-water interfaces using explicit and implicit descriptions for liquid water

Quantum mechanical simulations that include the effects of the liquid environment are highly relevant for the characterization of solid-liquid interfaces,which is crucial for the design of a wide range of devices.In this work we present a rigorous and systematic study of the band alignment of semiconductors in aqueous solutions by contrasting a range of hybrid explicit/implicit models against explicit atomistic simulations based on density-functional theory.We find that consistent results are obtained provided that the first solvation shell is treated explicitly.Interestingly,the first molecular layer of explicit water is only relevant for the pristine surfaces without dissociatively adsorbed water,hinting at the importance of saturating the surface with quantum mechanical bonds.By referencing the averaged electrostatic potentials of explicit and implicit water against vacuum,we provide absolute alignments,finding maximal differences of only~0.1–0.2 V.Furthermore,the implicit reference potential is shown to exhibit an intrinsic offset of−0.33 V with respect to vacuum,which is traced back to the absence of an explicit water surface in the implicit model.These results pave the way for accurate simulations of solid-liquid interfaces using minimalistic explicit/implicit models.

Photorealistic modelling of metals from first principles

The colours of metals have attracted the attention of humanity since ancient times,and coloured metals,in particular gold compounds,have been employed for tools and objects symbolizing the aesthetics of power.In this work,we develop a comprehensive framework to obtain the reflectivity and colour of metals,and show that the trends in optical properties and the colours can be predicted by straightforward first-principles techniques based on standard approximations.We apply this to predict reflectivity and colour of several elemental metals and of different types of metallic compounds(intermetallics,solid solutions and heterogeneous alloys),considering mainly binary alloys based on noble metals.We validate the numerical approach through an extensive comparison with experimental data and the photorealistic rendering of known coloured metals.

Automated high-throughput Wannierisation

Maximally-localised Wannier functions(MLWFs)are routinely used to compute from first-principles advanced materials properties that require very dense Brillouin zone integration and to build accurate tight-binding models for scale-bridging simulations.At the same time,high-throughput(HT)computational materials design is an emergent field that promises to accelerate reliable and cost-effective design and optimisation of new materials with target properties.The use of MLWFs in HT workflows has been hampered by the fact that generating MLWFs automatically and robustly without any user intervention and for arbitrary materials is,in general,very challenging.We address this problem directly by proposing a procedure for automatically generating MLWFs for HT frameworks.Our approach is based on the selected columns of the density matrix method and we present the details of its implementation in an AiiDA workflow.We apply our approach to a dataset of 200 bulk crystalline materials that span a wide structural and chemical space.We assess the quality of our MLWFs in terms of the accuracy of the band-structure interpolation that they provide as compared to the band-structure obtained via full first-principles calculations.Finally,we provide a downloadable virtual machine that can be used to reproduce the results of this paper,including all first-principles and atomistic simulations as well as the computational workflows.

Common workflows for computing material properties using different quantum engines

The prediction of material properties based on density-functional theory has become routinely common,thanks,in part,to the steady increase in the number and robustness of available simulation packages.This plurality of codes and methods is both a boon and a burden.While providing great opportunities for cross-verification,these packages adopt different methods,algorithms,and paradigms,making it challenging to choose,master,and efficiently use them.We demonstrate how developing common interfaces for workflows that automatically compute material properties greatly simplifies interoperability and cross-verification.We introduce design rules for reusable,code-agnostic,workflow interfaces to compute well-defined material properties,which we implement for eleven quantum engines and use to compute various material properties.Each implementation encodes carefully selected simulation parameters and workflow logic,making the implementer’s expertise of the quantum engine directly available to nonexperts.All workflows are made available as open-source and full reproducibility of the workflows is guaranteed through the use of the AiiDA infrastructure.

Electrosorption at metal surfaces from first principles

Electrosorption of solvated species at metal electrodes is a most fundamental class of processes in interfacial electrochemistry.Here,we use its sensitive dependence on the electric double layer to assess the performance of ab initio thermodynamics approaches increasingly used for the first-principles description of electrocatalysis.We show analytically that computational hydrogen electrode calculations at zero net-charge can be understood as a first-order approximation to a fully grand canonical approach.Notably,higher-order terms in the applied potential caused by the charging of the double layer include contributions from adsorbate-induced changes in the work function and in the interfacial capacitance.These contributions are essential to yield prominent electrochemical phenomena such as non-Nernstian shifts of electrosorption peaks and non-integer electrosorption valencies.We illustrate this by calculating peak shifts for H on Pt electrodes and electrosorption valencies of halide ions on Ag electrodes,obtaining qualitative agreement with experimental data already when considering only second order terms.The results demonstrate the agreement between classical electrochemistry concepts and a first-principles fully grand canonical description of electrified interfaces and shed new light on the widespread computational hydrogen electrode approach.

Anti-Jahn-Teller effect induced ultrafast insulator to metal transition in perovskite BaBiO_(3)

The Jahn-Teller(JT)effect involves the ions M with a degenerate electronic state distorting the corner-sharing MO_(6)octahedra to lift the degeneracy,inducing strong coupling of electrons to lattice,and mediating the exotic properties in perovskite oxides.Conversely,the anti-Jahn–Teller(AJT)effect refers to the deformation against the Jahn-Teller-distorted MO_(6)octahedra.However,it is difficult to experimentally execute both effects descending from the fine-tuning of crystal structures.We propose the AJT can be introduced by THz laser illumination at 11.71 THz in a candidate superconducting perovskite material BaBiO_(3)near room temperature.The illumination coherently drives the infrared-active phonon that excites the Raman breathing mode through the quadratic-linear nonlinear interaction.The process is characterized by the emergence of an AJT effect,accompanied by an insulator-to-metal transition occurring on the picosecond timescale.This study underlines the important role of crystal structure engineering by coherent phonon excitation in designing optoelectronic devices.