Tuning thermal transport in Si nanowires by isotope engineering

We study thermal transport in isotopically disordered Si nanowires, discussing the feasibility of phonon engineering for thermoelectric applications within these systems. To this purpose, we carry out atomistic molecular dynamics and nonequilibrium Green's function calculations to characterize the dependence of the thermal conductance as a function of the isotope concentration, isotope radial distribution and temperature. We show that a reduction of the conductivity of up to 20% can be achieved with suitable isotope blends at room temperature and approximately 50% at low temperature. Interestingly, precise control of the isotope composition or radial distribution is not needed. An isotope disordered nanowire roughly behaves like a low-pass filter, as isotope impurities are transparent for long wave-length acoustic phonons, while only mid- and high-frequency optical phonons undergo significant scattering.We acknowledge financial support from the Ministerio de Economía y Competitividad (MINECO) under grant FEDER-MAT2013-40581-P and the Severo Ochoa Centres of Excellence Program under Grant SEV-2015-0496 and from the Generalitat de Catalunya under grants no. 2014 SGR 301 and through the Beatriu de Pinós fellowship program (2014 BP_B 00101). The calculations were performed at the Barcelona Supercomputing Center (BSC-CNS) within the project “Thermal transport in isotopically disordered Si nanowires (FI-2016-1-0022)”. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI) ReferencesPeer Reviewe

ABINIT: Overview and focus on selected capabilities

Paper published as part of the special topic on Electronic Structure SoftwareABINIT is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe–Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the “temperaturedependent effective potential” approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which ABINIT relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The ABINIT DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library LIBPAW. ABINIT has strong links with many other software projects that are briefly mentioned.This work (A.H.R.) was supported by the DMREF-NSF Grant No. 1434897, National Science Foundation OAC-1740111, and U.S. Department of Energy DE-SC0016176 and DE-SC0019491 projects. N.A.P. and M.J.V. gratefully acknowledge funding from the Belgian Fonds National de la Recherche Scientifique (FNRS) under Grant No. PDR T.1077.15-1/7. M.J.V. also acknowledges a sabbatical “OUT” grant at ICN2 Barcelona as well as ULiège and the Communauté Française de Belgique (Grant No. ARC AIMED G.A. 15/19-09). X.G. and M.J.V. acknowledge funding from the FNRS under Grant No. T.0103.19-ALPS. X.G. and G.-M. R. acknowledge support from the Communauté française de Belgique through the SURFASCOPE Project (No. ARC 19/24-057). X.G. acknowledges the hospitality of the CEA DAM-DIF during the year 2017. G.H. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-05-CH11231 (Materials Project Program No. KC23MP). The Belgian authors acknowledge computational resources from supercomputing facilities of the University of Liège, the Consortium des Equipements de Calcul Intensif (Grant No. FRS-FNRS G.A. 2.5020.11), and Zenobe/CENAERO funded by the Walloon Region under Grant No. G.A. 1117545. M.C. and O.G. acknowledge support from the Fonds de Recherche du Québec Nature et Technologie (FRQ-NT), Canada, and the Natural Sciences and Engineering Research Council of Canada (NSERC) under Grant No. RGPIN-2016-06666. The implementation of the libpaw library (M.T., T.R., and D.C.) was supported by the ANR NEWCASTLE project (Grant No. ANR-2010-COSI-005-01) of the French National Research Agency. M.R. and M.S. acknowledge funding from Ministerio de Economia, Industria y Competitividad (MINECO-Spain) (Grants Nos. MAT2016-77100-C2-2-P and SEV-2015-0496) and Generalitat de Catalunya (Grant No. 2017 SGR1506). This work has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation program (Grant Agreement No. 724529). P.G. acknowledges support from FNRS Belgium through PDR (Grant No. HiT4FiT), ULiège and the Communauté française de Belgique through the ARC project AIMED, the EU and FNRS through M.ERA.NET project SIOX, and the European Funds for Regional Developments (FEDER) and the Walloon Region in the framework of the operational program “Wallonie-2020.EU” through the project Multifunctional thin films/LoCoTED. The Flatiron Institute is a division of the Simons Foundation. A large part of the data presented in this paper is available directly from the Abinit Web page www.abinit.org. Any other data not appearing in this web page can be provided by the corresponding author upon reasonable request.Peer reviewe

Theoretical modelling of electrons and holes in semiconductor nanostructures

En esta tesis se utiliza la aproximación de masa efectiva y función envolvente para estudiar teóricamente las propiedades optoelectrónicas de una gran variedad de nanoestructuras semiconductoras, muchas de las cuales son obtenibles en un laboratorio a día de hoy. El primer capítulo de la tesis se centra en el estudio de los efectos derivados de aplicar campos magnéticos externos sobre varias nanoestructuras formadas por anillos cuánticos: dos anillos acoplados lateralmente y verticalmente, y una red periódica bidimensional de anillos. El segundo capítulo constituye la parte más extensa e importante de la tesis y estudia la influencia del entorno dieléctrico sobre las propiedades optoelectrónicas de nanocristales sintetizados mediante técnicas de química coloidal con forma esférica y alargada. Mediante cálculos multipartícula basados en las metodologías DFT y CI, se estudia el efecto del confinamiento dieléctrico sobre nanocristales poblados con un alto número de electrones o con pares electrón hueco. Finalmente, el último capítulo de la tesis se centra en el estudio de los estados multipartícula y las transiciones de fase a lo largo de un proceso en el que un nanocristal esférico es alargado hasta formar una estructura casi unidimensional

Exact Long-Range Dielectric Screening and Interatomic Force Constants in Quasi-Two-Dimensional Crystals

We develop a fundamental theory of the long-range electrostatic interactions in two-dimensional crystals by performing a rigorous study of the nonanalyticities of the Coulomb kernel. We find that the dielectric functions are best represented by 2×2 matrices, with nonuniform macroscopic potentials that are two-component hyperbolic functions of the out-of-plane coordinate z. We demonstrate our arguments by deriving the long-range interatomic forces in the adiabatic regime, where we identify a formerly overlooked dipolar coupling involving the out-of-plane components of the dynamical charges. The resulting formula is exact up to an arbitrary multipolar order, which we illustrate in practice via the explicit inclusion of dynamical quadrupoles. By performing numerical tests on monolayer BN, SnS2, and BaTiO3 membranes, we show that our method allows for a drastic improvement in the description of the long-range electrostatic interactions, with comparable benefits to the quality of the interpolated phonon band structure.We are grateful to N. Marzari and S. Ponc´e for bringing this problem to our attention and for their critical reading of an early version of the manuscript. We also thank the two anonymous reviewers for their careful reading and helpful suggestions. We acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECOSpain) through Grants No. MAT2016-77100-C2-2-P and No. PID2019–108573 GB-C22 and Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S) and of Generalitat de Catalunya (Grant No. 2017 SGR1506). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia (CESGA).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Lattice-mediated bulk flexoelectricity from first principles

We present the derivation and code implementation of a first-principles methodology to calculate the lattice-mediated contributions to the bulk flexoelectric tensor. The approach is based on our recent analytical long-wavelength extension of density-functional perturbation theory [Royo and Stengel, Phys. Rev. X 9, 021050 (2019)], and avoids the cumbersome numerical derivatives with respect to the wave vector that were adopted in previous implementations. To substantiate our results, we revisit and numerically validate the sum rules that relate flexoelectricity and uniform elasticity by generalizing them to regimes where finite forces and stresses are present. We also revisit the definition of the elastic tensor under stress, especially in regards to the existing linear-response implementation. We demonstrate the performance of our method by applying it to representative cubic crystals and to the tetragonal low-temperature polymorph of SrTiO$_3$, obtaining excellent agreement with the available literature data.We acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECO-Spain) through Grant No. PID2019-108573GB-C22 and Severo Ochoa FUNFUTURE center of excellence (Grant No. CEX2019-000917-S); and of Generalitat de Catalunya (Grant No. 2017 SGR1506)). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia (CESGA).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

In-Plane Flexoelectricity in Two-Dimensional D3d Crystals

We predict a large in-plane polarization response to bending in a broad class of trigonal two-dimensional crystals. We define and compute the relevant flexoelectric coefficients from first principles as linear-response properties of the undistorted layer by using the primitive crystal cell. The ensuing response (evaluated for SnS_{2}, silicene, phosphorene, and RhI_{3} monolayers and for a hexagonal BN bilayer) is up to 1 order of magnitude larger than the out-of-plane components in the same material. We illustrate the topological implications of our findings by calculating the polarization textures that are associated with a variety of rippled and bent structures. We also determine the longitudinal electric fields induced by a flexural phonon at leading order in amplitude and momentum.We acknowledge support from Ministerio de Ciencia e Innovación (MICINN-Spain) through Grants No. PID2019- 108573GB-C22 and Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S); from Generalitat de Catalunya (Grant No. 2021 SGR 01519); and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Low-temperature thermal rectification by tailoring isotope distributions

We combine first-principles electronic structure calculated thermal conductivity data with a numerical solution of the one-dimensional heat equation to show that an asymmetric distribution of impurity scattering, if suitably designed, yields the conditions for a low-temperature thermal rectification. This happens as a result of the differences in the peaks of the temperature dependence of the thermal conductivity. We demonstrate the effectiveness of the method by probing the thermal rectification rendered by a silicon slab with a steplike position-dependent isotopic composition. The same conclusions are obtained by using experimentally measured values of the thermal conductivity of Si samples with different isotope distributions.Peer reviewe

Using High Multipolar Orders to Reconstruct the Sound Velocity in Piezoelectrics from Lattice Dynamics

Information over the phonon band structure is crucial to predicting many thermodynamic properties of materials, such as thermal transport coefficients. Highly accurate phonon dispersion curves can be, in principle, calculated in the framework of density-functional perturbation theory. However, well-established techniques can run into trouble (or even catastrophically fail) in the case of piezoelectric materials, where the acoustic branches hardly reproduce the physically correct sound velocity. Here we identify the culprit in the higher-order multipolar interactions between atoms and demonstrate an effective procedure that fixes the aforementioned issue. Our strategy drastically improves the predictive power of perturbative lattice-dynamical calculations in piezoelectric crystals and is directly implementable for high-throughput generation of materials databases.We acknowledge the support of Ministerio de Economia, Industria y Competitividad (MINECO-Spain), through Grants No. MAT2016-77100-C2-2-P and No. SEV-2015- 0496, and of Generalitat de Catalunya (Grant No. 2017 SGR1506). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program (Grant Agreement No. 724529). Part of the calculations were performed at the Supercomputing Center of Galicia (CESGA).Peer reviewe

Quasiballistic phonon transport from first principles

At short length scales phonon transport is ballistic: the thermal resistance of semiconductors and insulators is quantized and length independent. At long length scales, on the other hand, transport is diffusive and resistance arises as a result of the scattering processes experienced by phonons. In many cases of interest, however, these two transport regimes coexist. Here we propose a first-principles approach to treat quasiballistic phonon transport where diffusive and ballistic phonons receive separate theoretical treatments. Partitioning the overall phonon population for a given transport length is performed examining the mean free paths obtained from the solution of the Boltzmann transport equation and allowing only diffusive phonons to participate in anharmonic phonon-phonon scattering processes. We present results for Si and diamond, discussing the crossover from ballistic to diffusive transport as the length scale and/or the temperature increases and compute the relative contribution of ballistic and diffusive phonons to the thermal conductance in each transport condition.We acknowledge financial support by the Ministerio de Economía, Industria y Competitividad (MINECO) under Grant No. FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant No. SEV-2015-0496 and by the Generalitat de Catalunya under Grant No. 2017 SGR 1506. M.L.-S. was funded through a Juan de la Cierva fellowship. We thank the Centro de Supercomputación de Galicia (CESGA) for the use of their computational resources. P.T. acknowledges funding by the Canon Foundation in Europe. The authors thank M. Brandbyge and L. Colombo for critical reading of the paper and J. Carrete for useful discussions about technical details related with the implementation of shengbte.Peer reviewe

Accurate Prediction of Hall Mobilities in Two-Dimensional Materials through Gauge-Covariant Quadrupolar Contributions

6 pages and 2 figuresDespite considerable efforts, accurate computations of electron-phonon and carrier transport properties of low-dimensional materials from first principles have remained elusive. By building on recent advances in the description of long-range electrostatics, we develop a general approach to the calculation of electron-phonon couplings in two-dimensional materials. We show that the nonanalytic behavior of the electron-phonon matrix elements depends on the Wannier gauge, but that a missing Berry connection restores invariance to quadrupolar order. We showcase these contributions in a MoS_{2} monolayer, calculating intrinsic drift and Hall mobilities with precise Wannier interpolations. We also find that the contributions of dynamical quadrupoles to the scattering potential are essential, and that their neglect leads to errors of 23% and 76% in the room-temperature electron and hole Hall mobilities, respectively.S. P. acknowledges support from the funded by the Fonds de la Recherche Scientifique de Belgique (FRSFNRS) as well as from the European Unions Horizon 2020 Research and Innovation Program, under the Marie Skłodowska-Curie Grant Agreement (SELPH2D, No. 839217); N. M. acknowledges support from the Swiss National Science Foundation and the NCCR MARVEL; M. G. acknowledges support from the Italian Ministry for University and Research through the LeviMontalcini program; M. S. and M. R. acknowledge support from Ministerio de Ciencia y Innovación (MICINN-Spain) through Grant No. PID2019–108573 GB-C22; from Severo Ochoa FUNFUTURE center of excellence (CEX2019-000917-S); from Generalitat de Catalunya (Grant No. 2017 SGR1506); and from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant Agreement No. 724529). Computational resources have been provided by the PRACE-21 resources MareNostrum at BSC-CNS, the Supercomputing Center of Galicia (CESGA), and by the Consortium des Équipements de Calcul Intensif (CÉCI), funded by the FRS-FNRS under Grant No. 2.5020.11 and by the Walloon Region as well as computational resources awarded on the Belgian share of the EuroHPC LUMI supercomputerWith funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe