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Chiral Epitaxy: Enantioselective Growth of Chiral Nanowires on Low-Symmetry Two-Dimensional Materials
Authors:
Noya Ruth Itzhak,
Kate Reidy,
Maya Levy-Greenberg,
Paul Anthony Miller,
Chen Wei,
Juan Gomez Quispe,
Raphael Tromer,
Olle Hellman,
Shahar Joselevich,
Aliza Ashman,
Lothar Houben,
Ifat Kaplan-Ashiri,
Xiao-Meng Sui,
Olga Brontvein,
Katya Rechav,
Laurent Travers,
Pedro A. S. Autreto,
Douglas S. Galvão,
Federico Panciera,
Oded Hod,
Leeor Kronik,
Frances M. Ross,
Ernesto Joselevich
Abstract:
Chiral crystals exhibit useful handedness-dependent properties, including spin selectivity and circularly polarized light sensitivity, yet controlling which enantiomer forms during synthesis remains a central challenge. Existing approaches utilize molecules in solution to template crystal growth, which restricts processing conditions and introduces organic contaminants incompatible with device fab…
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Chiral crystals exhibit useful handedness-dependent properties, including spin selectivity and circularly polarized light sensitivity, yet controlling which enantiomer forms during synthesis remains a central challenge. Existing approaches utilize molecules in solution to template crystal growth, which restricts processing conditions and introduces organic contaminants incompatible with device fabrication. Enantioselective growth of a chiral crystal on a chiral surface via vapor-phase synthesis (chiral epitaxy) has not yet been demonstrated. Here, we show chiral epitaxy of aligned tellurium nanowires on a low-symmetry two-dimensional material, ReSe2. In situ electron microscopies suggest a mechanism where handedness is determined at nucleation by the interface energy difference between Te enantiomers and the chiral substrate surface. Chiral epitaxy provides a solvent-free, vapor-solid route to homochiral crystals compatible with semiconductor and quantum manufacturing processes.
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Submitted 25 March, 2026;
originally announced March 2026.
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Resolving Transient Electron-Phonon Coupling with Time-Resolved Spontaneous Raman Spectroscopy
Authors:
Guy Reuveni,
Maya Levy Greenberg,
Matan Menahem,
Olle Hellman,
Omer Yaffe
Abstract:
Understanding the interaction of charge carriers with lattice vibrations in the quasi-equilibrium regime is crucial for semiconductor functionality. However, the structural signatures of these interactions are often too subtle for conventional ultrafast techniques to detect. We developed a time-resolved spontaneous Raman technique based on time-correlated single-photon counting to track the spectr…
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Understanding the interaction of charge carriers with lattice vibrations in the quasi-equilibrium regime is crucial for semiconductor functionality. However, the structural signatures of these interactions are often too subtle for conventional ultrafast techniques to detect. We developed a time-resolved spontaneous Raman technique based on time-correlated single-photon counting to track the spectral response following photoexcitation, providing sub-wavenumber spectral resolution and a few-hundred-picosecond temporal resolution. Unlike traditional pump-probe schemes, our method utilizes a modulated continuous-wave probe to maintain high spectral resolution, enabling detection of low-frequency Raman shifts down to 10 cm$^{-1}$. Applied to lightly boron-doped silicon, we resolve intra-valence band and inter-valence band electronic transitions. A coupled-mode analysis of transient phonon asymmetry, resulting from interference with the inter-valence band transitions, reveals electron-phonon coupling parameters that directly relate to carrier recombination. By capturing these subtle dynamical shifts, we demonstrate that this platform offers a powerful probe for investigating electron-phonon interactions in long-lived excited states.
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Submitted 10 March, 2026;
originally announced March 2026.
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Opposite impact of thermal expansion and phonon anharmonicity on the phonon-limited resistivity of elemental metals from first principles
Authors:
Ao Wang,
Junwen Yin,
Félix Antoine Goudreault,
Michel Côté,
Olle Hellman,
Samuel Poncé
Abstract:
Understanding electrical resistivity in metals remains a central challenge in quantifying charge transport at finite temperature. Current first-principles calculations based on the Boltzmann transport equation often match experiments, yet they almost always neglect the effect of thermal expansion and phonon anharmonicity. We show that both effects exert an opposite impact on electron-phonon coupli…
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Understanding electrical resistivity in metals remains a central challenge in quantifying charge transport at finite temperature. Current first-principles calculations based on the Boltzmann transport equation often match experiments, yet they almost always neglect the effect of thermal expansion and phonon anharmonicity. We show that both effects exert an opposite impact on electron-phonon coupling and on electrical resistivity. Thermal expansion enhances the coupling and leads to overestimation of resistivity, whereas anharmonic effects reduce it. By explicitly incorporating both effects, we establish a more complete description of resistivity in elemental metals, demonstrated here for Pb, Nb, and Al.
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Submitted 3 February, 2026; v1 submitted 21 November, 2025;
originally announced November 2025.
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Abinit 2025: New Capabilities for the Predictive Modeling of Solids and Nanomaterials
Authors:
Matthieu J. Verstraete,
Joao Abreu,
Guillaume E. Allemand,
Bernard Amadon,
Gabriel Antonius,
Maryam Azizi,
Lucas Baguet,
Clementine Barat,
Louis Bastogne,
Romuald Bejaud,
Jean-Michel Beuken,
Jordan Bieder,
Augustin Blanchet,
Francois Bottin,
Johann Bouchet,
Julien Bouquiaux,
Eric Bousquet,
James Boust,
Fabien Brieuc,
Veronique Brousseau-Couture,
Nils Brouwer Fabien Bruneval,
Alois Castellano,
Emmanuel Castiel,
Jean-Baptiste Charraud,
Jean Clerouin
, et al. (49 additional authors not shown)
Abstract:
Abinit is a widely used scientific software package implementing density functional theory and many related functionalities for excited states and response properties. This paper presents the novel features and capabilities, both technical and scientific, which have been implemented over the past 5 years. This evolution occurred in the context of evolving hardware platforms, high-throughput calcul…
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Abinit is a widely used scientific software package implementing density functional theory and many related functionalities for excited states and response properties. This paper presents the novel features and capabilities, both technical and scientific, which have been implemented over the past 5 years. This evolution occurred in the context of evolving hardware platforms, high-throughput calculation campaigns, and the growing use of machine learning to predict properties based on databases of first principles results. We present new methodologies for ground states with constrained charge, spin or temperature; for density functional perturbation theory extensions to flexoelectricity and polarons; and for excited states in many-body frameworks including GW, dynamical mean field theory, and coupled cluster. Technical advances have extended abinit high-performance execution to graphical processing units and intensive parallelism. Second principles methods build effective models on top of first principles results to scale up in length and time scales. Finally, workflows have been developed in different community frameworks to automate \abinit calculations and enable users to simulate hundreds or thousands of materials in controlled and reproducible conditions.
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Submitted 11 July, 2025;
originally announced July 2025.
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Impact of anharmonicity on the carrier mobility of the Pb-free CsSnBr$_3$ perovskite
Authors:
Junwen Yin,
Olle Hellman,
Samuel Poncé
Abstract:
Charge carrier mobilities are critical parameters in halide perovskite solar cells, governing their average carrier velocity under an applied electric field and overall efficiency. Recent advances in first-principles calculations of electron-phonon interactions and carrier mobilities have enabled predictive computations for perovskite solar cells. However, the flexible octahedral frameworks and ca…
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Charge carrier mobilities are critical parameters in halide perovskite solar cells, governing their average carrier velocity under an applied electric field and overall efficiency. Recent advances in first-principles calculations of electron-phonon interactions and carrier mobilities have enabled predictive computations for perovskite solar cells. However, the flexible octahedral frameworks and cationic displacements in these materials challenge the harmonic approximation, leading to significant difficulties in accurately calculating transport properties. To address these issues, we combine temperature-dependent effective potentials with the ab initio Boltzmann transport equations to compute carrier mobilities in a representative lead-free perovskite, CsSnBr$_3$. At room temperature, the electron/hole Hall mobilities in CsSnBr$_3$ are 106/256 cm$^2$/Vs when neglecting anharmonic effects and 59/145 cm$^2$/Vs when included. This overestimation of the harmonic approximation arises from the neglect of scattering coming from soft modes. We provide a workflow for performing first-principles carrier mobility calculations in anharmonic systems, advancing the predictive modeling of perovskite solar cells.
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Submitted 26 May, 2025;
originally announced May 2025.
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Microscopic theory of phonon polaritons and long wavelength dielectric response
Authors:
Olle Hellman,
Leeor Kronik
Abstract:
We present a first-principles approach for calculating phonon-polariton dispersion relations. In this approach, phonon-photon interaction is described by quantization of a Hamiltonian that describes harmonic lattice vibrations coupled with the electromagnetic field inside the material. All Hamiltonian parameters are obtained from first-principles calculations, with diagonalization leading to non-i…
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We present a first-principles approach for calculating phonon-polariton dispersion relations. In this approach, phonon-photon interaction is described by quantization of a Hamiltonian that describes harmonic lattice vibrations coupled with the electromagnetic field inside the material. All Hamiltonian parameters are obtained from first-principles calculations, with diagonalization leading to non-interacting polariton quasiparticles. This method naturally includes retardation effects and resolves non-analytical behavior and ambiguities in phonon frequencies at the Brillouin zone center, especially in non-cubic and optically anisotropic materials. Furthermore, by incorporating higher-order terms in the Hamiltonian, we also account for quasiparticle interactions and spectral broadening. Specifically, we show how anharmonic effects in phonon polaritons lead to a dielectric response that challenges traditional models. The accuracy and consequences of the approach are demonstrated on GaP and GaN as harmonic test systems and PbTe and $β$-Ga$_2$O$_3$ as anharmonic test systems.
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Submitted 12 January, 2026; v1 submitted 6 May, 2025;
originally announced May 2025.
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Roadmap on Advancements of the FHI-aims Software Package
Authors:
Joseph W. Abbott,
Carlos Mera Acosta,
Alaa Akkoush,
Alberto Ambrosetti,
Viktor Atalla,
Alexej Bagrets,
Jörg Behler,
Daniel Berger,
Hannah Bertschi,
Björn Bieniek,
Jonas Björk,
Volker Blum,
Saeed Bohloul,
Connor L. Box,
Nicholas Boyer,
Danilo Simoes Brambila,
Gabriel A. Bramley,
Kyle R. Bryenton,
María Camarasa-Gómez,
Christian Carbogno,
Fabio Caruso,
Sucismita Chutia,
Michele Ceriotti,
Gábor Csányi,
William Dawson
, et al. (181 additional authors not shown)
Abstract:
Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precis…
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Electronic-structure theory is the foundation of the description of materials including multiscale modeling of their properties and functions. Obviously, without sufficient accuracy at the base, reliable predictions are unlikely at any level that follows. The software package FHI-aims has proven to be a game changer for accurate free-energy calculations because of its scalability, numerical precision, and its efficient handling of density functional theory (DFT) with hybrid functionals and van der Waals interactions. It treats molecules, clusters, and extended systems (solids and liquids) on an equal footing. Besides DFT, FHI-aims also includes quantum-chemistry methods, descriptions for excited states and vibrations, and calculations of various types of transport. Recent advancements address the integration of FHI-aims into an increasing number of workflows and various artificial intelligence (AI) methods. This Roadmap describes the state-of-the-art of FHI-aims and advancements that are currently ongoing or planned.
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Submitted 20 April, 2026; v1 submitted 30 April, 2025;
originally announced May 2025.
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Ab initio theory of the non-resonant Raman effect in crystals at finite temperature in comparison to experiment: The examples of GaN and BaZrS3
Authors:
Florian Knoop,
Nimrod Benshalom,
Matan Menahem,
Paul Gartner,
Tommaso Salzillo,
Omer Yaffe,
Olle Hellman
Abstract:
We present an ab initio theory of the non-resonant Raman scattering process in crystals at finite temperature in direct comparison with experiments. The theory incorporates the scattering geometry and polarization dependence of the Raman process and the small but finite wave vectors of the phonons for correctly describing the scattering with longitudinal optical (LO) modes in optically anisotropic…
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We present an ab initio theory of the non-resonant Raman scattering process in crystals at finite temperature in direct comparison with experiments. The theory incorporates the scattering geometry and polarization dependence of the Raman process and the small but finite wave vectors of the phonons for correctly describing the scattering with longitudinal optical (LO) modes in optically anisotropic solids. We implement the theory for first-order Raman scattering and showcase the approach for wurtzite Gallium Nitride and the complex chalcogenide perovskite BaZrS3 in comparison to experiment. We subsequently discuss several common estimates for second-order Raman scattering in complex materials, and highlight similarities and differences to established theoretical approaches and simulation protocols both from phonon theory and molecular dynamics.
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Submitted 23 December, 2024;
originally announced December 2024.
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Mode-coupling formulation of heat transport in anharmonic materials
Authors:
Aloïs Castellano,
J. P. Alvarinhas Batista,
Olle Hellman,
Matthieu J. Verstraete
Abstract:
The temperature-dependent phonons are a generalization of interatomic force constants varying in T, which as found widespread use in computing the thermal transport of materials. A formal justification for using this combination to access thermal conductivity in anharmonic crystals, beyond the harmonic approximation and perturbation theory, is still lacking. In this work, we derive a theory of hea…
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The temperature-dependent phonons are a generalization of interatomic force constants varying in T, which as found widespread use in computing the thermal transport of materials. A formal justification for using this combination to access thermal conductivity in anharmonic crystals, beyond the harmonic approximation and perturbation theory, is still lacking. In this work, we derive a theory of heat transport of anharmonic crystals, using the mode-coupling theory of anharmonic lattice dynamics. Starting from the Green-Kubo formula, we develop the thermal conductivity tensor based on the system's dynamical susceptibility, or spectral function. Our results account for both the diagonal and off-diagonal contributions of the heat current, with and without collective effects. We implement our theory in the TDEP package, and have notably introduced a Monte Carlo scheme to compute phonon scattering due to third- and fourth-order interactions, achieving a substantial reduction in computational cost which enables full convergence of such calculations for the first time. We apply our methodology to systems with varying regimes of anharmonicity and thermal conductivity to demonstrate its universality. These applications highlight the importance of the phonon renormalizations and their interactions beyond the harmonic order. Overall, our work advances the understanding of thermal conductivity in anharmonic crystals and provides a theoretically robust framework for predicting heat transport in complex materials.
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Submitted 30 January, 2025; v1 submitted 22 November, 2024;
originally announced November 2024.
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Vibrational properties differ between halide and chalcogenide perovskite semiconductors, and it matters for optoelectronic performance
Authors:
K. Ye,
M. Menahem,
T. Salzillo,
F. Knoop,
B. Zhao,
S. Niu,
O. Hellman,
J. Ravichandran,
R. Jaramillo,
O. Yaffe
Abstract:
We report a comparative study of temperature-dependent photoluminescence and structural dynamics of two perovskite semiconductors, the chalcogenide BaZrS$_3$ (BZS) and the halide CsPbBr$_3$ (CPB). These materials have similar crystal structures and direct band gaps, but we find that they have quite distinct optoelectronic and vibrational properties. Both materials exhibit thermally-activated non-r…
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We report a comparative study of temperature-dependent photoluminescence and structural dynamics of two perovskite semiconductors, the chalcogenide BaZrS$_3$ (BZS) and the halide CsPbBr$_3$ (CPB). These materials have similar crystal structures and direct band gaps, but we find that they have quite distinct optoelectronic and vibrational properties. Both materials exhibit thermally-activated non-radiative recombination, but the non-radiative recombination rate in BZS is between two and four orders of magnitude faster than in CPB. Raman spectroscopy reveals that the effects of phonon anharmonicity are far more pronounced in CPB than in BZS. Further, although both materials feature a large dielectric response due to low-energy polar optical phonons, the phonons in CPB are substantially lower in energy than in BZS. Our results suggest that electron-phonon coupling in BZS is more effective at non-radiative recombination than in CPB, and that BZS may also have a substantially higher concentration of non-radiative recombination centers than CPB. The low defect concentration in CPB may be related to the ease of lattice reconfiguration, typified by anharmonic bonding. It remains to be seen to what extent these differences are inherent to the chalcogenide and halide perovskites and to what extent they can be affected by materials processing; comparing BZS single-crystals and thin films provides reason for optimism.
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Submitted 14 April, 2024; v1 submitted 29 February, 2024;
originally announced February 2024.
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The Disorder Origin of Raman Scattering In Perovskites Single Crystals
Authors:
Matan Menahem,
Nimrod Benshalom,
Maor Asher,
Sigalit Aharon,
Roman Korobko,
Sam Safran,
Olle Hellman,
Omer Yaffe
Abstract:
The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the non-monotonic evolution with te…
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The anharmonic lattice dynamics of oxide and halide perovskites play a crucial role in their mechanical and optical properties. Raman spectroscopy is one of the key methods used to study these structural dynamics. However, despite decades of research, existing interpretations cannot explain the temperature dependence of the observed Raman spectra. We demonstrate the non-monotonic evolution with temperature of the scattering intensity and present a model for 2nd-order Raman scattering that accounts for this unique trend. By invoking a low-frequency anharmonic feature, we are able to reproduce the Raman spectral line-shapes and integrated intensity temperature dependence. Numerical simulations support our interpretation of this low-frequency mode as a transition between two minima of a double-well potential surface. The model can be applied to other dynamically disordered crystal phases, providing a better understanding of the structural dynamics, leading to favorable electronic, optical, and mechanical properties in functional materials.
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Submitted 10 August, 2022;
originally announced August 2022.
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Hybrid ab initio method for examining thermal properties in magnetic materials
Authors:
Matthew Heine,
Olle Hellman,
David Broido
Abstract:
A hybrid ab initio theoretical approach for examining thermal properties in magnetic systems of unknown entropy is presented. Commonly used theoretical approaches interrogate thermal properties from Gibbs/Helmholtz free energies, which require an accurate model of magnetic interactions. The present approach avoids this requirement by instead calculating system pressure from thermally disordered mi…
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A hybrid ab initio theoretical approach for examining thermal properties in magnetic systems of unknown entropy is presented. Commonly used theoretical approaches interrogate thermal properties from Gibbs/Helmholtz free energies, which require an accurate model of magnetic interactions. The present approach avoids this requirement by instead calculating system pressure from thermally disordered microstates that properly incorporate vibrational and spin subsystems at each temperature as well as the coupling between these subsystems. In place of a specific model for magnetic interactions, the approach integrates measurements of temperature dependent magnetization of the studied material. We apply the approach to calculate phonon modes and to investigate the anomalously low thermal expansion of the classical Invar alloy, Fe_0.65Ni_0.35. The calculated phonon dispersions for Invar are in excellent agreement with measured data. The Invar thermal expansion is shown to remain small between 50 K and room temperature, consistent with the experimentally observed low thermal expansion value in this same temperature range. This anomalously small thermal expansion is directly connected to a small positive contribution from lattice thermal disorder that is nearly canceled by a large negative magnetic disorder contribution. By contrast, calculations for bcc Fe show a much larger thermal expansion, consistent with experiment, which is dominated by a large contribution from lattice thermal disorder that is reduced only slightly by a small negative contribution from that of magnetism. These findings give insights into the unusual nature of magnetism and spin-lattice coupling in Invar and Fe, and they support the presented new methodology as a complementary way to investigate thermal properties of magnetic materials.
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Submitted 11 May, 2022;
originally announced May 2022.
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Phonon-phonon interactions in the polarizarion dependence of Raman scattering
Authors:
Nimrod Benshalom,
Maor Asher,
Rémy Jouclas,
Roman Korobko,
Guillaume Schweicher,
Jie Liu,
Yves Geerts,
Olle Hellman,
Omer Yaffe
Abstract:
We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank formalism. This generalization of the second-rank Raman tensor $\mathcal{R}$ stems from the effect of off-diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon-phonon intera…
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We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank formalism. This generalization of the second-rank Raman tensor $\mathcal{R}$ stems from the effect of off-diagonal components in the crystal self-energy on the light scattering mechanism. We thus establish a novel manifestation of phonon-phonon interaction in inelastic light scattering, markedly separate from the better-known phonon lifetime.
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Submitted 22 June, 2023; v1 submitted 26 April, 2022;
originally announced April 2022.
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Unraveling heat transport and dissipation in suspended MoSe$_2$ crystals from bulk to monolayer
Authors:
D. Saleta Reig,
S. Varghese,
R. Farris,
A. Block,
J. D. Mehew,
O. Hellman,
P. Woźniak,
M. Sledzinska,
A. El Sachat,
E. Chávez-Ángel,
S. O. Valenzuela,
N. F. Van Hulst,
P. Ordejón,
Z. Zanolli,
C. M. Sotomayor Torres,
M. J. Verstraete,
K. J. Tielrooij
Abstract:
Understanding thermal transport in layered transition metal dichalcogenide (TMD) crystals is crucial for a myriad of applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood. Here, we present a combined experimental-theoretical study of the intrinsic lattice thermal conductivity of the representative…
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Understanding thermal transport in layered transition metal dichalcogenide (TMD) crystals is crucial for a myriad of applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood. Here, we present a combined experimental-theoretical study of the intrinsic lattice thermal conductivity of the representative TMD MoSe$_2$, focusing on the effect of material thickness and the material's environment. We use Raman thermometry measurements on suspended crystals, where we identify and eliminate crucial artefacts, and perform $ab$ $initio$ simulations with phonons at finite, rather than zero, temperature. We find that phonon dispersions and lifetimes change strongly with thickness, yet (sub)nanometer thin TMD films exhibit a similar in-plane thermal conductivity ($\sim$20~Wm$^{-1}$K$^{-1}$) as bulk crystals ($\sim$40~Wm$^{-1}$K$^{-1}$). This is the result of compensating phonon contributions, in particular low-frequency modes with a surprisingly long mean free path of several micrometers that contribute significantly to thermal transport for monolayers. We furthermore demonstrate that out-of-plane heat dissipation to air is remarkably efficient, in particular for the thinnest crystals. These results are crucial for the design of TMD-based applications in thermal management, thermoelectrics and (opto)electronics.
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Submitted 19 September, 2021;
originally announced September 2021.
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Direct Observation of Chiral Phonons by Inelastic X-ray Scattering
Authors:
Qingan Cai,
Olle Hellman,
Bin Wei,
Qiyang Sun,
Ayman H. Said,
Thomas Gog,
Barry Winn,
Chen Li
Abstract:
Phonon chirality has attracted intensive attention since it breaks the traditional cognition that phonons are linear propagating bosons. This new quasiparticle property has been extensively studied theoretically and experimentally. However, characterization of the phonon chirality throughout the full Brillouin zone is still not possible due to the lack of available experimental tools. In this work…
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Phonon chirality has attracted intensive attention since it breaks the traditional cognition that phonons are linear propagating bosons. This new quasiparticle property has been extensively studied theoretically and experimentally. However, characterization of the phonon chirality throughout the full Brillouin zone is still not possible due to the lack of available experimental tools. In this work, phonon dispersion and chirality of tungsten carbide were investigated by millielectronvolt energy-resolution inelastic X-ray scattering. The atomistic calculation indicates that in-plane longitudinal and transverse acoustic phonons near K and K$^\prime$ points are circularly polarized due to the broken inversion symmetry. Anomalous inelastic X-ray scattering by these circularly polarized phonons was observed and attributed to their chirality. Our results show that inelastic X-ray scattering can be utilized to characterize phonon chirality in materials and suggest that a revision to the phonon scattering function is necessary.
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Submitted 14 August, 2021;
originally announced August 2021.
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The dielectric response of rock-salt crystals at finite temperatures from first principles
Authors:
Nimrod Benshalom,
Guy Reuveni,
Roman Korobko,
Omer Yaffe,
Olle Hellman
Abstract:
We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasi-harmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the diel…
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We combine ab initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature dependent dielectric response of a NaCl single crystal. We measure the temperature evolution of its Raman spectrum and compare it to both a quasi-harmonic and anharmonic model. Results demonstrate the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering. Our model fully captures the linear dielectric response of a crystal at finite temperatures and may therefore be used to calculate the temperature dependence of other material properties governed by it.
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Submitted 25 September, 2021; v1 submitted 10 August, 2021;
originally announced August 2021.
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The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe
Authors:
Đorđe Dangić,
Olle Hellman,
Stephen Fahy,
Ivana Savić
Abstract:
The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity $κ$. However, the $κ$ of GeTe increases at the ferroelectric phase transition near $700$ K. Using first-principles calculations with the temperature dependent effective potential method, we show that this…
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The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity $κ$. However, the $κ$ of GeTe increases at the ferroelectric phase transition near $700$ K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in $κ$ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Negative thermal expansion increases phonon group velocities, which counteracts enhanced anharmonicity of phonon modes and boosts $κ$ close to the phase transition in the rhombohedral phase. A drastic decrease in the anharmonic force constants in the cubic phase increases the phonon lifetimes and $κ$. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a novel method of calculating $κ$ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates $κ$ near the phase transition. Our findings elucidate the influence of structural phase transitions on $κ$ and provide guidance for design of better thermoelectric materials.
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Submitted 22 February, 2021; v1 submitted 15 December, 2020;
originally announced December 2020.
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Thermal transport in nanoporous holey silicon membranes investigated with optically-induced transient thermal gratings
Authors:
Ryan A. Duncan,
Giuseppe Romano,
Marianna Sledzinska,
Alexei A. Maznev,
Jean-Philippe M. Peraud,
Olle Hellman,
Clivia M. Sotomayor Torres,
Keith A. Nelson
Abstract:
In this study, we use the transient thermal grating optical technique \textemdash a non-contact, laser-based thermal metrology technique with intrinsically high accuracy \textemdash to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 100 nm and 250 nm respectively. We compare the experimental results to ab initio c…
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In this study, we use the transient thermal grating optical technique \textemdash a non-contact, laser-based thermal metrology technique with intrinsically high accuracy \textemdash to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon membranes with limiting dimensions of 100 nm and 250 nm respectively. We compare the experimental results to ab initio calculations of phonon-mediated thermal transport according to the phonon Boltzmann transport equation (BTE) using two different computational techniques. We find that the calculations conducted within the Casimir framework, i.e. based on the BTE with the bulk phonon dispersion and diffuse scattering from surfaces, are in quantitative agreement with the experimental data, and thus conclude that this framework is adequate for describing phonon-mediated thermal transport through holey silicon membranes with feature sizes on the order of 100 nm.
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Submitted 20 October, 2020; v1 submitted 12 December, 2019;
originally announced December 2019.
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Anharmonicity and Ultra-Low Thermal Conductivity in Lead-Free Halide Double Perovskites
Authors:
Johan Klarbring,
Olle Hellman,
Igor A. Abrikosov,
Sergei I. Simak
Abstract:
The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs$_2$AgBiBr$_6$ , the prototypical compound in this class, and show that the lattice dynamics of Cs$_2$AgBiBr$_6$ is highly anharmonic, l…
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The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs$_2$AgBiBr$_6$ , the prototypical compound in this class, and show that the lattice dynamics of Cs$_2$AgBiBr$_6$ is highly anharmonic, largely in regards to tilting of AgBr$_6$ and BiBr$_6$ octahedra. Using an energy and temperature dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs$_2$AgBiBr$_6$ yield an ultra-low thermal conductivity, unexpected of high symmetry cubic structures.
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Submitted 11 December, 2019;
originally announced December 2019.
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Impact of anharmonicity on sound wave velocities at extreme conditions
Authors:
Olle Hellman,
S. I. Simak
Abstract:
Theoretical calculations of sound-wave velocities of materials at extreme conditions are of great importance to various fields, in particular geophysics. For example, the seismic data on sound-wave propagation through the solid iron-rich Earth's inner core have been the main source for elucidating its properties and building models. As the laboratory experiments at very high temperatures and press…
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Theoretical calculations of sound-wave velocities of materials at extreme conditions are of great importance to various fields, in particular geophysics. For example, the seismic data on sound-wave propagation through the solid iron-rich Earth's inner core have been the main source for elucidating its properties and building models. As the laboratory experiments at very high temperatures and pressures are non-trivial, ab initio predictions are invaluable. The latter, however, tend to disagree with experiment. We notice that many attempts to calculate sound-wave velocities of matter at extreme conditions in the framework of quantum-mechanics based methods have not been taking into account the effect of anharmonic atomic vibrations. We show how anharmonic effects can be incorporated into ab initio calculations and demonstrate that in particular they might be non-negligible for iron in Earth's core. Therefore, we open an avenue to reconcile experiment and ab initio theory.
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Submitted 28 August, 2019;
originally announced August 2019.
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Thermoelectric transport trends in group 4 half-Heusler alloys
Authors:
Kristian Berland,
Nina Shulumba,
Olle Hellman,
Clas Persson,
Ole Martin Løvvik
Abstract:
The thermoelectric properties of 54 different group 4 half-Heusler (HH) alloys have been studied from first principles. Electronic transport was studied with density functional theory using hybrid functionals facilitated by the $\mathbf{k} \cdot \mathbf{p}$ method, while the temperature dependent effective potential method was used for the phonon contributions to the figure of merit $ZT$. The phon…
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The thermoelectric properties of 54 different group 4 half-Heusler (HH) alloys have been studied from first principles. Electronic transport was studied with density functional theory using hybrid functionals facilitated by the $\mathbf{k} \cdot \mathbf{p}$ method, while the temperature dependent effective potential method was used for the phonon contributions to the figure of merit $ZT$. The phonon thermal conductivity was calculated including anharmonic phonon-phonon, isotope, alloy and grain-boundary scattering. HH alloys have an ${\it XYZ}$ composition and those studied here are in the group 4-9-15 (Ti,Zr,Hf)(Co,Rh,Ir)(As,Sb,Bi) and group 4-10-14 (Ti,Zr,Hf)(Ni,Pd,Pt)(Ge,Sn,Pb). The electronic part of the thermal conductivity was found to significantly impact $ZT$ and thus the optimal doping level. Furthermore, the choice of functional was found to significantly affect thermoelectric properties, particularly for structures exhibiting band alignment features. The intrinsic thermal conductivity was significantly reduced when alloy and grain boundary scattering were accounted for, which also reduced the spread in thermal conductivity. It was found that sub-lattice disorder on the ${\it Z}$-site, i.e. the site occupied by group 14 or 15 elements, was more effective than ${\it X}$-site substitution, occupied by group 4 elements. The calculations confirmed that ZrNiSn, ZrCoSb and ZrCoBi based alloys display promising thermoelectric properties. A few other n-type and p-type compounds were also predicted to be potentially excellent thermoelectric materials, given that sufficiently high charge carrier concentrations can be achieved. This study provides insight into the thermoelectric potential of HH alloys and casts light on strategies to optimize thermoelectric performance of multicomponent alloys.
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Submitted 5 July, 2019;
originally announced July 2019.
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Vibrational Effects in X-ray Absorption Spectra of 2D Layered Materials
Authors:
Weine Olovsson,
Teruyasu Mizoguchi,
Martin Magnuson,
Stefan Kontur,
Olle Hellman,
Isao Tanaka,
Claudia Draxl
Abstract:
With the examples of the C $K$-edge in graphite and the B $K$-edge in hexagonal BN, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in 2D layered materials. Theoretical XANES spectra are obtained by solving the Bethe-Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated mo…
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With the examples of the C $K$-edge in graphite and the B $K$-edge in hexagonal BN, we demonstrate the impact of vibrational coupling and lattice distortions on the X-ray absorption near-edge structure (XANES) in 2D layered materials. Theoretical XANES spectra are obtained by solving the Bethe-Salpeter equation of many-body perturbation theory, including excitonic effects through the correlated motion of core-hole and excited electron. We show that accounting for zero-point motion is important for the interpretation and understanding of the measured X-ray absorption fine structure in both materials, in particular for describing the $σ^*$-peak structure.
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Submitted 23 April, 2019;
originally announced April 2019.
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Order-disorder transition in the prototypical antiferroelectric PbZrO$_3$
Authors:
Bin Xu,
Olle Hellman,
L. Bellaiche
Abstract:
The prototypical antiferroelectric PbZrO$_3$ has several unsettled questions, such as the nature of the antiferroelectric transition, possible intermediate phase and the microscopic origin of the Pbam ground state. Using first principles, we show that no phonon becomes truly soft at the cubic-to-Pbam transition temperature, and the order-disorder character of this transition is clearly demonstrate…
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The prototypical antiferroelectric PbZrO$_3$ has several unsettled questions, such as the nature of the antiferroelectric transition, possible intermediate phase and the microscopic origin of the Pbam ground state. Using first principles, we show that no phonon becomes truly soft at the cubic-to-Pbam transition temperature, and the order-disorder character of this transition is clearly demonstrated based on molecular dynamics simulations and potential energy surfaces. The out-of-phase octahedral tilting is an important degree of freedom, which can collaborate with other phonon distortions and form a complex energy landscape with multiple minima. Candidates of the possible intermediate phase are suggested based on the calculated kinetic barriers between energy minima, and the development of a first-principles-based effective Hamiltonian. The use of this latter scheme further reveals that specific bi-linear interactions between local dipoles and octahedral tiltings play a major role in the formation of the Pbam ground state, which contrasts with most of the previous explanations.
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Submitted 20 July, 2019; v1 submitted 23 February, 2019;
originally announced February 2019.
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Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO$_{3}$ Perovskite from First Principles
Authors:
Jin-Jian Zhou,
Olle Hellman,
Marco Bernardi
Abstract:
Structural phase transitions and soft phonon modes pose a longstanding challenge to computing electron-phonon (e-ph) interactions in strongly anharmonic crystals. Here we develop a first-principles approach to compute e-ph scattering and charge transport in materials with anharmonic lattice dynamics. Our approach employs renormalized phonons to compute the temperature-dependent e-ph coupling for a…
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Structural phase transitions and soft phonon modes pose a longstanding challenge to computing electron-phonon (e-ph) interactions in strongly anharmonic crystals. Here we develop a first-principles approach to compute e-ph scattering and charge transport in materials with anharmonic lattice dynamics. Our approach employs renormalized phonons to compute the temperature-dependent e-ph coupling for all phonon modes, including the soft modes associated with ferroelectricity and phase transitions. We show that the electron mobility in cubic SrTiO$_{3}$ is controlled by scattering with longitudinal optical phonons at room temperature and with ferroelectric soft phonons below 200~K. Our calculations can accurately predict the temperature dependence of the electron mobility in SrTiO$_{3}$ between 150$-$300~K, and reveal the microscopic origin of its roughly $T^{-3}$ trend. Our approach enables first-principles calculations of e-ph interactions and charge transport in broad classes of crystals with phase transitions and strongly anharmonic phonons.
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Submitted 30 November, 2018; v1 submitted 14 June, 2018;
originally announced June 2018.
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A nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon
Authors:
D. S. Kim,
O. Hellman,
J. Herriman,
H. L. Smith,
J. Y. Y. Lin,
N. Shulumba,
J. L. Niedziela,
C. W. Li,
D. L. Abernathy,
B. Fultz
Abstract:
Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well-understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron…
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Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well-understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100-1500K on a single-crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a fortuitous cancellation of contributions from individual phonons.
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Submitted 6 February, 2018; v1 submitted 27 October, 2016;
originally announced October 2016.
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Intrinsic localized mode and low thermal conductivity of PbSe
Authors:
Nina Shulumba,
Olle Hellman,
Austin J. Minnich
Abstract:
Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity despite being stiffer, a trend that prior first-principles calculations have not reproduced. Here, we use ab-initio calculations that explicitly account for stron…
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Lead chalcogenides such as PbS, PbSe, and PbTe are of interest for their exceptional thermoelectric properties and strongly anharmonic lattice dynamics. Although PbTe has received the most attention, PbSe has a lower thermal conductivity despite being stiffer, a trend that prior first-principles calculations have not reproduced. Here, we use ab-initio calculations that explicitly account for strong anharmonicity to identify the origin of this low thermal conductivity as an anomalously large anharmonic interaction, exceeding in strength that in PbTe, between the transverse optic and longitudinal acoustic branches. The strong anharmonicity is reflected in the striking observation of an intrinsic localized mode that forms in the acoustic frequencies. Our work shows the deep insights into thermal phonons that can be obtained from ab-initio calculations that are not confined to the weak limit of anharmonicity.
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Submitted 27 September, 2016;
originally announced September 2016.
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Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics
Authors:
Davide G. Sangiovanni,
Olle Hellman,
Björn Alling,
Igor A. Abrikosov
Abstract:
We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics (NE-AIMD), and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored-atom) is…
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We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics (NE-AIMD), and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored-atom) is accelerated toward the neighboring defect-site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k_{NE} increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k_{NE}(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures, and reaches a factor of four orders of magnitude at the lowest temperature considered in the present study.
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Submitted 13 February, 2016; v1 submitted 15 September, 2015;
originally announced September 2015.
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Anharmonicity changes the solid solubility of an alloy at high temperatures
Authors:
Nina Shulumba,
Olle Hellman,
Zamaan Raza,
Jenifer Barrirero,
Björn Alling,
Frank Mücklich,
Igor A. Abrikosov,
Magnus Odén
Abstract:
We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti$_{1-x}$Al$_x$N alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstra…
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We have developed a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti$_{1-x}$Al$_x$N alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy, corresponding to the true equilibrium state of the system. We demonstrate that the anharmonic contribution and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti$_{1-x}$Al$_x$N alloy, lowering the maximum temperature for the miscibility gap from 6560 K to 2860 K. Our local chemical composition measurements on thermally aged Ti$_{0.5}$Al$_{0.5}$N alloys agree with the calculated phase diagram.
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Submitted 23 September, 2015; v1 submitted 9 March, 2015;
originally announced March 2015.
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Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO$_2$
Authors:
Tian Lan,
C. W. Li,
O. Hellman,
D. Kim,
J. A. Muñoz,
H. Smith,
D. L. Abernathy,
B. Fultz
Abstract:
Although the rutile structure of TiO$_2$ is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000\,K. Inelastic neutron scattering was used to measure the temperature dependence of the phonon density o…
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Although the rutile structure of TiO$_2$ is stable at high temperatures, the conventional quasiharmonic approximation predicts that several acoustic phonons decrease anomalously to zero frequency with thermal expansion, incorrectly predicting a structural collapse at temperatures well below 1000\,K. Inelastic neutron scattering was used to measure the temperature dependence of the phonon density of states (DOS) of rutile TiO$_2$ from 300 to 1373\,K. Surprisingly, these anomalous acoustic phonons were found to increase in frequency with temperature. First-principles calculations showed that with lattice expansion, the potentials for the anomalous acoustic phonons transform from quadratic to quartic, stabilizing the rutile phase at high temperatures. In these modes, the vibrational displacements of adjacent Ti and O atoms cause variations in hybridization of $3d$ electrons of Ti and $2p$ electrons of O atoms. With thermal expansion, the energy variation in this "phonon-tracked hybridization" flattens the bottom of the interatomic potential well between Ti and O atoms, and induces a quarticity in the phonon potential.
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Submitted 16 August, 2015; v1 submitted 16 February, 2015;
originally announced February 2015.
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Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab-initio molecular dynamics
Authors:
Nina Shulumba,
Björn Alling,
Olle Hellman,
Elham Mozafari,
Peter Steneteg,
Magnus Odén,
Igor A. Abrikosov
Abstract:
We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab-initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics (DLM-MD) with…
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We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab-initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics (DLM-MD) with the temperature dependent effective potential (TDEP) method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.
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Submitted 19 March, 2014;
originally announced March 2014.
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Thermal Anharmonic Effects in PbTe from First Principles
Authors:
A. H. Romero,
E. K. U. Gross,
M. J. Verstraete,
Olle Hellman
Abstract:
We investigate the harmonic and anharmonic contributions to the phonon spectrum of lead telluride, and perform a complete characterization of how the anharmonic effects dominate the phonons in PbTe as temperature increases. This effect is the strongest factor in the favorable thermoelectric properties of PbTe: an optical-acoustic phonon band crossing reduces the speed of sound and the intrinsic th…
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We investigate the harmonic and anharmonic contributions to the phonon spectrum of lead telluride, and perform a complete characterization of how the anharmonic effects dominate the phonons in PbTe as temperature increases. This effect is the strongest factor in the favorable thermoelectric properties of PbTe: an optical-acoustic phonon band crossing reduces the speed of sound and the intrinsic thermal conductivity. We present the detailed temperature dependence of the dispersion relation and compare our calculated neutron scattering cross section with recent experimental measurements. We analyze the thermal resistivity's variation with temperature and clarify misconceptions about existing experimental literature. This quantitative prediction opens the way to phonon phase space engineering, to tailor the lifetimes of crucial heat carrying phonons.
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Submitted 22 February, 2014;
originally announced February 2014.
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Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics
Authors:
C. W. Li,
O. Hellman,
J. Ma,
A. F. May,
H. B. Cao,
X. Chen,
A. D. Christianson,
G. Ehlers,
D. J. Singh,
B. C. Sales,
O. Delaire
Abstract:
The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of t…
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The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.
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Submitted 28 December, 2013;
originally announced December 2013.
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Temperature dependent effective third order interatomic force constants from first principles
Authors:
Olle Hellman,
Igor A. Abrikosov
Abstract:
The temperature dependent effective potential (TDEP) method is generalized beyond pair interactions. The second and third order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the Mode Grüneisen parameters for Si. We show that the extension of TDEP to higher order all…
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The temperature dependent effective potential (TDEP) method is generalized beyond pair interactions. The second and third order force constants are determined consistently from ab initio molecular dynamics simulations at finite temperature. The reliability of the approach is demonstrated by calculations of the Mode Grüneisen parameters for Si. We show that the extension of TDEP to higher order allows for an efficient calculation of the phonon life time, in Si as well as in $ε$-FeSi, a system that exhibits anomalous softening with temperature.
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Submitted 25 August, 2013;
originally announced August 2013.
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Temperature dependent effective potential method for accurate free energy calculations of solids
Authors:
Olle Hellman,
Peter Steneteg,
Igor A. Abrikosov,
Sergei I. Simak
Abstract:
We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in details. A number of practical exampl…
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We have developed a thorough and accurate method of determining anharmonic free energies, the temperature dependent effective potential technique (TDEP). It is based on \emph{ab initio} molecular dynamics followed by a mapping onto a model Hamiltonian that describes the lattice dynamics. The formalism and the numerical aspects of the technique are described in details. A number of practical examples are given, and results are presented, which confirm the usefulness of TDEP within \emph{ab initio} and classical molecular dynamics frameworks. In particular, we examine from first-principles the behavior of force constants upon the dynamical stabilization of body centered phase of Zr, and show that they become more localized. We also calculate phase diagram for $^4$He modeled with the Aziz \emph{et al.} potential and obtain results which are in favorable agreement both with respect to experiment and established techniques.
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Submitted 5 March, 2013;
originally announced March 2013.
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Lattice dynamics of anharmonic solids from first principles
Authors:
Olle Hellman,
I. A. Abrikosov,
S. I. Simak
Abstract:
An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic - or higher order - potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximati…
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An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic - or higher order - potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated.
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Submitted 29 October, 2011; v1 submitted 29 March, 2011;
originally announced March 2011.