Olle Hellman
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View article: Hydrogen Bonds Induce Double-Well Spectroscopic Signatures in α-Glycine
Hydrogen Bonds Induce Double-Well Spectroscopic Signatures in α-Glycine Open
Hydrogen bonds in molecular crystals are often modeled as double-well potentials, yet direct evidence linking this potential form to vibrational spectroscopic features remains elusive. In this study, we investigate α-glycine, a hydrogen-bo…
View article: Abinit 2025: New capabilities for the predictive modeling of solids and nanomaterials
Abinit 2025: New capabilities for the predictive modeling of solids and nanomaterials Open
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…
View article: Impact of anharmonicity on the carrier mobility of the Pb-free CsSnBr$_3$ perovskite
Impact of anharmonicity on the carrier mobility of the Pb-free CsSnBr$_3$ perovskite Open
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 electr…
View article: Microscopic theory of phonon polaritons and long wavelength dielectric response
Microscopic theory of phonon polaritons and long wavelength dielectric response Open
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…
View article: Mode-coupling formulation of heat transport in anharmonic materials
Mode-coupling formulation of heat transport in anharmonic materials Open
peer reviewed
View article: Roadmap on Advancements of the FHI-aims Software Package
Roadmap on Advancements of the FHI-aims Software Package Open
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 lev…
View article: Ab initio theory of the non-resonant Raman effect in crystals at finite temperature in comparison to experiment: The examples of GaN and BaZrS3
Ab initio theory of the non-resonant Raman effect in crystals at finite temperature in comparison to experiment: The examples of GaN and BaZrS3 Open
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 p…
View article: Mode-coupling formulation of heat transport in anharmonic materials
Mode-coupling formulation of heat transport in anharmonic materials Open
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 …
View article: Differing vibrational properties of halide and chalcogenide perovskite semiconductors and impact on optoelectronic performance
Differing vibrational properties of halide and chalcogenide perovskite semiconductors and impact on optoelectronic performance Open
We report a comparative study of temperature-dependent photoluminescence and structural dynamics of two perovskite semiconductors, the chalcogenide BaZrS3 3 and the halide CsPbBr3. 3 . These materials have similar crystal structures and di…
View article: Improving the precision of forces in real-space pseudopotential density functional theory
Improving the precision of forces in real-space pseudopotential density functional theory Open
The high-order finite difference real-space pseudopotential density functional theory (DFT) approach is a valuable method for large-scale, massively parallel DFT calculations. A significant challenge in the approach is the oscillating “egg…
View article: Microscopic understanding of the in-plane thermal transport properties of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>2</mml:mn><mml:mi>H</mml:mi></mml:mrow></mml:math> transition metal dichalcogenides
Microscopic understanding of the in-plane thermal transport properties of transition metal dichalcogenides Open
Transition metal dichalcogenides (TMDs) are a class of layered materials that hold great promise for a wide range of applications. Their practical use can be limited by their thermal transport properties, which have proven challenging to d…
View article: Vibrational properties differ between halide and chalcogenide perovskite semiconductors, and it matters for optoelectronic performance
Vibrational properties differ between halide and chalcogenide perovskite semiconductors, and it matters for optoelectronic performance Open
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 struc…
View article: TDEP: Temperature Dependent EffectivePotentials
TDEP: Temperature Dependent EffectivePotentials Open
The Temperature Dependent Effective Potential (TDEP) method is a versatile and efficient approach to include temperature in a binitio materials simulations based on phonon theory. TDEP can be used to describe thermodynamic properties in cl…
View article: Phonon–Phonon Interactions in the Polarization Dependence of Raman Scattering
Phonon–Phonon Interactions in the Polarization Dependence of Raman Scattering Open
We have found that the polarization dependence of Raman scattering in organic crystals at finite temperatures can only be described by a fourth-rank tensor formalism. This generalization of the second-rank Raman tensor stems from the effec…
View article: Finite Difference Interpolation for Reduction of Grid-Related Errors in Real-Space Pseudopotential Density Functional Theory
Finite Difference Interpolation for Reduction of Grid-Related Errors in Real-Space Pseudopotential Density Functional Theory Open
The real-space pseudopotential approach is a well-known method for large-scale density functional theory (DFT) calculations. One of its main limitations, however, is the introduction of errors associated with the positioning of the underly…
View article: Theory of thermal properties of magnetic materials with unknown entropy
Theory of thermal properties of magnetic materials with unknown entropy Open
Theoretical approaches to study thermal properties of magnetic materials typically require accurate models of magnetic interactions in order to define the entropy. Here we introduce a complementary approach for examining thermal properties…
View article: The Disorder Origin of Raman Scattering In Perovskites Single Crystals
The Disorder Origin of Raman Scattering In Perovskites Single Crystals Open
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 …
View article: Thermal expansion and phonon anharmonicity of cuprite studied by inelastic neutron scattering and <i>ab initio</i> calculations
Thermal expansion and phonon anharmonicity of cuprite studied by inelastic neutron scattering and <i>ab initio</i> calculations Open
Inelastic neutron scattering measurements were performed with a time-of-flight chopper spectrometer to observe phonons in all parts of the Brillouin zone of a single crystal of cuprite Cu₂O. We reduced the experimental data to phonon dispe…
View article: Hybrid ab initio method for examining thermal properties in magnetic materials
Hybrid ab initio method for examining thermal properties in magnetic materials Open
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 req…
View article: Phonon-phonon interactions in the polarizarion dependence of Raman scattering
Phonon-phonon interactions in the polarizarion dependence of Raman scattering Open
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 th…
View article: Dielectric response of rock-salt crystals at finite temperatures from first principles
Dielectric response of rock-salt crystals at finite temperatures from first principles Open
We combine ab initio simulations and Raman scattering measurements to\ndemonstrate explicit anharmonic effects in the temperature dependent dielectric\nresponse of a NaCl single crystal. We measure the temperature evolution of its\nRaman s…
View article: Diverging Expressions of Anharmonicity in Halide Perovskites
Diverging Expressions of Anharmonicity in Halide Perovskites Open
Lead‐based halide perovskite crystals are shown to have strongly anharmonic structural dynamics. This behavior is important because it may be the origin of their exceptional photovoltaic properties. The double perovskite, Cs 2 AgBiBr 6 , h…
View article: Unraveling Heat Transport and Dissipation in Suspended MoSe<sub>2</sub> from Bulk to Monolayer
Unraveling Heat Transport and Dissipation in Suspended MoSe<sub>2</sub> from Bulk to Monolayer Open
Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not wel…
View article: Unraveling heat transport and dissipation in suspended MoSe$_2$ crystals from bulk to monolayer
Unraveling heat transport and dissipation in suspended MoSe$_2$ crystals from bulk to monolayer Open
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 a…
View article: Direct Observation of Chiral Phonons by Inelastic X-ray Scattering
Direct Observation of Chiral Phonons by Inelastic X-ray Scattering Open
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…
View article: The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe
The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe Open
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 …
View article: The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe
The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe Open
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 kappa. However, the kappa of GeTe increases …
View article: Thermal transport in nanoporous holey silicon membranes investigated with optically induced transient thermal gratings
Thermal transport in nanoporous holey silicon membranes investigated with optically induced transient thermal gratings Open
In this study, we use transient thermal gratings—a non-contact, laser-based thermal metrology technique with intrinsically high accuracy—to investigate room-temperature phonon-mediated thermal transport in two nanoporous holey silicon memb…
View article: Temperature-dependent phonon lifetimes and thermal conductivity of silicon by inelastic neutron scattering and <i>ab initio</i> calculations
Temperature-dependent phonon lifetimes and thermal conductivity of silicon by inelastic neutron scattering and <i>ab initio</i> calculations Open
Inelastic neutron scattering on a single crystal of silicon was performed at temperatures from 100 to 1500 K. These experimental data were reduced to obtain phonon spectral intensity at all wave vectors →Q and frequencies ω in the first Br…