arXiv subjects

Kristian Berland

Publications and source records attributed to Kristian Berland.

At least 19 recordsLinked to original sources

arXiv2026

High-fidelity k$\cdot$p representations of first-principles electronic band structures through covariant renormalization

The k$\cdot$p method can be applied directly to first-principles energies and momentum matrix elements, but the resulting models converge slowly with the number of bands and are inexact wherever the underlying Hamiltonian is nonlocal. We show that both limitations can be largely removed by renormalizing the eigenvalue spectra of the Hermitian momentum matrices. These spectra are gauge invariant, and are identical for symmetry-related Cartesian components, hence scaling the recurring magnitudes provides a modest set of parameters that preserves degeneracies and crystal symmetry without needing to construct a symmetry-adapted basis. We choose to renormalize the models against reference eigenvalues and band velocities on rays out of a high-symmetry point. For GaP, a 15-band model can reproduce the band structure in the near-gap reference regions within a few meV. For zincblende and wurtzite AlN, similar accuracy requires 30- or 66-band models, which in the case of zincblende is traced to the strong warping in the [110] direction. The scheme readily generalizes to rocksalt PbTe, which includes spin-orbit coupling, and has L-centered valence and conduction band extrema. Further, we demonstrate, for GaP, how compact four-band Hamiltonians can be obtained by downfolding within the same renormalization scheme. Moreover, by changing the fitting regime to encompass the entire Brillouin zone, the scheme can be used to generate full-zone models that reproduce the density of states. The practical utility is illustrated with a 59-band k.p model for GaP, which can be evaluated on meshes far denser than the reference calculation, that in turn can be used both to resolve fine features in the density of states and to compute the hole conductivity at low temperature.

cond-mat.mtrl-sci↗

arXiv2026

Plasmonic properties and correlation energies from a compact multipole representation of the dielectric response in 2D metals

Multipole-Padé approximants provide a compact representation of dynamical response functions in terms of a small number of collective modes. Here, we generalize this framework to incorporate momentum dependence across the full Brillouin zone of 2D metals by constructing a symmetry-conserving, anisotropic representation of the inverse dielectric function. This analytic form enables efficient and accurate evaluation of quantities involving dynamical screening, including spectral features and correlation energies. We construct such compact representations for a set of seven two dimensional metals spanning distinct electronic regimes, and show that a small number of dispersive plasmonic modes suffices to accurately describe the dielectric response across the full Brillouin zone, while also yielding accurate correlation energies. The proposed representation therefore establishes a direct bridge between {\it ab initio} calculations and analytical models of screening, opening new avenues for applications in condensed matter systems.

cond-mat.mtrl-sci↗

arXiv2026

Bulk plasmons in elemental metals

The spectral properties, momentum dispersion, and broadening of bulk plasmonic excitations of 26 elemental metals are studied from first principles calculations in the random-phase approximation. Spectral band structures are constructed from the resulting momentum- and frequency-dependent inverse dielectric function. We develop an effective analytical representation of the main collective excitations in the dielectric response, extending our earlier model based on multipole-Padé approximants (MPAs) to incorporate both momentum and frequency dependence [MPA($\q$)]. With this representation, we identify plasmonic quasiparticle dispersions exhibiting complex features, including non-parabolic energy and intensity dispersions, discontinuities due to anisotropy, and overlapping effects that lead to band crossings and anti-crossings. Comparing with available experimental data, mainly in the optical limit, we find good agreement with the computed spectra. The results for elemental metals and their effective MPA($\q$) representation establish a reference point that can guide both fundamental studies and practical applications in plasmonics and spectroscopy.

physics.optics↗

arXiv2026

Exceptional thermoelectric properties in Na$_2$TlSb enabled by quasi-1D band structure

Materials with reduced dimensionality offer beneficial density-of-states (DOS) profiles for thermoelectric energy conversion, but can be impractical in realistic devices. Encouragingly, bulk high-symmetry materials can also exhibit similar quasi-low-dimensional band structures. A striking example is the full-Heusler compound Na$_2$TlSb, whose valence-band energy isosurfaces can form intersecting two-dimensional pockets, i.e., a box-like structure. The individual energy isosurface sheets resemble those of 1D quantum wires. The combination of high electron velocities (perpendicular to the pockets) and a rapidly increasing DOS with energy in the transport regime (due to the low dimensionality) makes Na$_2$TlSb a representative case where the band structure gives rise to attractive electronic transport properties. However, these beneficial features could be counteracted by high electronic scattering rates due to the large scattering space. In this first principles study of Na$_2$TlSb we find that the electronic scattering rates remain modest. This result is linked to the reduced matrix elements of large-momentum ($\mathbf{q}$) scattering across the delocalized energy isosurfaces. The enhanced free-carrier screening due to the large DOS also contributes to reducing scattering. In combination, the low-dimensional features and modest scattering result in excellent electronic transport properties. Combined with an ultra-low lattice thermal conductivity of $κ_\ell < 1$ W/mK reported in the literature, we predict a thermoelectric figure of merit ranging from 2.4 at 300 K to a 4.4 at 600 K. The $n$-type properties are also favorable, with $zT$ values from 1.5 at 300 K to 3.0 at 600 K.

cond-mat.mtrl-sci↗

arXiv2026

Momentum- and frequency-resolved collective electronic excitations in solids: insights from spectroscopy and first-principles calculations

Collective electronic excitations, including plasmons, excitons, and intra- and interband transitions, play a central role in determining the dynamic screening, optical response, and energy transport properties of materials. Recent advances in momentum- and frequency-resolved spectroscopies, such as electron energy-loss spectroscopy (EELS) and inelastic x-ray scattering (IXS), together with progress in first-principles many-body perturbation theory (MBPT) calculations, now allow collective excitations to be mapped with considerable precision across the Brillouin zone. This topical review surveys current developments in the representation and interpretation of both experimental and theoretical dielectric-response spectra. Particular emphasis is placed on recent ways of representing spectral band structures (SBS) of the direct and inverse dielectric functions, such as analytical approaches based on multipole-Padé approximants in momentum and frequency (MPA($\q$)), which provide a combined band-like description of the dispersion of the main collective excitations. We discuss how features observed in metals, semiconductors, and low dimensional systems reflect the interplay between electronic structure, screening strength, and local-field effects, and how post-processing procedures can improve the quantitative comparison between experiment and theory. Finally, we provide perspectives on open challenges and potential developments in quantitative dielectric-function analyses.

cond-mat.mtrl-sci↗

arXiv2025

Van der Waals Density Functional for Molecular Crystals

Since the development of the nonlocal correlation functional vdW-DF, the family of van der Waals density functionals has grown to better describe a wide variety of systems. A recent generation of the vdW-DF family, vdW-DF3, featured a newly-constructed form of the nonlocal correlation that more accurately modeled molecular dimers, layered structures, and surface adsorption. However, it also revealed an intrinsic tradeoff in vdW-DF3's parametrization and inflexibility of exchange in the generalized gradient approximation (GGA), limiting its accuracy for molecular crystals. In this paper we propose a new optimization of vdW-DF3 that is tailored to 3D molecular crystals. This functional, called vdW-DF3-mc, contains a new, tunable form of the exchange enhancement factor with parameters that directly correspond to physically relevant qualities. In addition, within the nonlocal correlation, we prioritize smoothness of the kernel switching function as a means of restoring flexibility to vdW-DF3's design. Testing vdW-DF3-mc on several benchmark sets, we achieve highly accurate energetics and geometries for molecular crystals. This is particularly evident for the case of polymorphs of ice, for which errors in the volume and cohesive energy are on the order of only 1%, indicating very promising performance for important subcategories of molecular crystals, such as polymorphism and hydrogen-bonded solids.

cond-mat.mtrl-sci↗

arXiv2025

Common errors in BoltzTraP-based calculations

Boltzmann transport calculations based on band structures computed from first principles play an important role in modern thermoelectric materials research. Among available codes, the \textsc{BoltzTraP} code is the most widely adopted, but many recent studies contain systematic mistakes. We identify three error modes: (1) inserting the electronic thermal conductivity at zero electric field, $κ_0$, in place of the electronic thermal conductivity at zero electric current, $κ_e$, (2) computing the figure of merit $zT$ by combining a constant relaxation time of unity while keeping the lattice thermal conductivity $κ_\ell$ in standard units, and (3) doing both errors at once. We have found many examples of the third error, but since the first two are simpler, we suspect they are also present in the literature. For the single parabolic band model, we derive exact analytical limits in the non-degenerate and near-degenerate regimes, and we show how mistakes appear for the realistic case study of ZrNiSn. Our results illustrate how faulty calculations can appear reasonable at certain temperatures and Fermi levels, and we provide practical guidance for identifying faulty results and avoiding such pitfalls in thermoelectric transport studies.

cond-mat.mtrl-sci↗

arXiv2025

Self-consistent layer-projected scissors operator for band structures of complex 2D van der Waals materials

We introduce a computationally efficient method to calculate the quasiparticle (QP) band structure of general van der Waals (vdW) heterostructures. A layer-projected scissors (LAPS) operator, which depends on the one-body density matrix, is added to the density functional theory (DFT) Hamiltonian. The LAPS operator corrects the band edges of the individual layers for self-energy effects (both intralayer and interlayer) and unphysical strain fields stemming from the use of model supercells. The LAPS operator is treated self-consistently whereby charge redistribution and interlayer hybridization occurring in response to the band energy corrections are properly accounted for. We present several examples illustrating both the qualitative and quantitative performance of the method, including MoS$_2$ films with up to 20 layers, bilayer MoS$_2$ in an electric field, lattice-matched MoS$_2$/WS$_2$ and MoSe$_2$/WSe$_2$ bilayers, and MoSe$_2$/WS$_2$ moiré structures. Our work opens the way for predictive modeling of electronic, optical, and topological properties of complex and experimentally relevant vdW materials.

cond-mat.mtrl-sci↗

arXiv2025

Spectral properties from an efficient analytical representation of the $GW$ self-energy within a multipole approximation

We propose an efficient analytical representation of the frequency-dependent $GW$ self-energy $Σ$ via a multipole approximation (MPA-$Σ$). The multipole-Padé model for the self-energy is interpolated from a small set of numerical evaluations of $Σ$ in the complex frequency plane, similarly to the previously multipole representation developed for the screened Coulomb interaction (MPA-$W$) [Phys. Rev. B \textbf{104}, 115157 (2021)]. We show that, likewise MPA-$W$, an appropriate choice of frequency sampling in MPA-$Σ$ is critical to guarantee computational efficiency and high accuracy. The combined MPA-$W$ and MPA-$Σ$ scheme considerably reduces the cost of full-frequency self-energy calculations, especially for spectral band structures over a wide energy range. Crucially, MPA-$Σ$ enables a multipole representation for the interacting Green's function $G$ (MPA-$G$), providing a straightforward evaluation of all the spectral properties, and a more general way to define the renormalization factor $Z$. We validate the MPA-$Σ$ and MPA-$G$ approaches for diverse systems: bulk Si, Na and Cu, monolayer MoS$_2$, the NaCl ion-pair and the F$_2$ molecule. Moreover, we introduce toy MPA-$Σ$/$G$ models to examine the quasiparticle picture in different regimens of weak and strong correlation. With these models, we expose limitations in defining $Z$ from the local derivative of $Σ$.

cond-mat.mtrl-sci↗

arXiv2025

Thermoelectric transport of strained CsK$_2$Sb: The role of electron velocities and scattering within extended Fermi surfaces

In this first-principles study, we investigated the thermoelectric properties of the full-Heusler compound CsK$_2$Sb at different compressive strains. This material exhibits a valence band structure with significant effective mass anisotropy, forming tube-like energy isosurfaces below the band edge, akin to that of two-dimensional (2D) systems. Such systems can have a large number of high-mobility charge carriers and a beneficial density of states profile. In the calculations, we predicted a maximum p-type figure of merit ($zT$) of 2.6 at 800 K, in line with previous predictions of high $zT$. This high $zT$ arises from the low lattice thermal conductivity of 0.35 Wm$^{-1}$K$^{-1}$ and the beneficial electronic band structure. The high density of states significantly increased the electron-scattering space, but this effect was largely compensated by reduced scattering rates of electrons with large momentum ${\mathbf{q}}$. We further explored the effect of enhancing the low-dimensionality through compressive strain. This increased the p-type power factor by up to 66 %; partly due to more strongly pronounced 2D features of the valence band, but primarily due to increased Fermi velocities. However, compressive strain also increased phonon velocities and hence the lattice thermal conductivity. The maximum p-type $zT$ thus only increased slightly, to 2.7 at 1 % compressive strain. In the conduction band, strain aligned the $Γ$- and X-centered valleys, resulting in the optimal n-type $zT$ increasing from 0.9 to 2.3 at 2 % compressive strain. Thus, highly strained CsK$_2$Sb has the potential for both good p- and n-type thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2024

Design of novel organic proton-transfer acid-base (anti-)ferroelectric salts with crystal structure prediction

Organic molecular ferroelectrics, including organic proton-transfer ferroelectrics and antiferroelectrics, are potentially attractive in organic electronics and have significant chemical tunability. Among these, acid-base proton transfer (PT) salts stand out due to their low coercive fields and the possibility to tune their properties with different acid-base combinations. Using crystal structure prediction, combining small acid and base organic molecular species, we here predict three novel acid-base PT ferroelectric salts with higher polarization than existing materials. We also report two combinations that form antiferroelectric crystal structures. However, some combinations also result in unfavorable packing or the formation of co-crystal or in one case a divalent salt. The protonation state is found to be highly linked to the crystal structure, with cases where salt crystal structures have the same energetic preferability as co-crystals with a different crystalline packing.

cond-mat.mtrl-sci↗

arXiv2024

Dynamical Disorder in the Mesophase Ferroelectric HdabcoClO4: A Machine-Learned Force Field Study

Hybrid molecular ferroelectrics with orientationally disordered mesophases offer significant promise as lead-free alternatives to traditional inorganic ferroelectrics owing to properties such as room temperature ferroelectricity, low-energy synthesis, malleability, and potential for multiaxial polarization. The ferroelectric molecular salt HdabcoClO4 is of particular interest due to its ultrafast ferroelectric room-temperature switching. However, so far, there is limited understanding of the nature of dynamical disorder arising in these compounds. Here, we employ the neural network NeuralIL to train a machine-learned force field (MLFF) with training data generated using density functional theory. The resulting MLFF-MD simulations exhibit phase transitions and thermal expansion in line with earlier reported experimental results, for both a low-temperature phasetransition coinciding with the orientational disorder of ClO4- molecules and the onset of rotation of Hdabco+ and ClO4- molecules in a high-temperature phase transition. We also find proton transfer even in the low-temperature phase, which increases with temperature and leads to associated proton disorder as well as the onset of disorder in the direction of the hydrogen-bonded chains.

cond-mat.mtrl-sci↗

arXiv2024

Unraveling many-body effects in ZnO: Combined study using momentum-resolved electron energy-loss spectroscopy and first-principles calculations

We present a detailed study of the dielectric response of ZnO using a combination of low-loss momentum-resolved electron energy-loss spectroscopy (EELS) and first-principles calculations at several levels of theory, from the independent particle and the random phase approximation with different variants of density functional theory (DFT), including hybrid and DFT$+U$ schemes; to the Bethe-Salpeter equation (BSE). We use a method based on the $f$-sum rule to obtain the momentum-resolved experimental loss function and absorption spectra from EELS measurements. We characterize the main features in the direct and inverse dielectric functions of ZnO and their dispersion, associating them to single-particle features in the electronic band structure, while highlighting the important role of many-body effects such as plasmons and excitons. We discuss different signatures of the high anisotropy in the response function of ZnO, including the symmetry of the excitonic wave-functions.

cond-mat.mtrl-sci↗

arXiv2023

Lattice Thermal Conductivity from First Principles and Active Learning with Gaussian Process Regression

The lattice thermal conductivity ($κ_{\ell}$) is a key materials property in power electronics, thermal barriers, and thermoelectric devices. Identifying a wide pool of compounds with low $κ_{\ell}$ is particularly important in the development of materials with high thermoelectric efficiency. The present study contributed to this with a reliable machine learning (ML) model based on a training set consisting of 268 cubic compounds. For those, $κ_{\ell}$ was calculated from first principles using the temperature-dependent effective potential (TDEP) method based on forces and phonons calculated by density functional theory (DFT). 238 of these were preselected and used to train an initial ML model employing Gaussian process regression (GPR). The model was then improved with active learning (AL) by selecting the 30 compounds with the highest GPR uncertainty as new members of an expanded training set. This was used to predict $κ_{\ell}$ of the 1574 cubic compounds in the \textsc{Materials Project} (MP) database with a validation R2-score of 0.81 and Spearman correlation of 0.93. Out of these, 27 compounds were predicted to have very low values of $κ_{\ell}$ ($\leq 1.3$ at 300~K), which was verified by DFT calculations. Some of these have not previously been reported in the literature, suggesting further investigations of their electronic thermoelectric properties.

cond-mat.mtrl-sci↗

arXiv2023

Discovering Ferroelectric Plastic (Ionic) Crystals in the Cambridge Structural Database: Database Mining and Computational Assessment

Hybrid or organic plastic crystals have the potential as lead-free alternatives to conventional inorganic ferroelectrics. These materials are gaining attention for their multiaxial ferroelectricity, above-room-temperature Curie temperatures, and low-temperature synthesis. Here, we report a screening study of the Cambridge Structural Database (CSD) resulting in 55 new candidate plastic and plastic ionic ferroelectric molecular crystals, along with 16 previously reported ferroelectrics. With over 1.2 million entries in the CSD, the screening procedure involved many steps, including considerations of molecular geometry and size, space group, and hydrogen bonding pattern. The spontaneous polarization and electronic band gaps were predicted using density functional theory. 21 of the candidate ferroelectrics have a polarization greater than $10 \mathrm{μC/cm^2}$, out of which nine are reported at room temperature.

cond-mat.mtrl-sci↗

arXiv2023

Database mining and first-principles assessment of organic proton-transfer ferroelectrics

In organic proton-transfer ferroelectrics (OPTFe), molecules are linked together in a hydrogen-bonded network and proton transfer (PT) between molecules is the dominant mechanism of ferroelectric switching. Their fast switching frequencies make them attractive alternatives to conventional ceramic ferroelectrics, which contain rare and/or toxic elements, and require high processing temperatures. In this study, we mined the Cambridge Structural Database for potential OPTFes, uncovering all previously reported compounds, both tautomers and co-crystals, in addition to seven new candidate tautomers. The mining was based on identifying polar crystal structures with pseudo center-of-symmetry and viable PT paths. The spontaneous polarization and PT barriers were assessed using density functional theory.

cond-mat.mtrl-sci↗

arXiv2022

Purcell-induced suppression of superradiance for molecular overlayers on noble atom surfaces

We study the impact of an environment on the electromagnetic responses of a molecule in the presence of a dielectric medium. By applying the dipole-dipole coupling between the molecule's and the environment's degrees of freedom, we can reduce the complex system into its components and predict excitation lifetimes of single and few molecules attached to a dielectric surface by knowing the entire quantum-mechanical properties of the molecules, such as transition energies and dipole moments. The derived theory allows for the description of superradiance between two molecules depending on the geometric arrangement between both concerning their separation and orientation with respect to each other. We analyse the possibility of superradiance between two molecules bound to a dielectric sphere and determine a change of the relevant length scale where the usually considered wavelength in free space is replaced with the binding distance, drastically reducing the length scales at which collective effects can take place.

quant-ph↗

arXiv2022

Improved proton-transfer barriers with van der Waals density functionals: Role of repulsive non-local correlation

Proton-transfer (PT) between organic complexes is a common and important biochemical process. Unfortunately, PT energy barriers are difficult to accurately predict using density functional theory (DFT); in particular, the generalized gradient approximation (GGA) tends to underestimate PT barriers. Moreover, PT typically occurs in environments where dispersion forces contribute to the cohesion of the system; thus, a suitable exchange-correlation functional should accurately describe both dispersion forces and PT barriers. This paper provides benchmark results for the PT barriers of several density functionals including several variants of the van der Waals density functional (vdWDF). The benchmark set comprises small organic molecules with inter- and intra-molecular PT. The results show that replacing GGA correlation with a fully non-local vdW-DF correlation increases the PT barriers, making it closer to the quantum chemical reference values. In contrast, including nonlocal correlations with the Vydrov-Voorhis (VV) method or dispersion-corrections at the DFT-D3 or the Tkatchenko-Scheffler (TS) level has barely any impact on the PT barriers. Hybrid functionals also increase and improve the energies and the best performance is provided by a hybrid version of the consistent-exchange van der Waals density functional vdW-DF-cx. For the formic acid dimer PT system, we analyzed the GGA exchange and non-local correlation contributions. The analysis shows that the repulsive part of the non-local correlation kernel plays a key role in the PT energy barriers predicted with vdW-DF.

physics.chem-ph↗