arXiv subjects

Bo B. Iversen

Publications and source records attributed to Bo B. Iversen.

15 recordsLinked to original sources

arXiv2025

High-Quality Ultra-Fast Total Scattering and Pair Distribution Function Data using an X-ray Free Electron Laser

High-quality total scattering data, a key tool for understanding atomic-scale structure in disordered materials, require stable instrumentation and access to high momentum transfers. This is now routine at dedicated synchrotron instrumentation using high-energy X-ray beams, but it is very challenging to measure a total scattering dataset in less than a few microseconds. This limits their effectiveness for capturing structural changes that occur at the much faster timescales of atomic motion. Current X-ray free-electron lasers (XFELs) provide femtosecond-pulsed X-ray beams with maximum energies of approximately 24 keV, giving the potential to measure total scattering and the attendant pair distribution functions (PDFs) on femtosecond timescales. Here, we show that this potential has been realised using the HED scientific instrument at the European XFEL and present normalised total scattering data for 0.35 Å-1 < Q < 16.6 Å-1 and their PDFs from a broad spectrum of materials, including crystalline, nanocrystalline and amorphous solids, liquids, and clusters in solution. We analyse the data using a variety of methods, including Rietveld refinement, small-box PDF refinement, joint reciprocal-real space refinement, cluster refinement, and Debye scattering analysis. The resolution function of the setup is also characterised. We conclusively show that high-quality data can be obtained from a single approximately 30 fs XFEL pulse. Our efforts not only significantly increase the existing maximum reported Q-range for an S(Q) measured at an XFEL but also mean that XFELs are now a viable X-ray source for the broad community of people using reciprocal space total scattering and PDF methods in their research.

cond-mat.mtrl-sci↗

arXiv2022

Machine learning based approach for solving atomic structures of nanomaterials combining pair distribution functions with density functional theory

Determination of crystal structures of nanocrystalline or amorphous compounds is a great challenge in solid states chemistry and physics. Pair distribution function (PDF) analysis of X-Ray or neutron total scattering data has proven to be a key element in tackling this challenge. However, in most cases a reliable structural motif is needed as starting configuration for structure refinements. Here, we present an algorithm that is able to determine the crystal structure of an unknown compound by means of an on-the-fly trained machine learning model that combines density functional theory (DFT) calculations with comparison of calculated and measured PDFs for global optimization in an artificial landscape. Due to the nature of this landscape, even metastable configurations can be determined.

cond-mat.mtrl-sci↗

arXiv2021

On single crystal total scattering data reduction and correction protocols for analysis in direct space

We explore data reduction and correction steps and processed data reproducibility in the emerging single crystal total scattering based technique of three-dimensional differential atomic pair distribution function (3D-$Δ$PDF) analysis. All steps from sample measurement to data-processing are outlined in detail using a CuIr$_2$S$_4$ example crystal studied in a setup equipped with a high-energy x-ray beam and a flat panel area detector. Computational overhead as it pertains to data-sampling and the associated data processing steps is also discussed. Various aspects of the final 3D-$Δ$PDF reproducibility are explicitly tested by varying data-processing order and included steps, and by carrying out a crystal-to-crystal data comparison. We identify situations in which the 3D-$Δ$PDF is robust, and caution against a few particular cases which can lead to inconsistent 3D-$Δ$PDFs. Although not all the approaches applied here-in will be valid across all systems, and a more in-depth analysis of some of the effects of the data processing steps may still needed, the methods collected here-in represent the start of a more systematic discussion about data processing and corrections in this field.

cond-mat.mtrl-sci↗

arXiv2021

Chemical bonding origin of the thermoelectric power factor in Half-Heusler semiconductors

Intermetallic semiconductors with the cubic Half-Heusler structure (XYZ) have excellent thermoelectric properties. This has been attributed to the high degeneracy of the carrier pockets in the band structure, but large differences are found between different material compositions. Half-Heuslers are often interpreted within Zintl chemistry, making a clear distinction between an electropositive cation ($X^{n+}$) and an extended polyanion ($YZ^{n-}$). Based on quantitative real space chemical bonding analysis, we unravel large degrees of covalent bonding between the formal cation and anion, making the Zintl distinction clearly invalid. This covalence is shown to strongly affect the band structure, thermoelectric properties and response properties in the materials, with improved thermoelectric properties observed for those materials that least follow the Zintl concept. This expands our knowledge of the chemical bonding motifs governing physical properties, and gives a critical view on the simplistic chemical concepts too often applied for design of complex materials.

cond-mat.mtrl-sci↗

arXiv2020

Expression and interactions of stereo-chemically active lone pairs and their relation to structural distortions and thermal conductivity

Stereo-chemically active lone pairs are typically described as an important non-bonding effect, and large interest has centered on understanding the derived effect of lone pair expression on physical properties such as the thermal conductivity. To manipulate such properties, it is essential to understand the conditions that lead to lone pair expression and to provide a quantitative chemical description. Here we first use density functional theory calculations to establish the presence of stereo-chemically active lone pairs on antimony in $\text{MnSb}_{2}\text{O}_{4}$. The lone pairs are formed through a similar mechanism to those in binary post-transition metal compounds in an oxidation state of two less than their main group number, where the degree of orbital interaction determines the expression of the lone pair. In $\text{MnSb}_{2}\text{O}_{4}$ the Sb lone pairs interact through a void space in the crystal structure, and they minimize their mutual repulsion by introducing a deflection angle. This angle increases significantly with decreasing Sb-Sb distance, thus showing the highly destabilizing nature of the lone pair interactions. Analysis of the chemical bonding in the structure shows that it is dominated by polar covalent interactions. A database search of related ternary chalcogenide structures shows that for structures with a lone pair the degree of lone pair expression is largely determined by whether the antimony-chalcogen units are connected or not, suggesting a cooperative effect. Isolated $\text{SbX}_3$ units have larger X-Sb-X bond angles, and therefore weaker lone pair expression than connected units. Since increased lone pair expression is equivalent to an increased orbital interaction (covalent bonding), which typically leads to increased heat conduction, this can explain the previously established correlation between larger bond angles and lower thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2019

Solving the disordered structure of $β$-Cu$_{2-x}$Se using the three-dimensional difference pair distribution function

High-performing thermoelectric materials such as Zn$_4$Sb$_3$ and clathrates have atomic disorder as the root to their favorable properties. This makes it extremely difficult to understand and model their properties at a quantitative level, and thus effective structure-property relations are challenging to obtain. Cu$_{2-x}$Se is an intensely studied, cheap and non-toxic high performance thermoelectric, which exhibits highly peculiar transport properties, especially around the $β$ to $α$ phase transition around 400 K, which must be related to the detailed nature of the crystal structure. Attempts to solve the crystal structure of the low temperature phase, $β$-Cu$_{2-x}$Se, have been unsuccessful since 1936. So far all studies have assumed that $β$-Cu$_{2-x}$Se has a three-dimensional periodic structure, but here we show that the structure is ordered only in two dimensions while being disordered in the third dimension with a highly disordered stacking sequence. Using the three-dimensional difference pair distribution function (3D-$Δ$PDF) analysis method for diffuse single crystal X-ray scattering, we solve the structure of the ordered layer and show that there are two modes of stacking disorder present, which give rise to an average structure with higher symmetry. The present approach allows for a direct solution of structures with disorder in some dimensions and order in others. The local and extended structure of a solid determines its properties and Cu$_{2-x}$Se represents an example of a high-performing thermoelectric material where the local atomic structure differs significantly from the average periodic structure observed from Bragg crystallography.

cond-mat.mtrl-sci↗

arXiv2018

Nanoscale Surface Dynamics of Bi$_2$Te$_3$(111): Observation of a Prominent Surface Acoustic Wave and the Role of van der Waals Interactions

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi$_2$Te$_3$. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi$_2$Te$_3$(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

cond-mat.mtrl-sci↗

arXiv2018

A Helium-Surface Interaction Potential of Bi$_2$Te$_3$(111) from Ultrahigh-Resolution Spin-Echo Measurements

We have determined an atom-surface interaction potential for the He$-$Bi$_2$Te$_3$(111) system by analysing ultrahigh resolution measurements of selective adsorption resonances. The experimental measurements were obtained using $^3$He spin-echo spectrometry. Following an initial free-particle model analysis, we use elastic close-coupling calculations to obtain a three-dimensional potential. The three-dimensional potential is then further refined based on the experimental data set, giving rise to an optimised potential which fully reproduces the experimental data. Based on this analysis, the He$-$Bi$_2$Te$_3$(111) interaction potential can be described by a corrugated Morse potential with a well depth $D=(6.22\pm0.05)~\mathrm{meV}$, a stiffness $κ=(0.92\pm0.01)~\mathrmÅ^{-1}$ and a surface electronic corrugation of $(9.6\pm0.2)$% of the lattice constant. The improved uncertainties of the atom-surface interaction potential should also enable the use in inelastic close-coupled calculations in order to eventually study the temperature dependence and the line width of selective adsorption resonances.

cond-mat.mtrl-sci↗

arXiv2017

Electron-Phonon Coupling and Surface Debye Temperature of Bi$_2$Te$_3$(111) from Helium Atom Scattering

We have studied the topological insulator Bi$_2$Te$_3$(111) by means of helium atom scattering. The average electron-phonon coupling $λ$ of Bi$_2$Te$_3$(111) is determined by adapting a recently developed quantum-theoretical derivation of the helium scattering probabilities to the case of degenerate semiconductors. Based on the Debye-Waller attenuation of the elastic diffraction peaks of Bi$_2$Te$_3$(111), measured at surface temperatures between $110~\mbox{K}$ and $355~\mbox{K}$, we find $λ$ to be in the range of $0.04-0.11$. This method allows to extract a correctly averaged $λ$ and to address the discrepancy between previous studies. The relatively modest value of $λ$ is not surprising even though some individual phonons may provide a larger electron-phonon interaction. Furthermore, the surface Debye temperature of Bi$_2$Te$_3$(111) is determined as ${\rm Θ}_D = (81\pm6)~\mbox{K}$. The electronic surface corrugation was analysed based on close-coupling calculations. By using a corrugated Morse potential a peak-to-peak corrugation of 9% of the lattice constant is obtained.

cond-mat.mtrl-sci↗

arXiv2015

Large Seebeck Effect by Charge-Mobility Engineering

The Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient. Besides fundamental relevance in solid state physics, it serves as a key quantity to determine the performance of functional thermoelectric materials. In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts toward superior thermoelectrics through the engineering of electronic structure. Here, we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: A charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient. This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a dramatic mobility change occurs due to the crossover between two different charge-relaxation regimes. Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch. As the physical origin for the latter can vary greatly, our proposal provides a unifying framework for the understanding of a large panoply of thermoelectric phenomena. When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials for applications in solid-state cooling or power generation.

cond-mat.mtrl-sci↗

arXiv2014

Intra- and Interband Electron Scattering in the Complex Hybrid Topological Insulator Bismuth Bilayer on Bi$_2$Se$_3$

The band structure, intra- and interband scattering processes of the electrons at the surface of a bismuth-bilayer on Bi$_2$Se$_3$ have been experimentally investigated by low-temperature Fourier-transform scanning tunneling spectroscopy. The observed complex quasiparticle interference patterns are compared to a simulation based on the spin-dependent joint density of states approach using the surface-localized spectral function calculated from first principles as the only input. Thereby, the origin of the quasiparticle interferences can be traced back to intraband scattering in the bismuth bilayer valence band and Bi$_2$Se$_3$ conduction band, and to interband scattering between the two-dimensional topological state and the bismuth-bilayer valence band. The investigation reveals that the bilayer band gap, which is predicted to host one-dimensional topological states at the edges of the bilayer, is pushed several hundred milli-electronvolts above the Fermi level. This result is rationalized by an electron transfer from the bilayer to Bi$_2$Se$_3$ which also leads to a two-dimensional electron state in the Bi$_2$Se$_3$ conduction band with a strong Rashba spin-splitting, coexisting with the topological state and bilayer valence band.

cond-mat.mes-hall↗

arXiv2013

Highly Dispersive Electron Relaxation and Colossal Thermoelectricity in the Correlated Semiconductor FeSb$_2$

We show that the colossal thermoelectric power, $S(T)$, observed in the correlated semiconductor FeSb$_2$ below 30\,K is accompanied by a huge Nernst coefficient $ν(T)$ and magnetoresistance MR$(T)$. Markedly, the latter two quantities are enhanced in a strikingly similar manner. While in the same temperature range, $S(T)$ of the reference compound FeAs$_2$, which has a seven-times larger energy gap, amounts to nearly half of that of FeSb$_2$, its $ν(T)$ and MR$(T)$ are intrinsically different to FeSb$_2$: they are smaller by two orders of magnitude and have no common features. With the charge transport of FeAs$_2$ successfully captured by the density functional theory, we emphasize a significantly dispersive electron-relaxation time $τ(ε_k)$ due to electron-electron correlations to be at the heart of the peculiar thermoelectricity and magnetoresistance of FeSb$_2$.

cond-mat.str-el↗

arXiv2011

Enhanced electron correlations in FeSb$_2$

FeSb$_2$ has been recently identified as a new model system for studying many-body renormalizations in a $d$-electron based narrow gap semiconducting system, strongly resembling FeSi. The electron-electron correlations in FeSb$_2$ manifest themselves in a wide variety of physical properties including electrical and thermal transport, optical conductivity, magnetic susceptibility, specific heat and so on. We review some of the properties that form a set of experimental evidences revealing the crucial role of correlation effects in FeSb$_2$. The metallic state derived from slight Te doping in FeSb$_2$, which has large quasiparticle mass, will also be introduced.

cond-mat.str-el↗

arXiv2009

Huge Thermoelectric Power Factor: FeSb2 versus FeAs2 and RuSb2

The thermoelectric power factor of the narrow-gap semiconductor FeSb2 is greatly enhanced in comparison to the isostructural homologues FeAs2 and RuSb2. Comparative studies of magnetic and thermodynamic properties provide evidence that the narrow and correlated bands as well as the associated enhanced thermoelectricity are only specific to FeSb2. Our results point to the potential of FeSb2 for practical thermoelectric application at cryogenic temperatures and stimulate the search for new correlated semiconductors along the same lines.

cond-mat.str-el↗

arXiv2006

Electronic structure and transport in CsBi$_4$Te$_6$

The band structure of the novel low-temperature thermoelectric material, \CBT, is calculated and analyzed using the semi-classic transport equations. It is shown that to obtain a quantitative agreement with measured transport properties a band gap of 0.08 eV must be enforced. A gap in reasonable agreement with experiment was obtained using the generalized gradient functional of Engel and Vosko. We found that the experimental $p$-type sample has a carrier concentration close to optimal. Furthermore the conduction bands have a form equally well suited for thermoelectric properties and we predict that an optimally doped $n$-type compound could have thermoelectric properties exceeding those of the $p$-type.

cond-mat.mtrl-sci↗