arXiv subjects

Bo Brummerstedt Iversen

Publications and source records attributed to Bo Brummerstedt Iversen.

At least 19 recordsLinked to original sources

arXiv2026

$\mathcal{PT}$-symmetry as Effective Time Reversal Symmetry for Anderson Localization in a Collinear Antiferromagnet

We show that $\mathcal{PT}$-symmetry acts as the effective time-reversal symmetry governing Anderson localization in the antiferromagnet BaMn$_2$Bi$_2$, whereas the general time-reversal symmetry is broken by its long-range magnetic order. The magnetoconductance follows the laws established from breaking time-reversal symmetry throuhg the orbital coupling of charge carriers to the magnet vector potential, however, the degrees of symmetry breaking are qualitatively modulated by the magnetocrystalline anisotropy stabilizing $\mathcal{PT}$-related sublattices. The quantum interferences and hoping amplitudes governing the phenomena are drastically impaired by transverse magnetic fields, which readily cant $\mathcal{PT}$-related magnetic sublattices. Accordingly, when the $\mathcal{PT}$-related texture is weakly perturbed by longitudinal fields orienting along the sublattices, the localization is minutely affected. The robustness of the $\mathcal{PT}$-enforced degeneracies is thus governed by exchange and magnetocrystalline interactions, in contrast to the exact $\mathcal{PT}$-symmetry of the full Hamiltonian, which is broken by any finite field.

cond-mat.other↗

arXiv2026

Intrinsic single crystals of MnTe altermagnet

We report the synthesis methodology, structure, and intrinsic properties of ultra-high quality single crystals of MnTe, an archetypal altermagnet. The crystals, obtained from self-flux method, are nearly free from crystal imperfections and disproportionate chemical compositions as seen by various investigation methods. In measurements under quasi free-standing configuration minimizing stress induced effects, the crystals exhibit complex and anisotropic domain kinetics in both superheating and supercooling regimes around the altermagnetic transition at $T_{\mathrm{N}} = 310\,\mathrm{K}$. An Anderson insulating state is observed below $T_{\mathrm{MI}}\approx 150\,\mathrm{K}$ with a carrier density of about $1.6\times 10^{17}\,\mathrm{cm}^{-3}$, being sharply contrast to metallic states usually seen in Te-deficit samples. Nevertheless, hallmarks of altermagnetism, anomalous Hall effect and X-ray magnetic circular dichroism signal, are robust in this intrinsic limit, however with significantly reduced magnitudes.

cond-mat.str-el↗

arXiv2025

Intrinsic physical properties of flexible van der Waals semiconductor InSe

InSe is a van der Waals semiconductor in which mechanical flexibility, high electronic mobility, and non-trivial electronic structures converge, making it an attractive platform for both intriguing fundamental studies and promising device developments. However, the nucleation and growth of phase-pure, intrinsic InSe crystals require stringent thermodynamical conditions, and have therefore remained elusive. Since InSe melts incongruently, the widely used synthesis methods based on cooling of a 1:1 In-Se mixture will produce either aggregates of multiphase crystallites or uncontrolled In-rich, heavily electron-doped InSe. This fundamental thermodynamic constraint provides a compelling explanation for the large discrepancies observed in the reported physical properties of InSe. We overcome these limitations by utilizing the traveling solvent floating zone (TSFZ) method to produce high quality, centimeter-size InSe single crystals. Electrical, thermal, and thermoelectric transport measurements demonstrate that TSFZ-InSe single crystals closely approach the intrinsic limit, establishing it as a benchmark material for the future studies of this important material.

cond-mat.mtrl-sci↗

arXiv2025

Frustrated vacancy ordering creates novel quantum properties in Kutinaite, $\mathrm{Ag}_{6}\mathrm{Cu}_{14.4}\mathrm{As}_7$

Ideal crystals are fully ordered, but real-world crystals always contain defects breaking translational symmetry. Random defects in crystals have important implications and they e.g. provide the foundation for semiconductor-based electronic devices. Structurally correlated defects introduce an additional level of complexity, which may lead to novel materials properties, but rationalization of relations between correlated disorder and the emergent material properties are very rare. Here we report that the defect structure of the mineral Kutinaite, Ag6Cu14.4As7, exhibits unprecedented metallic diamagnetism, a hallmark of non-trivial electronic states that require delicate symmetrical protection. Using a combination of X-ray scattering methodologies, simulations, and physical property measurements, we deduced and verified subtle frustrated vacancy ordering of the Cu sublattice when cooling crystals below ~300 K. The vacancy frustration in Kutinaite leads to unique quantum properties, and our study calls for a reconsideration of the role of vacancies as quasi-chemical species in crystals.

cond-mat.dis-nn↗

arXiv2024

Structural origins of ultra-low glass-like thermal conductivity in AgGaGe3Se8

Materials with a low thermal conductivity are important for a variety of applications such as thermal barrier coatings and thermoelectrics, and understanding the underlying mechanisms of low heat transport, and relating them to structural features, remains a central goal within material science. Here, we report on the ultra-low thermal conductivity of the quarternary silver chalcogenide AgGaGe3Se8 with a remarkable value of only 0.2 Wm-1K-1 at room temperature, and an unusual glass-like thermal behavior from 2 K to 700 K. The ultra-low thermal conductivity is linked to a disordered nature of Ag in the structure, displaying extremely large Ag atomic displacement parameters obtained from multi-temperature synchrotron powder X-ray scattering measurements, and Ag ionic conductivity at elevated temperatures. Additionally, signs of structural anharmonicity and soft bonding are evident from a low temperature Boson peak in the heat capacity and a low Debye temperature of 147 K.

cond-mat.mtrl-sci↗

arXiv2023

Oxide layer formation prevents deteriorating ion migration in thermoelectric Cu$_2$Se during operation in air

Cu$_2$Se is a mixed ionic-electronic conductor with outstanding thermoelectric performance originally envisioned for space missions. Applications were discontinued due to material instability, where elemental Cu grows at the electrode interfaces during operation in vacuum. Here, we show that when Cu$_2$Se is operating in air, formation of an oxide surface layer suppresses Cu$^+$ migration along the current direction. In operando X-ray scattering and electrical resistivity measurements quantify Cu$^+$ migration through refinement of atomic occupancies and phase composition analysis. Cu deposition can be prevented during operation in air, irrespective of a critical voltage, if the thermal gradient is applied along the current direction. Maximum entropy electron density analysis provides experimental evidence that Cu$^+$ migration pathways under thermal and electrical gradients differ substantially from equilibrium diffusion. The study establishes new promise for inexpensive sustainable Cu$_2$Se in thermoelectric applications, and it underscores the importance of atomistic insight into materials during thermoelectric operating conditions.

cond-mat.mtrl-sci↗

arXiv2022

Direct visualization of magnetic correlations in frustrated spinel ZnFe$_2$O$_4$

Magnetic materials with the spinel structure (A$^{2+}$B$^{3+}_2$O$^4$) form the core of numerous magnetic devices, but ZnFe$_2$O$_4$ constitutes a peculiar example where the nature of the magnetism is still unresolved. Susceptibility measurements revealed a cusp around $T_c=13\;\mathrm{K}$ resembling an antiferromagnetic transition, despite the positive Curie-Weiss temperature determined to be $Θ_{CW}=102.8(1)\;\mathrm{K}$. Bifurcation of field-cooled and zero-field-cooled data below $T_c$ in conjunction with a frequency dependence of the peak position and a non-zero imaginary component below $T_c$ shows it is in fact associated with a spin-glass transition. Highly structured magnetic diffuse neutron scattering from single crystals develops between $50\;\mathrm{K}$ and $25\;\mathrm{K}$ revealing the presence of magnetic disorder which is correlated in nature. Here, the 3D-m$Δ$PDF method is used to visualize the local magnetic ordering preferences, and ferromagnetic nearest-neighbor and antiferromagnetic third nearest-neighbor correlations are shown to be dominant. Their temperature dependence is extraordinary with some flipping in sign, and a strongly varying correlation length. The correlations can be explained by orbital interaction mechanisms for the magnetic pathways, and a preferred spin cluster. Our study demonstrates the power of the 3D-m$Δ$PDF method in visualizing complex quantum phenomena thereby providing a way to obtain an atomic scale understanding of magnetic frustration.

cond-mat.mtrl-sci↗

arXiv2022

Improving the operational stability of thermoelectric Zn$_4$Sb$_3$ by segmentation

The mixed ionic-electronic conductor $β$-Zn$_4$Sb$_3$ is a cheap and high performing thermoelectric material, but under operating conditions with a temperature gradient and a running current, the material decomposes as Zn readily migrates in the structure. Here, we report an improved stability of $β$-Zn$_4$Sb$_3$ by introducing ion-blocking interfaces of stainless steel to segment the sample, produced by a rapid one-step Spark Plasma Sintering synthesis. The stability of the samples is tested under temperature gradients and electric currents, which reveals significantly improved stability of the segmented samples compared to unsegmented samples. The segmented samples are stable under temperature gradient from 250°C to room temperature with no external current, whereas the unsegmented sample decomposes into ZnSb and Zn under the same conditions. The thermoelectric figure of merit, zT, of the segmented sample is slightly reduced, mainly due to the increased thermal conductivity. In conclusion, a rapid one-step synthesis of segmented $β$-Zn$_4$Sb$_3$ is developed, which successfully improves the long-term operational stability by blocking the Zn ion migration.

cond-mat.mtrl-sci↗

arXiv2021

Effects of Voigt Diffraction Peak Profiles on the Pair Distribution Function

Powder X-ray Diffraction (PXRD) and Pair Distribution Function (PDF) analysis are well-established techniques for investigation of atomic configurations in crystalline materials, and the two are related by a Fourier transformation. In PXRD experiments, structural information, such as crystallite size and strain, is contained within the peak profile function of the diffraction peaks. However, the effects of the PXRD peak profile function on the PDF are not fully understood. Here, all the effects from a Voigt diffraction peak profile are solved analytically and verified experimentally through a high-quality X-ray total scattering measurements on strained Ni powder. The Lorentzian contribution to strain broadening is found to result in Voigt shaped PDF peaks. Furthermore, it is demonstrated that an improper description of the Voigt shape during model refinement leads to overestimation of the atomic displacement parameter.

cond-mat.mtrl-sci↗

arXiv2021

Tuneable local order in thermoelectric crystals

Although crystalline solids are characterized by their periodic structures, some are only periodic on average and deviate on a local scale. Distinct local orderings can exist with identical periodic structures making their differences hidden to normal diffraction methods. Using x-ray scattering we investigate the thermoelectric half-Heusler, Nb1-xCoSb, with high vacancy concentrations, where signs of local order have been attributed to differences in stoichiometry. We show that the composition is actually always very close to x = 1/6 irrespective of nominal sample composition, and that the synthesis method controls the local order. The vacancy distribution is shown to follow a vacancy repulsion model, and local structural relaxations around the vacancies are characterized. Control over the local structure provides a new frontier for tuning material properties.

cond-mat.mtrl-sci↗

arXiv2020

Probing efficient n-type lanthanide dopants for Mg$_3$Sb$_2$ thermoelectrics

The recent discovery of n-type Mg$_3$Sb$_2$ thermoelectric has ignited intensive research activities on searching for potential n-type dopants for this material. Using first-principles defect calculations, here we conduct a systematic computational screening of potential efficient n-type lanthanide dopants for Mg$_3$Sb$_2$. In addition to La, Ce, Pr, and Tm, we find that high electron concentration ($\geq$ 10$^{20}$ cm$^{-3}$ at the growth temperature of 900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which are generally more efficient than other lanthanide dopants and the anion-site dopant Te. Experimentally, we confirm Nd and Tm as effective n-type dopants for Mg$_3$Sb$_2$ since doping with Nd and Tm shows superior thermoelectric figure of merit zT $\geq$ 1.3 with higher electron concentration than doping with Te. Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and thermal conductivity reduction are achieved. As a result, we obtain high zT values of about 1.65 and 1.75 at 775 K in n-type Mg$_{3.5}$Nd$_{0.04}$Sb$_{1.97}$Te$_{0.03}$ and Mg$_{3.5}$Tm$_{0.03}$Sb$_{1.97}$Te$_{0.03}$, respectively, which are among the highest values for n-type Mg$_3$Sb$_2$ without alloying with Mg$_3$Bi$_2$. This work sheds light on exploring promising n-type dopants for the design of Mg$_3$Sb$_2$ thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2020

A simple model for vacancy order and disorder in defective half-Heusler systems

Defective half-Heusler systems X(1-x)YZ with large amounts of intrinsic vacancies, such as Nb(1-x)CoSb, Ti(1-x)NiSb and V(1-x)CoSb, are a group of promising thermoelectric materials. Even with high vacancy concentrations they maintain the average half-Heusler crystal structure. These systems show high electrical conductivity but low thermal conductivity arising from an ordered YZ lattice, which conducts electrons, while the large amounts of vacancies on the X sublattice effectively scatters phonons. Using electron scattering it was recently observed that in addition to Bragg diffraction from the average cubic half-Heusler structure, some of these samples show broad diffuse scattering indicating short-range vacancy order while other samples show sharp additional peaks, indicating long-range vacancy ordering. Here we show that both the short and long-range ordering can be explained using the same simple model, which assumes that vacancies on the X-sublattice avoid each other. The samples showing long-range vacancy order are in agreement with the predicted ground-state of the model, while short-range order samples are quenched high-temperature states of the system. A previous study showed that changes in sample stoichiometry affect whether the short or long-range vacancy structure is obtained, but the present model suggests that thermal treatment of samples should allow controlling the degree of vacancy order, and thereby the thermal conductivity, without changes in composition. This is important as the composition also dictates the amount of electrical carriers. Independent control of electrical carrier concentration and degree of vacancy order should allow further improvements in the thermoelectric properties of these systems.

cond-mat.mtrl-sci↗

arXiv2019

Rapid one-step synthesis and compaction of high-performance n-type Mg3Sb2 thermoelectrics

n-type Mg3Sb2-based compounds are emerging as a promising class of low-cost thermoelectric materials due to their extraordinary performance at low and intermediate temperatures. However, so far high thermoelectric performance has merely been reported in n-type Mg3Sb2-Mg3Bi2 alloys with a large amount of Bi. Moreover, current synthesis methods of n-type Mg3Sb2 bulk thermoelectrics involve multi-step processes that are time- and energy-consuming. Here we report a fast and straightforward approach to fabricate n-type Mg3Sb2 thermoelectrics using spark plasma sintering, which combines the synthesis and compaction in one step. Using this method, we achieve a high thermoelectric figure of merit zT of ~0.4-1.5 at 300-725 K in n-type (Sc, Te)-doped Mg3Sb2 without alloying with Mg3Bi2. In comparison with the currently reported synthesis methods, the complexity, process time, and cost of the new method are significantly reduced. This work demonstrates a simple, low-cost route for the potential large-scale production of n-type Mg3Sb2 thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2019

Fermi surface complexity, effective mass, and conduction band alignment in n-type thermoelectric Mg3Sb2-xBix from first principles calculations

Using first principles calculations, we study the conduction band alignment, effective mass, and Fermi surface complexity factor of n-type Mg3Sb2-xBix (x = 0, 1, and 2) from the full ab initio band structure. We find that with increasing the Bi content the K and M band minima moves away from the conduction band minimum CB1 while the singly-degenerate $Γ$ band minimum shifts rapidly downward and approaches the conduction band minimum. But the favorable six-fold degenerate CB1 band minimum keeps dominating the conduction band minimum and there is no band crossing between the $Γ$ and CB1 band minima. In addition, we show that the connection of the CB1 carrier pockets with the energy level close to the band minimum M can strongly enhance the carrier pocket anisotropy and Fermi surface complexity factor, which is likely the electronic origin for the local maximum in the theoretical power factor. Our calculations also show that the band gap, density of states effective mass, Seebeck coefficient, and Fermi surface complexity factor decrease with increasing the Bi content, which is unfavorable to the electrical transport. However, reducing the conductivity effective mass with increasing the Bi content is beneficial to the electrical transport by improving carrier mobility and weighted mobility as long as the detrimental bipolar effect is insignificant. As a result, in comparison with n-type Mg3Sb2, n-type Mg3SbBi shows higher power factors and a much lower optimal carrier concentration for the theoretical power factor at 300 K, which can be easily achieved by the experiment.

cond-mat.mtrl-sci↗

arXiv2018

Relating chemical bonding to physical properties: The origin of unexpected isotropic properties in layered materials

Layered materials span a very broad range of solids ranging from van der Waals materials to highly complex crystal structures such as clays. They are commonly believed to have highly anisotropic properties, which is essentially attributed to weak interlayer interactions. The layered Mg3Sb2 structure is currently being intensely scrutinized due to its outstanding thermoelectric properties. Based on quantitative chemical bonding analysis we unravel that Mg3Sb2 exhibits a nearly isotropic three-dimensional (3D) bonding network with the interlayer and intralayer bonds being surprisingly similar, and these unique chemical bonding features are the origin of the nearly isotropic structural and thermal properties. The isotropic 3D bonding network is found to be broadly applicable to many Mg-containing compounds with the layered CaAl2Si2-type structure. Intriguingly, a parameter based on the electron density can be used as an indicator measuring the anisotropy of lattice thermal conductivity in layered structures. This work extends our understanding of structure and properties based on chemical bonding analysis, and it will guide the search for, and design of, layered materials with tailored anisotropic properties.

cond-mat.mtrl-sci↗

arXiv2018

Model-free reconstruction of magnetic correlations in frustrated magnets

Frustrated magnetic systems exhibit extraordinary physical properties but quantification of their magnetic correlations poses a serious challenge to experiment and theory. Current insight into frustrated magnetic correlations relies on modelling techniques such as reverse Monte Carlo methods, which require knowledge about the exact ordered atomic structure. Here we present a method for direct reconstruction of magnetic correlations in frustrated magnets by three-dimensional difference pair distribution function analysis of neutron total scattering data. The methodology is applied to the disordered frustrated magnet bixbyite, (Mn1-xFex)2O3, which reveals nearest-neighbor antiferromagnetic correlations for the metal sites up to a range of approximately 15 Å. Importantly, this technique allows for magnetic correlations to be determined directly from the experimental data without any assumption about the atomic structure.

cond-mat.mtrl-sci↗

arXiv2016

Reorientation of the bicollinear antiferromagnetic structure at the surface of Fe$_{1+y}$Te bulk and thin films

Establishing the relation between the ubiquitous antiferromagnetism in the non-superconducting parent compounds of unconventional superconductors and their superconducting phase is believed to be important for the understanding of the complex physics in these materials. Going from the bulk systems to thin films strongly affects the phase diagram of unconventional superconductors. For Fe$_{1+y}$Te, the parent compound of the Fe$_{1+y}$Se$_{1-x}$Te$_x$ superconductors, bulk sensitive neutron diffraction has revealed an in-plane oriented bicollinear antiferromagnetic structure. Here, we show by spin-resolved scanning tunneling microscopy that on the surfaces of bulk Fe$_{1+y}$Te, as well as on thin films grown on the topological insulator Bi$_2$Te$_3$, the spin direction is canted both away from the surface plane and from the high-symmetry directions of the surface unit cell, while keeping the bicollinear magnetic structure. Our results demonstrate that the magnetism at the Fe-chalcogenide surface markedly deviates from a simple in-plane oriented bicollinear antiferromagnetic structure, which implies that the pairing at the surface of the related superconducting compounds might be different from that in the bulk.

cond-mat.str-el↗

arXiv2016

Evidence for interfacial superconductivity in a bi-collinear antiferromagnetically ordered FeTe monolayer on a topological insulator

The discovery of high-temperature superconductivity in Fe-based compounds [1,2] has triggered numerous investigations on the interplay between superconductivity and magnetism [3] and, more recently, on the enhancement of transition temperatures through interface effects [4]. It is widely believed that the emergence of optimal superconductivity is intimately linked to the suppression of long-range antiferromagnetic (AFM) order, although the exact microscopic picture of this relationship remains elusive [1] due to the lack of data with atomic spatial resolution [5-7]. Here, we present a spin-polarized scanning tunneling spectroscopy (SP-STS) study of ultrathin FeTe$_{1-x}$Se$_x$ (x = 0, 0.5) films grown on prototypical Bi-based bulk topological insulators. Surprisingly, we find an energy gap at the Fermi level indicating superconducting correlations up to Tc ~ 6 K for one unit cell thin FeTe layers grown on Bi2Te3 substrates, in contrast to the non-superconducting FeTe bulk compound [8]. Moreover, SP-STS reveals that the energy gap spatially coexists with bicollinear AFM order. This finding opens novel perspectives for theoretical studies of competing orders in Fe-based superconductors as well as for experimental investigations of exotic phases in heterostructures of topological insulators and superconducting layers.

cond-mat.supr-con↗