Materials Science
Be and Be-related impurities in diamond: density functional theory study
K. M. Etmimi, M. A. Ojalah and A. M. Abotruma
First-principles density functional simulations were employed to investigate the geometries, electrical properties, and hyperfine structures of various beryllium-doped diamond configurations, including interstitial (Be$_i$), substitutional (Be$_s$), and beryllium-nitrogen (Be-N) complexes. The incorporation of Be into the diamond
lattice is more favorable as a substitutional dopant than as an interstitial dopant, although both processes are endothermic. Interstitial Be could potentially exhibit motional averaging from planar to axial
symmetry with an activation energy of 0.1 eV. The most stable Be$_s$ configuration has $T_{d}$
symmetry with a
spin state of $S=1$. Co-doping with nitrogen reduces the formation energy of Be$_s$-N$_{n}$ $(n=1-4)$ complexes, which further decreases as the number of nitrogen atoms increases. This is attributed to the smaller covalent radius of nitrogen compared to carbon, resulting in reduced
lattice distortion. Be$_s$-N$_3$ and Be$_s$-N$_4$ co-doping introduces shallow donors, while Be$_s$ exhibits $n$-type semiconductivity, but the deep donor level renders it impractical for room-temperature applications. These findings provide valuable insights into the behavior of beryllium as a dopant in diamond and highlight the potential of beryllium-nitrogen co-doping for achieving $n$-type diamond
semiconductors.
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Materials Science · Cross-listed from cond-mat.mes-hall
Thermomagnonic Torques in Insulating Altermagnets
Edward Schwartz, Hamed Vakili and Alexey A. Kovalev
We develop a
symmetry-controlled theory of anisotropic thermomagnonic torques in insulating altermagnets. We identify a
spin-splitter magnonic torque linked to thermally generated, sublattice-odd
spin currents and an anisotropic entropic torque dictated by
crystal symmetry. These torques produce anisotropic
magnetic-texture responses to temperature gradients. In particular, thermally generated
spin currents induce domain-wall precession, which reduces domain-wall velocities for selected gradient directions. We also predict an anisotropic skyrmion
Hall response, with
symmetry-selected directions enabling fast skyrmion motion with strongly suppressed transverse deflection. Our results reveal experimentally testable
symmetry fingerprints of insulating altermagnets and extend more broadly to anisotropic magnets with exchange-driven
magnon spin splitting.
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Materials Science
Analysis and Uncertainty Quantification of Thermal Transport Measurements through Bayesian Parameter Estimation
Jeremy Drew, Shravan Godse, Yuxing Liang et al. (+3 more)
The thermal transport community is increasingly interested in rigorous uncertainty quantification (UQ) of their measurements. In this work, we argue that
Bayesian parameter estimation (BPE) represents a powerful framework for both analysis/fitting and UQ. We provide a detailed walkthrough of the technique (including code to duplicate our results) and example analysis based on measuring the thermal conductance of a gold/sapphire interface with FDTR. Comparisons are made against traditional analysis/UQ techniques adopted by the thermal transport community. Notable advantages of BPE include the interpretability of its results, including the capacity to indicate incorrect input assumptions, as well as a way to balance overall goodness of fit against prior knowledge of feasible parameter values. In some cases, incorporating this additional information can affect not only the magnitude of error bars but the inferred values themselves.
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All authors Jeremy Drew, Shravan Godse, Yuxing Liang, Abhishek Pathak, Jonathan A. Malen, Rachel C. Kurchin
Materials Science
On the origin of the unusual strain morphologies and polar Moiré patterns in twisted ferroelectrics
Sergey Prosandeev, Charles Paillard and Laurent Bellaiche
Density functional theory calculations are conducted to understand and reveal the origin of the complex shear strain morphology and of the polar Moiré topological pattern recently observed in twisted BaTiO$_3$
bilayers. Our first-principles calculations, along with an original analysis of them allowing the decomposition of forces into the acoustic and optical contributions, point out to the occurrence of forces mostly acting on the acoustic-related motions to produce the standing waves of the shear strain. Such acoustic waves naturally generate a striking self-organization of the shear strains, and hence create a peculiar gradient of these shear strains. A Moiré dipole pattern, consisting of the interpenetrated arrays of
vortices and antivortices made of the electric dipoles, then mostly arises due to the coupling of this gradient of the shear strain with the electric dipoles. Furthermore, other forces, namely acting on the motions associated with the optical
phonons, could also play a role in the formation of these polar
vortices and antivortices, but at a smaller extent.
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Materials Science
Size-strain characteristics of lead and gold under fast ramp compression
E. F. Talantsev and D. A. Komkova
Phase transitions in materials under fast ramp compression are an ongoing research topic, which is part of several global projects like inertial fusion. Currently, X-ray diffraction (XRD) examination of samples under fast ramp compression is limited to the determination of the sample phase state and the unit
cell lattice parameters. Here, we propose to extend this examination route by introducing the Williamson-
Hall analysis of the XRD data measured in samples under fast ramp rate conditions. To demonstrate the applicability of the method, we performed an analysis for ramp compressed lead $(P = 200 GPa)$ and gold $(P = 1003 GPa)$, which both exhibit a transition from the face-centred cubic (fcc)
lattice to the body-centred cubic (bcc)
lattice at the studied pressures. The analysis showed that lead under fast ramp compression has a nanocrystalline structure with a crystalline size of $D = (4 \pm 1) nm$ and
lattice strain $\varepsilon = 0.006 \pm 0.002 $. The effect of extreme hardening of bcc-Pb under fast ramp compression can be explained as the formation of an ultrafine grain structure in this metal. Elemental gold exhibits average crystalline size $D > 12 nm $ and unprecedentedly high, for a pure metallic element,
lattice strain $\varepsilon = 0.014 \pm 0.001 $ under fast ramp compression.
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Materials Science
Chemical Engineering of Altermagnetism in Two-Dimensional Metal-Organic Frameworks
Diego López-Alcalá, Alberto M. Ruiz, Andrei Shumilin et al. (+1 more)
Altermagnetism represents a novel class of collinear antiferromagnetism exhibiting non-relativistic
spin splitting without net magnetization, driven by
lattice symmetry rather than
spin-orbit coupling (SOC). Here, we introduce a coordination-driven chemical strategy to realize altermagnetic (AM)
spin splitting in two-dimensional (2D) planar tetracoordinated Cr-based metal-organic frameworks (MOFs). Using density functional theory (DFT) calculations, we demonstrate that replacing centrosymmetric pyrazine (pyz) ligands with non-centrosymmetric imidazole (imz) linkers in Cr-based MOFs reduces
lattice symmetry, enabling g-wave AM
spin splitting up to 65 meV. Furthermore, frontier molecular orbital engineering (FMOE) allows selective ligand
spin polarization, inducing a shift to d-wave AM anisotropy in polycyclic ligand-based 2D MOFs with
spin splitting up to 83.9 meV. Microscopic
magnetic exchange interactions (J) analysis reveals that ligand-mediated interactions dominate over metal-metal coupling, stabilizing AM order in systems with radical ligands. Interestingly, we further confirm AM
spin splitting in
spin wave spectrum, where
chiral magnon splitting is observed. Finally, we show that AM
spin splitting gives rise to experimentally accessible charge to
spin conversion, emerging as a linear response in d-wave and as a
symmetry-allowed nonlinear effect in g-wave 2D AM MOFs. This work establishes coordination chemistry as a powerful and versatile route to
symmetry control in 2D MOFs, enabling rational design of 2D molecular materials with tunable electronic and AM properties for next-generation spintronic devices.
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All authors Diego López-Alcalá, Alberto M. Ruiz, Andrei Shumilin, José J. Baldoví
Materials Science
Structural, Energetic, Electronic, and Vibrational Properties of Boron-Substituted Tungsten Clusters
Akshata M. Waghmare, Gautam S. Kamat, Sajeev S. Chacko et al. (+2 more)
We report a study of the density functional theory of the structural, electronic and vibrational properties of small tungsten-boron clusters using the B3LYP exchange-correlation functional together with the QZVP basis set. A large number of possible isomeric structures were generated using the USPEX code interfaced with Gaussian 03, and the lowest-energy configurations were selected for detailed analysis. The results show that the addition of boron significantly modifies the geometry of tungsten clusters by reducing their
symmetry and leading to the formation of shorter and stronger W-B and B-B bonds. With increasing boron concentration, boron atoms preferentially occupy edge and outer positions and gradually form interconnected B-B links within the clusters. These structural modifications improve the overall stability of the clusters, as reflected by the increase in binding energy, HOMO-LUMO energy gap, ionization potential, and chemical hardness with increasing boron concentration. The
eigenvalue spectra show a greater separation between the occupied and unoccupied electronic states after boron substitution, indicating enhanced electronic stabilization and stronger electron localization. Vibrational analysis further reveals a gradual shift from low-frequency W-W vibrational modes in pure tungsten clusters to higher-frequency W-B and B-B stretching modes in boron-rich clusters, indicating an increase in the covalent character of bonding. Overall, the results show that boron improves the stability of tungsten clusters by modifying their bonding and electronic properties at the atomic scale
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All authors Akshata M. Waghmare, Gautam S. Kamat, Sajeev S. Chacko, Chetan V. Gurada, Balasaheb J. Nagare
Materials Science · Cross-listed from cond-mat.str-el
Detection of Image Potential States above the vacuum level in GeTe
Frédéric Chassot, Aki Pulkkinen, Ján Minár et al. (+3 more)
The ferroelectric
semiconductor {\alpha}-GeTe(111) has attracted significant
attention in the last decade due to its unique properties, with extensive studies focusing on its occupied electronic bandstructure. In contrast, its unoccupied states - particularly those near the conduction band minimum - remain largely unexplored. In an effort to characterize those states, we surprisingly observe three image potential states (IPS) in {\alpha}-GeTe(111) extending up to 0.8 eV above the
vacuum level. Using time and angle-resolved photoemission spectroscopy, we resolve the full
parabolic dispersions of the first three IPS and determine their binding energies. Our analysis, combined with Bloch spectral function calculations, reveals that the unexpected persistence of IPS above the
vacuum level originates from strong dipole transitions and the presence of large electron reservoirs in GeTe.
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All authors Frédéric Chassot, Aki Pulkkinen, Ján Minár, Gunther Springholz, Matthias Hengsberger, Claude Monney
Materials Science
Ferrimagnetic Order in Tetragonal Antiperovskite Mn$_3$GeN
Shaun O'Donnell, Corlyn Regier, Sharad Mahatara et al. (+7 more)
The
crystal and
magnetic structures of the nitride antiperovskite Mn$_3$GeN reveals ferrimagnetic order stemming from a distorted kagome-derived
lattice of the Mn atoms. Polycrystalline Mn$_3$GeN was synthesized via a solid-state reaction and characterized using
neutron powder diffraction, DC magnetometry, and first-principles calculations. Rietveld refinement reveals near-stoichiometric composition (Mn$_3$GeN$_{0.94(1)}$) adopting a tetragonal $I4/mcm$ structure at $T$ = 500 K and below, featuring axially distorted and tilted [NMn$_6$] octahedra that result in a buckled Mn kagome
lattice. On heating, the tetragonal distortion and octahedral tilt angle decrease continuously before transitioning to the cubic $Pm\bar{3}m$ antiperovskite phase at $T \approx$ 524 K.
Neutron diffraction and magnetometry together reveal noncollinear ferrimagnetic ordering. For 30 K $\le T \le$ 500 K, the
magnetic structure is described by a single propagation vector, $k$ = (0, 0, 0), with inequivalent Mn1 and Mn2 sublattices that couple antiferromagnetically to yield a net moment. Density functional theory-based calculations show the different local moments originate from the bandwidths associated with the distinct Mn-N bond lengths. The temperature dependence of the sublattice moments indicates a compensation-like crossover between Mn1- and Mn2-derived magnetization near 380 K. These findings uncover a previously unrecognized subtlety in the
magnetic and structural behavior of Mn$_3$GeN, highlighting the interplay between structural distortions,
magnetic ordering, and electronic structure in kagome-derived antiperovskite materials.
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All authors Shaun O'Donnell, Corlyn Regier, Sharad Mahatara, H. Cein Mandujano, Efrain E. Rodriguez, Danielle R. Yahne, Stephan Lany, Sage R. Bauers, Rebecca W. Smaha, James R. Neilson
Materials Science
Accurate bandgaps of photovoltaic kesterites from first-principles DFT+U
Andrew C. Burgess, Lórien MacEnulty, Ethan D'Arcy et al. (+2 more)
Streamlined prediction of the electronic properties of photoactive materials warrants a Density Functional Theory (DFT) based approach that (i) yields reliable bandgaps, (ii) is free of empirically tuned parameters, and (iii) exhibits low computational overhead. Here we show that for Cu2ZnSnS4 and Cu2ZnGeS4 kesterite photovoltaic materials, all three of these demands are met by the DFT plus Hubbard U technique (DFT+U) with corrective parameters evaluated via minimum-tracking linear response. The predicted bandgaps are found to even marginally outperform those from the self-consistent GW approach. Key to this method's success is the application of Hubbard U corrections to all atomic subspaces that dominate the conduction and valence band edges, as opposed to the conventional approach of correcting 3d and 4f atomic states. Intriguingly, the inclusion of Hund's J corrections via the extended DFT+U+J functional significantly worsens these results. This under performance can be ameliorated through the use of the Burgess-Linscott-O'Regan (BLOR) flat-plane based Hubbard U plus Hund's J functional, with bandgap predictions in close agreement with the conventional DFT+U method. The DFT+U method is also used to predict defect-induced changes to the bandgap and associated formation energies, in 1,728-atom supercells.
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All authors Andrew C. Burgess, Lórien MacEnulty, Ethan D'Arcy, David Gavin, David D. O'Regan
Materials Science · Cross-listed from cond-mat.dis-nn
On the Boroxol Ring Fraction in Melt-Quenched B$_2$O$_3$ Glass
Debendra Meher, Nikhil V. S. Avula and Sundaram Balasubramanian
An atomistic structural model for melt-quenched B$_2$O$_3$ glass has eluded the simulation community so far. The difficulty lies in the abundance of the six-membered boroxol rings - an intermediate-range order motif suggested through Raman and NMR spectroscopy - which is challenging to obtain in atomistic molecular dynamics simulations. Here, we report the development of a DFT-accurate machine-learned potential for B$_2$O$_3$ and employ quench rates as low as 10$^{9}$ K/s to obtain B$_2$O$_3$ glasses with more than 30% of boron atoms in boroxol rings. Also, we show that the pressure, and consequently the boroxol fraction, in the deep potential molecular dynamics (DPMD) simulations critically depends on the range of the geometry descriptor used in the
embedding neural network, and at least a 9 $\unicode{x212B}$ range is required. The boroxol ring fraction increases with decreasing quench rate. Finally, amorphous B$_2$O$_3$ configurations display a minimum in energy at a boroxol fraction of 75%, intriguingly close to the experimental estimate in B$_2$O$_3$ glass.
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Materials Science · Cross-listed from cond-mat.mes-hall
Electrically tunable spin qubits in strain-engineered graphene p-n junctions
Myung-Chul Jung and Nojoon Myoung
Strain engineering enables
quantum confinement in pristine
graphene without degrading its intrinsic mobility and
spin coherence. Here, we extend previously proposed strain-induced charge-
qubit architectures by incorporating
spin degrees of freedom through Rashba
spin-orbit coupling (RSOC) and Zeeman fields, enabling
spin-
qubit operation in single-layer
graphene (SLG). In a
graphene p-n junction, a strain-induced nanobubble generates a pseudo-
magnetic field that forms double
quantum dots with gate-tunable level hybridization. Tight-binding
quantum transport simulations and a four-band model reveal two distinct avoided crossings:
spin-conserving gaps at zero detuning and
spin-flip gaps at finite detuning, the latter increasing with SOC strength while the former decreases. Time-domain simulations confirm detuning-dependent Rabi oscillations corresponding to these two operational regimes. These results demonstrate that strain-induced confinement combined with tunable SOC provides a viable mechanism for coherent
spin manipulation in pristine
graphene, positioning strained SLG as a promising platform for scalable
spin-based
quantum technologies.
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Materials Science
Multimode Jahn-Teller Effect in Negatively Charged Nitrogen-Vacancy Center in Diamond
Jianhua Zhang, Jun Liu, Z. Z. Zhu et al. (+3 more)
We present a first-principles study of the multimode Jahn-Teller (JT) effect in the exctied $^{3}E$ state of the negatively charged nitrogen-vacancy (NV) center in diamond. Using density functional theory combined with an intrinsic distortion path (IDP) analysis, we resolve the full activation pathways of the JT distortion and quantitatively decompose the distortion into contributions from individual vibrational modes. We find that multiple vibrational modes participate cooperatively in the JT dynamics, giving rise to a shallow adiabatic potential energy surface with low barriers, consistent with thermally activated pseudorotation. The dominant JT-active modes are found to closely correspond to vibrational features observed in two-dimensional electronic spectroscopy (2DES), in agreement with recent ab initio molecular dynamics simulations. Our results establish a microscopic, mode-resolved picture of vibronic coupling in the excited-state NV center and provide new insight into dephasing, relaxation, and optically driven dynamics relevant to solid-state
quantum technologies.
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All authors Jianhua Zhang, Jun Liu, Z. Z. Zhu, K. M. Ho, V. V. Dobrovitski, C. Z. Wang
Materials Science
Anomalous shift in scattering from topological nodal-ring semimetals
Runze Li, Chaoxi Cui, Ying Liu et al. (+2 more)
An electron beam may experience an anomalous spatial shift during an interface scattering process. Here, we investigate this phenomenon for reflection from mirror-
symmetry-protected nodal-ring semimetals, which are characterized by an integer topological charge $\chi_h$. We show that the shift is generally enhanced by the presence of nodal rings, and the ring's geometry can be inferred from the profile of shift vectors in the interface momentum plane. Importantly, the anomalous shift encodes the topological information of the ring, where the circulation of the shift vector field $\kappa_s$ over a semicircle is governed by the topological charge, with a simple relationship: $\kappa_s=-2\pi \chi_h$. Furthermore, we demonstrate that the shift and its circulation reflect distinct features of topological phase transitions of the charged rings. This study uncovers a novel physical signature of topological nodal rings and positions anomalous scattering shifts as a powerful tool for probing topological band structures.
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All authors Runze Li, Chaoxi Cui, Ying Liu, Zhi-Ming Yu, Shengyuan A. Yang
Materials Science
Low-temperature behavior of density-functional theory for metals based on density-functional perturbation theory and Sommerfeld expansion
Xavier Gonze, Christian Tantardini and Antoine Levitt
The temperature dependence of most solid-state properties is dominated by
lattice vibrations, but metals display notable purely electronic effects at low temperature, such as the linear specific heat and the linear
entropy, that were derived by Sommerfeld for the non-interacting electron gas via the low-temperature expansion of
Fermi-Dirac integrals. Here we treat temperature as a perturbation within density-functional perturbation theory (DFPT). For finite temperature, we show how self-consistency screens the bare, temperature-induced density change obtained in the non-interacting picture: the inverse transpose of the electronic dielectric operator, that includes Adler-Wiser and a term related to the shift in
Fermi level, links the self-consistent density response to the bare thermal density change. This approach is implemented in DFTK, and demonstrated by the computation of the second-order derivative of the free energy, and the first-order derivative of
entropy for aluminum. Then, we examine the $T\!\to\!0$ limit. The finite temperature formalism contains divergences, that we cure using the Sommerfeld expansion to analyze metallic systems at 0 K. The electronic free energy is quadratic in $T$ provided the
Fermi level is not at a Van Hove singularity of the density of states. If the latter happens, another temperature behavior might appear, depending on the type of Van Hove singularity, that we analyze. Our formulation applies to systems periodic in one, two, or three dimensions, and provides a basis for studying temperature-dependent electronic instabilities (e.g., charge-density waves) within density-functional theory and DFPT.
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Materials Science
Enrico Perfetto and Gianluca Stefanucci
Non-Hermitian physics is reshaping our understanding of
quantum systems by revealing states and phenomena without Hermitian counterparts. While non-Hermiticity is typically associated with gain-loss processes in open systems, we uncover a fundamentally different route to non-Hermitian behavior emerging from non-
equilibrium correlations. In photoexcited
semiconductors, the effective interaction between electrons and holes gives rise to a pseudo-Hermitian Bethe-Salpeter
Hamiltonian (PH-BSH) that governs excitonic states in the presence of excited populations. Within this framework, we identify a previously unknown class of excitonic quasiparticles - exceptional
excitons - corresponding to exceptional points embedded inside the electron-hole continuum. Exceptional
excitons emerge at the onset of population inversion, and represent the strongly renormalized counterparts of the system's
equilibrium excitons. They are spatially localized, protected against hybridization with the continuum, and remain long-lived even in regimes where conventional
excitons undergo a Mott transition. Crucially, exceptional
excitons appear only when the PH-BSH is evaluated with non-thermal, resonantly generated carrier populations that support an excitonic superfluid. Ab initio results for monolayer WS_2 explicitly demonstrate this scenario and show that exceptional
excitons can be realized with existing ultrafast pumping techniques. We also identify distinctive optical and photoemission signatures that enable their unambiguous detection.
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Materials Science · Cross-listed from physics.app-ph
Sensor free, self regulating thermal switching via anomalous Ettingshausen effect and spin reorientation in DyCo5
Shibo Wang, Hiroki Tsuchiura and Nobuaki Terakado
We propose a sensor free, self regulating thermal switch that combines the anomalous Ettingshausen effect (AEE) with a temperature driven
spin reorientation transition (SRT) in the rare earth cobalt compound DyCo$_5$. Using density functional theory and the Kubo linear-response formalism, we compute the anomalous
Hall conductivity $\sigma_{xy}(\varepsilon)$ and the finite temperature anomalous Nernst conductivity $\alpha_{xy}(T)$ for two magnetization directions, magnetization parallel and perpendicular to the crystallographic c axis. While the intrinsic $\sigma_{xy}$ at the
Fermi level remains sizable for both orientations, $\alpha_{xy}$ exhibits an about two orders of magnitude contrast in the SRT temperature window. This contrast is consistent with the low temperature Mott relation through the energy slope $\partial_\varepsilon \sigma_{xy}(\varepsilon)\rvert_{E_{\mathrm F}}$ and is traced to strongly peaked Berry
curvature hot spots generated by
spin orbit coupling induced avoided crossings of Co $3d$ bands. Combining $\alpha_{xy}$ with longitudinal transport coefficients, we estimate device level metrics, namely the anomalous Nernst thermopower $S_{\mathrm{ANE}}$ and the Ettingshausen coefficient $\Pi_{\mathrm{AEE}}=T S_{\mathrm{ANE}}$, and demonstrate robust orientation controlled switching under a fixed in plane bias current. These results establish a materials based route to compact thermal control without external sensors or feedback electronics and provide a concrete example that the proposed principle can be realized in an existing ferromagnet.
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Materials Science · Cross-listed from cond-mat.str-el
Pressure-induced hole delocalization in the strongly correlated quasicubic charge-transfer perovskite $LaBa_2Fe_3O_{8+\delta}$d
M. ElMassalami, S. Favre and M. B. Silva Neto
Analysis of the thermal and baric
evolution of resistance in $LaBa_2Fe_3O_{8+\delta}$ enabled the construction of its pressure-temperature (P-T) phase diagram, which prominently displays a critical boundary, $P^{MIT}_c(T)$, marking the transition from localized to hole-type extended states. The relatively low critical pressures [$P^{MIT}_c(T) \approx 3$-8 GPa] suggest that, as $P \rightarrow P_c$ in this narrow-gap, strongly correlated charge-transfer system, both the hybridization strength and the charge-transfer character are progressively enhanced - ultimately leading to the emergence of
metallicity. Emphasizing the electronic nature of this transition, pressure-dependent structural analyses at room temperature reveal no associated structural phase transition at $P^{MIT}_c(T)$; the system retains a (weakly tetragonally distorted) quasicubic perovskite structure with Murnaghan-type compressibility up to 30\,GPa. The emergence of hole delocalization and metallic conduction, coupled with suppressed antiferromagnetism, suggests proximity to
quantum criticality.
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Materials Science
Estimating Reaction Rate Constants from Impedance Spectra: Simulating the Multistep Oxygen Evolution Reaction
Freja Vandeputte, Bart van den Boorn, Matthijs van Berkel et al. (+3 more)
The efficiency of water electrolysis in a photoelectrochemical
cell is largely limited by the oxygen
evolution reaction (OER) at its
semiconductor photoanode. Reaction rate constants are key to investigating the slow kinetics of the multistep OER, as they indicate the rate-determining step. While these rate constants are usually calculated based on first-principles simulations, this research aims to estimate them from experimental electrochemical impedance spectroscopy (EIS) data. Starting from a microkinetic model for charge transfer at the
semiconductor-electrolyte interface, an expression for the impedance as a function of the rate constants is derived. At lower potentials, the order of this impedance model is reduced, thus eliminating the rate constants corresponding to the last reaction steps. Moreover, it is shown that EIS data from at least two potentials needs to be combined in order to uniquely identify the rate constants of a particular reduced order model. Therefore, this work details a sample maximum likelihood
estimator that integrates not only multiple frequencies, but also multiple potentials simultaneously. Measuring multiple periods of the current density and potential signals, allows this frequency domain
estimator to take measurement uncertainty into account. In addition, due to the large numerical range of the rate constants, various scaling methods are implemented to achieve numerical stability. To find suitable initial values for the highly nonlinear optimization problem, different global estimation methods are compared. The complete estimation procedure of the rate constants is illustrated on simulated EIS data of a hematite photoanode.
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All authors Freja Vandeputte, Bart van den Boorn, Matthijs van Berkel, Anja Bieberle-Hütter, Gerd Vandersteen, John Lataire
Materials Science · Cross-listed from cond-mat.other
Chiral orbital current driven topological Hall effect in Mn3Si2Te6
Arnab Das and Soumik Mukhopadhyay
Chiral orbital current (COC) plays a crucial role in governing the magnetization and transport behaviour in the layered ferrimagnetic nodal-line
semiconductor Mn3Si2Te6. Here, we observe that the topological
Hall effect (THE), typically attributed to Berry
curvature from
chiral spin textures, originates from COC, which produces an emergent
magnetic field for conduction electrons due to its real-space orbital textures. We find that the THE signal strengthens as we move down from bulk to nanoflakes, but tends to disappear with increasing current, along with the disappearance of the COC state. We also demonstrate a strong correlation between the colossal magnetoresistance (CMR) and the observed THE, suggesting that large Berry
curvature and topological transport can arise purely from orbital degrees of freedom, providing a new platform for engineering dissipationless transport in 2D magnets.
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Materials Science
Isotropic Dirac fermion and anomalous oscillator strength of zeroth Landau level transition
Zeping Shi, Wenbin Wu, Guangyi Wang et al. (+12 more)
Dirac
fermions, characterized by their linear dispersion and relativistic nature, have emerged as a prominent class of quasiparticles in condensed matter physics. While the Dirac equation, initially developed in the context of high-energy physics, provides a remarkable framework for describing the electronic properties of these materials, the inherent
symmetry constraints of condensed matter often lead to deviations from the idealized paradigm. In particular, three-dimensional Dirac
fermions in solids often exhibit anisotropic behavior, challenging the notion of perfect
symmetry inherent in the Dirac equation. Here, we report the observation of isotropic massive Dirac
fermions in LaAlSi through Landau level spectroscopy. The presence of three-dimensional massive Dirac
fermions across the
Fermi energy is demonstrated by quantized and semiclassical analyses of the
magnetic field
evolution of Landau level transitions. The isotropic topological nature,
Fermi velocity, and Dirac mass are evidenced by the identical magneto-infrared response among the Faraday and three Voigt geometries. Furthermore, we observe an unusually large oscillator strength in the zeroth Landau level transition of the Dirac
fermion, compared to transitions with higher indices. This phenomenon, supported by model calculations, can be attributed to the combined effects of the partial excitation of Dirac
fermion and the resonant dielectric coupling with the Weyl
plasma. Our work provides a strategy for realizing ideal quasiparticle excitations and their coupling effects in condensed matter systems, offering a platform for exploring relativistic physics.
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All authors Zeping Shi, Wenbin Wu, Guangyi Wang, Mykhaylo Ozerov, Jian Yuan, Wei Xia, Yuhan Du, Xianghao Meng, Xiangyu Jiang, Mingsen Zhou, Yuxi Chen, Hao Shen, Yanfeng Guo, Junhao Chu, Xiang Yuan
Materials Science
Quantifying the Rate Performance of Potassium-Ion Batteries
Caolin Ua Tuiscint and Jonathan N Coleman
Lithium-ion batteries dominate battery research and industry due to their long research history and high energy density. However, increasing demand and limited lithium resources have raised lithium prices and battery costs, motivating interest in alternative chemistries. Potassium-ion batteries have recently attracted
attention because potassium is more abundant and potentially lower cost, although the technology remains at an early stage and requires further insight for development. In this work, recently developed methods for rigorous analysis of electrode rate performance are applied to published rate-performance data from a wide range of potassium-ion batteries. Using specific capacity (mAh per g) versus charge and discharge rate curves, performance parameters are extracted to evaluate the applicability of the key model underlying these methods and to analyze opportunities for potassium-ion batteries. Relationships between these parameters and electrode properties are used to interpret performance trends and compare potassium-ion batteries with established technologies. The analysis shows that the model effectively fits potassium-ion battery rate-performance data and allows reliable extraction of performance parameters. While potassium-ion batteries generally exhibit lower specific capacities than lithium-ion and sodium-ion batteries, their upper limit of rate performance is found to exceed that of lithium-ion, sodium-ion, and related two-dimensional material electrodes, despite the larger atomic size and mass of potassium.
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Materials Science
Study of the acoustic and thermal response of an elastically anisotropic solid to a sub-nanosecond laser pulse in transient grating spectroscopy
Jakub Kušnír, Tomáš Grabec, Petr Sedlák et al. (+2 more)
Transient grating spectroscopy (TGS) is a material characterization technique based on
laser-induced thermoelastic excitation of thermal and acoustic gratings. On opaque samples, these gratings are dynamic surface displacements that reflect the sample's elastic and thermal properties, enabling both types of parameters to be determined from a single experiment. Here, we develop a detailed finite element model (FEM) of the TGS experiment that fully captures the coupling between the thermal and mechanical fields, as well as the optical detection of surface displacement using a heterodyning approach. Using custom-designed two-dimensional elements, the model is particularly suitable for analyzing TGS measurements on anisotropic media, for which analytical theory is insufficient. The simulation captures not only the anisotropic relaxation of the thermoelastic field but also several acoustic features that arise at very short (ultra-transient) timescales and provide additional information about the elastic properties of the examined material. The model offers new opportunities for the in silico testing of various modifications of TGS experiments and their applications to a broad class of materials.
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All authors Jakub Kušnír, Tomáš Grabec, Petr Sedlák, Pavla Stoklasová, Hanuš Seiner
Materials Science
Fully Compensated Ferrimagnetic Properties of (Cr,Fe)S Compound with a Pyrrhotite-type Structure
Weida Yin, Masato Miyakawa, Satoshi Semboshi et al. (+6 more)
To optimize the processing conditions for the (Cr,Fe)S non-
equilibrium phase with a pyrrhotite-type structure, the phase states and
magnetic properties of the specimens obtained at various sintering temperatures were investigated. A slightly off-stoichiometric composition of Cr23Fe23S54 (approximately (Cr,Fe)7S8) sintered and quenched from 1323 K indicates a single-phase pyrrhotite-type structure with a layered-NiAs-type structure in which vacancies occupy every two layers (C12/c1; the space number is 15). The compound shows fully compensated ferrimagnetic behavior at a magnetization compensated temperature of approximately 200 K. The
magnetic behavior exhibits a typical N-type ferrimagnet, as predicted by Néel. From X-ray photoelectron spectroscopy analyses, it is found that the compound is composed of Fe2+ and Cr3+. The large
magnetic coercivity of 38 kOe at 5 K is also unique and can be applied to spintronic devices. Furthermore, changing the quenching temperature enables control of the degree of order of the vacancies in the interlayer and results in tuning of the magnetization compensated temperature. First-principles calculations show a pseudo-gap located at the
Fermi level in the up-
spin band, suggesting high
spin polarization as well as the NiAs-type structure indicated in the previous our report.
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All authors Weida Yin, Masato Miyakawa, Satoshi Semboshi, Noriharu Yodoshi, Akira Masago, Yosuke Kawahito, Tetsuya Fukushima, Hisazumi Akai, Rie Y. Umetsu