Materials Science
Ultrahigh-Entropy Compositionally Complex Ceramics: Fluorite-Pyrochlore Phase Stability and Order-Disorder Transitions
Keqi Song and Jian Luo
Three groups of 32 ultrahigh-
entropy compositionally complex ceramics (CCCs) containing 16-19 components were synthesized and characterized. The first group of 15 CCCs forms single ultrahigh-
entropy fluorite or pyrochlore phases in 12 compositions, while two compositions exhibit fluorite-fluorite dual phases, observed for the first time. Compared with ternary A2B2O7 pyrochlore, these ultrahigh-
entropy CCCs are more prone to ordering (pyrochlore formation), surprisingly even more at higher size disorder (more
lattice distortion), with suppressed dual-phase formation relative to five-component high-
entropy ceramics. Two new 19-component series, denoted as "19CCC-P|Fy" and "19CCC-P|Fz", form single ultrahigh-
entropy fluorite or pyrochlore phases. The 19CCC-P|Fy series, maintaining a 2:2 ratio of 3+ and 4+ cations, exhibits a pyrochlore-to-fluorite order-disorder transition (ODT) at y ~ 0.84, consistent with the linear projection of the pyrochlore superstructure peak intensity as an order parameter. In contrast, the 19CCC-P|Fz series with non-2:2 ratios of 3+ and 4+ cations show an abrupt ODT at z ~ 0.41-0.44, despite a projected transition at z ~ 0.64 from the order parameter.
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Materials Science
Electric Field Induced Multi-Space Topological Phase Transitions in Janus Monolayer MnBi2Se2Te2
Kefan Zhang, Jian Wu and Weiyi Pan
Manipulating coexisting multi-space topological states within a single material is a critical frontier for low-power spintronic applications, for which perpendicular electric-field gating offers a highly precise tuning mechanism. Using first-principles calculations and atomistic
spin simulations, it is demonstrated that Janus monolayer MnBi2Se2Te2 can exhibit simultaneous momentum-space and real-space topological phase transitions under an external electric field. Specifically, the electric field drives momentum-space transitions across a topologically trivial state (C = 0), a low-Chern-number
quantum anomalous
Hall state (C = -1), and a high-Chern-number state (C = 2). Concurrently, by actively tuning the competition between the Dzyaloshinskii-Moriya interaction and
magnetic anisotropy, the electric field induces real-space
magnetic transitions from a uniform
ferromagnetic state to an isolated skyrmion state, and ultimately to a skyrmion-spiral domain coexistence phase. Electric-field-induced variations in both the QAHE and
magnetic textures may give rise to multiple topological phase transitions, involving distinct topological regimes: a purely k-space
topology, RK-joint skyrmions, and pure real-space skyrmions. These findings establish a powerful material platform and efficient route for the synergistic control of coexisting topological orders, making it highly promising for next generation multi-functional spintronic devices.
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Materials Science
Domain-Growth Kinetics and Scaling Laws Governing Pulse-Driven Accumulative Polarization Switching in HZO
Manish Anand, Balram Khattar and Abhishek Sharma
Accumulative polarization switching driven by sequential sub-coercive electric-field pulses offers a promising route toward low-power ferroelectric
memories and neuromorphic devices. However, the kinetic regimes governing this nonequilibrium process remain poorly understood. Here, we employ a phase-field model based on the time-dependent Landau-Ginzburg formalism to investigate pulse-driven accumulative switching in ferroelectric HZO. By systematically varying the initial domain configuration, pulse amplitude, pulse-on time, and pulse-off time, we establish a quantitative link between microscopic domain-wall dynamics and macroscopic polarization accumulation. We show that the effective switched-domain radius follows distinct scaling regimes characterized by the local kinetic exponent. Initially, a local exponent greater than 1 indicates superlinear domain growth driven by enhanced irreversible domain-wall propagation under successive pulses. As switching progresses, a local exponent close to unity marks steady self-similar growth, whereas a local exponent less than 1 signifies decelerating dynamics caused by
geometric confinement, depletion of switchable polarization, and relaxation-induced back switching. The transition between these regimes is governed by the competition between field-driven excitation during the pulse-on interval and spontaneous relaxation during the pulse-off interval. The initial domain geometry further influences this transition. Increasing the pulse amplitude or pulse-on duration extends the superlinear regime, whereas longer pulse-off times promote relaxation and suppress accumulation. These findings establish a unified scaling framework for pulse-driven accumulative switching, providing quantitative insight into nonequilibrium ferroelectric domain
evolution and design guidelines for HZO-based
memory and neuromorphic devices.
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Materials Science
Raman spectroscopy of the van der Waals altermagnet Co$_{1/4}$NbSe$_2$
Dushyanthini Balasundaram, Bishal Thapa, Resham Babu Regmi et al. (+3 more)
We investigate the influence of Co intercalation and altermagnetic order on the
lattice dynamics of the layered compound Co$_{1/4}$NbSe$_2$. Polarization-resolved Raman spectroscopy, supported by density-functional theory, enables identification of six Raman-active
phonons. Co intercalation drives a substantial reconstruction of the vibrational spectrum through zone folding of NbSe$_2$
phonons, producing hybridized modes with mixed zone-center and zone-boundary character. Despite this, Co atoms do not participate in any Raman-active modes by
symmetry, which is in marked contrast to related $1/3$ compounds where intercalant modes do contribute to the Raman spectrum. Temperature-dependent Raman measurements across the altermagnetic transition show no discontinuities, which is consistent with
magnetic fluctuations persisting above the Néel temperature. However, we find evidence for
spin-
phonon coupling in $A_{1g}$
symmetry modes owing to their out-of-plane Se displacements. Our work demonstrates the substantial impact of intercalation on the vibrational properties of transition metal dichalcogenides and the presence of
spin-
phonon interactions in a newly discovered altermagnetic material.
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All authors Dushyanthini Balasundaram, Bishal Thapa, Resham Babu Regmi, Nirmal J. Ghimire, Igor I. Mazin, Patrick M. Vora
Materials Science · Cross-listed from cs.LG
EquiFiLM: Charge-Conditioned Equivariant Force Fields via Feature-wise Linear Modulation
Samuel Sahel-Schackis, Ken-ichi Nomura, Aiichiro Nakano et al. (+2 more)
Foundation machine learning force fields (MLFFs) such as MACE-MP-0 and UMA cover broad chemical space at near density functional theory (DFT) accuracy. However, they assume
equilibrium ground-state physics and do not natively handle externally induced changes to the electronic state, such as charging, applied fields, or electronic excitation, which limits their use for driven processes such as photoexcitation and charge injection. We propose EquiFiLM, a lightweight extension that adds continuous external conditioning to any equivariant foundation MLFF via a per-layer Feature-wise Linear Modulation (FiLM) block, learning externally driven changes to the potential energy surface from minimal training data. The block modulates only scalar channels and preserves E(3)-equivariance exactly. We demonstrate the recipe on charged
liquid water with the foundation model MACE-MatPES as the backbone, yielding E-MACE. On the four training charges, E-MACE delivers a $3.1\times$ reduction in force RMSE ($21.3$ to $6.96$ meV/$\mathring{A}$) and a $61\times$ reduction in per-atom energy RMSE ($6.1$ to $0.1$ meV/atom) over a baseline without EquiFiLM trained on the same data, at indistinguishable inference cost. Across seven held-out interpolation and extrapolation charges, force RMSE stays within $18-61$ meV/$\mathring{A}$ and energy RMSE within $0.7-5.4$ meV/atom. The model runs stable molecular dynamics across the full range tested and predicts the charge-dependent first-shell response of the reduced pair distribution function probed by ultrafast electron diffraction. Adding this conditioning axis to the foundation requires only a few thousand DFT-labeled frames, against the $\approx 10^8$ structures of a charge-aware foundation trained from scratch. The recipe is backbone- and conditioning-agnostic: it applies without architectural change to any equivariant MLFF with scalar interaction-layer channels.
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All authors Samuel Sahel-Schackis, Ken-ichi Nomura, Aiichiro Nakano, Matthias F. Kling, Thomas Linker
Materials Science
Non-Hermitian Tight-Binding Bands in Graphene: Optical Conductivity, Strain Effects, and Bernal Bilayer Extension
Maolin Bo, Yaorui Tan, Sunxin Fan et al. (+4 more)
Within the tight binding framework of graphenes {\pi} electron nearest neighbors, the Tan Bo model parametrizes transition energies t(dr) based on bond lengths and angles via the Mobius transformation combined with exponential decay. Comparisons between isotropic , geometrically anisotropic , and Slater Koster scales reveal that B = 0 is equivalent to the SK scheme, with L(B) reaching its optimum at Bopt = 0. The Hermitian assembly maintains the Dirac cone at the K point.The Tan Bo geometry dependent transition model and non Hermitian TB assembly scheme developed in this study provide a reproducible single particle
benchmark and parameterization reference for future non Hermitian chemical calculations incorporating electron correlation effects in
graphene systems.
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All authors Maolin Bo, Yaorui Tan, Sunxin Fan, Xiang Chen, Yunhu Zhu, Zhongkai Huang, Chuang Yao
Materials Science
Magnetotransport and electronic band structure of EuNi$_2$As$_2$ antiferromagnet
Faheem Gul, Mane Sahakiyan, Orest Pavlosiuk et al. (+1 more)
We investigated the magnetotransport properties of single-
crystals of tetragonal van der Waals compound EuNi$_2$As$_2$, that orders antiferromagnetically below 14.6 K in an incommensurate helical structure. Metamagnetic transitions are revealed by the magnetization measured in the
magnetic field applied transverse to the axis of the helix, and are clearly reflected in the magnetoresistance. Overall, the magnetoresistance is small, but shows complex changes with the temperature, the strength, and the angle of the applied
magnetic field. In magnetically ordered state, magnetoresistance shows prominent anomalies related to the metamagnetic transitions. For temperatures above the Néel point the negative magnetoresistance can be modeled very well with de Gennes-Friedel mechanism of the
spin-disorder-scattering reduction.
Hall resistivity data indicate hole-dominated multi-band conductivity in
antiferromagnetic state and single-band one above the Néel temperature, with carrier concentrations of the order of 10$^{22}$cm$^{-3}$. This metallic character of the compound seems to obscure the plausible topological contribution to the
Hall resistivity. Our ab-initio calculations of electronic band structure showed that the electronic structure changes very strongly upon
magnetic ordering, but the density of states at the
Fermi level differs by a factor smaller than two, in agreement with experimental
Hall resistivity data. Meaningful changes in the density of states,
magnetic moments, and screening length of Eu-4$f$ orbitals are discussed in terms of the effects of Hubbard corrections.
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All authors Faheem Gul, Mane Sahakiyan, Orest Pavlosiuk, Piotr Wiśniewski
Materials Science
Data-driven atomistic modelling of hybrid halide perovskite passivation
Laura-Bianca Paşca, Henry J. Snaith and Volker L. Deringer
Molecular passivation of surface defects is key to improving the optoelectronic performance of hybrid halide perovskite materials, but the underlying atomistic mechanisms are incompletely understood. While machine-learned interatomic potentials are now widely used to simulate complex molecular and crystalline systems, their application to experimentally-realistic scenarios - such as molecules coordinating to perovskite surfaces - is still far from trivial. Here, we describe a multistep training pipeline, resembling continuous fine-tuning used for large
language models, to underpin atomistic modelling and computational experiments in this domain. Our protocol involves two components: (i) a large, curated, and open
dataset of diverse metal and hybrid halide perovskite structures ('hyP-26'); and (ii) a small, specialised
dataset for an amino-silane molecule passivating the surface, providing highly specific information for fine-tuning. We apply this approach to explore collective behaviour at a mixed-composition halide perovskite surface passivated with a varying coverage of amino-silane molecules, revealing an
evolution of interactions with increasing molecular surface coverage.
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Materials Science
High-temperature operation of III-nitride high-electron-mobility transistors
Yi-Chen Liu, Jacklyn Zhu, John Niroula et al. (+3 more)
High-electron-mobility transistors (HEMTs) made with III-nitride materials are of potential use in high-temperature electronic applications including power electronics, communications, aerospace and space exploration. However, the demands of such applications make it essential to understand the thermal limits and performance
evolution of III-nitride HEMTs. Here, we analyze the high-temperature operation of III-nitride HEMTs, examining the impact on material properties, device structure, and
circuit-level behavior. We explore the role of critical device layers - including barrier and channel engineering, substrate selection, and passivation strategies - in mitigating high-temperature-induced effects, and evaluate the thermal stability of III-nitride HEMTs in logic, radiofrequency, and power electronics applications. We also highlight key remaining challenges in the design and optimization of III-nitride devices for high-temperature applications.
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All authors Yi-Chen Liu, Jacklyn Zhu, John Niroula, Hridibrata Pal, Tomas Palacios, Savannah R. Eisner
Materials Science
Phase-field modeling of elastically driven abnormal grain growth
Yazhuo Liu, Yin Zhang, Kunqing Ding et al. (+6 more)
Grain-refined metals typically exhibit high strength, yet their engineering applications are often constrained by grain coarsening under thermo-mechanical loading. Recent experiments have revealed abnormal grain growth (AGG) in ultrafine-grained Ni thin films subjected to cyclic loading at room temperature. Unlike conventional AGG, which generally requires significant plastic deformation or high temperatures, this phenomenon occurs within the regime of macroscopic elastic deformation. This AGG is characterized by the preferential growth of grains with an in-plane <100> orientation aligned with the loading direction. Here, we investigate the underlying physical mechanisms by combining phase-field simulations with micromechanical analysis. The results indicate that elastic energy reduction provides a thermodynamically plausible driving force for this orientation-selective grain growth. Phase-field simulations reveal the
evolution kinetics of AGG and confirm that local grain geometry and stress states play critical roles in determining the grain growth pathway. By applying this framework to systems with varying elastic anisotropy, we establish a general approach for investigating elastically driven AGG in polycrystalline materials.
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All authors Yazhuo Liu, Yin Zhang, Kunqing Ding, Yichen Yang, Alejandro Barrios, Xavier Maeder, Olivier Pierron, Xing Liu, Ting Zhu
Materials Science
Semiconductor nanofilms as thermal phonon polarizers: competing effects of scattering selection rules and boundary mode conversion
Vasumathy Ravishankar and Navaneetha K. Ravichandran
Phonon scattering selection rules are known to control heat flow through bulk solids. Here we show that these selection rules also modulate heat flow through nanoscale
semiconductor films, although through a previously-unexplored mechanism. Using first-principles calculations, we expose a competition between these selection rules and
phonon mode conversion at boundaries of nanoscale films, that drives mode-polarized heat currents at cryogenic temperatures ($\le$ 100 K). This polarizing effect is stronger in materials like indium phosphide, where selection rules based on large velocity differences between
phonon branches amplifies the longitudinal acoustic (LA)
phonon contribution to thermal conductivity by restricting their intrinsic scattering events, while boundary mode conversion in nanoscale films suppresses it by depopulating the LA
phonons. The resulting transverse-polarized non-
equilibrium phonons will enable
symmetry-selective engineering of
phonon coupling to electrons, strains and defects in nanoscale films, that is difficult to achieve in bulk solids.
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Materials Science
A Distributional Framework for Generative Modeling of Molecular Crystals
Michael Kilgour, Alex Dong, Mark E. Tuckerman et al. (+1 more)
Molecular
crystals are a highly polymorphic class of materials, with a single molecule commonly crystallizing via multiple packing patterns, making structure and property prediction very challenging.
Crystal structure prediction typically comprises the production of sets of promising candidate structures, each considered in isolation rather than as samples in a thermodynamic distribution. Likewise, modern generative approaches to this problem, despite naturally sampling distributions of
crystals, lack a concrete formulation of the distributions being sampled. Two components are required to impart meaning to the distributions of
crystals generated under such models: a canonical parameterization, and a loss function which equilibrates the generated samples to some target distribution. We develop such a parameterization, and train energy-based generative flow networks (GFlowNets) to approximate the Boltzmann distribution over
crystal structures for target molecules and space groups. Combined, these components comprise our MXtalGFlow framework for molecular
crystal modeling. Going beyond sampling disconnected sets of low-energy structures, MXtalGFlow yields a thermodynamic distribution over
crystal structures. We sample and analyze distributions of
crystals for two molecules, each under two energy functions, a Lennard-Jones potential and the Universal Model for Atoms. We characterize the local structural basins about the known polymorphs, and identify additional as-yet un-reported packing modes with competitive probabilities to the known experimental structures. With MXtalGFlow, we illustrate how to define and train a model to sample a thermodynamically meaningful distribution of molecular
crystals, and analyze such a distribution to glean useful information.
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All authors Michael Kilgour, Alex Dong, Mark E. Tuckerman, Jutta Rogal
Materials Science · Cross-listed from physics.optics
Torsional selection rule for the spin–orbit conversion of light
Edilberto O. Silva
Standard Pancharatnam-Berry and linear-birefringent media convert optical
spin into orbital angular momentum (OAM) through an anisotropy director, a rank-two, headless field, and therefore obey the selection rule $\Delta\ell=2q$ per unit texture charge $q$. We show that a medium with
geometric torsion, the continuum limit of a screw-dislocation array, can convert
spin to OAM through the contortion of its material connection, which enters the effective paraxial dynamics as a rank-one vector field. The resulting selection rule is $\Delta\ell=q$. Its winding is fixed by geometry and
symmetry, not by a Pancharatnam–Berry director, and the process conserves the screw charge $\tilde J_z=L_z+(q/2)\sigma_z$ while exchanging $(2-q)\hbar$ of angular momentum per converted
photon with the defect
lattice. Paraxial simulations confirm the rule: a circular Gaussian input develops a stable, topologically quantized $\ell=+q$
vortex in the reversed helicity, with $83\%$ conversion over three Rayleigh ranges and no fine-tuning. We propose a polarization-resolved photonic-
lattice discriminator in which the slope of the measured OAM versus the independently written texture charge, one for torsion, two for birefringence, separates the two mechanisms.
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Materials Science · Cross-listed from physics.comp-ph
dpti: An Automated Thermodynamic Integration Workflow for Phase Diagram Calculations with Machine Learning Interatomic Potentials
Fengbo Yuan, Xin Zhong, Donghao Zheng et al. (+4 more)
Thermodynamic integration (TI) is a widely used approach for computing free energies and phase diagrams. However, TI calculations driven by machine learning interatomic potentials (MLIPs) remain technically challenging because they require careful design of reversible integration paths and many closely related molecular dynamics (MD) tasks for each phase and state point. To address these challenges, we present dpti, an open-source Python package that automates TI workflows for phase diagram calculations with MLIPs. dpti connects reference systems with analytically known free energies to MLIP-described atomic and molecular solids and
liquids through reversible integration paths. Given JSON input files, dpti generates and runs the required MD tasks, computes free energy contributions, estimates errors, and propagates coexistence points into phase boundaries. We demonstrate the usage of dpti with two examples driven by Deep Potential models: a silica phase diagram involving beta-quartz, coesite, and melt, and the ice Ih-
liquid water phase boundary. dpti provides a useful tool for automated phase diagram calculations of materials modeled by MLIPs.
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All authors Fengbo Yuan, Xin Zhong, Donghao Zheng, Jinzhe Zeng, Linfeng Zhang, Han Wang, Yifan Li
Materials Science · Cross-listed from cond-mat.mes-hall
Functionalization of $g$-wave altermagnets: spin-splitter effect enabled by surfaces
Sopheak Sorn
We investigate surfaces of a $g$-wave altermagnet(AM) and show that they provide a platform for realizing $d$-wave altermagnetism and the associated
spin-splitter functionality. Using the Kubo formalism applied to a minimal slab model, we evaluate the
spin-splitter effect(SSE) by computing the
spin conductivity corresponding to a transverse
spin current induced by a longitudinal electric field. We find a finite SSE, absent in the bulk, that emerges from surface-induced $d$-wave altermagnetism. Strikingly, the sign pattern of the $d$-wave altermagnetism on both surfaces of the slab geometry is identical to each other, leading to additive contributions to SSE from the two surfaces, with a
spin-splitter angle reaching up to 15 degrees. In addition, this response is intrinsically linked to an accompanying surface-induced weak ferromagnetism, which potentially enables control of altermagnetic domains via an external
magnetic field and provides a route to optimize the SSE functionality. These results can be understood in terms of a bulk-boundary correspondence between surface states and bulk altermagnetic order parameters, where the
magnetic multipolar character of the latter plays a central role. Our findings strongly suggest thin-film engineering as a viable strategy to functionalize non-$d$-wave AMs.
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Materials Science · Cross-listed from cond-mat.str-el
Chirped Floquet linear drives activate forbidden charge-to-spin conversions in Rashba two-dimensional electron gases
Mohsen Yarmohammadi
In Rashba two-dimensional electron gases (2DEGs), charge-to-
spin conversion via the Edelstein effect is conventionally limited to the transverse plane. Accessing longitudinal or out-of-plane pathways typically requires static
magnetic fields or interface engineering, which cause stray fields and lack tunability. While dynamic circular or elliptical Floquet drives can also unlock these forbidden pathways, a simple Floquet linear drive cannot. Here, we propose an alternative approach: a chirped Floquet linear drive. The chirp induces in-plane Floquet-Zeeman fields and an odd-parity momentum drift, which simultaneously break rotational and time-reversal
symmetries. This mechanism activates the forbidden Edelstein charge-to-
spin conversions in Rashba 2DEGs. Experimentally accessible via programmable spatial light modulators or optical delay lines, this tunable chirped linear drive offers a broadly applicable route to
spin-orbit torque switching and high-efficiency spintronics.
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Materials Science · Cross-listed from physics.optics
Ultra-high-speed line-scan Raman imaging
Qingyi Wu, Xusheng Tang, Francesco Masia et al. (+7 more)
Raman spectroscopic imaging has emerged as a potent tool due to its non-invasive nature and capability for chemical composition analysis. Line-scan Raman spectroscopy accelerates imaging speed by two orders of magnitude compared to point detection Raman methods. However, further enhancements in imaging speed were constrained by the readout speed of typically used charge-coupled device (CCD) spectroscopic detectors. We developed an ultra-fast line-scan Raman imaging technique based on recently available complementary metal-
oxide-
semiconductor (CMOS) detectors with low cost and read noise, and fast readout during exposure combined with a global shutter. Employing a high-efficiency transmissiongrating imaging spectrometer, we demonstrate imaging speeds up to two orders of magnitude faster than traditional line scan Raman imaging techniques and up to four orders of magnitude faster than point scan Raman methods, achieving Raman imaging up to 80 kHz spectral rate. We demonstrate that this technology is applicable to a variety of samples, including microplastics, biological
cells, and tablets, creating images in an extremely short time frame, showcasing exceptional detection capabilities and the ability to reveal detailed information.
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All authors Qingyi Wu, Xusheng Tang, Francesco Masia, Peng Liang, Hao Peng, Lindong Shang, Yuntong Wang, Yue Qu, Wolfgang Langbein, Bei Li
Materials Science
Different dielectric, magnetic, and magnetodielectric mechanisms in M-type BaFe12O19 hexaferrite regulated by doping Ga3+ and In3+ cations
Yang Yang, Run-Yu Lei, Jian-Ping Zhou et al. (+1 more)
We systematically investigated the
magnetic, dielectric, and MD properties of BaFe12-xMexO19 ceramics prepared by a solid-state reaction method. The Ga3+ cations with a smaller radius preferentially substitute the Fe3+ ions in FeO6 octahedra while the In3+ cations with a larger radius tend to replace the Fe3+ ions in FeO5 bipyramids of R blocks, inducing different physical characteristics. The pure BaFe12O19 and Ga-doped samples show ferrimagnetism in the temperature range from 10 K to 300 K. The In-doped samples exhibit a transition from non-collinear magnetism to collinear ferrimagnetism. The dielectric decrease of pure BaFe12O19 at around 10-175 K is attributed to the
quantum paraelectric state, and the shoulder peaks of loss at about 140-200 K are from electron hopping. The dipole glass state is responsible for the dielectric peak of Ga-doped samples at around 20-40 K. The dielectric increase and plateau of In-doped samples are mainly ascribed to the electron hopping at low temperatures. Their dielectric properties at high temperatures are all attributed to the interfacial polarization caused by the Maxwell-Wagner effect. The MD effect also has different origins for the various samples at low temperatures. For the pure BaFe12O19, the negative MD effect at extremely low temperatures and the positive MD effect after warming are ascribed to
spin-
phonon coupling and field-dependent electron hopping, respectively. The positive MD effect in Ga-doped hexaferrites results from the field-dependent electric dipoles inside FeO5 bipyramids. For the In-doped samples, the negative MD effect and subsequent transformation to the positive MD effect originate from the field-dependent non-collinear
spin ordering and electron hopping, respectively. The MD effect at high temperatures is attributed to the combination of magnetoresistance and Maxwell-Wagner effects.
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All authors Yang Yang, Run-Yu Lei, Jian-Ping Zhou, Xiao-Ming Chen
Materials Science
Disentangling Electronic and Lattice Contributions to Transient Absorption in Metal Halide Perovskites: A First-Principles Study of CH3NH3PbBr3
Lu Qiao, Ronaldo Rodrigues Pela and Claudia Draxl
Soft
lattices combined with strong electron-
phonon coupling in metal halide perovskites result in a complex interplay between electronic and
lattice degrees of freedom. This interplay complicates the interpretation of time-resolved excitation spectra like pump-probe spectra. Here, we develop a first-principles approach that combines a nonequilibrium extension of the Bethe-Salpeter equation with ab initio molecular dynamics to resolve the origin of transient absorption. This approach can quantitatively disentangle electronic and thermal
lattice contributions across femtosecond-to-picosecond timescales. Exemplified with \ce{CH3NH3PbBr3}, we find that on the femtosecond scale, both X-ray and optical transient absorption spectra are dominated by electronic contributions: Photoinduced Coulomb screening weakens the effective electron-hole interaction and blueshifts the excitonic resonances, whereas Pauli blocking is negligible. On the picosecond scale, thermal
lattice contributions become essential, with distinct mechanisms dominating different spectral regions:
Lattice vibrations lead to spectral redistribution in the X-ray transient absorption spectrum, whereas
lattice expansion blueshifts the optical transient absorption spectrum.
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Materials Science
Glassy Dynamics of LiCl.6H2O Solution in Nanoporous Media
Armin Mozhdehei, Mohammad Nadim Kamar, Ronan Lefort et al. (+4 more)
Understanding how nanoconfinement alters the dynamics of glass-forming aqueous electrolytes is essential for clarifying the interplay among ionic hydration, hydrogen-bond structure, and interfacial effects. Here, LiCl.6H2O was investigated in the bulk and under confinement in SBA-15 mesoporous silica with an average pore diameter of 8 nm. Differential scanning calorimetry, Raman spectroscopy, quasielastic
neutron scattering, 1 H
spin-
lattice relaxation, and pulsed-fieldgradient NMR were combined to probe thermal behavior, hydrogen-bond structure, local mobility, and translational transport over complementary time and length scales. The calorimetric results show that LiCl.6H2O remains glass-forming under confinement, while its thermal signature of the glass transition becomes slightly broader and shifted upward relative to the bulk. Raman spectra in the O-H stretching region indicate that the concentrated LiCl solution possesses a weakened and less tetrahedrally connected hydrogen-bond network compared with bulk water. On the subnanosecond timescale, elastic fixed-window analysis reveals reduced mean-squared displacements under confinement, demonstrating suppressed motional amplitudes inside the pores. Inelastic fixed-window
neutron scattering scans analyzed within a jump-
diffusion framework yield lower effective translational
diffusion coefficients and longer residence times for the confined
liquid, indicating that confinement mainly hinders translational escape from transient local environments. 1 H relaxometry further shows that confinement broadens the distribution of local
proton fluctuation times, while PFG-NMR confirms that the measured long-range water mobility in bulk LiCl.6H2O solution is reduced relative to bulk water. While the present data do not resolve distinct interfacial and pore-centered populations in confined LiCl.6H2O, its dynamics are markedly altered across timescales, from the glassy to the
liquid state, resulting in slower, spatially constrained, and more heterogeneous motions.
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All authors Armin Mozhdehei, Mohammad Nadim Kamar, Ronan Lefort, Alain Moréac, Arnaud Le Pottier, Agathe Belime, Denis Morineau
Materials Science
Formation and Thermodynamic Behavior of THF-Water Hydrates in Confined Mesoporous Media
Armin Mozhdehei, Oriana Osta, Thomas Marescot et al. (+3 more)
Tetrahydrofuran (THF) is a
benchmark guest for probing clathrate hydrate thermodynamics because a stoichiometric aqueous solution (THF.17H2O) forms structure-II (sII) hydrate at ambient pressure with a well-defined dissociation temperature. Here, we combine differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) in bulk and confined media to resolve how composition, pore filling, and cooling rate govern hydrate formation in SBA-15 mesoporous silica. Bulk DSC establishes mass-balanced enthalpies for ice and sII hydrate and confirms reversible dissociation/melting temperatures. In confinement, the heating traces separate into a Gibbs-Thomson depressed ice melt (= -14.7 $\pm$ 0.2 {\textdegree}C), an in-pore hydrate dissociation (= -13.2 $\pm$ 0.2 {\textdegree}C). Confined hydrate appears only when two criteria are met: near-percolating filling ($\phi$ = 1.0 -1.1 cm3/g) and sufficient THF ($\ge$ 1:16 mol:mol). Cooling-rate experiments (1.0 vs 0.5 {\textdegree}C/min) demonstrate that slower precooling increases the confined-hydrate fraction and reduces confined ice without shifting
equilibrium temperatures: at $\phi$ = 1.1, the hydrate enthalpy rises by ~60% at 1:11 and ~54% at 1:14, but by $\le$ 17% at 1:16. Temperature-cycling tests show invariant reheating peak positions, indicating that capillarity and composition, rather than kinetic history, fix the liquidus and dissociation temperatures. WAXS indicates that the phase formed in pores is crystallographically identical to bulk sII. Finally, the variation of melting points ($\Delta$T___) plotted against inverse pore radius follows the Gibbs-Thomson law for both ice melting and hydrate dissociation, quantitatively linking the observed shifts to crystalline size and clarifying how confinement, cooling rate, and composition govern the competition between hydrate formation and water crystallization.
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All authors Armin Mozhdehei, Oriana Osta, Thomas Marescot, Arnaud Desmedt, Denis Morineau, Christiane Alba-Simionesco
Materials Science
Chiral Phonons Coupled to Spin-Split Bands in Altermagnetic CrSb and MnTe
Armando Consiglio, Maximilian Ünzelmann, Giancarlo Panaccione et al. (+1 more)
Altermagnets exhibit momentum-dependent
spin splitting without net magnetization, providing a unique platform where
magnetic order, electronic structure and
lattice dynamics intertwine. Here, using first-principles calculations, we demonstrate that the prototypical altermagnets CrSb and MnTe host locally
chiral phonon modes carrying finite
phonon angular momentum with a six-lobes $f$-wave texture in momentum space. Our results show that the
chiral lattice motion originates from the pnictogen/chalcogen sublattice, while the altermagnetic
spin splitting is generated by the
magnetic transition-metal atoms, indicating that
chiral lattice dynamics and altermagnetic electronic states originate from different atomic sublattices of the same
crystal. In pristine compounds, at each valley, inversion
symmetry suppresses the net
phonon angular momentum despite local circular atomic motion. We further demonstrate that isoelectronic
symmetry lowering induced by chemical substitution lifts this cancellation and generates finite valley
phonon chirality, while keeping the altermagnetic nature of the compounds intact. Most importantly, we reveal that
chiral phonons couple to momentum-dependent
spin-split electronic bands through momentum-dependent electron-
phonon interaction, producing characteristic modifications of the electronic structure, possibly accessible by photoemission experiments. Our results establish altermagnets as a promising platform for
chiral phononics and
spin-selective
lattice control.
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All authors Armando Consiglio, Maximilian Ünzelmann, Giancarlo Panaccione, Domenico Di Sante
Materials Science · Cross-listed from cond-mat.mes-hall
Xing-Lei Ma, Jinchao Zhao, Bo-Qing Wu et al. (+2 more)
In conventional Friedel oscillations, the real-space charge density oscillations induced by an
impurity are characterized by an oscillation period set by the
Fermi momentum. In this work, we demonstrate that in metals with an isolated (nearly) flat band at the
Fermi energy,
quantum geometry induces a distinct type of oscillations, which we call the
quantum geometric Friedel oscillations (QGFOs). The period of the QGFOs is set by the momentum-space separation of the
quantum metric hot spots of the isolated band. The conventional and
quantum metric-induced oscillations can coexist at low temperatures. At higher temperatures, the conventional Friedel oscillation amplitudes away from the
impurity site are set by the thermal length such that the oscillations can be easily washed out by temperature effects. Remarkably, the QGFOs decay length is set by the
quantum metric length which is defined by the integration of the
quantum metric of the isolated band. As a result, the QGFOs can persist even at temperatures much larger than the bandwidth of the isolated flat band. Moreover, the decay length is invariant for a wide range of temperature which is a striking result. In conclusion, we show that the
quantum metric induces novel Friedel oscillations. Our work suggests that the measurement of the QGFOs is a powerful way to detect the
quantum metric length (which is associated with the integral of the
quantum metric) and the
quantum metric hot spot separations (which are associated with the distribution of the
quantum metric in the momentum space).
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All authors Xing-Lei Ma, Jinchao Zhao, Bo-Qing Wu, Patrick A. Lee, K. T. Law
Materials Science
Topological Electronic States and Phonon Mediated Superconductivity in Ru Based Ternary Pnictides: ZrRuAs and HfRuP
Jubair Hossan Abir, Tanvir Khan, Tauhidur Rahman et al. (+5 more)
This study investigates the structural, electronic, mechanical, optical, thermodynamic, and superconducting properties of ZrRuAs and HfRuP. Electronic band structure calculations reveal topological semimetallic behavior in both compounds with several bands crossing the
Fermi level. The inclusion of
spin orbit coupling lifts band degeneracies in both compounds. The
Fermi surface shows both electron and hole segments. The estimated Pugh ratio, Poisson ratio and machinability index indicate that both compounds are ductile in nature and exhibit excellent machinability. Optical results show metallic reflectivity and notable absorption in the ultraviolet region. Thermodynamic analysis indicates stable behavior. The estimated superconducting transition temperatures are 9.75 K for ZrRuAs and 9.03 K for HfRuP that indicates strong coupling conventional superconductivity. The results provide a detailed view of the physical and superconducting properties of ZrRuAs and HfRuP and help to close the existing research gaps in these topological superconducting materials.
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All authors Jubair Hossan Abir, Tanvir Khan, Tauhidur Rahman, Md. Kamrul Hassan, Sraboni Saha Moly, Mst. Maskura Khatun, Raihana Shams Islam, Saleh Hasan Naqib
Materials Science
VASP Plugins: Linking the Vienna ab-initio Simulation Package with Python
Sudarshan Vijay, Martijn Marsman, Georg Kresse et al. (+1 more)
Implementing novel features and experimental
algorithms into widely adopted density functional theory (DFT) codes is frequently hindered by complex legacy architectures and the use of compiled
languages such as Fortran. These production codes, while optimised for high-performance computing clusters, present significant hurdles for software development and rapid prototyping, often requiring deep expertise in the code's internal structure to modify. To address this challenge, we present a Python plugin infrastructure for the Vienna ab-initio Simulation Package (VASP) that combines computational efficiency with the flexibility of high-level scripting. Our architecture uses a C++ intermediate layer and pybind11 to expose VASP data as NumPy arrays via shared
memory buffers, ensuring high performance without data duplication. We implement two categories of plugins: those that modify quantities at the end of each converged self-consistent field (SCF) cycle, such as structure and force_and_stress, and those that operate during the SCF cycle, such as local_potential and occupancies. We demonstrate the utility of our implementation through three applications, structure relaxation using the scipy library, implementing an implicit solvent model, and adding the DFT-D4 dispersion corrections. This infrastructure effectively bridges the gap between high-performance electronic structure routines and the widespread scientific Python ecosystem.
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All authors Sudarshan Vijay, Martijn Marsman, Georg Kresse, Martin Schlipf