Materials Science
A high-entropy form of $R$Mn$_6$Sn$_6$ with distinct magnetotransport regimes correlated to different magnetic structures
Kyle W. Fruhling, Jonathan Gaudet, William D. Ratcliff et al. (+9 more)
The kagome $R$Mn$_6$Sn$_6$ material family has attracted significant
attention as high-temperature metallic magnets with a host of different
magnetic orderings and anisotropy. Theoretical studies point to the rare-earth ($R$) as the determining factor for both the direction of
magnetic anisotropy and the type of
magnetic ordering in a given compound. This motivates studying high-
entropy forms of $R$Mn$_6$Sn$_6$ to examine how the interplay of several rare-earth elements leads to different
magnetic states in a single
crystal. Here, we present a rare-earth mix of Tb, Dy, Ho, Er, Tm, and Lu that produces phase transitions from a paramagnet to an easy-plane ferrimagnet (FiM) below $T_{\text{C}}$ = 380 K, then to a FiM easy-axis state at $T_{\text{SR1}}$ = 207 K, to a canted FiM ground state below $T_{\text{SR2}}$ = 79 K. This behavior is consistent with previously reported high-
entropy $R$Mn$_6$Sn$_6$ compounds; however, uniquely, the rare-earth mix studied here exhibits a broad transition from easy-plane to easy-axis anisotropy from 270 K to 170 K, and reveals a nonmonotonic magnetoresistance. Using
neutron scattering data, we found that both observations correlate with an incommensurate modulated contribution to the
spin state due to competing rare-earth interactions. This magnetoresistive behavior and the correlated
spin structures underscore the potential for rare-earth engineering of magnetism.
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All authors Kyle W. Fruhling, Jonathan Gaudet, William D. Ratcliff, Jonathan S. White, Siddharth Nandanwar, Noah J. Fau, Gregory T. McCandless, Enrique O. González Delgado, Julia Y. Chan, Oksana Zaharko, Michael A. Susner, Fazel Tafti
Materials Science
Vibrational, structural, and chemical fingerprints of ion diffusion in crystalline solids
Gavin Winter, Juno Nam and Rafael Gómez-Bombarelli
Predicting mobile-ion self-diffusivity $D^*$ from molecular dynamics (MD) simulations is essential for identifying promising solid-state electrolytes, but directly simulating ion
diffusion is computationally expensive, particularly with high-accuracy machine learning interatomic potentials (MLIPs).
Diffusion is a slow, emergent process that requires long trajectories to converge. Thermodynamic properties, by contrast, converge much faster: the enthalpy $h$, vibrational
entropy $s_{vib}$, and 2-body, excess configurational
entropy $s^{ex}_{2,config}$ can be extracted from comparatively short MD trajectories, and they encode rich information about the free energy landscape from which transport properties like self-diffusivity ultimately arise. Intuitive correlations are discussed between these thermodynamic properties and ion
diffusion, motivating a data-driven approach to exploit this link. A simple neural network was trained to predict diffusivity from features computed over short MD trajectories: a vibrational fingerprint (the vibrational density of states, VDOS) and a structural fingerprint (the radial distribution function, RDF), conditioned on chemistry information encoded in the MLIP
embedding. This combination allows the model to predict the converged $\log_{10} D^*$ (cm$^2$/s) \textemdash\ normally obtained from significantly longer MD simulations \textemdash\ with a mean absolute error of 0.398 and a Spearman's rank correlation $\rho$ of 0.844.
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Materials Science
Optically Coupled Noise Spectroscopy Quantifies Defect Passivation in Monolayer MoS2 FETs
Shashwata Chattopadhyay, Soumyadip Hazra, Sraboni Dey et al. (+4 more)
Transition metal dichalcogenide monolayers are promising materials for electronic and photonic applications, yet the performance of chemical vapour deposition grown samples is severely limited by native sulphur vacancies that introduce mid-gap trap states, degrade carrier mobility, and elevate electrical noise. Here we investigate octanethiol passivation of sulphur vacancies in monolayer MoS2 field effect transistors, combining x-ray photoelectron spectroscopy, photoluminescence, and Raman scattering with electrical transport and optically coupled low-frequency noise spectroscopy. Thiol treatment reduces the sulphur vacancy concentration from 7% to 4%, reducing the free electron density. Electrically, it increases the device resistance 35-fold while restoring gate switching with an on/off ratio of 10^4 and improving field-effect mobility from 1 to 5 cm^2/Vs. Low frequency noise spectroscopy directly quantifies the defect suppression: the Hooge parameter alpha_H drops by more than two orders of magnitude after passivation. Gate-dependent noise confirms carrier mobility fluctuation as the dominant dark noise mechanism, while optical excitation drives a crossover to carrier number fluctuation dominated noise, reflecting preferential interaction of photogenerated carriers with residual vacancy states via generation-recombination trapping, a mechanistic distinction inaccessible to gate-bias measurements alone. Density functional theory calculations corroborate these findings, showing suppression of vacancy-induced mid-gap states by more than half and partial restoration of the intrinsic bandgap. These results establish optically coupled low-frequency noise spectroscopy as a sensitive, low-cost, and non-destructive tool for quantifying defect passivation in TMDC-based devices.
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All authors Shashwata Chattopadhyay, Soumyadip Hazra, Sraboni Dey, Kritika Sharu, Renjith Nadarajan, Arijit Kayal, Joy Mitra
Materials Science
Automated Analysis to Reveal Grain Boundary Phase Microstructures
R. Daniel Moore, Ian S. Winter, Robert E. Rudd et al. (+2 more)
We develop a method for analyzing grain boundary (GB) microstructures that identifies distinct interfacial phases and the dislocation line defects separating them. Similar to bulk materials, GBs can adopt multiple distinct interfacial phases and undergo first-order phase transitions. When these phases coexist, their spatial arrangement and phase junctions constitute a GB microstructure, characterized by variations in excess properties and line defects with associated dislocation content. Despite this intrinsic heterogeneity, our ability to quantitatively characterize GB microstructures remains limited, as it requires identification of individual GB phases, phase-resolved excess properties, and the Burgers content of phase junctions, capabilities not available in existing automated methods. Here, we present an automated tool that performs interfacial microstructure mapping for planar coincidence site
lattice GBs. The method identifies the spatial distribution of GB phases, quantifies phase-specific excess properties, and estimates the Burgers content of GB phase junctions. We demonstrate the approach using three representative cases: (i) quantification of mass transport during
diffusion-limited GB phase transformations; (ii) identification of structurally indistinguishable phases formed by vacancy and interstitial loops; and (iii) characterization of GB microstructures containing phase
nuclei. More broadly, this framework enables quantitative studies of GB
evolution processes, including spinodal decomposition and coarsening with direct implications for GB deformation, creep, and migration.
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All authors R. Daniel Moore, Ian S. Winter, Robert E. Rudd, Fadi Abdeljawad, Timofey Frolov
Materials Science
Machine-learned exchange-correlation functionals for molecules, solids, and reactive surfaces
Mohamed S. Abdallah, Zhuotao Jin, Boris Kozinsky et al. (+1 more)
The application of density functional theory to heterogeneous catalysis is hindered by the shortcomings of conventional density functional approximations. We combine machine learning with explicitly non-local physically informed descriptors and introduce an exchange-correlation functional (CIDER26SS) framework regularized for wide transferability. CIDER26SS is size-extensive, highly efficient, provides a balanced and accurate description of both molecular and solid-state systems, and is specifically well-optimized for transition metal surface chemistry. Surpassing existing conventional functionals, CIDER26SS resolves the CO/Pt puzzle, identifying the correct binding site for CO adsorption on the Pt(111) surface, along with an accurate adsorption energy, Pt
lattice constant, and surface energy. Predictions agree well with the experimental values, even when all bulk and surface data for Pt are excluded from the training set. Remarkably, CIDER26SS exceeds the accuracy of semilocal approximations even for systems far outside the training domain.
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All authors Mohamed S. Abdallah, Zhuotao Jin, Boris Kozinsky, Kyle Bystrom
Materials Science
All-Optical Control of Interfacial Polarization in MoS$_2$/WSe$_2$ Heterobilayers
Muhammad Sufyan Ramzan, Giancarlo Soavi and Caterina Cocchi
All-optical tuning of van der Waals
heterostructures with coherent radiation offers a promising path toward ultrafast
memory and optoelectronic devices. In the first-principles framework of real-time time-dependent density functional theory, we predict the induction of a persistent, long-lived out-of-plane polarization in MoS$_2$/WSe$_2$ heterobilayers, resonantly driven by intense ultrafast pulses. While weak fields preserve the intrinsic type-II band
alignment, intermediate intensities trigger a four-fold enhancement of interlayer charge transfer. By analyzing the high-harmonic generation spectrum, we identify a transition from the perturbative to the strong-field regime inducing photoinduced interfacial polarity. We additionally show that
lattice strain, ubiquitously present in heterobilayers, can be used as additional knob to adjust the resonant condition without compromising the permanent dipole induction. Our findings provide a theoretical blueprint for the all-optical manipulation of polar phases in low-dimensional
heterostructures at the femtosecond scale.
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Materials Science
Dirac Surface States and Nonlocal Quantum Tunneling in Topological Semiconductor Mo$_2$SeTe$_3$ for High-Performance Tunnel FETs
Zafar Sadik Mehrub, Suvodip Kundu Arnob, Md. Tareq Mahmud et al. (+2 more)
A first-principles and device-level study of the quasi-two-dimensional transition-metal chalcogenide Mo$_2$SeTe$_3$ is performed. The material is found to be a weak topological
semiconductor with a finite bulk band gap and
symmetry-protected Dirac surface states, indicating strong potential for next-generation low-power
quantum electronic devices. An SOC-driven band inversion accompanied by an indirect semiconducting gap of approximately 0.75 eV is observed. Topological nontriviality is rigorously confirmed through Wannier charge-center
evolution and $\mathbb{Z}_2$ invariant analysis, yielding weak topological indices of $(0;001)$, while iterative Green's-function surface-state calculations corroborate Dirac-cone conducting states traversing the bulk gap on
symmetry-preserving surfaces. Mo$_2$SeTe$_3$ additionally exhibits exceptional dynamical and mechanical stability, pronounced optical anisotropy, high dielectric polarizability, broad infrared-to-visible optical absorption, a large static dielectric constant, and substantial birefringence, making it favorable for photonic and optoelectronic applications. Thermoelectric transport analyses further reveal enhanced carrier mobility and a competitive figure of merit under $n$-type doping near room temperature. A dual-source tunnel field-effect transistor (TFET) is implemented via TCAD simulations with nonlocal band-to-band tunneling, yielding subthreshold switching below the thermionic limit, a high ON/OFF current ratio, and enhanced tunneling efficiency driven by SOC-induced orbital hybridization and topologically enhanced interband coupling. The concurrent realization of nontrivial bulk-boundary correspondence, robust transport properties, and steep-slope switching characteristics establishes Mo$_2$SeTe$_3$ as a multifunctional
quantum material platform for topological and next-generation energy-efficient nanoelectronic devices.
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All authors Zafar Sadik Mehrub, Suvodip Kundu Arnob, Md. Tareq Mahmud, Nazmul Hasan, Alamgir Kabir
Materials Science
Chemical Control of Electronic Structure and Topology in Tellurium-Encapsulated Silicene
Gabriel Elyas Gama Araujo, Andre Luis de Oliveira Batista, Willian Oliveira Santos et al. (+3 more)
We investigate chemical control of the electronic, optical, and topological properties of two-dimensional \ce{Si2X2Te2} (\ce{X} = \ce{B}, \ce{Al}, \ce{Ga}, and \ce{In}) monolayers using first-principles calculations. All compounds are dynamically stable
semiconductors, with their vibrational and electronic properties evolving systematically upon group-III substitution. Hybrid-functional calculations including
spin–orbit coupling reveal predominantly $p$-orbital band edges and increasingly pronounced relativistic effects from B to In. Most notably, the calculated $\mathbb{Z}_2$ invariant identifies \ce{Si2In2Te2} as a candidate
quantum spin Hall
insulator, while the B-, Al-, and Ga-based monolayers remain topologically trivial. Bethe–Salpeter calculations further show that electron–hole interactions redistribute oscillator strength near the absorption onset while preserving a weak in-plane optical anisotropy. Our results establish group-III substitution as a simple chemical route to tune the electronic structure and drive a transition from trivial to nontrivial
topology in \ce{Si2X2Te2} monolayers.
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All authors Gabriel Elyas Gama Araujo, Andre Luis de Oliveira Batista, Willian Oliveira Santos, Alexandre Amaral Leitao, Alexandre Cavalheiro Dias, Andreia Luisa da Rosa
Materials Science · Cross-listed from cond-mat.soft
Non-Monotonic Dynamical Correlations Across The Glass Crossover
Corentin C. L. Laudicina, Ilian Pihlajamaa, Liesbeth M. C. Janssen et al. (+2 more)
The dramatic slowing down of structural relaxation in supercooled
liquids is accompanied by the emergence of dynamic heterogeneity. A monotonically increasing dynamical correlation length, measured at the $\alpha$-timescale, is one of the remarkable features of this phenomenon. Here we show that this picture is incomplete: the dynamical correlation length measured in the $\beta$-relaxation regime exhibits a striking non-monotonic temperature dependence, reaching a maximum near the mode-coupling crossover temperature $T_c$ and decreasing upon further cooling, even as local dynamical fluctuations continue to intensify. This behavior suggests a crossover from spatially extended, maximally cooperative motion near $T_c$ to increasingly compact and localized relaxation events below it. We demonstrate that this
evolution is quantitatively captured by
stochastic beta-relaxation theory, an extension of mode-coupling theory beyond mean-field that explicitly predicts an avoided dynamical transition in finite dimensions. Our results provide the first direct spatial evidence in favor of the avoided-transition picture of the mode-coupling crossover, and establish the peak of the $\beta$-regime correlation length as a robust indicator of the mode-coupling crossover.
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All authors Corentin C. L. Laudicina, Ilian Pihlajamaa, Liesbeth M. C. Janssen, Thomas Voigtmann, Tommaso Rizzo
Materials Science · Cross-listed from math-ph
Andrea Marini, Kai Wu and Riccardo Reho
Enrico Fermi in 1932 used classical Gauss equation to derive the Coulomb density–density interaction from the longitudinal electromagnetic potential. In this work we extend the Fermi procedure to the transverse component of the vector potential. By using a fully
quantum canonical transformation, we replace the transverse vector potential with a current–current and current–current–density interactions. The transformed
Hamiltonian is, then, projected in the fermionic space providing a matter–only, totally incoherent and gauge respecting
Hamiltonian. The removal of coherence will avoid the breakdown of perturbation theory predicted by Haag's
theorem, and make possible to introduce, in the transformed space, the diagrammatic approach. After discussing the implications of the transverse interactions on the current theories based on the Fermi procedure, we conclude by showing how the transverse
Hamiltonian provides the
quantum origin of the electromagnetic longitudinal–transverse splitting of
phonons and other elemental excitations.
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Materials Science
Symmetry-engineering ferroelectricity in silicon dioxides
Yin Dai and Menghao Wu
It is a long-established rule for classical ferroelectricity that any ferroelectric
crystal must adopt one of the 10 specific polar point groups. Here we predict a unique type of ferroelectricity that can be generated in some
crystals belonging to nonpolar noncentrosymmetric groups. This principle can be applicable to many systems including silicon dioxides, the most widely used dielectric materials. Most of their crystalline phases do not belong the polar groups, while the nonlinear Si-O-Si configurations lead to multiple identical states. We show first-principles evidence that the
crystal symmetry forbidding the formation of polarizations, can be broken by either parallel surfaces in thin-films or applying a uniaxial strain. As a result, the multiple identical states are endowed with polarizations of different directions, and low-barrier ferroelectric switching can be realized via transition between them, which can be room-temperature robust down to the thickness of 1 nm. Our findings may not only enable low-cost and large-scale manufacture of ferroelectrics directly integrated in silicon chips, but also open a new avenue for exploring ferroelectricity in prevalent nonpolar materials.
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Materials Science · Cross-listed from cond-mat.mes-hall
Electric-Field-Induced Second Harmonic Generation at a Reconfigurable LaAlO$_3$/SrTiO$_3$ Nanojunction
Pubudu Wijesinghe, Melanie Dieterlen, Kyoungjun Lee et al. (+5 more)
Electrically tunable nonlinear optical responses at the nanoscale remain challenging to achieve because conventional nonlinear materials lack the combination of large susceptibility, nanoscale confinement, and in situ reconfigurability. Here we report electric-field-induced second harmonic (EFISH) generation from a nanoscale tunnel junction defined by conductive atomic force microscope lithography at the LaAlO$_3$/SrTiO$_3$ interface. A conducting channel written at the interface is interrupted by a nanoscale insulating gap, across which applied DC bias produces local electric fields exceeding $10^7$ V/m. The SHG signal is spatially localized at the junction, exhibits a quadratic bias dependence described by $I(2\omega) \propto |\chi^{(2)}_\mathrm{0} + \chi^{(3)} E_\mathrm{DC}|^2$ with no hysteresis, a modulation depth exceeding 380% at $|V_\mathrm{DC}| = 1$ V, and shows a two-lobed input-polarization pattern aligned with the junction axis, consistent with EFISH from a centrosymmetric host. Calibration against a BBO reference
crystal gives $|\chi^{(3)}| \approx 1\times10^{-19}$ m$^2$/V$^2$ at 6 K. These results establish cAFM-written oxide nanojunctions as a reconfigurable platform for nanoscale nonlinear optics in which the junction geometry sets the
symmetry of the response and the large field-induced $\chi^{(2)}$ of SrTiO$_3$ provides the optical nonlinearity. Because the nonlinearity is both generated and read out within the same nanoscale gap, the junction operates simultaneously as a subwavelength source and a near-field detector of optical nonlinearity.
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All authors Pubudu Wijesinghe, Melanie Dieterlen, Kyoungjun Lee, Ahmed Omran, Aswini Ramankutty, Chang-Beom Eom, Patrick Irvin, Jeremy Levy
Materials Science
Sulfur-rich Spirofluorene-Bridged N Heterotriangulene Redox-Active Polymers
Angelina Jocic, Tom Wickenhäuser, Sebastian Lindenthal et al. (+8 more)
Sulfur-rich spirofluorene-bridged N-heterotriangulene (FTN)
polymers featuring covalently linked oligosulfide units and a terthiophene-based analogue were synthesized via nucleophilic aromatic substitution and Stille cross-coupling polymerization. The resulting materials are amorphous, insoluble solids with high thermal stability and sulfur contents up to 25 wt%. Structural and compositional analyses by combustion analysis, ToF-SIMS, FT-IR, XPS, and solid-state NMR confirm the efficient incorporation of short oligosulfide to disulfide linkages and well-defined terthiophene units in the respective
polymers. Electrochemical characterization in lithium half-
cells reveals a reversible, high-voltage oxidation of the FTN unit at 3.8-4.0 V (vs. Li/Li+), accompanied by low-voltage sulfur- or terthiophene-based redox processes between 1.5-2.5 V (vs. Li/Li+). Sulfur incorporation markedly increases the theoretical and initial discharge capacities (up to 129 mA h g-1), while the sulfide conversion processes exhibit rapid fading and poor reversibility due to sulfide dissolution. In contrast, the terthiophene-linked
polymer shows only transient low-voltage activity while maintaining high Coulombic efficiencies (ca. 99.7%) governed by the persistent FTN backbone redox event. Our results highlight how different redox-active linkers influence the electrochemical behavior of FTN-based
polymers and provide insights into the design of functional organic cathode materials featuring multi-redox processes.
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All authors Angelina Jocic, Tom Wickenhäuser, Sebastian Lindenthal, Alexander Welle, Vanessa Trouillet, Ronald Curticean, Irene Wacker, Jana Zaumseil, Rasmus R. Schröder, Rüdiger Klingeler, Milan Kivala
Materials Science
Soft and chiral phonons in chiral phase of K3NiO2
Hong Dang Nguyen, Fedir Borodavka, Miroslav Lebeda et al. (+6 more)
Raman scattering measurements confirmed the theoretical prediction that the structural phase transition from the achiral tetragonal to the chiral tetragonal phase, which occurs near 400 K, is induced by a doubly degenerate soft
phonon at the Z point of the Brillouin zone. In the low-temperature chiral phase, the soft mode activates in Raman spectra, splits into two components with A1 and B1
symmetries and harden with cooling according to Cochran law. Circularly polarized Raman scattering did not reveal the angular momentum of these singly degenerate
phonons at the Gamma point, which is consistent with theory. We also calculated the
phonon branches in the whole Brillouin zone for both crystalline phases and compared the results with the
phonons observable in the Raman spectra. The calculations revealed that some
phonons with nonzero k have angular momentum in the chiral phase. A pronounced circular motion of atoms can be observed, for example, in a Dirac-type topological
phonon at the M-point of the Brillouin zone with a frequency of 168 cm-1.
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All authors Hong Dang Nguyen, Fedir Borodavka, Miroslav Lebeda, Nazar Zaremba, Peter Hoehn, Eteri Svanidze, Jan Drahokoupil, Ales Vlk, Stanislav Kamba
Materials Science
Identification of the length scale parameter of simplified strain gradient elasticity from standard Mode I fracture tests
Yury Solyaev, Kirill Shelkov and Pavel Polyakov
Recently, it was shown that additional material constants of strain gradient elasticity (SGE) can be identified for brittle and quasi-brittle materials based on the analysis of experimental data on the crack size effect. In the present paper, we perform precise numerical simulations within SGE and derive regression relations for processing experimental data from standard fracture mechanics tests under pure Mode I loading conditions (CCT, SENT, SENB). We consider the simplified SGE, whose constitutive relations contain a single length scale parameter $\ell$ in addition to the classical elastic constants. We show that, for brittle materials, this parameter can be explicitly identified as $\ell \approx 0.362 (K_{Ic}/\sigma_{ult})^2$. This identification ensures that the fracture loads predicted by classical linear elastic fracture mechanics (LEFM) and by the simplified SGE coincide for relatively long Mode I cracks. However, within the simplified SGE, these fracture loads are evaluated from the nonsingular stress field using the maximum principal stress criterion. For quasi-brittle materials, we derive regression relations that describe the non-classical size effect on strength. This effect is usually treated within nonlinear fracture mechanics but can be naturally captured by SGE. Examples of identification of the length scale parameter $\ell$ based on the established relations and the experimental data for chopped fiber composites and for porous and dense quasi-brittle ceramics are presented.
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Materials Science
On Transmission Function Amplitude and Phase Recovery in Multislice Electron Ptychography
Bridget R. Denzer, Colin Gilgenbach and James M. LeBeau
{Multislice electron ptychography commonly accounts for inelastic scattering by including an absorptive object potential in the reconstruction forward model that attenuates the elastic signal. However, it remains unclear how the reconstruction is quantitatively impacted by thermal scattering. Here, we use the
quantum excitation of
phonons (QEP) formalism, which explicitly separates elastic and inelastic (thermal diffuse) scattering within a single multislice propagation, to simulate four-dimensional scanning transmission electron microscopy (4D STEM)
datasets of PbTiO$_3$ and SrTiO$_3$ and reconstruct their phase and amplitude. We find that reconstructions of the QEP total and elastic-only
datasets are nearly indistinguishable across sample thicknesses from 11 to 40 nm and across atomic species, demonstrating that the reconstruction is largely insensitive to incoherent thermal diffuse scattering within the collected angular range. Reconstructions from multislice phase-only simulations further confirm that appreciable amplitude does not arise from multiple elastic scattering, but instead reflects attenuation of the coherent elastic signal due to thermal scattering. Further comparison of the QEP elastic-only reconstruction with absorptive multislice simulations reveals substantial amplitude contrast deviations for heavy Pb columns (up to 17%), arising from the approximations used to evaluate the absorptive potential for the 4D STEM simulation. These results thus indicate that reconstruction phase and amplitude accuracy are not significantly limited by the absorptive potential forward model, even in the presence of thermal diffuse scattering.
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Materials Science
Scalable photoexcitation-induced molecular dynamics with machine-learned Hamiltonians
Leyu Cai, Yunzhe Jia, Daqiang Chen et al. (+1 more)
Ultrafast photoexcitation offers a controllable route to steer structural dynamics in solids, yet predicting how nonequilibrium electronic excitation drives
lattice motion across extended spatial and temporal scales remains a major computational challenge. Here we introduce time-dependent ab-initio propagation with electronic machine learning (TDAP-eML), a framework that explicitly incorporates electronic
evolution into scalable simulations of photoexcitation-induced
lattice dynamics. By integrating machine-learned electronic structure with atomistic propagation, TDAP-eML describes how photoexcitation reshapes the evolving energy landscapes and forces governing structural motion. Across representative examples including silicon and FeSe, the framework reproduces key photoexcited
lattice responses obtained from first-principles time-dependent density functional theory calculations and captures coherent
phonon dynamics together with their dependence on excitation conditions. Its computational advantage increases with system size, reaching nearly three orders of magnitude reduction in computational cost for the large systems examined. TDAP-eML thus establishes a scalable framework for coupled electronic and
lattice evolution, linking nonequilibrium excitation to photoinduced forces, predictive structural dynamics, and experimentally accessible observables.
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All authors Leyu Cai, Yunzhe Jia, Daqiang Chen, Sheng Meng
Materials Science
Quasi-one-dimensional topological band structure and van Hove singularities in monolayer TaIrTe$_4$ from laser $\mu$-ARPES
Honey Boban, Tanguy Prongué, Amarjyoti Choudhury et al. (+9 more)
Recent transport experiments reported a
quantum spin Hall
insulator phase in monolayer 1T-TaIrTe$_4$ gated away from charge neutrality. This phase is not predicted by band structure calculations and has been attributed to an electronic instability induced by strong correlations at a putative van Hove singularity. Here, we investigate the electronic structure of exfoliated monolayer 1T-TaIrTe$_4$ using micro-focus
laser angle resolved photoemission. We find a strongly anisotropic band structure susceptible to density wave instabilities. Our data further reveal a saddle point singularity in the density of states. However, we find that the saddle point lies at a density for which transport experiments found no anomalies. Moreover, the quasiparticle line widths suggest weak electron correlations only. This points to a secondary role of van Hove singularities and electron correlations in the transport phase diagram of monolayer 1T-TaIrTe$_4$.
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All authors Honey Boban, Tanguy Prongué, Amarjyoti Choudhury, Dario Marchiani, Andrés Bareño, Felix Eder, Enrico Giannini, Fabian O. von Rohr, Alberto F. Morpurgo, Marco Gibertini, Anna Tamai, Felix Baumberger
Materials Science
Scalable Cyclic Olefin Copolymer Encapsulation for High Optical Quality of TMD Monolayers
Suprova Das, Md Tarik Hossain, Zlata Fedorova et al. (+8 more)
Monolayer transition metal dichalcogenides (TMDs) combine a direct bandgap, strongly bound
excitons, and pronounced second-order optical nonlinearity, which makes them promising materials for ultrathin optoelectronic and nanophotonic devices. However, their optical performance is often degraded by environmental exposure and substrate-induced charge trapping, motivating the development of scalable encapsulation strategies. Here, we investigate
spin-coated cyclic olefin copolymer (COC) as a scalable encapsulant for TMDs. Room-temperature and cryogenic optical spectroscopy reveal enhanced photoluminescence and second-harmonic generation, accompanied by excitonic linewidth narrowing and an increased
exciton-to-trion ratio. In addition, COC encapsulation induces an excitonic peak splitting and an overall spectral blueshift. First-principles calculations attribute these spectral modifications to local
symmetry breaking at the chalcogen interface and macroscopic compressive strain, respectively. These findings establish
spin-coated COC as an effective, scalable encapsulation strategy and a potential platform for post-growth excitonic and band-structure engineering.
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All authors Suprova Das, Md Tarik Hossain, Zlata Fedorova, Zifei Zhang, Axel Printschler, Begimai Adilbekova, Honey Jayeshkumer Shah, Stefan Velja, Caterina Cocchi, Andrey Turchanin, Isabelle Staude
Materials Science
Broken Inversion Symmetry via a Magic Methyl Effect
Calum J Gibb, Jordan Hobbs, Caitlin O Brien et al. (+4 more)
Polar
liquid crystals - fluid phases in which molecular dipoles organise into ferro- or antiferroelectric states - present a highly constrained molecular design space, where small changes can entirely suppress polar organisation. Here we demonstrate, contrary to expectations, that installation of a methyl group in the 5-position of a 1,3-dioxane ring substantially increases the onset temperature of polar order. Remarkably, this effect proves transferable across several
liquid-
crystal families, including examples where the methylated derivative exhibits a polar phase whereas its parent compound does not. Across 8 matched pairs, we find that a 5-methyl group can enhance the onset temperature of polar order in 1,3-dioxane materials by up to 120 {\deg}C. These findings establish a simple and general molecular design strategy for enhancing and enabling spontaneous dipolar ordering in
liquid crystals.
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All authors Calum J Gibb, Jordan Hobbs, Caitlin O Brien, Kate Hille, Benji Maher, Christopher M Pask, Richard J Mandle
Materials Science · Cross-listed from cond-mat.dis-nn
The Trachenko-Zaccone equation: nonlinear relaxation from glasses to complex systems
Alessio Zaccone, Valeriy V. Ginzburg, Oleg V. Gendelman et al. (+2 more)
The Trachenko-Zaccone equation provides a compact nonlinear dynamical framework for describing non-Debye relaxation in disordered condensed matter. Originally developed to rationalize stretched- and compressed-exponential relaxation in
liquids and glasses from the dynamics of interacting local relaxation events, the same equation has subsequently appeared in broader contexts, including
polymer relaxation and nonlinear models of global population dynamics. This review retraces the conceptual development of the equation, with particular emphasis on its physical origin in Kostya Trachenko's feed-forward interaction mechanism, its mathematical structure, and its possible generalizations. We also include personal recollections of the work with Kostya Trachenko at Queen Mary University of London in August 2019, during which the equation emerged in essentially its present form. After reviewing applications to stress relaxation, glassy materials,
polymer relaxation and population dynamics, we discuss future directions including time-dependent feedback parameters, coupled order parameters, heterogeneous and spatially resolved formulations, flux terms, network versions and
stochastic extensions. The central theme of the review is that the Trachenko-Zaccone equation should be viewed not only as a model of glassy relaxation, but as a general nonlinear feedback equation with potential applications across complex systems.
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All authors Alessio Zaccone, Valeriy V. Ginzburg, Oleg V. Gendelman, Sofia F. Mauro, John C. Mauro
Materials Science
Systematic and accurate anharmonic formation free energies of metastable defects via a constrained Bayesian Adaptive Biasing Force framework
Clovis Lapointe, Anruo Zhong, Manuel Athènes et al. (+1 more)
Computing the formation free energies of metastable defects at finite temperature remains challenging due to anharmonicity, the multiplicity of basins and the frequent occurrence of migration events. Using the previously introduced constrained
Bayesian Adaptive Biasing Force method (BABFc), we establish a practical thermodynamic integration framework for computing restricted anharmonic formation free energies associated with individual metastable defect basins. We demonstrate that BABFc can be exploited as a robust, systematic and numerically efficient tool for addressing a long standing bottleneck in atomistic materials science: the finite temperature thermodynamic characterization of metastable defects in complex, highly anharmonic energy landscapes. The proposed workflow requires only a reference local minimum and a confinement strategy and does not rely on defect specific collective variables. It therefore provides a general route to assign well-defined free energies to individual metastable basins, even when these basins are separated by low barriers and embedded in a dense landscape of competing configurations. In this setting, BABFc enables stable, bias corrected free energy estimates with statistical accuracies at the meV/atom level and a very low failure rate, including for systems containing approximately one thousand atoms. We demonstrate this capability through thousands of independent free energy calculations covering hundreds of four-interstitial and four-vacancy configurations in bcc $\alpha$-Fe over a broad temperature range, using both a traditional empirical potential and a data driven force field.
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All authors Clovis Lapointe, Anruo Zhong, Manuel Athènes, Mihai-Cosmin Marinica
Materials Science · Cross-listed from cond-mat.mes-hall
Towards Ultra Scalability of Non-Volatile Magnetic Tunnel Junctions with a 3D Storage Layer
Nuno Caçoilo, Shunsuke Fukami, Olivier Fruchart et al. (+1 more)
The perpendicular
Spin Transfer Torque
Magnetic Random Access
Memory is one of the most promising emerging non-volatile
memory technologies, based on ultra-thin
magnetic tunnel junctions. However, as these devices are limited by their thermal stability factor at technological nodes smaller than 20 nm, their scalability is compromised. A possible solution to this limitation relies on taking advantage of the shape anisotropy, by increasing substantially the thickness of the storage layer. Thanks to the combination of a vertical aspect-ratio and enhanced volume, high thermal stability can be maintained at sub-10 nm nodes. Here, we present the technological advancements and understanding of the magnetisation reversal that led to faster switching speeds at reduced switching voltage, providing a viable approach for dense arrays of ultra-small
magnetic tunnel junctions.
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All authors Nuno Caçoilo, Shunsuke Fukami, Olivier Fruchart, Ioan-Lucian Prejbeanu
Materials Science
First-principles design of main-group dimer defects in ZnO as candidate quantum defects
Taejoon Park, Erik Alfredo Perez, Shimin Zhang et al. (+2 more)
Zinc oxide (ZnO), a wide-band-gap
semiconductor with mature growth techniques, is a promising host for optically active
quantum spins. Yet, optically active
quantum defects in ZnO remain largely unexplored. Here, we identify and characterize a family of double substitutional impurities in ZnO, formed by main-group donor-acceptor (DA) pairs, as candidates for optically active
quantum defects. Using hybrid density functional theory (DFT), we systematically investigate double substitutional DA complexes and their defect physics, including electronic structure, thermodynamic stability, and optical properties. The proposed defects exhibit isolated defect states, strong
spin localization on the acceptor site, and $C_{3v}$
symmetry. Importantly, the electronic structure of the DA pairs is largely determined by the atomic properties of their constituent atoms. We further examine their optical characteristics, including zero-
phonon lines (ZPLs), radiative lifetimes, and nonradiative decay to assess their viability as color centers. Notably, among the dimers, (Si$_{Zn}$-B$_O$)$^+$ and (Ge$_{Zn}$-B$_O$)$^+$ exhibit visible optical transitions with sub-microsecond radiative lifetimes and robust charge states against optical ionization, while (Si$_{Zn}$-C$_O$)$^{2+}$ shows the smallest Huang-Rhys factor, approximately 5.6. Our results propose a new family of main-group donor-acceptor defects in ZnO as promising candidates for optically active
spin defects.
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All authors Taejoon Park, Erik Alfredo Perez, Shimin Zhang, Yuan Ping, Hosung Seo
Materials Science · Cross-listed from cond-mat.mes-hall
Yasuhiro Todaka, Motoki Asano, Isamu Yasuda et al. (+3 more)
We study the phase of the coupling between
magnons and surface acoustic wave (SAW)
phonons in
magnetic thin films. The coupling constant changes from a real to a complex number as the external
magnetic field is reduced. Below a transition field, the imaginary coupling constant allows SAW
phonons to couple to overdamped
magnons whose resonance frequency is close to zero and far from the SAW resonance. The strength of the imaginary coupling constant and the magnitude of the transition field both scale with
magnetic damping. We find the coupling produces a broad, pronounced minimum in the SAW transmittance spectrum near zero
magnetic field in a Ni/Ru/Ni synthetic antiferromagnet with large
magnetic damping. These results demonstrate that the phase of the complex
magnon-
phonon coupling constant can be tuned via
magnetic field in strongly damped magnets, offering a platform to explore novel regimes of
magnon-
phonon interactions.
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All authors Yasuhiro Todaka, Motoki Asano, Isamu Yasuda, Masashi Kawaguchi, Daiki Hatanaka, Masamitsu Hayashi
Materials Science
Prediction of a layer nonlinear Hall effect in bilayer nonmagnetic or antiferromagnetic systems
Zehou Li, Shenda He, Pan Zhou et al. (+3 more)
Nonlinear Hall effects provide a powerful probe of
quantum geometry in solids and enable rectification phenomena beyond the constraints of linear response. In this Letter, we predict a layer nonlinear Hall effect (LNHE) in stacked bilayer systems composed of nonmagnetic or antiferromagnetic materials with a vanishing linear Hall conductivity. In such systems, the second- or third-order nonlinear Hall responses are intrinsically layer odd: the contributions from the two constituent layers have equal magnitude but opposite sign, resulting in exact cancellation under layer-exchange
symmetry. An out-of-plane electric field $E_z$ can break this
symmetry, thereby unveiling the hidden response and converting it into a switchable macroscopic nonlinear Hall signal. Using a minimal $k\!\cdot\!p$ model, we demonstrate that the LNHE can originate from the Berry
curvature dipole mechanism. A systematic
symmetry analysis of all 80 layer groups further yields a complete classification of the
symmetry constraints and stacking configurations that allow for this type of LNHE. Beyond this mechanism, additional
symmetry analysis reveals that LNHE may also arise from
quantum metric dipole or inversed mass dipole. Remarkably, even in cases where second-order nonlinear Hall responses are
symmetry forbidden, a third-order LNHE can still survive in certain stacked bilayer configurations. First-principles calculations on representative bilayers—nonmagnetic 1T$'$-WTe$_2$ and 1T$'$-ReS$_2$—explicitly demonstrate electrically reversible second-order LNHE, in full agreement with our
symmetry-based predictions. Overall, our results establish LNHE as a universal phenomenon in a wide range of layered
quantum materials and provide a robust route toward electrically tunable nonlinear transport.
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All authors Zehou Li, Shenda He, Pan Zhou, Baoru Pan, Rui Tan, Lizhong Sun
Materials Science
Magnetic-Field Selection of Magnetic Order in Altermagnets and Noncollinear Antiferromagnets
Qiu-Shi Huang, Chaoxi Cui, Yilin Han et al. (+3 more)
Conventional field selection of
magnetic order relies on the Zeeman coupling, which, however, vanishes in magnets without net magnetization, a rapidly growing class including altermagnets (AMs), noncollinear antiferromagnets (nc-AFMs), and PT-symmetric antiferromagnets (PT-AFMs). Here we show that the quantity that fundamentally couples a magnet to a uniform
magnetic field is not the magnetization, but the binary order parameter eta that labels the two time-reversal-related minima of the Landau free energy. We develop a Landau theory of order selection based on eta under the constraints of
magnetic point-group (MPG)
symmetry, in which eta couples to odd-degree
polynomials in the
magnetic field. Within this framework, the linear term is the ferromagnetic Zeeman coupling, while higher-order couplings with leading degree n = 3, 5, 7, and 9 naturally appear in AMs and nc-AFMs. In contrast, combined PT
symmetry forbids any such coupling. Consequently, it is the order-(n-1)
magnetic susceptibility, rather than the net magnetization, that serves as the primary experimental observable for identifying the
magnetic order of AMs and nc-AFMs. For all 122 MPGs, we classify the leading coupling degree and the corresponding
polynomial forms. We demonstrate our framework in two representative materials: the AM MnF2 and the nc-AFM MnTe2. We further construct a
symmetry-allowed
spin model for an AM system to reveal the microscopic origin of the higher-order coupling and establish the coupling coefficient explicitly in terms of the
spin-model parameters. Our work unifies the description of
magnetic-order selection across magnets with and without net magnetization, offers a microscopic origin for this counterintuitive physics, and provides fingerprints for distinguishing intrinsic field selection from extrinsic switching.
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All authors Qiu-Shi Huang, Chaoxi Cui, Yilin Han, Junxi Duan, Zhi-Ming Yu, Yugui Yao
Materials Science
Orbital-Selective Spin Splitting in the Altermagnetic Ti$_2$XX' Monolayers
Xin Zhang and Shihao Zhang
Altermagnets have attracted intensive
attention recently, owing to their unique combination of zero net
magnetic moment of antiferromagnets and momentum space
spin-splitting of ferromagnets. Herein, we systematically explore monolayer altermagnetic Ti$_2$XX' (X/X' = F, Cl, Br, I) materials for novel electronic properties. Especifically,
spin-splitting exists in the valence bands of all studied materials, whereas several compositions exhibit nearly
spin-degenerate conduction bands near the Fermi level, which is markedly different from conventional altermagnets. This phenomenon can be attributed to the bond-angle-sensitive super-exchange occurring in the $d_{xz}$ or $d_{yz}$ orbitals, while the super-exchange of the $d_{x^2-y^2}$ orbital is not sensitive to the Ti-X-Ti bond angle. Motivated by this intriguing phenomenon, Fermi level modulation via doping or
heterostructure construction enables the transition between two electronic states, endowing these materials great application potential in information storage devices.
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