Materials Science
Structured-Light Control of Goniopolar Thermoelectricity in NaSn\texorpdfstring{$_2$}{2}As\texorpdfstring{$_2$}{2}
Atoumane Ndiaye and Edwin Fohtung
Goniopolar metals exhibit opposite thermoelectric polarities along different crystallographic directions, enabling zero-field transverse thermoelectricity but offering few means for external control. Here we show that a spatially structured Laguerre–Gaussian vector potential can programmably reconstruct the goniopolar phase space of NaSn$_2$As$_2$. First-principles-derived Wannier transport with bond-dependent Peierls coupling reveals two OAM-dependent spatial scaling laws: the radial response follows the Laguerre–Gaussian radius $r_{\max}\propto\sqrt{|\ell|}$, while calculations for $|\ell|=2$–5 yield a dominant angular harmonic $m_{\mathrm{dom}}=2|\ell|$, encoding the optical winding in a frequency-doubled thermoelectric response. Increasing $|\ell|$ simultaneously reconstructs pre-existing goniopolar windows, enhancing a representative window by approximately 17% at $|\ell|=5$. By contrast, reversing $\ell$ at fixed polarization produces only a small correction that approximately interchanges upon polarization reversal. Energy-resolved transport reveals that the structured field redistributes in-plane and cross-plane electronic velocities, shifting the directional Seebeck-zero boundaries that define the goniopolar state. These results establish
vortex position and OAM magnitude as programmable control coordinates for goniopolar thermoelectricity.
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Materials Science
Theory of Chirality-Induced Spin Selectivity in Trefoil-knot Molecules
Xi Sun, Shu-Zheng Zhou, Kai-Yuan Zhang et al. (+1 more)
The origin of chirality-induced
spin selectivity (CISS) remains elusive, and ultrahigh
spin polarization (SP) in topologically knotted molecules is unexplained. We develop a discrete
geometric spin-orbit coupling (SOC) framework for molecular junctions, resolving site-specific
curvature and current-partition effects beyond continuous models. For trefoil-
knot molecules, it quantifies site-resolved
geometric SOC, reaching $\sim$120 meV, nearly two orders of magnitude larger than intrinsic SOC of light atoms. The substrate-coupled benzene unit, carrying the largest current and remarkable
geometric SOC, dominates the CISS effect. The framework quantitatively reproduces measured SP, temperature-dependent magnetoresistance (MR) and $\Delta$MR trends, establishing discrete
geometric SOC as a predictive tool for CISS in topologically knotted molecules.
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All authors Xi Sun, Shu-Zheng Zhou, Kai-Yuan Zhang, Hua-Hua Fu
Materials Science
Decoding Thermal Stability: In situ Insights into Phase Controlled Phosphine-free Colloidal Bi-Te Nanosheets
Fagui He, Kevin Oldenburg, Rostyslav Lesyuk et al. (+1 more)
Bismuth telluride (Bi2Te3) is a prototypical V-VI
semiconductor of interest for both thermoelectric and topological applications; however, phase-selective synthesis and thermal stability across the bismuth-telluride homologous series remain poorly understood. We report a facile and phosphine-free colloidal synthesis approach for a phase-selective synthesis of Bi2Te3, BiTe, and Bi4Te5 nanosheets with well-defined hexagonal morphology. By controlling precursor chemistry and reaction temperature, we achieved the selective formation of different phases within the (Bi2)m(Bi2Te3)n homologous series. Based on in situ heating studies, BiTe and Bi4Te5 nanosheets transform into Bi2Te3 at about 340 degree C, followed by preferential Te sublimation under
vacuum or oxidation in air at higher temperatures. We discuss plausible mechanisms for these phase transformations. EDS analysis and FFT analysis of STEM images provide direct evidence for the temperature-dependent compositional and structural changes and highlight the close thermal relationship among these phases. These results not only advance fundamental understanding of phase stability and thermal behavior in bismuth-tellurides at the nanoscale but also establish a framework for understanding structural
evolution in related homologous series, providing insights into their potential future applications in thermoelectric, spintronic, and topological systems.
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All authors Fagui He, Kevin Oldenburg, Rostyslav Lesyuk, Christian Klinke
Materials Science
Complex magnetic phase diagrams in Tb$_2$IrAl$_4$Ge$_2$ and Er$_2$IrAl$_4$Ge$_2$
Karolina Gornicka, Matthew S. Cook, Brenden R. Ortiz et al. (+2 more)
We report the synthesis and comprehensive investigation of single-crystalline Tb$_2$IrAl$_4$Ge$_2$ and Er$_2$IrAl$_4$Ge$_2$, the first Ir-based members of the $Ln_2M$Al$_4$Ge$_2$ family. Both compounds crystallize in the tetragonal Tb$_2$NiAl$_4$Ge$_2$-type structure (space group $I4/mmm$) and exhibit easy-axis
magnetic anisotropy with the crystallographic $c$ axis as the preferred magnetization direction. Tb$_2$IrAl$_4$Ge$_2$ undergoes antiferromagnetic ordering below $T_N = 25$ K and develops a sequence of field-induced metamagnetic transitions accompanied by pronounced magnetotransport anomalies, including a nonlinear Hall response. In contrast, Er$_2$IrAl$_4$Ge$_2$ exhibits three successive zero-field
magnetic transitions and a substantially richer low-temperature
magnetic phase diagram comprising multiple competing field-induced phases confined to relatively modest
magnetic fields. The differences between the Tb- and Er-based compounds demonstrate the wide range of
magnetic behavior that can be realized within the \ce{Tb2NiAl4Ge2}-type structure. More broadly, the successful realization of Ir-based $Ln_2M$Al$_4$Ge$_2$ compounds considerably expands the accessible chemical space of this family and opens a route toward the exploration of related $4d$- and $5d$-based aluminogermanides.
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All authors Karolina Gornicka, Matthew S. Cook, Brenden R. Ortiz, Andrew D. Christianson, Andrew F. May
Materials Science · Cross-listed from cond-mat.str-el
Why polar excitons stay sharp: parity protection of the center-of-mass recoil channel in exciton-phonon scattering
Michael O. Atambo
In polar
semiconductors the Fröhlich interaction is the dominant electron–
phonon coupling, yet excitonic resonances in materials such as halide perovskites remain anomalously sharp. We show that standard frozen-center-of-mass treatments of the
exciton–
phonon problem miss the decisive kinematic degree of freedom: restoring the exact center-of-mass (COM) recoil reveals a universally open, parameter-free $1s\to1s$ absorption channel at recoil momentum $q_*=\sqrt{2M_{\rm ex}\hbar\omega_{\rm LO}}/\hbar$, whose rate scales as $N_{\rm LO}(T)$. We prove that this recoil channel is controlled by destructive electron–hole interference: the recoil linewidth vanishes with the mass asymmetry as $\gamma_{\rm LO}^{\rm recoil}\propto\mathcal{F}_{1s,1s}(q_*)^2$, and the elastic dressing obeys the exact suppression law $S_X/S_{\rm ind}=\eta^2(6-\eta^2)/5$ within the hydrogenic Fröhlich model. The theory establishes a hierarchy of scattering regimes. In mass-asymmetric materials (GaAs, $\eta=-0.74$) the recoil channel is active ($\gamma_{\rm LO}^{\rm recoil}=2.2$~meV); in mass-symmetric materials (FAPbI$_3$, $\eta=0$; MAPbI$_3$, $\eta=-0.11$) it is killed by interference (0.00 and 0.12 meV), showing that the observed 27–40 meV perovskite linewidths cannot be accounted for by COM recoil and therefore require internal-state-changing and other inelastic channels, of which the constructive, $\eta$-robust $1s\to np$ resonance is the leading candidate within the present model. The Fröhlich constant $\alpha$ alone is therefore insufficient as a figure of merit: after projection onto the correlated
exciton, the controlling parameters are $\eta$, $q_*a_X$, and the Rydberg detuning.
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Materials Science · Cross-listed from cond-mat.str-el
Room-temperature Magnetoelastic Coupling in UIr$_4$Al$_{15}$
Mingyu Xu, Tomasz Klimczuk, M. Brian Maple et al. (+1 more)
The interplay between electronic and structural degrees of freedom underpins many emergent phenomena in
quantum materials, yet magnetoelastic coupling in metallic systems is typically weak and confined to low temperatures or microscopic length scales. Here, we report giant magnetoelastic coupling slightly above room temperature in the uranium-based intermetallic compound UIr$_4$Al$_{15}$. Using temperature-dependent single-
crystal X-ray diffraction together with anisotropic
magnetic susceptibility measurements, we directly resolve subtle but reproducible structural distortions coupled to
magnetic alignment. Despite the absence of crystallographic
symmetry breaking, pronounced anomalies emerge in
lattice parameters and selected chemical bond distances near the
magnetic transition region, revealing an unusual sensitivity of the
crystal structure to
magnetic orientation. The coupling enables direct probing of how atomic distances and local chemical bonding govern the electronic and
magnetic states in a bulk intermetallic material. Our results establish UIr$_4$Al$_{15}$ as a rare platform in which magnetism and
lattice distortions are strongly intertwined near room temperature, opening new opportunities for magnetically responsive
quantum materials and functional
magnetic sensing applications.
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All authors Mingyu Xu, Tomasz Klimczuk, M. Brian Maple, Weiwei Xie
Materials Science · Cross-listed from cond-mat.stat-mech
Learning ab initio phase-field models
Mengyi Chen, Peichen Zhong, Zihan Zhang et al. (+1 more)
Simulating microstructure
evolution requires
quantum-mechanical accuracy and mesoscopic reach in length and time scales, a combination that no current method achieves. Classical phase-field models provide this reach, but their accuracy is limited by phenomenological free energies and mobilities. Here we develop a framework for learning ab initio phase-field models, where the mesoscopic equation is not postulated but derived from a Mori-Zwanzig projection of molecular dynamics onto species-density fields under explicit assumptions. The nonlocal free energy and mobility left unspecified by this equation are parametrized by neural networks and learned from short molecular dynamics trajectories generated with machine-learning interatomic potentials of ab initio accuracy. We demonstrate the framework on an iron-boron melt and on hydrogen-helium mixtures under planetary conditions. For iron-boron, the model shows that the melt at the FeB$_4$ composition is spinodally unstable at ambient pressure but stabilized at 10 GPa, offering a thermodynamic rationale for why FeB$_4$ has been synthesized only under high pressure. For hydrogen-helium, the model predicts the immiscibility boundary and captures droplet nucleation and growth in helium-rain simulations of a column corresponding to 2.2 million atoms, far beyond the scale of atomistic modeling at comparable accuracy. Trained across compositions and conditions, such models could provide a mesoscopic counterpart to ab initio molecular dynamics.
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All authors Mengyi Chen, Peichen Zhong, Zihan Zhang, Qianxiao Li
Materials Science · Cross-listed from cond-mat.str-el
Na$_2$Ir$^\text{IV}$Cl$_6$: The Missing Member of the Perfectly Cubic Vacancy Ordered $A_2$IrCl$_6$ Family, Another Potential $J_{eff}$ = $\frac{1}{2}$ Ground State Candidate
Siddhartha Sankar Soren, Aditya Patra, Rounak Das et al. (+5 more)
Vacancy ordered double perovskites (VODPs) have been known for more than 100 years, but their exact
crystal structures and
magnetic ground state remained controversial till date. Anhydrous VODP iridates ($A_2$Ir$X_6$; $A$ = K$^+$, (NH$_4$)$^+$, Cs$^+$, Rb$^+$, and $X$ = F$^-$, Cl$^-$, Br$^-$), having $face-centered~cubic$ (fcc) antiferromagnetic ground state, offer a chemically well-defined platform to investigate the influence of on-site Coulomb repulsion,
crystal field,
spin-orbit coupling, and competing Heisenberg and Kitaev interactions. Interestingly, among these, the Na-based VODP iridates seem to possess an unusual hydration dependent structural and
magnetic uncertainty. As a result, a cubic Na$_2$IrCl$_6$ system having fcc antiferromagnetism could not be stabilized ever. Here, for the first time, we report the synthesis and comprehensive characterization of anhydrous Na$_2$IrCl$_6$ single
crystals having fcc structure with antiferromagnetic transition temperature of 1.91 K. Thermogravimetric analysis establishes the absence of
lattice water and also demonstrates thermal stability of the
crystals up to 453 K. The cubic phase reported here contrasts with previously described 'anhydrous' monoclinic Na$_2$IrCl$_6$ and highlights the strong structural and physical property relationship in Na-based VODP iridates. This new material offers an exciting playing ground for exploring another potential perfect $J_{eff}$ = $\frac{1}{2}$ octahedra and the consequent strong Kitaev interaction within the nearest-neighbor (nn) pairs of Ir ions.
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All authors Siddhartha Sankar Soren, Aditya Patra, Rounak Das, Ritwik Das, Tanmay Chanda, Jens Buck, Indra Dasgupta, Sugata Ray
Materials Science
Benchmarking average atom potentials derived from atomic cluster expansions
Deepak Somani, Lorenzo Piersante and Anirudh Raju Natarajan
Average atom (A-atom) potentials provide a mean-field description of a chemically disordered
alloy and are used to predict the properties of solid solutions without short-range order. Such potentials are usually averaged from an existing interatomic potential and are therefore only as accurate as the parent model. Accurate interatomic potentials are themselves difficult to parameterize and can require large training
datasets. Here we
benchmark a recently developed formalism that computes an exact A-atom potential directly from a linear atomic cluster expansion (ACE). We first fit a linear ACE to Fe-W data generated with an embedded atom method (EAM) potential. The resulting A-atom potential reproduces the properties of the disordered phase computed from an explicit random supercell and from a conventional A-atom potential averaged from the same EAM potential. We then fit a linear ACE to energies and forces computed from electronic structure calculations for about 1500 small Mo-Nb structures with an average of 7 atoms per structure. The A-atom potential derived from this ACE reproduces the DFT elastic constants,
lattice parameter, mixing enthalpy, and Bain path of special quasirandom structures. Of the three chemical site bases, only the occupation basis also reproduces the DFT surface energies of the
alloy. For Mo-Nb, the A-atom potential also predicts that the ideal solution
entropy outweighs the destabilizing vibrational contribution to the finite-temperature free energy of the disordered phase. These
benchmarks show that the properties of disordered
alloys can be recovered from small training
datasets when the chemical site basis is chosen carefully.
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Materials Science
Domain-engineered ferroelectric BiFeO$_3$ thin films for efficient bias-free THz emission
Z. Abdul Hadi, N. Rezi, M. Monti et al. (+10 more)
Ultrashort light pulses can drive transient photocurrents in solids, the basis of modern terahertz (THz) emitters. Ferroelectrics have recently emerged in this context as sources that require neither the bias voltage of photoconductive
antennas nor the
magnetic field of spintronic emitters, yet the ultrafast photocurrent generating the emission has remained debated. Here, we resolve this question in epitaxial BiFeO$_3$ thin films with engineered ferroelectric domain configurations, from single-domain to periodic stripe patterns. The stripe-domain film emits a THz field four times stronger than the single-domain configurations, reaching about half the output of a metallic spintronic reference emitter while remaining unsaturated at our highest excitation fluences. Resolving the emitted THz field in amplitude and phase as a function of the pump light polarization and the
crystal orientation, we separate the coexisting photocurrents by their
symmetry. This identifies the ultrafast screening of the ferroelectric polarization by photocarriers as the dominant emission mechanism in the in-plane-polarized films, enhanced in multidomain films by the strong built-in fields at the domain walls, while an out-of-plane polarization favors the bulk photovoltaic current. Our results establish the ferroelectric domain configuration as a design parameter setting both the strength and the microscopic origin of light-driven THz emission.
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All authors Z. Abdul Hadi, N. Rezi, M. Monti, G. Vaudel, A. Abdelsamie, C. Carrétéro, D. Sando, M. Viret, V. Garcia, S. Fusil, M. C. Weber, P. Ruello, V. Juvé
Materials Science · Cross-listed from cond-mat.str-el
Robustness of the Verwey transition against remanent strain-induced defects in magnetite
M. A. Gala, K. J. Grzywa, K. Komędera et al. (+16 more)
The Verwey transition in magnetite is a
benchmark electronically driven phase transition that is highly sensitive to
lattice imperfections and external perturbations. Doping, deviations from oxygen stoichiometry, and irradiation-induced point defects are known to lower the Verwey transition temperature, $T_{\rm V}$. By contrast, the role of remanent strain fields and extended defect structures generated by uniaxial stress has remained largely unexplored. Here we combine dark-field x-ray microscopy (DFXM) and ac
magnetic susceptibility to determine how strain-induced defects affect the Verwey transition in stoichiometric single-crystalline magnetite after uniaxial compression and unloading. The DFXM measurements were performed on two samples compressed along the same $[011]$ direction: loading to $200~\mathrm{MPa}$ generated stable line-like defects and remanent strain fields, whereas loading beyond the fracture threshold produced denser defect networks and mechanical fracture. To test the effect of a different loading geometry, we also measured the ac susceptibility of a sample fractured by compression along $[001]$. Real-space DFXM imaging,
lattice-orientation mapping, and residual strain mapping show substantial remanent structural disorder after compression. Nevertheless, neither the transition temperature $T_{\rm V}$ nor the sharpness of the transition changes measurably in any of the compressed or fractured samples. This behavior contrasts with the strong suppression of $T_{\rm V}$ by point-like disorder. Our results show that extended line-like defects and remanent strain fields produced by plastic deformation do not measurably perturb the macroscopic electronic ordering responsible for the Verwey transition, thereby distinguishing the effects of point defects from those of extended slip-related defects in magnetite.
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All authors M. A. Gala, K. J. Grzywa, K. Komędera, R. Zalecki, A. Baczmański, Z. Kąkol, A. Pacanowska, C. M. N. Kumar, R. Rodriguez Lamas, C. Detlefs, C. Yildirim, C. Marin, J. Debray, G. Beutier, L. Ortega, A. Kozłowski, N. Barišić, J. E. Lorenzo, W. Tabiś
Materials Science
Pressure induced structural transitions, coordination crossover, and optoelectronic response in divalent metavanadate CaV$_2$O$_6$
Bhagyashri Giri, Asish Kumar Mishra, Bidisha Mukherjee et al. (+3 more)
High-pressure synchrotron X-ray diffraction, Raman spectroscopy, photoluminescence (PL), and UV-Vis absorption measurements were carried out on the divalent metavanadate CaV$_2$O$_6$ up to 31 GPa at room temperature. Ambient CaV$_2$O$_6$ crystallizes in a centrosymmetric pseudo-brannerite monoclinic structure ($C2/m$), where vanadium atoms reside in fivefold-coordinated VO$_5$ trigonal bipyramids. Under hydrostatic compression, the system undergoes a structural phase transition starting at $\sim$1 GPa to a lower-
symmetry, non-centrosymmetric monoclinic phase ($C2$), which becomes the single stable phase above 5.2 GPa. This transition involves an increase in vanadium coordination from VO$_5$ bipyramids to edge-sharing distorted VO$_6$ octahedra. High-pressure Raman scattering corroborates the structural phase boundaries, exhibiting mode softening ($\gamma_3 = -0.42$, $\gamma_9 = -0.46$) within the mixed-phase regime (1.0 - 4.4 GPa) and sharp anomalies in mode frequencies and linewidths across 5.2 GPa. The optical measurements show complete PL quenching near 1 GPa due to non-radiative recombination pathways introduced by local polyhedral distortions, alongside a non-monotonic
evolution of the indirect optical band gap. Above 20 GPa, an anomaly in polyhedral distortion and internal strain drives an isosymmetric structural modification within the $C2$ phase, marked by an alteration in bulk compressibility and stabilization of the optical gap. Comparative high-pressure band-gap measurements on the sister compound MgV$_2$O$_6$ validate the explicit coupling between cation coordination changes and optoelectronic response in divalent metavanadates.
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All authors Bhagyashri Giri, Asish Kumar Mishra, Bidisha Mukherjee, Debabrata Samanta, Nico Giordano, Goutam Dev Mukherjee
Materials Science · Cross-listed from cond-mat.mes-hall
Synthesis of Metal-Semiconductor Heterojunctions in Single Graphene Nanoribbons
Ziyi Wang, Kaitlin Slicker, Weichen Tang et al. (+9 more)
Graphene nanoribbons (GNRs) are a highly tunable class of one-dimensional (1D)
quantum materials that can be fabricated through bottom-up synthesis. Precise control over GNR band gaps and band-edge
alignments has established them as a promising nanoelectronics platform, but forming high-quality electronic interfaces remains challenging. Here we combine sequential on-surface synthesis and scanning tunneling microscopy (STM)-induced dehydrogenation to directly write metallic 7-iGNR segments into otherwise semiconducting H$_2$-7-iGNRs without changing the GNR width or carbon backbone connectivity. The resulting metallic segments exhibit nonzero spectral weight at the Fermi level ($E_\mathrm{F}$) and spatially extended electronic states, consistent with two dispersive bands crossing $E_\mathrm{F}$. The new metallic states are well described by an extended Su–Schrieffer–Heeger zigzag-ladder
Hamiltonian. Scanning tunnelling spectroscopy (STS) and first-principles calculations show that the valence band edge of adjacent, connected semiconducting GNR segments lies close to $E_\mathrm{F}$ for the metallic segments. STS spectra recover bulk GNR properties within $\sim$1 nm on either side of the metal-
semiconductor GNR interface, with no discernible depletion region. The observed GNR band edge
alignment is consistent with a small energy barrier for hole injection, thus suggesting a possible route towards atomically precise, low-barrier p-type contacts for future GNR-based electronic devices.
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All authors Ziyi Wang, Kaitlin Slicker, Weichen Tang, Boyu Qie, Rafal Zuzak, Haowen Pang, Yudi Huang, Xinheng Li, Peter H. Jacobse, Steven G. Louie, Felix R. Fischer, Michael F. Crommie
Materials Science
Additive solid immersion lenses for enhanced collection efficiency of shallow NV centers by pulsed laser deposition and structurization of high-k amorphous oxides
Michael S. Bar, Nicole Raatz, Max Kneiss et al. (+3 more)
The negatively charged nitrogen-vacancy ($\mathrm{NV}^-$) defect in diamond can be used for
quantum information processing via the readout of the radiative decay of the excited state into the ground state. The large refractive index mismatch of diamond and air causes low
photon extraction efficiency due to total internal reflection losses. Here, we investigate the
photon extraction numerically and report additive solid immersion lenses (SILs) consisting of the high-k dielectric yttria-stabilized-zirconia (YSZ) for surface modification and enhanced light extraction of shallow implanted nitrogen-vacancy centers. We demonstrate an average increase of 63% in saturation intensity using a scalable bottum up room temperature pulsed
laser deposition and photolithography structurization process. The average necessary
laser intensity for saturation is reduced by 33% by the SILs.
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All authors Michael S. Bar, Nicole Raatz, Max Kneiss, Andreas Koenig, Marius Grundmann, Holger von Wenckstern
Materials Science · Cross-listed from cond-mat.mes-hall
Charge and spin dynamics in spintronic THz emitters from Time-Dependent Density Functional Theory
Ali Kefayati
Spintronic terahertz (THz) emitters based on ferromagnet/heavy-metal
heterostructures provide efficient broadband sources of THz radiation, yet the microscopic relation between ultrafast
spin transport and charge redistribution remains incompletely understood. Here, we employ time-dependent density functional theory to resolve the coupled charge,
spin, and magnetization dynamics of Co/Pt and Co/W bilayers in real time and space. Despite qualitatively similar interlayer
spin-current dynamics, the two
heterostructures exhibit markedly different interlayer charge transfer: Co/Pt shows a relatively small and partially reversed transfer from Pt to Co, whereas Co/W exhibits a larger and persistent transfer from Co to W. We trace these differences to the material-dependent ground-state electronic structure and the
spin- and orbital-resolved phase space accessible to photoexcited carriers.
Spin-resolved dynamics further reveal coexisting
spin-orbit-mediated redistribution and optically induced intersite
spin transfer, including a delayed minority-
spin transfer from the heavy metal to Co common to both systems. While net electronic
spin angular momentum is transferred between the layers, we find no corresponding net transfer of electronic orbital angular momentum. In Co/W, the persistent redistribution of electronic density from interstitial to atom-centered states additionally provides strong evidence for light-induced electronic localization. These results demonstrate that ultrafast
spin transport does not uniquely determine charge or angular-momentum redistribution in spintronic THz emitters, which instead depends critically on the material-specific electronic phase space.
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Materials Science
Topo-Spectral Percolation Descriptors for Mechanistic Ion Transport Pathways from Static Crystal Structures
Diptendu Roy, Chiku Parida, Juan María García-Lastra et al. (+1 more)
Ion transport controls the macroscopic performance of solid electrolytes, battery electrodes, ion-selective membranes, electrolysis ceramics, mixed conductors, porous sorbents, and biological channels. In each case, ion transport is determined by structural characteristics, namely the available
diffusion pathways and the specific bottleneck atoms that limit the ionic jumps. These are intrinsic properties of the crystalline framework, whereas temperature, carrier concentration, and correlated motion govern the transport rate. Ab initio molecular dynamics (AIMD) resolves the mechanism directly but is too costly for materials screening, and most surrogate models give only a single transport number. Here we present a calibration-free method for determining the mechanism, based on Topo-Spectral Percolation Descriptors (TSPD). TSPD obtains the transport mechanism from a single static structure in seconds. It constructs a periodic migration network whose edge barriers are computed from physics-based energetics. We then analyze the barrier-threshold
topology of this network and the spectrum of its barrier-weighted graph
Laplacian. To establish its validity for computational solid-state ionics, we test TSPD against AIMD ion densities and
neutron diffraction data for eight materials, including cathodes and solid electrolytes, whose transport ranges from one- to three-dimensional. TSPD reproduces the established mechanism in every case and correctly identifies structures in which
geometric openness does not imply viable ion percolation. The method is most informative for pathway-limited, direction-dependent transport, and it complements full dynamical simulations for strongly cooperative or nearly isotropic transport.
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All authors Diptendu Roy, Chiku Parida, Juan María García-Lastra, Arghya Bhowmik
Materials Science · Cross-listed from physics.optics
Parallax Depth Sectioning and 3D Reconstruction in 4D-STEM
Desheng Ma, Chia-Hao Lee, Zixiao Shi et al. (+2 more)
Three-dimensional (3D) information is encoded in four-dimensional scanning transmission electron microscopy (4D-STEM) through parallax between the virtual images formed at each detector pixel. In this paper we connect the encoding and
retrieval of depth-dependent information in 4D-STEM to two other widely-used 3D imaging methods,
tomography and light-field photography, and derive its 3D phase contrast transfer function. Viewing the 4D data as a collection of angular slices yields a collection of sinograms, providing an intuitive visual representation of the 3D information transfer. Within the small-angle approximation, tilt-corrected bright-field (tcBF) depth sectioning is equivalent to non-iterative tomographic reconstruction. Alternatively, the 4D data also can be mapped to light-field (or plenoptic) imaging, which acquires spatially resolved diffraction data in parallel using an array of lenslets. Adapting a light-field imaging
algorithm, we show that tcBF depth sections can be obtained by extracting two-dimensional slices from a single four-dimensional fast
Fourier transform (4D FFT) of the
dataset, yielding a substantial speedup when reconstructing a large number of depth slices. These differing perspectives distinguish the
geometric aspects of depth resolution from wave-optical effects that are present in phase-contrast imaging. Depth-resolved aberration-corrected bright-field (acBF) imaging yields improved 3D reconstructions compared to tcBF, removing contrast oscillations and reducing the variation in response with depth. We demonstrate acBF depth sectioning using simulated and experimental data and resolve distinct layers in stacked oxide films and nanoparticle assemblies. These non-iterative volumetric reconstructions from a single 4D-STEM
dataset show promise for improved imaging of thick, weakly-scattering samples and for reconstructing low-dose acquisitions.
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All authors Desheng Ma, Chia-Hao Lee, Zixiao Shi, David A. Muller, Steven E. Zeltmann
Materials Science
Landau-like formalism for the thermal-runaway and filamentation instabilities in switching Mott devices
Aniket Bajaj, Anudeep Tullibilli, Arup Basak et al. (+2 more)
Correlated oxides can switch to a low-resistance state when exposed to a voltage, making these
insulator-to-metal transition materials promising candidates for memristor technology. Switching in materials such as VO2 occurs because of a thermal-runaway instability that leads to the abrupt formation of a hot current-carrying metallic filament within the
insulator. We study, theoretically and experimentally, this bias-induced switching as a non-linear dynamics stability problem. We construct Landau free energy-like Lyapunov functions to show that both the on-state ("set") and the off-state ("reset") transitions can be described as separate saddle-node bifurcations. Starting from the reaction-
diffusion equation for the heat transport, the abrupt snap-back (reset) to the insulating state is systematically formulated as a mean field Landau-like dynamics with the filament width playing the role of the order parameter. The formulation predicts an abrupt first-order phase transition with a break-off of a finite width filament across the current-controlled reset transition. The formulation is quantitatively compared against our electrical and optical microscopy experiments on VO2 thin film devices. Multi-cycle switching dynamics experiments further reveal the emergence of a stable
stochastic regime following an initial electro-spatial forming phase. Beyond establishing a cross-disciplinary dynamical framework that may be readily compared to instabilities in various reaction-
diffusion scenarios, the work also provides a predictive foundation for the rational design and optimization of Mott-transition-based technologies.
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All authors Aniket Bajaj, Anudeep Tullibilli, Arup Basak, Pavan Nukala, Bhavtosh Bansal
Materials Science · Cross-listed from cond-mat.soft
Anisotropic medium-range order uncovers dynamic crossovers in glass-forming liquids
Kamlesh Mishra, Rajesh Ganapathy and Walter Kob
On cooling
liquids towards their glass-transition temperature, the dramatic increase of their relaxation times is accompanied by changes in particle dynamics at two distinct temperatures: A high-T crossover, where particles become temporarily caged by their neighbors, and a low-T crossover, where the cage escape mechanism changes. While there is some evidence that the former is associated with a change in local particle arrangement, no structural modification has so far been detected across the low-T crossover, fueling scepticism about the relevance of structure for glassy dynamics. Here, we introduce a novel four-point correlation function which allows to determine a structural length scale characterizing cage anisotropy. Extensive molecular dynamics simulations reveal that this scale, as well as the mean structural length scale of the glass-former, extends into the medium range, i.e., significantly exceeds the particle size. Strikingly, the difference between these two scales - a measure of the degree of cage anisotropy - peaks at both crossover temperatures. We discuss how the presence of these peaks enables understanding the nature of the change in the microscopic transport mechanism at the two temperatures, thereby linking both dynamical crossovers to a single structural observable, the anisotropic medium-range order. This fundamental insight demonstrates that structure, extending well-beyond the local cage, is an essential ingredient for understanding the relaxation dynamics of deeply supercooled
liquids.
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Materials Science
Train for Accuracy, Execute at Scale: Architecture-Preserving Inference for Equivariant Atomistic Foundation Models
Lei Fu, Zihui Feng, Yongheng Li et al. (+8 more)
Equivariant atomistic foundation models provide broadly transferable interatomic potentials trained against
quantum-mechanical reference data, but their repeated execution at simulation scale remains computationally and
memory intensive. We present Symmetrix-XL, an inference engine that scales pretrained MACE checkpoints without retraining,
distillation, or modification of their learned weights. It combines streamed-edge execution to avoid graph-wide materialization of expanded edge intermediates, model-specialized code generation to compile checkpoint-specific operators, and tiled execution to reuse a bounded device-
memory workspace. On complete LAMMPS-step
benchmarks, Symmetrix-XL reduces inference time by 3.1-5.0 times relative to ML-IAP + cuEquivariance across tested A100 and RTX 5090 workloads. On a single A100 80 GB GPU, the tested MACE-OMAT-0 capacity boundary increases from 24,565 atoms to 11.24 million atoms. This substantially lowers the hardware threshold for simulations that would otherwise require spatial decomposition across many GPUs and compute nodes. The same backend weak-scales to 703 million atoms on 64 A800 GPUs at 93.8 percent efficiency. Energy, force, stress, molecular-dynamics stability, and Matbench Discovery evaluations reproduce reference MACE behavior within measured tolerances. Case studies spanning solid-state, interfacial, and reactive systems demonstrate the increased capacity in realistic workflows. These results establish post-training execution as a scaling axis complementary to model redesign, compression, and distributed scale-out, and show that the practical accuracy-cost frontier depends on both model architecture and inference execution.
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All authors Lei Fu, Zihui Feng, Yongheng Li, Hongwei Du, Xin He, Junyi Wu, Kejie Bao, Yueyu Zhang, Zeyu Deng, Ziheng Lu, Bonan Zhu
Materials Science
HVPE Homoepitaxy of $\beta$-Ga$_2$O$_3$: High-Temperature Growth and Orientation-Dependent Surface Evolution
Sourav Sarker, Saleh Ahmed Khan, Ahmed Ibreljic et al. (+2 more)
In this work, we systematically investigate homoepitaxial $\beta$-Ga$_2$O$_3$ growth by halide vapor phase epitaxy (HVPE) on (010), (001), and ($\bar{2}$01) native substrates over a broad growth parameter space, extending the growth temperature as high as 1150 $^\circ$C. The growth kinetics exhibit strong dependence on precursor supply, temperature, reactor pressure, and Ga-source-to-substrate distance. Increasing the HCl flow from 80 to 300 sccm enhances the growth rate by up to seven-fold, while increasing the temperature from 1050 to 1150 $^\circ$C nearly doubles the deposition rate. At 1150 $^\circ$C, atmospheric-pressure growth yields rates more than 2.5 times higher than those at 100 Torr, while reducing the O$_2$ flow to 5 sccm further increases the growth rate to 14.3 $\mu$m/h, producing films up to 42.9 $\mu$m thick in 3 h. Notably, phase-pure homoepitaxial layers with good crystalline quality are obtained across all three orientations, with low rocking-curve full width at half maximum (FWHM) values of 63.4, 66.6, and 42.8 arcsec for the (010), (001), and ($\bar{2}$01) epilayers, respectively. The surface morphology exhibits a pronounced crystallographic dependence and evolves strongly with the HVPE growth conditions. In particular, the ($\bar{2}$01) surface develops well-defined step-and-terrace morphology with local RMS roughness as low as 0.17 nm. These results extend $\beta$-Ga$_2$O$_3$ HVPE homoepitaxy into a higher-temperature regime and demonstrate a broad process window for high-rate growth of thick, high-quality epilayers with strongly orientation-dependent surface
evolution.
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All authors Sourav Sarker, Saleh Ahmed Khan, Ahmed Ibreljic, Pronoy Kanti Das, Anhar Bhuiyan
Materials Science
Layer-by-layer growth of highly aligned MoS$_2$ nanoribbon arrays
Kaichi Yamamoto, Pablo Solís-Fernández, Haiming Sun et al. (+10 more)
One-dimensional transition metal dichalcogenides (TMD) nanoribbons (NRs) offer a promising route to aggressive channel-width scaling in nanoscale transistors. However, controlling their layer number, a key determinant of device performance, has remained elusive. Here, we demonstrate a chemical vapor deposition (CVD) approach for layer-by-layer growth of MoS$_2$ NRs on a low-
symmetry sapphire surface. This approach enables the oriented growth of single-crystalline bilayer NRs with a 2H stacking configuration, while maintaining widths below 20 nm. Increasing the MoO$_3$ precursor supply was found to be crucial for promoting the growth of the second and subsequent layers. By controlling the growth time and precursor supply, the layer number of MoS$_2$ NRs can be increased up to four layers. This method can also be extended to the growth of vertical MoS$_2$/WS$_2$ hetero-NRs. The average carrier mobility and current density of bilayer NR field-effect transistors (FETs) are two- and three-times higher than those of monolayer counterparts, respectively, and a maximum mobility of 86.7 cm$^2$V$^{-1}$s$^{-1}$ was attained with a bilayer NR-FET. These results highlight layer number as a key parameter for optimizing NR device performance. Our thickness-controlled growth strategy provides a general route towards TMD NR-based transistors with enhanced scalability and performance.
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All authors Kaichi Yamamoto, Pablo Solís-Fernández, Haiming Sun, Yanlin Gao, Yeri Lee, Yushan Tseng, Aika Uchida, Masahiro Hara, Sunmin Ryu, Yung-Chang Lin, Susumu Okada, Kazu Suenaga, Hiroki Ago
Materials Science
Topological robustness of thermally disordered lattices: From average structures to ensemble electronic properties
Oleg Rubel and Karekin Sadikian
Band
topology is usually assigned to a single
crystal structure, yet at finite temperature a
crystal is an ensemble of thermally disordered configurations with a fluctuating band gap. We ask whether the thermally averaged structure is a faithful proxy for this ensemble, taking Bi$_2$Se$_3$ as a representative topological
insulator and classifying band ordering in each ab initio molecular dynamics snapshot by scaling the
spin-orbit coupling. The averaged structure fails in two ways: it misses the Rashba-like
spin splitting that instantaneous
symmetry breaking produces in individual configurations, and it substantially underestimates the band-gap renormalization, whereas a small ensemble of harmonic Monte Carlo configurations reproduces both the mean and the spread of the gap. Using a band-inversion survival probability, we find that the band inversion of Bi$_2$Se$_3$ holds in every sampled configuration up to 600 K, above which individual configurations cross into normal band ordering well before the average gap closes. Topological robustness at finite temperature is thus a property of the distribution over configurations rather than of any single structure.
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Materials Science
Complex Magnetic Phases and Unconventional Skyrmion Thermodynamics in Magnetically Intercalated Cr$_{1+x}$Te$_2$Compounds
Clayton Conner, Santosh Karki Chhetri, Avinash Sah et al. (+13 more)
Chemical intercalation provides a powerful route for tuning the electronic and
magnetic properties of transition metal dichalcogenides (TMDs), enabling the emergence of
magnetic phases and phenomena that are otherwise inaccessible in their pristine forms. Here, we investigate the structural, electronic, and
magnetic properties of magnetically intercalated Cr$_{1+x}$Te$_2$ through a combination of scanning tunneling microscopy, angle-resolved photoemission spectroscopy, magnetization, electrical transport, and Lorentz transmission electron microscopy measurements. We uncover a rich
magnetic phase diagram comprising paramagnetic, in-plane
spin-fluctuation, out-of-plane ferromagnetic, and field-induced Néel-type skyrmion phases. In particular, Lorentz transmission electron microscopy directly reveals the formation of Néel-type skyrmion textures slightly below the Curie temperature, with their configurations evolving strongly with temperature and
magnetic field. The skyrmions exhibit pronounced thermal hysteresis and coexist with stripe domains, indicative of a complex
magnetic energy landscape arising from the competition among disorder-induced local Dzyaloshinskii-Moriya interactions, perpendicular
magnetic anisotropy, and dipolar interactions. Our results demonstrate that chemical intercalation can profoundly reshape the
magnetic interactions and phase behavior of TMDs, establishing Cr$_{1+x}$Te$_2$ as a versatile platform for exploring tunable magnetism, skyrmion physics, and unconventional thermodynamic phenomena in low-dimensional
quantum materials.
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All authors Clayton Conner, Santosh Karki Chhetri, Avinash Sah, Manoj Gadtoula, Dilan M. Gamachchi, Indeewari M. Karunarathne, Steven Kelley, Andrew C. Meng, Jacob Cook, Cheng Zhang, Jin Hu, Yue Li, Hoyeon Jeon, An-Ping Li, Zheng Gai, Guang Bian
Materials Science · Cross-listed from physics.comp-ph
From Energy-Force Weighting to Primal-Dual Optimization of Machine-Learned Interatomic Potentials
Chenyu Wang, Yangshuai Wang and Lei Zhang
Machine-learned interatomic potentials are commonly fitted by weighted-sum scalarization, which combines energy and force errors in a single loss. A nominal weight, however, identifies a potential only relative to the complete fitting protocol. We therefore treat energy–force balancing as a protocol-dependent problem of physical model selection. For fixed-basis atomic cluster expansion models of molten LiCl,
liquid H$_2$O, and Si, the resulting scalarization paths contain dominated states at extreme weights. Their nondominated subsets depend on the solver, and the out-of-distribution Si path is nonmonotone. We replace direct weight selection by minimizing the regularized energy objective subject to an upper bound on the normalized force loss. Projected dual ascent adjusts the Lagrange multiplier from the force-constraint residual. A frozen-multiplier limited-
memory quasi-Newton refinement then returns the final model. Across the three systems, the constrained procedure reaches the solver-matched low-error region and gives measured speedups of $6.7$–$8.9$ over completed scalarization scans under the stated timing convention. Physical-property calculations show that first-shell geometry is comparatively insensitive to the selected balance, whereas transport and solid-state observables vary more strongly. These results identify the fitted model–protocol pair as the relevant object of energy–force model selection and support force-constrained fitting as an explicit selection rule.
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