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Physics Condensed Matter Materials Science

cond-mat.mtrl-sci Watch Materials Science

Monday 15 June

Materials Science

GaN Nucleation Landscape on Patterned Sapphire Shaped by the Growth Temperature of Directly Grown Boron-Compound Masks

Yunjin Heo, Donghoi Kim, Hyeonoh Jo et al. (+4 more)

The growth temperature of directly grown boron-compound masks on patterned sapphire can modify the local accessibility of the underlying sapphire surface and thereby alter the subsequent nucleation behavior of GaN. In this work, we investigate how ammonia-borane-derived boron-compound masks grown at different temperatures shape the GaN nucleation landscape within circular SiO$_2$ openings during the initial stage of epitaxial lateral overgrowth. The preferential nucleation position of GaN changes systematically with mask growth temperature: masks grown at 700–750$^\circ$C produce pronounced edge-biased distributions, whereas higher-temperature masks lead to more inward-shifted and spatially sparse GaN domains. Quantitative analysis of the GaN areal fraction, the number of visibly isolated domains, and the radial distribution of domain centers shows that the mask growth temperature affects both the amount of GaN coverage and the spatial arrangement of GaN domains within each opening. The nonmonotonic change in the number of visibly isolated domains is interpreted as a consequence of competition between reduced lateral merging and reduced effective substrate accessibility, rather than as a direct measure of the number of active nucleation sites. Kinetic Monte Carlo simulations reproduce the essential experimental trends by varying the effective density and radial distribution of substrate-accessible sites. These results suggest that the growth temperature of directly formed boron-compound masks provides a practical means of reshaping the intra-opening GaN nucleation landscape by controlling the spatial distribution and effectiveness of local pathways through which GaN precursors can access the underlying sapphire surface.
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All authors Yunjin Heo, Donghoi Kim, Hyeonoh Jo, Aelim Ha, Soohyung Park, Jaewu Choi, Chinkyo Kim

Materials Science

Activated Migration of Localized Ligand-Field Excitons in Atomically Thin CrCl3

Hyesun Kim, Renlong Liu, Sangho Yoon et al. (+6 more)

Two-dimensional crystals with densely packed atoms exhibit a range of emerging properties, particularly a wide variety of excitonic behaviors. Thickness-variable layered chromium trihalides with finite surface recombination sites provide an ideal system for understanding how excitons confined in octahedral ligand fields migrate on nanometer length scales, a regime that defies conventional transport probes. In this work, we demonstrate that Cr3+-derived photoluminescence in CrCl3 is spectrally thickness-independent, but its relaxation dynamics are strongly sensitive to thickness and temperature, thereby indicating significant activated migration. A diffusion-coupled surface recombination model reveals an effective out-of-plane diffusivity of 4.5 x 10-6 cm2/s for the ligand-field excitons and a diffusion activation energy of 130 meV. The latter is comparable to the reorganization energy independently estimated from optical Stokes shifts, suggesting that exciton transport is coupled to local lattice relaxation. Furthermore, we show that the relaxation dynamics can be systematically tuned by either enhancing or suppressing surface recombination through controlled surface reactions or encapsulation. This work not only reveals the nanoscopic transport of localized ligand-field excitons but also establishes a spectroscopic transport probe applicable to various 2D materials.
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All authors Hyesun Kim, Renlong Liu, Sangho Yoon, Hyunjong Lim, Takashi Taniguchi, Kenji Watanabe, Jonghwan Kim, Changgu Lee, Sunmin Ryu

Materials Science · Cross-listed from cs.CL

Sycophancy as Material Failure under Pushback Loading: A Multi-Axis Characterization Across Three Loading Cases and up to Seventeen Material Charges

Ferdinand M. Schessl

Sycophancy in LLMs is documented across 70+ papers, but expert agreement on construct boundaries remains low (ICC=.184; Ye et al., 2026). The construct fragments because behavioral classification depends on which surface form is privileged. We adopt a materials-science framing: conversation as test specimen under load, LLM-model as material charge, pushback as progressive load, stance-flip as material failure. We characterize this failure across three loading cases (debate n=1000; false-presuppositions n=3400; ethical-setting n=3400; 10-17 material charges per case; 7800 specimens total) using 14 turn-level axis-measurements spanning velocity, damage accumulation, frame-drift, brittleness, and direction stability, plus three speaker-resolved axes from an independent pipeline. The measurements are Hooke-coupled ($\sigma = E \cdot \varepsilon$ analog) and reproduce across loading cases with effects up to $|r_{rb}| = 0.35$ on debate; the sign structure adds a second pattern: the ethical-setting case inverts the velocity and accumulation blocks. Variance composition partitions into two profiles: debate is charge-dominated (brittle-fracture-like: the material grade decides), false-presuppositions and ethical-setting are topic-dominated (creep-like: the load decides); the ratios (2.03 vs 0.13/0.17) are estimator-dependent, for debate even in direction. Cross-judge reliability (GPT-4o vs Haiku 4.5) shows debate scoring is judge-robust (Cohen's $\kappa = 0.88$) while false-presupposition scoring is judge-sensitive ($\kappa = 0.36$) – a caveat single-judge benchmarks must report. This is the methodological move Ye et al.'s diagnosis calls for: a multi-axis characterization that does not depend on which surface form of the construct one privileges.
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Materials Science · Cross-listed from cond-mat.mes-hall

Interfacial Magnetotransport in a NiI$_2$/Graphene Heterostructure

Stasiu Thomas Chyczewski, Xiaotong Xu and Wenjuan Zhu

We investigate magnetotransport in a van der Waals heterostructure composed of monolayer graphene and the insulating helical antiferromagnet nickel iodide (NiI$_2$). While NiI$_2$ is highly resistive and thus poorly suited for direct transport measurements, we demonstrate that magnetotransport in an adjacent graphene layer provides an electrical readout of magnetic-state-dependent interfacial behavior. Most notably, first-harmonic longitudinal magnetoresistance under in-plane magnetic fields exhibits large, anisotropic low-field peaks that are absent from a monolayer graphene/h-BN control device and are suppressed above the multiferroic transition temperature of NiI$_2$. Temperature-dependent harmonic measurements provide complementary evidence: the second-harmonic resistance shows the clearest nonlinear contrast relative to the control device, while the third harmonic contains a larger generic nonlinear and thermal background that is nevertheless modified in the heterostructure. These results demonstrate that graphene-based transport measurements offer a sensitive, non-invasive probe of magnetic phase behavior in electrically insulating van der Waals magnets, opening routes toward spintronic devices based on insulating vdW multiferroics.
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Materials Science

Moiré trapping of quadrupolar excitons in van der Waals trilayers

Indrajit Maity, Arash A. Mostofi, Ángel Rubio et al. (+1 more)

Quadrupolar excitons in van der Waals heterostructures - quantum superpositions of anti-aligned dipolar excitons - offer a novel platform to explore exotic many-body physics, with applications to quantum sensing and photonic devices. Yet their internal structure, symmetry, and real-space localisation remain largely unknown. Here, we reveal the atomic-scale structure of quadrupolar excitons in twisted WSe2/WS2/WSe2 trilayers by solving the Bethe-Salpeter equation within a large-scale atomistic framework. We discover that large atomic relaxations at small twist angles give rise to two distinct quadrupolar excitons trapped at moiré lattice sites, differing in the in-plane symmetry of the electron density about the hole: one azimuthally symmetric, with the density maximal at the hole, and one threefold symmetric, with a node at the hole. Moiré trapping, neglected in commonly used models of quadrupolar exciton formation, is critical to their many-exciton phases. Without moiré trapping, quadrupolar excitons transition into anti-parallel dipolar excitons on a bipartite square lattice, while with trapping, the same dipoles are confined to a triangular lattice and experience geometric frustration. Our study uncovers the highly non-trivial nature of quadrupolar excitons, with direct implications for simulating frustrated quantum magnetism in a fully tunable excitonic platform.
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All authors Indrajit Maity, Arash A. Mostofi, Ángel Rubio, Johannes Lischner

Materials Science

Lattice Matching Dictates the Growth Mode and Quality of Deuterium Crystallization in Confined Spherical Shells

Peng Bi, Yu-Shen Wan, Wei Zhang et al. (+3 more)

Cryogenic hydrogen isotope fuel layers with high structural integrity and atomic-scale smoothness are prerequisites for symmetric implosion and ignition in inertial confinement fusion (ICF). Using deuterium (D$_2$) as model fuel, we perform large-scale molecular dynamics simulations with a Feynman-Hibbs corrected Silvera-Goldman potential to describe nuclear quantum effects at low temperatures, systematically investigating D$_2$ crystallization inside spherical ablator capsules. By varying substrate lattice constant from 3.1 angstrom to 3.9 angstrom, we demonstrate that lattice matching dictates the transition from coherent epitaxial growth to polycrystalline formation, establishing it as the primary design principle for high-performance targets. When the substrate lattice closely matches the equilibrium hexagonal-close-packed (HCP) spacing of cryogenic D$_2$ (approximately 3.5 angstrom), D$_2$ forms coherent layer-by-layer epitaxial growth consistent with Ostwald's stepwise nucleation theory, yielding HCP-dominated near-single crystals with minimal dislocations and ultra-smooth inner surfaces. In contrast, large lattice mismatch destabilizes coherent growth and causes island-like growth, producing polycrystalline structures with mixed HCP/FCC phases, elevated defects, and greatly increased surface roughness. Radial stress analysis shows that interfacial stress from mismatch localizes within 2-3 molecular layers near the interface, triggering subsequent defect-mediated growth. These findings highlight substrate lattice matching in regulating confined solid growth and crystallization quality, establish it as a key principle for ablator inner-surface engineering in ICF cryogenic targets, and offer atomic guidance for growing high-quality single-crystal deuterium-tritium (DT) fuel layers with optimal smoothness.
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All authors Peng Bi, Yu-Shen Wan, Wei Zhang, Jian Chen, Yong Yi, Qi-Feng Chen

Materials Science

Atom Probe Tomography as an Emerging Tool for Understanding Defect-driven Mechanisms in HfO$_{2}$-based Ferroelectrics

Kasper A. Hunnestad, Catherine Dubourdieu, Alexei Gruverman et al. (+1 more)

HfO$_{2}$-based ferroelectrics are essential for the next generation of CMOS-compatible memory and logic devices, yet their performance is governed by a complex interplay between oxygen vacancies, dopants, and structural defects that remains an active area of investigation. These defects shape the function-critical dynamic phenomena, such as polar phase stabilization, wake-up, fatigue, and imprint. In this Perspective, we review the limitations of established high-resolution structural characterization techniques and propose atom probe tomography (APT) as a powerful tool for the three-dimensional (3D), atomic-scale mapping of all constituent species in hafnia-based ferroelectric systems. By resolving individual dopants, vacancy clustering, and interfacial segregation, APT can facilitate a quantitative understanding of defect-property relations in hafnia-based ferroelectrics. We discuss current experimental challenges for APT application to ferroelectric oxides, demonstrate a proof-of-concept of atomic-scale reconstruction in a hafnia-based device stack, and highlight the potential of APT to guide the development of ferroelectric structures with enhanced reliability and performance.
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All authors Kasper A. Hunnestad, Catherine Dubourdieu, Alexei Gruverman, Dennis Meier

Materials Science

Twistronic control of shift current in multilayer moiré system

Michele Bagaglini, Cesare Tresca, Federico Bisti et al. (+1 more)

The bulk photovoltaic effect in non-centrosymmetric materials provides an alternative mechanism for the conversion of light into a current response compared to p-n junctions. Among its various contributions, the shift current is particularly attractive because it is governed by the geometric properties of electronic wavefunctions and can generate large photocurrents in low-dimensional materials. Here, we investigate the evolution of the shift current response in mono-, bi-, and trilayer H-MoS2, as well as in twisted moiré bilayers and trilayers. To describe large moiré supercells we develop a Slater-Koster tight-binding model parametrized from first-principles calculations. The resulting electronic structures and shift-current responses are compared with density functional theory calculations and Wannier-interpolated results to verify the accuracy of the approach. The model accurately reproduces the electronic structure near the band edges and captures the main spectral features of the shift current conductivity. We show that twisting breaks the crystal symmetry and activates additional conductivity tensor components that are forbidden in untwisted structures, leading to new tunable in-plane photocurrent components. Analysis of the shift distance reveals a direct connection between the twist-induced modification of the electronic wavefunctions and the increase of the nonlinear response. Our results establish the twist angle as an effective parameter for engineering shift current generation in multilayer transition-metal dichalcogenide base systems and demonstrate that tight-binding approaches provide a practical route for exploring nonlinear optical phenomena in large-scale moiré materials beyond the limits of conventional first-principles calculations.
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All authors Michele Bagaglini, Cesare Tresca, Federico Bisti, Gianni Profeta

Materials Science · Cross-listed from cond-mat.str-el

Quantum oscillations in proximity to high-angular-momentum band inversion

Jiahao Yang and Gang v. Chen

Quantum oscillations provide a fundamental probe of electronic structure in magnetic fields, revealing the Fermi surface topology in metals, and/or the quasiparticle properties even in the insulating regimes. Here we study quantum oscillations in minimal models of high-angular-momentum (HAM) band inversion for both a chiral two-band model and a time-reversal-invariant four-band model. In the former case, finite oscillations can appear at the hybridized Chern-insulating regime due to thermal-activated excitations. In the latter case, interference between the two time-reversal-related blocks drives a strong deviation from the Lifshitz-Kosevich form, producing a non-monotonic temperature dependence of the oscillation amplitude and characteristic suppression temperatures whose number follows the angular momentum $l$. These results identify experimentally accessible signatures of HAM band inversion and provide a framework for other discrete-symmetry-related hybridizations and excitonic pairings.
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Materials Science

Melt-Quench Failures and Practical Solutions for Universal Machine-Learning Interatomic Potentials in Amorphous Structure Generation

Shuwei Li, Yuqi An, Xingyu Guo et al. (+2 more)

Generating experimentally relevant amorphous structures via melt-quench molecular dynamics is prohibitively expensive at the first-principles level. Universal machine-learning interatomic potentials (uMLIPs) could accelerate such simulations, but their reliability under non-equilibrium conditions remains unclear. Here, we examine eight leading uMLIPs for generating amorphous IrO2, using this electrocatalytically relevant oxide as a diagnostic case. Under the conventional melt-quench protocol, all models yield unphysically expanded structures with densities of 1-4 g/cm3, far below the ab initio molecular dynamics (AIMD) reference value of 10.04 g/cm3. Comparisons against ab initio references show that accurate energies and forces alone do not ensure stable NPT dynamics; correct energy-volume responses and pressure predictions are also essential. We identify two practical remedies: pressure-targeted fine-tuning and a revised NVT-quench/NPT-equilibration protocol that avoids unphysical volume expansion without additional ab initio training data. Both recover IrO2 densities and local structures consistent with AIMD. Across 30 chemically diverse materials, the volume-expansion failure proves general, and the revised protocol substantially improves density predictions, reducing the AIMD-referenced MAE from 2.46 to 0.35 g/cm3. This work establishes practical validation criteria and simulation strategies for robust uMLIP-driven amorphous structure generation.
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All authors Shuwei Li, Yuqi An, Xingyu Guo, Wenqiang Yang, Zhenbin Wang

Materials Science · Cross-listed from cs.CE

Phase-field analysis of fracture in heterogeneous wellbore systems: effects of casing eccentricity and cement-formation interface strength

Tharunsarathy Sachithanantham, Wasim Niyaz Munshi, Chandrasekhar Annavarapu et al. (+1 more)

Predicting the initiation and propagation of cracks in heterogeneous wellbore systems under complex in-situ conditions remains challenging. We present a hybrid phase-field fracture framework to model crack growth in heterogeneous wellbore systems with weak interfaces. The framework is first validated against benchmark problems with available analytical and numerical solutions. Subsequently, numerical experiments are conducted to isolate the effects of interface strength and casing eccentricity on crack growth. The results show that casing eccentricity strongly influences both the pressure at crack initiation and the resulting crack paths, reducing the crack initiation pressure by up to 30% relative to the concentric configuration. For the examples considered here, beyond a critical eccentricity threshold of 50%, localized shear stresses drive the nucleation of inclined cracks in the rock formation in addition to radial cracking – a failure mode absent in concentric configurations. For sufficiently weak interfaces (i.e., interfaces with 30% of the toughness of the surrounding bulk material), radially propagating cracks in the cement sheath are deflected along the interface rather than penetrating into the rock formation. This deflection delays stress relaxation within the sheath, promotes the nucleation of additional radial cracks, and increases the risk of sustained casing pressure and wellbore failure. Finally, a three-dimensional simulation reveals depth-dependent crack nucleation, stress-shadow effects that suppress full-depth crack growth, and crack coalescence along the cement-rock interface – phenomena that are fundamentally inaccessible under plane-strain assumptions – demonstrating the applicability of the framework to realistic heterogeneous wellbore systems.
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All authors Tharunsarathy Sachithanantham, Wasim Niyaz Munshi, Chandrasekhar Annavarapu, Birendra Jha

Materials Science

Implementation of rotational invariance for first-principles phonons and application to low-dimensional materials

Benoit Van Troeye, Xavier Gonze and Geoffrey Pourtois

The acoustic flexural (phonon) modes of low-dimensional materials should show a quadratic dispersion close to the Brillouin zone center. Any departure from this behavior in Density Functional Theory calculations is typically associated with a breaking of the rotational invariance, with few methods available to correct it. In this work, we reexamine this issue based on a reciprocal-space imposition of this condition, with corrections on the zone-center IFCs and their first and second derivatives with respect to the phonon wavevector. We propose two correction schemes for the short-range part of the interatomic force constants, one based on the Moore-Penrose pseudoinverse, that we implement in the ABINIT software package, and one based on the straight modification of the on-site antisymmetric part of the first-order derivative of the dynamical matrix with respect to the wavevector. We investigate the impact of imposing rotational invariance for different system dimensionalities and pseudopotentials. Finally, we discuss rotational invariance in the context of long-range electrostatics contribution to the IFCs. We observe that the usual treatment of electrostatics is by construction not rotationally invariant. This is found especially critical when correcting the second derivatives of the IFCs.
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Materials Science

Dynamically suppressed lattice rotations in SrTiO$_3$ as a basis for photo-induced ferroelectricity

Huaiyu Hugo Wang, Michael Fechner, Giovanni De Vecchi et al. (+22 more)

Photo-induced ferroelectricity in the quantum paraelectric SrTiO$_3$ involves the dynamical interplay between a coherently driven Ti-O stretching vibration and multiple structural degrees of freedom, including antiferrodistortive rotations, strain, and the polar mode instability. In the high-temperature cubic phase, in the absence of average antiferrodistortion, time-resolved X-ray diffuse scattering has evidenced a correlation between a photo-induced reduction in antiferrodistortive fluctuations and the emergence of ferroelectric order. Here, we complement these measurements with time-resolved elastic X-ray diffraction in the low-temperature tetragonal phase, in which antiferrodistortive fluctuations are small but a finite average rotation has set in. In this phase, we observe a long-lived reduction of the equilibrium antiferrodistortive rotation angle. A unified theory of the nonlinear lattice dynamics based on first-principles calculations describes the dynamics in both high-temperature cubic and low-temperature tetragonal phases, providing a basis for light-induced ferroelectricity in SrTiO$_3$.
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All authors Huaiyu Hugo Wang, Michael Fechner, Giovanni De Vecchi, Sylvia L. Griffitt, Gal Orenstein, Jade Stanton, Viktor Krapivin, Man T. Wong, Zhuquan Zhang, Mina Bionta, Vincent Esposito, Meredith Henstridge, Matthias C. Hoffmann, Patrick L. Kramer, Zach Porter, Ryan A. Duncan, Takahiro Sato, Soyeun K. Kim, Hasan Yavas, Samuel Teitelbaum, Keith Nelson, Ankit S. Disa, Michael F"orst, Mariano Trigo, Andrea Cavalleri

Materials Science · Cross-listed from cond-mat.str-el

Orbital-selective band evolution and out-of-plane correlation in the FeGe-family kagome antiferromagnet ScFe$_6$Ge$_6$

Jae Hyuck Lee, Ze Yan, Tongrui Li et al. (+10 more)

In strongly correlated materials such as high-temperature superconductors, the relation between charge density wave (CDW) order and magnetism remains an important unresolved problem. FeGe is the first kagome metal known to exhibit CDW order deep within an antiferromagnetic state, accompanied by an unconventional evolution of lattice symmetry. To elucidate the general conditions governing such spin-correlated CDW order, we investigate ScFe$_6$Ge$_6$, which lacks CDW order and therefore exhibits reduced involvement of charge degrees of freedom while retaining other properties of FeGe. Instead of a CDW order, ScFe$_6$Ge$_6$ undergoes a magnetic transition at $T^*$ = 195 K. Across this transition, angle-resolved photoemission and Raman spectroscopy reveal orbital-selective behavior confined to a single kagome Dirac band, together with electron-phonon and magnetoelastic coupling to an out-of-plane phonon mode. These results suggest that orbital-selective physics and out-of-plane correlations play enhanced roles in realizing spin-correlated CDW order in magnetic kagome metals.
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All authors Jae Hyuck Lee, Ze Yan, Tongrui Li, Yichen Yang, Dirk Wulferding, Jongkeun Jung, Zhicheng Jiang, Mao Ye, Zhengtai Liu, Changyoung Kim, Soohyun Cho, Yanfeng Guo, Dawei Shen

Materials Science · Cross-listed from cond-mat.str-el

Evaluating the Structural Basis for Polar Altermagnet Candidate Ca$_{3}$(Ru,Ti)$_{2}$O$_{7}$

Akash Saha, Yihuang Xiong, Vladimir A. Stoica et al. (+15 more)

The interplay between polar and altermagnetic orders remains largely unexplored in the broader landscape of correlated electron systems. Ca$_{3}$Ru$_{2}$O$_{7}$ has been proposed by density functional theory (DFT) as a polar altermagnet, reliant on the transformation of experimentally reported $Bb2_{1}m$ phase to a lower symmetry $Pn2_{1}a$ structure. Here, we perform a targeted search for the $Pn2_{1}a$ phase using synchrotron X-ray diffraction on single crystals of Ca$_{3}$Ru$_{2}$O$_{7}$ and Ca$_{3}$(Ru$_{0.99}$Ti$_{0.01}$)$_{2}$O$_{7}$. No diffraction signature of the $Pn2_{1}a$ structure is detected down to 20 K within experimental limits of $\sim$60-200 fm atomic displacements, significantly smaller than the DFT prediction of $\sim$1 pm. Combined with recent nonlinear transport measurements, our structural study suggests Ca$_{3}$Ru$_{2}$O$_{7}$ as a unique system where strong electron correlations drive an electronic phase transition without any measurable lattice symmetry change. With Ti substitution exceeding $\sim$3%, a chemically tunable altermagnetic phase with $Bb2_{1}m$ structure emerges. The study highlights the importance of sub-picometer metrology towards de-convolving structural versus electronic origins of altermagnets.
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All authors Akash Saha, Yihuang Xiong, Vladimir A. Stoica, Subin Mali, Aaron Pearre, Saugata Sarker, Huaiyu Wang, Yufei Zhao, Evguenia Karapetrova, Yu Wang, Jadupati Nag, Zachary W. Hazenstab, Seng Huat Lee, Long-Qing Chen, Geoffroy Hautier, Binghai Yan, Zhiqiang Mao, Venkatraman Gopalan

Materials Science

InvDesMobility: a reliability-gated first-principles feedback framework for closed-loop materials discovery

Wen-Kao Li, Ze-Feng Gao, Peng-Jie Guo et al. (+2 more)

Inverse materials design starts from target functionality and searches for structures that can realize it. Its value in closed-loop discovery depends not only on prediction performance, but also on whether expensive first-principles results are independently validated, provenance-recorded, and admitted as feedback only when evidence is sufficient. This is especially important for composite properties such as carrier mobility, where a final scalar value hides intermediate quantities, fit quality, convergence history, and workflow assumptions. Here we present InvDesMobility, a reliability-gated first-principles feedback framework that integrates multi-agent automated DFT, evidence stratification, generative structure proposal, acquisition ranking, and auditable release. Using 516 2DMatPedia-derived candidates, the workflow produced 280 QC-passed materials and 573 retained carrier-direction seed channels after channel-level reliability gating. These records were split into two feedback objects: relaxed structures updated the generative model, while retained mobility channels trained the acquisition model and set validation priority. Over multiple iterations, InvDesMobility screened 2.4 x 10^6 structures, submitted 102 candidates for DFT validation, and retained 86 reliability-gated generated channels across 41 formulas. Overall, the main contribution is not a fixed list of high-mobility materials, but a transferable feedback contract that makes closed-loop inverse design both useful and auditable when learning from expensive calculated properties. All source data, retained feedback records, and workflows are available at https://github.com/DreamLufei/invDesMobility, with an accompanying evidence website at https://dreamlufei.github.io/invDesMobility/.
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All authors Wen-Kao Li, Ze-Feng Gao, Peng-Jie Guo, Wei Ji, Zhong-Yi Lu

Materials Science

Collective relaxation eigenmodes and anisotropic magnon thermal transport in $\alpha$-MnTe

Rui-Jie Yao, Lei Wang and Bo-Ye Sun

The intrinsic magnon thermal transport in the altermagnet $\alpha$-MnTe is studied by solving the three-dimensional linearized Boltzmann transport equation with the four-magnon collision matrix. Diagonalizing the collision matrix gives direct access to the relaxation eigenmodes beyond the relaxation-time approximation. We show that Umklapp scattering lifts the momentum-related zero modes of the Normal-only collision operator and substantially modifies the low-lying relaxation spectrum. Using the same collision matrix, we compute the magnon thermal conductivity tensor. The full linearized Boltzmann transport equation result exceeds the relaxation-time approximation by more than an order of magnitude at low temperature and reveals a strong transport anisotropy, with the out-of-plane thermal conductivity remaining larger than the in-plane component over the studied temperature range. A mode-resolved analysis shows that the dominant heat-carrying modes retain momentum-like character inherited from the Normal-only zero modes, and that the larger out-of-plane conductivity mainly originates from the stronger out-of-plane group-velocity contribution, rather than from a large difference in relaxation lifetimes.
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Materials Science

Optically Active Fractional Wannier-Center Displacement Drives Giant Second-Harmonic Generation

Hongsheng Pang and Lixin He

Electric polarization is a static ground-state Berry-phase property, whereas second-harmonic generation (SHG) and shift current are dynamical optical responses. Their connection is encoded in the shift vector, whose Brillouin-zone average is governed by the band-resolved Berry-phase polarization difference between the optically connected initial and final states. Here we exploit this geometric relation in quantized formal polarization (QFP) crystals, where symmetry-quantized formal-polarization branches correspond to fractional Wannier-center sectors. First-principles screening identifies noncentrosymmetric QFP materials with giant SHG responses, including $\mathrm{InNbBr}_6$ and $\mathrm{InPS}_3$. Band-resolved Berry-phase analysis shows that their dominant optical transitions connect occupied and low-lying unoccupied states whose Wannier centers lie at distinct fractional Wyckoff positions, producing a large transition-resolved Wannier-center displacement. This displacement gives rise to a large shift vector and a dominant shift-vector-related intraband contribution to the static SHG susceptibility. Our results show that symmetry-quantized formal polarization can become optically active through transitions between fractional Wannier-center sectors, providing a symmetry-guided route to giant SHG and shift-current responses.
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Sunday 14 June

Materials Science · Cross-listed from quant-ph

Adiabatically-induced Kawaguchi geometry and jerk in quantum-classical systems

Athanasios Chatzistavrakidis, Larisa Jonke and Ryan Requist

Adiabatically eliminating the quantum degrees of freedom in a mixed quantum-classical system produces an effective force in the classical equation of motion. The elimination can be made to any order in the adiabatic parameter, generating a series of higher order forces. By applying a sequence of near-identity unitary transformations to the quantum state, we derive a hierarchy of increasingly accurate effective actions for the classical variables. The third order Euler-Lagrange equation is non-Newtonian as the force depends on the jerk, the third order time derivative of position. We find that the third order terms induce a special kind of Kawaguchi geometry on the space of classical variables. This geometry is characterized by an almost symplectic structure and a differential line element that depends on the acceleration in addition to the velocity. Our results can be used to efficiently capture higher order nonadiabatic effects in molecular dynamics simulations.
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Materials Science · Cross-listed from quant-ph

Quantum Algorithm for Open-System Battery Cathodes by Modeling Multiple Strongly Coupled Holstein Polarons with Chain-Mapped Caldeira-Leggett Dynamics

Joshua M. Courtney

Cathode lithiation occupies a chemical regime of tightly localized orbitals, narrow bandwidths, and strong electron-lattice coupling. The defining electrochemical observables (open-circuit voltage and differential capacity) are open-system, reservoir-equilibration quantities that closed-Hamiltonian quantum simulation cannot produce, set by exchange with electron, Li$^+$, and phonon baths. We present a fault-tolerant quantum algorithm that recovers them through a unitary chain-mapped Caldeira-Leggett embedding, rendering the baths Trotterizable. The resulting fourth-order Trotter step has a T-gate count polynomial in system size, validating its open-system dynamics against hierarchical equations of motion (HEOM) at strong coupling and the Lindblad limit at weak coupling. For single-carrier olivine LiFePO$_4$, a single voltage anchor on an otherwise DFT-fixed Hamiltonian places the differential-capacity peak within the $\pm5$ mV reproducibility of the experimental plateau. For multi-carrier spinel LiMn$_2$O$_4$, whose $1{:}1$ Mn$^{3+}$/Mn$^{4+}$ filling makes the inter-site Coulomb repulsion dynamically active, the same kernel yields a two-plateau voltage curve with a $125$ mV split, within $17\%$ of the observed $150$ mV. We deliver an end-to-end fault-tolerant resource estimate for such a multi-carrier, three-reservoir observable: $368$ logical qubits and $\sim3\times10^5$ T-gates per step, or $\sim1.7\times10^{12}$ T-gates for a full voltage curve (parallelizable over $\sim10^3$ trajectories), leaving the production-scale dynamical run as a milestone for future hardware. The same kernel reproduces macroscopic quantum coherence, two-band superconductivity, and the Mikheyev-Smirnov-Wolfenstein resonance without modification, placing dynamical battery chemistry and similar Hamiltonians within scope for fault-tolerant quantum simulation.
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Materials Science

Confined Oxygen-Vacancy Migration Drives Ferroelectric Switching

Hyeonhu Bae and Binghai Yan

Conventional ferroelectricity arises from intrinsic lattice distortions, whereas oxygen vacancies are generally regarded as detrimental because their migration induces leakage currents and polarization degradation. However, the recently discovered ultrathin van der Waals ferroelectric Bi2SeO5 exhibits robust out-of-plane polarization switching despite its pristine crystal symmetry forbidding the corresponding displacive ferroelectric instability. Here we show that this apparent contradiction originates from confined oxygen-vacancy migration. We find that oxygen vacancies preferentially form within SeO3 units and undergo reversible low-barrier rearrangements between nearly degenerate configurations. These localized vacancy dynamics generate a large switchable out-of-plane polarization, while long-range vacancy diffusion is suppressed by substantially higher migration barriers. At a representative vacancy concentration of 2.5%, the resulting polarization reaches approximately 16 uC cm^-2, consistent with experiment. Our results identify confined oxygen-vacancy migration as the microscopic origin of ferroelectric switching in Bi2SeO5 and establish defect-enabled ferroelectricity as a general mechanism for layered van der Waals oxides.
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Materials Science · Cross-listed from quant-ph

Learning ground state observables from quantum computing experiments

Ben Jaderberg, Freya Shah, Minjun Jeon et al. (+3 more)

Recent theoretical progress has established conditions under which machine learning models can efficiently predict ground-state properties of gapped local Hamiltonians when trained on quantum-generated data. Previous experimental demonstrations in this paradigm, however, have largely been limited to small systems or highly structured states, due to the difficulty of preparing many-body ground states on quantum processors. In this work, we demonstrate learning from experimental quantum data generated from approximate ground states of the two-dimensional Heisenberg XXZ model with system sizes up to 115 qubits. We construct a dataset of single-site expectation values, two-point correlations, and 12-body loop correlations across the antiferromagnetic phase. We then train neural networks on this data and show that they can accurately predict spatially resolved observables for previously unseen Hamiltonian parameters, both within the training distribution and in an out-of-distribution regime approaching the phase boundary. Our results demonstrate the practical realization of learning from quantum data for an interacting two-dimensional many-body system at scale, motivating a path toward regimes where quantum processors could provide training data beyond the reach of classical approximation methods.
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All authors Ben Jaderberg, Freya Shah, Minjun Jeon, M. Emre Sahin, Christa Zoufal, Kunal Sharma

Materials Science · Cross-listed from physics.chem-ph

High-sensitivity hydrogen gas permeation: system development, sample preparation, and influence of testing variables

R. Li, A. Zafra, Z. D. Harris et al. (+1 more)

There is a need to develop quantitative, high-resolution hydrogen gas (H2) permeation techniques to provide a better understanding of hydrogen-material interactions. This study aims to develop and validate a high-sensitivity H2 permeation system, which is then leveraged to systematically quantify the influence of surface condition and key testing variables (surface oxides, residual gas impurities, pressure, and temperature) on hydrogen permeation. A gas permeation system capable of operating at pressures up to 50 bar and temperatures up to 250 C was developed, incorporating high-sensitivity mass spectrometric detection and controlled surface preparation protocols. Permeation transients were analysed in a model material (annealed pure Fe) to determine hydrogen diffusivity and permeability under systematically varied surface states, oxygen contents, pressures and temperatures. Surface oxides are shown to play a dominant role in controlling hydrogen permeation at room temperature. The presence of oxide layers can severely hinder or completely suppress hydrogen uptake, with measurable permeation requiring oxide removal via pickling and Pd coating on both surfaces, or activation through hydrogen-assisted reduction at elevated temperature. Residual oxygen present prior to hydrogen exposure further reduces permeability by modifying surface boundary conditions, indicating strongly surface-controlled kinetics. Under optimised surface conditions, hydrogen transport follows bulk diffusion-controlled behaviour, with steady-state flux obeying Sieverts' law at 25 C (1-5 bar) and diffusivity and permeability exhibiting Arrhenius behaviour between 25 and 150 C at 5 bar, indicating bulk diffusion-controlled transport. The developed system resolves hydrogen fluxes as low as 1.98x10-9 mol(m2 s) and provides a robust platform for investigating surface, mechanical and environmental effects on H2 permeation.
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All authors R. Li, A. Zafra, Z. D. Harris, E. Martínez-Pañeda

Materials Science · Cross-listed from cond-mat.str-el

Probing Interfacial Magnetic Anisotropy in \texorpdfstring{CoV$_{2}$O$_{4}$}{CoV2O4} using Spin Hall Magnetoresistance

Sairam Ithineni, Krishna Jha, Aditya A. Wagh et al. (+5 more)

Spin Hall magnetoresistance (SMR) has emerged as a powerful probe for investigating interfacial spin transport and magnetic anisotropy in complex oxide heterostructures. In this work, we investigate the interfacial magnetic anisotropy in Pt/CVO through angle-dependent magnetotransport measurements. Unlike the bulk-sensitive magnetic measurements on both strained CVO and Pt/CVO films, which exhibit a ferrimagnetic transition at $T_{C} \approx 150$ K accompanied by out-of-plane anisotropy that reorients toward in-plane anisotropy below 90 K, SMR reveals a distinct interfacial magnetic anisotropy. The rotational scans of the in-plane transverse SMR at 20 K exhibit substantial hysteresis about [100], while no hysteresis is observed along [110] and [1$\bar{1}$0], indicating a biaxial anisotropy with easy axes along [110] and [1$\bar{1}$0]. Furthermore, the absence of sharp discontinuities in both the in-plane longitudinal and transverse SMR, together with pronounced discontinuities near the in-plane [010] direction during out-of-plane rotation, strongly indicates the presence of in-plane anisotropy. This behavior persists up to 120 K. The discrepancy between the bulk-sensitive magnetic measurements and the SMR response suggests that the Pt/CVO interface retains a magnetic anisotropy distinct from the bulk, highlighting the interfacial sensitivity of SMR. Additionally, the spin mixing conductance is found to be of the order of $10^{14}$ $\Omega^{-1}\mathrm{m}^{-2}$, comparable to other oxide-based spintronic systems. These findings highlight the crucial role of interfacial effects in spin transport and establish Pt/CVO as a promising platform for spintronic applications.
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All authors Sairam Ithineni, Krishna Jha, Aditya A. Wagh, Shwetha G. Bhat, Debashree Nayak, K. Senapati, P.S. Anil Kumar, D. Samal

Materials Science

Surface-Sensitive Mapping of Anisotropic Phonon Cascades in T$_{d}$-WTe$_{2}$

Alp Akbiyik, Felix Kurtz, Nandana Veena Uday et al. (+3 more)

Understanding how energy flows from photoexcited carriers into the lattice is essential for describing nonequilibrium phenomena in low-symmetry quantum materials. Here, we use ultrafast low-energy electron diffraction and diffuse scattering to probe momentum-resolved phonon dynamics at the surface of T$_d$-WTe$_2$, a strongly anisotropic semimetal. Following optical excitation, the Debye–Waller suppression of Bragg peaks exhibits a biexponential increase of the mean-squared atomic displacement, indicating sequential lattice relaxation. Analysis of the diffuse background reveals a preferential intensity build-up parallel to the tungsten-chain axis in the material, attributed to anisotropic electron–phonon coupling during electronic cooling which precedes anharmonic phonon–phonon scattering and subsequent thermalization across the surface Brillouin zone. The results identify a hierarchical relaxation pathway in which energy is first deposited into selected finite-momentum phonons before spreading through the broader lattice bath. Our work highlights the importance of momentum-resolved diffuse scattering for disentangling electron–phonon and phonon–phonon relaxation in anisotropic topological semimetals.
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All authors Alp Akbiyik, Felix Kurtz, Nandana Veena Uday, Sergey V. Yalunin, Claus Ropers, Hannes Böckmann

Materials Science

Intrinsic linewidths of confined phonons in few-layer hBN

Aleksandar Radic, Jack Kelsall and Akshay Rao

Understanding lattice vibrations in two-dimensional (2D) materials is essential for controlling thermal transport, mechanical response, and energy dissipation in nanoscale devices. However, the intrinsic lifetimes of low-energy phonon modes, particularly those that are optically silent, remain largely unexplored. Here we use helium-3 spin-echo spectroscopy to resolve low-energy phonons at the surface of hexagonal boron nitride (hBN) and measure their intrinsic linewidths. We observe the flexural and Rayleigh wave modes and extract the bending rigidity of a quasi-freestanding hBN monolayer. We further report the simultaneous observation of multiple surface-confined interlayer shear modes whose energies agree closely with linear-chain model predictions. By resolving their intrinsic linewidths, we demonstrate a strong confinement-induced reduction in phonon lifetimes, with a near order of magnitude increase in linewidth between the four- and two-layer modes. The temperature dependence of the linewidths indicates that phonon-phonon scattering dominates between 160-360K, while the systematic broadening with decreasing layer number reveals the impact of confinement on phonon decay. These results reveal how reduced dimensionality affects the decay of interlayer shearing modes in hBN, providing direct insight into the phonon lifetimes, confinement effects, and dissipation pathways that govern the dynamical behaviour of two-dimensional materials.
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Materials Science · Cross-listed from cond-mat.str-el

Stabilizing Itinerant Electrons in a Corner-Sharing Kagomé Oxide Nd4Os3ZnO14

Ryutaro Okuma, Yuita Fujisawa, Chia-Hsiu Hsu et al. (+8 more)

Kagome oxides provide a fertile platform for exploring exotic electronic states arising from geometrical frustration and characteristic band topology. Here, we report the synthesis of a 5d transition-metal kagome oxide, Nd4Os3ZnO14, obtained via high-temperature, high-pressure hydrothermal synthesis. Single-crystal X-ray diffraction reveals a two-dimensional kagome network formed by corner-sharing OsO6 octahedra, with a nominal osmium valence of +4.67. In-plane resistivity and hard X-ray photoelectron spectroscopy measurements indicate that the semimetallic electronic structure at room temperature evolves into a semiconducting ground state upon cooling, accompanied by a pronounced enhancement of hole mobility. Magnetic susceptibility measurements demonstrate localized Nd3+ moments without long-range magnetic order down to 2 K. The coexistence of a metallic kagome plane, strong spin-orbit coupling inherent to 5d electrons, and rare-earth magnetism establishes Nd4Os3ZnO14 as a promising platform for investigating correlated electron phenomena in kagome oxides within the itinerant regime.
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All authors Ryutaro Okuma, Yuita Fujisawa, Chia-Hsiu Hsu, Keita Kojima, Daisuke Nishio-Hamane, Kazuki Sumida, Akio Kimura, Akira Yasui, Kohei Yamagami, Jun-ichi Yamaura, Yoshihiko Okamoto

Materials Science · Cross-listed from cond-mat.mes-hall

Mapping the Stability of Spin Qubits in Superconducting Pseudogap Systems

Chen-How Huang and Miguel A. Cazalilla

Superconducting spin qubits, realized as Yu-Shiba-Rusinov spin-doublet states in quantum-dot-superconductor systems, represent a cornerstone of current research in quantum technologies. We analyze these ground states of quantum impurities in superconducting pseudogap systems, namely systems with a pseudogap tunneling density of states $\rho(\epsilon) \sim |\epsilon|^r$ for energies $|\epsilon|\gg \Delta$ ($\Delta$ being a $s$-wave pairing potential). For $r=1$, these hosts are realized as Dirac materials (graphene or 3D topological insulator surfaces) in proximity to conventional superconductors, or as $d+i s$ superconductors. Using effective field theory and numerical renormalization group, we map the phase diagram against the pseudogap exponent $r > 0$ and particle-hole symmetry-breaking perturbations. At particle-hole symmetry, increasing $r$ also increases the critical value, $J_c$, of the Kondo coupling that triggers the transition from spin doublet to singlet. Unlike the gapless pseudogap Kondo systems, numerical and analytical evidence suggest that Andreev reflection stabilizes a singlet ground state at $J$ for all $r > 0$. Breaking particle-hole symmetry – by potential scattering or chemical potential – eventually restores the transition at lower $J_c$. Our results indicate that coupling to superconducting hosts with large pseudogap exponents enhances the stability of spin qubits at large Kondo coupling.
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Materials Science

Van der waals engineering of valley polarization in WSe2 via kagome V2O3 monolayer heterostructure through magnetic proximity effect

Fazle Subhan

To achieve a robust valley splitting in transition metal dichalcogenides (TMDs) at zero magnetic field is a challenging and elusive goal for valleytronic and spintronic devices. Therefore, in the current study, using the first principles calculations, we introduce an oxide-driven valleytronic platform by employing a two-dimensional ferromagnetic oxide (V2O3) as a magnetic proximity partner for WSe2. We found that the intrinsic ferromagnetism of V2O3 induces a prominent and spontaneous valley splitting of ~ 10.41 meV in WSe2. This pronounced valley polarization mainly originates from the strong interfacial exchange coupling interaction and charge redistribution mediated by V 3d electrons, coupled with the intrinsic spin-orbit coupling of WSe2. Interestingly, this value is significantly greater than the previously reported value for CrI3/WSe2, which corresponds to an effective magnetic field of ~ 10 T. Besides, we also have a high Curie temperature of 500 K, and an out-of-plane magnetic anisotropy energy (MAE) of 0.31 meV, indicating that this oxide-based heterostructure can also be used for near-room temperature operation. Importantly, under the external electric field with a step of 0.1 eV/Å, the ferromagnetism is preserved and an enhancement in Curie temperature and MAE makes this oxide-based heterostructure more valuable for the next generation valleytronic devices. Therefore, these findings establish a new paradigm for realizing tunable, robust, and magnetic field-free valleytronic and spintronic devices for this oxide-based heterostructure. Moreover, this concept can be generalized to other correlated oxide-based TMD systems, providing a versatile strategy for next-generation quantum and functional materials.
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Materials Science

Carbon Layer Orientation and Closed-Pore Construction Achieving Ultra-Low Specific Surface Area Hard Carbon for High-Performance Na-ion Storage

Bowen Wang, Zihan Yang, Minghui Zhao et al. (+9 more)

Addressing the critical trade-off between initial Coulombic efficiency (ICE) and reversible capacity in hard carbon anodes for Na-ion batteries (NIBs), we introduce a novel coupling strategy that combines carbon layer orientation reconstruction with closed-pore construction to produce hard carbon with an ultra-low specific surface area. We demonstrate that the nanographite domains within the hard carbon precursor undergo entropy-driven orientation reconstruction through the synergistic regulation of heteroatom doping and medium-temperature carbonization. This process not only increases interlayer spacing and promotes structural disorder but also enables the formation of dense, closed pores and ultramicropores at domain boundaries via confined atomic migration, while simultaneously encapsulating surface open pores within internal closed ones. Due to this unique pore architecture, our hard carbon exhibits an ultra-low specific surface area of 1.89 m2 g-1 with a markedly higher proportion of closed pores. As a result, our hard carbon achieves a remarkable reversible capacity of 342.3 mAh g-1 at 20 mA g-1, with an exceptional ICE of 90.4% and a dominant plateau capacity of 262.3 mAh g-1 (76.6%) for NIBs. We believe this coupling strategy provides a new paradigm for the structural engineering of high-ICE anode materials in advanced NIBs.
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All authors Bowen Wang, Zihan Yang, Minghui Zhao, Wenjie Mai, Qing Xu, Huan Li, Liang Zhang, Chul Gyu Jhun, Le Chen, Wentao Zhang, Jingtai Zhao, Jinliang Li