Materials Science
Vibrational spectroscopy identifies the bond asymmetry of hexagonal diamond
Li Zhu
Bulk hexagonal diamond has been synthesized by independent routes, but its structure remains contested: the two recent refinements disagree even on the sign of the difference between its two inequivalent bond lengths, 238~mÅ apart, and both depart from an earlier 2003 refinement. Here we test the competing structures with first-principles
lattice dynamics. Relaxed hexagonal diamond has an interlayer bond longer than the intralayer bonds by 24~mÅ in both functionals, an effect of its eclipsed conformation that scales with polytype hexagonality. The bright zone-center $A_{1g}$ mode gauges the interlayer bond at $\approx\!-2{,}100$~\icm~Å$^{-1}$, and neither refined coordinate reproduces the full pattern of measured modes. The only structure matching the twinned sample's three bands requires tens-of-gigapascals confining stress and
lattice constants excluded by its own diffraction. Raman spectroscopy and diffraction jointly select a small positive bond asymmetry: inverting the spectrum of the phase-pure sample gives $\OB-\OA=24\pm3$~mÅ (95% interval), and two determinations on separate samples give $+33\pm8$ and $+60\pm45$~mÅ. The 1{,}529~\icm{} feature cannot be assigned to homogeneous ideal 2H diamond, and the local HRTEM observation remains an open puzzle.
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Materials Science
Wrinkling of Randomly Heterogeneous Film-Substrate Systems
Xinyu Xing, Liyu Zhong, Feng Deng et al. (+1 more)
Wrinkling instabilities in stiff films on compliant substrates are strongly affected by spatial fluctuations in film stiffness. We develop a homogenized instability theory for one-dimensional film–substrate systems with random bending stiffness. The heterogeneous stability equation is reformulated as a Lippmann–Schwinger equation for the
curvature field, and a strong-contrast expansion is derived using a local–nonlocal kernel decomposition and a cavity-field formulation. Truncation at third order yields an effective polarizability and a Dyson-type dispersion relation for predicting the critical load and wavenumber. The stiffness is modeled as an exponentially mapped Gaussian random field, allowing the required two- and three-point connected statistics to be obtained analytically. The theory is validated against generalized
eigenvalue calculations and
Fourier spectral simulations. Increasing stiffness contrast lowers the critical load and shifts the instability toward higher wavenumbers, producing shorter wrinkles. At weak contrast, the threshold follows the universal scaling $N_c^{(0)}-N_c\sim\varepsilon^2$, whereas at moderate and strong contrast the third-order approximation is more accurate than the second-order theory. Wavelength selection is controlled by the ratio of the dominant material wavelength $\lambda^\ast$ to the harmonic-mean reference wavelength $\lambda_H$. For $\lambda^\ast/\lambda_H<1$, the harmonic-mean model accurately predicts the wrinkle wavelength. The framework provides a mechanics-based tool for reliability assessment and design of statistically heterogeneous film–substrate systems.
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All authors Xinyu Xing, Liyu Zhong, Feng Deng, Sheng Mao
Materials Science
AC Field-driven orientational crossover and energy dissipation in suspended magnetic nanoparticles
Iago López-Vázquez, Siraj Ul Haq, Kazuya Okada et al. (+4 more)
By combining the Landau–Lifshitz–Gilbert equation with Brownian rotational dynamics of
magnetic nanoparticles (MNPs), we theoretically investigate the role of particle rotation through easy-axis reorientation in
magnetic fluid hyperthermia (MFH). Our results reveal a field-driven crossover in the stationary orientation of the easy axes, from predominantly perpendicular to predominantly parallel or antiparallel to the applied field as the field amplitude increases. Although the precise crossover field depends on particle size and excitation frequency, it occurs at approximately $0.5H_k$, where $H_k$ is the uniaxial anisotropy field. These orientational regimes are directly linked to the underlying microscopic dynamics and the associated MFH performance through the occurrence of switching and non-switching hysteresis cycles, predominantly associated with Néel magnetization reversal and Brownian particle rotation, respectively. The relative importance of these dissipation mechanisms also depends on frequency: at $f=1$ MHz, Brownian heating dominates at low field amplitudes, whereas Néel heating dominates at high fields. By contrast, at $f=100$ kHz, both contributions remain comparable over most of the investigated field range.
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All authors Iago López-Vázquez, Siraj Ul Haq, Kazuya Okada, Sergiu Ruta, Roy W. Chantrell, Òscar Iglesias, David Serantes
Materials Science
Centimeter-scale fully suspended metal and metal oxide thin films by one-step transfer-free liquid metal capillary forming
Chunlei Song, Zhenqi Guo, Yuanting Su et al. (+4 more)
Fully suspended thin films can decouple substrate effects and provide additional tuning degrees of freedom compared with their substrate-supported counterparts, making them unique platforms for next-generation thin film devices. Here we report one-step, transfer-free and substrate-free fabrication of centimeter-scale ultrathin fully suspended metal and metal oxide film structures via
liquid metal capillary forming. We show that, analogous to soap film formation, the instantaneously developed few-nanometer-thick native surface oxide can laminate various
liquid metals into micrometer-thick metallic films. Surprisingly, the surfactant-like metal oxide bilayer can survive dewetting-induced
liquid metal drainage, forming suspended two-dimensional films featuring an enormous lateral size-to-thickness ratio on the order of 10^7. We further demonstrate rapid prototyping of metallic minimal-surface thin-walled structures and ultra-sensitive acoustic wave detection with these suspended thin film platforms.
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All authors Chunlei Song, Zhenqi Guo, Yuanting Su, Changren Tian, Yeqi Zhu, Liang Lei, Jianbo Tang
Materials Science · Cross-listed from cond-mat.str-el
Magnetic properties of a quasi-two-dimensional spin-1/2 antiferromagnet Y2CuGe4O12
J. Khatua, Changhyun Koo, Suyoung Kim et al. (+7 more)
Competing
magnetic interactions and frustration-induced
quantum fluctuations in spatially anisotropic low-dimensional magnets often give rise to exotic
magnetic phenomena, including field-induced phases. Here, we present
crystal structure,
magnetic susceptibility, specific heat, and electron
spin resonance (ESR) measurements on polycrystalline Y$2$CuGe$4$O${12}$, supported by density functional theory (DFT) calculations. In this compound, the Cu$^{2+}$ ions form a distorted triangular
lattice with competing intraplanar ferromagnetic ($J_1 \approx 0.138$ K and $J_2 \approx 0.01$ K) and antiferromagnetic ($J_3 \approx -3.22$ K) exchange interactions, together with a weaker interplanar antiferromagnetic coupling ($J_4 \approx -1.56$ K). These interactions account for the small Curie–Weiss temperature, $\theta{\rm CW}=-1.8$ K. Despite the dominant antiferromagnetic interactions, no signature of long-range
magnetic ordering is observed down to 0.4 K. Instead, broad maxima in both the
magnetic susceptibility and
magnetic specific heat reveal the development of short-range
spin correlations, further supported by the critical ESR linewidth broadening characteristic of low-dimensional frustrated magnets. Application of an external
magnetic field progressively suppresses the broad maximum in the
magnetic specific heat, reflecting competition between the Zeeman and exchange energy scales, and drives the system into a field-polarized state above the saturation field, $\mu_0H_{\rm s}=2.6$ T. In this regime, the
magnetic specific heat exhibits an exponential temperature dependence, consistent with gapped
magnon excitations. These results establish Y$_2$CuGe$4$O${12}$ as a rare distorted triangular-
lattice magnet in which further-neighbor exchange interactions dominate the
magnetic behavior, providing a promising platform for exploring frustration-driven
quantum phenomena.
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All authors J. Khatua, Changhyun Koo, Suyoung Kim, Eundeok Mun, Yugo Oshima, V. K. Sahu, Heung-Sik Kim, B. Koteswararao, Kwang-Yong Choi, P. Khuntia
Materials Science · Cross-listed from cond-mat.str-el
Study of the Anomalous Hall effect by tuning the spin orientation in the Altermagnetic material CrSb
Sreedevi Chintalapudi, Upasana Agrawal and Suvadip Das
Recent development in the field of altermagnetism, and increased demand for the search of applications of anomalous hall effect have ushered in a new era for novel
quantum phases in materials.
Quantum materials previously anticipated to be scientifically predictable have unfolded novel properties that brought them into the spotlight. These manifestations have led us to rethink our understanding of existing classification of
magnetic materials and preexisting notions about anomalous hall effect in the light of topologically nontrivial phases of matter. One such recent de- velopment lies in the novel class of alter-
magnetic materials with prospect for
quantum computing. In this article, we delineate the
spin and orbital resolved electronic spectrum, mode-decomposed
phonon dispersion relations, geometrical berry
curvature and topological surface states and their implications on anomalous Hall conductivity in the promising altermagnetic compound CrSb. We further utilize first principles calculations coupled with computationally efficient maximally localized wannier states of numerous
magnetic configurations of the altermagnet to simulate the effect of external fields and elucidate the fact that the linear behaviour of anomalous hall conductivity with magnetization does not necessarily hold true for all
magnetic classes, such as altermagnets.
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Materials Science
Hidden chiral signatures in ferroaxial K2Zr(PO4)2
Nora Taufertshöfer, Awadhesh Narayan and Nicola A. Spaldin
We use first-principles calculations and multipole analyses to demonstrate the relationship between ferroaxiality and chirality in the prototypical ferroaxial material K2Zr(PO4)2. Using the atomic-site electric toroidal monopole as a measure of electronic chirality in real space, we show that, while the paraxial phase of K2Zr(PO4)2 is non-chiral, the ferroaxial phase is antiferro-chiral. By applying an electric field, we induce a ferri-chiral state with net electronic chirality which is opposite for opposite underlying ferroaxial domains and can be tuned by the direction and strength of the electric field. Associated with the real-space induced chirality, we find a distinct response in momentum space, with induced non-zero components in the Berry
curvature dipole tensor that switch sign between opposite ferri-chiral domains. Our findings therefore reveal hidden chirality in ferroaxial materials in both real and momentum space.
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Materials Science
Ordered-to-disordered transfer learning with graph neural networks for formation-energy and HOMO-LUMO gap prediction in high-entropy perovskite oxides
Panupol Untarabut, Narjes Jomaa, Sylvian Cadars et al. (+4 more)
High-
entropy perovskite oxides (HEPOs) represent a chemically complex class of materials with promising functional properties, yet their vast compositional space and, chemical/structural disorder pose significant challenge for accurate property prediction. Graph neural networks (GNNs) enable rapid exploration of materials space but are often limited by the availability of representative training data. Here, we investigate ordered-to-disordered transfer learning using GNNs for formation-energy and HOMO-LUMO gap prediction in HEPOs by transferring knowledge learned from chemically ordered perovskites. Four representative GNN models, including CGCNN, GATGNN, ALIGNN and M3GNet are evaluated to understand the role of structural representations, spanning pairwise two-body and angular three-body interactions in transfer performance. We find strong property-dependent transfer behavior: formation-energy prediction transfers effectively to disordered HEPOs, whereas HOMO-LUMO gap prediction shows limited transferability due to its sensitivity to local chemical environments. Incorporating a small HEPO-specific training
dataset substantially improves HOMO-LUMO gap prediction. Representation-level analysis using UMAP further highlights the importance of encoding three-body
geometric information such as in ALIGNN for capturing complex structure-property relationships and improving transferability.
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All authors Panupol Untarabut, Narjes Jomaa, Sylvian Cadars, Olivier Masson, Samuel Bernard, Assil Bouzid, Santanu Saha
Materials Science
Tuneable magnetic behaviour, electronic structure and nitrogen vacancy formation in Gd$_{x}$Sm$_{1-x}$N
Omri Porat, Elma Joshy, Jackson Miller et al. (+2 more)
The rare earth nitrides are the only series of intrinsic ferromagnetic
semiconductors where the interplay of
spin and unquenched orbital angular momentum provides access to a range of
magnetic behaviour. Furthermore, the
magnetic properties can be finely tuned through the combination of multiple lanthanide ions in the nitride. Here we present a combined computational and experimental study on the electronic and
magnetic properties of Gd$_x$Sm$_{1-x}$N and discuss the effect of cation substitution on the internal exchange field and band structure. We find that as the coercive field of Gd$_x$Sm$_{1-x}$N changes over orders of magnitude via cation substitution the internal exchange field changes by $\sim$20%. Control of these material properties is vital in the field of superconducting spintronics. Finally, motivated by an enhanced concentration of nitrogen vacancies in films with higher Sm content, we investigate computationally the formation of nitrogen vacancy defects in Gd$_x$Sm$_{1-x}$N finding that the formation energy is significantly reduced for vacancy sites adjacent to Sm ions rather than Gd ions.
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All authors Omri Porat, Elma Joshy, Jackson Miller, Simon Granville, William Holmes-Hewett
Materials Science
A Unified Symmetry Framework For In-plane Anomalous Hall effect
Xun-Jiang Luo, Yong-Ting Shi, Ding-Fu Shao et al. (+3 more)
The in-plane anomalous Hall effect (IPAHE), driven by an in-plane net magnetization or an applied
magnetic field, challenges the conventional anomalous Hall paradigm. Despite growing interest, a unified
symmetry principle governing these phenomena has remained elusive. Here, we establish a comprehensive
symmetry framework that bridges the
spin space group, which dictates the
magnetic geometry, with the
magnetic space group, which governs the anomalous Hall response. We show that spontaneous IPAHE can emerge in ferromagnets when
spin-orbit-coupling-induced
spin-group
symmetry breaking permits additional net magnetization directions. For field-induced IPAHE, we analyze how an applied
magnetic field reduces the
symmetries of all 122
magnetic point groups and identify 54 groups that support IPAHE. Our framework naturally predicts IPAHE in a broad class of unconventional magnets, including altermagnets and odd-parity magnets. In particular,
symmetry analysis reveals characteristic one-, two-, or three-fold angular harmonics of the Hall conductance under an in-plane rotating field, providing a
symmetry-resolved fingerprint for unconventional magnetism. Using this framework, we screen the MAGNDATA database and identify candidate materials supporting spontaneous or field-induced IPAHE, encompassing ferromagnets, antiferromagnets, and unconventional magnets. Finally, we validate the
symmetry predictions through first-principles calculations for two representative materials.
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All authors Xun-Jiang Luo, Yong-Ting Shi, Ding-Fu Shao, Ping Zhang, Ning Hao, Mingliang Tian
Materials Science · Cross-listed from physics.optics
Ascorbic acid mitigates oxidative structural degradation in bovine spermatozoa: a label-free quantitative phase microscopy study
Harpreet Kaur, Shubham Tiwari, Aman Kumar et al. (+3 more)
Oxidative stress is a key factor in low fertility outcomes during assisted reproduction technology (ART) and contributes to poor sperm quality. Conventional assessment relies on bright-field microscopy, which lacks the quantitative sensitivity to resolve subcellular structural and biophysical changes without exogenous contrast
agents, a requirement that can introduce cytotoxic effects and compromise
cell viability. This study uses QPM as a label-free, high-throughput method to demonstrate how ascorbic acid reduces the impact of oxidative structural degradation in Sahiwal bovine spermatozoa. To illustrate this pathology, severe oxidative stress was experimentally induced using hydrogen peroxide (H2O2), and two doses of ascorbic acid (1 mg/ml and 8 mg/ml) were tested to evaluate dose-dependent antioxidant recovery in structurally damaged sperm. QPM was used to quantify structural changes in key biophysical parameters, including dry mass, optical thickness, volume, surface area, sphericity, and surface area-to-volume ratio, as well as intracellular texture parameters. Under acute oxidative stress, bovine spermatozoa exhibited significant reductions in optical thickness and dry mass, alongside measurable changes in intracellular structural organization, collectively indicative of oxidative stress-induced morphological degradation. Co-treatment with ascorbic acid resulted in partial, dose-dependent structural preservation, with the 8 mg/ml formulation demonstrating statistically significant attenuation of these structural changes compared to the 1 mg/ml treatment group. These findings suggest that QPM-derived biophysical parameters may serve as promising, label-free structural indicators for characterizing oxidative damage in bovine spermatozoa. Future studies should incorporate functional validation to determine whether this structural preservation translates to improved outcomes in ART.
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All authors Harpreet Kaur, Shubham Tiwari, Aman Kumar, Sandeep Kumar Jha, Satish Kumar Dubey, Dalip Singh Mehta
Materials Science · Cross-listed from cond-mat.dis-nn
Boundary Kerr Signatures of the Interband-Coherence Hall Effect
Ivan Iorsh and Mikhail Titov
We identify a Hall response carried by optically induced interband coherence rather than by a non-
equilibrium band population. In a weakly doped zinc-blende
semiconductor, a longitudinal dc field drives a transverse flux of the conduction–valence coherence created by near-gap light. Angular averaging eliminates the homogeneous coherence density, while a lateral boundary converts the transverse flux into an antisymmetric, edge-localized, helicity-odd polarization. The resulting Kerr signal requires neither
spin-orbit coupling nor a
spin, valley, or orbital accumulation within an individual band. Within the eight-band Kane model, we derive the boundary kinetic equation and obtain a complex propagation length controlled by optical detuning and interband dephasing. The edge profile is monotonic at optical resonance and develops damped spatial oscillations away from it. The response is enhanced by electron–hole asymmetry and by strong interband mixing, making narrow-gap
semiconductors especially favorable to observe the effect. These results establish dc-driven Kerr microscopy as a direct probe of an interband-coherence Hall effect and of multiband
quantum kinetics in real space.
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Materials Science
A versatile generalized digital twin for Electron Microscopy
Thomas W. Pfeifer, Andrew R. Lupini and Eric R. Hoglund
Novel experiments in transmission electron microscopy often require the development of specialized imaging and spectroscopy states, such as for flexible momentum-resolved high energy-resolution spectroscopy. This task is complicated by the need to align and configure many different lenses, and by ambiguity around the locations of various imaging and diffraction planes. Here we develop a versatile digital twin for simulating the electron beam trajectory throughout an electron microscope, and we develop the calibration procedures required for accurate prediction of microscope states. This allows the user to quickly and easily determine the correct lens values for setting up new condenser or projector modes. Automatic procedures to change lens settings and measure the resulting changes can be used to close the loop. With the accelerating development of machine learning and artificial intelligence tools, we also believe a physically-informed model of the microscope can serve as a valuable tool for automated microscopy and AI/ML integration.
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Materials Science
Layer- and Field-Dependent Magnetic Order in 2D CrSBr Revealed by Pulsed Nanocalorimetry
Hugo Gomez-Torres, Roop K. Mech, Alessandra Canetta et al. (+9 more)
Understanding the
evolution of
magnetic order in the two-dimensional limit remains a central challenge in van der Waals magnets, where thermodynamic measurements are constrained by the femtogram-scale mass of exfoliated flakes. Here, microsecond pulse-heating nanocalorimetry is used to measure the heat capacity and
magnetic entropy of CrSBr flakes down to the monolayer limit. The measurements reveal the
entropy landscape associated with
magnetic ordering, uncovering a decreasing interlayer transition temperature toward the monolayer limit and an
entropy-derived effective moment per layer that increases with thickness toward the expected
spin-only scale. Thermodynamic anomalies capture a crossover from bulk-like interlayer antiferromagnetism to a regime dominated by intralayer ferromagnetic correlations. A pronounced layer-parity effect further emerges, with odd-layer samples displaying an additional high-temperature contribution associated with uncompensated
magnetic layers. Under in-plane
magnetic fields applied along the easy axis, antiferromagnetic order is progressively suppressed, allowing extraction of a thickness-dependent characteristic suppression field reflecting weakened interlayer exchange coupling.
Entropy analysis further reveals an extended regime of
magnetic fluctuations persisting well above the interlayer ordering transition. Together, these results establish nanocalorimetry as a powerful thermodynamic probe of low-dimensional magnetism, providing direct access to
magnetic entropy, exchange interactions, and dimensional crossover in atomically thin van der Waals magnets.
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All authors Hugo Gomez-Torres, Roop K. Mech, Alessandra Canetta, Llibertat Abad, Carles Navau, Daniel G. Chica, Xavier Roy, Pascal Gehring, Kenji Watanabe, Takashi Taniguchi, Aitor Lopeandia, Javier Rodriguez-Viejo
Materials Science
Resolving competing distortions in Ca0.4Sr0.6TiO3 using complementary electron and X-ray techniques
Robin Sjökvist, Catriona A. Crawford, Richard Beanland et al. (+1 more)
We present a study of the perovskite Ca0.4Sr0.6TiO3 using variable temperature transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). At room temperature and below, PXRD shows that the material adopts an orthorhombic Pbcm structure analogous to the P-phase of NaNbO3. Above 380 K the material transforms to a tetragonal I4/mcm phase. The structural distortions of these phases can be described as a combination of modes and order parameters associated with the M, T, ${\Delta}$ and R-points of the Brillouin zone, each of which can be associated with a different set of superstructure reflections visible in X-ray and electron diffraction patterns. For the I4/mcm phase only the expected R-point reflections are observed in PXRD while both M and R- reflections are observed in electron diffraction. Using ${\Delta}$ and R dark field TEM images we show that the phase transition proceeds by a loss of coherence of TiO6 octahedral tilting along the c-axis, leading to a microstructure of thin nanoscale platelets with a different local
symmetry to the macroscopic structure. These persist well above the phase transition temperature and are probably responsible for the long-standing discrepancy between Raman spectroscopy and diffraction measurements in this materials system, as well as other secondary effects.
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All authors Robin Sjökvist, Catriona A. Crawford, Richard Beanland, Mark S. Senn
Materials Science
Savi-Bhransha: Graph-Theoretic Dislocation-Loop Characterization in Crystals
Utkarsh Bhardwaj
Dislocation loops govern the properties of crystalline materials, but extracting their detailed characteristics from atomistic simulations is difficult when loops are fragmented or embedded in compact defect debris. We present Savi-Bhransha, a graph-theoretic method that reconstructs interstitial and vacancy loops directly from local defect-displacement motifs, without constructing a global interface mesh. The method identifies Burgers-vector family, habit plane, loop size, segment-wise edge/screw character, and boundary and bulk defect populations for BCC, FCC, and HCP
crystals. We apply it to single-cascade simulations over a range of energies in BCC W and HCP Zr, and to successive collision cascades in BCC W and FCC FeNiCr. We
benchmark the method against the Dislocation Extraction
Algorithm (DXA). Total dislocation lengths remain strongly correlated between the two methods, while Savi-Bhransha returns more stable loop-level objects in complex environments where DXA returns fragmented, overlapping open segments. Savi-Bhransha also better resolves mixed-morphology defects and dislocations near other defects, including vacancy clusters. Median runtime speedups are 6.34x for BCC W and 8.86x for HCP Zr, with peak-
memory reductions up to 7.77x. In successive W cascades, the resolved boundary-defect concentration brackets transient-grating-spectroscopy measurements and the predicted Burgers-vector fraction agrees with room-temperature TEM. In FCC FeNiCr, the method resolves Heidenreich-Shockley dissociation, with a Shockley-pair signature in about 91% of surviving <110>-family interstitial clusters. Savi-Bhransha therefore enables efficient,
topology-resolved analysis of large radiation-damage simulations and direct comparison with experimentally accessible observables.
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Materials Science · Cross-listed from cond-mat.soft
Mechanical isotropy of heterogeneous octahedral materials
Jehoon Moon, Seunghwan Lee, Gisoo Lee et al. (+2 more)
An octahedral network has been widely used as a fundamental building block for diverse architected materials. Here, we demonstrate that heterogeneous octahedral materials can achieve complete elastic isotropy at a critical constituent volume fraction, nearly independent of the constituent stiffness ratio. The anisotropy ratio, a, transitions from a > 1 to a < 1 at the critical constituent volume fraction, due to a change in the dominant deformation modes. Microstructural analysis reveals that the geometry and connectivity of the heterogeneous octahedral materials are very similar to those of heterogeneous materials constructed on combined simple cubic (SC) and body-centered cubic (BCC)
lattices exhibiting opposite elastic anisotropy. More importantly, we demonstrate that varying the constituent volume fractions in the octahedral materials governs elastic anisotropy, similarly to tuning the BCC-to-SC composition ratio in the SC-BCC materials widely employed for designing mechanically isotropic architected materials. The mechanical isotropy of the heterogeneous octahedral materials is further assessed using 3D-printed prototypes.
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All authors Jehoon Moon, Seunghwan Lee, Gisoo Lee, Jeongun Lee, Hansohl Cho
Materials Science · Cross-listed from cond-mat.str-el
Beyond Stoner-Wohlfarth: Machine-Learning Models and Symbolic Regression of Hard-Magnet Properties
Samuel J. R. Holt, Christina Winkler, Timoteo Colnaghi et al. (+7 more)
Predicting the extrinsic properties from hysteresis loops of a
magnetic grain, namely the coercive field, remanent magnetisation, and maximum energy product, from its intrinsic micromagnetic parameters is a central problem in permanent-magnet modelling. Established analytical models provide useful estimates but often neglect nonuniform magnetisation processes, whereas direct micromagnetic simulations are computationally expensive. In this work, we train machine-learning models on 12012 micromagnetic simulations of an idealised cubic grain, spanning broad ranges of the saturation magnetisation, exchange constant, and uniaxial anisotropy constant. Benchmarked against the analytical models on identical held-out data, the machine-learning models predict all three extrinsic properties with substantially lower errors. Symbolic regression recovers the Kronmüller form of the coercive field, with an effective demagnetising factor that depends on the material, and finds new closed-form expressions for the remanence and maximum energy product. Each law contains at most two fitted constants yet approaches the accuracy of the machine-learning models. We also investigate the inverse problem of recovering the intrinsic parameters from the three extrinsic properties. The saturation magnetisation and anisotropy constant are recovered accurately, whereas the exchange constant is not, because it influences the extrinsic properties only weakly. The trained models are released through the mammos-ai Python package, enabling thousands of candidate parameter sets to be screened in seconds rather than the hours or days required by direct micromagnetic simulation.
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All authors Samuel J. R. Holt, Christina Winkler, Timoteo Colnaghi, Martin Lang, Swapneel A. Pathak, Andrea Petrocchi, Michael Adams, Thomas Schrefl, Andreas Marek, Hans Fangohr
Materials Science
Quadratic piezoelectricity from stacking-engineered interference in multilayer sliding ferroelectrics
Jiaxuan Fan, Yuexing Zhao, Xiao-Ping Li et al. (+3 more)
Designing nonlinear piezoelectricity requires suppressing the linear piezoelectric coefficient without extinguishing higher-order electromechanical response, yet a general and reconfigurable route remains lacking. Here we introduce stacking-engineered piezoelectric interference as such a mechanism in multilayer sliding ferroelectrics. Combining first-principles calculations with a generalized Ginzburg–Landau framework, we show that each interlayer gap acts as a local piezoelectric channel whose sign and magnitude are determined by stacking. Constructive interference between same-signed channels produces a linear-dominated response, whereas destructive interference between oppositely signed channels suppresses the linear coefficient while preserving a finite quadratic response. Representative MoS$_2$ and NiTe$_2$ multilayers approach the parabolic limit, with BAAC-stacked MoS$_2$ reducing the linear-to-quadratic crossover strain by a factor of 25 relative to CBA-stacked MoS$_2$. Experimentally accessible tetralayer MoS$_2$ sliding pathways further connect linear-dominated, quadratic-dominated and sign-inverted states. Here, we identify stacking-engineered interference as a design principle for programmable nonlinear electromechanics in layered materials.
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All authors Jiaxuan Fan, Yuexing Zhao, Xiao-Ping Li, Yurong Yang, Xing-Qiu Chen, Lei Wang
Materials Science
Strength-degradation phase-field regularization of cohesive fracture: the antiplane case
Blaise Bourdin and Corrado Maurini
Phase-field approaches to fracture, initially designed as regularization of the Griffith model of brittle fracture, are now commonly viewed as gradient-damage models whose regularization length becomes a material property driving crack nucleation. One weakness of this approach is that the strength surface cannot be arbitrary: its shape is dictated by the elastic energy, and its magnitude by the regularization length. We focus on the antiplane version of the model introduced by Bourdin, Marigo, Maurini and Zolesi (arXiv:2506.22558), which handles crack propagation along unknown paths and nucleation governed by an arbitrary convex strength surface by degrading the strength instead of the stiffness. It can be interpreted as a regularization of softening plasticity in which localization bands obey an equivalent cohesive law set by the strength domain and the toughness, while the role of the regularization length, when small compared to the elasto-cohesive length, is purely numerical. Strength, stiffness, and toughness thus become independent material data, and limit analysis, perfect plasticity, cohesive fracture, and brittle fracture merge into a single variational framework. We derive closed-form solutions for a simple shear problem, propose a numerical scheme combining alternate minimization and conic programming, and numerically verify the equivalent cohesive law, its independence of the regularization, and the size effect governed by the elasto-cohesive length. A "surfing" simulation highlights the structure of the propagating crack while a re-entrant V-notch is used to show how the model bridges small-scale yielding, cohesive fracture, and brittle fracture without a priori hypotheses.
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Materials Science
Domain-Selective Enhancement of Second Harmonic Generation in Monolayer MoS$_2$ via Ferroelectricity-Controlled Photodoping
David Hernández-Pinilla, Line Jelver, María Jesús Martínez-Morillo et al. (+6 more)
Hybrid
heterostructures combining two-dimensional
semiconductors with ferroelectric materials offer a versatile route to actively control light-matter interactions at the nanoscale. Here, we report all-optical, light-induced domain-selective control of second-harmonic generation (SHG) in monolayer molybdenum disulfide (MoS$_2$) integrated with periodically poled lithium niobate (LiNbO$_3$). Spatially resolved SHG imaging reveals a pronounced modulation of the nonlinear optical response of monolayer MoS$_2$ governed by the ferroelectric domain pattern of the underlying substrate. A strong SHG contrast is observed between domains of opposite polarization, with a marked dependence on both the excitation wavelength and the incident optical power. The comparison between ferroelectric domains that either enable or do not exhibit light-driven photodoping in the MoS$_2$ monolayer provides a direct assessment of the role of carrier density in the nonlinear optical response. We find that ferroelectric-polarization-controlled photodoping at the MoS$_2$/LiNbO$_3$ interface enhances the effective second-order susceptibility, $\chi^2$, producing an increase in SHG intensity of up to ~70% under resonant excitation conditions. Ab initio calculations corroborate that charge doping modifies the electronic band structure of MoS$_2$ and strongly affects $\chi^2$ in the resonant regime, providing microscopic support for the experimentally observed modulation. The results highlight the combination of light intensity and ferroelectricity as a powerful knob for band-structure modulation in 2D materials and reconfigurable nonlinear optical responses, opening pathways toward programmable frequency conversion, smart light modulators, and advanced nonlinear photonic functionalities in integrated hybrid platforms.
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All authors David Hernández-Pinilla, Line Jelver, María Jesús Martínez-Morillo, César Hernando-Fuente, Miquel Cherta, Guillermo López-Polin, Joel D. Cox, Luisa E. Bausá, Mariola O Ramírez
Materials Science · Cross-listed from cond-mat.mes-hall
A low-temperature ultra-high-vacuum scanning probe microscope with in situ electronic transport capabilities: from macro to nano in 10 minutes
Diego Exposito, Oscar Custance and Ivan Brihuega
We have developed a low temperature (LT), ultra-high-
vacuum (UHV) system that combines two complementary techniques, scanning probe microscopy (SPM) and electrical transport measurements, within a single platform. By providing simultaneous access to the atomic-scale surface landscape and the macroscopic device response of the same sample, the setup enables direct correlations between local structural/spectroscopic signatures and global electronic transport behavior in two-dimensional (2D) devices. The system allows experiments where atomic-scale modifications or controlled manipulations are performed while continuously monitoring their impact on device-scale performance. The setup consists of two interconnected UHV chambers: a dedicated preparation chamber and a separate measurement chamber that houses a
liquid-helium cryostat and the SPM/transport stage. Base pressure is 1x10^-11 Torr. A key feature is direct optical access to the sample, enabling rapid and reliable tip positioning with an accuracy of 5 microns x 5 microns within 10 minutes. The SPM, operated using custom-built electronics, can track the exact same sample region across a temperature range from 2.9 K to 400 K, with mechanical stability below 1 pm. System performance is demonstrated on
graphene devices, and bulk Pb is used to determine energy resolution. Using superconducting tips, scanning tunneling spectroscopy measures the superconducting gap with an energy resolution of 30 microV. Transport measurements track the temperature dependence of both resistivity and critical current across the superconducting transition of an in-situ prepared Pb nanowire.
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Materials Science
Charge-Density-Wave Phase Selection by Janus-Induced Intrinsic Strain in Monolayer NbSSiAs$_2$
Chun-Jie Zhang, Bing Zhang, Dongliang Mao et al. (+3 more)
Controlling phase selection among competing charge-density-wave (CDW) instabilities remains challenging in two-dimensional materials. Here, first-principles calculations show that Janus-induced intrinsic tensile strain redirects the off-M soft-mode tendency of NbS$_2$ to the M point in NbSSiAs$_2$, selecting a $2\times2$ CDW reconstruction. Electron-
phonon coupling analysis identifies momentum-selective coupling between Nb-derived states and a longitudinal acoustic mode as the origin of the M-point instability. The reconstructed phase hosts two nearly degenerate Nb-trimerized configurations whose relative stability is tuned by biaxial strain. Both configurations retain
phonon-mediated superconductivity on the 6-7 K scale, indicating the coexistence of CDW order and superconductivity. Compressive strain favors the 1+3-hollow configuration and induces a band-inverted, $Z_2$-nontrivial state while preserving superconductivity. Together, these results identify Janus-induced intrinsic strain as an internal structural route for CDW phase selection, whereas external strain provides access to a regime in which CDW order,
topology, and superconductivity coexist.
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All authors Chun-Jie Zhang, Bing Zhang, Dongliang Mao, Yapeng Wu, Xiao-Ping Li, Lei Wang
Materials Science
First-principles carrier mobility and optical absorption of strained ZnO with self-consistent Hubbard interactions
Hong-Guk Min, Wooil Yang, Sabyasachi Tiwari et al. (+2 more)
Carrier mobility and optical absorption are key performance parameters of oxide
semiconductors in transparent and flexible displays. We use a newly developed density-functional perturbation theory with a self-consistent Hubbard correction (DFPT+U) to study
phonon-limited electron transport and
phonon-assisted optical absorption in strained zinc oxide (ZnO). This parameter-free approach accounts for electron-
phonon interactions and on-site correlation effects simultaneously. Electronic structures and
phonon dispersions are computed under three distinct uniaxial strain directions. Uniaxial tensile strain up to 4.8% along [\bar110] is found to increase the room-temperature electron mobility by 19% while leaving visible-range optical absorption essentially unchanged. These results demonstrate that moderate strain can selectively enhance carrier transport without degrading optical transparency, and establish DFPT+U as an effective framework for predicting strain-dependent transport and optical properties in wide-band-gap oxides with implications for strain-engineered display and optoelectronic applications.
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All authors Hong-Guk Min, Wooil Yang, Sabyasachi Tiwari, Feliciano Giustino, Young-Woo Son
Materials Science
One-Step Epitaxial Access to Rhombohedral Graphene Flat-Band States on Step-Bunched SiC
Hao Zhong, Xingzhe Wang, Hanbin Deng et al. (+15 more)
Rhombohedral
graphene multilayers provide a moiré-free platform for correlated and topological flat-band physics, but direct, transfer-free epitaxial access to thickness-tunable multilayers remains limited. Here we report a one-step graphitization route on 4$^\circ$ off-axis 4H-SiC, in which high-temperature flash annealing simultaneously drives self-organized step bunching and multilayer
graphene formation. Atomic-resolution cross-sectional scanning transmission electron microscopy identify local ABC registry and distinguish rhombohedral from Bernal stacking. The thickness is tuned from bilayer to more than twenty layers by varying single parameter, the annealing temperature. Angle-resolved photoemission spectroscopy directly tracks the thickness-dependent
evolution from interface-dominated low-energy states toward pronounced near-Fermi-level flat-band spectral weight in thick multilayers. Low-temperature scanning tunneling microscopy and spectroscopy on a 17-layer film further reveal a 13.4 meV low-energy spectral reconstruction and a $\sqrt{3} \times \sqrt{3}$ Kekulé-like modulation, providing microscopic signatures consistent with an intervalley-mixed electronic texture. This one-step, transfer-free approach establishes step-bunched SiC as an epitaxial platform that links stacking engineering with moiré-free correlated flat-band electronic states.
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All authors Hao Zhong, Xingzhe Wang, Hanbin Deng, Tianyu Yang, Haixuan Cao, Renzhe Li, Qiang Wan, Shangkun Mo, Keming Zhao, Shuming Yu, Dingkun Qin, Guang Zhu, Yifan Zhou, Jianping Shi, Shuangfeng Jia, He Zheng, Jia-Xin Yin, Nan Xu
Materials Science
Mechanism-Dependent Descriptors Enable Predictive Design of Oxygen Capacity in Perovskite Oxides
Shuiping Gong, Yi Li, Jie Su et al. (+9 more)
Perovskite oxides can reversibly accommodate substantial changes in oxygen stoichiometry, making them attractive for clean-energy technologies including chemical looping and oxygen storage. Despite extensive efforts to optimize their redox properties, predictive descriptors capable of assessing oxygen capacity across diverse compositions remain under development. Here, we combine experiments and first-principles calculations to establish composition and oxygen-capacity relationships in the model perovskite series LnxSr1-xCoO3. We confirm that increasing Sr2+ content promotes the formation of high-valence Co4+, expanding the cationic redox reservoir available during oxygen release and thereby enhancing oxygen capacity. In this regime, oxygen-vacancy formation energy captures the observed trend because oxygen release is primarily compensated by Co4+/Co3+/Co2+ redox. Across the rare-earth series, however, oxygen capacity decreases from La to Lu despite progressively lower oxygen-vacancy formation energies. We reveal that this counterintuitive behavior originates from an alternative charge-compensation pathway, in which
lattice oxygen is partially oxidized to O1- -like species during oxygen removal. Heavy rare-earth compositions (Tb-Lu) preferentially stabilize these oxygen-hole species through distinct local bonding environments, with charge compensation involving both oxidized
lattice oxygen and reduced rare-earth and cobalt cations, thereby suppressing net oxygen release despite favorable vacancy thermodynamics. We further identify average metal-oxygen bond strength, quantified by integrated
crystal orbital Hamilton population, as a physically meaningful descriptor for oxygen capacity when anionic redox becomes dominant.
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All authors Shuiping Gong, Yi Li, Jie Su, Zhenhao Zhou, Xiaobo Liao, Jian Deng, Chris Wolverton, Tao Deng, Jiangang He, Runxia Cai, Chaochao Dun, Zhenpeng Yao
Materials Science
Molecular beam epitaxy synthesis of ternary nitride PrTaN$_2$ and its crystal structure determination
Kosuke Takiguchi, Yoshiharu Krockenberger, Eisuke Magome et al. (+2 more)
We report the discovery of a novel ternary nitride PrTaN$_2$ synthesized as a thin film using molecular beam epitaxy. The combination of e-beam evaporation for refractory elements and a
radio-frequency nitrogen radical source enables growth under a highly nitriding environment, providing access to phases not readily obtained in bulk synthesis. Structural characterization by X-ray diffraction and high-angle annular dark-field scanning transmission electron microscopy reveals that the compound crystallizes in an orthorhombic structure and grows with a well-defined orientation on YAlO$_3$ substrates, while remaining essentially strain-free. To determine the
crystal structure from limited thin-film diffraction data, we developed a fitting procedure based on structure factors. By combining extinction rules with constraints from Wyckoff positions, the number of fitting parameters is significantly reduced, enabling reliable structure determination. Systematic exclusion of alternative candidate phases in the Pr-Ta-N system, together with structure factor fitting, identifies the space group as $P222$ and determines the atomic coordinates. The present results demonstrate that thin-film growth with molecular beam epitaxy can stabilize previously unexplored ternary nitrides, and establish a practical approach for structural determination in such systems. This work provides a pathway for the exploration of new complex nitride materials.
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All authors Kosuke Takiguchi, Yoshiharu Krockenberger, Eisuke Magome, Yoji Kunihashi, Hideki Yamamoto
Materials Science
Knudsen-Controlled Switching of Thermal Conductivity Response by Targeted Phonon Excitation
Shixian Liu, Tianhao Li, Fei Yin et al. (+4 more)
Targeted
phonon excitation offers a route to dynamically control heat conduction, yet no general principle predicts whether a spectrally selective nonequilibrium
phonon population will enhance or suppress thermal transport. A Knudsen-controlled competition between the increased contribution of long-mean-free-path
phonons and excitation-enhanced intrinsic scattering governs the sign of the thermal-conductivity response. First-principles three-
phonon scattering rates combined with
phonon-tracking Monte Carlo simulations are used to examine Ge, Si, and 3C–SiC from bulk
crystals to confined nanofilms. In bulk systems, excitation-enhanced scattering dominates and thermal conductivity is predominantly suppressed. In nanofilms, by contrast, low-frequency excitation can increase the contribution of quasi-ballistic heat-carrying channels and enhance thermal conductivity, whereas higher-frequency excitation is predominantly suppressive. At fixed background temperature and excitation strength, these opposite responses are organized in a frequency–Knudsen map based on the normalized target frequency, $\omega_{\mathrm t}/\omega_{\mathrm D}$, and the Knudsen number, $\mathrm{Kn}$. The resulting framework provides a general physical basis for controlling nonequilibrium heat transport beyond static
phonon engineering.
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All authors Shixian Liu, Tianhao Li, Fei Yin, Yu He, Alexander A. Barinov, Han Meng, Nuo Yang