Materials Science · Cross-listed from cond-mat.mes-hall
Resonant Far-Infrared Spectroscopy of Flat-Band Fermions in Magic Angle Graphene
Ayelet J. Uzan-Narovlansky, Ipsita Das, Juan F. Mendez Valderrama et al. (+10 more)
Moiré engineering in twisted two-dimensional (2D) materials radically alters low-energy bands, interactions and topological
quantum states. Despite extensive studies, optical spectroscopy of interacting moiré bands in the characteristic far-infrared (FIR) regime has remained largely unexplored due to extreme experimental challenges. Using a newly developed millikelvin FIR platform, we report the observation of the long-sought-after characteristic FIR resonances of flat-band electrons in magic-angle twisted bilayer
graphene (MATBG). We observe highly tunable spectroscopic signatures of interacting light and heavy
fermions that constitute the flat bands in MATBG. Using the topological heavy-
fermion model (THF), we show that itinerant topological electrons act as an "
antenna" that couples strongly to the optical field, with resonant frequencies renormalized by the hybridization with localized heavy electrons. We establish optical selection rules of MATBG which uncovers the key
symmetry governing light-heavy
fermion hybridization. At charge neutrality, we observe pronounced resonances at energies below the on-site Coulomb energy, implying the emergence of new many-body modes. Our experiments and modeling provide a fundamental understanding of light-matter interactions in MATBG and enable resonant optical spectroscopy of moiré bands down to millikelvin temperatures.
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All authors Ayelet J. Uzan-Narovlansky, Ipsita Das, Juan F. Mendez Valderrama, Yue Tang, Jonah Herzog-Arbeitman, Haoyu Hu, Pengjie Wang, Zhaoyi Joy Zheng, Haosen Guan, Kenji Watanabe, Takashi Taniguchi, B. Andrei Bernevig, Sanfeng Wu
Materials Science · Cross-listed from cond-mat.str-el
Defect-induced optical magnons and local magnetic correlations in MnSb2Te4
Dhurba R. Jaishi, Bing Li, Tianxiong Han et al. (+4 more)
Magnetic defect engineering offers a route to manipulate and control
magnetic and electronic states in
quantum materials. In the Mn(Bi,Sb)2Te4 family of topological
magnetic insulators, Sb substitution facilitates site mixing between Mn and Sb atoms that affects
magnetic order and band
topology. Here we directly probe these defect-induced
magnetic interactions using inelastic
neutron scattering on single
crystals of MnSb2Te4. We find that antisite mixing generates inequivalent and disordered
magnetic sublattices where strong defect-induced antiferromagnetic coupling produces an optical
magnon with a large gap. Semi-classical
spin-dynamics simulations accurately capture
magnetic excitations in the ordered and paramagnetic states and identify that linear Mn-Te-Mn bonds mediate coupling to antisite
magnetic defects.
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All authors Dhurba R. Jaishi, Bing Li, Tianxiong Han, S. X. M. Riberolles, D. M. Pajerowski, Jiaqiang Yan, R. J. McQueeney
Materials Science
Mechanisms of Nanoscroll Formation and Particle Encapsulation in Janus MXenes
Sasan Rezaee, Fatemeh Mohammad Dezashibi, Ould el Moctar et al. (+1 more)
Morphology transfer of 2D Janus MXenes into nanoscrolls unlocks unusual properties. Although a scalable synthesis route has been experimentally verified, the atomistic mechanism underlying nanoscroll formation remains poorly understood. We use large-scale reactive molecular dynamics simulations, validated against density functional theory (DFT) and experimental structural and elastic properties, to investigate stability and quantify the driving forces and geometry governing nanoscroll formation in three Janus MXenes, (Tx)Ti2C(Ty), where (Tx) and (Ty) denote the bottom and top surface terminations among bare (-b), -O, and -OH. Both square and infinitely wide flakes with lengths ranging from 10 to over 120 nm are simulated. We find that 1-7%
lattice-induced strain generates a bending moment in these structures. The sheet scrolls, curves, or forms a nanotube depending on the resulting
curvature and initial sheet size. For MXenes with an initial length of 120 nm, multiwalled nanoscrolls form with interlayer distances of around 0.7 nm and inner diameters of about 7 nm for (O)Ti2C(OH) and (b)Ti2C(OH), whereas (b)Ti2C(O) instead produces a much larger interlayer distance of around 1.7 nm and an inner diameter exceeding 20 nm. We show that spontaneous scrolling of a Janus MXene in the presence of an anchored nanoparticle produces a core@shell composite, in which the particle locally deforms the nanoscroll and widens the interlayer channels. This locally tunable, enlarged interlayer spacing offers a promising design route for MXene-based energy-storage electrodes. However, our simulations reveal H2 gas release during encapsulation, which promotes nanobubble formation that can reduce battery life.
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All authors Sasan Rezaee, Fatemeh Mohammad Dezashibi, Ould el Moctar, Hossein Darban
Materials Science
PACE-SIMS: Checkpoint-Gated Autonomous SIMS Characterization with AI-Agent Quality Control
Anton V Ievlev, Heather Hare, Yiyang Li et al. (+1 more)
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is widely used for local chemical investigations across a broad range of materials and systems. However, its operation is expensive in expert time: a trained researcher must supervise the acquisition continuously, tuning parameters as the experiment proceeds, often across a campaign spanning multiple days. Here, we present PACE-SIMS (Pause-Assess-Correct-Execute for SIMS), an agentic workflow that runs a SIMS study as a human-AI collaboration. In this workflow, the researcher specifies the scientific questions and quality requirements, and an AI
agent builds the plan and, after approval, executes it autonomously, pausing at checkpoints to judge each measurement and to correct, retry, or escalate. To validate the approach, we applied it to a study of chemical composition in 18O-enriched WOx films. During this randomized, blind two-polarity study (8.1 hours, 35 measurements), the
agent applied three rule-governed corrections and diagnosed an unanticipated source excursion without requiring event-specific control logic to be programmed in advance. The same run returned transferable measurement science, including a composition calibration and the deposition's tracer-delivery mechanism, from less than two hours of researcher
attention. The developed agentic architecture is not specific to SIMS and can be applied to other analytical techniques, with the primary target being destructive measurements, for which optimization-based methods are poorly suited.
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All authors Anton V Ievlev, Heather Hare, Yiyang Li, Sergei V Kalinin
Materials Science · Cross-listed from physics.app-ph
Measurement of third-order elastic constants using thermal modulation of ultrasonic waves
Bibo Zhong and Jinying Zhu
Third-order elastic constants (TOEC) play an important role in nonlinear material characterization, but measurements of TOEC are laborious with large error margins. This Letter presents the equations of wave velocity changes caused by homogeneous temperature variation and uniaxial stress in isotropic media and the expression of TOEC in terms of thermally induced velocity change and thermal strain. TOEC of an aluminum sample were experimentally determined by measuring ultrasonic wave velocity changes in the uniaxial loading test and the thermal modulation test. Experimental results showed good agreement between the two test methods. Owing to the simple test setup and high measurement sensitivity, the thermal modulation test is a potential experimental method to determine TOEC and absolute acoustic nonlinearity parameters.
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Materials Science · Cross-listed from math.DS
Effective Dynamics of Disclination Pairs
Nicolò Briatico, Pierluigi Cesana and Marco Morandotti
The dissipative dynamics of a pair of wedge disclinations with opposite Frank angles confined to a circular domain is investigated under the assumption of radially symmetric motion. Two distinct dynamical regimes are identified: a diverging-disclination regime and an annihilating-dipole regime. The stationary points associated with both regimes are determined and fully characterized in terms of their stability, and quantitative estimates for the characteristic
evolution times in their vicinity are derived. In the annihilating-dipole regime, it is shown that, after a suitable time rescaling, the resulting law of motion for the dipole coincides with that of an edge dislocation.
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Materials Science · Cross-listed from eess.SP
Effects of Tool Wear on the Surface Texture in Turning: A Feature Characterization Approach Based on ISO 21920-2
Alexander Müller, Maximilian Berndt, Hagen Schmidt et al. (+5 more)
The surface texture of a turned component acts as a fingerprint of both the process parameters and the tool wear condition, imaging the cutting edge. Classical surface parameters such as $R_\mathrm{a}$ or $R_\mathrm{q}$ describe the topography only globally and allow no spatially resolved evaluation of the process-induced deterministic structures. This work investigates how far the feature characterization standardized in ISO 21920-2 makes this wear information accessible and physically interpretable. The database comprises roughness profiles of twelve AlTiN-coated carbide inserts (CNMG120408) machining normalized AISI 1045 steel, measured at nine wear states over the entire tool life, with three replicate profiles per state. The correlation of standardized field and feature parameters with crater wear, flank wear, and cutting time is first examined. Watershed segmentation is then adapted to extract the rotational tool grooves and evaluate their geometry statistically. A newly developed mean-feature approach decomposes the profile into a deterministic and a
stochastic component. Wear-induced changes are almost entirely carried by the deterministic component, and within it by the trailing flank of the cutting groove. A comparison with confocal measurements confirms that the mean feature reconstructs the engaged cutting edge geometry, with the trailing-flank steepening attributable to notch wear on the secondary cutting edge. An exhaustive evaluation of more than 920,000 feature characterization combinations and multivariate models reveals that the groove-level mean maximum absolute gradient $\overline{R_\mathrm{dt}}_\mathrm{groove}$ alone explains 83-90% of the variance of the wear indicators, so that a single, physically motivated parameter suffices for robust wear estimation. A follow-up study will investigate inline monitoring using scattered light sensors.
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All authors Alexander Müller, Maximilian Berndt, Hagen Schmidt, Lars Müller, Matthias Eifler, Eberhard Kerscher, Benjamin Kirsch, Jörg Seewig
Materials Science
The electrical transport of intrinsic two-dimensional ferroelectric metal PtBi2
Dan Li, Liu Yang and Lei Li
Breaking the conventional stereotype that ferroelectrics are necessarily insulating, two-dimensional (2D) ferroelectric metals combine seemingly incompatible switchable electric polarization and metallic conductivity, providing a fertile ground for the discovery of novel electrical transport phenomena and the development of innovative electronic devices. Using the semiclassical Boltzmann equation and first-principles calculations, we systematically investigate the linear and nonlinear transport responses of the intrinsic 2D ferroelectric metal \ch{PtBi2} to an applied electric field. Our ab initio molecular dynamics simulations reveal that it possesses a high Curie temperature reaching $800~\text{K}$. We propose that the
crystal structure of its high-temperature paraelectric phase can be explicitly distinguished through simple measurements of the in-plane electrical conductivity. Quantitative calculations of the Edelstein effect and the intrinsic
spin Hall effect demonstrate a sizable charge-to-
spin conversion efficiency, highlighting its potential in spintronics. We also find that a Berry
curvature dipole-induced nonlinear Hall effect emerges in uniaxially strained \ch{PtBi2}. Furthermore, we highlight the unique advantages of 2D ferroelectric metals in gate-controlled transport applications. Based on the domain wall scattering mechanism, we conceptually design a novel ferroelectric metal field-effect transistor (FEM-FET) capable of nonvolatile switching between high-resistance and low-resistance states under a gate voltage. Our work not only unveils the rich transport physics in 2D ferroelectric metals but also provides valuable insights into the design of next-generation nonvolatile
memory and spintronic devices.
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Materials Science
Beam Routing through Excitons in Transition Metal Dichalcogenide Monolayers
Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez et al. (+6 more)
Routing light at the nanoscale typically relies on nanostructured surfaces to imprint directionality on the emission. Using low-temperature, angle-resolved cathodoluminescence spectroscopy, we show that the intrinsic excitonic transitions of a
semiconductor can themselves produce routed emission. We probe monolayers of WSe$_2$, MoSe$_2$, and MoTe$_2$ and resolve the excitonic species of monolayer WSe$_2$ – the bright
exciton, the trion, and the
spin-forbidden dark
exciton – through their distinct angular emission profiles. While the in-plane transition dipoles of the bright
exciton and trion radiate predominantly toward the surface normal, the out-of-plane dipole of the dark
exciton, inaccessible under normal-incidence optical excitation, produces a directional emission channel at large angles. We further tune the balance between neutral and charged
exciton emission through the local dielectric environment. Our results establish dark
excitons in TMD monolayers as a platform for directional light emission in compact photonic architectures without additional nanostructuring.
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All authors Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez, Maria Papadopoulou, Kenji Watanabe, Takashi Taniguchi, N. Asger Mortensen, Sergii Morozov, Nicolas Ubrig
Materials Science
Thermoelastic Harvesting Outperforming Thermoelectric Generators Below 100 {\deg}C
Bruno Neumann, Andreas Henschke, Morik Nikolic et al. (+1 more)
Low-grade waste heat below 100 degC is one of the largest untapped opportunities in solid-state energy conversion. Three ferroic routes are candidates for recovering this resource: thermomagnetic, pyroelectric, and thermoelastic harvesters. The last has remained the most unexplored, despite decades of progress on the underlying NiTi shape-
memory alloy wires. Three system-design changes close this gap: a protagonist-antagonist architecture that recovers the energy for prestraining, a continuously tunable prestrain mechanism that sets the force-strain balance, and transversal water flow that decouples cycle frequency from wire length. The resulting harvester delivers a directly measured power density of 366 mW/cm^3 with respect to the active material, about 1.7 times the next-best thermoelastic device, ahead of every reported thermomagnetic and pyroelectric generator, and outperforming the best thermoelectric generators in this temperature range also with respect to power per material cost. The system maps the parameter space directly through force and displacement measurements, without using material-property estimates, giving a quantitative picture of how the
alloy responds while the device is doing work.
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All authors Bruno Neumann, Andreas Henschke, Morik Nikolic, Sebastian Fähler
Materials Science
Energy-Dependent Dechanneling in Cu: Insights from Monte Carlo Channeling Simulations
Przemyslaw Jozwik, Cyprian Mieszczynski, Renata Ratajczak et al. (+1 more)
Ion channeling and backscattering techniques are powerful tools for studying
crystal lattice disorders and defect structures in crystalline materials. However, the accurate interpretation of channeling phenomena necessitates the utilization of simulation models that account for the intricate interactions between point defects, dislocations, and extended defect clusters. The present paper introduces a Monte Carlo method that reproduces experimental spectra over a wide range of analyzing beam energies and enables quantitative identification of defect types and distributions. The simulations reveal characteristic energy dependencies that distinguish point defects from extended defects, offering a novel perspective on disturbances caused, for example, by ion implantation in metals and
semiconductors. To this end, the McChasy code has been developed as a flexible and accessible tool for scientists, enabling the modeling of various
crystal systems, including complex
semiconductors, multilayer epitaxial films, and oxide
crystals. The program's integration of experimental data on ion channeling with defect modeling establishes a robust framework for defect analysis in materials science. The present article expounds upon the simulation capabilities of the program by reproducing the characteristic "elbows" in channeling spectra that were previously observed in experiments conducted on Cu
crystals.
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All authors Przemyslaw Jozwik, Cyprian Mieszczynski, Renata Ratajczak, Andrzej Turos
Materials Science · Cross-listed from cond-mat.mes-hall
Mismatch between Raman shear modes and ferroelectric polarization in 3R-MoS$_{2}$
Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi et al. (+7 more)
Sliding ferroelectricity in parallel-stacked two-dimensional van der Waals materials enables a broad range of novel device concepts, but exploiting it requires reliable, non-destructive assignment of the underlying stacking order and polarization state. Here, we combine Kelvin-probe force microscopy (KPFM) with low-frequency Raman spectroscopy to probe the polarization domains and stacking configurations of a exfoliated trilayer 3R-MoS$_{2}$ flake on a hBN substrate. We find that ABA and BAB - both stackings with zero net polarization - are indistinguishable in KPFM, yet show drastically different low-frequency shear modes. This observation is reproduced across multiple flakes and is corroborated by low-temperature photoluminescence. Notably, the standard bond-polarizability model does not account for the difference in shear-mode activity between the ABA and BAB configurations, indicating that the interlayer Raman response of these stackings is governed by physics beyond a simple polarizability picture. Our results show that none of the here-used individual techniques alone is sufficient to assign sliding-ferroelectric stacking order and motivate a combined spectroscopic-scanning-probe approach.
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All authors Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi, Swarup Deb, Elisabeth Grune, Kenji Watanabe, Takashi Taniguchi, Rico Schwartz, Oliver Kühn, Tobias Korn
Materials Science
Electronic structure, band offset, and interface electron population of the LaInO$_3$/BaSnO$_3$ system
G. Hoffmann, A. A. Riaz, C. Kalha et al. (+7 more)
Perovskite oxides and their
heterostructures exhibit a wide range of functional properties. Among these materials, BaSnO$_3$/LaInO$_3$
heterostructures form high-mobility two-dimensional electron gases (2DEGs) at their interfaces. In particular, room-temperature electron mobilities exceeding 100~cm$^2$/Vs were enabled by recent advances in thin-film growth. This work presents a combined experimental and theoretical study of the electronic structure of BaSnO$_3$, LaInO$_3$, and BaSnO$_3$/LaInO$_3$
heterostructures with varying LaInO$_3$ overlayer thicknesses. Soft and hard X-ray photoelectron spectroscopy (SXPS and HAXPES) measurements are combined with densities of states (DOS) derived from hybrid density functional theory (DFT) calculations. The analysis of core, semi-core, and valence states allows to arrive at a comprehensive understanding of the chemical bonding and electronic structure in the parent oxides as well as the formed
heterostructures. For the BaSnO$_3$/LaInO$_3$
heterostructure, the band offset and population of 2DEG states at the interface is directly probed using HAXPES.
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All authors G. Hoffmann, A. A. Riaz, C. Kalha, A. Gloskovskii, C. Schlueter, O. Brandt, W. Aggoune, C. Draxl, O. Bierwagen, A. Regoutz
Materials Science · Cross-listed from cond-mat.mes-hall
Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2
Tianxing Jiang, Lianchuang Li, Haiyan Zhu et al. (+10 more)
Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices.
Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine
spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the
spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of
spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the
spin spiral wavevector, which is caused by local
curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics.
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All authors Tianxing Jiang, Lianchuang Li, Haiyan Zhu, Hongyu Wang, Junchao Tian, Wenzhao Wang, Weiyi Pan, Haitao Wang, Changlin Zheng, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang
Materials Science
Resonant Raman signatures of bright and momentum-dark exciton coupled by intervalley phonon scattering in monolayer WSe2
Hendrik Lambers, Nihit Saigal, Lara Blinov et al. (+3 more)
Exciton-
phonon coupling in atomically thin transition metal dichalcogenides governs key processes such as
exciton thermalization and intervalley scattering and remains challenging to access directly by optical spectroscopy. Here, we employ resonance Raman spectroscopy at cryogenic temperatures to probe
exciton-
phonon coupling in hBN-encapsulated WSe_2 monolayers. Tuning the excitation
laser across the bright
exciton X_KK resonance, we observe rich Raman spectra and focus on the resonance profile of the degenerate A_1'/E' optical
phonon mode. The profile exhibits two asymmetric resonance peaks whose energetic separation significantly exceeds the
phonon energy - a feature that cannot be explained by first-order Raman scattering alone. We demonstrate that this discrepancy is resolved by including third-order Raman scattering, in which intervalley scattering enabled by a finite-momentum
phonon couples the bright
exciton X_KK to a momentum-dark
exciton X_d. Fitting the experimental resonance profiles of two independent samples with a model comprising coherent first- and third-order scattering yields consistent
exciton energies and linewidths, with a momentum-dark
exciton X_d approximately 45meV to 55meV below the bright
exciton X_KK. The results indicate efficient bright-to-dark
exciton coupling. Our findings provide a microscopic framework for understanding the anomalously bright emission spectra of WSe_2 monolayers despite its
spin-forbidden lowest
exciton transition and highlight the role of momentum-dark
excitons in resonant light-matter interaction.
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All authors Hendrik Lambers, Nihit Saigal, Lara Blinov, Jonas Kiemle, Alexander W. Holleitner, Ursula Wurstbauer
Materials Science
Magneto-optical magnetoelectric voltage sensor
Michael P. Path and Jeffrey McCord
Applications in high-voltage and electromagnetically harsh environments require reliable galvanically isolated voltage sensing, which can be achieved using optical readout. While established optical voltage sensors rely on electro-optic effects or piezoelectric strain with direct optical detection, strain-mediated magnetoelectric coupling combined with magneto-optical readout offers an alternative voltage sensing principle that remains largely unexplored. Here, such a sensor based on a bismuth-substituted yttrium iron garnet magneto-optical indicator film mechanically coupled to a piezoelectric actuator is presented. Voltage induced stress results in changes of the out-of-plane magnetization via magnetoelastic coupling which is detected through magneto-optical Faraday rotation. A critical state of the domain structure is set via an applied bias field, in which voltage-induced nucleation and domain-wall motion dominates the response. In this high sensitivity regime, both AC and DC voltage readout modes are demonstrated, based on either voltage-driven magnetization reversal or voltage-induced modifications of the magnetization loop shape. Equivalent voltage noise densities in the millivolt per root hertz range are achieved. The results establish strain-mediated magneto-optical voltage sensing as a distinct approach to optically isolated voltage measurement.
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Materials Science
Strain-controlled sign reversal of the anomalous Hall effect in Ru/[Co/Ni]$_N$ multilayers
Jingying Zhang, Sigang Wang, Yue Xiang et al. (+4 more)
The anomalous Hall effect (AHE) is a hallmark transport phenomenon in ferromagnets arising from relativistic
spin-orbit interaction. Here, we report an unexpected sign reversal of the AHE in Ru/[Co/Ni]$_N$ multilayers controlled by the stacking sequence of the Ru layer. When Ru is placed beneath, rather than atop, the Co/Ni multilayers, the anomalous Hall signal switches from positive to negative. By systematically varying the multilayer repeat number N and combining transport measurements with first-principles calculations, we show that this reversal originates from in-plane tensile strain imposed by the Ru underlayer, which reshapes the electronic structure and redistributes Berry
curvature near the Fermi level. Our findings establish interfacial strain as an effective knob for tuning Berry-
curvature-driven transport and suggest a pathway toward strain-controlled topological transport phenomena in
magnetic multilayers.
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All authors Jingying Zhang, Sigang Wang, Yue Xiang, Wenhui Xie, Zhe Yuan, Yi Liu, Zongzhi Zhang
Materials Science
Optically Tunable Threshold Switching and Thermally Activated Transport in Planar Ag/MAPbI$_3$ Thin Single-Crystal Devices
Ofelia Durante, Valeria Demontis, Sebastiano De Stefano et al. (+14 more)
Halide perovskites have enabled major advances in optoelectronics, extending well beyond photovoltaics. Their mixed ionic-electronic conduction, once regarded as detrimental to device stability, is increasingly viewed as a functional degree of freedom for
memory and neuromorphic-inspired devices, especially when coupled to external stimuli such as light. Specifically, single
crystals are attractive models because they suppress grain-boundary effects and microstructural disorder that can mask intrinsic transport and interfacial mechanisms in polycrystalline films. Here, we report the growth of thin methylammonium lead iodide (MAPbI$_3$) single
crystals by a space-confined method and their integration into planar two-terminal devices with directly deposited Ag contacts. At room temperature, the devices exhibit ultra-low dark currents ($10^{-13}-10^{-12}$ A) and negligible hysteresis in the dark. Under illumination, the current increases due to photogeneration and the I-V characteristics develop a pronounced polarity-dependent hysteresis and a threshold-like transition between two conductance states. Temperature-dependent dark measurements (300-400 K) show thermionically activated, contact-influenced transport and a weakly varying normalized hysteresis metric. Together with the back-to-back Schottky-diode analysis and control devices using more inert contact materials, these results support a transport model in which Ag/perovskite interfaces play a central role and the hysteretic response is influenced by coupled interfacial and ionic processes.
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All authors Ofelia Durante, Valeria Demontis, Sebastiano De Stefano, Selene Matta, Adolfo Mazzotti, Daniela Marongiu, Emanuele Meloni, Elisa Pili, Fang Liu, Nicola Sestu, Angelica Simbula, Mauro Carta, Michele Saba, Andrea Mura, Massimiliano Di Ventra, Giovanni Bongiovanni, Antonio Di Bartolomeo
Materials Science
Data-Efficient Adaptation of DPA-4 Force Fields to DFT+U Energetics: A Case Study in NiO
Fengyu Xie, Peiheng Jiang and Zhicheng Zhong
Foundation machine-learned force fields (MLFFs) are often pretrained on broad materials
datasets whose electronic-structure conventions may not reproduce the phase energetics required for a specific correlated material. Using NiO as a case study, we examine whether incorrect source-level phase energetics can be corrected efficiently through target-level fine-tuning. Along a common structural interpolation, non-
spin-polarized PBE and ferromagnetic PBE+U predict opposite energetic orderings of the octahedral Oct and square-planar Sqr phases. Pretrained DPA-4 models adapt rapidly to the NiO PBE+U surface, reaching energy and force root-mean-square errors (RMSEs) of approximately 0.5 meV/atom and 30 meV/Å, respectively, with approximately 170 PBE+U labels. Crucially, models previously fine-tuned to the opposing no-U surface recover the qualitative PBE+U phase ordering with nearly the same target-data efficiency as models fine-tuned directly from their respective pretrained initializations. Our results show that incorrect source-level phase energetics can be reversed through target-level fine-tuning, and suggest a practical multi-fidelity strategy in which pretraining prioritizes broad, consistent, and affordable data, while compact target-level
datasets impose energetics through application-specific fine-tuning.
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Materials Science
Lithography-free patterning of SrTiO$_3$-based two-dimensional electron gases using direct atomic layer processing
Anshu Gupta, Karolis Parfeniukas, Amit Chanda et al. (+5 more)
We present a scalable and lithography-free strategy for the realization of a two-dimensional electron gas (2DEG) in TiO$_2$-patterned SrTiO$_3$ (100) via Al deposition using magnetron sputtering. A 15 nm thick TiO$_2$ layer, deposited by direct atomic layer processing, is employed to spatially define the conducting regions, enabling direct transport measurements without post-growth microfabrication. Upon Al deposition, an insulating AlO$_x$ overlayer is formed, and the region lacking the TiO$_2$ pattern leads to the creation of oxygen vacancies in SrTiO$_3$. These oxygen vacancies act as electron donors, populating the Ti 3$d$ conduction bands and giving rise to a confined 2DEG at the interface. Magneto-transport measurements reveal a sheet carrier density on the order of $\approx5-7\times10^{13}$ cm$^{-2}$, comparable to values typically achieved in pulsed
laser deposition-grown SrTiO$_3$-based
heterostructures, along with effective electrostatic tunability. This work demonstrates a simple, cost-effective, and industry-compatible route for engineering oxide 2DEGs, providing a versatile platform for scalable device fabrication and interfacial transport studies.
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All authors Anshu Gupta, Karolis Parfeniukas, Amit Chanda, Thor Hvid-Olsen, Mira Baraket, Maksym Plakhotnyuk, Kasper S. Pedersen, Felix Trier
Materials Science · Cross-listed from physics.optics
Aggregation-engineered loss-tolerant strong coupling in metallic microcavities
Andrea Betti, Eleonora Cara, Giulia Serrano et al. (+7 more)
Room-temperature strong coupling in organic microcavities is usually achieved by combining high-quality optical resonators with highly ordered excitonic media, a requirement that limits scalability and processing flexibility. Here we show that this constraint can be relaxed by using molecular aggregation as a design parameter rather than treating it as a parasitic effect. We realize solution-processed Rhodamine 6G-poly(vinyl alcohol) films embedded in low-quality-factor silver Fabry-Perot microcavities and demonstrate clear angle-resolved anticrossing with coupling energies up to 324 meV despite the large optical losses of the metallic mirrors. A two-
exciton coupled-oscillator model shows that the relative weight of these species controls the collective coupling strength and can be tuned through dye loading and
spin-coating conditions. In contrast, angle-resolved photoluminescence is dominated by a broad, red-shifted lower-polariton emission, consistent with relaxation through excimer-like states formed in densely packed molecular domains. These results identify molecular aggregation as a practical design lever for loss-tolerant strong coupling in wet-processed metallic cavities and suggest that ground-state aggregates and excited-state excimer-like species play distinct roles in polariton formation and emission.
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All authors Andrea Betti, Eleonora Cara, Giulia Serrano, Lorenzo Poggini, Alessia Valzelli, Natascia De Leo, Paolo Bartolini, Andrea Taschin, Renato Torre, Alice Boschetti
Materials Science
Advancing in situ hydrogen embrittlement studies through an integrated charging cell for SEM micromechanical testing
Lavakumar Bathini, Guillaume Kermouche, Sergio Sao-Joao et al. (+2 more)
A comprehensive understanding of hydrogen-deformation interactions at the microscale is essential for revealing hydrogen embrittlement mechanisms. In situ micromechanics with simultaneous hydrogen (H-) charging has therefore gained traction in recent times. In the present study, we aim to address the drawbacks of current in situ H-charging setups by developing a more robust 3-electrode-based back-side charging system for a scanning electron microscope to perform various micromechanical tests. The development of the novel setup is discussed and demonstrated through micropillar compression of an Fe-25Cr single
crystal (110) during H-charging. H has increased the yield strength and the apparent strain-hardening rate. H activates multiple slip systems and enhances dislocation density and
entanglement, leading to pronounced forest hardening as revealed by electron microscopy. Estimation of activation volume from strain-rate jump tests indicates that the deformation is controlled by the solute drag effect on kink mobility and dislocation forest hardening.
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All authors Lavakumar Bathini, Guillaume Kermouche, Sergio Sao-Joao, Frédéric Christien, Szilvia Kalácska
Materials Science
Deep-Learning-Accelerated Dopant Selection for High-k HfO2 Dielectrics: A Disorder-Resolved Study of Y, Si and Al
Zunair Masroor, Bonwook Gu, Wonjoong Kim et al. (+3 more)
Hafnium oxide (HfO2) is the cornerstone high-k dielectric in modern silicon technology. Since the constraints of silicon device fabrication rule out replacing the material itself, dopant incorporation is the principal means available to engineer its band gap and dielectric constant within existing process flows. However, dopant selection is still largely empirical due to the coupled interplay among thermodynamic stability, electronic insulation, and dielectric response. Here, we present a high-throughput computational framework integrating special quasi-random structures (SQS), machine-learning potentials (SevenNet), and graph neural networks (ALIGNN) to systematically evaluate doped-HfO2 compositions across three dopants (Al, Si, Y) and two technologically relevant polymorphs (monoclinic and orthorhombic). Our analysis uncovers a fundamental design principle: formation energy, band gap, and dielectric constant are decoupled parameters requiring application-specific prioritization rather than simultaneous optimization. Yttrium achieves the lowest formation energy (-3.763 eV/atom) and favors orthorhombic phase stabilization at process-compatible thermal budgets; silicon preserves near-pristine band gaps (around 5.72 eV) critical for suppressing leakage in gate dielectric applications; and aluminum enables concentration-tunable band gap widening (5.6-5.9 eV) suited for voltage scaling. Validation against experimental literature and density functional theory (DFT) confirms quantitative accuracy (0.02 eV band gap error for Si-doping, mean absolute error less than 0.001 eV/atom formation energy). This framework provides rational, property-decoupled guidance for dopant engineering in HfO2-based dielectrics and related high-k oxide systems.
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All authors Zunair Masroor, Bonwook Gu, Wonjoong Kim, Trinh Ngoc Le, Summal Zoha, Han-Bo-Ram Lee
Materials Science
First-principles cumulant approach to the vibronic structure of spin defects
Jinsoo Park, Yu Jin, Arpan Kundu et al. (+2 more)
Color centers in wide-band-gap
semiconductors are leading platforms for solid-state
quantum technologies, yet a quantitative description of their vibronic structure has remained elusive due to the complexity of multi-
phonon processes in localized defect states. Here we present a first-principles Green's function framework based on the retarded cumulant ansatz (RCA) to describe electron-
phonon interactions in
spin defects; our approach goes beyond the adiabatic and lowest-order perturbation theory approximations underlying widely used approaches. Applied to the negatively charged nitrogen-vacancy (NV$^-$) center in diamond, our method reveals that multi-
phonon satellites persist over a 400 meV energy window even at zero temperature, driven by
quantum zero-point fluctuations. We demonstrate that accurate spectral functions require mode-, momentum-,
spin-, and orbital-resolved electron-
phonon matrix elements sampled across the full Brillouin zone, to account for hybridized and propagating
phonon channels. We find that the vibronic structure of the NV$^-$ center exhibits strong
spin and orbital anisotropy, with different orbitals coupling to qualitatively distinct parts of the
phonon spectrum, and
spin-selective coupling affecting both sideband positions and intensities.
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All authors Jinsoo Park, Yu Jin, Arpan Kundu, Jorge O. Sofo, Giulia Galli
Materials Science · Cross-listed from cond-mat.mes-hall
Spin lifetime anisotropy in graphene induced by the SiO2 interface
Aron W. Cummings, Chunhao Guo, Andrew Grieder et al. (+5 more)
Understanding how common dielectric substrates influence the
spin transport properties of
graphene is essential for advancing
graphene-based spintronic technologies. Here we use a comprehensive set of numerical simulations to reveal how a SiO$_2$ substrate modifies the
spin texture and governs
spin relaxation in
graphene. Using first-principles density
matrix dynamics simulations, as well as tight-binding (TB) transport simulations, we quantify the effects of electron-
phonon scattering, impurity scattering, and electrostatic disorder on the
spin relaxation process. We find that a 2D SiO$_2$ substrate induces a predominantly Rashba-type helical
spin texture in
graphene, leading to a
spin lifetime anisotropy of 1/2. Meanwhile, bulk SiO$_2$ breaks in-plane
symmetry in
graphene, leading to anisotropic in-plane and out-of-plane components in the
spin texture, which we capture with a newly-developed TB model of
graphene. Transport simulations under realistic disorder conditions reveal a
spin lifetime anisotropy between 0.5 and 1, similar to what is seen in measurements of
graphene spin valves on a SiO$_2$ substrate. Our results reveal a more complex picture of
spin relaxation at the ubiquitous
graphene/SiO$_2$ interface, beyond the standard Rashba model, providing critical insight for interpreting experiments and guiding substrate engineering for
graphene spintronics.
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All authors Aron W. Cummings, Chunhao Guo, Andrew Grieder, Shihao Tu, Mayank Gupta, Junqing Xu, Juan Marmolejo-Tejada, Yuan Ping
Materials Science · Cross-listed from stat.AP
Process-fracture mapping of a DLP-printed photopolymer using Bayesian active learning and surrogate-based sensitivity analysis
Ethan Blackwell, Yogesh C. Chandrashekar, Guoqiang Li et al. (+1 more)
Digital light processing (DLP) enables rapid fabrication of
polymer structures, but fracture performance depends on multiple interacting processing variables, making exhaustive experimental characterization impractical. This work presents a data-efficient framework for process-fracture mapping of a DLP-printed photopolymer using
Bayesian active learning and digital image correlation (DIC)-assisted Mode I fracture experiments. Four processing parameters were considered: layer angle, UV exposure time, layer height, and print temperature. Fracture resistance was quantified by the critical J-integral, $J_c$, obtained from three-point-bending tests with DIC-based evaluation of crack-mouth opening displacement and hinge-point kinematics. Beginning with two randomly selected conditions, Gaussian process regression (GPR) and a modified upper confidence bound (UCB)-style acquisition function selected 26 additional experiments, yielding 28 processing conditions with three replicates each. The final GPR surrogate reproduced the training data with $R^2=0.99$ and achieved leave-one-out cross-validation performance of $R^2=0.63$ and Pearson $r=0.81$. Surrogate-based sensitivity analysis quantified parameter effects and global contributions. One-at-a-time response curves revealed nonlinear conditional trends, while global Sobol analysis identified UV exposure time as the dominant processing variable, with first-order and total-order indices of 0.6780 and 0.7581, respectively. Based on total-order influence, the parameters ranked as UV exposure time, layer angle, print temperature, and layer height. The first-order Sobol indices summed to 0.8058, indicating non-negligible interaction and higher-order effects. These results demonstrate that
Bayesian-active-learning-guided experimentation can efficiently recover process-fracture relationships and parameter interactions from a sparse experimental campaign.
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All authors Ethan Blackwell, Yogesh C. Chandrashekar, Guoqiang Li, Kshitiz Upadhyay
Materials Science
Magnetoelastic coupling descriptor for high-throughput ab initio search of magnetocaloric materials
Debajit Chakraborty, Kirill D. Belashchenko and Aleksander L. Wysocki
We use Landau theory of phase transitions to design a magnetoelastic coupling descriptor that can identify
magnetic materials on the brink of a first-order transition, where a strong magnetocaloric effect (MCE) can arise. The descriptor can be computed from electronic structure calculations and requires structural optimization in the paramagnetic state, which we model using special quasi-random collinear
spin configurations. We first evaluate the descriptor for a set of known magnetocaloric materials and identify compounds in which the MCE is driven by magnetoelastic coupling. We then apply the descriptor in a high-throughput ab initio screening of
magnetic L1$_2$ compounds. This search identifies the L1$_2$ cubic phases of Mn$_3$Ge and Mn$_3$Sb as materials with strong magnetoelastic coupling and potential for a large magnetocaloric response.
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