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Physics Condensed Matter

cond-mat Watch Condensed Matter

Thursday 30 July

Soft Condensed Matter

Molecular Hyperpolarisability as a Screening Descriptor for Second-Order Nonlinear Optics in Ferroelectric Nematic Liquid Crystals

Charles Parton-Barr, Nerea Sebastian and Richard Mandle

Ferroelectric nematic liquid crystals combine fluidity with macroscopic polar order. Although the archetypal NF material RM734 exhibits large nonlinear optical coefficients, most known ferroelectric nematics were designed without consideration of optical nonlinearity. Here, we assess whether electronic-structure calculations can be used to identify promising nonlinear optical candidates within known polar liquid-crystal materials. Frequency-dependent molecular hyperpolarisability tensors were calculated using a range of DFT methods and basis sets, then converted to macroscopic d-coefficients using an oriented-gas model with empirical <P1> and <P3> values. Calculated values were benchmarked against available experimental d33, d15, and d13/d31 coefficients for representative materials. We find that absolute values depend strongly on method and bulk-parameter assumptions, whereas relative trends are more useful for screening. Explicit conformer averaging does not consistently improve agreement with experiment. Applying the best-performing single-conformer protocols to a broader polar liquid-crystal dataset identifies candidate materials with enhanced predicted nonlinear optical response and reveals simple design rules based on donor-acceptor asymmetry, conjugation length, and linker choice.
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Statistical Mechanics · Cross-listed from quant-ph

Bridging continuous control and Floquet driving for charging many-body spin chains

Sebastián V. Romero, Xi Chen and Yue Ban

Recent advances in quantum information and quantum thermodynamics have reshaped the understanding of energy storage at the microscopic scale, paving the way toward protocols for storing and transferring energy in quantum devices. These systems, known as quantum batteries, offer a conceptual alternative to conventional macroscopic chemical batteries by exploiting quantum coherence, correlations, and many-body dynamics. By navigating the landscape of established spin-based quantum batteries, we review existing charging and work-extraction protocols, as well as the impact of external factors on their performance. Motivated by Floquet-engineered proposals, we further establish a connection between continuously and periodically driven spin chains as platforms for quantum energy storage. Finally, we survey experimental realizations and proposals, highlighting their implementability and scalability in current and near-term quantum technologies.
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Materials Science

Tunable Conformal Graphene Growth on Oxide Nanotube scaffolds: Towards Superwettable Hierarchical 2D-3D Architectures

Fernando Nunez Galvez, Muhammad Hamza, Johannes Berndt et al. (+7 more)

Hierarchical hybrid nanoarchitectures that integrate vertically oriented graphene nanowalls, GNWs, with metal oxide, MeOx, nanotube scaffolds offer versatile platform for smart surfaces, nanoelectronics, and electrochemical technologies. Herein we present rapid, dry, plasma-assisted fabrication route that enables direct and conformal growth of GNWs on mechanically robust MeOx nanoforests. The method combines supported single-crystalline organic nanowires as a 1D soft template with sequential plasma-enabled oxide deposition and GNW growth, all performed under mild temperature, power, and vacuum conditions. This approach yields an unprecedented 2D-3D hierarchical architecture consisting of tunable-thickness MeOx nanotubes uniformly decorated with radially oriented graphene nanosheets, forming re-entrant, multiscale surface. Resulting hierarchical roughness imparts fluorine-free, long-term omniphobicity, with contact angles exceeding 170 degree for water, bovine serum, and other complex fluids. GNWs dominate the wetting response across TiO2, Al2O3, and SiO2 nanotube scaffolds, effectively decoupling surface behavior from intrinsic oxide chemistry and maintaining robust repellency under UV irradiation and water condensation. Comprehensive SEM, TEM, XPS, angle-resolved NEXAFS, and Raman analyses elucidate growth mechanism and confirm preservation of the sp2 graphitic framework, together with controlled degree of edge functionalization. Overall, this work establishes universal, substrate-compatible, low-temperature, and scalable route for the fabrication of tunable graphene-metal oxide nano-microstructured multifunctional surfaces.
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All authors Fernando Nunez Galvez, Muhammad Hamza, Johannes Berndt, Thomas Strunskus, Franziska Traeger, Nicolo di Novo, Juan R. Sanchez-Valencia, Carmen Lopez-Santos, Ana Borras, Eva Kovacevic

Mesoscale and Nanoscale Physics

Probing the Potential Profile of Twisted Bilayer Graphene via Fabry-Pérot Interference

Curtis McDowell, Alina Mreńca-Kolasińska, Kenji Watanabe et al. (+3 more)

We use Fabry-Pérot interference to probe the internal potential profile of large-angle twisted bilayer graphene. We trace anomalous resistance oscillations in the nominally unipolar regime to a hidden cavity formed by unintentional local doping in our device. By analyzing zero-field interference patterns, we determine the location and size of this inhomogeneity. Magnetotransport measurements support the model, distinguishing local cavity modes from global resonances through their different magnetic dependence. Simulations using our extracted profile reproduce the experimental features. Our results highlight interference spectroscopy as a simple, non-invasive probe for identifying local defects and internal potential barriers in ballistic devices.
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All authors Curtis McDowell, Alina Mreńca-Kolasińska, Kenji Watanabe, Takashi Taniguchi, Ming-Hao Liu, Thiti Taychatanapat

Materials Science

STM study of single phosphorus incorporation into silicon by heating PBr3 on Si(100)

Tatiana V. Pavlova and Vladimir M. Shevlyuga

The objective of miniaturizing doped areas in silicon, with the ultimate goal of achieving atomic-precision doping, requires a fundamental understanding of the dopant incorporation process at the atomic level. We present a combined scanning tunneling microscopy (STM) and density functional theory (DFT) investigation of single phosphorus atom incorporation into the Si(100) surface. Phosphorus was supplied via PBr3 molecules, which completely dissociate on Si(100) at room temperature. By performing in situ annealing within the STM, we directly tracked the same phosphorus atom before and after heating. Upon annealing, the P atom undergoes an exchange with a nearby Si atom, forming a stable P-Si-Br complex with a Br atom located atop the Si atom of the heterodimer. The activation barrier calculated using DFT is consistent with our observation of doping starting at temperatures as low as 175 C. These results provide detailed atomic-scale insight into the phosphorus incorporation pathway and offer a foundation for improving methods of precise, single-atom doping in silicon.
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Strongly Correlated Electrons

Finite-size effects and interaction-driven crossovers in quarter-filled attractive Hubbard model: Exact diagonalization, DMRG and machine-learning analysis

Md Fahad Equbal, M. A. H. Ahsan and Satoru Hayami

We investigate the quarter-filled attractive Hubbard model on finite-width cylindrical lattices using exact diagonalization (ED), density-matrix renormalization group (DMRG) and unsupervised machine-learning-based techniques. Analysis of the ground-state energetics, local observables and correlation functions reveals a continuous interaction-driven crossover from weakly correlated fermions to a regime dominated by tightly bound singlet pairs. This crossover originates from the competition between kinetic-energy-driven fermionic itinerancy and interaction-driven onsite pair formation and exhibits behavior consistent with the BCS–BEC crossover in the thermodynamic limit. Hole-binding-energy calculations provide direct energetic evidence for pair formation: the two-hole binding energy remains negative throughout the attractive regime whereas three-hole binding emerges only at sufficiently strong attraction and exhibits pronounced finite-size dependence. To obtain an unbiased characterization of the correlation landscape, we apply principal component analysis (PCA) and uniform manifold approximation and projection (UMAP) to the real-space correlation matrices. PCA reveals a systematic redistribution of correlation variance whereas UMAP identifies a clear separation between weak- and strong-pairing regimes. Both machine-learning-based approaches independently identify the same crossover region inferred from conventional observables while providing an order-parameter-independent characterization of the underlying reorganization of many-body correlations. Finite-size scaling analyses of the pairing structure factor and the leading PCA variance ratio demonstrate that these signatures remain robust with increasing system size.
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Materials Science

Oxygen Sensing Without Organic Molecules: Mixed-phase TiO2 as Cost-effective Ultrasensitive Optical Sensors

R.Rega, A.Fioravanti, F.Borbone et al. (+2 more)

Oxygen (O2) detection is commonly carried out via either fluorescence-based optical sensors or chemoresistive sensors. Each approach has its own limitations. Optical sensors require the molecular design and synthesis of specific organic fluorescent species which can be costly and less stable. Chemoresistive sensors, despite presenting advantages in using more cost-effective inorganic materials, are often limited by low sensitivities to O2 at ppm concentrations and by their inability to operate at room temperature. In this work, we demonstrate the detection of O2 at concentrations as low as a few tens of ppm at room temperature by using titanium dioxide (TiO2) mixed-phase nanoparticles as optical sensors. By simultaneously measuring the photoluminescence of nanoparticles in rutile and anatase phase, O2 detection was achieved in the concentration range of 30-500 ppm, with a response curve well-calibrated by a Langmuir function. Good response promptness and repeatability are also demonstrated. This approach to O2 optical sensing offers two intrinsic advantages over the most commonly used methodologies: (1) use of a cost-effective, easy-to-prepare and stable of the sensitive material compared to those typically employed in optical sensing, and (2) improved room-temperature detection efficiency in the low O2 concentration range, outperforming most commonly-used chemoresistive sensors.
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All authors R.Rega, A.Fioravanti, F.Borbone, M.Mazzocchi, S.Lettieri

Mesoscale and Nanoscale Physics

Hyperchaos in a Magnetic Nanodisk Driven by Ferromagnetic Resonance

A Kolli, Hugo Merbouche, Salvatore Perna et al. (+3 more)

We investigate the chaotic dynamics driven in the nonlinear regime of ferromagnetic resonance of an out-of-plane magnetized nanodisk in detail. By combining extensive micromagnetic simulations with time-series analysis across the control parameter space, we map the topological transitions from stable periodic orbits to strange attractors and quantify the dynamical complexity. Despite the simplicity of our nanoscale system, we evidence that it can exhibit hyperchaotic dynamics with up to three positive Lyapunov exponents in vast regions of the control plane accessible to experimental studies. Using a mode projection technique, we unveil that the generated complexity is related to the number of quantized spin-wave modes participating in the dynamics. Our findings establish magnon-spintronic nanodevices as versatile entropy sources for unconventional processing of information.
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All authors A Kolli, Hugo Merbouche, Salvatore Perna, Claudio Serpico, Damien Rontani, Grégoire de Loubens

Materials Science

High-Field EPR/ENDOR of N/Be Centers for Defect Engineering in 6H-SiC

Yuliya Ermakova, Ekaterina Dmitrieva, Margarita Sadovnikova et al. (+5 more)

Silicon carbide (SiC) in its various structural modifications is widely used in power semiconductor electronics, operating under extreme conditions of high temperature, high voltage, and intense radiation. The discovery of spin defects (S>0) with unique optical and coherent properties has further positioned SiC as a promising platform for quantum technologies. Here, we investigate a 6H-SiC single crystal co-doped with nitrogen and beryllium at concentrations of 1018 cm-3, using continuous-wave and pulsed electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). To enhance spectral resolution, experiments were conducted in the W-band (94 GHz; B = 3.4 T). Pulsed EPR identified nitrogen donors and beryllium acceptors in various lattice positions, allowing for the determination of their phase coherence and spin-lattice relaxation times. ENDOR measurements elucidated the electron-nuclear interactions with the local silicon and carbon environment, including distant coordination spheres. The observed hyperfine structures indicated highly delocalized spin density within the supercell. The TRIPLE resonance spectra verify coupled nuclear spin subspaces from different coordination spheres due to defect spin density. These results demonstrate the feasibility of incorporating dual impurities with distinct functional roles while preserving the crystal lattice`s structural features.
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All authors Yuliya Ermakova, Ekaterina Dmitrieva, Margarita Sadovnikova, Fadis Murzakhanov, George Mamin, Sergey Nagalyuk, Evgeny Mokhov, Marat Gafurov

Mesoscale and Nanoscale Physics

Giant nonlinear Hall effect in a Pt/ferrimagnetic insulator bilayer under Zeeman-exchange frustration

Takayuki Shiino, Matteo Fettizio, Weronika Janus et al. (+1 more)

Competing magnetic interactions can create metastable or unstable states and render magnetic systems highly susceptible to external perturbations. Here we show that Zeeman-exchange frustration in an Al-substituted terbium iron garnet with a compositional gradient across its thickness gives rise to a giant nonlinear Hall response in an adjacent Pt layer. Near magnetic compensation, a field-induced spin-flip transition is accompanied by unusually large higher-order odd harmonic voltages, with the third, fifth, and seventh harmonics reaching amplitudes comparable to that of the first harmonic. The field, temperature, and current dependences collectively identify Joule heating as the parametric drive of the harmonic response. Macrospin-chain simulations further show that current-induced thermal modulation periodically switches the interfacial Fe magnetization between exchange- and Zeeman-dominated states and reproduces the observed harmonic signals. These results demonstrate how frustration can convert a weak thermal perturbation into a large nonlinear electrical response, providing a route to nonlinear magnetotransport in compensated ferrimagnets.
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All authors Takayuki Shiino, Matteo Fettizio, Weronika Janus, Can Onur Avci

Mesoscale and Nanoscale Physics · Cross-listed from cs.ET

Nanoparticle Networks for Neuromorphic Computing

Jonas Mensing, Wilfred G. van der Wiel and Andreas Heuer

Physical computing leverages complex dynamical systems for energy-efficient data processing. In this work, we present a neuromorphic architecture based on metallic nanoparticles interconnected by molecular junctions on a $\text{SiO}_2$/Si substrate. We demonstrate that surrounding static control electrodes transform this nanoparticle network from a passive reservoir into a tunable nonlinear dynamical system. By analyzing how these electrodes route simple one-dimensional voltage inputs into multidimensional signal responses, we establish three core design rules to maximize computational performance. First, operating near the system's cutoff frequency achieves an optimal balance between nonlinear charge tunneling and linear capacitive memory. Second, tuning the underlying $\text{SiO}_2$ thickness sets the electrostatic screening length and dictates the memory type. Thick oxide layers reduce the screening length, causing networks larger than this length to transition into a persistent, non-volatile-like regime. Conversely, networks smaller than the screening length exhibit only fading memory. Third, introducing structural disorder via heterogeneous molecular junctions overcomes inherent limits on expressivity. While a network's computational expressivity scales with its physical size, it is ultimately capped by the screening length. Breaking internal spatial symmetries with localized disorder bypasses this saturation, allowing control voltages to independently manipulate specific signal amplitudes and phases, universally maximizing performance for dynamic neuromorphic applications.
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Statistical Mechanics

Ensemble theory of thermodiffusion

R. Belousov, J. Elliott and A. Erzberger

Heat flow sustained by a temperature difference generates a gradient of matter as observed, for example, in atomistic simulations of hard-core gases. We use path-ensemble theory to represent the coupled thermal and diffusive transport by dynamics on a graph. Calibrated from a single equilibrium simulation, this model accurately predicts nonequilibrium mass distribution in a hard-core gas across a wide range of applied temperature gradients. It also enables estimation of transport properties, such as the Soret coefficient, using analytical expressions.
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Mesoscale and Nanoscale Physics

Scalar-spin-chirality-driven fractional Chern insulator on a kagome lattice

Shinnosuke Tsutsumi, Koji Kudo and Kentaro Nomura

Fractional Chern insulators (FCIs) are the lattice analogs of the fractional quantum Hall states, emerging even without an external magnetic field. In this work, we demonstrate the emergence of the FCI states in a kagome magnet with a noncoplanar magnetic order that induces a finite scalar spin chirality. By incorporating in our model both electron-electron interactions and the effect of band dispersion, we find that stronger interactions relative to the band dispersion stabilize the FCI state over a broader range of scalar spin chirality. We characterize the emergent FCI state by calculating overlap with representative states, identifying the ground-state degeneracy and the finite energy gap in the thermodynamic limit, and tracking the spectral flow under multiple flux-quantum insertions. Our results suggest that kagome magnets with scalar spin chirality can be promising platforms for realizing FCIs.
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Materials Science

Deep Learning for Accelerated Long-Horizon Forecasting of Multicomponent Multiphase Microstructure Evolution in High-Entropy Alloys

Hamidreza Razavi and Nele Moelans

Phase-field modeling provides a powerful approach for predicting microstructure evolution but becomes computationally prohibitive for multicomponent and multiphase systems over large spatial and temporal scales. This work presents an AE-GCN-LSTM surrogate framework for long-horizon forecasting of microstructure evolution in the multicomponent AlCrFeNi high-entropy alloy system containing coexisting BCC and FCC phases. A multi-head autoencoder compresses the four elemental concentration fields and phase-field order parameter into latent representations, which are formulated as graphs for learning their spatial and temporal evolution. The framework accurately forecasts microstructure evolution over horizons extending to 3,000,000 simulation timesteps. Its robustness is systematically evaluated under previously unseen conditions without retraining, fine-tuning, or parameter adaptation. These evaluations include variations in FCC precipitate size and initial position, microstructures containing one, two, and five FCC precipitates, and complex phase interactions involving precipitate merging and splitting. Although trained only on 100 x 100 computational domains containing a single nominal alloy composition, the framework is successfully transferred to larger 256 x 256 and 512 x 512 systems and to previously unseen AlCrFeNi compositions. Across the evaluated configurations, the model preserves the dominant phase morphology and compositional evolution while providing computational speedups ranging from approximately 7200 to 62300 relative to conventional phase-field simulations. These results demonstrate that latent graph-based AE-GCN-LSTM forecasting provides a scalable and computationally efficient surrogate for long-horizon simulation of multicomponent, multiphase microstructures and offers a promising foundation for high-throughput alloy design.
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Soft Condensed Matter

Partial vision leads to an unexpected emergent collective behavior in active aligning particles

Raúl Molina-Prados Lallena, José Martín-Roca and Chantal Valeriani

The Vicsek Model represents a paradigmatic framework for understanding the collective motion of active aligning particles, traditionally assuming isotropic interaction fields. Inspired by biological systems characterized by limited perception and blind spots, we propose a generalized Vicsek model featuring two distinct, non-overlapping angular vision cones. We systematically investigate the non-equilibrium phase behavior of this system by tuning the aperture area ({\alpha}) and the front-back orientation (e̱ta}) of the cones. Our results reveal that restricting the lateral vision area destabilizes global order, shifts the critical noise, and induces highly dense traveling bands. Furthermore, breaking the front-back symmetry introduces non-reciprocal interactions that profoundly alter the emergent spatial structures: forward-biased vision drives strong clustering through "follow the leader" alignment, whereas backward-biased alignment stabilizes an exceptionally homogeneous flocking state with suppressed density fluctuations. Finally, we incorporate short-range volume exclusion, demonstrating that the structural integrity of these novel tightly-clustered phases is highly sensitive to steric interactions. Our work provides new insights into the interplay between non-reciprocal perception, spatial anisotropy, and physical constraints in active matter.
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Mesoscale and Nanoscale Physics

Significant ordinary Nernst effect contribution to spin-orbit torque harmonic Hall measurements in metallic structures

Igor Lyalin, C. C. Chiang and C. L. Chien

The harmonic Hall measurements are commonly used to quantify the spin-orbit torque in ferromagnet/normal metal bilayers. The contribution from ordinary Nernst effect to the second harmonic voltages is usually assumed to be negligible and omitted in the data analysis. We show that in Cr/Co bilayers the ordinary Nernst effect cannot be neglected and can lead to largely exaggerated values of spin-orbit torque efficiency. Conducting additional experiments, we estimate the Nernst coefficient of Cr and find it comparable to the values reported for other metals. Consequently, the ordinary Nernst effect should be carefully considered when employing the harmonic Hall technique to quantify spin-orbit torque in metals, while the results of some previous works might need to be re-examined.
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Disordered Systems and Neural Networks · Cross-listed from nlin.CD

Chaos in reason: How chain-of-thought LLMs can look for an answer

Gregorio Jaca, Kristóf Benedek and János Török

Large Language Models (LLMs) have achieved remarkable performance across a wide range of tasks, yet their internal dynamics remain poorly understood. In this work, we apply the tools of nonlinear dynamics and chaos theory to LLMs. By analyzing both text and hidden state trajectories, we demonstrate that LLMs exhibit hallmark signatures of chaos, including strong sensitivity to initial conditions, manifested as intermittent, jump-like divergence of nearby trajectories combined with bounded evolution, with consistent results across different distance metrics. An exact Jacobian analysis of the Transformer's sub-blocks shows that self-attention and the feed-forward network expand and propagate perturbations, while normalization and residual connections counteract this expansion and promote stability. Recurrence plots show structural similarities between LLMs and canonical chaotic systems such as the Lorenz attractor, while dimension analysis reveals fractal structures in the hidden state space, particularly pronounced in the last layers. We propose that the nonlinear coupling induced by attention mechanisms plays a key role in driving this chaotic behavior.
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Materials Science

Finite-temperature bulk moduli from an EOS-based Grüneisen function

Çetin K{\i}l{\i}ç

An equation-of-state (EOS)-based construction of the Grüneisen function is developed and assessed through predictions of finite-temperature bulk moduli. In this approach, the volume dependence of the Grüneisen function is expressed analytically in terms of static EOS information, anchored by Debye temperatures obtained from elastic data sampled near equilibrium, and constrained by the infinite-compression limit. The resulting form requires as material-specific input only static energy-volume data and near-equilibrium elastic properties, with no parameters adjusted to thermal data. Within a Mie–Grüneisen–Debye framework, the approach is examined for diamond, magnesium oxide, silicon, and sodium chloride, chosen to span a broad range of stiffness, using machine-learning interatomic potentials from the Universal Models for Atoms (UMA) and Universal Point Edge Transformer (UPET) families, together with a dispersion-corrected variant of UMA. The calculated bulk moduli reproduce the expected experimental softening trends and capture the overall scale of the bulk-modulus temperature derivatives, although the level of quantitative agreement depends on the material, the underlying interatomic potential, and the selected analytic EOS form. These results show that an EOS-based construction of the Grüneisen function can capture the leading thermal-softening behavior of bulk moduli without fitting to thermal data. The observed sensitivity to the underlying static description further suggests that the framework may help identify deficiencies relevant to thermoelastic transferability and thereby inform future training and validation strategies for universal machine-learning interatomic potentials.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Quantum Magnonics: Quantum States Generation and Applications

Zi-Xu Lu, Xuan Zuo, Xin-Lei Hei et al. (+5 more)

Hybrid systems based on magnons in ferromagnetic materials, such as yttrium iron garnet, have achieved remarkable development in the last decade. These include the coupling of magnons to microwave and optical photons, superconducting qubits, phonons, spins, the center-of-mass motion of a ferromagnet, etc. Here, we review both the experimental and theoretical progress in this field, focusing on the generation of magnonic quantum states and their applications in a broad range of fields. Since the strong coupling is a prerequisite for achieving coherent quantum control of magnons and preparing magnonic quantum states, we start by introducing representative strong-coupling experiments in cavity magnonics, then review a series of protocols for creating various magnonic quantum states, such as Fock, cat, squeezed, and entangled states, and discuss their potential applications in macroscopic quantum studies, quantum information science, quantum sensing, magnonic quantum devices, dark matter detection, and so on. Finally, we summarize the review and give an outlook for the future study of quantum magnonics.
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All authors Zi-Xu Lu, Xuan Zuo, Xin-Lei Hei, Gang Liu, Zeng-Xing Liu, Qi Guo, Peng-Bo Li, Jie Li

Superconductivity

Cavity Tuning of the CDW–Superconductivity Interplay in a Kagome Metal

Lan-Ting Shi, I-Te Lu, Xin Wang et al. (+3 more)

Kagome metals host competing electronic orders, including charge-density-wave (CDW) order and superconductivity, shaped by intertwined lattice, electronic-correlation, and kagome-geometric effects. Here, using quantum electrodynamical density functional theory, we identify an equilibrium cavity route for reshaping this balance in the kagome metal CsV$_3$Sb$_5$. An out-of-plane polarized single-mode cavity selectively softens CDW-related phonons, counteracting pressure-induced hardening and extending the CDW instability toward higher pressures. In the high-pressure regime where the CDW instability is otherwise suppressed, cavity coupling redistributes Eliashberg spectral weight toward lower frequencies, enhances the total electron–phonon coupling (EPC), and increases the EPC-based Allen–Dynes estimate of $T_c$. This response originates from a charge-density redistribution induced by the out-of-plane photon mode, which modifies lattice restoring forces and drives the phonon and EPC renormalization. These results establish cavity quantum electrodynamics as a viable equilibrium route for tuning intertwined charge order, lattice dynamics, and superconductivity in kagome materials.
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All authors Lan-Ting Shi, I-Te Lu, Xin Wang, Dongbin Shin, Lede Xian, Angel Rubio

Materials Science

A Combined Microbeam and Phase-Field Approach to Identify the Toughness and Ultimate Strength of Amorphous Silica

Gustavo Alberto Rosales-Sosa, Gergely Molnar, Yoshinari Kato et al. (+7 more)

This work presents a new approach to evaluating the toughness, described by the critical energy release rate ($G_c$), and ultimate tensile strength ($\sigma_c$) of amorphous silica (SiO$_2$ glass), combining microbeam tests and phase-field calculations. The latter provides a numerical route to brittle fracture without prescribing explicit fracture surfaces a priori, enabling crack initiation and propagation to be tracked. Single-notched microbeams and newly designed bone-shaped microbeams with a notch-free gauge section were fabricated by Focused Ion Beam (FIB) milling nd tested under bending in air, probing the brittle-fracture and strength-controlled regimes, respectively. Both geometries were modeled by Finite Element Analysis (FEA) coupled with a phase-field formulation. We found $G_c = 5.1$~J/m$^2$ (critical stress intensity factor $K_{IC} = 0.61$~MPa$\cdot$m$^{1/2}$), an intrinsic material length scale $\ell_c = 9.1$~nm, and $\sigma_c = 6.8$~GPa, consistent with previously reported brittle properties of silica glass. Through a parametric study, we show the effect of notch geometry on the fracture response of the microbeams and the impact of dimensional measurement error on the determined toughness. Unlike conventional micromechanical methods that yield only $K_{IC}$, our combined microbeam geometries and phase-field approach simultaneously deliver $G_c$ and $\sigma_c$, bridging brittle-fracture characterization and the strength-controlled regime inaccessible to toughness-only techniques.
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All authors Gustavo Alberto Rosales-Sosa, Gergely Molnar, Yoshinari Kato, Taichi Nakashima, Takahito Ohmura, Etienne Barthel, Guillaume Kermouche, Sergio Sao Joao, Shingo Nakane, Hiroki Yamazaki

Materials Science

Tilt-driven ferrielectricity in PbZrO$_3$

Huazhang Zhang, Ying Liu, Carson Carroll et al. (+6 more)

We reveal a tilt-driven mechanism for ferrielectricity in prototypical antiferroelectric PbZrO$_3$. Specifically, introducing an additional octahedral tilt into the antiferroelectric $Pbam$ phase breaks the symmetry constraint that enforces equal antiparallel dipoles, converting the compensated “$\uparrow \uparrow \downarrow \downarrow$” nonpolar configuration into an uncompensated “$\uparrow \uparrow \downarrow \downarrow$” polar $Pmc2_1$ phase. First-principles calculations show that the $Pmc2_1$ phase becomes stabilized under lattice contraction and gains increasing free-energy advantage over competing phases at finite temperatures. Atomic-scale imaging directly confirms the presence of $Pmc2_1$-like structures in thin films and single crystals. This work identifies a symmetry-governed, kinetically easily accessible pathway to ferrielectricity and establishes a form of “competitive” improper ferroelectricity, with broad implications for antiferroelectrics.
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All authors Huazhang Zhang, Ying Liu, Carson Carroll, Saptam Ganguly, Bin Xu, Xiaozhou Liao, Gustau Catalan, Philippe Ghosez, Nicholas C. Bristowe

Strongly Correlated Electrons

Trion Excitations in Twisted Bilayer Graphene: A Quantum Monte Carlo Study

Shibo Shan, Cheng Huang, Patrick Ledwith et al. (+1 more)

Determining the nature of charge carriers is a fundamental goal in the study of strongly correlated electron systems. Here, we employ the continuous-field momentum-space quantum Monte Carlo method to reveal exotic "Dirac trion" excitations in the finite-temperature normal state of twisted bilayer graphene. While the ground state is a symmetry-breaking insulator with gapped ($\sim$ 20 meV) electron-like excitations, we show that a small temperature ($\sim$ 3 meV), well below the interaction scale, drives the system into a strongly fluctuating symmetric normal state. We demonstrate that this normal state hosts gapless excitations consisting of three-particle bound states, two electrons and one hole, that are exactly orthogonal to the higher-energy electrons at the zero-momentum gapless point. These Dirac trions have the remarkable property of being arbitrarily light despite being composed of heavy constituents, and their spectra can be easily tuned by varying the twist angle and interlayer hopping strength. Our unbiased quantum many-body computation sheds light on the Dirac trions in a projected correlated flat-band setting and opens the door for further investigation of many-body excitations in strongly correlated topological bands beyond Landau levels.
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All authors Shibo Shan, Cheng Huang, Patrick Ledwith, Zi Yang Meng

Statistical Mechanics

Non-reciprocity drives a Brownian dimer out of equilibrium

Suman Pramanik, Soham Dutta and Arnab Saha

We consider the minimal model of a two dimensional Brownian dimer consisting of two overdamped monomers, trapped in an isotropic harmonic potential and mutually coupled by a non-reciprocal harmonic spring that violates Newton's action-reaction principle. We have shown that the non-reciprocal interaction alone can drive the system far from equilibrium, in the absence of any external time dependent drive and being in contact with a single thermal bath. The exact steady state probability distribution and current are explicitly calculated for the zero-rest-length limit of the spring, which eventually maps our model to another non-equilibrium phenomenon, called Brownian gyration. For a spring with finite rest length, these quantities are calculated numerically.
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Materials Science

Optimization of magneto-electric properties in Lead-free (x)Co1.2Ti0.2Fe1.6O4 - (100-x)BaTiO3 based composites

Rajeev Dwivedi, Samanway Mohanta, Ashutosh Anand et al. (+7 more)

This work presents a systematic study of lead-free multiferroic composites of (x)Co1.2Ti0.2Fe1.6O4 - (100-x)BaTiO3 (x = 10, 20, 30), which were synthesized by a solid-state reaction method to investigate the effects of composition and sintering temperature on their structural , electrical, magnetic, and magnetoelectric (ME) properties. X-ray diffraction along with Rietveld refinement confirms the coexistence of tetragonal BaTiO3 (BTO) and cubic spinel Co1.2Ti0.2Fe1.6O4 (CTFO) phases. Microstructural analysis shows that densification and grain growth are better at higher sintering temperatures, leading to better coupling between the two phases. Dielectric and ferroelectric studies indicate lossy polarization-electric field (P-E) behaviour due to leakage from the conductive phase, while magnetic properties show increased magnetization with increasing ferrite content. All composites exhibit ME coefficients, which depend on the composition and sintering conditions; the highest ME coefficient (~1.28 mV/cm.Oe) was observed for the 30CTFO - 70BTO composite sintered at 1200 {\deg}C. This improvement is due to the optimal balance between magnetostrictive and piezoelectric responses and improved interfacial strain transfer. These results demonstrate that simultaneous optimization of dopant-modified composition and sintering conditions is essential for achieving improved magnetoelectric coupling in bulk multiferroic composites. Moreover, the results demonstrate the potential of lead-free composites for multifunctional device applications in next-generation, low-power technologies, including high-density non-volatile memory (e.g. FeRAM/MRAM), magnetic field sensors, spintronic devices, and actuators.
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All authors Rajeev Dwivedi, Samanway Mohanta, Ashutosh Anand, Abhinash Tripathy, Najnin Bano, Dharmendra Kumar, Hemant Singh, R. Venkatesh, Sachin Gupta, Dinesh Kumar Shukla

Materials Science

Unconventional and Fragile Magnetic Exciton in a van der Waals Quantum Magnet

Kai-Xuan Zhang, Min Zhang, Minjae Kim et al. (+25 more)

The recently discovered magnetic exciton in the van der Waals (vdW) antiferromagnet NiPS3 exemplifies these phenomena, exhibiting several distinctive characteristics. Despite extensive investigation, much of its physics remains unresolved, with key questions about why the NiPS3 magnetic exciton is so sharp and optically bright despite the nominally spin-forbidden transition, posing significant challenges to a proper understanding and practical manipulation of the exciton. An urgent question is to what extent it is due to chemical disorder, magnetic weakening, lattice modification, or intrinsic instability of the bright exciton itself: answers to which will put stringent constraints on possible theoretical models. Here we address these questions using hydrostatic pressure as a clean, continuous, reversible, and in-situ tuning parameter. We find that the sharp photoluminescence peak is drastically suppressed by as little as 0.4 GPa and completely quenched by 1.5 GPa, with demonstrating its reversibility. Crucially, this bright-to-dark conversion occurs without magnetic, crystallographic, or electronic reconstruction despite an increase in the Neel temperature, as established by Raman, X-ray absorption, nuclear magnetic resonance spectroscopy, and first-principles many-body calculations. Our results demonstrate that the optical brightness of the magnetic exciton is independent of chemical disorder, lattice expansion, and weakening of magnetic order, indicating that a higher-order correlated mechanism governs the bright exciton. We further propose experimentally constrained microscopic scenarios involving exciton pairing, crystal-field-controlled spin-orbit mixing, and symmetry breaking, providing a framework for future tests of entangled magnetic exciton in correlated quantum magnets.
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All authors Kai-Xuan Zhang, Min Zhang, Minjae Kim, Yong-Hyun Kim, Junghyun Kim, Heejun Yang, Pyeongjae Park, Chaebin Kim, Mangesh Diware, Junik Hwang, Youjin Lee, Byeong-Gwan Cho, Hyeong-Do Kim, Tae-Yeong Koo, Chunhua Chen, Mingtao Li, Xujie Lü, Wenge Yang, Kee-Hoon Kim, Seung-Ho Baek, Hyeonsik Cheong, Sung-Keun Lee, Beom Hyun Kim, Christopher Lane, Jian-Xin Zhu, Zhaorong Yang, Young-Woo Son, Je-Geun Park

Materials Science

A Universal Crystal-Field Design Principle for Orbital-Order-Driven Altermagnetism

Shantanu Pathak and Saswata Bhattacharya

Altermagnets combine collinear antiferromagnetic order with nonrelativistic spin splitting, enabling spintronic functionalities without relying on spin–orbit coupling. While staggered orbital ordering has recently emerged as an alternative route to altermagnetism, its generality has remained unexplored. Here, we establish a universal crystal-field design principle for orbital-order-driven altermagnetism. We show that structural relaxation consistently reconstructs the crystal-field landscape, activating a common $d_{xz}/d_{yz}$ orbital manifold that drives spontaneous staggered orbital ordering and robust $d$-wave nonrelativistic spin splitting across transition-metal compounds spanning electron fillings from $d^1$ to $d^7$. By introducing a unified symmetry framework based on layer-dependent magnetic and orbital order parameters, we demonstrate how interlayer stacking determines whether the system realizes a bulk altermagnetic state or a globally compensated antialtermagnetic phase. Furthermore, we reveal that this symmetry-protected spin-split texture gives rise to highly anisotropic spin-polarized conductivities. Our results establish crystal-field engineering as a predictive design strategy for discovering and engineering orbital-order-driven altermagnets.
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Strongly Correlated Electrons

Iterative minimization in reduced density matrix functional theory for periodic systems

Kai Luo, Jingang Han, Peize Lin et al. (+3 more)

Reduced density matrix functional theory (RDMFT) offers a route beyond Kohn-Sham density functional theory for strongly correlated systems, yet practical calculations for periodic solids are still out of reach. We formulate RDMFT for extended systems in a basis-independent way and present a planewave implementation using iterative minimization for periodic solids, evaluating nonlocal exchange-correlation functionals through the existing adaptive compressed exchange machinery. Natural occupations are optimized under N-representability constraints with a spectral projected gradient (SPG) method or an first-order explicit-by-implicit (EBI) map, while natural orbitals are updated by Riemannian optimization on the complex Stiefel manifolds. Benchmarks on typical systems of \ce{H2}, silicon, and sodium with the Hartree-Fock functional show that SPG reproduces converged hybrid references, whereas EBI can stall when occupations approach $0$ or $1$. With the power and Müller functionals, SPG yields lower energies and more stable convergence than EBI. Applications to fractionally charged \ce{LiH}, dissociating \ce{H2} and \ce{N2} molecules, and equation of state of silicon show that the algorithm presented in this implementation is reliable and robust.
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All authors Kai Luo, Jingang Han, Peize Lin, Daye Zheng, Mohan Chen, Xinguo Ren

Quantum Gases · Cross-listed from hep-th

Non-relativistic Floquet Conformal Field Theory

Diptarka Das, Sumit R. Das, Arnab Kundu et al. (+1 more)

We develop a formalism for studying Floquet dynamics for systems with non-relativistic conformal invariance in d spatial dimensions. Our analysis indicates the existence of two dynamical phases, hyperbolic and elliptic, separated by a parabolic transition surface. We demonstrate this by studying the fidelity of the driven state and the expectation value of a conformal generator in the many body ground state during the drive. Stroboscopically, they behave exponentially in the hyperbolic phase, show oscillatory behavior in the elliptic phase, and exhibit power-law on the transition surface. Our analysis is completely universal and can be directly applied to several systems including trapped fermions near unitarity and resonant anyons. The former can provide experimental signatures of these dynamical phases. We also comment on a holographic perspective of such driven non-relativistic CFTs and demonstrate that the hyperbolic phase is associated with a timelike stationary-limit surface, such as an ergosphere, in the bulk, while the parabolic phase corresponds to an extremal Killing horizon.
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All authors Diptarka Das, Sumit R. Das, Arnab Kundu, Krishnendu Sengupta

Mesoscale and Nanoscale Physics

Third-order nonlinear transport in a percolative two-dimensional superconductor

Wenjun Liu, Chenghe Wang, Xiubin Li et al. (+5 more)

Percolative superconductivity frequently arises in two-dimensional van der Waals materials due to reduced dimensionality, enhanced quantum fluctuations, and complex electron-phonon interactions, providing a unique platform where normal electrons coexist with Cooper pairs. We report the observation of substantial third-order nonlinear transport in a trilayer $1T^\prime$-MoTe$_2$ superconductor within its percolative transition regime. The third-harmonic longitudinal voltage ($V_{\|}^{3\omega}$) exhibits a clear cubic dependence on excitation current below a threshold, with both its magnitude and nonlinear coefficient strongly correlated with the superconducting state. This nonlinear response is semiquantitatively captured by the superconducting fluctuation within the time-dependent Ginzburg-Landau theory, where nonlinear transport arises due to fluctuating Cooper pairs. Our results demonstrate that third-order nonlinear transport serves as a sensitive probe of superconducting transitions in percolative systems and establish a foundation for exploring higher-order transport phenomena in strongly correlated systems.
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All authors Wenjun Liu, Chenghe Wang, Xiubin Li, Kenji Watanabe, Takashi Taniguchi, Tao Zhang, Xiao-Xiao Zhang, Jing Li