Disordered Systems and Neural Networks
Defect states in three-dimensional diamond photonic band gap crystals
Julia Rocha, Bart A. van Tiggelen, Ad Lagendijk et al. (+2 more)
We perform a theoretical study of defect states within the photonic band gap of three-dimensional diamond
crystals composed of point scatterers and doped with substitutional defects. The defects introduce localized states inside the photonic band gap, whose existence conditions and eigenfrequencies are expressed in terms of the on-site Green's function of the ideal defect-free
crystal. Off-site Green's functions are also calculated as function of distance and are shown to vanish within approximately two unit
cells. Finite-size effects are analyzed by comparing the results obtained in the infinite-
crystal limit with numerical simulations based on the coupled-dipole method. The latter not only reproduce the eigenfrequencies of the defect states within the band gap, but also provide their lifetimes originating from the finite
crystal size. The lifetimes of the defect states increase exponentially with
crystal size, becoming very long for large
crystals. In addition to defect states in the three-dimensional photonic band gap, the defects also give rise to strongly detuned states outside the gap, which decouple from the spectrum of the ideal defect-free
crystal.
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All authors Julia Rocha, Bart A. van Tiggelen, Ad Lagendijk, Willem L. Vos, Sergey E. Skipetrov
Materials Science
Direct current thermo-mechanical testing: Principles, uncertainty hierarchy, and its role in advanced materials characterisation
Abdalrhaman Koko, Sodiq Abiodun Kareem, Olajesu Favor Olanrewaju et al. (+4 more)
Direct current thermo-mechanical testing (DC-TMT), based on resistive Joule heating, enables rapid heating and cooling, steep thermal gradients and simultaneous mechanical loading, making it a powerful tool for probing deformation, phase transformations, oxidation-assisted damage and creep under conditions inaccessible to conventional furnace-based methods. Despite its growing use, DC-TMT lacks formal standardisation and is often misinterpreted as equivalent to bulk isothermal testing, overlooking intrinsic differences in thermal and mechanical fields. This review addresses that gap by consolidating four decades of research on specimen geometry, temperature measurement, strain characterisation and environmental control, and by classifying uncertainty sources as dominant, secondary and conditional. Evidence from modelling and experiment shows that temperature gradients, heating rate and gauge representativeness govern the reliability of inferred material behaviour. Applications across aluminium, steels, nickel-based superalloys, titanium
alloys, hardmetals, zirconium
alloys, shape
memory alloys and additively manufactured systems are critically assessed. The review highlights domains where DC-TMT provides reproducible mechanistic insight and conditions where direct equivalence with bulk data is not warranted. Implications include the need for transparent reporting, multi-sensor temperature validation and integration with electro-thermal modelling to enable rigorous, mechanism-focused interpretation.
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All authors Abdalrhaman Koko, Sodiq Abiodun Kareem, Olajesu Favor Olanrewaju, Rachael Williams, Justus Uchenna Anaele, Yuanbo T. Tang, Bryan Roebuck
Disordered Systems and Neural Networks · Cross-listed from quant-ph
Superextensive learning in quantum reservoirs at the onset of information scrambling
Jonas Freiheit and Francesco Campaioli
The idea that information processing is optimised near the boundary between order and chaos has emerged as a recurring principle across neuroscience, complex systems, and machine learning. Here we test this hypothesis in
quantum many-body systems, numerically simulating two-dimensional Ising networks of up to $N=20$
spins, operated as
quantum reservoirs for time-series forecasting. Using out-of-time-order correlators (OTOCs), we locate the onset of information scrambling as the input strength is swept, separating regimes where information is frozen and scrambled across the whole reservoir state. We show that prediction precision peaks at the onset of scrambling, along with the number of computational-basis states that the reservoir actively populates. We then show that prediction precision grows as a power law $\sim N^{\alpha}$ in the reservoir size, superextensively $(\alpha>1)$ at the onset of scrambling and only sublinearly $(\alpha<1)$ in either neighbouring regime. Finally, we show that scrambling enhances the nonlinear components of the reservoir
memory while reducing its linear capacity. At the onset, the total
memory capacity grows superextensively, provided that the necessary “forgetting” mechanism is supplied by a collective relaxation channel. These results consolidate the role of information scrambling in learning systems, turning it from an operating point into a scaling law for the performance of
quantum reservoirs.
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Materials Science · Cross-listed from physics.optics
Generalizing Thermal Transport in High-Contrast Metamaterials through Interfacial Fresnel Reflection
Seung Hyeon Ham, Yu Min Kim, In Hyeok Choi et al. (+1 more)
The rapid growth of generative AI has intensified the need for efficient heat dissipation in large-scale data centers. To control heat flow, thermal metamaterials with layered structures have been widely used, which impart the anisotropic properties of thermal conductivities. However, the conventional effective medium approximation (EMA) often fails to provide accurate predictions in systems with a high thermal conductivity contrast between adjacent layers embedded in a background medium. Here, we generalize the EMA by introducing two corrective coefficients that extend its validity to regimes where the conventional EMA was previously inapplicable, i.e., high-contrast thermal metamaterials with the background medium. Notably, one of these coefficients that we proposed has the same mathematical form as the Fresnel reflection coefficient in optics. This allows us to interpret the "reflection-like" behavior of heat flow as it penetrates adjacent layers with high thermal contrast. Our findings suggest that heat
diffusion, traditionally viewed as a purely dissipative process, can be understood intuitively through the framework of ray optics.
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All authors Seung Hyeon Ham, Yu Min Kim, In Hyeok Choi, Jeong Woo Han
Materials Science · Cross-listed from physics.chem-ph
Battery Recycling: Mechanistic Modelling of LiCoO$_2$ Leaching with Coupled Diffusion-Reaction Kinetics and Film Passivation
Uddipta Sarma and Ganesh Madabattula
We developed a new mechanistic model for lithium and cobalt recovery during acidic-reductive leaching from LiCoO$_2$. The system considers HCl as an acid and H$_2$O$_2$ as a reducing
agent. The model tracks conversion of LCO, the particle radius, the acid, H$_2$O$_2$, formation-dissolution dynamics of Co$_3$O$_4$ film, the film thickness, recovery of lithium and cobalt, and moles of O$_2$. We introduced the film passivation effects for reduced recovery in the absence of H$_2$O$_2$. We validated the model against the experimental data of recovered Li and Co reported in literature at three acid concentrations (0.5 M, 1.5 M, and 2.5 M) and four H$_2$O$_2$ concentrations (0%, 0.2%, 0.4%, and 0.6% (v/v)). The model works reasonably well at 0.5 M and 1.5 M HCl for the 0% and 0.6% concentrations of H$_2$O$_2$. At 2.5 M HCl, the model over-predicts the data in the presence of H$_2$O$_2$, while it works well at 0% H$_2$O$_2$. We suggest pathways for further improvements in the model. The comprehensive mechanistic modelling framework for the leaching with a full list of the equations, the parameters, and the variables, reported for the first time, can be extended to other cathode chemistries and acid-reductive leaching systems.
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Materials Science
Parity-controlled electron-hole interference in exciton-phonon coupling
Michael O. Atambo
We demonstrate that
exciton-
phonon coupling in polar
semiconductors is governed by a parity-controlled interference selection rule. By performing an exact multipole expansion of the excitonic form factor and validating it against numerical integration of hydrogenic envelope functions, we show that the long-range infrared divergence of the Fröhlich interaction is exactly canceled for elastic scattering between excitonic states of the same parity. The ground-state
exciton is thereby protected from long-wavelength polar
phonons by destructive electron-hole interference. In contrast, transitions between states of opposite parity exhibit constructive interference, preserving a finite, robust coupling to macroscopic polar fields independent of band-structure details. Mass asymmetry between the electron and hole activates higher-order multipole terms in the elastic channel but leaves the constructive inelastic channel essentially unaffected. The selection rule is dimensionally invariant, applying to bulk and two-dimensional systems alike, and naturally explains the anomalously weak
phonon dressing observed in halide perovskites as well as the strong
phonon sidebands in transition-metal dichalcogenides. Our framework provides a universal, analytically exact criterion for
exciton-
phonon coupling strength, offering a design principle for engineering excitonic materials with tailored
phonon interactions.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
The temperature-dependent non-Abelian gauge potential
Zheng-Chuan Wang
This study shows that the usual time
evolution operator can act as a U(1) gauge transformation on the initial wavefuntion. When considering the
spin freedom in the system, the time
evolution operator corresponds to a non-Abelian SU(2) gauge transformation for
spin-1/2 particles and a SU(3) gauge transformation for
spin-1 particles. If we adopt the adiabatic approximation in the magnetization dynamics of a ferromagnetic system driven by a
spin-polarized current, a temperature-dependent non-Abelian thermal gauge potential will appear. We can employ a temperature-dependent Landau-Lifshitz-Gilbert (LLG) equation to describe the magnetization dynamics in the semi-classical limit, a linear approximated solution for this equation is given analytically.
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Mesoscale and Nanoscale Physics
Non-Hermitian topological Euler insulators
Longwen Zhou
Topological Euler
insulators emerge in multiband systems with real Bloch
Hamiltonians and wavefunctions. Their fragile
topologies are characterized by the Euler class of degenerate bands and protected by the $PT$ or $C_2T$
symmetry in two dimensions, which go beyond the tenfold $K$-theory classification of topological matter. In this work, we extend the conception of topological Euler
insulators to non-Hermitian systems and propose a theoretical framework to unlock their nontrivial Euler
topology. Focusing on two-dimensional, three-band non-Hermitian
lattice models with symmetric
Hamiltonians, we formulate a comprehensive description of their topological Euler bands,
entanglement spectrum and bulk-boundary correspondence. Three typical models of non-Hermitian Euler
insulators are constructed and investigated explicitly to illustrate our theory. Unique topological phase transitions and anomalous edge-band overlaps with non-Hermitian origins are further identified. Our study establishes the presence of topological Euler bands in non-Hermitian systems and unveils their intriguing physical characteristics, thereby broadening the existing territory of topological matter in non-Hermitian open systems.
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Quantum Gases
Dynamical development of long-range spatial coherence in non-equilibrium bosonic condensation
Bianca Rae Fabricante, Dąbrówka Biegańska, Paolo Comaron et al. (+14 more)
Development of spontaneous coherence is one of the hallmarks of bosonic condensation in a variety of physical systems, such as cold atoms, confined
photons, and hybrid light-matter quasiparticles like
exciton polaritons in
semiconductors. While spatial coherence is well understood once a steady-state condensate has been established, its temporal
evolution as the condensate forms is largely unexplored. Here, we explore the dynamical formation of a non-
equilibrium, driven-dissipative
exciton-polariton condensate through both time-resolved experiments and numerical modeling. Our study reveals that the spatial coherence is established through two distinct stages. The early-time stage is interaction-driven and features transient oscillations in spatial coherence. This stage is followed by a steady-state regime characterized by a spatially-uniform high degree of coherence that extends over the entire size of the system and persists over time. These stages of spatial coherence development occur in both free flowing and confined
exciton-polariton systems that undergo a quench - rapid growth of the condensate starting from two different initial settings. Our study offers a deep insight into the process by which long-range spatial coherence is established in a non-
equilibrium bosonic condensate.
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All authors Bianca Rae Fabricante, Dąbrówka Biegańska, Paolo Comaron, Mateusz Król, Matthias Wurdack, Maciej Pieczarka, Mark Steger, David W. Snoke, Kenneth West, Daniele Sanvitto, Dario Ballarini, Dimitrios Trypogeorgos, Loren N. Pfeiffer, Andrew G. Truscott, Marzena Szymańska, Elena A. Ostrovskaya, Eliezer Estrecho
Materials Science
Interpretable physics-informed retrieval-augmented generation language model for end-to-end inorganic crystal synthesis planning
Wei-Jian Jiang, Ye-Nan Sha, Hui Guo et al. (+7 more)
Synthesis planning for inorganic materials requires predicting both synthesizability and viable routes by linking microscopic thermodynamic stability with macroscopic synthesis methods, precursors, and processing conditions. Here, we develop an interpretable Physics-Informed
Retrieval-Augmented Generation
Language Model (PIRAG-LM) for end-to-end inorganic
crystal synthesis planning. We construct a material-centered Structured Synthesis Knowledge Base (SSKB) containing route-level records for 13,820 experimentally synthesized inorganic
crystals. PIRAG-LM retrieves historical precedents using chemical, structural, and thermodynamic similarity, then employs a structured LLM
reasoning module to propose routes, precursors, and processing conditions and assess thermodynamic feasibility, kinetics, and accessibility. It achieves 91.4% accuracy in synthesis-method prediction, compared with 72.1% for the LLM alone, and generalizes to materials reported after the knowledge cutoff. Because the framework relies on
retrieval rather than parametric memorization, its performance can be improved by expanding the SSKB without retraining the
language model. Guided by PIRAG-LM, we experimentally synthesize five new compounds: BaMo0.3In0.7O2.95, BaNb0.4In0.6O2.9, Hg[B(CN)4]2, CoCo(CN)6, and SrNb2Fe2(PO4)6, via solid-state and solution routes. These results demonstrate an interpretable machine-learning approach that helps bridge computational materials discovery and experimental realization.
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All authors Wei-Jian Jiang, Ye-Nan Sha, Hui Guo, Jie Chen, Yu-Cai Liang, Ke Zhou, Qi-Long Gao, Dong-Lin Han, Xin-Gao Gong, Wan-Jian Yin
Mesoscale and Nanoscale Physics
Quantum geometric signatures of Link-Unlink transitions and nonlinear Hall response in Hopf-link semimetals
Kamalesh Bera, Arijit Saha and Debashree Chowdhury
Quantum geometry, comprising of
quantum metric and Berry
curvature, plays a significant role in the electronic transport properties of solids. In this work, we theoretically investigate the
quantum geometric properties of a Hopf-link semimetal, a distinct topological class that is charecterized by a nodal link-unlink transition. We compute the interband optical conductivity of the Hopf link, which effectively distinguishes between linked and trivial phases. While recent studies establish
quantum metric dipole-mediated scattering-free nonlinear Hall effect, this effect becomes even more fascinating in systems where the Berry-
curvature-dipole contribution to nonlinear Hall conductivity vanishes. Owing to the underlying $PT$
symmetry of the Hopf-link semimetal, the Berry
curvature and its corresponding contribution to the nonlinear Hall effect are entirely suppressed. Consequently, by introducing an appropriate perturbation, a finite nonlinear Hall conductivity emerges solely due to the
quantum metric in the Hopf semimetal. Notably, this purely intrinsic,
symmetry-driven nonlinear response remains entirely unmixed with extrinsic components.
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Materials Science
Realization of Air-Stable Two-Dimensional Superconductor Nb2Pd3Te5 With Quasi-One-Dimensional Pair Density Modulation
Jiayi Wang, Hui Guo, Hao Zhang et al. (+12 more)
Two-dimensional (2D)
superconductors provide a fertile platform for exploring reduced-dimensional superconductivity and emergent
quantum phenomena. Incorporating quasi-one-dimensional (quasi-1D) structural motifs into 2D
superconductors offers a powerful route to engineer strong electronic anisotropy, enabling unconventional superconducting states and anisotropic superconducting transport functionalities. However, such systems remain rarely realized. Here we report the realization of a 2D
superconductor Nb2Pd3Te5, exhibiting an intrinsic quasi-1D pair density modulation. Monolayer and bilayer Nb2Pd3Te5 is synthesized via van-der-Waals epitaxy. Using ultralow-temperature scanning tunneling microscopy/spectroscopy, we observe the quasi-1D
crystal structure and superconductivity below ~0.6 K with a pronounced quasi-1D pair density modulation. Remarkably, both monolayer and bilayer Nb2Pd3Te5 show strong air stability. Our findings establish atomically 2D Nb2Pd3Te5 as a robust and promising platform for exploring novel low-dimensional
quantum phenomena and anisotropy-enabled superconducting devices.
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All authors Jiayi Wang, Hui Guo, Hao Zhang, Haowei Chen, Peixuan Li, Xianghe Han, Ziang Wang, Siyu Xu, Qian Fang, Haohao Xu, Shixuan Du, Chengmin Shen, Hui Chen, Wang Yao, Hong-Jun Gao
Soft Condensed Matter
Multiple pattern formation in quorum sensing of density enhanced motility
Itay Azizi
Using Langevin dynamics simulations, I investigate nonequilibrium systems of particles following a density-enhanced motility (DEM) rule: particles are passive below a critical local density and active above it. This mechanism represents an inverse of the conventional quorum-sensing rule. I explore specifically quorums much larger than particle size and at two levels of activity. Above critical values of the density, quorum size, and activity, the system undergoes phase separation into low- and high-energy regions. The passive particles organize into distinct spatial patterns, including holes, stripes, and labyrinthine structures, while the active particles form a gas. I characterize the resulting steady states and identify the qualitative mechanisms governing the selection of different morphological regimes. I further discuss the relevance of these results to biological systems governed by analogous quorum-sensing mechanisms and to biological systems exhibiting related forms of spatial organization. Finally, I present directions for future investigation.
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Materials Science
Beyond linear stability: Heterogeneity-induced fingering of crack fronts
Manish Vasoya, Laurent Ponson and Veronique Lazarus
We investigate the stability of elastic interfaces beyond the linear regime by considering penny-shaped crack fronts propagating through toughness heterogeneities. Using fracture mechanics simulations, we drive crack fronts through arrays of obstacles with tunable toughness contrast. At low contrast, the crack front stiffness remains finite and stabilizes front perturbations. Above a critical threshold, however, the stiffness vanishes and the front destabilizes into long fingers that evolve into daughter cracks propagating between obstacles while the original crack remains pinned. Near threshold, the crack front response displays the characteristic square-root scaling behavior of classical saddle-node bifurcations. Yet, our analysis reveals a fundamentally different mechanism: the stable energy-minimizing crack-front configuration disappears without colliding with an unstable counterpart. Instead, the instability originates from a global loss of Griffith-compatible
equilibria governed by the nonlocal interactions along the crack front. Beyond fracture mechanics, these findings point toward a broader class of collective global bifurcations in nonlocal elastic interfaces and may help rationalize the brittle-to-quasibrittle transition in heterogeneous solids.
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Soft Condensed Matter
Splay-induced charging of ferroelectric droplets in highly non-uniform electric fields
Lorenzo Fiorentini, Elia Rocchetti, Raouf Barboza et al. (+2 more)
Ferroelectric sessile droplets on a superhydrophobic surface can be put into motion by highly non-uniform electric fields, in a direction that depends on the field sign. This indicates that droplets carry a non-vanishing electrostatic charge, which we interpret as acquired through contact electrification at the interface with the superhydrophobic substrate. This phenomenon is not observed in the presence of a uniform or quasi-uniform electric field, suggesting that droplets charging is favored in highly nonuniform fields able to induce localized distortions of the ferroelectric polarization, in turn producing localized charge accumulation sites. Electrostatic charging is thus understood as a splay-induced contact electrification and interpreted as a strategy to minimize the accumulated charge. Electrophoretic-like motion is observed in about 30% of the experiments, meaning that splay-induced charging requires the proper combination of conditions such as droplets size, distance from the field source, freshness of the substrate coating and other experimental details that are not easily quantified. When charging does not occur, effects related to droplet polarization such as dielectrophoresis, ejection of polarized fluid jets and electrostriction-like deformations are observed.
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All authors Lorenzo Fiorentini, Elia Rocchetti, Raouf Barboza, Roberta Galeazzi, Liana Lucchetti
Superconductivity
Distinct Surface and Bulk Superconductivity in the Kagome Superconductor SrSn$_3$
Qun Zhu, Yong-Wei Wang, Ji-Hai Zhang et al. (+6 more)
Surface and bulk superconductivity may possess fundamentally different superconducting properties in
quantum materials with nontrivial electronic structures, yet their superimposed spectroscopic signatures often prevent direct experimental access to each superconducting channel. Here we reveal, in epitaxial films of the kagome
superconductor SrSn$_3$, distinct surface and bulk superconducting channels with markedly different superconducting gaps, upper critical fields, and
vortex-core electronic states by tuning the tunneling junction resistance in scanning tunneling spectroscopy. The surface superconductivity is characterized by a thickness-independent superconducting gap and an enhanced upper critical field, whereas the bulk superconducting channel exhibits a larger superconducting gap that decreases with reducing film thickness and a much lower upper critical field. Within
magnetic vortex cores, robust non-split zero-bias conductance peaks are observed exclusively in the surface superconducting channel, while pronounced zero-bias suppression is consistently associated with the bulk superconducting channel. These findings demonstrate that the
vortex-core electronic structure depends sensitively on the underlying superconducting channel, providing new insight into
vortex-bound states in topological
quantum materials.
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All authors Qun Zhu, Yong-Wei Wang, Ji-Hai Zhang, Qiang-Jun Cheng, Chen-Yu Hu, Jun-Zhong Wang, Qi-Kun Xue, Xu-Cun Ma, Can-Li Song
Materials Science
High-throughput Discovery of Magnetic Rare Earth Transition Metal Alloys
Shuo Tao, Osman Goni Ridwan, Liqin Ke et al. (+1 more)
We present an accelerated materials discovery framework that combines
diffusion-based
crystal structure generation with hierarchical screening to identify new rare-earth–transition-metal magnets simultaneously achieving high magnetization and thermodynamic stability. Using this workflow, we systematically explored over 3000 binary (R-T) and ternary (R-T-T$'$) compositions spanning R~$\in \{\text{Y, Sm}\}$, T~$\in \{\text{Fe, Co, Ni}\}$, and T$' \in \{\text{Ti, V, Cr, Mn, Cu, Zn}\}$, and filtered approximately 240{,}000 generated
crystal structures through machine-learning interatomic potential prescreening and
spin-polarized density functional theory validation. We identify 300+ low-energy
magnetic candidates within 0.1~eV/atom above the convex hull at the DFT level, including 5 thermodynamically stable phases. The highest saturation magnetization reaches ${\sim}1.8$~T in Fe-rich binary and ternary phases (SmFe$_{12}$, YFe$_{12}$, YFe$_{18}$Ti and Sm$_2$Fe$_{16}$Mn).
Symmetry analysis reveals that the majority of ternary candidates are subgroup derivatives of known binary prototypes through Wyckoff site splitting that accommodates T$'$ substitution. Site-resolved
magnetic moment analysis further shows that Mn aligns ferromagnetically with the Fe sublattice with minimal magnetization loss, whereas Cr couples antiferromagnetically, providing systematic guidance for dopant selection. These findings demonstrate a generalizable strategy for targeted
magnetic materials discovery and suggest that extending generative searches to larger unit
cells ($>$20 atoms) with higher Fe fractions is a promising route toward stable phases with saturation magnetization exceeding 1.8~T.
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All authors Shuo Tao, Osman Goni Ridwan, Liqin Ke, Qiang Zhu
Superconductivity
Grain Boundary Engineering Effect on Vortex Matter in Superconducting Films
Qun Wang, Ting Chen, Ya-Xun He et al. (+6 more)
Grain boundaries (GBs) in polycrystalline superconducting films act as a double-edged sword: they can pin
vortices or degrade superconductivity through Josephson-like weak-link coupling. Here, we demonstrate that sputtering pressure tunes GB coupling in NbTiN films and visualize its consequences for
vortex matter. The 5 mTorr film exhibits dispersed grain orientations and a two-step resistive transition under field, signaling intergranular weak-link behavior. In contrast, the 7 mTorr film develops a (111) texture, a single-step transition, higher critical current density, a second magnetization peak, and a {\delta}l-type pinning response consistent with improved GB coupling. Cryogenic
magnetic force microscopy reveals a spatially heterogeneous, cluster-like
vortex configuration in the 5 mTorr film, whereas the 7 mTorr film hosts a more uniform distribution with enhanced local order. These results establish a connection between deposition-controlled GB connectivity, macroscopic weak-link transport, and microscopic
vortex organization, providing a practical route to tailor
vortex pinning in polycrystalline superconducting films.
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All authors Qun Wang, Ting Chen, Ya-Xun He, Xing-Jian Liu, Jian-Wen Sun, Kang-Hong Yin, Fang-Ting Lin, Shi-Xun Cao, Jun-Yi Ge
Strongly Correlated Electrons
Electric excitation of spin resonance in altermagnetic and antiferromagnetic conductors
R. Ramazashvili, V. Shablenko and Ya. B. Bazaliy
We predict electric-dipole
spin resonance (EDSR) in altermagnetic conductors: in a
magnetic field ${\mathbf H}_\perp$, perpendicular to the magnetization axis, an AC electric field will induce a
spin resonance peak above the lower threshold frequency $\omega_- = 2H_\perp$. In sufficiently clean samples, this peak shall be clearly visible on the background of ohmic absorption. EDSR can thus serve as a diagnostic of altermagnetism in conducting materials and as a means to distinguish it from higher-
symmetry antiferromagnetic order.
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Mesoscale and Nanoscale Physics
Metastable magnetic domains and the anomalous $B_\parallel=0$ resistance peak in twisted double bilayer graphene
Zhenxiang Gao, Christopher Coleman, Silvia Lüscher et al. (+7 more)
In
graphene moirés, valley polarization gives rise to orbital magnetism, manifested as an anomalous Hall effect and resulting in Barkhausen jumps in longitudinal resistance when changing domain configurations modify quasiparticle scattering. Beyond a simple picture of polarized domains, however,
spin and valley textures within and between the domains are less well understood, as is the effect of these textures on transport. In the valley-polarized quarter-metal state of twisted double bilayer
graphene, a sharp and metastable peak in longitudinal resistance often appears at zero in-plane
magnetic field, whose microscopic origin has yet to be identified. Here, we show that this peak depends on the configuration of domains of orbital magnetism, which is itself set by the gate-voltage trajectory used to enter the ordered state and by the
magnetic field — particularly the in-plane component — present during that trajectory. The sensitivity of the effect to in-plane
magnetic field components points to
spin, linked to valley polarization through
spin-orbit coupling, as the key degree of freedom in both the domain formation and the resistance peak.
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All authors Zhenxiang Gao, Christopher Coleman, Silvia Lüscher, Ruiheng Su, Manabendra Kuiri, Kenji Watanabe, Takashi Taniguchi, Nemin Wei, Chunli Huang, Joshua Folk
Mesoscale and Nanoscale Physics
Signatures of a ferro-Josephson effect in twisted graphene
Ruiheng Su, Zhenxiang Gao, Christopher Coleman et al. (+9 more)
When a
spin-polarized current is driven across a
magnetic domain wall, the resulting
spin-transfer torque may, beyond a critical threshold, set the wall's moments into precession. This precession modulates the Berry
curvature experienced by electrons traversing the wall, producing an electromotive force that is topological in nature and proportional to the precession frequency, mapping precisely onto the DC Josephson effect and leading to the name ferro-Josephson effect. We report signatures consistent with this effect in a twisted
graphene van der Waals
heterostructure, where
spin and valley textures are linked by exchange, Hund's coupling, and
spin-orbit interactions. Tuned to fillings where the isospin degeneracy is spontaneously broken, the samples develop a sharp peak in the longitudinal resistance within a fraction of a millitesla of $B_\parallel=0$—a peak that disappears as the current is reduced toward zero. In differential resistance the feature resolves into sharp resonances that disperse with $B_\parallel$ on microtesla and picoampere scales. We argue that these arise from the current-driven precession of
spin-domain-wall moments, in competition with the in-plane anisotropy set by a minuscule applied field, and that they establish nonlinear transport as a sensitive probe of isospin domain-wall dynamics at energy scales far below $k_BT$.
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All authors Ruiheng Su, Zhenxiang Gao, Christopher Coleman, Manabendra Kuiri, Dacen Waters, Kenji Watanabe, Takashi Taniguchi, Matthew Yankowitz, Nemin Wei, Chunli Huang, Allan H. MacDonald, Joshua Folk
Soft Condensed Matter
Dynamical and conformational behavior of a polymer in a crowded solution
Setarehalsadat Changizrezaei and Colin Denniston
We investigate the structure and dynamics of a
polymer in a fluid containing mobile spherical colloidal crowders of radius $R$. We compare and contrast the behavior with Langevin dynamics (LD) and
lattice–Boltzmann molecular dynamics (LBMD), the latter incorporating long-range hydrodynamic interactions. Both the colloid size relative to the monomer radius $r$ and the volume fraction $\phi$ are varied to determine how crowding modifies
polymer behavior. Increasing volume fraction induces
polymer compaction, with the mechanism strongly dependent on the size ratio $R/r$. Small colloids primarily modify the short-wavelength
polymer conformation, causing self-avoiding-walk-like behavior to persist to shorter length scales, whereas large colloids reduce the effective long-wavelength Flory exponent, indicating degraded solvent quality consistent with a confinement-blob picture.
Polymer diffusion exhibits distinct behavior in LD and LBMD. In LD,
diffusion decreases rapidly and depends strongly on $R/r$; a phenomenological scaling involving $\ln(1+R/r)$ captures this size dependence, and additional scaling with $R_g$ reduces scatter, indicating
polymer-scale correlations induced by crowding. In contrast, LBMD
diffusion follows an effective-medium-like exponential dependence on concentration, governed by hydrodynamic coupling. Rouse-mode analysis identifies three regimes: scaling breakdown at low volume fraction, Zimm-like behavior at intermediate density in both LD and LB, and at high density hydrodynamic screening in LB with confinement-dominated dynamics in LD.
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