Papers

The common New England sight of a cranberry bog presents a rich tapestry of fluid dynamics and soft matter phenomena. Here, we present four connected problems exploring the behavior of cranberries in their stages of harvest: the buoyant rise of a cranberry in a flooded bog, the stable floating configuration of a cranberry on the surface, the aggregation and interaction between many floating cranberries collected with a boom, and the piling of cranberries onto a truck for transportation. We model these phenomena from first principles and develop simple computational simulations of their collective behaviors. Additionally, we describe tabletop experiments to accompany these problems, either as in-class demonstrations or lab activities. Throughout, we draw connections to broader physical principles in soft condensed matter and fluids, allowing the real-world example of the cranberry bog to serve as a bridge between the undergraduate curriculum and topics in soft matter research.

Collective organization in physical, biophysical, and biological systems often emerges from many weak, local interactions, yet the resulting global structures display striking regularities and apparent limits in diversity. Existing theoretical approaches typically emphasize specific mechanisms, detailed dynamics, or energetic optimization, making it difficult to identify constraints that are independent of microscopic realization. Here we develop a general theoretical framework showing that, when effective interactions among system components compress into a low-dimensional interaction space, global organization is governed by geometric constraints rather than detailed dynamics. We formalize interaction spaces as metric manifolds derived from coarse-grained effective couplings and show that low interaction dimensionality imposes upper bounds on the number, separability, and robustness of distinct collective organizations. These results yield impossibility statements: many conceivable macroscopic organizations are excluded a priori, even when locally compatible interactions exist. The framework applies across equilibrium and nonequilibrium systems without assuming specific symmetries or conservation laws. By shifting the explanatory focus from generative mechanisms to structural constraints, this work establishes a general, geometry-based perspective on collective organization.

Identifying physical mechanisms that protect the information carried by various forms of structured light is one of the cornerstones of today's classical and quantum communications. Here we show that the purity of orbital angular momentum (OAM) modes can be protected against degradation in random media by leveraging two fundamental features of their own Schrödinger Hamiltonian dynamics, namely, Zeno effect – frequent observations slow down the evolution – , and Gouy phase – the back-action of the observation. Repeated, OAM-dependent Gouy phase kicks imparted along the disturbing path by simple imaging systems trigger the optical Zeno effect that protects the input OAM mode against mode cross-talk that would broaden the OAM spectrum. Given the universality of the mechanism, the Gouy phase-assisted Zeno effect would protect propagation modes other than those of OAM, and the diverse forms of structured light built with them.

Efficient resolution of neuroinflammation and debris clearance is a key determinant of successful central nervous system regeneration. Regenerative vertebrates such as Danio rerio often exhibit faster immune resolution and debris clearance than mammals, yet the molecular determinants underlying these differences remain incompletely understood. TAM receptor tyrosine kinases (Tyro3, Axl, and Mertk) and their ligands Gas6 and Protein S are central regulators of phagocytosis and immune resolution in the nervous system, but whether intrinsic structural properties of these receptor-ligand complexes contribute to regenerative efficiency has not been systematically explored. Here, we present a comparative in silico analysis of TAM receptors and ligands from zebrafish, human, and mouse, integrating sequence evolution, high-confidence structural modeling, interface characterization, and electrostatic analysis. Despite substantial sequence divergence, ligand-binding domains display strong structural conservation, supporting a conserved global mode of TAM-ligand engagement. At the interface level, zebrafish complexes show enhanced electrostatic contributions and increased salt-bridge density, particularly in the Tyro3-Protein S interaction. Residue-level electrostatic analysis reveals clustered interface hotspots that are spatially conserved across species despite evolutionary rewiring of individual contacts. Together, these results suggest that TAM receptor-ligand interfaces are evolutionarily tuned through subtle electrostatic and geometric optimization rather than large-scale structural changes, providing a conserved yet adaptable framework for species-specific modulation of phagocytic signaling.

We explore a rigorous formulation of agent-based SIR epidemic dynamics as a discrete-state Markov process, capturing the stochastic propagation of infection or an invading agent on networks. Using indicator functions and corresponding marginal probabilities, we derive a hierarchy of evolution equations that resembles the classical BBGKY hierarchy in statistical mechanics. The structure of these equations clarifies the challenges of closure and highlights the principal problem of systemic complexity arising from stochastic but generally not fully chaotic interactions. Monte Carlo simulations are used to validate simplified closures and approximations, offering a unified perspective on the interplay between network topology, stochasticity, and infection dynamics. We also explore the impact of lockdown measures within a networked agent framework, illustrating how SIR dynamics and structural complexity of the network shape epidemic with propagation of the COVID-19 pandemic in Northern Italy taken as an example.

Aggregates of light harvesting 2 (LH2) complexes form the major exciton-relaying domain in the photosynthetic unit of purple bacteria. Application of a generalized master equation to pairs of the B850 units of LH2 complexes, where excitons predominantly reside, provide quantitative information on how the inter-LH2 exciton transfer depends on the distance, relative rotational angle, and the relative energies of the two LH2s. The distance dependence demonstrates significant enhancement of the rate due to quantum delocalization of excitons, the qualitative nature of which remains robust against the disorder. The angle dependence reflects isotropic nature of exciton transfer, which remains similar for the ensemble of disorder. The variation of the rate on relative excitation energies of LH2 exhibits resonance peaks, which however is fragile as the disorder becomes significant. Overall, the average transfer times between two LH2s are estimated to be in the range of 4 - 25 ps for physically plausible inter-LH2 distances.

Higher-order interactions provide a nuanced understanding of the relational structure of complex systems beyond traditional pairwise interactions. However, higher-order network analyses also incur more cumbersome interpretations and greater computational demands than their pairwise counterparts. Here we present an information-theoretic framework for determining the extent to which a hypergraph representation of a networked system is structurally redundant, and for identifying its most critical higher orders of interaction that allow us to remove these redundancies while preserving essential higher-order structure.

Radiofrequency-compressed keV electron sources for ultrafast electron diffraction (UED) face competing demands: short pulses require low charge, yet weak scatterers demand high flux; high repetition rates enable signal averaging, yet most systems operate $\lesssim$1 kHz with low detection efficiency. Here, we demonstrate a 90 keV DC-RF source operating at 40 kHz with direct electron detection that address these challenges simultaneously. THz streaking retrieves compressed pulse durations of 97 $\pm$ 3 fs (FWHM) at 370 aC and 114 $\pm$ 47 fs (FWHM) at 2.8 fC. Long-term $t_0$ timing drifts, characterized independently both by convolution analysis of compression data and direct THz streaking measurements, lie between 65 - 95 fs (FWHM), among the lowest reported for RF-based systems. At low charge (17 aC), we report an intrinsic pulse duration of 56 fs (FWHM) from comparison of simulations to measured compression data, among the shortest for keV UED at $>$16 aC. Moreover, 2.8 fC bunches, combined with 40 kHz repetition rate and direct detection, produce a detectable normalized throughput that is one (three-to-four) orders of magnitude higher than existing keV (MeV) sources. This enables practical UED studies of weakly scattering samples and processes previously impractical due to low cross-sections and long acquisition times.

Motivated by breakthroughs in inertial confinement fusion (ICF), first achieving ignition conditions in National Ignition Facility (NIF) shot N210808 and then laser energy breakeven in N221204, modeling efforts here investigate the effect of imposed magnetic fields on integrated hohlraum simulations of igniting systems. Previous NIF experiments have shown yield and hotspot temperature to increase in magnetized, gas-filled capsules in line with scalings. In this work, we use the 2D radiation-magnetohydrodynamics code Lasnex with a Livermore ICF common model. Simulations are tuned to closely approximate data from unmagnetized experiments. Investigated here is the effect of imposed axial fields of up to 100 T on the fusion output of high-performing ICF shots, specifically the record BigFoot shot N180128, and HYBRID-E shots N210808 and N221204. The main observed effect is an increase in the hotspot temperature due to magnetic insulation. Namely, electron heat flow is constrained perpendicular to the magnetic field and alpha trajectories transition to gyro-orbits, enhancing energy deposition. In addition, we investigate the impact of applied magnetic fields to future NIF designs, specifically an example Enhanced Yield Capability design with 3 MJ of laser energy as well as a high-h̊o}R, low implosion velocity "Pushered Single Shell" design. In conclusion, magnetization with field strengths of 5-75 T is found to increase the burn-averaged ion temperature by 50% and the neutron yield by 2-12. Specifically, we see yield enhancement of at least 50% with only a 5-10 T applied magnetic field for N221204, while a 65 T field on N210808 with symmetrization gives an 8 increase in yield. This is all without further design optimization to best take advantage of an applied B field, which promises even greater improvements for designs tailored specifically towards magnetization.

Synchronization of self-sustained oscillators under fixed-frequency and amplitude forcing is well understood, but how time-varying forcing mangles phase locking has been much less explored. Theory predicts that slow, deterministic modulation of the drive amplitude or frequency can lead to a peculiar synchronization regime characterized by intermittent locking of the oscillation phase beyond the Arnold-tongue boundaries associated with fixed harmonic forcing. We test these predictions in a controllable aeroacoustic self oscillator, i.e, a whistle, that exhibits a robust limit cycle and is subject to external acoustic forcing with programmable frequency and amplitude modulation. Under both slowly varying frequency or amplitude of the forcing, three regimes are observed: (i) strict synchronization (ii) intermittent synchronization, characterized by alternating phase locking and brief phase slip episodes and (iii) no synchronization, with regular phase slips. Particularly in strict synchronization regime, the phase of the oscillator will follow arbitrary slowly-varying drive phase and under amplitude modulation its amplitude fluctuations are strongly suppressed.

We present a proposal for a nanomechanical membrane resonator integrated into a moderate-finesse ($\mathcal{F}\sim 10$) optical cavity as a versatile platform for detecting high-frequency gravitational waves and vector dark matter. Gravitational-wave sensitivity arises from cavity-length modulation, which resonantly drives membrane motion via the radiation-pressure force. This force also enables in situ tuning of the membrane's resonance frequency by nearly a factor of two, allowing a frequency coverage from 0.5 to 40 kHz using six membranes. The detector achieves a peak strain sensitivity of $2\times 10^{-23}/\sqrt{\text{Hz}}$ at 40 kHz. Using a silicon membrane positioned near a gallium-arsenide input mirror additionally provides sensitivity to vector dark matter via differential acceleration from their differing atomic-to-mass number ratios. The projected reach surpasses the existing limits in the range of $2\times 10^{-12}$ to $2\times 10^{-10}$ $\text{eV}/c^2$ for a one-year measurement. Consequently, the proposed detector offers a unified approach to searching for physics beyond the Standard Model, probing both high-frequency gravitational waves and vector dark matter.

The San Fernando Valley (SFV) in Southern California is a complex sedimentary basin whose shape strongly influences ground shaking. We develop a fully quantitative, probabilistic graph-regularized inference model that integrates both gravity and receiver function (RF) constraints and evaluate its ability to determine the basin's shape. The sediment-basement interface in single-station RFs is often difficult to interpret due to scattering and noise, which can render isolated stations unusable. By using RFs from a dense seismic array and incorporating gravity, we address the issue of non-uniqueness in converting the times of RF phases to layer thickness by comparing the predicted gravity to observations at each station. In areas where the density contrast may change, Bayesian inference with a graph Laplacian allows us to determine the effective density contrast by taking into account its neighbors' picks and densities. This method promotes spatial smoothness between neighboring stations, while preserving sharp contrasts in locations supported by the RF and gravity data. We applied this method to a dataset that was acquired in fall 2023, when 140 nodes were installed in the SFV. Our results show the deep Sylmar sub-basin, the San Fernando sub-basin, and the Leadwell high found in a previous study (Juárez-Zúñiga and Persaud, 2025), and our results also show good agreement with the industry seismic reflection profiles across the valley. This method demonstrates how to incorporate gravity with lateral density variations into receiver function interpretation to better map interfaces in the subsurface.

Driven by the growing demand for high-speed 3D measurement in advanced manufacturing, optical metrology algorithms must deliver high accuracy and robustness under dynamic conditions. Fringe projection profilometry (FPP) offers high precision, yet the 2pi ambiguity of the wrapped phase means that conventional absolute phase recovery typically relies on multiple coded patterns, sacrificing temporal resolution. Deep learning-based composite FPP (CFPP) shows promise for single-shot phase recovery from a composite fringe, but limited interpretability makes it difficult to assess reconstruction reliability or trace error sources in the absence of ground truth. To address this, we propose HSURE-CFPP (Heteroscedastic Snapshot-ensemble Uncertainty-aware Ratio Estimation for CFPP). HSURE-CFPP predicts the numerator-denominator ratio used for wrapped-phase computation with a heteroscedastic snapshot-ensemble network, enabling ultra-fast 3D imaging from a single composite fringe and producing pixel-wise uncertainty maps for confidence assessment and unreliable-region identification. Specifically, a heteroscedastic likelihood jointly estimates pixel-wise noise variance to capture data uncertainty, while a snapshot ensemble quantifies model uncertainty via dispersion across snapshots, yielding total predictive uncertainty as an interpretable reliability measure. Experiments on static and dynamic scenes demonstrate that HSURE-CFPP achieves high-accuracy reconstruction at high speed and that the predicted uncertainty correlates well with reconstruction errors, providing a deployable quality-assessment mechanism for deep-learning-based FPP.

We present an all-optical detection approach to determine the position and spatial profile of an electron beam based on quantum properties of alkali metal atoms. To measure the electric field, produced by an electron beam, we excite thermal rubidium atoms to a highly excited Rydberg state via a two-photon ladder transition and detect Stark shifts of Rydberg states by monitoring frequencies of the corresponding electromagnetically induced transparency (EIT) transmission peaks. We addressed several technical challenges in this approach. First, we use crossed laser beams to obtain spatial information about the electron beam position and geometry. Second, by pulsing the electron beam and using phase-sensitive optical detection, we separate the true electron beam electric signature from the parasitic electric fields due to photoelectric charges on the windows. Finally, we use a principle component analysis to further improve signal quality. We test this method to detect the current and to reconstruct a 2D profile of a 20 keV electron beam with currents ranging from 25 - 100 uA. While this technique provides less spatial resolution than fluorescence-based measurements, thanks to their speed and limited optical access requirements it can be useful for real-time non-invasive diagnostics of charged particle beams at accelerator facilities.

We study relaxation toward statistical equilibrium states of inviscid generalised two-dimensional flows, where the generalised vorticity $q$ is related to the streamfunction $\psi$ via $q=(-\nabla^2)^{\frac{\alpha}{2}}\psi$, with the parameter $\alpha$ controlling the strength of the nonlinear interactions. The equilibrium solutions exhibit an $\alpha \mapsto -\alpha$ symmetry, under which generalised energy $E_G$ and enstrophy $\Omega_G$ are interchanged. For initial conditions that produce condensates, we find long-lived quasi-equilibrium states far from the thermalised solutions we derive using canonical ensemble theory. Using numerical simulations we find that in the limit of vanishing nonlinearity, as $\alpha \to 0$, the time required for partial thermalisation $\tau_{th}$ scales like $1/\alpha$. So, the relaxation of the system toward equilibrium becomes increasingly slow as the system approaches the weakly nonlinear limit. This behaviour is also captured by a reduced model we derive using multiple scale asymptotics. These findings highlight the role of nonlinearity in controlling the relaxation toward equilibrium and that the inherent symmetry of the statistical equilibria determines the direction of the turbulent cascades.

While fluids near a solid surface are at the core of applications in energy storage/conversion, electrochemistry/electrowetting and adsorption/catalysis, their nanoscale behavior remains only partially deciphered. Beyond conventional effects (e.g. adsorption/reaction, interfacial transport, phase transition shifts), recent experimental and theoretical studies on metallic surfaces have unraveled exotic peculiarities such as complex electrostatic screening, unexpected wetting transition, and interfacial quantum friction. These novel features require developing and embarking new tools to tackle the coupling between charge relaxation in the metal and molecular behavior in the vicinal fluid. Here, using the concept of Virtual Thomas-Fermi fluids, we employ a molecular simulation approach to investigate interfacial transport of fluid molecules and metal charge carriers at their interface–including the underlying electrostatically-driven dynamic friction and the coupling between charge current/hydrodynamic flow (the so-called electrohydrodynamic drag). While conventional numerical techniques consider either insulating materials or metallic materials described as polarizable, non-conducting media, our atom-scale strategy provides an effective yet realistic description of the solid excitation spectrum–including charge relaxation modes and conductivity. By applying this approach to water near metallic surfaces of various electrostatic screening lengths, we unveil a non-monotonous dependence of the fluid/solid friction on the metallicity with a maximum occurring as the charge dynamic structure factors of the solid and fluid strongly overlap. Moreover, we report a direct observation of the electrohydrodynamic drag which arises from the momentum transfer between the solid and liquid through dynamic electrostatic interactions and the underlying interfacial friction.

Purpose: Annotation of medical breast images is an essential step toward better diagnostic but a time consuming task. This research aims to focus on different selecting sample strategies within deep active learning on Breast Region Segmentation (BRS) to lessen computational cost of training and effective use of resources. Methods: The Stavanger breast MRI dataset containing 59 patients was used in this study, with FCN-ResNet50 adopted as a sustainable deep learning (DL) model. A novel sample selection approach based on Breast Anatomy Geometry (BAG) analysis was introduced to group data with similar informative features for DL. Patient positioning and Breast Size were considered the key selection criteria in this process. Four selection strategies including Random Selection, Nearest Point, Breast Size, and a hybrid of all three strategies were evaluated using an active learning framework. Four training data proportions of 10%, 20%, 30%, and 40% were used for model training, with the remaining data reserved for testing. Model performance was assessed using Dice score, Intersection over Union, precision, and recall, along with 5-fold cross-validation to enhance generalizability. Results: Increasing the training data proportion from 10% to 40% improved segmentation performance for nearly all strategies, except for Random Selection. The Nearest Point strategy consistently achieved the lowest carbon footprint at 30% and 40% data proportions. Overall, combining the Nearest Point strategy with 30% of the training data provided the best balance between segmentation performance, efficiency, and environmental sustainability. Keywords: Deep Active Learning, Breast Region Segmentation, Human-center analysis

Coherent control, a central concept in physics and chemistry, has sparked significant interest due to its ability to fine-tune interference effects in atoms and individual molecules for applications ranging from light-harvesting complexes to molecular qubits. However, precise characterization of the system's dissipative dynamics is required for its implementation, especially at high temperature. In a quantum control experiment, this means learning system-bath parameters and driving coupling strengths. Here, we demonstrate how to infer key physical parameters of a single molecule driven by spectrally modulated pulses at room temperature. We develop and compare two computational approaches based on two-photon absorption photoluminescence signals: an optimization-based minimization scheme and a feed-forward neural network. The robustness of our approach highlights the importance of reliable parameter estimation in designing effective coherent control protocols. Our results have direct applications in ultrafast spectroscopy, quantum materials and technology.

We derive and analyze well-posed, energy- and entropy-stable boundary conditions (BCs) for the two-dimensional linear and nonlinear rotating shallow water equations (RSWE) in vector invariant form. The focus of the study is on subcritical flows, which are commonly observed in atmospheric, oceanic, and geostrophic flow applications. We consider spatial domains with smooth boundaries and formulate both linear and nonlinear BCs using mass flux, Riemann's invariants, and Bernoulli's potential, ensuring that the resulting initial boundary value problem (IBVP) is provably entropy- and energy-stable. The linear analysis is comprehensive, providing sufficient conditions to establish the existence, uniqueness, and energy stability of solutions to the linear IBVP. For the nonlinear IBVP, which admits more general solutions, our goal is to develop nonlinear BCs that guarantee entropy stability. We introduce the concepts of linear consistency and linear stability for nonlinear IBVPs, demonstrating that if a nonlinear IBVP is both linearly consistent and linearly stable, then, for sufficiently regular initial and boundary data over a finite time interval, a unique smooth solution exists. Both the linear and nonlinear IBVPs can be efficiently solved using high-order accurate numerical methods. By employing high-order summation-by-parts operators to discretize spatial derivatives and implementing weak enforcement of BCs via penalty techniques, we develop provably energy- and entropy-stable numerical schemes on curvilinear meshes. Extensive numerical experiments are presented to verify the accuracy of the methods and to demonstrate the robustness of the proposed BCs and numerical schemes.

We report a femtosecond time-resolved strong-field study of ammonia borane (AB, BH3NH3) following both single and double ionization, revealing ultrafast fragmentation dynamics and hydrogen release. Mass spectrometry, combined with fragment correlation analysis and ab initio molecular dynamics simulations, is used to identify the molecular origin of the neutral and ionic products. Singly ionized AB produces neutral H and H2, while doubly ionized AB produces neutral H and H2 along with H+, H2+, and H3+, all within 1 ps. Electronic-structure calculations show that H, H+, H2, H2+, and H3+ originate predominantly from hydrogen atoms bound to the boron center and that their formation proceeds through hydrogen migration and, in some channels, neutral H2 roaming. The calculations further indicate that the dication meets the structural and energetic requirements for neutral H2 release, a prerequisite for forming astrochemically relevant H3+. However, the large adiabatic relaxation energy causes most roaming H2 to dissociate before proton abstraction, suppressing H3+ formation. These results provide new insight into dissociative ionization pathways in hydrogen-rich molecules, extend mechanistic principles developed for halogenated alkanes to ammonia borane, and suggest implications for hydrogen-release chemistry in ammonia-borane-based storage materials.

Despite the large body of research showing that students in STEM classes at all levels learn better via active learning than they do via lecture, post-secondary physics and astronomy (P&A) faculty members continue to primarily use teacher-focused, lecture pedagogy in their classes. Methods include answers from eight faculty members, and interviews with five faculty members who self-identified as primarily using lecture were conducted to determine their perceptions of why they use lecture. During analysis coding, results show that an unanticipated theme not sufficiently represented in the pre-existing literature rose to the forefront: that many of these faculty members feel the decision of pedagogy is out of their control. In conclusion, a grounded theory was developed and is proposed herein that these faculty feel a sense of powerlessness. Reasons offered include administrators often make decisions based on the financial needs of the school, which then force the faculty into using lecture as their primary pedagogy. Implications include that providing professional development in active pedagogies may not be sufficient to help faculty members change pedagogy, as they may need to be convinced that they have the power to make change and use student-centered, active learning pedagogies within their own individual constraints and settings. Understanding that some instructors may feel powerless in choosing how to teach is an important step for professional development providers toward ensuring that faculty have a voice and can choose the best teaching methods for their classrooms.

Stellar-mass binaries evolving in the vicinity of supermassive black holes (SMBHs) may be common in the universe, either in active galactic nuclei or in other astrophysical environments. Here, we study in detail the resonant excitation of SMBH modes driven by a nearby stellar-mass binary. The resulting resonant energy fluxes vary with the orbital location and frequency of the binary, exhibiting a rich and complex structure. In particular, we find that the total energy flux radiated to infinity is maximized at a gravitational-wave frequency that is close to, but not exactly equal to, the real part of the corresponding quasinormal-mode frequency. Moreover, as the binary is moved farther away from the SMBH, this offset from the mode frequency becomes increasingly pronounced. In addition, for suitable orientations, the binary can effectively “feed” the light ring of the SMBH, selectively exciting particular oscillation modes. For rotating (Kerr) black holes, the mode spectrum is significantly more intricate; however, individual modes are also less strongly damped, leading to an enhanced – but more difficult to interpret – resonant response.

Starting from smooth initial data, we investigate the complex-time analytic structure of the one-dimensional Hou–Luo (HL) model, a wall approximation of the three-dimensional axisymmetric Euler equations. While the finite-time blow-up in this setting has been already established, here we chart the entire singular landscape. This analysis is enabled by a novel formulation of the HL model in Lagrangian coordinates, in which the time-Taylor coefficients of the flow fields are evaluated symbolically to high truncation order. Our results are threefold. First, we show that the Lagrangian series for the vorticity converges within the complex-time disc of radius~$t_\star >0$ and is free from (early-time) resonances that impede the Eulerian formulation. Second, applying asymptotic analysis on the series, we recover both the blow-up time and the singularity exponent with high accuracy. This also enables a quantitative assessment of the Beale–Kato–Majda criterion, which we find correctly identifies the blow-up time, but washes out the local singularity exponent, as it relies on a spatial supremum. Third, and most importantly, we develop a Lagrangian singularity theory that predicts the eye-shaped singularity profile observed in Eulerian coordinates by exploiting the driving mechanism of the blow-up: The accumulation of multiple fluid particles at the same Eulerian position. The employed techniques extend recently introduced methods for the inviscid Burgers equation [C. Rampf et al., Phys. Rev. Fluids 7 (2022) 10, 104610], and can be further adapted to higher spatial dimensions or other hydrodynamical equations.

Relativistic magnetic reconnection is thought to power various multi-wavelength emission signatures from neutron stars and black holes. Relativistic resistive magnetohydrodynamics (RRMHD) offers the simplest model of reconnection. However, a small uniform resistivity underestimates the reconnection rate compared to first-principles kinetic models. By employing an effective resistivity based on kinetic models - which connects the reconnection electric field to the charge-starved current density - we show that RRMHD can reproduce the increased reconnection rate of kinetic models, both in local current sheets and in global black hole magnetospheres.

Band degeneracies, ranging from Hermitian Dirac points to non-Hermitian exceptional points (EPs), play a central role in topological phase transitions. Beyond the topology of individual degeneracies, their mutual interactions yield richer phenomena. A representative example is the anomalous non-annihilating collision of pairwise-created degeneracies, previously believed to occur only in non-Abelian multiband systems. Here, we theoretically reveal and experimentally demonstrate that such an anomalous collision can emerge even in a simple two-band system without non-Abelian nature. In a two-dimensional non-Hermitian lattice whose Brillouin zone forms a nonorientable Klein bottle, two EPs with opposite topological charges, pairwise created from a hybrid point, merge into a new vortex point upon re-encounter, instead of annihilating. Remarkably, the hybrid point is a defective degeneracy featuring no eigenenergy braiding, whereas the vortex point is a non-defective degeneracy yet exhibits nontrivial eigenenergy braiding. This process manifests a non-Hermitian phase transition from a gapped phase to a gapless phase, a scenario that we directly observe in a hybrid-dimensional acoustic lattice via momentum-resolved band braid and Berry phase measurements. Our findings identify nonorientability as a new arena for engineering band degeneracies and topological phases, and pave the way for experimentally exploring the interplay between exceptional and nonorientable topology.

We have extended the origin-invariant length gauge (LG(OI)) approach – originally developed by Caricato and co-workers for optical rotation (OR) and electronic circular dichroism (ECD) – to vibrational circular dichroism (VCD). This approach avoids the need for gauge-including atomic orbitals (GIAOs), which are typically required to circumvent the unphysical dependence of the CD rotatory strengths on the arbitrary choice of coordinate origin for length gauge (LG) computations. Benchmark VCD spectra are presented for (P)-hydrogen peroxide, (S)-methyloxirane, (1R, 5R)-{\alpha}-pinene, and (1R, 4R)-camphor using Hartree-Fock (HF) theory and density functional theory (DFT) methods across a range of basis sets and compared to those obtained from LG, velocity-gauge (VG), and GIAO computations. These analyses show that for VCD the LG(OI) approach does not converge to the basis-set limit as rapidly as the GIAO approach, but does yield similar quality spectra as GIAO for all major VCD peaks for quadruple-zeta-quality basis sets. The LG(OI) and VG VCD spectra are less reliable compared to GIAOs for smaller basis sets.

With a view to identifying and understanding the linear receptivity and amplification mechanisms that underpin laminar-to-turbulent transition over blunt bodies in hypersonic flow, we use resolvent analysis to study the flow over a blunt cone with 7{\deg} half-angle at Mach number $M_{\infty} = 6$, zero angle of attack, and nose-radius-based Reynolds number $Re_{R_n} = 90000$. Optimal forcing and responses are obtained for frequencies up to 330 kHz and azimuthal wavenumbers between 0 and 200. Wall-temperature effects are accounted for by considering both isothermal ($T_w =$ 300 K) and adiabatic wall conditions. The resolvent analysis shows that stationary streak modes are the most amplified in the isothermal case, followed by entropy-layer modes between 20 and 140 kHz. In the adiabatic case, the $1^{st}$ Mack mode is the most amplified. The entropy layer, caused by the nose-tip bluntness, has a profound influence on the receptivity structures. For the optimal streak mode, the most intense receptivity structures lie deep in the entropy layer, further away from the boundary-layer edge compared to the equivalent sharp-cone streak mode. This indicates that atmospheric disturbances may excite streak-like instability without fully penetrating the boundary layer. For the entropy-layer modes, the dominant receptivity and fluctuation signatures are located within the entropy layer. An energy-budget analysis reveals that these modes are most susceptible to kinetic disturbances and they sign the most on temperature fluctuations. These modes are found to leverage a temperature mixing mechanism that exploits the baseflow's wall-normal temperature gradient in the entropy layer to grow.

In the merry-go-round fictitious forces are acting like centrifugal force and Coriolis force. Like the Lorentz force Coriolis force is velocity dependent and, following Arnold, can be modeled by twisting the symplectic form. If the merry-go-round is accelerated an additional fictitious force shows up, the Euler force. In this article we explain how one deals symplectically with the Euler force by considering time-dependent symplectic forms. It will turn out that to treat the Euler force one also needs time-dependent primitives of the time-dependent symplectic forms.

Scaling femtosecond terahertz (THz) and ultraviolet (UV) sources to high repetition rates is essential for high-throughput ultrafast spectroscopy and imaging applications. Yet, their efficient generation at high average power remains limited by thermal effects, phase-matching constraints, and material damage. Here, we demonstrate broadband THz and UV generation driven by a common Yb:KGW laser operating from at 40 - 600 kHz. THz radiation is produced by optical rectification in stoichiometric MgO:LiNbO$_3$ using a line-focus geometry, yielding single-cycle pulses of 55 - 92 nJ energy with peak electric fields of 37 - 90 kV/cm. Electro-optic sampling and beam-quality measurements reveal tunable control between central frequency, bandwidth and field amplitude by translating the generation region transversely within the crystal. Using shorter pump pulses preserves THz conversion efficiency, while longer pulses at 100 kHz reduce THz output by up to a factor of four due to cumulative thermal effects. Femtosecond 257.5 nm UV pulses are generated by cascaded fourth-harmonic generation in $\beta$-barium borate with conversion efficiencies exceeding 10% at 40 kHz and stable operation up to 600 kHz. These results demonstrate a compact, thermally robust platform for high-average-power nonlinear conversion and are directly relevant to next-generation high-repetition-rate ultrafast electron diffraction and spectroscopy systems.

Sea ice harbours a rich community of well-adapted microorganisms that inhabit liquid micro-spaces where extreme conditions prevail. Currently at risk under climate change, the sea-ice microbiome holds mysteries about evolution of life on Earth and possibly elsewhere, which require methodological innovation to be unravelled. Gaining microscopic insight into the internal structure and biology of sea ice has traditionally been limited to destructive and extrusive ice core sampling methods. Here we present an in situ microscopic imaging system to observe undisturbed living microorganisms directly within sea the ice matrix. The complex and heterogeneous nature of sea ice, including its water crystal lattice, brine channels, air bubbles, and various impurities, presents engineering challenges for the development of this imaging system. Despite the fragile nature of the sea-ice matrix, we could successfully deploy, test and use the new in situ microscope during a recent expedition on the icepack in Arctic. We collected numerous images of live and intact single-celled and colony-forming diatoms, and documented for the first time at such a high resolution some microphysical features of sea ice. The hardware and software design of the endoscope is presented along with acquisition results of the microstructure and diatom images. These findings collectively demonstrate the potential for this new in situ microscopic imaging system to transform the way we study sea ice and to allow a deeper understanding of its complex microstructure and living microorganisms.