Computational Physics · Cross-listed from cs.LG
Uncertainty-aware Multi-fidelity Closure via Conditional Normalizing Flows
Jice Zeng, Shady E. Ahmed, David Barajas-Solano et al. (+1 more)
Reduced-order models (ROMs) provide efficient surrogates for complex multiscale systems, but their predictive accuracy is often compromised by truncation errors and the inadequate representation of interactions between resolved and unresolved scales. The missing effect of truncated (unresolved) scales on ROM (resolved) scales is often denoted as the closure problem. In this work, we formulate ROM closure modeling as a multi-fidelity (MF) learning problem and propose an uncertainty-aware MF framework based on conditional normalizing flows to enhance ROM predictive accuracy. The proposed approach learns a probabilistic mapping from low-fidelity (LF) ROM coefficients to high-fidelity (HF) coefficients, thereby improving predictive fidelity while quantifying the uncertainty associated with the learned closure. Two correction strategies are investigated: direct learning, in which HF coefficients are predicted directly from LF inputs, and residual learning, which learns the discrepancy between LF and HF coefficients. The framework is demonstrated on a double shear layer and
vortex merging problems governed by the two-dimensional NavierStokes equations. Results show that both correction strategies improve ROM accuracy over uncorrected ROM, with residual learning achieving consistently better performance than direct learning. Moreover, the two proposed deep generative model-based strategies provide uncertainty quantification for the corrected ROM coefficients, which is critical for assessing prediction confidence and supporting the reliable use of ROMs in practical applications.
Read the full abstract ▾
All authors Jice Zeng, Shady E. Ahmed, David Barajas-Solano, Panos Stinis
Physics and Society
Symbiosis as a systemic catalyst and the impossibility of coalitions in optimal networks
Giulia Palma, Antonio Rizzo and Chiara Mocenni
The stability of complex systems hinges on the tension between individual incentives and collective welfare. Modeling these dynamics through strategic network interactions based on anti-coordination, we formally prove that any globally optimal configuration constitutes a Strong Nash
Equilibrium, creating topological barriers against collective deviations. However, in sub-optimal states, strictly individualistic
agents remain trapped in stagnant
equilibria. We show that coalition formation acts as a vital catalyst for global efficiency. Paralleling Tomasello's evolutionary theory of shared intentionality, the emergence of symbiotic joint agency overcomes selfish stagnation and drives the system toward optimal niche partitioning. We validate our framework through extensive computational simulations and apply it to an empirical pollination network, demonstrating how symbiosis may steer real-world ecosystems toward maximum resilience. We uncover metastable dynamics where coalitions continuously reconfigure, revealing that biological
evolution relies on a perpetual, adaptive balance between competition and cooperation.
Read the full abstract ▾
General Physics
Effective Constrained Scalar–Gauss–Bonnet Inflation Motivated by $f(R,\mathcal{G})$ Gravity
G.G.L. Nashed, Sudan Hansraj and Amare Abebe
We develop an effective framework for inflation in a constrained scalar–Gauss–Bonnet theory motivated by a restricted sector of $f(R,\mathcal{G})$
gravity. Using unified parametrizations of the Hubble expansion rate and the Gauss–Bonnet coupling function within a generalized slow-roll formalism, we derive analytical expressions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$, and study their dependence on the model parameters. We show that the Hubble parametrization mainly controls the scalar sector through the slow-roll parameter $\epsilon_1$, while the Gauss–Bonnet-induced contribution $\epsilon_4$ can significantly affect the scalar tilt and strongly suppress
primordial tensor modes, naturally leading to very small values of $r$. A representative
benchmark solution yields $n_s \simeq 0.958$ and $r \simeq 2.7 \times 10^{-4}$, marginally compatible with current Planck, ACT, and BICEP/Keck constraints. We further investigate the scalar perturbation structure of the exactly constrained theory, where the Lagrange-multiplier constraint forces the lapse perturbation to vanish and, together with the gravitational momentum constraint, implies $\dot{\mathcal{R}}=0$, eliminating the propagating scalar degree of freedom at linear order. This exact result clarifies that the generalized slow-roll treatment should be interpreted as an effective softly constrained description. We also discuss perturbative stability conditions, including the positivity of the relevant kinetic coefficients and propagation speeds. Our results demonstrate that the effective constrained scalar–Gauss–Bonnet framework provides a flexible and observationally viable description of inflation while clarifying the distinction between the exact constrained limit and its effective slow-roll realization.
Read the full abstract ▾
Applied Physics · Cross-listed from cond-mat.soft
Round-Robin Test of a Light-Emitting Electrochemical Cell: Establishing a Reference Protocol for Quality Research
Anton Kirch, Kumar Saumya, Joan Ràfols-Ribé et al. (+26 more)
Emerging technologies benefit from a jointly established reference protocol, which can lower the bar of entry for new researchers while serving as a calibration standard for established actors. The light-emitting electrochemical
cell (LEC) combines electrochemistry and optoelectronics in an intricate manner, and it can by that enable sustainable and commercially relevant printing fabrication of emissive thin-film devices. However, LEC performance is sensitive to a range of material and processing parameters, which frequently results in inadequate, or even erroneous, device evaluation. With this in mind, we present herein a LEC reference protocol, which details the sourcing of materials and the procedures and parameters for robust device fabrication and operation. The protocol has been tested across nine international research groups, and the collected results from this interlaboratory round-robin test confirm that good LEC performance can be reproducibly obtained following our protocol. We also identify common pitfalls that can arise during LEC development, and present practical steps for attaining optimum LEC performance. We hope this reference protocol will improve the quality of future LEC research and serve as a guide for future researchers entering this vibrant field.
Read the full abstract ▾
All authors Anton Kirch, Kumar Saumya, Joan Ràfols-Ribé, Shi Tang, Christian Larsen, Ajay Kumar Poonia, Nicolò Maccaferri, Chang-Ki Moon, João Pedro Ferreira Assunção, Frank Nüesch, Sandra Gellner, Rubing Bai, Weiao Yang, Zuowei Liu, Daniel Tordera, Sergio Martínez-Saiz, Shun-ichiro Ito, Koshi Oi, Felix Hergenhan, Karl S. Schellhammer, Sebastian Reineke, Taishi Takenobu, Henk J. Bolink, Yufeng Hu, Zhiwei Liu, Ekaterina Nannen, Roland Hany, Malte C. Gather, Ludvig Edman
Instrumentation and Detectors
Polariton spectroscopy at the diamond K-edge via X-ray parametric down-conversion
Fridtjof Kerker, Dietrich Krebs, Xenia Brockmüller et al. (+4 more)
It has recently been shown that x-ray parametric down-conversion (XPDC) provides access to high-energy polaritons, resulting from the hybridization of down-converted
photons with electronic excitations in a nonlinear medium. Here, we present a spectrally resolved study of this effect around the K-shell absorption edge in diamond. Our results exhibit pronounced signatures of polaritonic hybridization, which we visualize by introducing a polariton spectral map and analyze by help of theoretical modelling. We find that the hybridization at this absorption edge results in substantially higher coupling strength than previously reported for a non-resonant case and reaches well into the strong-coupling regime. In addition, we demonstrate how our measurements of polaritonic XPDC allow us to extract the refractive index for bulk diamond at high spectral resolution around the carbon K-edge.
Read the full abstract ▾
All authors Fridtjof Kerker, Dietrich Krebs, Xenia Brockmüller, Ankita Negi, Christoph J. Sahle, Blanka Detlefs, Christina Boemer
Chemical Physics · Cross-listed from cs.CE
Closed-Loop Molecular Design with Calibrated Deference
Newman Cheng, Gordon Broadbent IV, Jason Dong et al. (+11 more)
We present Cognitive Loop via In-Situ Optimization (CLIO), an
agent that couples a continuously-updated belief-state graph with a recursive plan-then-act loop. The result is a
reasoning agent that can contribute something qualitatively different, which we term calibrated deference: the capacity to recognize when its own tools or assumptions are failing, to adapt its strategy in response, and to generate mechanistic hypotheses that guide experimental revision. We tested CLIO in a closed-loop human-AI campaign to design an aqueous organic redox flow battery (AORFB) negolyte, with CLIO leading proposal and interpretation in close partnership with chemists who synthesized, characterized, and weighed in on design choices. Across 17 candidates over three rounds, CLIO converged on a top phosphonate candidate; characterization confirmed a 130~mV improvement in redox potential over the literature baseline. Characterization then revealed unexpectedly poor electrochemical reversibility – a regression no property predictor had flagged. CLIO generated competing mechanistic hypotheses, prioritized discriminating diagnostics, traced the failure to phosphonate-potassium ion pairing, and prescribed a sulfonate replacement. The resulting compound showed substantially improved electrochemical reversibility and maintained a 90~mV improvement in redox potential, closing the design-make-test-redesign loop.
Read the full abstract ▾
All authors Newman Cheng, Gordon Broadbent IV, Jason Dong, Syed Mohammed Ali Hussaini, Farman Ullah, Morris Sharp, Gabrielle Barnes, Nanlin Guo, Deyu Zou, Karin Strauss, William Chappell, David G. Kwabi, Bichlien H. Nguyen, Jake A. Smith
Computational Physics · Cross-listed from quant-ph
A Variational Quantum Algorithm for Nonlinear Finite Element Analysis of Hyperelastic Materials
Uditnarayan Kouskiya and Caglar Oskay
This manuscript explores a variational
quantum formulation for nonlinear elasticity problems arising from hyperelastic material models. The approach leverages the potential energy structure of hyperelasticity and employs a hybrid
quantum classical framework in which the energy functional is evaluated using parameterized
quantum circuits and optimized through classical routines. To enable a hybrid (classical
quantum implementation),
polynomial approximations of the nonlinear terms in strain energy density are introduced, yielding a representation compatible with variational
quantum algorithms. The methodology is demonstrated on a special case of the NeoHookean material model in a one dimensional setting using finite element discretizations with first and second order shape functions and nonhomogeneous boundary conditions. Numerical experiments investigate the influence of the
polynomial approximation order on the accuracy and efficiency of the proposed approach, illustrating its feasibility for near-term
quantum devices.
Read the full abstract ▾
Atmospheric and Oceanic Physics · Cross-listed from cs.LG
Probabilistic adjustment of seasonal forecasts using cVAE with a non-parametric decoder: A case study of Arctic sea ice predictions
Parsa Gooya and Reinel Sospedra-Alfonso
Seasonal
climate predictions support planning and risk management by offering early information of the likely
climate conditions in the coming months, and associated uncertainties. Ensemble forecasts enable this by simulating many plausible outcomes, allowing predictions to be expressed as usable probabilities. Large ensembles and high-resolution forecasts strengthen this by better sampling uncertainty and capturing finer-scale processes but come with significant computational cost. Moreover, forecast ensembles drift and exhibit systematic biases and spatio-temporal errors that grow with lead time, requiring careful post-processing and calibration. A probabilistic post-processing framework based on conditional Variational Autoencoders (cVAEs) was developed at the Canadian Center for
Climate Modeling and Analysis to generate large ensembles of adjusted seasonal predictions of Arctic sea ice. The generative model was designed to learn the observational distribution conditioned on the biased model prediction. This enables generation of arbitrarily large ensembles of calibrated, bias corrected forecasts with improved skill. Here, we extend this framework to a Generalized Variational Inference problem to address the loss of fine-scale energy and the characteristic blurriness, a known limitation of standard cVAEs. Specifically, we employ a generator in place of the Gaussian parametrized decoder in cVAE and use Continuous Ranked Probability Score (CRPS) in the objective function instead of the Gaussian log-likelihood. We further use a higher resolution target
dataset compared to the raw forecast. We show that the adjusted forecasts are better calibrated, more consistent with the observational distribution, and exhibit smaller errors than
benchmark predictions, while also enhancing the resolution of the raw forecasts and improving sharpness and spectral power relative to the standard cVAE.
Read the full abstract ▾
Optics
Cavity optomechanics in an infinite cylinder
Samar Deep, Emily Eadie and Pablo Bianucci
We report the observation of optically-induced mechanical oscillations in flame-produced surface nanoscale axial photonic resonators (SNAPRs). The frequency of the optically-excited mechanical modes in these SNAPRs is not affected by the particulars of the optical axial confinement, and matches with the theoretically predicted frequency for the fundamental breathing mode of an infinite solid cylinder. When the SNAPR fabrication recipe is changed, changing the optical mode spectrum, the behaviour of the observed mechanical modes remains unchanged. This shows a degree of geometrical uncoupling between the optical and mechanical degrees of freedom, which in turn implies the effective radius variation causing axial optical confinement in our SNAPRs is dominated by a refractive index change rather than a physical radius change.
Read the full abstract ▾
Computational Physics · Cross-listed from cs.LG
Unveiling Multi-regime Patterns in SciML: Distinct Failure Modes and Regime-specific Optimization
Yuxin Wang, Yuanzhe Hu, Xiaokun Zhong et al. (+7 more)
Neural networks trained under different hyperparameter settings can fall into distinct training "regimes," with consistent behavior within regimes and qualitative differences across regimes. In this paper, we study such multi-regime behavior in scientific machine learning (SciML) models through a regime-aware diagnostic framework that jointly analyzes performance, training dynamics, and loss-landscape geometry. We identify three key findings: (i) a consistent three-regime structure emerges across many standard SciML models, different constraint enforcements, and various optimizer designs; (ii) optimization effectiveness is regime-specific, with no single method performing well across all regimes; and (iii) SciML models can exhibit fine-grained failure modes that can challenge conventional interpretations of standard loss-landscape metrics. Our results provide an approach to establish a unified, task-oblivious perspective on failure modes in SciML and to inform regime-aware guidance for improving robustness. We validate these findings across widely-used SciML models, including physics-informed neural networks, neural operators, and neural ordinary differential equations, on
benchmarks spanning representative ordinary and partial differential equations.
Read the full abstract ▾
All authors Yuxin Wang, Yuanzhe Hu, Xiaokun Zhong, Xiaopeng Wang, Haiquan Lu, Tianyu Pang, Michael W. Mahoney, Yujun Yan, Pu Ren, Yaoqing Yang
Atmospheric and Oceanic Physics · Cross-listed from astro-ph.EP
Shock wave formation in the thermosphere by an earthgrazing fireball: Empirical evidence for volatile-enhanced hydrodynamic shielding
Elizabeth A. Silber, Denis Vida, Miro Ronac Giannone et al. (+7 more)
Hydrodynamic shielding is a theoretically well-established but observationally elusive and experimentally difficult-to-replicate phenomenon with implications that extend far beyond meteor physics. Rare earthgrazing meteoroids with infrasound signatures that penetrate to the ground can be used to probe hydrodynamic shielding that leads to strong shock formation at high altitude. Here, we report the first coordinated optical and multi-station infrasound observations of a centimeter-scale earthgrazing fireball that generated sustained cylindrical line shock at thermospheric altitudes near 92 km. The event was recorded by numerous optical stations and three infrasound arrays, allowing trajectory reconstruction, ablation behavior, acoustic source localization, and shock characteristics. Optical observations indicate early mechanical erosion and ablation/evaporation at exceptionally low dynamic pressure, consistent with a cometary or a porous, volatile-bearing CM chondritic object. Independent infrasound detections localize shock generation to multiple points along a 164 km trajectory segment near perigee. Weak-shock modeling yields a consistent blast radius of ~30 m, implying an acoustic-equivalent source size far exceeding the physical dimensions of the ~45 g
nucleus. We demonstrate that classical gas dynamics and ablation-driven hydrodynamic shielding alone cannot account for these observations under ambient thermospheric conditions. We show that volatile release provides the additional flow-field density enhancement required to amplify hydrodynamic shielding, reduce the effective local Knudsen number, and sustain a shock envelope capable of radiating detectable infrasound. These results demonstrate that small, volatile-rich meteoroids can transiently establish continuum-like flow in rarefied environments.
Read the full abstract ▾
All authors Elizabeth A. Silber, Denis Vida, Miro Ronac Giannone, Jamie Shepherd, Sarah Albert, Daniel C. Bowman, Tammy Do, Margaret Campbell-Brown, Peter Jenniskens, Reynold E. Silber
Atmospheric and Oceanic Physics
Density and sea level changes weaken the Atlantic Meridional Overturning Circulation through ocean bottom pressure
Qianjiang Xing, Shane Elipot, William E. Johns et al. (+6 more)
Observations from the RAPID array at 26.5$^\circ$N indicate a linear decline in the Atlantic Meridional Overturning Circulation (AMOC) over the past two decades, linked to contrasting boundary changes: a weakening western-boundary contribution that is partly compensated by strengthening at the eastern boundary. Yet it remains unclear whether this partial compensation reflects a basin-wide adjustment or a regional feature, and what processes drive it. Here we use a high-resolution ocean model to investigate the spatial structure and underlying mechanisms of the AMOC change across the mid-latitude North Atlantic. The model reproduces a meridionally coherent decline in western-boundary deep overturning transport together with a partially compensating strengthening at the eastern boundary, consistent with observations at 26.5$^\circ$N. These opposing trends arise from a vertically coherent ocean bottom pressure trend shaped by two competing drivers: rising coastal sea level and decreasing interior density. Through geostrophic balance, this mechanism produces partial boundary compensation across latitudes, yielding a basin-wide AMOC decline throughout the mid-latitude North Atlantic.
Read the full abstract ▾
All authors Qianjiang Xing, Shane Elipot, William E. Johns, Bablu Sinha, Jules Kajtar, Adam T. Blaker, Tillys Petit, Ben I. Moat, David A. Smeed
Medical Physics · Cross-listed from math.OC
Improving depth-resolution, in-plane contrast, and reducing non-uniformity artifacts for wide-angle DBT
Emil Y. Sidky, Adrian A. Sanchez, John Paul Phillips et al. (+4 more)
Purpose: This work aims to develop an image reconstruction
algorithm for wide-angle digital breast tomosynthesis (DBT) that has improved depth resolution and in-plane contrast while reducing non-uniformity artifacts. Approach: The image reconstruction
algorithm is an extension of our prior work on sparsity-regularized iterative image reconstruction. The
algorithm is performed in two stages as explained in a prior work. The first stage consists of a low-resolution reconstruction that exploits sparsity for quantitative accuracy. In this work, this first stage is augmented with a formulation that includes the estimation of a "background" image, which absorbs low-frequency artifacts that cause image non-uniformity. Results: The new
algorithm is demonstrated on three patient cases for which the data are acquired on a wide-angle DBT system. Conclusion: The results on the shown cases indicate that the
algorithm design goals have been met, but additional empirical results and task-based assessment are needed to strengthen this conclusion.
Read the full abstract ▾
All authors Emil Y. Sidky, Adrian A. Sanchez, John Paul Phillips, Zheng Zhang, Dan Xia, Ingrid S. Reiser, Xiaochuan Pan
Space Physics
Optical and Radar Observations of the February 2025 Falcon 9 Upper-Stage Re-entry
Juha Vierinen, Dabrowka Knach, Jorge L. Chau et al. (+9 more)
We investigate the February 19, 2025, re-entry of a Falcon 9 upper stage using optical observations from 43 meteor cameras across central Europe together with radar detections of re-entry
plasma obtained with the 32.55 MHz SIMONe Germany multistatic radar system. Optical observations of fragment emissions between 85 and 36 km altitude were used to reconstruct 30 fragment trajectories, identify two main fragment families, and fit ballistic trajectories to estimate kinetic energy loss per unit mass. The optical detection-height distribution peaks near 60 km with a standard deviation of 10 km, and both optical and radar signatures occur in the same broad altitude region as the maximum kinetic-energy loss. Radar echoes were detected at altitudes between 55 and 75 km, and the radar-derived positions are consistent with those obtained from optical observations. Two distinct radar echo types associated with the re-entry
plasma were identified: (1) specular trail echoes from overdense wake
plasma, with radar cross-sections (RCS) of up to 60 dBsm, and (2) short-lived non-specular trail echoes with RCS values of 20–30 dBsm, exhibiting a delay of 1–2 s compared to optical signatures. The characteristic decay time of both echo types is approximately 1 s. In the radar-echo altitude range, the estimated Knudsen numbers for meter-scale fragments are well below unity, consistent with continuum-flow conditions and shock-driven
plasma production rather than ordinary meteor-like impact ionization. These serendipitous radar observations demonstrate that the atmospheric re-entry of other spacecraft, including objects smaller than the Falcon 9 upper stage such as Starlink satellites, may likewise be detectable using comparable multistatic meteor radar systems deployed globally.
Read the full abstract ▾
All authors Juha Vierinen, Dabrowka Knach, Jorge L. Chau, Gerd Baumgarten, Devin Huyghebaert, Matthias Clahsen, Nico Pfeffer, Toralf Renkwitz, Robin Wing, Kenneth S. Obenberger, Björn Gustavsson, Daniel Kastinen
Space Physics
Estimating sub-frame time differences in camera image sequences
Juha Vierinen, Pavithiran Sivasothy and Björn Gustavsson
Some optical measurements require relative timing of intensity variations with accuracy much finer than the camera frame period. One motivating example is dynamic aurora, where different
prompt emissions are expected to originate from different altitude regions and can therefore have millisecond-scale relative delays caused by finite energetic-electron velocities and other electron-transport effects. These delays are predicted to be a small fraction of the frame duration of typical auroral
video cameras. We present a cross-spectral technique for estimating the relative delay between two time-varying optical intensity signals recorded by one or more image sensors. The method is validated with a calibration device that generates two pseudorandomly pulsed optical emissions with a known relative delay, recorded using a consumer smartphone camera. For the tested recordings, the method estimates relative delays between image-sensor regions with better than $50$~$\mu$s accuracy. Although developed for high-frame-rate auroral imaging, the technique has numerous other imaging applications, including camera timing calibration and measurements of time-varying optical signals. The single-camera tests demonstrate that the method can characterize sub-frame timing differences across an image sensor, such as those produced by rolling-shutter readout. The same analysis applies to separate cameras when they observe the same time-varying signal and are synchronized to a shared clock.
Read the full abstract ▾
Optics
Lensless wide-field 3D fiber endoscopy through scattering media using synthetic wavelength holography
Muralidhar Madabhushi Balaji, Parker Liu, Patrick Cornwall et al. (+3 more)
Minimally invasive imaging with fiber optic endoscopes is crucial for in vivo visualization of tissue morphology, as it supports applications such as early detection of tumors. However, imaging performance of conventional fiber endoscopes is limited when scattering layers are present between target and the distal end of endoscope. This limitation is particularly relevant in biomedicine, where targets such as early stage lesions or blood clots can be obscured by scattering tissue. To address this challenge, we present a lensless endoscopic imaging approach based on synthetic wavelength holography (SWH). SWH is a computational imaging technique in which two optical fields acquired at closely spaced wavelengths are combined to synthesize a field at a longer synthetic wavelength. As the field at longer wavelengths is less sensitive to path length perturbations, this approach can enable endoscopic recovery of
holographic information despite scattering in the intervening tissue. In addition, because the synthetic field is assembled from scattered optical fields with larger optical étendue, our approach can extend the field of view (FoV) beyond the numerical aperture of the fiber. In this paper, we present the first demonstration of an SWH-based lensless endoscope using a multicore multimode fiber. We experimentally recover three-dimensional images of objects hidden behind scattering layers and through real biological tissue, with a spatial resolution of $\approx 500~\mu m$. We further demonstrate recovery of object information over an extended FoV of 46{\deg} without any distal optics. These results suggest a practical path toward extending fiber endoscopy for wide-field, three-dimensional imaging through scattering media.
Read the full abstract ▾
All authors Muralidhar Madabhushi Balaji, Parker Liu, Patrick Cornwall, Tianyi Wang, Jürgen Czarske, Florian Willomitzer
Applied Physics
Comparing Spatially Periodic Feedback and Space-Time Modulation for Unidirectional Wave Propagation in a 1D Mass-Spring-Damper System
João H.S. Brandão, Danilo Braghini and José R.F. Arruda
Unidirectional wave propagation has emerged as a key concept in the dynamics of non-reciprocal mechanical and acoustic metamaterials. This work investigates two fundamentally distinct strategies for achieving directional wave propagation in a periodic one-dimensional mass-spring-damper lumped system: space-time modulation and spatially periodic feedback. In the first approach, the stiffness is modulated periodically in both space and time. The resulting space-time periodic system is analyzed using a Plane Wave Expansion (PWE) formulation based on the Bloch-Floquet theory to obtain the dispersion relation. The traveling modulation produces asymmetric dispersion diagrams and directional band gaps, within which elastic waves propagate preferentially in a single direction due to broken time-reversal
symmetry. In the second approach, non-reciprocity is introduced through a spatially periodic feedback action. The force can depend on the displacement and/or its derivatives, such as velocity or acceleration, and is applied to the masses along the system. The lumped system with a finite number of unit
cells is modeled using classical mechanics principles, yielding a state-space model of the system. The active mechanism can generate directional amplification or attenuation via the non-Hermitian skin effect (NHSE), characterized by boundary-localized modes identified by a topological invariant, the winding number. The stability of the space-time periodic system is assessed through the Lyapunov-Floquet theory. In the periodic feedback case, stability is investigated using the
eigenvalues of the state
matrix. These results provide design guidelines for directional wave propagation in elastic waveguides.
Read the full abstract ▾