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Physics Physics

physics Watch Physics

Thursday 16 April

Biological Physics

Deformation of Bacterial Cell Membranes by Action of Metal Surface under Plasmon Resonance Condition

Taras Vasyliev, Saulius Juodkazis and Valeri Lozovski

This paper is devoted to studies of the mechanical deformation of the S. aureus cell wall. The bacterium is modelled as a thin elastic membrane containing cytoplasm, which is treated as an incompressible fluid. Deformation occurs via Van der Waals interactions between the bacterium and a solid metallic surface, both with and without the influence of surface plasmon resonance (SPR). Our modelling results indicate that the excitation of surface plasmons significantly increases the effective interaction area between the bacterial membrane and the nanostructured surface. The elastic and dielectric properties of the bacterium's components are uninvestigated. Therefore, theoretical calculations are performed in wide, physically meaningful ranges. Thus, the results of studies give only a qualitative estimation. However, they are novel and, with further experiments, can solve the inverse problem of obtaining physical properties. The paper highlights the potential of SPR to enhance antibacterial strategies, inspiring further research and innovation.
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Chemical Physics

Toward Accurate RIXS Spectra at Heavy Element Edges: A Relativistic Four-Component and Exact Two-Component TDDFT Approach

Lukas Konecny, Muhammed A. Dada, Daniel R. Nascimento et al. (+1 more)

We present a relativistic time-dependent density functional theory (TDDFT) approach for the simulation of resonant inelastic X-ray scattering (RIXS) spectra, based on both a full four-component (4c) Dirac-Coulomb Hamiltonian and a modern atomic mean-field exact two-component (amfX2C) Hamiltonian model. The approach builds on the pseudo-wavefunction formalism and a core-valence separation scheme, enabling the efficient evaluation of couplings between two manifolds of excited states relative to a common ground state, as required for solving the Kramers-Heisenberg equation for RIXS. The relativistic formulation provides a variational description of scalar and spin-orbit relativistic effects, which are essential for accurately describing inner-shell excitations involved in RIXS processes. Its transformation to the 2c regime via the amfX2C Hamiltonian significantly reduces the computational cost while offering 4c-quality results by accounting for two-electron and exchange-correlation picture-change effects arising from the X2C transformation. In addition to two-dimensional RIXS maps, the methodology enables the direct evaluation of high-energy-resolution fluorescence detection (HERFD) and resonant X-ray emission spectra (RXES). Applications to 2p3d and 3d4f RIXS maps of selected ruthenium and uranium complexes demonstrate that the amfX2C approach reproduces reference 4c results and experimental spectra with high accuracy, capturing all key spectral features and providing reliable peak assignments.
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All authors Lukas Konecny, Muhammed A. Dada, Daniel R. Nascimento, Michal Repisky

Computational Physics · Cross-listed from math.NA

Asymptotic gauge-invariant Hybrid High-Order method for magnetic Schrödinger equations

Joubine Aghili

We introduce a Hybrid High-Order (HHO) method for the Schrödinger equation in the presence of a magnetic vector potential. In quantum mechanics, physical observables are invariant under continuous gauge transformations, which must be kept at the discrete level to avoid unphysical artifacts. To address this, we construct a discrete covariant gradient operator on arbitrary polyhedral meshes. We prove that the resulting discrete bilinear form guarantees gauge covariance asymptotically at the discrete level. The resulting scheme achieves optimal convergence rates and preserves a discrete Garding inequality, guaranteeing a stable ground state. The theoretical properties of the scheme are corroborated by numerical experiments, including the computation of the Fock-Darwin fundamental energy and replicating the Aharonov-Bohm effect.
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Optics

Roadmap on Attosecond Science

Rocio Borrego Varillas, Pierre Agostini, Fernando Ardana-Lamas et al. (+108 more)

Twenty-five years have passed since the first experimental demonstration of attosecond pulses, marking the advent of our ability to resolve and control electron motion in real time. What began as a technological breakthrough - generating the shortest flashes ever produced - has evolved into a powerful approach for probing and steering electronic dynamics in atoms, molecules, and solids. This roadmap, authored by leading experts in the field, surveys the recent rapid progress in the generation and characterization of attosecond pulses, emerging attosecond measurement and control techniques, and their expanding range of applications. It reviews current and future developments in attosecond light sources, including novel laser technologies, waveform synthesizers, new schemes for high-order harmonic generation, attosecond pulse generation at free-electron lasers, and structured light. Advances in attosecond measurement methodologies are also discussed, encompassing all-attosecond pump-probe spectroscopy, attosecond four-wave mixing, attosecond microscopy, spectroscopy with light transients, and attosecond interferometry. Furthermore, the roadmap addresses applications of attosecond spectroscopy to reveal electron dynamics in molecules and condensed matter systems from both theoretical and experimental perspectives, and highlights emerging directions at the interface with quantum optics and quantum entanglement. Overall, this work aims to serve as a comprehensive resource for navigating the evolving landscape of attosecond science.
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All authors Rocio Borrego Varillas, Pierre Agostini, Fernando Ardana-Lamas, Cord L. Arnold, David Ayuso, Maurizio Reduzzi, Jakub Benda, Jens Biegert, Charles Bourassin-Bouchet, Thomas Brabec, Christian Brahms, Andrew C. Brown, David Busto, Jérémie Caillat, Francesca Calegari, Carlo Callegari, Stefanos Carlström, Zenghu Chang, Ming-Chang Chen, Anna G. Ciriolo, Paul Corkum, Gabriele Crippa, Rafael de Q. Garcia, Louis DiMauro, Nirit Dudovich, Per Eng-Johnsson, Davide Faccialà, Philip Flores, Titouan Gadeyne, Gianluca Aldo Geloni, Chase Geirger, Shima Gholam-Mirzaei, Jimena D. Gorfinkiel, Eleftherios Goulielmakis, Mohammed Hassan, Carlos Hernández-García, Phay Ho, Dandan Hui, Lynda R. Hutcheson, Misha Ivanov, Subhendu Kahaly, Henry Kapteyn, Nicholas Karpowicz, Franz X. Kärtner, Matthias Kling, Omer Kneller, Dong Hyuk Ko, Peter M. Kraus, Maximilian Kubullek, Stephen R. Leone, Franck Lépine, Anne L'Huillier, Chen-Ting Liao, Thomas Linker, Alexander Gabriel Lohr, Matteo Lucchini, Lars Bojer Madsen, Roland E. Mainz, Balázs Major, Jon P. Marangos, David Marco, Hugo Marroux, Sean Marshallsay, Rebeca Martínez Vázquez, Rodrigo Martín-Hernández, Zdeněk Mašín, Michael Meyer, Felipe Morales Moreno, Margaret Murnane, Daniel M. Neumark, Mauro Nisoli, Marcus Ossiander, Sreelakshmi Palakka, Serguei Patchkovskii, Zekun Pi, Luis Plaja, Julita Poborska, Miguel A. Porras, Kevin C. Prince, David N. Purschke, Nicolette G. Puskar, Giulio Maria Rossi, Jérémy R. Rouxel, Thierry Ruchon, Patrick Rupprecht, Pascal Salières, Giuseppe Sansone, Fabian Scheiba, Martin Schultze, Bernd Schütte, Svitozar Serkez, Miguel A. Silva-Toledo, Olga Smirnova, Salvatore Stagira, Andrea Trabattoni, John C. Travers, Igor Tyulnev, Morgane Vacher, Giulio Vampa, Hugo W. van der Hart, Katalin Varjú, Anne-Lise Viotti, Vartika Vishnoi, Marc Vrakking, Vincent Wanie, Stefan Witte, Fei Xu, Vladislav S. Yakovlev, Linda Young, Diling Zhu, Caterina Vozzi

Optics

Optical Modulation Due to Energy Exchange Between Photonic and Exciton Modes in the Intermediate Coupling Regime

Evripidis Michail, Sander A. Mann, Kamyar Rashidi et al. (+4 more)

Actively tunable photonic devices are vital for next-generation optoelectronics requiring rapid switching and high bandwidth. Although organic optoelectronic devices have found wide application, their use as optical modulators has been limited by low absorption in the critical near-infrared (NIR) region, slow response time, and weak nonlinearities. To address these limitations, we developed a scheme based on intermediate exciton-photon coupling in a NIR absorbing squaraine-dye based photonic structure. Using energy-momentum resolved pump-probe spectroscopy, we show that the sign and magnitude of the optical response of our system depends strongly on the energy detuning between the excitonic and photonic modes. These data are analyzed using temporal coupled-mode theory to show that near resonance, a distinct energy exchange process emerges in the cross-over regime between strong and weak light-matter coupling. This effect enables dynamical control over the photoinduced response, providing a pathway for broadband optical signal modulation extending into the NIR spectral region.
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All authors Evripidis Michail, Sander A. Mann, Kamyar Rashidi, Christoph Lambert, Vinod M. Menon, Andrea Alu, Matthew Y. Sfeir

Plasma Physics

Laboratory evidence of electron pressure anisotropy driving plasmoid mediated magnetic reconnection

A. Sladkov, T. Waltenspiel, H. Ahmed et al. (+8 more)

Plasmoid-driven magnetic reconnection in elongated current sheets is suspected to be an ubiquitous phenomenon in space and astrophysical plasmas, but the mechanisms driving its onset and dynamics are still debated. Deciphering the physical mechanisms dominating the destabilization and fragmentation of the current sheet, as well as its evolution, would have a wide impact into our understanding of the induced plasma turbulence and particle acceleration. Here, by coupling 3D hybrid simulations with laser-driven experiments that involve counterflowing high-energy-density magnetized plasmas with a long aspect ratio of their contact layer, we show that electron pressure anisotropy is the driving factor of the growth rate of the tearing instability, and will sustain the reconnection process even without classical resistivity. Dissipative mechanisms, such as resistivity and isotropization, are further found to stabilize the sheet to varying degrees, thus modifying plasmoid formation. By identifying the roles of pressure anisotropy, dissipation, and large-scale geometry, our work lays the groundwork for the evaluation of plasmoid-driven reconnection impact on the dynamics of laboratory and astrophysical plasmas.
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All authors A. Sladkov, T. Waltenspiel, H. Ahmed, A. Alexandrova, V. Anthonippillai, P. Antici, S. N. Chen, I.Cohen, E. d'Humières, W. Yao, J. Fuchs

Plasma Physics

On nonlinear saturation of toroidal Alfvén eigenmode due to thermal plasma nonlinearities

Ningfei Chen, Thomas Hayward-Schneider, Fulvio Zonca et al. (+7 more)

The nonlinear saturation of toroidal Alfven eigenmode (TAE) due to thermal plasma nonlinearities is investigated using gyrokinetic particle-in-cell simulations and theoretical analysis. In the single toroidal mode number simulations with zonal fields filtered out, we find that the saturation level of TAE is governed by thermal plasma nonlinearities for gamma_L/omega_n > 0.47%, which has weak dependence on the linear drive gamma_L, i.e., "stiffness" in saturation level. We find that the frequency of TAE decreases as the amplitude of it increases, which is induced by the phase-space zonal structure (PSZS) of thermal plasmas universally existed in particle-in-cell simulations. The saturation of TAE can be finally reached when the mode merges into the continuum. Following this process, the separation of neighboring poloidal harmonics and mode transition to energetic particle modes can be observed. In simulations with zonal fields, zonal fields can essentially counteract the effects of PSZS of thermal plasmas, leading to roughly a factor of 2 enhancement of the TAE saturation level compared to the single toroidal mode number simulation, implying the necessity of including zonal modes in evaluating the saturation level of TAE.
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All authors Ningfei Chen, Thomas Hayward-Schneider, Fulvio Zonca, Zhiyong Qiu, Zhixin Lu, Xin Wang, Alessandro Biancalani, Alexey Mishchenko, Alberto Bottino, Philipp Lauber

Classical Physics · Cross-listed from cond-mat.stat-mech

Thermodynamic Geometry of Relaxation

Hao Wang, Li Zhao, Shuai Deng et al. (+1 more)

While the geometry of equilibrium states and driven non-equilibrium processes is clearly understood, a geometric description for relaxation towards equilibrium is still lacking. Here, we propose a thermo-geometric measure based on the Rayleigh quotient, reformulating relaxation as a fundamental competition between entropic stiffness and frictional dissipation. Taking a van der Waals gas with two dissipation channels as an example, we explicitly demonstrate its relaxation landscape. Particularly, we find that upon approaching the critical temperature $T_c$, the slow-mode relaxation rate vanishes linearly as $\lambda_s \propto (T-T_c)/T_c$, indicating critical slowing down. This study completes the thermodynamic geometry framework, providing a general tool for characterizing the relaxation dynamics of complex systems.
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All authors Hao Wang, Li Zhao, Shuai Deng, Yu-Han Ma

Optics

Toroidal Plasmonic Nanodimers for Enhanced Near-Infrared Emission in Heterostructured InP Quantum Dots

Arda Gulucu and Emre Ozan Polat

Near-infrared (NIR) emitters operating in the 650-900 nm range are highly attractive for imaging and sensing in turbid media; however, cadmium-free InP-based quantum dots (QDs) often suffer from limited brightness due to nonradiative pathways and inefficient photon outcoupling. In particular, heterostructured InP QDs can exhibit band alignments that induce partial spatial separation of charge carriers, leading to reduced electron-hole wavefunction overlap. This modifies intrinsic recombination dynamics and enhances the sensitivity of their emission to the surrounding photonic environment. Here, we investigate silver toroidal plasmonic nanoantenna dimers (Ag TPNDs) through finite-difference-time-domain (FDTD) simulations as a geometry-tunable platform for enhancing NIR emission of heterostructured InP-based QDs. The coupled toroidal geometry supports strongly confined bonding modes that generate intense nanogap hotspots, while its resonance can be systematically tuned through the toroid aspect ratio. By spectrally aligning the antenna response with QD emission bands (675-845 nm), we achieve large Purcell enhancements together with high quantum efficiencies, demonstrating efficient conversion of enhanced decay rates into radiative emission. We further show that nanometer-scale variations in emitter-antenna separation strongly modulate the radiative rates and spectral response. These results establish toroidal plasmonic nanodimers as a topology-driven platform for controlling emission in NIR quantum emitters and for advancing NIR nanophotonic applications.
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Applied Physics

Spontaneous Emission, Work Potential and Relaxation-Limited Processes in Setting Limits on Solar Energy Conversion Efficiency

Sumanta Mukherjee

Understanding the thermodynamics of radiation and the quantum-mechanical interactions between light and matter is important both for theoretical purposes and for technological advances, such as determining the limits of key processes like light-to-usable-energy conversion efficiencies. In this report, we discuss the physics of these two aspects, considering spontaneous emission as a pathway, and highlight the limitations of such descriptions in assessing energy-harvesting efficiency. In view of these limitations, we adopt a simplified approach to evaluate the exergy and work potential of radiation, providing a framework for assessing various aspects of light-to-usable-energy conversion efficiency. Our approach allows a theoretical estimate of the thermodynamic maximum limit for light-to-usable-energy conversion, which is approximately 76%. We validate these exergy and work potential estimates by modeling and accurately reproducing the Shockley-Queisser limit (~ 33.3%), which imposes a practical constraint on solar-to-usable-energy conversion efficiency. Beyond exergy considerations, our model incorporates processes such as spontaneous emission, nonradiative thermal losses, and photon upconversion, allowing us to evaluate their roles. The model further suggests that, under certain conditions, the maximum conversion efficiency can reach approximately 48%, for example with multijunction solar cells or via photon upconversion. These findings further suggest that the true thermodynamic limit for light-to-usable-energy conversion may be much higher (approximately 76%). However, accurately estimating this limit requires a more complete understanding of the thermodynamics of light, light-matter interactions, and the connection between them.
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Computational Physics · Cross-listed from cond-mat.mtrl-sci

Towards Non-van der Waals 2D Topological Insulators

Mani Lokamani, Gustav Bihlmayer, Gregor Michalicek et al. (+3 more)

Non-van der Waals two-dimensional (2D) materials derived from strongly bonded non-layered crystals have recently emerged as a novel and rising platform for nanoscale research. While uncovering and tuning their (opto-)electronic, catalytic, and magnetic properties has been the focus of intense research, the impact of spin-orbit coupling (SOC) onto their electronic structure has not yet been explored in detail. Studying these effects is, however, particularly relevant due to their surface cation termination and the presence of heavy elements in several representative compounds. Here, we investigate the effect of SOC onto the electronic structure of 2D AgBiO3, NaBiO3, and SbTlO3. While the first two systems show negligible band renormalization upon inclusion of relativistic effects around the band gap, SbTlO3 showcases a large SOC induced splitting (229meV) for the lowest conduction bands associated with a band inversion. Substitution of Tl with Pb forming SbPbO3 brings the band-inverted feature to the Fermi level. Analysis of topological invariants and investigation of edge states of zig-zag and armchair ribbons within the 200meV gap confirms the topological nature of the band splitting. Our work thus establishes a foundation for the systematic study of robust non-van der Waals 2D topological insulators.
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All authors Mani Lokamani, Gustav Bihlmayer, Gregor Michalicek, Daniel Wortmann, Stefan Blügel, Rico Friedrich

Optics · Cross-listed from quant-ph

Oscillation-Free Unconventional Photon Blockade from Quadrature Driving of a Kerr Dimer

Hamid Ohadi

We demonstrate unconventional photon blockade in a symmetric Kerr dimer driven with equal-amplitude fields at a $90^\circ$ phase difference. The minimum inter-cavity coupling is $J_{\min} = \gamma/4$ at a Kerr nonlinearity $U \ll \gamma$ achievable in standard photonic molecules. The quadrature-driven site emits strongly antibunched light with a smooth, oscillation-free second-order correlator directly resolvable with standard detectors. The scheme operates under continuous-wave and pulsed excitation, and fabrication disorder can be fully compensated by re-tuning the drive phase and amplitude ratio, removing the need for post-fabrication cavity trimming
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Optics

High-Integration multimode waveguide grating based CWDM4 MUX/DEMUX with Flat Wide Passband and Ultra-Low Crosstalk for 2xFR4 Module Applications

Xudong Gao, Chuanneng Luo, Mengxue Tao et al. (+5 more)

This work presents a compact, low-crosstalk CWDM4 MUX/DEMUX utilizing cascaded multimode waveguide grating filters. The individual filters are designed with finite Gaussian apodization and positive dispersion, enabling strong unilateral sidelobe suppression while maintaining a minimum feature size compatible with UV lithography. By cascading these filters, we demonstrate a DEMUX that achieves channel crosstalk below -25 dB, insertion loss under 1 dB, and a flat-top bandwidth of approximately 18 nm. The entire device occupies a compact footprint of only 1.6 mm x 40 um, with a channel spacing compatible with commercial TIA and driver chips. Furthermore, a series-parallel hybrid cascade configuration can further suppress the crosstalk to -40 dB.
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All authors Xudong Gao, Chuanneng Luo, Mengxue Tao, Haijiang Yu, Qin Li, Yang Wu, Hongyu Wang, Changfei Hu

Applied Physics · Cross-listed from cond-mat.mtrl-sci

Magneto-optical imaging of macroscopic altermagnetic domains in MnTe

Gakuto Watanabe, Soichiro Yamane, Ryotaro Maki et al. (+6 more)

Altermagnets are a new class of magnets accompanying global time-reversal symmetry breaking (TRSB) without net magnetization. The TRSB results in formation of novel altermagnetic domains. Features of altermagnetic domains, in particular their responses to external stimuli, are essentially important but yet unexplored. Here, we report visualization of bulk altermagnetic domains in MnTe based on scanning magneto-optical Kerr-effect microscopy using telecom infrared wavelength. We found two distinct TRSB domains with large Kerr rotations that do not scale with its tiny bulk magnetization. We also revealed controllability and stability of domains against magnetic or thermal perturbations. Our first observation of altermagnetic domains using a laboratory-scale simple optical technique showing their movable nature provide firm bases for future fundamental and application studies of altermagnets.
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All authors Gakuto Watanabe, Soichiro Yamane, Ryotaro Maki, Atsutoshi Ikeda, Akimitsu Kirikoshi, Junya Otsuki, Takuya Aoyama, Kenya Ohgushi, Shingo Yonezawa

Computational Physics

Sharp-interface VOF method for phase-change simulations on unstructured meshes

Jan Kren, Bojan Ničeno and Yohei Sato

Unstructured meshes are among the most versatile approaches for capturing non-canonical geometries in fluid dynamics simulations. Despite this, most high-fidelity first-principles phase-change models are developed and applied on structured meshes. We present a phase-change simulation method for unstructured meshes that combines the algebraic Volume-of-Fluid (VOF) technique with geometric interface reconstruction, implemented in an in-house open-source CFD code. Phase-change rates are computed from local temperature gradients evaluated at the reconstructed interface, without empirical closure models, using a reconstruction procedure that operates on arbitrary polyhedral cells. Because the method relies on the standard finite-volume framework, it can be integrated into other cell-centred codes supporting unstructured meshes. The approach is validated against the one-dimensional Stefan and Sucking problems and the three-dimensional Scriven bubble growth on both hexahedral and polyhedral meshes, showing good agreement with analytical solutions in all three cases. A detailed analysis of the Scriven problem reveals that the interface-modified least-squares gradient stencil on Cartesian meshes overestimates the interfacial temperature gradient, producing a persistent overshoot of the analytical bubble radius and a coherent four-fold anisotropy that elongates the bubble along grid diagonals. On polyhedral meshes, the irregular face orientations eliminate both effects, yielding isotropic growth and monotonic convergence. Finally, we demonstrate the framework on turbulent upward co-current annular boiling flow, where early transient results are qualitatively consistent with a previous LES study and experimental observations of wave-modulated evaporation.
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Computational Physics

Spectrally Accurate Simulation of Axisymmetric Vesicle Dynamics

M.A. Shishkin

We present a meshless numerical method for simulating the dynamics of axisymmetric vesicles in a viscous medium. Key innovations include: (1) adaptive reparameterization based on local length scales, reducing the number of required harmonics; (2) gauge dynamics for maintaining optimal parameterization; (3) error control near the symmetry axis; and (4) spectrally accurate quadrature schemes for singular integrals. The method achieves high accuracy and computational efficiency for simulating lipid bilayer dynamics and related problems in soft matter physics.
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Computational Physics · Cross-listed from quant-ph

Split-Evolution Quantum Phase Estimation for Particle-Conserving Hamiltonians

Megan Cerys Rowe, Carlo A. Gaggioli, Ludmila Szulakowska et al. (+2 more)

We present a hardware demonstration and resource analysis of split-evolution quantum phase estimation (SE-QPE) on a Quantinuum System Model H2 quantum computer. SE-QPE is a modification to canonical QPE for particle-conserving Hamiltonians in which controlled time evolution is replaced by CSWAP-based interference between a target register and a reference register. For factorizations of time evolution with a shared eigenbasis, SE-QPE preserves the phase-register outcome distribution of canonical QPE and, unlike with compute–uncompute substitutions, it remains compatible with non-exact eigenstates. The substitution removes controlled-simulation overhead and enables parallel evolution on two registers, reducing the depth of each phase-kickback block. Resource analysis for Trotterized double-factorized chemistry Hamiltonians shows that the substitution becomes increasingly favorable at higher phase powers and combining QPE and SE-QPE implementations can be a useful option. Over a range of FeMoco active spaces, SE-QPE reduces time evolution resources, with asymptotic reductions of about 33% in CX count, 25% in $T$ count, and an asymptotic depth ratio of $3/N$ for CX layers. On Quantinuum H2-2, a four-qubit model ethylene demonstration with explicit inverse QFT and repeated phase-kickback steps up to 8 phase bits yields distinct energies and shows the auxiliary registers provide useful error detection filters.
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All authors Megan Cerys Rowe, Carlo A. Gaggioli, Ludmila Szulakowska, David Muñoz Ramo, David Zsolt Manrique

Biological Physics

Self-propelled particles driven by light

Jannis Fischer, Alejandro Jurado and Timo Betz

Recent advances in the field of active soft matter promise a lot. Both, experimental advances and theoretical understanding point towards new material classes in reach, for example self-healing materials that might switch their properties from elastic to solid easily or switch their macroscopic shapes. All these materials require an active force to propel parts of themselves on the micrometer scale. While chemical fuels are often used to generate these active forces, applying energy in a simple and continuous way remains unsolved. Here we explore using light as such an energy source. Overall, generating active driven, self-propelled particles is hence not only of great interest but also a general challenge. Moreover, controlling such particles even within living tissue would open new worlds, for example to enable specific drug delivery or the design of micro-robots. One recently proposed method to establish light driven self propelled particles is to create specific shaped and transparent objects, that move when illuminated with homogeneous light. In these particles, the refraction of the light leads to a momentum transfer, which then drives the active movement. Here, we show both in simulation and experiments that the production of such particles is possible and demonstrate the feasibility of this propulsion effect, while investigating different shapes. Our experiments show that breaking the shape-symmetry of the particles creates a refraction-based propulsion under homogeneous illumination. Subsequent simulations reveal that total reflection leads to the largest momentum transfer among all different geometries considered. Overall, our study introduces the proof-of-principle for refraction-propelled particles, which has the potential to benefit many fields of study including cellular behaviour, collective dynamics and the understanding of disease mechanisms.
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Optics

Light-propelled microparticles based on symmetry-broken refractive index profiles

Julian Jeggle, Matthias Rüschenbaum, Adrian Paskert et al. (+7 more)

Active colloidal microparticles require reliable actuation to sustain directed motion. Light-based propulsion is particularly attractive as it provides persistent energy supply and enables direct spatiotemporal control. Here, we introduce 3D-printable particles with symmetry-broken refractive index profiles (SBRIP particles) that achieve propulsion through direct momentum transfer from asymmetric light refraction. Internal refractive-index gradients provide optical symmetry breaking independent of external shape, fundamentally decoupling propulsion from particle geometry. Geometrically symmetry-broken particles with a homogeneous refractive index are another special case, where propulsion originates from refractive contrast at the boundary instead of within the particle. Unlike conventional systems relying on absorption or reflection, this transparency-based mechanism minimizes heating and mitigates shadowing in bulk suspensions. We present a theoretical framework for refractive propulsion as well as numerical simulations of the SBRIP particles using raytracing and the finite volume method. This is complemented by experiments, validating the momentum transfer mechanism using particles with geometric symmetry breaking. The high transparency of our particles ensures deep light penetration, enabling the realization of volumetric active matter. This opens pathways toward adaptive nonlinear optical materials where light-driven particle reorganization modulates the local refractive index, establishing a dynamic feedback loop between the optical field and the material structure.
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All authors Julian Jeggle, Matthias Rüschenbaum, Adrian Paskert, Ivan Kalthoff, Elena Vinnemeier, Jesco Schönfelder, Jörg Imbrock, Cornelia Denz, Marcel Rey, Raphael Wittkowski

Biological Physics

Unraveling the Mechanism of Drug Binding to SARS-CoV-2 RNA Pseudoknot with Thermodynamics-Driven Machine Learning

Mariia Ivonina and Jakub Rydzewski

The pseudoknot secondary structure in SARS-CoV-2 RNA is essential for regulating protein synthesis through $-$1 programmed ribosomal frameshifting ($-1$ PRF), a mechanism that allows the virus to generate both structural and non-structural proteins from overlapping reading frames. This pseudoknot exhibits both threaded and unthreaded long-lived topologies. The influence of ligand binding on its folding is a process critical for the development of $-$1 PRF small-molecule inhibitors. Understanding this process through unbiased molecular dynamics (MD) simulations can be facilitated by introducing collective variables (CVs) that capture the corresponding slowest dynamical modes. Here, we use spectral map (SM), a thermodynamics-driven machine learning technique, to learn such CVs directly from all-atom MD trajectories of the SARS-CoV-2 RNA pseudoknot in complex with the $-$1 PRF inhibitor merafloxacin and its two structural analogs in neutral and ionized forms. Free-energy landscapes (FELs) derived from the learned CVs indicate that ligand-induced destabilization is topology-selective. In the threaded pseudoknot, the inhibitors destabilize the S2 stem, while in the unthreaded pseudoknot, destabilization occurs in the S1 and S3 stems. Furthermore, the extent to which each ligand reshapes the FEL matches experimentally reported antiviral potency, whereas the protonation state qualitatively alters dynamics within the same RNA topology. Overall, our results show how pseudoknot topology, ligand type, and protonation state collectively influence the slow conformational dynamics of viral RNA and establish physiological protonation as a critical factor for modeling RNA-targeted drug action.
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Chemical Physics

Frozen density embedding with pCCD electron densities

Rahul Chakraborty and Paweł Tecmer

The pair-coupled-cluster doubles (pCCD) method has emerged as a viable approach for quantum-chemical studies of strongly correlated systems. Despite its lower formal scaling (O(N$^4$)) compared to other versions of coupled cluster (CC) theory, applications to large chemical structures are still expensive. Fragmentation and embedding strategies offer a viable approach in such cases. In this work, we present a simple and efficient density-embedding scheme based on pCCD electron densities. The main computational benefit arises from the fact that pCCD response $\Lambda$-equations are much cheaper to compute than those of standard CC methods, providing easy access to one-electron properties. The pCCD densities of the individual subsystems are used to generate static embedding potentials that capture the environment's effect on the embedded system. The individual fragment energies are then iteratively converged in a self-consistent fashion. We demonstrate the reliable performance of this scheme with the estimation of dipole moments of the weakly bound CO2$\cdots$Rg (Rg = He, Ne, Ar, and Kr) complexes and with the modeling of vertical excitations of some microsolvated molecules.
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Optics

End-to-End Inverse Designed Single-Layered Metasurface for High-Compression Snapshot RGB-Achromatic Full-Stokes Polarization Imaging

Xingyu Chai, Jirong Bao, Haining Yang et al. (+1 more)

Snapshot full-Stokes polarimetry across multiple wavelengths remains challenging because conventional architectures rely on multiplexed measurements and bulky optics. We present an end-to-end inverse designed single-layered metasurface that reconstructs RGB full-Stokes images from a snapshot sensor measurement. A metasurface modeled by a multilayer perceptron (MLP) is employed to encode the full-Stokes polarization information. The system jointly optimizes a differentiable metasurface frontend with a U-Net backend. On a real-world dataset, our design achieves a high compression ratio of 12 for snapshot RGB-achromatic polarization imaging using a single-layered metasurface. These results show that end-to-end optical-digital co-design enables high-compression snapshot full-Stokes polarization imaging with a compact footprint.
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All authors Xingyu Chai, Jirong Bao, Haining Yang, Mengdi Sun

Chemical Physics · Cross-listed from cond-mat.soft

Highly coarse-grained polarisable water models for mesoscopic simulations

Michael A. Seaton, Benjamin T. Speake and Ilian T. Todorov

Modelling micro- and meso-scopic scale thermodynamic and transport properties of soft condensed matter hinges upon its representation. This is especially relevant for polar solvents such as water, since these require effective representation of their dielectric nature as driven by molecular charge distributions and molecular network structuring. The dielectric nature of a medium leads to complex phenomena such as local polarisability response and restructuring near interfaces in reaction to changes in local charge distributions. Inclusion of such phenomena when using larger-than-atomistic techniques such as coarse-grained molecular dynamics (CG-MD) and dissipative particle dynamics (DPD) is still an open question, to which we provide a novel way to consider and justify the necessary and suitable coarse-graining level, enabling us to compare new polar CG models' performance against that of an underlying atomistic model. We polarise our previous non-polar nDPD water model to prepare it for use in simulations of liquid electrolytes as well as solvated organic membranes and measure its fitness to serve as a dielectric medium by comparing its properties to those of the TIP3P water model, while simultaneously observing changes to properties already represented well by the non-polar model.
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Fluid Dynamics

Deformation and instability of sessile soap bubbles in an electric field

Hongsik Kim and Sunghwan Jung

Interfacial deformation under electric fields is a common phenomenon in many industrial processes. Particularly, we are interested in the dynamics of sessile soap bubbles in a parallel-plate electric field which exhibits a stable deformation regime followed by conical instability. Using side-view imaging, we track the equilibrium shapes, the transition to the unstable regime, and the pre-jet apex dynamics within one experimental system. In the stable regime, the meridional profile is well described by a spheroidal fit, and the aspect ratio collapses across initial bubble sizes onto a single steady-state branch when plotted against the dimensionless field $E^\ast = \sqrt{\mathrm{Bo}_e}$ for data acquired within a fixed ambient session where the electric Bond number $\mathrm{Bo}_e$ is defined as $\varepsilon_0 E_0^2 R_0/(2\gamma)$. The endpoint of this branch marks the transition to the unstable regime. Above onset of instability, the apex sharpens into a cone with half-angle $30.0^{\circ}$ $\pm$ $0.6^{\circ}$, below the classical Taylor value. To quantify the late pre-jet stage, we define the axial distance $b(t)$ from the instantaneous apex to a fixed reference vertex determined from the terminal cone geometry and measure its evolution. The corresponding rate grows as jetting is approached, and a near-tip inertia-capillary model captures the observed logarithmic trend as an approximation. Together, these measurements establish a single-system experimental benchmark in which stable electrocapillary deformation is organized by a single steady-state branch that leads into conical instability and pre-jet dynamics.
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Chemical Physics · Cross-listed from cond-mat.stat-mech

Emergence of Open Chemical Reaction Network Thermodynamics within Closed Systems

Benedikt Remlein, Massimiliano Esposito and Francesco Avanzini

Under which conditions do the dynamics and thermodynamics of open chemical reaction networks (CRNs), grounded on the notion of idealized chemostats that exchange selected species, emerge from underlying closed CRNs? While open CRNs provide the standard framework to describe out-of-equilibrium chemical systems, real systems are finite and ultimately relax to equilibrium, leaving the status of this description conceptually unresolved. Here, we address this fundamental question. We show that open-CRN behavior arises as an asymptotic regime of closed CRNs when two physically minimal and transparent conditions are met: a time-scale separation, whereby fast reactions effectively act as exchange mechanisms, and an abundance separation, whereby a subset of species behaves as chemostats with diverging chemical capacity. In this regime, both the stochastic dynamics and the thermodynamic structure - including local detailed balance, entropy production, and free-energy balance - emerge to leading order from the underlying closed CRN. We further predict that this regime persists up to a characteristic time proportional to the scale of the abundance separation, thereby providing an experimentally accessible criterion for estimating how long a finite closed system behaves as an open CRN. Our results apply to arbitrary stoichiometries. They show that chemostats need not be introduced as external idealizations, but instead arise as emergent thermodynamic structures within closed systems, providing a unified and physically grounded foundation for the nonequilibrium thermodynamics of CRNs.
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Optics

Low voltage and high-bandwidth thin-film lithium tantalate modulator on a silicon dioxide substrate

Zihan Li, Alexander Kotz, Adrian Schwarzenberger et al. (+2 more)

Modern communication networks demand ever-increasing transmission bandwidth, placing stringent requirements on low-cost, high-performance electro-optic modulators. Substantial advances have been made in integrated photonics employing lithium niobate on insulator. In contrast, photonic integrated circuits based on lithium tantalate – a material already commercially adopted for wireless filters – have been developed, offering reduced DC drift, higher optical power handling, and lower birefringence. These advantages enable more complex and dense photonic integrated circuits, and make lithium tantalate a promising material platform for next-generation integrated electro-optic modulators. However, in contrast to the extensively studied thin-film lithium niobate platform, thin-film lithium tantalate modulators have only been explored on silicon substrates. Here, we report the first fabrication and characterization of thin-film lithium tantalate electro-optic modulators manufactured on a 4-inch (100 mm) fused-silica substrate for adapting a low-loss slow-wave microwave electrode to improve the electro-optic bandwidth. By employing a slow-wave electrode design to achieve velocity matching between microwave and optical signals, the demonstrated modulator achieves a 3-dB electro-optic bandwidth of 64 GHz with a low half-wave voltage of 1.53 V, with potential to operate at the measured 100 GHz electrical bandwidth, if the employed spectral biasing is removed. The modulator moreover exhibits low bias drift, with a constant switching voltage down to 10 mHz. This performance enables high-speed data transmission comparable to state-of-the-art lithium niobate modulators fabricated on quartz substrates. Using the fabricated devices, a net single lane data rate of 440.6 Gbps is achieved using PAM8 signaling.
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All authors Zihan Li, Alexander Kotz, Adrian Schwarzenberger, Christian Koos, Tobias J. Kippenberg

Optics

Spectral and spatial filtering of whispering gallery modes in precision-engineered microbubble resonators

Ramgopal Madugani, Amal Jose, Christophe Pin et al. (+3 more)

Similar to microspheres, thin-walled microbubble resonators support whispering gallery modes (WGMs) that combine ultrahigh Q-factors and small effective mode volumes. In contrast, their hollow nature enables enhanced interactions with encapsulated materials and lower spectral mode density due to the tight radial confinement of the optical modes. However, the existence of a high axial-mode density still leads to significant mode mixing and modal interference that can complicate spectral shift measurements, thereby limiting sensing applications. To address this limitation, we have fabricated geometric filters directly on the surface of microbubbles using focused ion beam (FIB) milling. Based on numerical calculations, we first designed and then fabricated large tapered patterns, such as circular surface dips or holes, that could effectively filter modes while minimizing optical scattering losses. Local lateral mode confinement and partial recovery of high Q-factors were experimentally achieved by adding shallow slit patterns. Using few-mode engineered microbubble resonators, we subsequently demonstrated pressure sensing and wide spectral tuning of WGMs free from mode-mixing artifacts. This precision engineering approach promises improved mode isolation, tunable directional emission, and ultrasensitive measurements in microbubble resonator devices.
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All authors Ramgopal Madugani, Amal Jose, Christophe Pin, Metin Ozer, Martina Hentschel, Síle Nic Chormaic

Medical Physics · Cross-listed from eess.IV

Generative Modeling of Complex-Valued Brain MRI Data

Marco Schlimbach, Moritz Rempe, Jessica Mnischek et al. (+4 more)

Objective. Standard Magnetic Resonance Imaging (MRI) reconstruction pipelines discard phase information captured during acquisition, despite evidence that it encodes tissue properties relevant to tumor diagnosis. Current machine learning approaches inherit this limitation by operating exclusively on reconstructed magnitude images. The aim of this study is to build a generative framework which is capable of jointly modeling magnitude and phase information of complex-valued MRI scans. Approach. The proposed generative framework combines a conditional variational autoencoder, which compresses complex-valued MRI scans into compact latent representations while preserving phase coherence, with a flow-matching-based generative model. Synthetic sample quality is assessed via a real-versus-synthetic classifier and by training downstream classifiers on synthetic data for abnormal tissue detection. Main results. The autoencoder preserves phase coherence above 0.997. Real-versus-synthetic classification yields low AUROC values between 0.50 and 0.66 across all acquisition sequences, indicating generated samples are nearly indistinguishable from real data. In downstream normal-versus-abnormal classification, classifiers trained entirely on synthetic data achieve an AUROC of 0.880, surpassing the real-data baseline of 0.842 on a publicly available dataset (fastMRI). This advantage persists on an independent external test set from a different institution with biopsy-confirmed labels. Significance. The proposed framework demonstrates the feasibility of jointly modeling magnitude and phase information for normal and abnormal complex-valued brain MRI data. Beyond synthetic data generation, it establishes a foundation for the usage of complete brain MRI information in future diagnostic applications and enables systematic investigation of how magnitude and phase jointly encode pathology-specific features.
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All authors Marco Schlimbach, Moritz Rempe, Jessica Mnischek, Lukas T. Rotkopf, Jens Weingarten, Jens Kleesiek, Kevin Kröninger

Computational Physics · Cross-listed from math.NA

High-order kernel regularization of singular and hypersingular Helmholtz boundary integral operators

Luiz M. Faria, Carlos Perez-Arancibia and Svetlana Tlupova

This paper extends and analyzes the high-order kernel regularization framework of Beale & Tlupova (arXiv:2510.13639) to all four on-surface boundary integral operators of the Helmholtz Calderon calculus in three dimensions: the single-layer, double-layer, adjoint double-layer, and hypersingular operators. To the best of our knowledge, this work provides the first high-order kernel regularization of the hypersingular operator for both the Helmholtz and Laplace equations in three dimensions. The regularization replaces each singular kernel with a smooth modification constructed from error functions together with a polynomial correction whose coefficients are determined through moment conditions. Alongside the derivation of the regularizing functions, the paper provides a unified error analysis of the combined regularization and quadrature discretization procedure. By coupling the regularization parameter to the mesh size, the two error contributions can be balanced, leading to explicit overall convergence rates that depend jointly on the order of the regularization and the degree of exactness of the surface quadrature rule. A key practical feature of the method is its implementation simplicity. Once the regularizing functions are determined, the numerical task reduces entirely to the evaluation of smooth surface integrals using standard quadrature, without the need for element-local solves, singularity-specific precomputations, or specialized quadrature rules. Although the modified kernel is generally incompatible with kernel-specific fast methods, this limitation is addressed through H-matrix acceleration, applicable in a black-box manner. Numerical examples – including verification of the predicted convergence rates and solution of sound-soft and sound-hard scattering problems by smooth obstacles – demonstrate the accuracy and practicality of the proposed methodology.
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Chemical Physics

Interfacial Electric Fields in Water Nanodroplets are Weakly Dependent on Curvature and pH

Gabriele Amante, Fortunata Panzera, Gabriele Centi et al. (+4 more)

The origin of enhanced reactivity in aqueous microdroplets remains debated, with interfacial electric fields (IEFs) often invoked as catalytic drivers. Here, we provide a quantum-mechanical, spatially resolved characterization of the electric field at air-water interfaces by combining deep-learning molecular dynamics with ab initio re-sampling. Across planar interfaces and nanodroplets of varying curvature and charge state, we find an outward-oriented field of $\sim 1.0$–$1.2$ V/Å along the intrinsic surface normal. Crucially, its magnitude scales linearly with the average number of hydrogen bonds per interfacial molecule, directly tying the field to the local hydrogen-bond network. Despite its large magnitude and contrary to common expectations, we find that curvature and pH exert only a minor influence on the IEF, becoming negligible at experimentally relevant droplet sizes and pH. Consequently, the reactivity differences observed in $\mu$m-sized droplets cannot be ascribed to variations in the IEF, which changes by a factor of only $\sim10^{-5}$ between $3$ and $40\mu$m-sized droplets. Moreover, the IEF is localized inside the interfacial region and rapidly vanishes within a few Å. This strong spatial confinement renders the IEF strongly tied to the local electronic structure, identifying it as a local property of the air-water boundary rather than an independent physical driver of “on-water” catalysis.
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All authors Gabriele Amante, Fortunata Panzera, Gabriele Centi, Jing Xie, Ali Hassanali, A. Marco Saitta, Giuseppe Cassone