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Hydrogen bonding plays a crucial role in biology and technology, yet it remains poorly understood and quantified despite its fundamental importance. Traditional models, which describe hydrogen bonds as electrostatic interactions between electropositive hydrogen and electronegative acceptors, fail to quantitatively capture bond strength, directionality, or cooperativity, and cannot predict the properties of complex hydrogen-bonded materials. Here, we introduce a novel approach that conceptualizes the effect of hydrogen bonds as elastic dipoles in an electric field, which captures a wide range of hydrogen bonding phenomena in various water systems. Using gypsum, a hydrogen bond heterostructure with two-dimensional structural crystalline water, we calibrate the hydrogen bond strength through an externally applied electric field. We show that our approach quantifies the strength of hydrogen bonds directly from spectroscopic measurements and reproduces a wide range of key properties of confined water reported in the literature. Using only the stretching vibration frequency of confined water, we can predict hydrogen bond strength, local electric field, O-H bond length, and dipole moment. Our work also introduces hydrogen bond heterostructures - a new class of electrically and chemically tunable materials that offer stronger, more directional bonding compared to van der Waals heterostructures, with potential applications in areas such as catalysis, separation, and energy storage.

Low symmetry crystals have recently emerged as a platform for exploring novel light-matter interactions in the form of hyperbolic shear polaritons. These excitations exhibit unique optical properties such as frequency-dispersive optical axes and asymmetric light propagation and energy dissipation, which arise from the presence of non-orthogonal resonances. However, only non-vdW materials have been demonstrated to support hyperbolic shear polaritons, limiting their exotic properties and potential applications. Here we introduce for the first time novel shear phenomena in low symmetry crystal thin films by demonstrating the existence of elliptical and canalized shear phonon polaritons in gypsum, an exfoliable monoclinic sulphate mineral. Our results unveil a topological transition from hyperbolic shear to elliptical shear polaritons, passing through a canalization regime with strong field confinement. Importantly, we observe a significant slowdown of group velocity, reaching values as low as 0.0005c, highlighting the potential of gypsum for "slow light" applications and extreme light-matter interaction control. These findings expand the application scope of low-symmetry crystals with the benefits that an exfoliable material provides, such as stronger field confinement, tunability, and versatility for its incorporation in complex photonic devices that might unlock new optical phenomena at the nanoscale.

Spin dynamics in Al-substituted yttrium iron garnets is investigated using broadband ferromagnetic resonance measurements in the temperature range T=200-360 K. Using the measured data, the resonance field and linewidth as well as their temperature dependence are determined, with implications for the uniformity and overall quality of the samples prepared via different chemical fabrication routes. These key parameters governing the spin dynamics in the material are important for its applications in high-speed spintronic and magnonic devices., Comment: 8 pages, 3 figures; accepted for publication in Journal of Low Temperature Physics (2025)

The dynamic magnetic properties of Full Heusler alloy thin films of Co$_2$FeGe, grown on MgO (001) substrates under different thermal conditions, were investigated. Brillouin light scattering and ferromagnetic resonance measurements revealed that depositing at room temperature followed by annealing at 300 deg C for 1 hour produces the best results for maximizing magnetization, exchange stiffness, and minimizing spin-dynamic dissipation in the films, which are desirable characteristics for high-speed spintronic devices. Additionally, strong hybridization of spin waves in the Damon-Eshbach geometry was observed, which is attractive for applications in magnonic signal processing circuits., Comment: 13 pages, 6 figures

Evaluating Video Language Models (VLMs) is a challenging task. Due to its transparency, Multiple-Choice Question Answering (MCQA) is widely used to measure the performance of these models through accuracy. However, existing MCQA benchmarks fail to capture the full reasoning capabilities of VLMs due to selection bias, when models disproportionately favor certain answer options based on positional patterns observed during training. In this work, we conduct a comprehensive empirical analysis of several VLM architectures across major datasets designed to assess complex video-focused reasoning. We identify where the bias is most pronounced and demonstrate to what extent model responses reflect genuine understanding of video content and related questions, as opposed to reliance on arbitrary patterns or superficial cues, such as answer position. By decomposing the MCQA task and adapting fairness bias metrics to VLMs, we introduce a post-processing calibration technique BOLD to balance this bias. Our results show that reducing selection bias improves not only debiasing metrics but also overall model performance, including Accuracy and F1 Mean score. Our method, by suppressing "blind guessing", offers a more cost- and time-effective approach to mitigating selection bias compared to existing techniques. This study represents the first focused investigation of selection bias in video-to-text LLM-powered models.

Magnetic properties of Y$_3$AlFe$_4$O$_{12}$ garnet ceramics have been studied over a wide range of temperature ($3-370$ K) and magnetic fields (up to $2$ kOe). Effects of varying the temperature on some of the application-specific magnetic characteristics have been analyzed in detail. With the decrease in temperature, the effective anisotropy constant, $K_{Eff}$, is found to sharply rise below $\sim150$ K, while the saturation magnetization, $M_s$, changes only slightly ($<20\%$ within $3-250$ K). The exchange stiffness, $A_{ex}$, and spin wave stiffness, $D$, parameters mirror the relatively smooth behavior of the magnetization at low temperatures but display a rapid drop when $T$ approaches $T_C\approx436$ K. The temperature dependence of the domain wall thickness and the critical single-domain size correlate with the corresponding values for pure Y$_3$Fe$_5$O$_{12}$ (YIG) and are in agreement with the theoretical estimates. We conclude by discussing the ways of how to use the obtained results for tailoring the magnetic properties of YIG-based materials for technological applications., Comment: 16 pages, 6 figures, full-length article submitted to a journal

Numerous methods have been proposed to adapt a pre-trained foundational CLIP model for few-shot classification. As CLIP is trained on a large corpus, it generalises well through adaptation to few-shot classification. In this work, we analyse the intra-modal overlap in image space in terms of embedding representation. Our analysis shows that, due to contrastive learning, embeddings from CLIP model exhibit high cosine similarity distribution overlap in the image space between paired and unpaired examples affecting the performance of few-shot training-free classification methods which rely on similarity in the image space for their predictions. To tackle intra-modal overlap we propose to train a lightweight adapter on a generic set of samples from the Google Open Images dataset demonstrating that this improves accuracy for few-shot training-free classification. We validate our contribution through extensive empirical analysis and demonstrate that reducing the intra-modal overlap leads to a) improved performance on a number of standard datasets, b) increased robustness to distribution shift and c) higher feature variance rendering the features more discriminative for downstream tasks., Comment: BMVC 2024, Oral

Machine unlearning is a crucial area of research. It is driven by the need to remove sensitive information from models to safeguard individuals' right to be forgotten under rigorous regulations such as GDPR. In this work, we focus on unlearning within CLIP, a dual vision-language encoder model trained on a massive dataset of image-text pairs using contrastive loss. To achieve forgetting we expand the application of Lipschitz regularization to the multimodal context of CLIP. Specifically, we ensure the smoothing of both visual and textual embeddings associated with the class intended to be forgotten relative to the perturbation introduced to the samples from that class. Additionally, importantly, we remove the necessity for real forgetting data by generating synthetic samples through gradient ascent maximizing the target class. Our forgetting procedure is iterative, where we track accuracy on a synthetic forget set and stop when accuracy falls below a chosen threshold. We employ a selective layers update strategy based on their average absolute gradient value to mitigate over-forgetting. We validate our approach on several standard datasets and provide thorough ablation analysis and comparisons with previous work., Comment: Published in WACV 2025 conference

We investigate the switching of a magnetic nanoparticle comprising the middle free layer of a memory cell based on a double magnetic tunnel junction under the combined effect of spin-polarized current and weak on-chip magnetic field. We obtain the timing and amplitude parameters for the current and field pulses needed to achieve 100 ps range \emph{inertia-free} switching under \emph{least-power} dissipation. The considered method does not rely on the stochastics of thermal agitation of the magnetic nanoparticle typically accompanying spin-torque switching. The regime of ultimate switching speed-efficiency found in this work is promising for applications in high-performance nonvolatile memory., Comment: 4 pages, 2 figures

Recent experiments demonstrated that proton transport through graphene electrodes can be accelerated by over an order of magnitude with low intensity illumination. Here we show that this photo-effect can be suppressed for a tuneable fraction of the infrared spectrum by applying a voltage bias. Using photocurrent measurements and Raman spectroscopy, we show that such fraction can be selected by tuning the Fermi energy of electrons in graphene with a bias, a phenomenon controlled by Pauli blocking of photo-excited electrons. These findings demonstrate a dependence between graphene's electronic and proton transport properties and provide fundamental insights into molecularly thin electrode-electrolyte interfaces and their interaction with light.

Strong coupling of molecules to vacuum fields has been widely reported to lead to modified chemical properties such as reaction rates. However, some recent attempts to reproduce infrared strong coupling results have not been successful, suggesting that factors other than strong coupling may sometimes be involved. Here we re-examine the first of these vacuum-modified chemistry experiments in which changes to a molecular photoisomerisation process in the UV-vis spectral range were attributed to strong coupling of the molecules to visible light. We observed significant variations in photoisomerisation rates consistent with the original work; however, we found no evidence that these changes need to be attributed to strong coupling. Instead, we suggest that the photoisomerisation rates involved are most strongly influenced by the absorption of ultraviolet radiation in the cavity. Our results indicate that care must be taken to rule out non-polaritonic effects before invoking strong coupling to explain any changes of chemical properties arising in cavity-based experiments., Comment: 33 pages, 17 figures

This paper provides a robust, scalable Bluetooth Low-Energy (BLE) based indoor localization solution using commodity hardware. While WiFi-based indoor localization has been widely studied, BLE has emerged a key technology for contact-tracing in the current pandemic. To accurately estimate distance using BLE on commercial devices, systems today rely on Receiver Signal Strength Indicator(RSSI) which suffers from sampling bias and multipath effects. We propose a new metric: Packet Reception Probability (PRP) that builds on a counter-intuitive idea that we can exploit packet loss to estimate distance. We localize using a Bayesian-PRP formulation that also incorporates an explicit model of the multipath. To make deployment easy, we do not require any hardware, firmware, or driver-level changes to off-the-shelf devices, and require minimal training. PRP can achieve meter level accuracy with just 6 devices with known locations and 12 training locations. We show that fusing PRP with RSSI is beneficial at short distances < 2m. Beyond 2m, fusion is worse than PRP, as RSSI becomes effectively de-correlated with distance. Robust location accuracy at all distances and ease of deployment with PRP can help enable wide range indoor localization solutions using BLE., Comment: 14 pages, 10 figures

We present all-electrical operation of a Fe$_x$Cr$_{1-x}$-based Curie switch at room temperature. More specifically, we study the current-induced thermally-driven transition from ferromagnetic to antiferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) indirect coupling in a Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe multilayer. Magnetometry measurements at different temperatures show that the transition from the ferromagnetic to the antiferromagnetic coupling at zero field is observed at $\sim$325K. Analytical modelling confirms that the observed temperature-dependent transition from indirect ferromagnetic to indirect antiferromangetic interlayer exchange coupling originates from the modification of the effective interlayer exchange constant through the ferromagnetic-to-paramagnetic transition in the Fe$_{17.5}$Cr$_{82.5}$ spacer with minor contributions from the thermally-driven variations of the magnetization and magnetic anisotropy of the Fe layers. Room-temperature current-in-plane magnetotransport measurements on the patterned Fe/Cr/Fe$_{17.5}$Cr$_{82.5}$/Cr/Fe strips show the transition from the 'low-resistance' parallel to the 'high-resistance' antiparallel remanent magnetization configuration, upon increased probing current density. Quantitative comparison of the switching fields, obtained by magnetometry and magnetotransport, confirms that the Joule heating is the main mechanism responsible for the observed current-induced resistive switching., Comment: 8 pages, 4 figures

We investigate the interlayer coupling between two thin ferromagnetic (F) films mediated by an antiferromagnetic (AF) spacer in F*/AF/F trilayers and show how it transitions between different regimes on changing the AF thickness. Employing layer-selective Kerr magnetometry and ferromagnetic-resonance techniques in a complementary manner enables us to distinguish between three functionally distinct regimes of such ferromagnetic interlayer coupling. The F layers are found to be individually and independently exchange-biased for thick FeMn spacers - the first regime of no interlayer F-F* coupling. F-F* coupling appears on decreasing the FeMn thickness below 9 nm. In this second regime found in structures with 6.0-9.0 nm thick FeMn spacers, the interlayer coupling exists only in a finite temperature interval just below the effective N\'eel temperature of the spacer, which is due to magnon-mediated exchange through the thermally softened antiferromagnetic spacer, vanishing at lower temperatures. The third regime, with FeMn thinner than 4 nm, is characterized by a much stronger interlayer coupling in the entire temperature interval, which is attributed to a magnetic-proximity induced ferromagnetic exchange. These experimental results, spanning the key geometrical parameters and thermal regimes of the F*/AF/F nanostructure, complemented by a comprehensive theoretical analysis, should broaden the understanding of the interlayer exchange in magnetic multilayers and potentially be useful for applications in spin-thermionics., Comment: 14 pages, 9 figures

We report on the single-step fabrication of homogeneous and highly ordered Laser Induced Periodic Surface Structures (LIPSS) over large areas on Au nanolayers, that can be used for plasmonic sensing. A comprehensive study on LIPSS formation on 32 nm Au film upon double, 170 fs pulse irradiation, unveiled the key importance of interpulse delay as the determining factor behind the homogeneity of laser induced structures and confirmed that highly ordered, functional LIPSS occur solely upon double pulse irradiation. In particular, the impact of pulse overlap, fluence and interpulse delay reveals that homogeneous LIPSS formation is optimized within a specific interpulse delay range. At the same time, examination of nanoscale features of the structures points out a significant differentiation of the LIPSS formation characteristics between single and double pulse irradiation. Theoretical investigation complements experimental results providing insights on the structure formation mechanism. Ellipsometric measurements validate that such structures exhibit characteristic plasmon resonances that can be exploited for sensing applications. The presented data demonstrate a novel functionality of LIPSS, while providing strong evidence of the capabilities of femtosecond double pulse irradiation as a valuable and low-cost tool for the precise fabrication of highly ordered structures.

The study concerns dynamics of standing spin waves in arrays of sub-100 nm elliptic synthetic-antiferromagnet (SAF) nanodisks. We performed a detailed ferromagnetic resonance analysis in conjunction with micromagnetic modeling to find out several prominent traits of such systems. One broad line is shown to be the sole resonant response for a SAF of the considered sizes. We demonstrate that this mode is degenerated, and its excitation map resembles a superposition of in-center and edge-type oscillations. We also show how this hybrid excitation leads to almost twofold enhancement in the shape-induced anisotropy of the mode., Comment: 3 pages, 1 figure, submitted to 2022 IEEE 12th International Conference Nanomaterials: Applications & Properties (NAP)

Geometric phase optical elements made of space-variant anisotropic media customarily find their optimal operating conditions when the half-wave retardance condition is fulfilled, which allows imparting polarization-dependent changes to an incident wavefront. In practice, intrinsic limitations of man-made manufacturing process or the finite spectrum of the light source lead to a deviation from the ideal behavior. This implies the implementation of strategies to compensate for the associated efficiency losses. Here we report on how the intrinsic tunable features of self-engineered liquid crystal topological defects can be used to enhance the rejection capabilities of spectrally adaptive vector vortex coronagraphs. We also discuss the extent of which current models enable to design efficient devices.

The demagnetization and associated magnetocaloric effect in strong-weak-strong ferromagnetic trilayers, upon a reorientation of the strong ferromagnets from parallel to antiparallel magnetization, is simulated using atomistic spin dynamics. The simulations yield non-trivial spin distributions in the antiparallel state, which in turn allows entropy to be calculated directly. Empirical functional forms are obtained for the magnetization distribution in the spacer, differing significantly from some of the commonly used models. Finally, we find that the magnetocaloric effect in the system can be significantly improved by allowing the local exchange to vary through the spacer, which in practice can be implemented by spatially tailoring the spacer's magnetic dilution., Comment: 8 pages, 6 figures