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Non-Gaussian statistics of nanohertz stochastic gravitational waves
No full textMultiple pulsar timing arrays (PTAs) have recently reported evidence for nHz stochastic gravitational wave background (SGWB), stimulating intensive discussions about its physical origin. In principle, the sources may be either supermassive black hole binaries (SMBHBs) or processes in the early Universe. One key difference between the two lies in the statistics of the SGWB frequency power spectrum. In particular, the often assumed Gaussian random SGWB does not accurately describe the distribution of the collective SMBHB emission. This work presents a semianalytical framework for calculating the non-Gaussian statistics of SGWB power expected from SMBHBs. We find that (a) wave interference between individual SMBHBs with indistinguishable observed frequencies and (b) the Poisson fluctuation of the source numbers, together shape the non-Gaussian statistics. Implementing the non-Gaussian statistics developed in this work, we investigate the sensitivity of current and future PTA datasets in distinguishing the origin of the SGWB through non-Gaussian information. Additionally, we find an interesting approximation of the non-Gaussian statistics, which has implications for accurately and practically treating non-Gaussianity in PTA Bayesian analyses
Hydrostatic behaviour of highly inert Fomblin and Halocarbon fluids as pressure-transmitting media in high-pressure experiments
No full textA pressure-transmitting medium (PTM) plays an important role in diamond anvil cell (DAC) experiments as it ensures that the sample is exposed to hydrostatic pressure. Although PTMs that are liquids under ambient conditions are the easiest to handle and load, the selection of chemically inert liquid media with established hydrostatic properties is limited. To widen the choice of highly inert PTMs for high-pressure experiments, the hydrostatic behaviours of Fomblin Z60, Fomblin Z25, Fomblin Y LVAC 06/6 and Halocarbon Oil 11-14 were investigated. The ruby fluorescence method was used to monitor the evolution of pressure gradients across the DAC sample chamber during compression and decompression. Fomblin Z60 and Fomblin Z25 perfluoropolyethers, which are hydrostatic to 1.7 and 1.5 GPa, respectively, exhibited the best hydrostatic performance, followed by Halocarbon Oil 11-14 with a limit of 1.2 GPa, whereas the non-hydrostatic behaviour of Fomblin Y LVAC 06/6 was observed above only 0.6 GPa
Virtual pulse reconstruction diagnostic for single-shot measurement of free electron laser radiation power
No full textAccurate characterization of radiation pulse profiles is crucial for optimizing beam quality and enhancing experimental outcomes in free electron laser (FEL) research. In this paper, we present a unique approach that employs machine learning techniques for real-time virtual diagnostics of FEL radiation pulses. Our simple artificial intelligence (AI)-based diagnostic tool utilizes longitudinal phase space data obtained from the X-band transverse deflecting structure to reconstruct the temporal profile of FEL pulses in real time. Unlike traditional single-shot methods, this AI-driven solution provides a noninvasive, highly efficient alternative for pulse characterization. By leveraging state-of-the-art machine learning models, our method facilitates precise, single-shot measurements of FEL pulse power, offering significant advantages for FEL science research. This work outlines the conceptual framework, methodology, and validation results of our virtual diagnostic tool, demonstrating its potential to significantly impact FEL research
A high-frequency artificial nerve based on homogeneously integrated organic electrochemical transistors
No full textArtificial nerves that are capable of sensing, processing and memory functions at bio-realistic frequencies are of potential use in nerve repair and brain–machine interfaces. n-type organic electrochemical transistors are a possible building block for artificial nerves, as their positive-potential-triggered potentiation behaviour can mimic that of biological cells. However, the devices are limited by weak ionic and electronic transport and storage properties, which leads to poor volatile and non-volatile performance and, in particular, a slow response. We describe a high-frequency artificial nerve based on homogeneously integrated organic electrochemical transistors. We fabricate a vertical n-type organic electrochemical transistor with a gradient-intermixed bicontinuous structure that simultaneously enhances the ionic and electronic transport and the ion storage. The transistor exhibits a volatile response of 27 μs, a 100-kHz non-volatile memory frequency and a long state-retention time. Our integrated artificial nerve, which contains vertical n-type and p-type organic electrochemical transistors, offers sensing, processing and memory functions in the high-frequency domain. We also show that the artificial nerve can be integrated into animal models with compromised neural functions and that it can mimic basic conditioned reflex behaviour
Ferroelectricity of wurtzite AlHfN heterovalent alloys
No full textThin films of aluminum hafnium nitride (AlHfN) were synthesized via reactive magnetron sputtering for Hf contents up to x = 0.13. X-ray diffraction showed a single c-axis oriented wurtzite phase for all films. Hard x-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent–and not just trivalent–alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices
Biopolymer‐Templated Hierarchical 3D‐Structured Gold Nanoparticle/Graphene Oxide Hybrid Materials for Ultrasensitive Surface‐Enhanced Raman Scattering
No full textSurface-enhanced Raman scattering (SERS) is a highly advantageous analytical technique for detecting trace biological and chemical compounds. However, significant challenges remain in the cost-effective fabrication of large-area and homogenous SERS substrates. A simple and scalable approach utilizing a layer-by-layer spray deposition followed by thermal annealing is proposed to fabricate cellulose nanofibril (CNF) films loaded with gold nanoparticles (Au NPs) and graphene oxide (GO) hybrids as SERS substrates. These hybrid 3D structures comprising CNF/Au NPs/GO significantly enhance SERS sensitivity by both electromagnetic enhancement and chemical enhancement. Incorporating CNF as a 3D network enables a more uniform distribution of Au NPs/GO. Thermal annealing further induces hotspots. For instance, the annealed CNF/Au NPs/GO hybrid thin films achieve a detection limit of 1.0 × 10 m and a high enhancement factor of 4.97 × 1011 for Rhodamine 6G. Grazing incidence small-angle X-ray scattering combined with nano-Fourier-transform infrared spectroscopy is first used to confirm the combined Raman enhancement mechanism of localized surface plasmon resonance and interface charge transfer with high spatial resolution. Therefore, the proposed methodology establishes a robust framework for the scalable fabrication of ultrasensitive SERS substrates
Defect‐Mediated Scintillation in Fully Inorganic Perovskites via Water‐Induced 0D/3D Phase Modulation
No full textScintillation detectors are essential tools in high-energy physics, medical imaging, and security, due their efficiency in converting ionizing radiation into visible light. Lead-based inorganic perovskites, particularly 3D CsPbBr3, have emerged as promising next-generation scintillators due to their high photon attenuation and fast emission properties. In contrast, the 0D phase, Cs4PbBr6, exhibits unique emission characteristics and defect-mediated behavior, offering additional opportunities to tune scintillation performance in hybrid systems. However, the role of the 0D Cs4PbBr6 phase in scintillation has remained largely unexplored, and the mechanism of the emission is not well understood. Herein, a simple and reproducible synthesis of polycrystalline perovskite powders is developed with the specific scope of modulating the 3D/0D CsPbBr3/Cs4PbBr6 phases in the samples, aiming to clarify the role of the 0D phase in the emission properties of the materials. The method relies on a solvent-antisolvent approach, in which incremental water additions selectively promote the formation of the 3D phase over the 0D one. The scintillation properties of the resulting powders are evaluated, revealing an increased scintillation yield for low water volumes used in the synthesis and an ultrafast decay time under X-ray radiation. Cathodoluminescence and temperature-dependent radioluminescence highlight defect-driven scintillation mechanisms, providing insights for future material optimization
Interplay of T and 1T ′ Phases Influencing the Transport Properties of MoWTe Weyl Semimetals
No full textWTe2 and MoTe2 in their orthorhombic (Td) phases have emerged as prominent candidates for type-II Weyl semimetals due to their unique quantum transport phenomena. The distinctive electronic structure leads to unusual electron behavior under a magnetic field, resulting in extreme magnetoresistance (XMR). This study addresses a critical gap in understanding the impact of structural modulation on MR behavior, which has important implications for optimizing materials in quantum devices and spintronics. We successfully synthesized miscible MoxW1–xTe2 (x = 0.1 to 0.5) single crystals and investigated their structural evolution and transport properties. Raman and powder XRD analyses reveal that Mo doping induced a transition from the Td phase to an intermediate monoclinic 1T′ phase. Notably, the x = 0.3 sample exhibits the highest stability toward the Td phase. Transport measurements reveal a significant reduction in MR and residual resistivity ratio (RRR), alongside an increase in electron–hole asymmetry with higher Mo content, attributed to the reduced crystal quality and enhancement of impurity scattering. Enhanced structural stabilization toward the Td phase significantly improves MR and positively influences transport properties. These findings offer new insights into the role of structural tuning in the electronic properties of Weyl semimetals with implications for spintronics and quantum materials research
The promise of deep-stacking for neutrino astronomy
No full textThe detection of high-energy astrophysical neutrinos by IceCube has opened new windows for neutrino astronomy, but their sources remains largely unresolved. We study a methodology to address this — deep-stacking — that exploits correlations between observed neutrinos and comprehensive catalogs of potential source populations, including faint, high-redshift sources. By stacking signals from numerous weak sources and optimizing source weighting, significant gains in sensitivity can be achieved, particularly in the low-background regime where individual high-energy neutrinos dominate. We provide a semi-analytic framework to estimate sensitivity improvements for populations of sources under various background scenarios and redshift evolutions. Our analysis demonstrates that deep-stacking can increase detection sensitivity by a factor of 3–5, enabling detailed population studies. Furthermore, we discuss the potential to resolve the diffuse neutrino flux and investigate the redshift evolution of source populations. This approach offers a direct path toward identifying the primary sites of cosmic-ray acceleration and the mechanisms responsible for high-energy neutrino production
Puzzling Variation of Gamma Rays from the Sun over the Solar Cycle Revealed with Fermi-LAT
No full textThe steady-state gamma-ray emission from the Sun is thought to consist of two emission components due to interactions with Galactic cosmic rays: (1) a hadronic disk component, and (2) a leptonic extended component peaking at the solar edge and extending into the heliosphere. The flux of these components is expected to vary with the 11 yr solar cycle, being highest during solar minimum and lowest during solar maximum, as it varies with the cosmic-ray flux. No study has yet analyzed the flux variation of each component over solar cycles. In this work, we measure the temporal variations of the flux of each component over 15 yr of Fermi Large Area Telescope observations and compare them with the sunspot number and Galactic cosmic-ray flux from AMS-02 near Earth. We find that the flux variation of the disk anticorrelates with the sunspot number and correlates with cosmic-ray protons, as expected, confirming its emission mechanism. In contrast, the extended component exhibits a more complex variation: despite an initial anticorrelation with the sunspot number, we find neither anticorrelation with the sunspot number nor correlation with cosmic-ray electrons over the full 15 yr period. This most likely suggests that cosmic-ray transport and modulation in the inner heliosphere are unexpectedly complex and may differ for electrons and protons or, alternatively, that there is an additional, unknown component of gamma rays or cosmic rays. These findings impact space weather research and emphasize the need for close monitoring of Cycle 25 and the ongoing polarity reversal