DSpace community: 物理學系暨研究所

Dirac's variational approach to semiclassical Kramers problem in Smoluchowski limit

title: Dirac's variational approach to semiclassical Kramers problem in Smoluchowski limit abstract: Kramers escape from a metastable state in the presence of both thermal and quantum fluctuations under strong damping is treated as a thermally activated process in a quantum modified semiclassical potential. Dirac’s time-dependent variational method together with the Jackiw-Kerman function is employed to derive the semiclassical potential. Quantum correction is incorporated in the drift potential, and is determined by quasi-stationary conditions and minimal uncertainty relation. The semiclassical rate obtained here is consistent in form with those from the quantum Smoluchowski equations deduced heuristically by modifying the diffusion coefficient using the path-integral method. Unlike approaches using the path-integral, which involves continuation into imaginary time, the approach here is simpler and more easily understood in terms of classical picture.

Localized Deviations from the CO─Polycyclic Aromatic Hydrocarbon Relation in PHANGS-JWST Galaxies: Faint Polycyclic Aromatic Hydrocarbon Emission or Elevated CO Emissivity?

title: Localized Deviations from the CO─Polycyclic Aromatic Hydrocarbon Relation in PHANGS-JWST Galaxies: Faint Polycyclic Aromatic Hydrocarbon Emission or Elevated CO Emissivity? abstract: Polycyclic aromatic hydrocarbon (PAH) emission is widely used to trace the distribution of molecular gas in the interstellar medium, exhibiting a tight correlation with CO(2–1) emission across nearby galaxies. Using PHANGS-JWST and PHANGS-Atacama Large Millimeter/submillimeter Array (ALMA) data, we identify localized regions where this correlation fails, with CO flux exceeding that predicted from 7.7 μm PAH emission by more than an order of magnitude. These outlier regions are found in 20 out of 70 galaxies and are located in galaxy centers and bars, without signs of massive star formation. We explore two scenarios to explain the elevated CO-to-PAH ratios, which can either be due to suppressed PAH emission or enhanced CO emissivity. We examine PAH emission in other bands (3.3 and 11.3 μm) and the dust-continuum-dominated bands (10 and 21 μm), finding consistently high CO-to-PAH (or CO-to-dust continuum) emission ratios, suggesting that 7.7 μm PAH emission is not particularly suppressed. In some outlier regions, PAH sizes and spectral energy distribution of the radiation differ slightly from nearby control regions with normal CO-to-PAH ratios, though without a consistent trend. We find that the outlier regions show higher CO velocity dispersions (ΔvCO). This increase in ΔvCO lowers CO optical depth and raises its emissivity for a given gas mass. Our results favor a scenario where shear along the bar lanes and shocks at the bar ends elevate CO emissivity, leading to the breakdown of the CO–PAH correlation. Future JWST spectroscopy and deep ALMA observations of CO isotopologues will provide critical tests of this scenario.

Constraining the Subgalactic Relationship between Star Formation and the Hot Interstellar Medium in NGC 4254

title: Constraining the Subgalactic Relationship between Star Formation and the Hot Interstellar Medium in NGC 4254

Stellar associations powering H II regions: II. Escape fraction of ionising photons

title: Stellar associations powering H II regions: II. Escape fraction of ionising photons abstract: Newly formed stars profoundly affect their environment by depositing energy and momentum into the surrounding gas. However, only a fraction of the stellar feedback is retained in the cloud, and observational constraints are needed to improve our understanding of this process. In a sample of 19 nearby galaxies, we matched H II regions from PHANGS–MUSE to their ionizing stellar source from PHANGS–HST and measured the percentage of ionizing radiation that leaks into the surrounding diffuse ionised gas (DIG). Based on a catalogue in which each H II region is powered by a single young and massive stellar association, we measure a photon escape fraction of fesc = 82−24+12 %. We obtain comparable results when using different procedures to match the ionised gas to its source. All samples in our study contain a substantial fraction of objects (up to 20%), in which the stellar source is insufficient to produce the H α flux observed from the nebula. Many of these cases are probably related to uncertain age estimates, but we also find numerous regions for which a significant fraction of the ionising photon budget is contributed by stars that reside outside the boundaries of the H II region. This finding motivates the use of an alternative galaxy-wide approach in which we include all H II regions and stellar sources, not just those that show a clear overlap. When we sum the ionisation budget over entire galaxies, we measure slightly lower, but consistent values.

The ALMaQUEST Survey. XVII. Unveiling Multiple Quenching Pathways in Green Valley Galaxies via Molecular Gas and Quenching Timescale Analyses

title: The ALMaQUEST Survey. XVII. Unveiling Multiple Quenching Pathways in Green Valley Galaxies via Molecular Gas and Quenching Timescale Analyses abstract: Statistically, green valley (GV) galaxies exhibit lower molecular gas fractions (fgas) and reduced star formation efficiency (SFE) compared to star-forming galaxies. However, it remains unclear whether quenching is primarily driven by one factor or results from a combination of mechanisms in individual GV galaxies. In this study, we address this question by examining the spatial distributions of star formation and molecular gas in 28 GVs selected from the ALMaQUEST survey and additional literature samples. For each galaxy, we identify regions with suppressed specific star formation rate (sSFR) and measure Δfgas and ΔSFE—offsets from the resolved scaling relations of the star-forming main-sequence galaxies. By comparing the fraction of regions with negative Δfgas and ΔSFE, we classify 35.7% ± 13.2% (57.1% ± 17.9%) of GV galaxies as fgas driven, 39.3% ± 14.0% (39.3% ± 14.0%) as SFE driven, and 25.0% ± 10.6% (3.6% ± 3.6%) as mixed mode when adopting a fixed (variable) CO-to-H2 conversion factor (αCO). These results indicate that GVs undergo quenching through multiple pathways. As sSFR decreases from the main sequence to the GV, we observe a transition toward predominantly SFE-driven quenching, possibly linked to internal processes such as morphological quenching or active galactic nucleus activity. We further estimate the quenching timescale (τdecay), defined as the time from the peak star formation rate to 1 e–1 (approximately 37%) of its value, using integrated MaNGA spectra. SFE-driven quenching is typically associated with short τdecay, while fgas-driven quenching shows a broader range. Overall, 75% of GVs exhibit τdecay shorter than 1 Gyr, suggesting that quenching in most GVs proceeds rapidly, challenging purely slow-quenching scenarios like starvation.

Molecular gas and star formation in central rings across nearby galaxies

title: Molecular gas and star formation in central rings across nearby galaxies abstract: Context. Nearby galaxies exhibit a variety of structures, including so-called central or (circum-)nuclear rings that are similar to the Milky Way (MW) Central Molecular Zone (CMZ). These rings are common in barred galaxies and can be gas-rich and highly star-forming. Aims. We aim to study the molecular gas content and star formation rate of central rings within nearby galaxies and link them to global galaxy properties, especially the bar morphology. Methods. We utilized 1″(≲100 pc) resolution CO(2–1) observations from the PHANGS-ALMA survey, visually identifying 20 central rings and determine their properties. For 14 of these rings, MUSE observations tracing star formation rate (SFR) surface density were available. We derived the rings’ geometry, integrated molecular gas masses, SFRs, depletion times, and compared them to host galaxy and bar properties from the literature. Results. Molecular gas is an effective tracer for central rings. Previous studies have used ionized gas and dust tracers to identify central rings in galaxies of similar morphological types as the PHANGS galaxies (numerical Hubble type T ∼ −3 to T ∼ 9). In comparison, molecular gas yields similar fractions of galaxies hosting central rings and similar radii distributions. The gaseous central rings have typical radii of ∼ 400+250−150pc, molecular gas masses of log(M/M⊙) ∼ 8.1+0.17−0.23, and SFRs of ∼ 0.21+0.15−0.16 M⊙/yr. As a result, they contribute 5.6+4.5−2.1% and 13+10−5% to their host galaxies’ molecular gas mass and SFR, respectively. While the MW CMZ sits at the lower end of the radius, molecular gas mass, and SFR distribution, it matches well in terms of ring molecular gas mass and SFR fraction, and depletion time. Longer bars contain more massive molecular central rings, but there is no correlation between the classical bar strength parameters (Qb, εbar, A2max) and the ring’s molecular gas content. Conclusions. Although absolute central ring properties (ring radius, molecular gas mass, SFR) likely depend on host galaxy properties, the similarities between the MW CMZ and PHANGS central rings in relative parameters (molecular gas and SFR fraction, depletion time) suggest that the processes of gas inflow and star formation are similar for central rings across nearby galaxies.

Resolved H II Regions in NGC 253: Ionized Gas Structure and Suggestions of a Universal Density─Surface Brightness Relation

title: Resolved H II Regions in NGC 253: Ionized Gas Structure and Suggestions of a Universal Density─Surface Brightness Relation abstract: We use the full-disk Very Large Telescope/MUSE mosaic of NGC 253 to identify 2492 H II regions and study their resolved structure. With an average physical resolution of 17 pc, this is one of the largest samples of highly resolved spectrally mapped extragalactic H II regions. Regions of all luminosities exhibit a characteristic emission profile described by a double Gaussian with a marginally resolved or unresolved core with radius < 10 pc surrounded by a more extended halo of emission with radius = 20─30 pc. Approximately 80% of the emission of a region originates from the halo component. As a result of this compact structure, the luminosity─radius relations for core and effective radii of H II regions depend sensitively on the adopted methodology. Only the isophotal radius yields a robust relationship in NGC 253, but this measurement has an ambiguous physical meaning. We invert the measured emission profiles to infer density profiles and find central densities of ne ≍ 10─100 cm−3. In the brightest regions, these agree well with densities inferred from the [S II] λλ6716, 6730 doublet. The central density of H II regions correlates well with the surface brightness within the effective radius. We show that this same scaling relation applies to the recent MUSE + Hubble Space Telescope catalog for 19 nearby galaxies. We also discuss potential limitations, including completeness, impacts of background subtraction and spatial resolution, and the generality of our results when applied to other galaxies.

Duration and properties of the embedded phase of star formation in 37 nearby galaxies from PHANGS-JWST

title: Duration and properties of the embedded phase of star formation in 37 nearby galaxies from PHANGS-JWST abstract: Light reprocessed by dust grains emitting in the infrared enables the study of the physics at play in dusty embedded regions, where ultraviolet and optical wavelengths are attenuated. Infrared telescopes such as JWST have made it possible to study the earliest feedback phases, when stars are shielded by cocoons of gas and dust. Comprehending this phase is crucial for unravelling the effects of feedback from young stars that leads to their emergence and the dispersal of their host molecular clouds. Here we show that the transition from the embedded to the exposed phase of star formation is short (< 4 Myr) and sometimes almost absent (< 1 Myr) across a sample of 37 nearby star-forming galaxies covering a wide range of morphologies, from massive barred spirals to irregular dwarfs. The short duration of the dust-clearing timescales suggests a predominant role of pre-supernova feedback mechanisms in revealing newborn stars, confirming previous results on smaller samples and allowing, for the first time, a statistical analysis of their dependencies. We find that the timescales associated with mid-infrared emission at 21 μm, tracing a dust-embedded feedback phase, are controlled by a complex interplay between giant molecular cloud properties (masses and velocity dispersions) and galaxy morphology. We report relatively longer durations of the embedded phase of star formation in barred spiral galaxies, while this phase is significantly reduced in low-mass irregular dwarf galaxies. We discuss tentative trends with gas-phase metallicity, which may favor faster cloud dispersal at low metallicities.

The PHANGS-MUSE/HST-Hα nebulae catalogue: Parsec-scale resolved structure, physical conditions, and stellar associations across nearby galaxies

title: The PHANGS-MUSE/HST-Hα nebulae catalogue: Parsec-scale resolved structure, physical conditions, and stellar associations across nearby galaxies abstract: We present the PHANGS-MUSE/HST-Hα nebulae catalogue, comprising 5177 spatially resolved nebulae across 19 nearby star-forming galaxies (D < 20 Mpc), based on high-resolution Hα imaging from HST, homogenised to a fixed (10 pc) physical resolution and sensitivity. Combined with MUSE integral field spectroscopy, this enables robust classification of 4882 H II regions and the separation of planetary nebulae and supernova remnants. We derive electron densities for 2544 H II regions using [S II] diagnostics and adopt direct or representative electron temperatures for consistent physical characterisation. Nebular sizes are measured using circularised radii and intensity-weighted second moments, yielding a median radius of approximately 20 pc and extending down to (sub-)parsec (deconvolved) radii. A structural complexity score is introduced via hierarchical segmentation to trace substructure, highlighting that around a third of the regions are H II complexes containing several individual clusters and bubbles, with an increased fraction of these regions in galactic centres. A luminosity–size relation, calibrated using the resolved HST sample, is applied to 30 790 MUSE nebulae, allowing the recovery of nebular sizes down to ~1 pc and providing statistical completeness beyond the HST detection limit. Comparisons with classical Strömgren radii indicate that observed sizes are systematically larger, corresponding to typical volume filling factors with a median of ϵ ~ 0.22 (10th–90th percentile 0.06–0.78), with larger regions exhibiting progressively lower values. We associate 3349 H II regions with stellar populations from the PHANGS-HST association catalogue, finding median ages of ~3 Myr and typical stellar masses of around 104–105 M⊙, supporting the link between ionised nebular and young stellar populations. We also assess the impact of diffuse ionised gas on emission-line diagnostics and after removing confirmed supernova remnants, find no strong variation in line ratios with nebular resolution, indicating minimal systematic bias in the MUSE catalogue. This dataset establishes a detailed, spatially resolved connection between nebular structure and ionising sources, and provides a benchmark for future studies of feedback, DIG contributions, and star formation regulation in the ISM, especially in combination with matched high-resolution observations. The full catalogue is made publicly available in machine-readable format.

PHANGS-JWST: The largest extragalactic molecular cloud catalog traced by polycyclic aromatic hydrocarbon emission

title: PHANGS-JWST: The largest extragalactic molecular cloud catalog traced by polycyclic aromatic hydrocarbon emission abstract: High-resolution JWST images of nearby spiral galaxies reveal polycyclic aromatic hydrocarbon (PAH) structures that potentially trace molecular clouds, even CO-dark regions. For this paper, we identified ISM cloud structures in PHANGS-JWST 7.7 μm PAH emission maps for 66 galaxies, smoothed to a common physical resolution of 30 pc and at native resolution. We extracted 108 466 cloud structures in the 30 pc sample and 146 040 clouds in the native resolution sample. We then calculated their molecular properties following a linear conversion from PAH to CO. Given the tendency for clouds in galaxy centers to overlap in velocity space, we opted to flag these clouds and omit them from the analysis in this work. The remaining clouds correspond to giant molecular clouds, such as those detected in CO(2 − 1) emission by ALMA, or lower surface density clouds that either fall below the ALMA detection limits of existing maps or genuinely have no molecular counterpart. We specifically used the homogenized sample for our analysis. Upon cross-matching the PAH clouds to the ALMA CO clouds at a homogenized resolution of 90 pc in 27 galaxies, we find that 41% of the PAH clouds are associated with a CO counterpart. We also show that the converted molecular cloud properties of the PAH clouds do not differ much when compared in different galactic environments. However, outside the central environment, the highest molecular mass surface density clouds are preferentially found in spiral arms. We further apply a lognormal fit to the mass spectra to an unprecedented extragalactic completeness limit of 2 × 103 M⊙, and find that spiral arms contain the most massive clouds compared to other galactic environments. Our findings support the idea that spiral arm gravitational potentials foster the formation of high surface density clouds, and that lower surface density clouds form in the interarm regions. The cloud Σmol values show a decline of a factor of ∼1.5 − 2 toward the outer 2 − 3 Re. However, the trend largely varies in individual galaxies, with flat, decreasing, and even no trend as a function of Rgal. Factors such as large-scale processes, galaxy types, and morphologies might influence the observed trends. We note that combining homogenized molecular properties of individual galaxies leads to the loss of information about the physical processes that are driving deviations in trends of those properties across different galactic environments. We published two catalogs at the CDS, one at the common resolution of 30 pc and another at the native resolution. We expect them to have broad utility for future studies of PAH clouds, molecular clouds, and star formation.

Moiré excitons in generalized Wigner crystals

title: Moiré excitons in generalized Wigner crystals abstract: Moiré superlattices of transition-metal dichalcogenide bilayers host strong Coulomb interactions residing in narrow electron bands, leading to correlated insulating states at fractional carrier doping densities, known as generalized Wigner crystals. In excited states, the formation of moiré excitons can be fundamentally shaped by the Wigner-crystal ground states, manifesting an intricate interplay between electronic and excitonic correlations. However, the microscopic description of these Wigner crystalline excitons (WCEs) remains elusive, largely subject to speculations, and is further needed for the understanding of exotic excitonic phases (e.g., exciton insulators and exciton density waves) and their unique properties (e.g., anomalous exciton diffusion). Here, using first-principles many-body GW–Bethe–Salpeter equation calculations, we directly reveal the internal structures of WCEs in angle-aligned MoSe2/MoS2 moiré heterostructure at hole fillings of 1/3 and 2/3. Our results uncover the propagation of correlation effects from the ground state to excited states, shaping the real-space characteristics of WCEs. The strong two-particle excitonic correlations dominate over the kinetic energy of free electron–hole pairs, in analog to the strong single-particle correlations of flat bands. We propose that such unusual excited-state correlation effects of WCEs can be experimentally probed by photocurrent tunneling microscopy (PTM). Our work provides a microscopic understanding of strongly correlated WCEs, suggesting them as a highly tunable mixed boson-fermion platform to study many-body interactions and phenomena.

Masses, Star Formation Efficiencies, and Dynamical Evolution of 18,000 H II Regions

title: Masses, Star Formation Efficiencies, and Dynamical Evolution of 18,000 H II Regions abstract: We present measurements of the masses associated with ∼18,000 H ii regions across 19 nearby star-forming galaxies by combining data from JWST, Hubble Space Telescope, MUSE, Atacama Large Millimeter/submillimeter Array, Very Large Array, and MeerKAT from the multiwavelength PHANGS survey. We report 10 pc-scale measurements of the mass of young stars, ionized gas, and older disk stars coincident with each H ii region, as well as the initial and current mass of molecular gas, atomic gas, and swept-up shell material, estimated from lower-resolution data. We find that the mass of older stars dominates over young stars at ≳10 pc scales, and ionized gas exceeds the stellar mass in most optically bright H ii regions. Combining our mass measurements for a statistically large sample of H ii regions, we derive 10 pc-scale star formation efficiencies of ≈6%–17% for individual H ii regions. Comparing each region’s self-gravity with the ambient interstellar medium (ISM) pressure and total pressure from presupernova stellar feedback, we show that most optically bright H ii regions are overpressured relative to their own self-gravity and the ambient ISM pressure and that they are hence likely expanding into their surroundings. Larger H ii regions in galaxy centers approach dynamical equilibrium. The self-gravity of regions is expected to dominate over presupernova stellar feedback pressure at ≳130 and 60 pc scales in galaxy disks and centers, respectively, but is always subdominant to the ambient ISM pressure on H ii region scales. Our measurements have direct implications for the dynamical evolution of star-forming regions and the efficiency of stellar feedback in ionizing and clearing cold gas.

Polycyclic aromatic hydrocarbon destruction in star-forming regions across 42 nearby galaxies

title: Polycyclic aromatic hydrocarbon destruction in star-forming regions across 42 nearby galaxies abstract: Polycyclic aromatic hydrocarbons (PAHs) are widespread in the interstellar medium (ISM) of near solar metallicity galaxies, where they play a critical role in ISM heating, cooling, and reprocessing stellar radiation. The PAH fraction, the abundance of PAHs relative to total dust mass, is a key parameter in ISM physics. Using JWST and MUSE observations of 42 galaxies from the PHANGS survey, we analyzed the PAH fraction in over 17 000 H II regions spanning a gas-phase oxygen abundance of 12 + log(O/H) = 8.0–8.8 (Z ∼ 0.2–1.3 Z⊙), and ∼400 isolated supernova remnants (SNRs). We find a significantly lower PAH fraction toward H II regions compared to a reference sample of diffuse ISM areas at matched metallicity. At 12 + log(O/H) > 8.2, the PAH fraction toward H II regions is strongly anti-correlated with the local ionization parameter, suggesting that PAH destruction is correlated with ionized gas and/or hydrogen-ionizing UV radiation. At lower metallicities, the PAH fraction declines steeply in H II regions and in the diffuse ISM, likely reflecting less efficient PAH formation in metal-poor environments. Carefully isolating dust emission from the vicinity of optically identified supernova remnants, we see evidence of selective PAH destruction from measurements of lower PAH fractions, which is, however, indistinguishable at ∼50 pc scales. Overall, our results point to ionizing radiation as the dominant agent of PAH destruction within H II regions; metallicity plays a key role in their global abundance in galaxies.

Temperature-based radial metallicity gradients in nearby galaxies

title: Temperature-based radial metallicity gradients in nearby galaxies abstract: Context. Gas-phase abundances provide insights into the baryon cycle, with radial gradients and 2D metallicity distributions tracking how metals are built up and redistributed across galaxy disks over cosmic time. Aims. We use a catalog of 22 958 H II regions across 19 nearby spiral galaxies to examine how precisely the radial abundance gradients can be traced when using only the [N II] λ5755 electron temperature as a proxy for temperature-based, direct method metallicities. Methods. Using 534 direct detections of the temperature sensitive [N II] λ5755 auroral line, we measured gradients in 15 of the galaxies. Leveraging our large catalog of individual H II regions, we carried out a stacking procedure in bins of the H II region [N II] λ6583 luminosity and radius to recover stacked radial gradients. Results. We found a good agreement between the metallicity gradients from the stacked spectra and those gradients from individual regions and those from strong-line methods. In addition, particularly in the stacked Te[N II] measurements, some galaxies show very low (< 0.05 dex) scatter in metallicities, indicative of a well-mixed ISM. We examined the individual high confidence (S/N > 5) outliers and identified 13 regions across nine galaxies with anomalously low metallicities, although this is not strongly reflected in the strong-line method metallicities. By stacking arm and interarm regions, we found no systematic evidence for offsets in metallicity between these environments, suggesting that enrichment within spiral arms is due to very localized processes. Conclusions. This work demonstrates the potential to systematically exploit the single [N II] λ5755 auroral line for detailed gas-phase abundance studies of galaxies. It provides strong validation of previous results, based on the strong-line calibrations, of a well-mixed ISM across typical star-forming spiral galaxies.

Characterization of Two Cool Galaxy Outflow Candidates Using Mid-infrared Emission from Polycyclic Aromatic Hydrocarbons

title: Characterization of Two Cool Galaxy Outflow Candidates Using Mid-infrared Emission from Polycyclic Aromatic Hydrocarbons abstract: We characterize two candidate cool galactic outflows in two relatively low-mass, highly inclined Virgo cluster galaxies: NGC 4424 and NGC 4694. Previous analyses of observations using the Atacama Large Millimeter/submillimeter Array carbon monoxide (CO) line emission maps did not classify these sources as cool outflow hosts. Using new high-sensitivity, high-spatial-resolution, JWST mid-infrared photometry in the polycyclic aromatic hydrocarbon (PAH)–tracing F770W band, we identify extended structures present off of the stellar disk. The identified structures are bright in the MIRI F770W and F2100W bands, suggesting they include PAHs as well as other dust grains. As PAHs have been shown to be destroyed in hot, ionized gas, these structures are likely to be outflows of cool (T ≤ 104 K) gas. This work represents an exciting possibility for using mid-infrared observations to identify and measure outflows in lower-mass, lower star formation galaxies.

Physical Properties of Gas Departing from Circular Rotation at 50 pc Scales Using the PHANGS-MUSE Galaxies

title: Physical Properties of Gas Departing from Circular Rotation at 50 pc Scales Using the PHANGS-MUSE Galaxies abstract: Noncircular (NC) motions have been observed across various spatial scales in disk galaxies, yet the physical properties of the gas involved in these motions remain poorly constrained. Using data from 19 galaxies from the PHANGS-MUSE sample, we investigated the prevalence of NC flows at spatial resolutions of tens of parsecs. We developed a new tool for 3D kinematic modeling of data cubes and applied to the PHANGS-MUSE Hα spectral lines to recover the underlying circular, NC motions, as well as the intrinsic velocity dispersion in these objects. The PHANGS-MUSE galaxies exhibit rotation-supported disks with Vrot/σintrin ratios ≳5. Our analysis revealed ionized gas exhibiting NC motions at different amplitudes, with low velocity amplitudes of about 5 km s−1 associated with the axisymmetric rotation component, deviations of ∼10 km s−1 primarily linked to interarm regions and spiral arms, and larger deviations (>20 km s−1), found in the central and bar regions. We found that the velocity dispersion and the strength of ionization correlate with the amplitude of NC motions, suggesting that the underlying dynamics of the warm gas are closely tied to its physical properties.

Stellar structures, molecular gas, and star formation across the PHANGS sample of nearby galaxies (Corrigendum)

title: Stellar structures, molecular gas, and star formation across the PHANGS sample of nearby galaxies (Corrigendum)

Surveying the Whirlpool at Arcseconds with NOEMA (SWAN): III. 13CO/C18O ratio variations across the M51 galaxy

title: Surveying the Whirlpool at Arcseconds with NOEMA (SWAN): III. 13CO/C18O ratio variations across the M51 galaxy abstract: Context. CO isotopologues are common tracers of the bulk molecular gas in extragalactic studies, providing insights into the physical and chemical conditions of the cold molecular gas, a reservoir for star formation. Aims. Since star formation occurs within molecular clouds, mapping CO isotopologues on the scale of clouds is important to understanding the processes driving star formation. However, achieving this mapping at such scales is challenging and time-intensive. The Surveying the Whirlpool Galaxy at Arcseconds with NOEMA (SWAN) survey addresses this by using the Institut de radioastronomie millimétrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) to map the 13CO(1−0) and C18O(1−0) isotopologues, alongside several dense gas tracers, in the nearby star-forming galaxy M51 at high sensitivity and spatial resolution (≈125 pc). Methods. We examine the 13CO(1−0) to C18O(1−0) line emission ratio as a function of galactocentric radius and star formation rate surface density to infer how different chemical and physical processes affect this ratio at cloud scales across different galactic environments: nuclear bar, molecular ring, and northern and southern spiral arms. Results. In line with previous studies conducted at kiloparsec scales for nearby star-forming galaxies, we find a moderate positive correlation with galactocentric radius and a moderate negative correlation with star formation rate surface density across the field of view (FoV), with slight variations depending on the galactic environment. Conclusions. We propose that selective nucleosynthesis and changes in the opacity of the gas are the primary drivers of the observed variations in the ratio.

Constraining Resolved Extragalactic R21 Variation with Well-calibrated ALMA Observations

title: Constraining Resolved Extragalactic R21 Variation with Well-calibrated ALMA Observations abstract: CO(1–0) and CO(2–1) are commonly used as bulk molecular gas tracers. The CO line ratios (especially CO(2–1)/CO(1–0)–R21) vary within and among galaxies, yet previous studies on R21 and alike often rely on measurements constructed by combining data from facilities with substantial relative calibration uncertainties that have the same order as physical line ratio variations. Hence, robustly determining systematic R21 variations is challenging. Here, we compare CO(1–0) and CO(2–1) mapping data from ALMA for 14 nearby galaxies, at a common physical resolution of 1.7 kpc. Our data set includes new ALMA (7 m+TP) CO(1–0) maps of 12 galaxies. We investigate R21 variation to understand its dependence on global galaxy properties, kiloparsec-scale environmental factors, and its correlation with star formation rate (SFR) surface density and metallicity. We find that the galaxy-to-galaxy scatter is 0.05 dex. This is lower than previous studies, which reported over 0.1 dex variation, likely reflecting significant flux calibration uncertainties in single-dish surveys. Within individual galaxies, R21 has a typical mean value of ∼0.64 and 0.1 dex variation, with an increase to ∼0.75 toward galactic centers. We find strong correlations between R21 and various galactic parameters, particularly SFR surface density, which shows a power-law slope of 0.10–0.11 depending on the adopted binning/fitting methods. Our findings suggest that, for studies covering main-sequence galaxy samples, assuming a fixed R21 = 0.64 does not significantly bias kiloparsec-scale molecular gas mass estimates from CO(2–1). Instead, systematic uncertainties from flux calibration and the CO-to-H2 conversion factor account for more systematic scatter of CO-derived molecular gas properties.

Band Unfolding in Finite Nanostructures: Visualizing Dirac, Spin–Valley, and Rashba Features

title: Band Unfolding in Finite Nanostructures: Visualizing Dirac, Spin–Valley, and Rashba Features abstract: Nanomaterials possess unique electronic properties distinct from bulk systems, and spatially resolved techniques such as nano-ARPES now allow for local band structure measurements at the nanoscale. However, theoretical tools for interpreting band dispersion in finite, aperiodic systems remain limited. Here, we propose a "giant molecule band unfolding" (GMBU) procedure that enables the extraction of band dispersion from molecular orbital levels of finite systems without assuming periodic boundary conditions. Using first-principles calculations for graphene, tungsten disulfide, and bismuth/silver surface alloy nanoflakes, we successfully reproduced the characteristic band structures of Dirac cones, spin-valley locking, and Rashba spin splitting, respectively. Our spin-resolved formulation visualizes spin textures and provides an efficient framework for analyzing spintronic and valleytronic properties. GMBU enabled visualization of band dispersion, even when nanoflakes were bent. The method bridges discrete and continuous electronic descriptions and is applicable across dimensionalities and symmetry classes, offering new possibilities for understanding and designing functional nanoscale materials.

CO Isotopologue-derived Molecular Gas Conditions and CO-to-H2 Conversion Factors in M51

title: CO Isotopologue-derived Molecular Gas Conditions and CO-to-H2 Conversion Factors in M51 abstract: Over the past decade, several millimeter interferometer programs have mapped the nearby star-forming galaxy M51 at a spatial resolution of ≤170 pc. This study combines observations from three major programs: the PdBI Arcsecond Whirlpool Survey, the SMA M51 large program, and the Surveying the Whirlpool at Arcseconds with NOEMA. The data set includes the (1–0) and (2–1) rotational transitions of 12CO, 13CO, and C18O isotopologues. The observations cover the r < 3 kpc region, including the center and part of the disk, thereby ensuring strong detections of the weaker 13CO and C18O lines. All observations are convolved in this analysis to an angular resolution of 4″, corresponding to a physical scale of 170 pc. We investigate empirical line ratio relations and quantitatively evaluate molecular gas conditions such as temperature, density, and the CO-to-H2 conversion factor (αCO). We employ two approaches to study the molecular gas conditions: (i) assuming local thermodynamic equilibrium (LTE) to analytically determine the CO column density and αCO, and (ii) using non-LTE modeling with RADEX to fit physical conditions to observed CO isotopologue intensities. We find that the αCO values in the center and along the inner spiral arm are ∼0.5 dex (LTE) and 0.1 dex (non-LTE) below the Milky Way inner disk value. The average non-LTE αCO is 2.4 ± 0.5 M⊙ pc−2 (K km s−1)−1. While both methods show dispersion due to underlying assumptions, the scatter is larger for LTE-derived values. This study underscores the necessity for robust CO line modeling to accurately constrain the molecular interstellar medium’s physical and chemical conditions in nearby galaxies.

Bilayer germanene growth as a chiral heterostructure

title: Bilayer germanene growth as a chiral heterostructure abstract: Growing a bilayer Xene other than the well-established bilayer graphene is a tremendous challenge, but with equally great potential toward the development of new two-dimensional materials. We have used angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling microscopy (STM) to investigate the growth of bilayer germanene (BLGE) on Ag(111) with the assistance of a Pb layer. AB-stacked BLGE was successfully stabilized atop a commensurate Pb(111) monolayer, itself commensurate to a 30° rotated germanene layer in quasifreestanding phase (QP) below. Surprisingly, this four-layer configuration resulted in BLGE with a lattice constant of ∼3.5 Å, which is 15% compressed relative to the freestanding counterpart. Relaxation of these four layers by density functional theory (DFT) naturally converges to the same lattice constants from measurement, despite the large compressive strain in the BLGE. ARPES spectra in two symmetry directions match those predicted by the same DFT model—in particular a characteristic eye-shaped feature resulting from the evolved Dirac cone in the ΓK BLGE direction. Thus, we have demonstrated a novel method to grow a new Xene bilayer as part of a unique heterostructure which stabilizes the bilayer against substantial compressive strain and further induces chirality.

Orthorhombic TaAs: A New Topological Phase of the Archetypical Weyl Semimetal

title: Orthorhombic TaAs: A New Topological Phase of the Archetypical Weyl Semimetal abstract: We report on a new, topologically nontrivial phase of TaAs identified in thin TaAs layers grown by molecular beam epitaxy. Structural investigations clearly show a new atom arrangement, confirmed by the presence of 104 reflections in the X-ray diffraction pattern, forbidden for the well-known tetragonal phase. Density functional theory confirms the presence of a new orthorhombic phase and reveals that its formation energy is slightly higher (by ∼0.6 meV per atom) than for the tetragonal phase. The orthorhombic TaAs is a topological Weyl semimetal with 20 Weyl nodes. Weak antilocalization of a topological origin is observed at low temperatures. With the Fermi energy relatively deep in the valence band, no other signatures of the chiral properties are resolved. The demonstration of the new phase, combined with the molecular beam epitaxy capabilities of doping and strain adjustment, opens a new way toward the fine-tuning of Weyl semimetal layers and heterostructures.

Anisotropy of Second-Harmonic Generation in SnSe Flakes with Ferroelectric Stacking

title: Anisotropy of Second-Harmonic Generation in SnSe Flakes with Ferroelectric Stacking abstract: The second-harmonic generation (SHG) susceptibilities of few-layer SnSe with ferroelectric stacking are investigated using both experimental and theoretical approaches. Theoretical calculations predict a maximum bulk SHG susceptibility of 2444 pm V−1 at 1.2 eV, which is three orders of magnitude larger than that of typical nonlinear crystals. Experimentally, a maximum value of 1424 pm V−1 at 1.19 eV in close agreement with the theoretical prediction is measured. The anisotropic SHG patterns observed experimentally align with theoretical predictions based on the material's point group symmetry. The photon-energy dependence of SHG patterns is also measured within the range of 1.19 to 1.55 eV to explore the relative strengths of various SHG susceptibilities. Notably, the measured is significantly larger than the theoretical value of bulk AC-SnSe, likely due to the strain effects and the mixing of ferroelectric and antiferroelectric stacking configurations in the practical SnSe few-layer samples.

Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides

title: Efficient light upconversion via resonant exciton-exciton annihilation of dark excitons in few-layer transition metal dichalcogenides abstract: Materials capable of light upconversion—transforming low-energy photons into higher-energy ones—are pivotal in advancing optoelectronics, energy solutions, and photocatalysis. However, the discovery in various materials pays little attention on few-layer transition metal dichalcogenides, primarily due to their indirect bandgaps and weaker light-matter interactions. Here, we report a pronounced light upconversion in few-layer transition metal dichalcogenides through upconversion photoluminescence spectroscopy. Our joint theory-experiment study attributes the upconversion photoluminescence to a resonant exciton-exciton annihilation involving a pair of dark excitons with opposite momenta, followed by the spontaneous emission of upconverted bright excitons, which can have a high upconversion efficiency. Additionally, the upconversion photoluminescence is generic in MoS2, MoSe2, WS2, and WSe2, showing a high tuneability from green to ultraviolet light (2.34–3.1 eV). The findings pave the way for further exploration of light upconversion regarding fundamental properties and device applications in two-dimensional semiconductors.

Epitaxial Ferroelectric Hexagonal Boron Nitride Grown on Graphene

title: Epitaxial Ferroelectric Hexagonal Boron Nitride Grown on Graphene abstract: Ferroelectricity realized in van der Waals (vdW) materials with non-centrosymmetric stacking configurations holds promise for future 2D devices with nonvolatile and reconfigurable functionalities. However, the epitaxial growth of ferroelectric vdW materials often struggles to achieve an energetically unfavorable stacking configuration that enables electric polarization. This challenge is particularly evident when performing heteroepitaxy on another vdW substrate to create versatile and scalable ferroelectric building blocks designed for large-area, atomic-scale thicknesses. Here, epitaxial hexagonal boron nitride (h-BN) multilayer films are successfully grew on single-crystal graphene synthesized on a miscut SiC (0001) substrate. Theoretical calculations illustrate that the moiré-patterned h-BN/graphene hetero-interface intrinsically exhibits polarization, leading to a polarized AB stacking in multilayer h-BN films to minimize the total formation energy, which is validated experimentally by the layer-dependent band dispersions. The as-grown multilayer h-BN layers demonstrated robust, homogeneous ferroelectricity with switchable out-of-plane polarization via interlayer sliding. This study establishes an effective route for stacking-controlled heteroepitaxy, enabling the large-scale integration of vdW materials with ferroelectricity and versatile functionalities, offering a promising platform for next-generation 2D ferroelectric devices

Operando X-ray and Mass Spectroscopy of Reduced Graphene Oxide (rGO)-Mediated Cobalt Catalysts for Boosting the Hydrogen Evolution Reaction

title: Operando X-ray and Mass Spectroscopy of Reduced Graphene Oxide (rGO)-Mediated Cobalt Catalysts for Boosting the Hydrogen Evolution Reaction abstract: Inserting underlying reduced graphene oxide (rGO) into Co aims to regulate the chemical integrity and catalytic ability of the Co upper layer for hydrogen evolution reaction (HER) as a green-hydrogen goal. Principally, an operando mass spectrometer indicates 3.8 times more considerable hydrogen generation in Co/rGO than in Co. The spectroscopical approaches, combining operando soft and hard X-ray probing, illustrate the chemical oxidation evolution of electronic Co-3d and Co-4p states differently regarding the underlying rGO contribution. A unique examination is regarded as the phase transition from the initial middle to high oxidation and to deoxidation, related to the intermediate Co0 existence and H2 generation. The chemical adsorption of Co–O⁡(H), Co–Hads, and H2 molecules desorption have been assigned their spectral significances. The rGO mediation indicates two significant metal Co and Co–O⁡(H) blocks in the two-dimensional R-k domain. Density-functional-theory (DFT) calculation provides the regeneration, sustained stability, and decreasing energy barrier of Co–Hads catalysts due to the rGO incorporation, thereby augmenting the HER enhancement through the alternative Volmer-Heyrovsky process. The in-situ experiment, including mass spectrometer, soft, and hard X-ray, provides evidence regarding the catalyst’s HER enhancement. This study offers insights into the chemical composition, electronic structure, and active role of Co bonded with or without the extinct OH and H bonds, advancing our comprehension of electrocatalytic reactions, thus taking our knowledge of composite materials to stepwise electrocatalytic reactions forward. This cutting-edge experiment under in-situ environment and DFT studies gives critical information regarding the catalytic mechanism and chemical stability of the Co and rGO materials.

Improving electrochromic properties of V2O5 smart film through Ti incorporation: local atomic and electronic perspectives

title: Improving electrochromic properties of V2O5 smart film through Ti incorporation: local atomic and electronic perspectives abstract: In this work, vanadium pentoxide (V2O5) and titanium-modified V2O5 thin films were synthesized using the sol-gel spin coating route. The effect of Ti-doping concentration on the electrochromic optical properties and atomic/electronic structures of V2O5 smart thin films is examined. As the doping concentration of Ti increases, the surface roughness of the films is reduced. The structure of the films is analyzed using X-ray diffraction (XRD) and Raman spectroscopy, while the electrochromic modulation of atomic and electronic structures is elucidated through Raman and X-ray absorption spectroscopy (XAS) conducted during lithiation and delithiation. The XRD patterns demonstrate that an increase in Ti concentration leads to a more amorphous structure of the films and a shift of the main diffraction peak to a lower angle, attributable to the enlarged spacing between the stacking layers resulting from the incorporation of Ti ions. Soft X-ray absorption spectroscopy (XAS) and in situ hard XAS of the V L-edge, the O K-edge, and the V K-edge revealed a reduction in the charge state of V and local atomic structural symmetry modification upon lithated coloration and delithiated bleaching process. The critical insights provided by in situ XAS provides reveal that a small amount of Ti has the ability to modify the interlayer distance and local atomic structure of V2O5, thereby improving its electrochromic switching rate and stability when utilized in smart windows.

Fidelity and Entanglement of Random Bipartite Pure States: Insights and Applications

title: Fidelity and Entanglement of Random Bipartite Pure States: Insights and Applications abstract: We investigate the fidelity of Haar random bipartite pure states from a fixed reference quantum state and their bipartite entanglement. By plotting the fidelity and entanglement on perpendicular axes, we observe that the resulting plots exhibit non-uniform distributions. The distribution depends on the entanglement of the fixed reference quantum state used to quantify the fidelity of the random pure bipartite states. We find that the average fidelity of typical random pure bipartite qubits within a narrow entanglement range with respect to a randomly chosen fixed bipartite qubit is . Extending our study to higher dimensional bipartite qudits, we find that the average fidelity of typical random pure bipartite qudits with respect to a randomly chosen fixed bipartite qudit remains constant within a narrow entanglement range. The values of these constants are , with d being the dimension of the local Hilbert space of the bipartite qudit system, suggesting a consistent relationship between entanglement and fidelity across different dimensions. The probability distribution functions of fidelity with respect to a product state are analytically studied and used as a reference for the benchmarking of distributed quantum computing devices.

Distributing circuits over heterogeneous, modular quantum computing network architectures

title: Distributing circuits over heterogeneous, modular quantum computing network architectures abstract: We consider a heterogeneous network of quantum computing modules, sparsely connected via Bell states. Operations across these connections constitute a computational bottleneck and they are likely to add more noise to the computation than operations performed within a module. We introduce several techniques for transforming a given quantum circuit into one implementable on such a network, minimising the number of Bell states required to do so. We extend previous works on circuit distribution to the case of heterogeneous networks. On the one hand, we extend the hypergraph approach of Andres-Martinez and Heunen (2019 Phys. Rev. A 100 032308) to arbitrary network topologies, and we propose the use of Steiner trees to detect and reuse common connections, further reducing the cost of entanglement sharing within the network. On the other hand, we extend the embedding techniques of Wu et al (2023 Quantum7 1196) to networks with more than two modules. We show that, with careful manipulation of trade-offs, these two new approaches can be combined into a single automated framework. Our proposal is implemented and benchmarked; the results confirm that our contributions make noticeable improvements upon the aforementioned works and complement their weaknesses.

Out-of-plane coordination of iridium single atoms with organic molecules and cobalt–iron hydroxides to boost oxygen evolution reaction

title: Out-of-plane coordination of iridium single atoms with organic molecules and cobalt–iron hydroxides to boost oxygen evolution reaction abstract: Advancements in single-atom-based catalysts are crucial for enhancing oxygen evolution reaction (OER) performance while reducing precious metal usage. A comprehensive understanding of underlying mechanisms will expedite this progress further. Here we report Ir single atoms coordinated out-of-plane with dimethylimidazole (MI) on CoFe hydroxide (Ir1/(Co,Fe)-OH/MI). This Ir1/(Co,Fe)-OH/MI catalyst, which was prepared using a simple immersion method, delivers ultralow overpotentials of 179 mV at a current density of 10 mA cm−2 and 257 mV at 600 mA cm−2 as well as an ultra-small Tafel slope of 24 mV dec−1. Furthermore, Ir1/(Co,Fe)-OH/MI has a total mass activity exceeding that of commercial IrO2 by a factor of 58.4. Ab initio simulations indicate that the coordination of MI leads to electron redistribution around the Ir sites. This causes a positive shift in the d-band centre at adjacent Ir and Co sites, facilitating an optimal energy pathway for OER.

Formation of highly stable interfacial nitrogen gas hydrate overlayers under ambient conditions

title: Formation of highly stable interfacial nitrogen gas hydrate overlayers under ambient conditions abstract: Surfaces (interfaces) dictate many physical and chemical properties of solid materials and adsorbates considerably affect these properties. Nitrogen molecules, which are the most abundant constituent in ambient air, are considered to be inert. Our study combining atomic force microscopy (AFM), X-ray photoemission spectroscopy (XPS), and thermal desorption spectroscopy (TDS) revealed that nitrogen and water molecules can self-assemble into two-dimensional domains, forming ordered stripe structures on graphitic surfaces in both water and ambient air. The stripe structures of this study were composed of approximately 90 % and 10 % water and nitrogen molecules, respectively, and survived in ultra-high vacuum (UHV) conditions at temperatures up to approximately 350 K. Because pure water molecules completely desorb from graphitic surfaces in a UHV at temperatures lower than 200 K, our results indicate that the incorporation of nitrogen molecules substantially enhanced the stability of the crystalline water hydrogen bonding network on graphitic surfaces. Additional studies on interfacial gas hydrates can provide deeper insight into the mechanisms underlying formation of gas hydrates.

DNA-Anchored Single-Molecule Iron Phthalocyanine As an Efficient Electrocatalyst for Alkaline Fuel Cells

title: DNA-Anchored Single-Molecule Iron Phthalocyanine As an Efficient Electrocatalyst for Alkaline Fuel Cells abstract: Molecular catalysts have attracted significant attention because of their high activity, selectivity, and tunability. However, in heterogeneous catalysis, the uniform dispersion and immobilization of molecular catalysts on the supporting substrate remain a significant challenge due to their aggregation tendency. Here, we present a facile strategy to molecularly disperse and immobilize a series of macrocyclic metal complexes onto reduced graphene oxide (rGO) by using DNA as a mediator. The electroactive amounts of molecularly dispersed iron phthalocyanine (FePc) molecules are increased by ∼50 times greater than that of pristine FePc catalyst. As a result, the single-molecule catalyst demonstrates a notable power density (∼290 mW cm–2) in an H2/O2 alkaline polymer electrolyte fuel cell. Operando X-ray absorption spectroscopy experiments combined with density functional theory calculations reveal that the coordination interaction between FePc and DNA enables the molecular dispersion and immobilization of FePc on the surface of rGO, and consequently improves the activity by regulating the electronic structure of active centers. This study points out a facile strategy to tackle the fundamental challenges facing molecular catalysts in long-lasting energy conversion technologies.

Atomic Insights into the Competitive Edge of Nanosheets Splitting Water

title: Atomic Insights into the Competitive Edge of Nanosheets Splitting Water abstract: The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxohydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with the atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain the performance. However, it is not fully understood how activity and stability are related at the atomic level, hampering rational design. Herein, we provide simple design rules (Figure 12) derived from the literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that IrOOH signals the chemical stability of crystalline IOHs while surpassing the activity of amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3Δ-O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e., μ1-O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.

Role of annealing environments on the local electronic and optical properties of zinc oxide films

title: Role of annealing environments on the local electronic and optical properties of zinc oxide films abstract: The exploration of zinc oxide (ZnO) has seen rapid growth due to its wide bandgap, high thermal conductivity, and high electron mobility within its nanostructures, making it highly valuable for applications in electronic and optical devices. This study involves the deposition of ZnO nanostructures onto a quartz substrate using the thermal evaporation deposition technique. The research delves into the influence of various annealing environments on morphology and electronic/atomic structures. X-ray Diffraction (XRD) analysis reveals the development of the ZnO phase with a preferred orientation when annealed in a mixture of hydrogen and argon gases. It was observed that the films do not transform into complete ZnO for the as-prepared and for lower temperatures (< 400 °C). Annealing at lower temperatures or room temperature retains the metallic Zn. When subjected to higher annealing temperatures in the presence of oxygen and ambient conditions, formation of ZnO hexagonal wurtzite structures was evident. A wide variation in the morphology was observed for films annealed in different annealing environments, from petal-like structures to rods. Both the band edge and defect emissions using photoluminescence were observed to vary with varying annealing conditions. In-situ XANES uncovers the reduction of Zn and metallic nature noticed in the as-prepared sample and argon annealing at 400 °C. A higher charge transfer by Zn with a prominent delocalization of Zn 4p state was observed in oxygen annealing environment at 400 °C with an enhanced coordination of Zn–O and Zn–Zn. These distinct photoluminescence emissions from the films were correlated to the electronic structure and local atomic structure. Further, this investigation provides a path to develop next-generation optoelectronic devices.

Ultra-low-potential methanol oxidation on single-Ir-atom catalyst

title: Ultra-low-potential methanol oxidation on single-Ir-atom catalyst abstract: Methanol oxidation plays a central role to implement sustainable energy economy, which is restricted by the sluggish reaction kinetics due to the multi-electron transfer process accompanied by numerous sequential intermediate. In this study, an efficient cascade methanol oxidation reaction is catalyzed by single-Ir-atom catalyst at ultra-low potential (<0.1 V) with the promotion of the thermal and electrochemical integration in a high temperature polymer electrolyte membrane electrolyzer. At the elevated temperature, the electron deficient Ir site with higher methanol affinity could spontaneous catalyze the CH3OH dehydrogenation to CO under the voltage, then the generated CO and H2 was electrochemically oxidized to CO2 and proton. However, the methanol cannot thermally decompose with the voltage absence, which confirm the indispensable of the coupling of thermal and electrochemical integration for the methanol oxidation. By assembling the methanol oxidation reaction with hydrogen evolution reaction with single-Ir-atom catalysts in the anode chamber, a max hydrogen production rate reaches 18 mol gIr−1 h−1, which is much greater than that of Ir nanoparticles and commercial Pt/C. This study also demonstrated the electrochemical methanol oxidation activity of the single atom catalysts, which broadens the renewable energy devices and the catalyst design by an integration concept.

Main Group SnN4O Single Sites with Optimized Charge Distribution for Boosting the Oxygen Reduction Reaction

title: Main Group SnN4O Single Sites with Optimized Charge Distribution for Boosting the Oxygen Reduction Reaction abstract: Transition metal single-atom catalysts (SACs) have been regarded as possible alternatives to platinum-based materials due to their satisfactory performance of the oxygen reduction reaction (ORR). By contrast, main-group metal elements are rarely studied due to their unfavorable surface and electronic states. Herein, a main-group Sn-based SAC with penta-coordinated and asymmetric first-shell ligands is reported as an efficient and robust ORR catalyst. The introduction of the vertical oxygen atom breaks the symmetric charge balance, modulating the binding strength to oxygen intermediates and decreasing the energy barrier for the ORR process. As expected, the prepared Sn SAC exhibits outstanding ORR activity with a high half-wave potential of 0.912 V (vs RHE) and an excellent mass activity of 13.1 A mgSn–1 at 0.850 V (vs RHE), which surpasses that of commercial Pt/C and most reported transition-metal-based SACs. Additionally, the reported Sn SAC shows excellent ORR stability due to the strong interaction between Sn sites and the carbon support with oxygen atom as the bridge. The excellent ORR performance of Sn SAC was also proven by both liquid- and solid-state zinc–air battery (ZAB) measurements, indicating its great potential in practical applications.

Peculiar spin glass phase emerging in FeCo/FePt driven via nanoconfined crystallographic distortions

title: Peculiar spin glass phase emerging in FeCo/FePt driven via nanoconfined crystallographic distortions abstract: We explore the existence of spin glass phase in FeCo/FePt bilayers arising due to disordered ferromagnet. The non-ergodic and highly degenerate landscape of the spin glass phase at low temperature explains the origin of complex magnetic texture in the FeCo/FePt system. Upon cooling the bilayered system, the magnetic texture undergoes spin freezing below 120 K as evident from the bifurcations in zero field cooling and field cooling magnetizations at low magnetic field as a manifestation of broken ergodicity. The uncompensated magnetic moments originating in the spin glass state result in slow time dynamics of thermoremanent magnetization. Consequently, the bilayers demonstrate an intriguing magnetic memory effect in which the magnetic state of the system could be retrieved upon isothermal ageing below 120 K after reversing the temperature cycle. Thermal treatment deteriorates the spin glass behaviour and shows a transition to strong ferromagnetic character in FeCo/FePt bilayers.

Structural and optical tunability of Ag-ZnO nanocomposite thin films for surface-enhanced Raman studies

title: Structural and optical tunability of Ag-ZnO nanocomposite thin films for surface-enhanced Raman studies abstract: The Ag-ZnO nanocomposite thin films of varying Ag contents were synthesized successfully using simultaneous RF- and DC-magnetron sputterings on quartz substrates. The as-prepared nanocomposites were then annealed in a mixture of hydrogen plus argon environment (95%Ar+5%H2) at a temperature of 300 °C. The pristine and annealed films were then subjected to synchrotron XRD to study for any possible structural modifications induced in the films. The crystalline behavior of the hexagonal wurtzite structure of ZnO, in the pristine and annealed samples of Ag-ZnO nanocomposites, was evident in samples prepared at low Ag content, but with increasing Ag content, the crystalline behavior of the hexagonal wurtzite structure was suppressed. FESEM utilized to study the formation of nanoparticles in the films revealed the non-uniform distribution and agglomeration of nanoparticles increases as the Ag content increases. A broad surface plasmon region in the range of 380–450 nm was observed for different compositions with remarkable blue and red shifts using UV-visible spectroscopy. These high-quality Ag-ZnO nanocomposite thin films with tunable optical properties were then used as surface-enhanced Raman spectroscopy (SERS) substrates for the detection of rhodamine B (RhB) at low concentrations of 10−6 M. The detection of RhB in trace amounts is important in the current scenario as wastewater discharge is a serious threat to our ecosystem.

Molecularly engineered potential of d-orbital modulated iron-bridged delaminated MBene for rechargeable Zn-Air batteries

title: Molecularly engineered potential of d-orbital modulated iron-bridged delaminated MBene for rechargeable Zn-Air batteries abstract: An air cathode with high catalytic activity and reversibility toward the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is designed using an engineered Fe d-orbital strategy through the typical coordinations of iron phthalocyanine molecule (FeMc) crystals on delaminated MoAl1−xB MBene sheets (FeMc–MoAl1−xB). The hybridization induces unique electronic guest–host interactions with significant charge donation by MoAl1−xB, resulting in remarkable regulation of the Fe3d charge delocalization and spin-state Fe(II) ion transition, thereby optimizing the adsorption/desorption of oxygenated intermediates with a balanced *OOH–*O transformation for simultaneously boosting both ORR and OER kinetics with high reversibility. FeMc–MoAl1−xB achieves a remarkable OER overpotential of 0.356 V at 10 mA cm−2 while exhibiting a favorable half-wave potential of 0.862 V for the ORR in an alkaline electrolyte. An aqueous Zn–air battery (ZAB) assembled with an FeMc–MoAl1−xB air cathode demonstrates a high peak power density of 168.2 mW cm−2 and a long cycling durability of 800 h, overperforming the (Pt/C + RuO2) counterpart. These findings highlight the significance of such novel efficient air–cathode catalysts by performing electronic structure engineering of FeMc with MoAl1−xB to develop rechargeable ZAB devices with enhanced performance and cost effectiveness.