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ABC-stacked rhombohedral graphene multilayers exhibit a wide variety of electronic ground states characterized by broken isospin symmetry and superconductivity. Recently, indirect evidence of inter-valley coherent (IVC) order has been reported in rhombohedral trilayer graphene (RTG), with possible implications for the origin of superconductivity. Here, we report the direct visualization of IVC order in RTG using scanning tunneling microscopy and spectroscopy. Tuning the chemical potential through the Van Hove singularity near the edge of the valence band, we observe a cascade of phase transitions associated with the formation of half- and quarter-metal states. IVC phases, distinguished by an enlarged real space unit cell, are directly imaged near both the high- and low-density boundaries of the half-metal phase. At high hole density, we precisely reconstruct the IVC band structure through quasiparticle interference. Intriguingly, the charge density modulations reveal a C3-symmetric incommensurate IVC order that agrees with the recent prediction of an IVC-crystal phase. Our findings demonstrate that IVC phases are a widespread symmetry-broken ground state within graphene systems., Comment: 22 pages, 4 figures

Inducing superconducting correlations in chiral edge states is predicted to generate topologically protected zero energy modes with exotic quantum statistics. Experimental efforts to date have focused on engineering interfaces between superconducting materials, typically amorphous metals, and semiconducting quantum Hall or quantum anomalous Hall (QAH) systems. However, the strong interfacial disorder inherent can prevent the formation of isolated topological modes. An appealing alternative is to use low-density flat band materials where the ground state can be tuned between intrinsic superconducting and quantum anomalous Hall states using only the electric field effect. However, quantized transport and superconductivity have not been simultaneously achieved. Here, we show that rhombohedral tetralayer graphene aligned to a hexagonal boron nitride substrate hosts a quantized anomalous Hall state at superlattice filling $\nu = -1$ as well as a superconducting state at $\nu \approx -3.1$ at zero magnetic field. Remarkably, gate voltage can also be used to actuate nonvolatile switching of the chirality in the quantum anomalous Hall state, allowing arbitrarily reconfigurable networks of topological edge modes in locally gated devices. Thermodynamic compressibility measurements further reveal a topologically ordered fractional Chern insulator at $\nu = 2/3$, also stable at zero magnetic field, enabling proximity coupling between superconductivity and fractionally charged edge modes. Finally, we show that integrating an additional transition metal dichalcogenide layer to the heterostructure enhances the maximum superconducting critical temperature and nucleates new superconducting pockets, while leaving the topology of $\nu = -1$ the quantum anomalous Hall state intact. Our results pave the way for hybrid interfaces between superconductors and topological edge states in the low-disorder limit.

Graphene and transition metal dichalcogenide flat-band systems show similar phase diagrams, replete with magnetic and superconducting phases. An abiding question has been whether magnetic ordering competes with superconductivity or facilitates pairing. The advent of crystalline graphene superconductors enables a new generation of controlled experiments to probe the microscopic origin of superconductivity. For example, recent studies of Bernal bilayer graphene show a dramatic increase in the observed domain and critical temperature $T_c$ of superconducting states in the presence of enhanced spin-orbit coupling; the mechanism for this enhancement, however, remains unclear. Here, we show that introducing spin-orbit coupling in rhombohedral trilayer graphene (RTG) via substrate proximity effect generates new superconducting pockets for both electron and hole doping, with maximal $T_c\approx$ 300mK three times larger than in RTG encapsulated by hexagonal boron nitride alone. Using local magnetometry and thermodynamic compressibility measurements, we show that superconductivity straddles an apparently continuous transition between a spin-canted state with a finite in-plane magnetic moment and a state with complete spin-valley locking. This transition is reproduced in our Hartree-Fock calculations, where it is driven by the competition between spin-orbit coupling and the carrier-density-tuned Hund's interaction. Our experiment suggests that the enhancement of superconductivity by spin-orbit coupling is driven not by a change in the ground state symmetry or degeneracy but rather by a quantitative change in the canting angle. These results align with a recently proposed mechanism for the enhancement of superconductivity in spin-orbit coupled rhombohedral multilayers, in which fluctuations in the spin-canting order contribute to the pairing interaction.

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wave functions in f- and d-atomic orbitals. In rhombohedral multilayer graphene (RMG), in contrast, magnetism-manifesting as spontaneous polarization into one or more spin and valley flavors[1-7]-originates from itinerant electrons near a Van Hove singularity. Here, we show experimentally that the electronic entropy in this system shows signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution $\Delta$ S $\approx$ 1k$_B$/charge carrier that onsets at the Curie temperature and survives over one order of magnitude in temperature. First order phase transitions show an isospin `Pomeranchuk effect' in which the fluctuating moment phase is entropically favored over the nearby symmetric Fermi liquid[8, 9]. Our results imply that despite the itinerant nature of the electron wave functions, the spin- and valley polarization of individual electrons are decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He[10]. Transport measurements, surprisingly, show a finite temperature resistance minimum within the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat band systems.

In the absence of time reversal symmetry, orbital magnetization provides a sensitive probe of topology and interactions, with particularly rich phenomenology in Chern insulators where topological edge states carry large equilibrium currents. Here, we use a nanoscale superconducting sensor to map the magnetic fringe fields in twisted bilayers of MoTe$_2$, where transport and optical sensing experiments have revealed the formation of fractional Chern insulator (FCI) states at zero magnetic field. At a temperature of 1.6K, we observe oscillations in the local magnetic field associated with fillings $\nu=-1,-2/3,-3/5,-4/7$ and $-5/9$ of the first moir\'e hole band, consistent with the formation of FCIs at these fillings. By quantitatively reconstructing the magnetization, we determine the local thermodynamic gaps of the most robust FCI state at $\nu=-2/3$, finding $^{-2/3}\Delta$ as large as 7 meV. Spatial mapping of the charge density- and displacement field-tuned magnetic phase diagram further allows us to characterize sample disorder, which we find to be dominated by both inhomogeneity in the effective unit cell area as well as inhomogeneity in the band edge offset and bound dipole moment. Our results highlight both the challenges posed by structural disorder in the study of twisted homobilayer moir\'e systems and the opportunities afforded by the remarkably robust nature of the underlying correlated topological states.

Spin and valley flavor polarization plays a central role in the many-body physics of flat band graphene, with fermi surface reconstructions often accompanied by quantized anomalous Hall and superconducting state observed in a variety of experimental systems. Here we describe an optical technique that sensitively and selectively detects flavor textures via the exciton response of a proximal transition metal dichalcogenide layer. Through a systematic study of rhombohedral and rotationally faulted graphene bilayers and trilayers, we show that when the semiconducting dichalcogenide is in direct contact with the graphene, the exciton response is most sensitive to the large momentum rearrangement of the Fermi surface, providing information that is distinct from and complementary to electrical compressibility measurements. The wide-field imaging capability of optical probes allows us to obtain spatial maps of flavor orders with high throughput, and with broad temperature and device compatibility. Our work paves the way for optical probing and imaging of flavor orders in flat band graphene systems., Comment: 29 pages, 4 figures, with supplementary materials

The standard flare model, despite its success, is limited in comprehensively explaining the various processes involving nonthermal particles. One such missing ingredient is a detailed understanding of the various processes involved during the transport of accelerated electrons from their site of acceleration to different parts of the flare region. Here we use simultaneous radio and X-ray observations from the Expanded Owens Valley Solar Array (EOVSA) and Spectrometer/Telescope for Imaging X-rays (STIX) onboard the Solar Orbiter (SolO), respectively, from two distinct viewing perspectives to study the electron transport processes. Through detailed spectral modeling of the coronal source using radio data and footpoint sources using X-ray spectra, we compare the nonthermal electron distribution at the coronal and footpoint sources. We find that the flux of nonthermal electrons precipitated at the footpoint is an order of magnitude greater than that trapped in the looptop, consistent with earlier works which primarily used X-ray for their studies. In addition, we find that the electron spectral indices obtained from X-ray footpoints is significantly softer than the spectral hardness of the nonthermal electron distribution in the corona. We interpret these differences based on transport effects and the difference in sensitivity of microwave and X-ray observations to different regimes of electron energies. Such an understanding is crucial for leveraging different diagnostic methods of nonthermal electrons simultaneously to achieve a more comprehensive understanding of the electron acceleration and transport processes of solar flares., Comment: Accepted for publication in the Astrophysical Journal

Anyons are two dimensional particles with fractional exchange statistics that emerge as elementary excitations of fractional quantum Hall phases. Experimentally, anyonic statistics manifest directly in the edge-state Fabry-P\'erot interferometer geometry, where the presence of $N_{qp}$ localized anyons in the interferometer bulk contributes a phase $N_{qp} \theta_a$ to the observed interference pattern, where $\theta_a$ is twice the statistical exchange phase. Here, we report a measurement of $\theta_a$ in a monolayer graphene Fabry-P\'erot interferometer at $\nu$ = 1/3. We find a preponderance of phase slips with magnitudes $\Delta \theta \approx 2 \pi / 3$, confirming the result of past experiments in GaAs quantum wells and consistent with expectations for the tunneling of Abelian anyons into the interferometer bulk. In contrast to prior work, however, single anyon tunneling events manifest as instantaneous and irreversible phase slips, indicative of quasiparticle equilibration times exceeding 20 minutes in some cases. We use the discrepancy between the quasiparticle equilibration rate and our measurement speed to vary the interferometer area and $N_{qp}$ independently, allowing us to precisely determine the interferometer phase and monitor the entry and exit of individual anyons to the interferometer loop in the time domain. Besides providing a replication of previous interferometric measurements sensitive to $\theta_a$ in GaAs, our results bring anyon dynamics into the experimental regime and suggest that the average `topological charge' of a mesoscopic quantum Hall device can be held constant over hour long timescales., Comment: Main Text: 9 pages, 4 figures. Supplementary Info: 20 pages, 15 figures

Aims: This work aims to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Here, by using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from ~ 7 s in the impulsive phase to ~ 35 s later in the flare. Methods: The temporal, spatial and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare are analysed. Images are reconstructed for individual peaks in the impulsive phase and spectral fitting is performed at high cadence throughout the first phase of pulsations. Results: Imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission changes in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux showing a "soft-hard-soft" spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Conclusions: Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration in time related to the associated filament eruption., Comment: 15 pages, 8 figures, accepted by A&A

The Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter (SolO) provides a unique opportunity to systematically perform stereoscopic X-ray observations of solar flares with current and upcoming X-ray missions at Earth. These observations will produce the first reliable measurements of hard X-ray (HXR) directivity in decades, providing a new diagnostic of the flare-accelerated electron angular distribution and helping to constrain the processes that accelerate electrons in flares. However, such observations must be compared to modelling, taking into account electron and X-ray transport effects and realistic plasma conditions, all of which can change the properties of the measured HXR directivity. Here, we show how HXR directivity, defined as the ratio of X-ray spectra at different spacecraft viewing angles, varies with different electron and flare properties (e.g., electron angular distribution, highest energy electrons, and magnetic configuration), and how modelling can be used to extract these typically unknown properties from the data. Lastly, we present a preliminary HXR directivity analysis of two flares, observed by the Fermi Gamma-ray Burst Monitor (GBM) and SolO/STIX, demonstrating the feasibility and challenges associated with such observations, and how HXR directivity can be extracted by comparison with the modelling presented here., Comment: Accepted for publication in ApJ (January 2024)

Quantum point contacts (QPCs) are an essential component in mesoscopic devices. Here, we study the transmission of quantum Hall edge modes through a gate-defined QPC in monolayer graphene. We observe resonant tunneling peaks and a nonlinear conductance pattern characteristic of Coulomb-blockaded localized states. The in-plane electric polarizability reveals the states are localized at a classically-unstable electrostatic saddle point. We explain this unexpected finding within a self-consistent Thomas-Fermi model, finding that localization of a zero-dimensional state at the saddle point is favored whenever the applied confinement potential is sufficiently soft compared to the Coulomb energy. Our results provide a direct demonstration of Coulomb-driven reconstruction at the boundary of a quantum Hall system., Comment: 15 pages, 10 figures. arXiv admin note: text overlap with arXiv:2204.10296

The calculation of loop corrections to the correlation functions of quantum fields during inflation or in the de~Sitter background presents greater challenges than in flat space due to the more complicated form of the mode functions. While in flat space highly sophisticated approaches to Feynman integrals exist, similar tools still remain to be developed for cosmological correlators. However, usually only their late-time limit is of interest. We introduce the method-of-region expansion for cosmological correlators as a tool to extract the late-time limit, and illustrate it with several examples for the interacting, massless, minimally coupled scalar field in de~Sitter space. In particular, we consider the in-in correlator $\langle\phi^2(\eta,q)\phi(\eta,k_1)\phi(\eta,k_2)\rangle$, whose region structure is relevant to anomalous dimensions and matching coefficients in Soft de Sitter effective theory., Comment: 44 pages, 4 figures, LaTeX

In the last decades, an increasing scientific interest has been growing in the elusive population of "dark" (i.e. lacking an optical/NIR counterpart) Dusty Star-Forming Galaxies (DSFGs). Although extremely promising for their likely contribution to the cosmic Star Formation Rate Density and for their possible role in the evolution of the first massive and passive galaxies around $z\sim3$, the difficulty in selecting statistically significant samples of dark DSFGs is limiting their scientific potentialities. This work presents the first panchromatic study of a sample of 263 Radio-Selected NIRdark galaxies discovered in the COSMOS field following the procedure by Talia+21. These sources are selected as radio-bright galaxies (S(3GHz)>12.65 uJy) with no counterpart in the NIR-selected COSMOS2020 catalog (Ks > 25.5 mag). For these sources, we build a new photometric catalog including accurate photometry from the optical to the radio obtained with a new deblending pipeline (PhoEBO: Photometry Extractor for Blended Objects). We employ this catalog to estimate the photo-zs and the physical properties of the galaxies through an SED-fitting procedure performed with two different codes (Magphys and Cigale). Finally, we estimate the AGN contamination in our sample by performing a series of complementary tests. The high values of the median extinction (Av ~ 4) and star formation rate (SFR ~ 500 Msun/yr) confirm the likely DSFG nature of the RS-NIRdark galaxies. The median photo-z (z~3) and the presence of a significant tail of high-z candidates (z>4.5) suggest that these sources are important contributors to the cosmic SFRD and the evolutionary path of galaxies at high redshifts., Comment: 22 pages, 9 figures, 3 tables. Accepted for publication in ApJ

Shared autonomy enables novice remote users to conduct deep-ocean science operations with robotic manipulators.

The phase diagram of an interacting two-dimensional electron system in a high magnetic field is enriched by the varying form of the effective Coulomb interaction, which depends strongly on the Landau level index. While the fractional quantum Hall states that dominate in the lower energy Landau levels have been explored experimentally in a variety of two-dimensional systems, much less work has been done to explore electron solids owing to their subtle transport signatures and extreme sensitivity to disorder. Here we use chemical potential measurements to map the phase diagram of electron solid states in $N=2$, $N=3$, and $N=4$ Landau levels in monolayer graphene. Direct comparison between our data and theoretical calculations reveals a cascade of density-tuned phase transitions between electron bubble phases up to two, three or four electrons per bubble in the N=2, 3 and 4 Landau levels respectively. Finite temperature measurements are consistent with melting of the solids for T$\approx$1K., Comment: 5 pages, 4 figures in main text, 7 pages, 3 figures in supplementary material

About 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals and dust. How efficient was this process? How fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the Star Formation Rate Density (SFRD) back to its widely unknown high redshift tail, traditionally observed in the Near-InfraRed (NIR), Optical and UV bands. Thus, the objects with a high amount of dust were missing. We aim to fill this knowledge gap by studying Radio Selected NIR-dark (\textit{RS-NIRdark}) sources, i.e. sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample by Talia et al. (2021) from 197 to 272 objects in the COSMic evolution Survey (COSMOS) field, including also photometrically contaminated sources, previously excluded. Another important step forward consists in the visual inspection of each source in the bands from u* to MIPS-24$\mu$m. According to their "environment" in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark to the Cosmic SFRD at 3$<$z$<$5 is $\sim$10--25$\%$ of that based on UV-selected galaxies.

Bernal bilayer graphene hosts even denominator fractional quantum Hall states thought to be described by a Pfaffian wave function with nonabelian quasiparticle excitations. Here we report the quantitative determination of fractional quantum Hall energy gaps in bilayer graphene using both thermally activated transport and by direct measurement of the chemical potential. We find a transport activation gap of 5.1K at B = 12T for a half-filled N=1 Landau level, consistent with density matrix renormalization group calculations for the Pfaffian state. However, the measured thermodynamic gap of 11.6K is smaller than theoretical expectations for the clean limit by approximately a factor of two. We analyze the chemical potential data near fractional filling within a simplified model of a Wigner crystal of fractional quasiparticles with long-wavelength disorder, explaining this discrepancy. Our results quantitatively establish bilayer graphene as a robust platform for probing the non-Abelian anyons expected to arise as the elementary excitations of the even-denominator state.

The B[e] phenomenon is manifested by a heterogeneous group of stars surrounded by gaseous and dusty circumstellar envelopes with similar physical conditions. Among these stars, the FS CMa-type objects are suspected to be binary systems, which could be experiencing or have undergone a mass-transfer process that could explain the large amount of material surrounding them. We aim to contribute to the knowledge of a recently confirmed binary, MWC 645, which could be undergoing an active mass-transfer process. We present near-infrared and optical spectra, identify atomic and molecular spectral features, and derive different quantitative properties of line profiles. Based on publicly available photometric data, we search for periodicity in the light curve and model the spectral energy distribution. We have detected molecular bands of CO in absorption at 1.62 $\mu$m and 2.3 $\mu$m for the first time. We derive an upper limit for the effective temperature of the cool binary component. We found a correlation between the enhancement of the H$\alpha$ emission and the decrease in optical brightness that could be associated with mass-ejection events or an increase in mass loss. We outline the global properties of the envelope, possibly responsible for brightness variations due to a variable extinction, and briefly speculate on different possible scenarios., Comment: 20 pages, 11 figures, 2 tables, published in Galaxies

Massive stars expel large amounts of mass during their late evolutionary phases. We aim to unveil the physical conditions within the warm molecular environments of B[e] supergiants (B[e]SGs) and yellow hypergiants (YHGs), which are known to be embedded in circumstellar shells and disks. We present K-band spectra of two B[e]SGs from the Large Magellanic Cloud and four Galactic YHGs. The CO band emission detected from the B[e]SGs LHA 120-S 12 and LHA 120-S 134 suggests that these stars are surrounded by stable rotating molecular rings. The spectra of the YHGs display a rather diverse appearance. The objects 6 Cas and V509 Cas lack any molecular features. The star [FMR2006] 15 displays blue-shifted CO bands in emission, which might be explained by a possible close to pole-on oriented bipolar outflow. In contrast, HD 179821 shows blue-shifted CO bands in absorption. While the star itself is too hot to form molecules in its outer atmosphere, we propose that it might have experienced a recent outburst. We speculate that we currently can only see the approaching part of the expelled matter because the star itself might still block the receding parts of a (possibly) expanding gas shell., Comment: 23 pages, 6 figures, published in Galaxies

Accessibility is one of the most important features in the design of robots and their interfaces. This thesis proposes methods that improve the accessibility of robots for three different target audiences: consumers, researchers, and learners. In order for humans and robots to work together effectively, they both must be able to communicate with each other. We tackle the problem of generating route instructions that are readily understandable by novice humans for the navigation of a priori unknown indoor environments. We then move on to the related problem of enabling robots to understand natural language utterances in the context of learning to operate articulated objects (e.g., fridges, drawers) by leveraging kinematic models. Next, we turn our focus to the development of accessible and reproducible robotic platforms for scientific research. We propose a new concept for reproducible robotics research that integrates development and benchmarking, so that reproducibility is obtained "by design" from the beginning of the research and development process. We then propose a framework called SHARC (SHared Autonomy for Remote Collaboration), to improve accessibility for underwater robotic intervention operations. SHARC allows multiple remote scientists to efficiently plan and execute high-level sampling procedures using an underwater manipulator while deferring low-level control to the robot. Lastly, we developed the first hardware-based MOOC in AI and robotics. This course allows learners to study autonomy hands-on by making real robots make their own decisions and accomplish broadly defined tasks. We design a new robotic platform from the ground up to support this new learning experience. A fully browser-based interface, based on leading tools and technologies for code development, testing, validation, and deployment serves to maximize the accessibility of these educational resources., Comment: Ph.D. Thesis

Recent experiments on Bernal bilayer graphene (BLG) deposited on monolayer WSe$_2$ revealed robust, ultra-clean superconductivity coexisting with sizable induced spin-orbit coupling. Here we propose BLG/WSe$_2$ as a platform to engineer gate-defined planar topological Josephson junctions, where the normal and superconducting regions descend from a common material. More precisely, we show that if superconductivity in BLG/WSe$_2$ is gapped and emerges from a parent state with inter-valley coherence, then Majorana zero modes can form in the barrier region upon applying weak in-plane magnetic fields. Our results spotlight a potential pathway for `internally engineered' topological superconductivity that minimizes detrimental disorder and orbital-magnetic-field effects., Comment: 7 pages, 4 figures, plus supplementary material

Magic-angle twisted trilayer graphene (MATTG) exhibits a range of strongly correlated electronic phases that spontaneously break its underlying symmetries. The microscopic nature of these phases and their residual symmetries stands as a key outstanding puzzle whose resolution promises to shed light on the origin of superconductivity in twisted materials. Here we investigate correlated phases of MATTG using scanning tunneling microscopy and identify striking signatures of interaction-driven spatial symmetry breaking. In low-strain samples, over a filling range of about 2-3 electrons or holes per moir\'e unit cell, we observe atomic-scale reconstruction of the graphene lattice that accompanies a correlated gap in the tunneling spectrum. This short-scale restructuring appears as a Kekul\'e supercell -- implying spontaneous inter-valley coherence between electrons -- and persists in a wide range of magnetic fields and temperatures that coincide with the development of the gap. Large-scale maps covering several moir\'e unit cells further reveal a slow evolution of the Kekul\'e pattern, indicating that atomic-scale reconstruction coexists with translation symmetry breaking at the much longer moir\'e scale. We employ auto-correlation and Fourier analyses to extract the intrinsic periodicity of these phases and find that they are consistent with the theoretically proposed incommensurate Kekul\'e spiral order. Moreover, we find that the wavelength characterizing moir\'e-scale modulations monotonically decreases with hole doping away from half-filling of the bands and depends only weakly on the magnetic field. Our results provide essential insights into the nature of MATTG correlated phases in the presence of strain and imply that superconductivity emerges from an inter-valley coherent parent state., Comment: the main text, extended data figures, and SI

Superconductivity (SC) is a ubiquitous feature of graphite allotropes, having been observed in Bernal bilayers[1], rhombohedral trilayers[2], and a wide variety of angle-misaligned multilayers[3-6]. Despite significant differences in the electronic structure across these systems, supporting the graphite layer on a WSe$_2$ substrate has been consistently observed to expand the range of SC in carrier density and temperature[7-10]. Here, we report the observation of two distinct superconducting states (denoted SC$_1$ and SC$_2$) in Bernal bilayer graphene with strong proximity-induced Ising spin-orbit coupling. Quantum oscillations show that while the normal state of SC$_1$ is consistent with the single-particle band structure, SC$_2$ emerges from a nematic normal state with broken rotational symmetry. Both superconductors are robust to in-plane magnetic fields, violating the paramagnetic limit; however, neither reach fields expected for spin-valley locked Ising superconductors. We use our knowledge of the Fermi surface geometry of SC$_1$ to argue that superconductivity is limited by orbital depairing arising from the imperfect layer polarization of the electron wavefunctions. Finally, a comparative analysis of transport and thermodynamic compressibility measurements in SC$_2$ shows that the proximity to the observed isospin phase boundaries, observed in other rhombohedral graphene allotropes, is likely coincidental, constraining theories of unconventional superconducting pairing mechanisms in theses systems.

Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy is challenging when samples are physically smaller than the diffraction limit of ~0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 GHz to 750 GHz. We measure the optical conductivity of a 7.5-$\mu$m wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.

Aims: The aim of this work is to develop a method to systematically detect and characterise fast-time variations ($\gtrsim 1$s) in the non-thermal hard X-ray (HXR) time profiles of solar flares using high-resolution data from Solar Orbiter's Spectrometer/Telescope for Imaging X-rays (STIX). Methods: The HXR time profiles were smoothed using Gaussian Process (GP) regression. The time profiles were then fitted with a linear combination of Gaussians to decompose the time profile. From the Gaussian decomposition, key characteristics such as the periodicity, full width at half maximum (FWHM), time evolution, and amplitude can be derived. Results: We present the outcome of applying this method to four M and X GOES-class flares from the first year of Solar Orbiter science operations. The HXR time profiles of these flares were decomposed into individual Gaussians and their periods were derived. The quality of fit is quantified by the standard deviation of the residuals (difference between observed and fitted curve, normalised by the error on the observed data), for which we obtain $\leq 1.8$ for all flares presented. In this work, the first detection of fast-time variations with Solar Orbiter's STIX instrument has been made on timescales across the range of 4-128s. Conclusions: A new method for identifying and characterising fast-time variations in the non-thermal HXR profiles of solar flares has been developed, in which the time profiles are fit with a linear combination of Gaussian bursts. The opportunity to study time variations in flares has greatly improved with the new observations from STIX on Solar Orbiter., Comment: 12 pages, 10 figures, A&A journal