arXiv subjects

Haichao Xu

Publications and source records attributed to Haichao Xu.

At least 19 recordsLinked to original sources

arXiv2026

A Disconnected Superconducting Regime at the Parent Limit of Infinite-Layer Nickelates

Infinite-layer nickelates have been widely viewed as cuprate analogs in which superconductivity emerges and forms a superconducting dome centered around 10-20% cation substitution. Here we show that pristine and stoichiometric PrNiO2, without cation substitution, exhibits intrinsic superconductivity characterized by zero resistance and diamagnetism in uncapped films. Through heterostructure engineering, we further exclude an interfacial origin of the superconductivity. Remarkably, zero-resistance superconductivity is consistently observed in trivalent-substituted PrNiO2, whereas it is rapidly suppressed by dilute divalent substitution. Combined with angle-resolved photoemission studies, these results indicate that such a new superconducting regime is confined to within 3% additional hole doping from pristine PrNiO2. Furthermore, this phase is separated from the previously established superconducting dome around ~ 20% divalent doping by a non-superconducting region in the phase diagram, and is further distinguished by a remarkably stronger upper-critical-field anisotropy. These findings establish a unique separated superconducting regime, suggesting that infinite-layer nickelates are not merely cuprate analogs but host distinct superconducting physics.

cond-mat.supr-con↗

arXiv2026

Marginal-Fermi-Liquid-like Behavior without Pseudogap in Infinite-Layer Nickelates

Pseudogap formation, strange-metal behavior and unconventional superconductivity are closely intertwined in hole-doped cuprates, yet their relationship remains unresolved. Infinite-layer nickelates offer a distinct 3d9-derived platform to address this question by combining a cuprate-like Ni dx2-y2 Fermi surface with multiband electronic degrees of freedom. Here we use angle-resolved photoemission spectroscopy to resolve the low-energy spectral function of superconducting La0.8Ca0.2NiO2 and parent LaNiO2 thin films. In La0.8Ca0.2NiO2, the electronic self-energy Im Sigma(omega) is approximately linear in energy and its slope increases from (pi/2, pi/2) to (pi, 0), revealing momentum-dependent marginal-Fermi-liquid-like scattering. Both films show a progressive suppression of low-energy spectral weight from the diagonal direction toward (pi, 0), with stronger suppression in parent LaNiO2. However, finite Fermi-level spectral weight persists around the entire Fermi surface, with no leading-edge shift or back-bending indicative of pseudogap formation in either the electron pocket or the cuprate-like hole band. Our results demonstrate that momentum-selective correlations and marginal-Fermi-liquid-like scattering can occur without a detectable cuprate-like pseudogap, providing a benchmark for identifying the essential normal-state electronic ingredients of high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2026

CoFEND: A Cross-Modal Fusion End-to-End Network for Cold-Start Drug-Drug Interaction Prediction

Cold-start drug-drug interaction (DDI) prediction for new drugs is critical for minimizing unexpected adverse drug reactions. The key challenge is to capture similarity between new and known drugs. However, such similarity is closely associated with complex relationships and mechanisms among drugs, enzymes, transporters, molecular structures, and other biomedical entities. Existing methods have three limitations in capturing such similarity: (1) only partial relationships and mechanisms are considered, which overlooks cross-modal information and yields incomplete or biased similarity modeling; (2) similarity computation between new and known drugs is conducted separately across modalities and performed offline for cold-start DDI prediction, leading to misalignment between similarity computation and DDI prediction; and (3) existing interpretability analyses are typically single-modality and focus primarily on key determinants of the perpetrator drug, while the underlying causes of susceptibility for the victim drug are seldom investigated. To address these issues, this paper proposes a novel Cross-Modal-Fused End-to-End Learning Network (CMF-ELN) with three components. First, diverse multimodal information is leveraged to construct four types of drug-centered knowledge graphs, enabling comprehensive similarity modeling under reconstruction-based supervision. Second, a four-channel graph autoencoder is designed to fuse cross-modal similarity within an end-to-end learning framework. Finally, a two-stage interpretability scheme is devised to precisely localize key factors for both perpetrator and victim drugs. Extensive experiments on two real datasets demonstrate that CMF-ELN achieves significantly higher prediction accuracy and more comprehensive interpretability of mechanisms than its peers.

cs.LG↗

arXiv2026

Photon Escape from Slab Thomson Media: A Scattering-order-resolved Recursive Formalism for Comptonization Applications

The scattering history of photons in slab media plays an important role in modelling Comptonized spectra and disc-corona radiative feedback. We develop a recursive formalism that evolves the post-scattering depth--direction distribution in slab Thomson media and yields boundary- and angle-resolved escape probabilities at each scattering order. For azimuth-integrated problems, the angular dependence closes within a two-component basis, reducing the transport problem to an efficient depth-kernel recursion. We apply the method to normally incident beam injection, Lambert-law boundary injection, and a vertically uniform isotropic internal source, and verify the results with Monte Carlo radiative-transfer simulations. The resulting distributions provide a photon-number-conserving route to semi-analytic Comptonized spectra and estimates of the Compton amplification factor and the fraction of downwardly scattered luminosity. We also derive the mean scattering number within this framework, obtaining the exact result $\langle N\rangle=2\tau$ for Lambert-law injection, while the uniform internal source changes from an optically thin $\tau\ln(1/\tau)$ behaviour to an optically thick $\tau^2/4$ scaling. At high scattering orders, the recursion is controlled by a dominant eigenmode: $P_n/P_{n-1}\rightarrow\lambda(\tau)$, where $\lambda(\tau)$ is the spectral radius of the slab recursion operator. This eigenmode also determines a limiting normalized angular distribution, so that viewing angle and escape boundary primarily affect the normalization of the high-order X-ray component, while spectral-shape differences are mainly confined to the unscattered and low-order components. These eigenvalue and eigenfunction results provide transport ingredients for future energy-dependent slab Comptonization models.

astro-ph.HE↗

arXiv2026

Electronic Origin of Ferromagnetic Excitations in the Candidate Spin-Triplet Superconductor CeSb2

The origin of quasi-one-dimensional (q1D) ferromagnetic (FM) excitations in the candidate spin-triplet superconductor CeSb$_2$ has remained unclear. Here we report an electronic mechanism for emergent q1D magnetism in the quasi-two-dimensional lattice of CeSb$_2$, revealed by angle-resolved photoemission spectroscopy (ARPES). High-resolution ARPES resolves no spin-density-wave gap on the dispersive Fermi pockets, disfavoring a nesting-driven mechanism for the q1D FM excitations. Instead, resonant ARPES reveals a pronounced selective enhancement of Ce 4$f$ spectral weight on the $C_2$-distributed Fermi pockets aligned with the Ce ladder. This observation signifies band-selective Kondo coupling that generates strongly anisotropic magnetic exchange interactions, which can naturally account for both the q1D ferromagnetic excitations and the competing magnetic orders. Our results identify a band-selective Kondo coupling mechanism for emergent low-dimensional magnetism in correlated $f$-electron systems.

cond-mat.str-el↗

arXiv2026

A narrow Comptonization locus in Seyfert slab coronae: implications for dominant coronal dissipation and reduced feedback

The thermal state of active galactic nucleus (AGN) coronae is commonly described by the electron temperature $kT_{\rm e}$, the Thomson optical depth $\tau_{\rm T}$, and the geometry of the Comptonizing medium. We compile a literature sample of radio-quiet Seyfert galaxies with direct slab-geometry thermal Comptonization measurements of $kT_{\rm e}$ and $\tau_{\rm T}$. For bottom-illuminated slab coronae, we adopt a geometry-motivated effective Comptonization parameter, $y=4\theta\tau_{\rm T}\,{K_3(1/\theta)} / {K_2(1/\theta)}$, where $\theta=kT_{\rm e}/m_{\rm e}c^2$ and $K_n$ is the modified Bessel function of the second kind. We find that the compiled slab-corona measurements are distributed along a narrow anti-correlated ridge in the $kT_{\rm e}-\tau_{\rm T}$ plane. For the cleaned primary \texttt{compTT} sample, this ridge corresponds to $\langle y\rangle=0.770$ with a logarithmic dispersion of only 0.086 dex. We then compare the observed ridge with reduced-feedback slab-equilibrium calculations and find that the locus favours a corona-dominated local energy budget, $f\simeq1$, together with a feedback factor well below the full-covering value, $g\ll1$. We interpret the constant-$y$ locus as implications for dominant local coronal dissipation and reduced disc-corona radiative feedback, consistent with a patchy corona. Supplementary pair-balance calculations indicate that the low-temperature, high-optical-depth part of the sample is unlikely to be sustained by a purely thermal pair plasma alone, suggesting that the fitted optical depths are more plausibly dominated by electron--ion plasma.

astro-ph.HE↗

arXiv2026

Unifying the X-ray coronae and ultra-fast outflows: a PBI-enhanced outflow-based corona model for the inner accretion disc

The fact that luminous X-ray coronae and Ultra-Fast Outflows (UFOs) are both inferred to originate from the innermost regions of active galactic nuclei (AGNs) suggests a deep physical connection between them. However, standard magnetic buoyancy models struggle to transport sufficient energy through the radiation-pressure-dominated inner disc to sustain both the phenomena, creating a theoretical energy deficit. In this work, we propose an outflow-based model with energy transport enhanced by the Photon Bubble Instability (PBI) in the inner region. By coupling this enhanced energy supply with the MHD turbulence-driven mass-loading mechanism appropriate for weakly magnetized standard discs, we solve the dynamical and thermodynamic structure of the corona. We find that the model can successfully launch high speed winds matching observed UFO kinematics provided the mechanical acceleration efficiency is high ($f_{\rm acc}\gtrsim 0.5$). Furthermore, the model naturally reproduces the observed spectral evolution found in AGN coronae: as the accretion rate increases, the corona becomes optically thicker and cooler and produces a softer spectrum. Our results support an extended slab-like coronal geometry and suggest that UFOs and X-ray coronae in the inner discs are manifestations of the same magnetic activity.

astro-ph.HE↗

arXiv2025

Evolution of accretion disc-corona in the TDE Candidate AT 2019avd

X-ray observations of the tidal disruption event (TDE) candidate AT 2019avd show drastic variabilities in flux and spectral shape over hundreds of days, providing clues on the accretion disc-corona evolution. We utilize a disc-corona model, in which a fraction of the gravitational energy released in the disc is transported into the hot corona above/below. Some soft photons emitted from the disc are upscattered to X-ray photons by the hot electrons in the optically thin corona. By fitting the NICER observations of AT 2019avd during epochs when the spectra exhibit significant hardening, we derive the evolution of the mass accretion rate, $\dot{m}$, and the coronal energy fraction, $f$. Our results show that $f$ decreases with increasing $\dot{m}$, which is qualitatively consistent with that observed in active galactic nuclei (AGNs), while the slope of this source, $f\propto \dot{m}^{-0.30}$, is much shallower than that of AGNs. We also find that the non-thermal X-ray spectrum in this source is significantly softer than those typically seen in AGNs and black-hole X-ray binaries. We argue that these quantitative differences can be a powerful diagnostic of the underlying magnetic turbulence, which may imply a stronger magnetic field within the TDE accretion disc than that in typical AGNs. It is also found that the evolution of the fitted neutral hydrogen column density follows a similar pattern to that of the accretion rate evolution, which may reflect the accumulation of absorbing material originating from the inflowing streams of stellar debris and/or other related sources.

astro-ph.HE↗

arXiv2025

Correlation-driven 3d Heavy Fermion behavior in LiV2O4

LiV2O4 is a spinel-structured compound that stands out as the first known 3d-electron system exhibiting typical heavy fermion behavior. A central question is how such strong mass renormalization emerges in the absence of f-electrons. In this work, we investigate the three-dimensional electronic structure of LiV2O4 thin films using angle-resolved photoemission spectroscopy (ARPES). We identify that an electron-like flat band is derived from a1g orbitals, along with a highly dispersive e'g band strongly coupled with phonons. The overall agreement with dynamical mean-field theory (DMFT) calculations highlights the essential role of inter-orbital Hund's coupling in reducing the a1g bandwidth to 25 meV, approaching a Mott state. Notably, we find that heavy-fermion behavior arises from additional renormalization at the a1g band near the Fermi level, likely driven by many-body interactions at energy scales down to a few meV and potentially linked to geometric frustration inherent to the spinel lattice. These results provide crucial insights into the origin of the heavy fermion behavior in 3d-electron systems.

cond-mat.str-el↗

arXiv2025

Thermoelectricity evidence for quantum criticality in clean infinite-layer nickelate films

We investigate the Seebeck coefficient ($S$) in infinite-layer nickelate films with different disorder levels. The disordered NdNiO$_{2}$ film exhibits a flat $S/T$ curve, whereas cleaner samples display a logarithmic divergence with decreasing temperature, followed by a pronounced ``hump'' near 25 K. These distinct behaviors reveal a disorder-driven transition from band-structure-dominated transport to quantum-critical-dominated transport. Below the ``hump'' temperature, four-fold symmetry breaking is observed in the in-plane angular magnetoresistance, indicating the presence of short-range antiferromagnetic order in parent infinite-layer nickelate films. Furthermore, the logarithmic divergence in $S/T$ is also observed in a clean superconducting Sm$_{0.73}$Ca$_{0.05}$Eu$_{0.22}$NiO$_{2}$ film, where it coexists with linear-in-temperature resistivity over the same temperature range. These findings demonstrate the existence of quantum criticality over a wide doping range in clean infinite-layer nickelate films, similar to cuprates, which highlights the central role of antiferromagnetic spin correlations in their superconducting pairing mechanisms.

cond-mat.supr-con↗

arXiv2025

Observation of Electride-like $s$ States Coexisting with Correlated $d$ Electrons in NdNiO$_2$

Despite exhibiting a similar $d_{x^2-y^2}$ band character to cuprates, infinite-layer nickelates host additional electron pockets that distinguish them from single-band cuprates. The elusive orbital origin of these electron pockets has led to competing theoretical scenarios. Here, using polarization-dependent and resonant angle-resolved photoemission spectroscopy (ARPES), we determine the orbital character of the Fermi surfaces in NdNiO$_2$. Our data reveal that the electron-like pocket arises predominantly from interstitial $s$ states, with negligible contributions from rare-earth 5$d$ and 4$f$ orbitals near the Fermi level. The observation of well-defined quantum well states indicates a uniform distribution of these interstitial electrons throughout the film thickness. By comparing with electronic structure of LaNiO$_2$, we find that the rare-earth element modulates the Ni-derived bands and hopping integrals through a chemical pressure effect. These findings clarify the role of rare-earth elements in shaping the low-energy electronic structure and establish the presence of electride-like interstitial $s$ states in a correlated oxide system, where electrons occupy lattice voids rather than atomic orbitals. The electride-like character offer new insight into the self-doping and superconductivity in infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2025

High Temperature Superconductivity Dominated by Inner Underdoped CuO$_2$ Planes in Quadruple-Layer Cuprate (Cu,C)Ba$_2$Ca$_3$Cu$_4$O$_{11+\delta}$

The superconducting transition temperature ($T_{\mathrm{c}}$) of trilayer or quadruple-layer cuprates typically surpasses that of single-layer or bilayer systems. This observation is often interpreted within the ``composite picture", where strong proximity effect between inner CuO$_2$ planes (IPs) and outer CuO$_2$ planes (OPs) is crucial. Albeit intriguing, a straightforward scrutinization of this composite picture is still lacking. In this study, using angle-resolved photoemission spectroscopy to investigate (Cu,C)Ba$_2$Ca$_3$Cu$_4$O$_{11+\delta}$ (CuC-1234) with a high $T_{\mathrm{c}}$ of 110~K, we found that the OPs are not superconducting at the $T_{\mathrm{c}}$ of the material. Instead, the large pairing strength and phase coherence concurrently emerge at the underdoped IPs, suggesting that the high $T_{\mathrm{c}}$ is primarily driven by these underdoped IPs. Given that the $T_{\mathrm{c}}$ of CuC-1234 is comparable to other trilayer or quadruple-layer cuprates, our findings suggest that the conventional ``composite picture" is not universally required for achieving high $T_{\mathrm{c}}$. More importantly, we demonstrate that CuO$_2$ planes free of apical oxygen can support superconductivity up to 110~K even at a doping level of 0.07 holes per Cu, a level that lies deep in the underdoped regime of single- and bilayer cuprates. These findings provide new insights into the origin of high $T_{\mathrm{c}}$ in multilayer cuprates.

cond-mat.supr-con↗

arXiv2024

Interface suppressed nematicity and enhanced superconductivity of FeSe/NdFeO3 in the low doping regime

The discovery of interface-enhanced superconductivity in single-layer FeSe/oxides has generated intensive research interests. Beyond the family of FeSe interfaced with various TiO$_2$ terminated oxides, high pairing temperature up to 80~K has been recently observed in FeSe interfaced with FeO$_x$-terminated LaFeO$_3$. Here we successfully extend the FeSe/FeO$_x$ superconducting interface to FeSe/NdFeO$_3$, by constructing 1uc-FeSe/6uc-NdFeO$_3$/Nb:SrTiO$_3$ heterostructures. Intriguingly, well-annealed FeSe/NdFeO$_3$ exhibits a low doping level of 0.038$\sim$0.046 ~e$^-/$Fe which deviates universally magic doping level (0.10$\sim$0.12 e$^-/\rm{Fe}$) and provides a new playground for studying the FeSe/oxide interface in the low electron-doped regime. Comparing it with thick FeSe films at the comparable electron doping level induced by surface potassium dosing, FeSe/NdFeO$_3$ shows a larger superconducting gap and the absence of a nematic gap, indicating an enhancement of the superconductivity and suppression of nematicity by the FeSe/FeO$_x$ interface. These results not only expand the FeSe/FeO$_x$ superconducting family but also enrich the current understanding on the roles of the oxide interface.

cond-mat.supr-con↗

arXiv2024

Inferior interfacial superconductivity in 1 UC FeSe/SrVO$_3$/SrTiO$_3$ with screened interfacial electron-phonon coupling

Monolayer FeSe/TiO$_x$ and FeSe/FeO$_x$ interfaces exhibit significant superconductivity enhancement compared to bulk FeSe, with interfacial electron-phonon coupling (EPC) playing a crucial role. However, the reduced dimensionality in monolayer FeSe, which may drive superconducting fluctuations, complicates the understanding of the enhancement mechanisms. Here we construct a new superconducting interface: monolayer FeSe/SrVO$_3$/SrTiO$_3$, in which the itinerant electrons of highly metallic SrVO$_3$ films can screen all the high-energy Fuchs-Kliewer phonons, including those of SrTiO$_3$, making it the first FeSe/oxide system with screened interfacial EPC while maintaining the monolayer FeSe thickness. Despite comparable doping levels, the heavily electron-doped monolayer FeSe/SrVO$_3$ exhibits a lower pairing temperature ($T_\mathrm{g}$ $\sim$ 48 K) than FeSe/SrTiO$_3$ and FeSe/LaFeO$_3$. Our findings disentangle the contributions of interfacial EPC from dimensionality on enhancing $T_\mathrm{g}$ in FeSe/oxide interfaces, underscoring the importance of interfacial EPC in $T_\mathrm{g}$ enhancement. This FeSe/VO$_x$ interface also provides a platform for studying the interfacial superconductivity.

cond-mat.supr-con↗

arXiv2024

Adaptive Frequency Bin Interval in FFT via Dense Sampling Factor $α$

The Fast Fourier Transform (FFT) is a fundamental tool for signal analysis, widely used across various fields. However, traditional FFT methods encounter challenges in adjusting the frequency bin interval, which may impede accurate spectral analysis. In this study, we propose a method for adjusting the frequency bin interval in FFT by introducing a parameter $α$. We elucidate the underlying principles of the proposed method and discuss its potential applications across various contexts. Our findings suggest that the proposed method offers a promising approach to overcome the limitations of traditional FFT methods and enhance spectral analysis accuracy.

cs.DS↗

arXiv2023

A new type of window functions constructed with exponential function

The Discrete Fourier Transform (DFT) is widely utilized for signal analysis but is plagued by spectral leakage, leading to inaccuracies in signal approximation. Window functions play a crucial role in mitigating spectral leakage by providing weighting mechanisms for discrete signals. In this paper, we introduce a novel window type based on exponential function, allowing for adjustable parameters and diverse variations. We present the formulation, properties, and motivation behind the design of the new window functions. Additionally, we analyze their behavior and evaluate their performance by comparing them with mainstream window functions using six parameters. Our findings demonstrate that these new window functions exhibit outstanding flexibility and versatility in signal analysis.

eess.SP↗

arXiv2023

Orientation-dependent electron-phonon coupling in interfacial superconductors LaAlO3/KTaO3

The emergent superconductivity at the LaAlO3/KTaO3 interfaces exhibits a mysterious dependence on the KTaO3 crystallographic orientations. Here we show, by soft X-ray angle-resolved photoemission spectroscopy, that the interfacial superconductivity is contributed by mobile electrons with unexpected quasi-three-dimensional character, beyond the "two-dimensional electron gas" scenario in describing oxide interfaces. At differently-oriented interfaces, the quasi-three-dimensional electron gas ubiquitously exists and spatially overlaps with the small q Fuchs-Kliewer surface phonons. Intriguingly, electrons and the Fuchs-Kliewer phonons couple with different strengths depending on the interfacial orientations, and the stronger coupling correlates with the higher superconducting transition temperature. Our results provide a natural explanation for the orientation-dependent superconductivity, and the first evidence that interfacial orientations can affect electron-phonon coupling strength over several nanometers, which may have profound implications for the applications of oxide interfaces in general.

cond-mat.supr-con↗

arXiv2021

Electronic structure and signature of Tomonaga-Luttinger liquid state in epitaxial CoSb$_{1-x}$ nanoribbons

Recently, monolayer CoSb/SrTiO$_3$ has been proposed as a candidate harboring interfacial superconductivity in analogy with monolayer FeSe/SrTiO$_3$. Experimentally, while the CoSb-based compounds manifesting as nanowires and thin films have been realized on SrTiO$_3$ substrates, serving as a rich playground, their electronic structures are still unknown and yet to be resolved. Here, we have fabricated CoSb$_{1-x}$ nanoribbons with quasi-one-dimensional stripes on SrTiO$_3$(001) substrates using molecular beam epitaxy, and investigated the electronic structure by in situ angle-resolved photoemission spectroscopy. Straight Fermi surfaces without lateral dispersions are observed. CoSb$_{1-x}$/SrTiO$_3$ is slightly hole doped, where the interfacial charge transfer is opposite to that in monolayer FeSe/SrTiO$_3$. The spectral weight near Fermi level exhibits power-law-like suppression and obeys a universal temperature scaling, serving as the signature of Tomonaga-Luttinger liquid (TLL) state. The obtained TLL parameter of $\sim$0.21 shows the underlying strong correlations. Our results not only suggest CoSb$_{1-x}$ nanoribbon as a representative TLL system, but also provide clues for further investigations on the CoSb-related interface.

cond-mat.str-el↗