arXiv subjects

Dao-Xin Yao

Publications and source records attributed to Dao-Xin Yao.

At least 19 recordsLinked to original sources

arXiv2026

Stacking-Controlled Altermagnetism and Topological Magnons in Bilayer CrI$_3$

Stacked van der Waals magnets provide a tunable route to altermagnetism, a compensated magnetic order characterized by momentum-dependent spin splitting, and to the topological magnetic excitations that such order can host. In bilayer CrI$_3$, first-principles calculations and linear spin-wave theory reveal stacking-controlled altermagnetic order and associated magnon band topology. Combining band-representation analysis with calculations of the dynamic structure factor relevant to inelastic neutron scattering, we further characterize the chirally split topological magnons and quantify their energy corrections and lifetimes using a many-body Green's-function approach. The interlayer magnetic ground state is highly sensitive to the stacking geometry, thereby controlling the magnon band topology and transport responses. We further show that magnon--magnon interactions renormalize the magnon dispersion and dynamic structure factor, with a particular focus on magnon decay. Using van der Waals bilayer CrI$_3$ as a representative platform, our results establish stacking engineering as a structural route for tuning altermagnetism and associated topological magnon excitations, opening avenues toward stacking-controlled spintronic and magnonic devices.

cond-mat.mtrl-sci↗

arXiv2026

Electronic structure and two-orbital model of the quadlayer La$_5$Ni$_4$O$_{13}$

The discovery of pressure-induced superconductivity in Ruddlesden--Popper (RP) nickelates has stimulated extensive interest in high-T$_c$ superconductors. Here, we systematically study the electronic properties of the quadlayer RP nickelate La$_5$Ni$_4$O$_{13}$ under ambient pressure, 5% isotropic compressive strain, and 4% $c$-axis uniaxial strain using density functional theory (DFT) and random phase approximation (RPA) calculations. DFT calculations show that isotropic strain broadens the Ni-$e_g$ bands and induces charge transfer from O-$p$ to Ni-$d$ orbitals, whereas $c$-axis uniaxial strain selectively shifts the $d_{z^2}$-derived bonding1 band upward while leaving the $d_{x^2-y^2}$ dispersion nearly unchanged. From Wannier downfolding, we construct a quadlayer two-orbital model that reproduces the low-energy Ni-$e_g$ bands. Our model reveals that under ambient pressure and 5% isotropic strain, the Fermi surface consists of two electron pockets ($α$ and $δ$) and three hole pockets ($β$, $β^{\prime}$, and $β^{\prime \prime}$), while under uniaxial strain, a $γ$ hole pocket with $d_{z^2}$ orbital character emerges. RPA calculations reveal that the leading spin response shifts from $\mathbf{q}\approx(2π/3,2π/3)$ at ambient pressure to $\mathbf{q}\approx(π,π)$ under both strain conditions and is enhanced under $c$-axis compression. These results suggest that $c$-axis compression may provide a favorable route to superconductivity in the quadlayer nickelate analogous to that in bilayer and trilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Identifying the structure of La3Ni2O7 in the pressurized superconducting state

The crystal structure of La3Ni2O7 in its high-pressure superconducting state has been the subject of intense debate, with conflicting reports proposing orthorhombic (Amam or Fmmm) and tetragonal (I4/mmm) symmetries. Here, using high-pressure Raman spectroscopy down to 3 K, we resolve this controversy by tracking the structural evolution of La3Ni2O7 up to 32.7 GPa. Leveraging rigorous symmetry-based selection rules, we identify a single structural transition from the orthorhombic Amam phase to the Fmmm phase at ~14.5 GPa, signaled by a profound phonon renormalization. Crucially, the persistence of D2h symmetry across the transition rules out the tetragonal I4/mmm phase in the superconducting state in our measurements. The emergence of bulk superconductivity coincides precisely with this transition. Our results establish the orthorhombic Fmmm structure as the intrinsic host of superconductivity in La3Ni2O7 below 19.45 GPa, resolving a central structural controversy and providing a critical foundation for understanding the superconducting mechanism in bilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Heavily Sr-Doped La$_{2}$SrNi$_{2}$O$_{7-δ}$ as a Tetragonal Ruddlesden-Popper Phase at Ambient Pressure

High-temperature superconductivity has been found in bilayer Ruddlesden-Popper (RP) nickelates in bulk samples under high pressure, or in thin films via compressive strain. In the superconducting state, a tetragonal structure with a straight Ni-O-Ni bond along c-axis has been commonly observed, together with the suppression or diminishing of the density-wave orders. Therefore, it remains an open question whether these factors are sufficient for achieving superconductivity at ambient pressure. Here we report the first successful synthesis of heavily Sr-doped La$_{2}$SrNi$_{2}$O$_{7-δ}$ under high-pressure and high-temperature (HPHT) conditions with a flux method. X-ray diffraction and scanning transmission electron microscopy (STEM) confirm that the material adopts a tetragonal (I4/mmm) structure with an 180$^{\circ}$ Ni-O-Ni bond angle along c-axis. Resistance measurements reveal metallic behavior with a low-temperature upturn and no density-wave features are observed. However, neither pressure nor oxygen variation induces superconductivity. Density functional theory calculations indicate that the holes introduced by Sr doping are predominantly doped into the Ni-3d$_{z^2}$ orbital, leading to the appearance of a very large $γ$ pocket on the Fermi surface at ambient pressure and significantly reducing the occupation of the Ni-3d$_{z^2 }$ orbital. Combining the experimental observations with theoretical calculations, we attribute the absence of superconductivity to the serious deviation from the half-filling state of the Ni-3d$_{z^2 }$ band, which is crucial for the interlayer antiferromagnetic interaction and thus for pairing. Our work unravels important issues for achieving superconductivity in bilayer nickelate system.

cond-mat.supr-con↗

arXiv2026

Scaling of the disorder operator at (3+1)D O(3) quantum criticality

The disorder operator, as an easily measured nonlocal observable, displays great potential in detecting intrinsic information of field theories. It has been systematically studied in one- and two-dimensional (1D and 2D) quantum systems, while the knowledge of 3D is still limited. The disorder operator associated with U(1) global symmetry exhibits rich geometric dependence on the shape of the spatial region at a quantum critical point, meanwhile, (3+1)D is the upper critical dimension for O(N) criticality, both of which pose a challenge for exploring the disorder operator in high dimensions. In this Letter, we investigate the scaling behaviors of disorder operators in (3+1)D O(3) models through large-scale quantum Monte Carlo simulation combined with theoretical analysis. Although the upper critical dimension introduces logarithmic corrections in correlations, we analytically prove and numerically demonstrate that the corrections do not modify the universal trihedral-corner contribution of the disorder operator. The universal contributions, such as the current central charge, have been revealed in our calculation, which establishes a concrete link between lattice simulations and continuum field theory. This work opens promising directions for the experimental and numerical exploration of universal properties at quantum critical points in (3+1)D models.

cond-mat.str-el↗

arXiv2026

Pairing symmetry and superconductivity in La$_3$Ni$_2$O$_7$ thin films

The recent discovery of superconductivity with a transition temperature $T_c$ over 40 K in La$_3$Ni$_2$O$_7$ and (La,Pr)$_{3}$Ni$_2$O$_7$ thin films at ambient pressure marks an important step in the field of nickelate superconductors. Here, we perform a renormalized mean-field theory study of the superconductivity in $\mathrm{La_3Ni_2O_7}$ thin films, using a bilayer two-orbital $t-J$ model. Our result reveals an $s_\pm$-wave pairing symmetry driven by the strong interlayer superexchange coupling of $d_{z^2}$ orbital, resembling the pressurized bulk case. Also, we roughly reproduce the experimentally observed nodeless shape of the superconducting gap at the $β$ pocket and the superconducting $T_c$. In addition, by analysing the orbital-resolved pairing configurations and their projections onto Fermi surface, we find that the nodeless feature of $β$ pocket is related to the interlayer pairing within both $d_{z^2}$ and $d_{x^2-y^2}$ orbitals. Moreover, we identify a formation of the inplane inter-orbital $d$-wave pairing between $d_{z^2}$ and $d_{x^2-y^2}$ orbitals, which can even enhance the dominated interlayer $s_\pm$-wave. Our study particularly highlights the diverse relations of different pairing channels in $\mathrm{La_3Ni_2O_7}$ that holds a complex Fermi surface.

cond-mat.supr-con↗

arXiv2026

Chiral Magnons: Mechanisms and Research Progress

Chiral magnons are distinctive collective spin excitations in magnetic ordered systems, whose dispersion relations break momentum-inversion symmetry, $ω(\boldsymbol{k}) \neq ω(-\boldsymbol{k})$, resulting in essential non-reciprocal spin-wave propagation. This built-in directionality provides new opportunities for spin information transfer, thermal-spin interconversion, and low-dissipation non-reciprocal microwave devices, which complement but differ from topological magnonics. In recent years, the proposal and rapid development of altermagnetism have broadened the physical origin and research framework of chiral magnons, making them a research frontier in condensed matter physics. This review presents a unified framework for chiral magnons, covering symmetry-breaking mechanisms, material implementation, experimental characterization, transport response, and many-body non-Hermitian dynamics, and evaluates routes toward room-temperature and device-related platforms. The discussion is based on symmetry analysis, model Hamiltonians, and spin-wave theory, combined with first-principles calculations as well as recent spectroscopic (e.g., inelastic and polarized neutron scattering, Brillouin light scattering) and transport measurements. This review further summarizes bulk-gap and Berry-curvature induced chiral magnon edge states, the enhancement of non-reciprocity via chiral spin pumping and cavity-magnon hybrids, as well as non-Hermitian features arising from multiparticle damping and gain-loss competition. This review provides a comprehensive reference for elucidating the underlying mechanisms of chiral magnons, advancing the synthesis and experimental characterization of novel materials, and also guiding the design of next-generation non-reciprocal magnonic devices.

cond-mat.str-el↗

arXiv2026

Pairing mechanism and superconductivity in 1313 phase La$_3$Ni$_2$O$_7$

Recently, the observation of superconductivity (SC) with $T_c$ $\approx$ 3.6 K in the pressurized 1313 La$_3$Ni$_2$O$_7$ has attracted considerable interest. Here, we systematically investigate the electronic properties and superconducting mechanism of 1313 La$_3$Ni$_2$O$_7$ using density functional theory plus dynamical mean-field theory (DFT+DMFT) and random phase approximation (RPA). Our DFT+DMFT calculations reveal that the single-layer (SL) subsystem exhibits nearly insulating behavior, with the $d_{z^2}$ orbital showing Mott physics, while the trilayer (TL) subsystem remains metallic. This indicates that SC primarily resides in the TL subsystem, whose Ni-$e_g$ orbitals are found to be hole-doped relative to bulk La$_4$Ni$_3$O$_{10}$. Based on DFT+DMFT-derived low-energy Hamiltonian, RPA-based analysis yields an $s^{\pm}$-wave pairing symmetry within the TL subsystem. Importantly, we identify two key factors that contribute to the significant suppression of $T_c$ in 1313 La$_3$Ni$_2$O$_7$ compared to bulk La$_4$Ni$_3$O$_{10}$. First, the hole doping in the TL subsystem, as established by DMFT, leads to a decreased pairing strength, as confirmed by RPA calculations -- a trend resembling that in bulk La$_4$Ni$_3$O$_{10}$. Second, the SL subsystem acts as a bridge connecting adjacent superconducting TL subsystems, thereby forming an S-N-S Josephson junction. The resulting interlayer Josephson coupling governs the phase coherence between TL subsystems and further suppresses the global $T_c$. Combinedly, our findings suggest that the high-$T_c$ phase in the RP La$_3$Ni$_2$O$_7$ family should be attributed to the 2222 La$_3$Ni$_2$O$_7$ rather than the 1313 La$_3$Ni$_2$O$_7$.

cond-mat.supr-con↗

arXiv2026

Logarithmic corrections to bulk and surface criticality in a three-dimensional quantum Heisenberg antiferromagnet

At the bulk upper critical dimension, marginally irrelevant interactions generate multiplicative logarithmic corrections to mean-field scaling. While these corrections are well understood for bulk observables, their consequences for boundary criticality, particularly for finite-size scaling, remain much less explored. Here we combine large-scale quantum Monte Carlo simulations with boundary renormalization-group analysis to study a (3 + 1)D O(3) quantum critical point. After verifying the known logarithmically modified bulk finite-size scaling, including the correlation-length scaling governed by the logarithmic finite-size exponent \hat{\coppa}, we tune the surface coupling to identify ordinary, special, and extraordinary boundary regimes. For the ordinary and special transitions, we derive logarithmic correction exponents and \hat{\coppa}-dependent finite-size scaling forms for boundary correlations, including results that have not been systematically established before. These predictions are quantitatively supported by Monte Carlo data. In the extraordinary regime, we find long-range surface magnetic order and a logarithmically enhanced surface-bulk correlation.

cond-mat.str-el↗

arXiv2026

Spiral Phase and Phase Diagram of the $S$=1/2 XXZ Model on the Shastry-Sutherland Lattice

We investigate the ground-state phase diagram of the $S$=1/2 XXZ model on the two-dimensional Shastry-Sutherland lattice using exact diagonalization (ED), density-matrix renormalization group (DMRG), and cluster mean-field theory (CMFT) with DMRG as a solver. In the isotropic case ($Δ=1$), CMFT results reveal an intermediate empty plaquette (EP) phase that has a lower energy than the full plaquette (FP) phase. However, due to mean-field artifacts, CMFT alone is not suitable for accurately determining phase boundaries. Therefore, we combined three methods to map out the reliable phase diagram. Our calculations show that the EP phase narrows as $Δ$ deviates from unity and eventually vanishes. More importantly, we identify a spiral phase at small $Δ$, which has not been reported in previous studies. This phase is clearly captured by DMRG simulations on long cylindrical geometries. The competition between the EP, spiral, and $xy$-AFM phases near their boundaries provides a plausible explanation for the emergent spin-liquid-like behavior in RE$_2$Be$_2$GeO$_2$, while shedding new light on the role of XXZ anisotropy in the Shastry-Sutherland XXZ model.

cond-mat.str-el↗

arXiv2026

Doping evolution of spin excitations in La$_{3-x}$Sr$_{x}$Ni$_2$O$_7$/SrLaAlO$_4$ superconducting thin films

Ambient-pressure superconductivity in compressively strained bilayer nickelate films provides a unique platform to test pairing scenarios, yet the evolution of magnetism with carrier doping remains largely unexplored. Here, we utilize Ni $L_3$-edge resonant inelastic x-ray scattering to systematically track the evolution of spin and electronic excitations in coherently strained La$_{3-x}$Sr$_x$Ni$_2$O$_7$/SrLaAlO$_4$ thin films, spanning the superconducting ($x \le 0.21$) and overdoped non-superconducting ($x = 0.38$) regimes. We reveal that dispersive spin excitations, characterized by double-stripe correlations and nearly doping-independent exchange scales, persist robustly throughout the entire superconducting dome. In stark contrast, upon entering the overdoped non-superconducting state, this coherent magnetic framework undergoes an abrupt collapse, melting into a heavily damped, low-spectral-weight continuum. We show that this magnetic breakdown is fundamentally driven by a selective doping-induced orbital reconstruction. While the invariant $\sim\!1.0$~eV intra-atomic $dd$ peak confirms an intact local octahedral crystal field, the concurrent quenching of the $\sim\!0.4$~eV and $\sim\!1.6$~eV features signifies a severe degradation of the apical-oxygen-mediated $d_{z^2}$--$p_z$--$d_{z^2}$ singlet sector and bilayer charge-transfer coherence. The synchronized demise of coherent spin excitations and macroscopic pairing establishes a direct, doping-controlled link, underscoring that maintaining the localized $d_{z^2}$ magnetic framework and robust apical-oxygen coupling is the fundamental prerequisite for high-$T_c$ superconductivity in bilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Progress of ambient-pressure superconductivity in bilayer nickelate thin films

This review summarizes recent progress of ambient-pressure superconductivity in bilayer nickelate La$_3$Ni$_2$O$_7$ thin films, a major advancement following the discovery of high-pressure superconductivity in bulk La$_3$Ni$_2$O$_7$. First, we explain how epitaxial strain engineering enables ambient-pressure superconductivity in La$_3$Ni$_2$O$_7$ thin films, with compressive strain from substrates like SrLaAlO$_4$ stabilizing superconductivity. Next, we review experimental characterizations of related systems, with particular emphasis on ARPES measurements that have shown conflicting Fermi surface topologies. We then discuss progress in increasing the superconducting transition temperature $T_c$. Finally, we summarize theoretical studies of the electronic structure and pairing symmetry of La$_3$Ni$_2$O$_7$ thin films. Together, these advances establish bilayer nickelate thin films as a highly tunable and promising platform for exploring high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2026

Electronic Nematicity Revealed by Polarized Ultrafast Spectroscopy in Bilayer La$_3$Ni$_2$O$_7$

We report a polarized ultrafast pump-probe study of the normal-state electronic dynamics in bilayer La$_3$Ni$_2$O$_7$ and trilayer La$_4$Ni$_3$O$_{10}$ single crystals at ambient pressure. While both nickelates exhibit density-wave (DW) transitions accompanied by the opening of a quasiparticle relaxation bottleneck, their electronic responses display strikingly different symmetry properties. La4Ni3O10 maintains an isotropic optical response across the entire temperature range. In contrast, La$_3$Ni$_2$O$_7$ exhibits a pronounced twofold ($C_2$) anisotropy in its lowtemperature electronic dynamics. This electronic nematicity, evident in both the relaxation dynamics and the polarization-dependent effective bottleneck energy scales, is strongly modified below 115 K, suggesting coupling or competition with a secondary DW-like instability reported by complementary probes. The presence of macroscopic electronic anisotropy in the bilayer system, and its absence in the trilayer system, suggests a possible relation between electronic nematic correlations and the superconducting normal state in a$_3$Ni$_2$O$_7$ that deserves further exploration.

cond-mat.str-el↗

arXiv2026

Corner Charge Fluctuations in Higher Dimensions

Measuring charge fluctuations within a subregion provides a powerful probe of quantum many-body systems. In two spatial dimensions, the shape dependence of the dimensionless corner contribution encodes universal data of quantum critical points and reveals observables of quantum geometry in various quantum phases. Here, we systematically extend this framework to higher dimensions. In three dimensions, we derive the universal angle dependence associated with trihedral corners of a generic parallelepiped and benchmark the predictions against Monte Carlo simulations of lattice models at the O(3) quantum critical point. We further identify a wedge-corner contribution that directly probes the quantum metric, supported by numerical results for a lattice Weyl semimetal model. More generally, we obtain angle functions for polyhedral corners of arbitrary parallelotopes in general dimensions and clarify the scaling of the corner contribution across phases of matter. While insulators and conformal critical points exhibit similar behavior across dimensions, metals display a characteristic even-odd dimensional effect.

cond-mat.str-el↗

arXiv2026

Spin order, spin excitations, and RIXS spectra of spin-1/2 tetramer chains

We investigate the spin dynamics of a 1D spin-1/2 Heisenberg tetramer chain. Employing a combination of Density Matrix Renormalization Group, quantum renormalization group, and perturbation theory techniques, we compute the energy levels and the quantum phase diagram, analyze the phase transitions, and evaluate the $L$ and $K$ -edge resonant inelastic x-ray scattering (RIXS) spectrum of fractionalized and collective (single and multi-particle) excitations. Our calculations suggest that the chain can transition between a hidden $Z_2\times Z_2$ discrete symmetry preserving tetramer phase and a Haldane phase with non-vanishing string order that breaks the hidden symmetry. These two gapped phases are intervened by an intermediate deconfined quantum critical state comprising of free spins and three-site doublets, which is a gapless critical phase with deconfined spinons. We find that the tetramer chain can support fractionalized (spinon) and collective (triplon and quinton) excitations. In the ferromagnetic intra-tetramer limit, the chain can support a quinton excitation which has a five-fold degenerate excited state. String order parameter calculations suggest CuInVO$_5$ to be in a Haldane-like phase whose $L$ -edge RIXS spectrum can support observable triplon and quinton excitations. We also identify possible two-particle excitations (two-singlon, two-triplon, triplon-quinton, and two-quinton excitations) resulting from the double spin-flip effect in the $K$ -edge RIXS spectrum.

cond-mat.str-el↗

arXiv2026

Spin Fluctuations in the Rare-Earth Doped Bilayer Nickelates

Spin fluctuations have been generally believed as the pairing glue of high-$T_c$ superconductivity. Recent inelastic neutron scattering (INS) studies have revealed a weak flat spin-fluctuation signal around 45 meV in the bilayer nickelate La$_3$Ni$_2$O$_{7-δ}$, suggesting strong interlayer and weak intralayer magnetic couplings ($SJ_{\perp}\approx$ 60 meV, $SJ_{\parallel}\leq$ 3.5 meV) in contrast to cuprate and pnictide superconductors. Here, we report further INS studies on the Pr and Nd doped La$_3$Ni$_2$O$_{7-δ}$ powder samples at ambient pressure. Besides the crystalline electric field excitations at low energies, we have found that the 45 meV flat mode splits into two modes in doped compounds, along with another weak mode at about 60 meV, where the spin fluctuations in La$_2$NdNi$_2$O$_{7-δ}$ are stronger than La$_3$Ni$_2$O$_{7-δ}$ and La$_2$PrNi$_2$O$_{7-δ}$. Our results are consistent with an enhanced interlayer coupling $SJ_{\perp}$ within the stripe-type Heisenberg model framework, where the estimated $SJ_{\perp}$ value is in the range of about 69 to 73 meV for the rare-earth doped bilayer nickelates.

cond-mat.supr-con↗

arXiv2026

Worldline deconfinement and emergent long-range interaction in the entanglement Hamiltonian and in the entanglement spectrum

The entanglement spectrum (ES) is a powerful tool for probing topological phases. While its behavior in gapped systems is well understood, its properties in gapless regimes remain unclear. In this work, we employ a quantum Monte Carlo method to study the ES of a two-dimensional square-octagon lattice Heisenberg model at quantum criticality and in the Néel phase. We find that the ES exhibits an M-shape magnon mode with a distinct sublinear dispersion, deviating from the conventional linear magnon. This behavior, similar to that of a one-dimensional long-range Heisenberg chain, reveals the emergence of relevant long-range interactions in the entanglement Hamiltonian. We demonstrate that the mechanism underlying short- and long-range interactions in the entanglement Hamiltonian can be interpreted as the confinement/deconfinement of worldlines in the path integral formulation. Our results reveal that gapless modes can fundamentally change the entanglement Hamiltonian and its spectrum, thereby offering insight into this general phenomenon.

cond-mat.str-el↗

arXiv2026

Giant Magnetocrystalline Anisotropy in Honeycomb Iridate NiIrO3 with Large Coercive Field Exceeding 17 T

The realization of unconventional quantum phases in frustrated and spin-orbit coupled materials remains at the forefront of quantum materials research. Here we report the synthesis and discovery of NiIrO3, the first honeycomb iridate with coupled 3d-5d magnetic sublattices, through a soft topotactic reaction. Structural analysis reveals an ilmenite-type stacking of edge-sharing NiO6 and IrO6 octahedral honeycomb sublattices in a Kitaev geometry. Comprehensive magnetic and electrical transport measurements unveil its long-range ferrimagnetic order below 213 K, which is in sharp contrast to the predominantly antiferromagnetic order in the known honeycomb iridates. Notably, the titled compound displays an exceptionally large magnetocrystalline anisotropy energy of 32.2 meV/f.u. and a giant coercivity with coercive field exceeding 17.3 T below 4.2 K, both ranking among the highest observed in iridates to date. Combined experimental and theoretical investigations indicate that the exceptional anisotropy and coercivity originate from the synergistic effect between strong lattice frustration in the coupled 3d-5d honeycomb lattice network and the robust spin-orbit coupling of the Ir4+ (Jeff = 1/2) state. This work positions NiIrO3 as a promising platform to investigate low-dimensional and frustrated quantum spin systems, and highlights its potential for spintronic applications through the targeted engineering of 3d-5d interactions.

cond-mat.str-el↗