arXiv subjects

Dao-Xin Yao

Publications and source records attributed to Dao-Xin Yao.

At least 55 records · Page 3Linked to original sources

arXiv2025

Exact diagonalization study of triangular Heisenberg model with four-spin ring-exchange interaction

Using Lanczos exact diagonalization (ED), we study the spin-1/2 $J_1$-$J_2$ Heisenberg model with the four-spin ring-exchange interaction $J_r$ on triangular lattice. We mainly use the level spectroscopic technique of two 36-site tori to investigate the ground-state phase diagram, and further characterize phases by spin, dimer and chiral correlation functions. The ground state has rich phases including several magnetic ordered phases like zigzag phase and tetrahedral phase, as well as several novel nonmagnetic phases, some of which exhibit valence bond solid behavior in their dimer correlation functions. However, we do not find direct evidence of a quantum spin liquid phase with spinon Fermi surface in this model. Our results can give a better understanding of the ground-state properties of the triangular Heisenberg model with ring-exchange interaction, and help to understand the relevant triangular materials.

cond-mat.str-el↗

arXiv2025

The effect of Carrier Doping and Thickness on the Electronic Structures of La$3$Ni$2$O$7$ Thin Films

Recently, the superconductivity of bilayer nickelate La3Ni2O7 has been observed in the thin film at ambient pressure, facilitated by epitaxial strain. Here, we investigate the effects of film thickness and carrier doping on the electronic structure of La3Ni2O7 thin films with thickness of 0.5-3 unit cells (UC) using first-principles calculations. At an optimal doping concentration of 0.4 holes per formula unit for 2UC film, the Ni-"d" _("z" ^"2" ) interlayer bonding state metallizes, leading to the formation of γ pockets at the Fermi surface, which quantitatively matches the experimental results of angle-resolved photoemission spectroscopy (ARPES). These findings provide theoretical support for recent experimental observations of ambient-pressure superconductivity in La3Ni2O7 thin films and highlight the crucial role of film thickness and carrier doping in modulating electronic properties.

cond-mat.supr-con↗

arXiv2025

Topological magnons in a collinear altermagnet

We propose a model with Weyl magnons and nodal-line magnons ($\mathbb{Z}_2$) in a collinear altermagnet on the honeycomb lattice. Altermagnetic magnon bands can be realized by breaking the symmetry of the second nearest neighbor exchange couplings without the Dzyaloshinskii-Moriya interaction (DMI). In addition to the Chern number and $\mathbb{Z}_2$ invariant, chirality is important to describe the magnon topology. The model shows the nonzero magnon spin Nernst effect when a longitudinal temperature gradient exists. We calculate the differential gyromagnetic ratio induced purely by the topology of altermagnetic magnons, which can be probed through the Einstein-de Haas (EdH) effect.

cond-mat.mes-hall↗

arXiv2025

Origin of the density wave instability in trilayer nickelate La$_{4}$Ni$_{3}$O$_{10}$ revealed by optical and ultrafast spectroscopy

In the intricate phase diagram of unconventional superconductors characterized by intertwined electronic orders and superconductivity, a key step in understanding the superconducting mechanism is to investigate the parent compounds from which superconductivity emerges through doping or pressure. In this study, we employed optical spectroscopy and ultrafast reflectivity measurements to examine the density wave instability in the trilayer nickelate La$_{4}$Ni$_{3}$O$_{10}$, which displays pressure-induced superconductivity up to 30 K. Our optical spectroscopy measurements reveal that La$_{4}$Ni$_{3}$O$_{10}$ behaves as a metal with a high plasma frequency. Upon cooling, we observed a distinct formation of a density wave energy gap in both optical conductivity and pump-probe measurements. The gap feature is more pronounced compared to the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$. Through a comparison of the experimentally determined plasma frequency with first-principles calculations, we classify La$_{4}$Ni$_{3}$O$_{10}$ as a moderately electron-correlated material, resembling the parent compound of iron-based superconductors but exhibiting weaker correlation than the bilayer nickelate La$_{3}$Ni$_{2}$O$_{7}$. The enhanced gap feature and weaker electronic correlation in La$_{4}$Ni$_{3}$O$_{10}$ may explain its lower superconductivity transition temperature under high pressure. These findings significantly advance our comprehension of the density wave and superconductivity mechanisms in the trilayer nickelate La$_{4}$Ni$_{3}$O$_{10}$.

cond-mat.str-el↗

arXiv2025

Einstein-de Haas effect: a bridge linking mechanics, magnetism, and topology

The Einstein-de Haas (EdH) effect is a fascinating phenomenon that links mechanics and magnetism. Despite being discovered over a century ago, it remains significant in contemporary science, particularly within the fields of spintronics and ultrafast magnetism. Recent predictions suggest that the EdH effect may be realized in topological magnon systems, potentially leading to even richer properties. In this perspective, we introduce recent advancements in the EdH effect and discuss its developments in three key aspects: the microscopic mechanism, its manifestation in topological systems, and chirality-selective magnon-phonon coupling. Our discussions aim to inspire further explorations of the EdH effect and highlight its promising applications in different areas.

cond-mat.mes-hall↗

arXiv2025

Correlation Effects in a Simplified Bilayer Two-Orbital Hubbard Model at Half Filling

Motivated by the discovery of high-temperature superconductivity in bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure, we investigate the ground-state phase diagram and correlation effects using determinant quantum Monte Carlo simulations in a simplified bilayer two-orbital Hubbard model at half filling. Our results reveal the emergence of a nonmagnetic weakly insulating phase at weak on-site Hubbard interactions, transitioning to an antiferromagnetic Mott insulating phase as the interaction strength exceeds a critical value $U/t_1^x\approx 4.15$. This phase transition is consistent with the 3D O(3) Heisenberg universality class. Additionally, we analyze dynamical properties such as the single-particle spectral function and dynamic spin structure factor. The pronounced inter-layer correlation of $d_{3z^2-r^2}$ orbitals results in a downward trend and an extended flatness in the $γ$ band, mirroring the angle-resolved photoemission spectroscopy findings under ambient pressure. Our numerical results provide important clues for understanding the strong correlation effects in La$_3$Ni$_2$O$_7$.

cond-mat.str-el↗

arXiv2025

Resonant inelastic x-ray scattering spectra of spinon, doublon, and quarton excitations of a spin-$\frac{1}{2}$ antiferromagnetic Heisenberg trimer chain

We investigate the excitation spectra of a spin-1/2 antiferromagnetic Heisenberg trimer spin chain by employing a combination of numerical and theoretical techniques. Utilizing the Krylov-space correction-vector method in density matrix renormalization group (DMRG), we calculate both the direct and indirect resonant inelastic x-ray scattering (RIXS) spectra for the trimer spin chain. To interpret the observed features in the RIXS spectra, we perform a theoretical perturbative analysis to compute the energy dispersions which are then utilized to determine the density of states (DOS) for both the single-particle and the two-particle excitation spectra. Our results show that the single-particle continua of the direct RIXS spectrum align with the energy levels observed in the DOS spectra of spinon, doublon, and quarton excitations. Furthermore, the two-particle continua are revealed in the indirect RIXS process, where all possible single particle excitations combine to form the various two-particle excitations of the trimer spin chain. Based on our calculations, we propose the RIXS mechanism of generating the fractionalized (spinon) and collective (doublon and quarton) excitations in the trimer spin chain at both the $L$-edge and the $K$-edge, including discussing the interplay of these excitations in the RIXS spectrum for various trimer coupling strength. The computed energy range of the excitations suggest the possibility of experimental detection at both the $L$-edge and the $K$-edge within the current capabilities of RIXS instrumentation resolution.

cond-mat.str-el↗

arXiv2025

Microscopic origin of magnetoferroelectricity in monolayer NiBr$_{2}$ and NiI$_{2}$

We investigate the magnetoelectric properties of the monolayer NiX$_{2}$ (X = Br, I) through first-principles calculations. Our calculations predict that the NiBr$_{2}$ monolayer exhibits a cycloidal magnetic ground state. For the NiI$_{2}$ monolayer, a proper-screw helical magnetic ground state with modulation vector $\boldsymbol{Q} = (q, 0, 0)$ is adopted, approximated based on experimental observations. The electric polarization in NiBr$_{2}$ shows a linear dependence on the spin-orbit coupling strength $λ_{\text{SOC}}$, which can be adequately described by the generalized Katsura-Nagaosa-Balatsky (gKNB) model, considering contributions from up to the third nearest-neighbor spin pairs. In contrast, the electric polarization in NiI$_{2}$ exhibits a distinct dependence on $q$ and $λ_{\text{SOC}}$, which cannot be fully explained by the gKNB mechanism alone. To address this, the $p$-$d$ hybridization mechanism is extended to NiI$_{2}$ to explain the observed behavior. The respective contributions from the $p$-$d$ hybridization and the gKNB mechanism in NiI$_{2}$ are then quantitatively evaluated. Overall, our work elucidates the microscopic mechanisms underlying multiferroicity in NiBr$_{2}$ and NiI$_{2}$ monolayers, with the conclusions readily applicable to their bulk forms.

cond-mat.mtrl-sci↗

arXiv2025

Large upper critical fields and strong coupling superconductivity in the medium-entropy alloy (Ti1/3Hf1/3Ta1/3)1-xNbx

Since the discovery of high-entropy superconductors in 2014, superconductivity has remained a focal point of interest in medium- and high-entropy alloys (MEAs-HEAs). Here, we report a series of (Ti0.33Hf0.33Ta0.33)1-xNbx MEA superconductors crystallized in the BCC structure, whose superconductivity was characterized by resistivity, magnetization, and specific heat measurements. The study found that the (Ti0.33Hf0.33Ta0.33)1-xNbx MEAs exhibit bulk superconductivity. With the doping of Nb, the superconducting transition temperature (Tc) increases from 5.31 K to 9.11 K, and the normalized Cel jumps at Tc, and the logarithmically averaged characteristic phonon frequency exhibit dome-shaped curves. Results from specific heat measurements indicate that the superconductivity is of a strongly coupled s-wave type observed. Furthermore, at low Nb content, the upper critical field of the samples is larger than the Pauli paramagnetic limit. The strongly coupling behavior and large upper critical field in s-wave type (Ti0.33Hf0.33Ta0.33)1-xNbx MEA superconductors are unusual, as they typically occur in other unconventional superconductors. Thus, (Ti0.33Hf0.33Ta0.33)1-xNbx may have significant potential in the research and understanding of physical mechanisms.

cond-mat.supr-con↗

arXiv2024

Spin density wave in the bilayered nickelate La$_3$Ni$_2$O$_{7-δ}$ at ambient pressure

The recent discovery of high-temperature superconductivity in high-pressurized La$_3$Ni$_2$O$_{7-δ}$ has garnered significant attention. Using density functional theory, we investigate the magnetic properties of La$_3$Ni$_2$O$_{7-δ}$ at ambient pressure. Our calculations suggest that with $δ=0$, the double spin stripe phase is favored as the magnetic ground state. Oxygen vacancies may effectively turn nearest Ni spins into \textit{charge} sites. Consequently, with moderate $δ$ values, our theoretical magnetic ground state exhibits characteristics of both double spin stripe and spin-charge stripe configurations, providing a natural explanation to reconcile the seemingly contradictory experimental findings that suggest both the configurations as candidates for the spin-density-wave phase. With higher $δ$ values, we anticipate the ground state to become a spin-glass-like noncollinear magnetic phase with only short-range order. The oxygen vacancies are expected to significantly impact the magnetic excitations and the transition temperatures $T_{SDW}$. Notably, the magnetic ordering also induces concomitant charge ordering and orbital ordering, driven by spin-lattice coupling under the low symmetry magnetic order. We further offer a plausible explanation for the experimental observations that the measured $T_{SDW}$ appears insensitive to the variation of samples and the lack of direct evidence for long-range magnetic ordering.

cond-mat.mtrl-sci↗

arXiv2024

Quantum phase transitions and composite excitations of antiferromagnetic spin trimer chains in a magnetic field

Motivated by recent advancements in theoretical and experimental studies of the high-energy excitations on an antiferromagnetic trimer chain, we numerically investigate the quantum phase transition and composite dynamics in this system by applying a magnetic field. The numerical methods we used include the exact diagonalization, density matrix renormalization group, time-dependent variational principle, and cluster perturbation theory. From calculating the entanglement entropy, we have revealed the phase diagram which includes the XY-I, $1/3$ magnetization plateau, XY-II, and ferromagnetic phases. Both the critical XY-I and XY-II phases are characterized by the conformal field theory with a central charge $c \simeq 1$. By analyzing the dynamic spin structure factor, we elucidate the distinct features of spin dynamics across different phases. In the regime with weak intertrimer interaction, we identify the intermediate-energy and high-energy modes in the XY-I and $1/3$ magnetization plateau phases as internal trimer excitations, corresponding to the propagating of doublons and quartons, respectively. Notably, applying a magnetic field splits the high-energy spectrum into two branches, labeled as the upper quarton and lower quarton. Furthermore, we explore the spin dynamics of a frustrated trimerized model closely related to the quantum magnet \ce{Na_2Cu_3Ge_4O_12}. In the end, we extend our discuss on the possibility of the quarton Bose-Einstein condensation in the trimer systems. Our results are expected to be further verified through the inelastic neutron scattering and resonant inelastic X-ray scattering, and also provide valuable insights for exploring high-energy exotic excitations.

cond-mat.str-el↗

arXiv2024

Tuning magnetic anisotropy in Fe$_{5}$GeTe$_{2}$ monolayer through doping and strain

Controlling magnetic anisotropy energy (MAE) in two-dimensional (2D) ferromagnetic materials is crucial for designing novel spintronic devices. Using first-principles calculations, we systematically investigate the magnetic properties of monolayer Fe$_5$GeTe$_2$ (F5GT) under two scenarios: (I) Co and Ni doping, and (II) compressive and tensile strains. Our results show that the F5GT monolayer exhibits a weak in-plane MAE, which can be significantly enhanced by Co doping. Additionally, a $1\%$ compressive strain switches the magnetic easy axis from in-plane to out-of-plane, while $4\%$ compressive strain can further enhance the out-of-plane MAE. Spin-orbit coupling (SOC) matrix analysis reveals that the enhancement of in-plane MAE in Co-doped F5GT (Co-F5GT) arises from changes in $ \left\langle {\it p}_{x} \left\vert L_z\right\vert {\it p}_{y} \right\rangle $ of Te and $ \left\langle {\it d}_{xy} \left\vert L_z\right\vert {\it d}_{x^2+y^2} \right\rangle $ of Fe(2) and Fe(3). The effect of compressive strain is primarily attributed to a substantial increase in the positive contribution from $ \left\langle {\it d}_{xy} \left\vert L_z\right\vert {\it d}_{x^2+y^2} \right\rangle $ of Fe(1).

cond-mat.mtrl-sci↗

arXiv2024

Direct observation of topological magnon edge states

Magnon Chern insulators (MCIs) exhibit unique topological magnon band structures featuring chiral edge states. Direct observations of the topologically protected magnon edge states have long been pursued. Here, we report the spatially resolved detection of magnon edge states in a two-dimensional ferromagnet with honeycomb lattice (single-layer chromium triiodide). Using scanning tunneling microscopy, we observed magnon-assisted inelastic tunneling conductance and revealed the gapped magnon spectra with enhanced signals at the van Hove singularities. Extra tunneling conductance contributed from the magnon edge states was detected at three different edge configurations. Our work provided direct evidence proving the existence of MCI states down to the single-layer limit, initiating spatially-resolved explorations on exotic properties arising from topological edge states of MCIs.

cond-mat.mes-hall↗

arXiv2024

Fractional Chern insulator candidate in twisted bilayer checkboard lattice

We investigate a fractional Chern insulator (FCI) candidate arising from Moiré bands with higher Chern number C=2 on a magic angle twisted bilayer checkboard lattice (MATBCB). There are two nearly flat low lying bands in the single particle energy spectrum under the first magic angle $ϕ\approx 1.608^{\circ}$ and chiral limit. We find MATBCB hosts a nearly uniform Berry curvature distribution and exhibits tiny violation of quantum geometric trace condition in the first moiré Brillourin Zone (mBZ), indicating that there is a nearly ideal quantum geometry in MATBCB in single particle level. Turning on projected Coulomb interactions, we perform exact diagonalization and find a ten-fold ground state quasi-degeneracy in many body energy spectrum with filling fraction $ν=1/5$. The ten-fold quasi-degenrate ground states further show spectra flow under flux pumping. By diagnosing the particle entanglement spectrum (PES) of the ground states, we obtain a clear PES gap and quasi-hole state counting consistent with Halperin spin singlet generalized Pauli principle, suggesting that a fractional Chern insulator is realized in this system.

cond-mat.str-el↗

arXiv2024

Superband: an Electronic-band and Fermi surface structure database of superconductors

In comparison to simpler data such as chemical formulas and lattice structures, electronic band structure data provide a more fundamental and intuitive insight into superconducting phenomena. In this work, we generate superconductor's lattice structure files optimized for density functional theory (DFT) calculations. Through DFT, we obtain electronic band superconductors, including band structures, density of states (DOS), and Fermi surface data. Additionally, we outline efficient methodologies for acquiring structure data, establish high-throughput DFT computational protocols, and introduce tools for extracting this data from large-scale DFT calculations. As an example, we have curated a dataset containing information on 2474 superconductors along with their experimentally determined superconducting transition temperatures, which is well-suited for machine learning applications. This work also provides guidelines for accessing and utilizing this dataset. Furthermore, we present a neural network model designed for training with this data. All the aforementioned data and code are publicly available at http://www.superband.work.

cond-mat.supr-con↗

arXiv2024

A deep learning approach to search for superconductors from electronic bands

Energy band theory is a foundational framework in condensed matter physics. In this work, we employ a deep learning method, BNAS, to find a direct correlation between electronic band structure and superconducting transition temperature. Our findings suggest that electronic band structures can act as primary indicators of superconductivity. To avoid overfitting, we utilize a relatively simple deep learning neural network model, which, despite its simplicity, demonstrates predictive capabilities for superconducting properties. By leveraging the attention mechanism within deep learning, we are able to identify specific regions of the electronic band structure most correlated with superconductivity. This novel approach provides new insights into the mechanisms driving superconductivity from an alternative perspective. Moreover, we predict several potential superconductors that may serve as candidates for future experimental synthesis.

cond-mat.supr-con↗

arXiv2024

Magnetization Plateaus in the Two-dimensional S = 1/2 Heisenberg Model with a 3$\times$3 Checkerboard Structure

We investigate the S=1/2 antiferromagnetic Heisenberg model with a 3$\times$3 checkerboard lattice structure in a longitudinal magnetic field. By using the stochastic series expansion quantum Monte Carlo (SSE-QMC) method, we obtain the properties of the non-plateau XY phase, 1/9, 3/9, 5/9, 7/9 magnetization plateau phases, and fully polarized phase. Then, we determine the precise phase transition critical points belonging to the 3D XY universality class through finite-size scaling. Moreover, we study the longitudinal and transverse dynamic spin structure factors of this model in different phases. For the non-plateau XY phase, the energy spectra present a gap between the low-energy gapless branch and the high-energy part under the competition of magnetic field and interaction. The gapless branch can be described by the spin wave theory in the canted antiferromagnetic phase of the effective "block spin" model. In the magnetization plateau phase, we identify that the excitation arises from localized disturbances within the sublattice, which are capable of spreading in momentum space. This study offers theoretical insights and interpretations for the characteristics of the ground state and the inelastic neutron scattering spectrum in two-dimensional quantum magnetic materials. Specifically, it focuses on materials with a checkerboard unit cell structure and odd-spin configurations under the influence of a longitudinal magnetic field.

cond-mat.str-el↗

arXiv2024

High-T$_C$ superconductivity in $\mathrm{La_3Ni_2O_7}$ based on the bilayer two-orbital t-J model

The recently discovered high-T$_C$ superconductor La$_3$Ni$_2$O$_7$ has sparked renewed interest in the unconventional superconductivity. Here we study superconductivity in pressurized La$_3$Ni$_2$O$_7$ based on a bilayer two-orbital $t-J$ model, using the renormalized mean-field theory. Our results reveal a robust $s^\pm-$wave pairing driven by the inter-layer $d_{z^2}$ magnetic coupling, which exhibits a transition temperature within the same order of magnitude as the experimentally observed $T_c \sim 80$ K. We establish a comprehensive superconducting phase diagram in the doping plane. Notably, the La$_3$Ni$_2$O$_7$ under pressure is found situated roughly in the optimal doping regime of the phase diagram. When the $d_{x^2-y^2}$ orbital becomes close to half-filling, $d-$wave and $d+is$ pairing can emerge from the system. We discuss the interplay between Fermi surface topology and different pairing symmetries. The stability of the $s^\pm-$wave pairing against Hund's coupling and other magnetic exchange couplings is discussed.

cond-mat.supr-con↗