arXiv subjects

Tianxing Ma

Publications and source records attributed to Tianxing Ma.

At least 37 records · Page 2Linked to original sources

arXiv2025

Charge stripe and superconductivity tuned by interlayer interaction in a sign-problem-free bilayer extended Hubbard model

Competing orders represent a central challenge in understanding strongly correlated systems. In this work, we employ projector quantum Monte Carlo simulations to study a sign-problem-free bilayer extended Hubbard model. In this model, a charge stripe phase, characterized by a peak at momentum $k_x=2πδ$ is induced by highly anisotropic interlayer spin-exchange coupling $J_z$, and strongly suppressed upon introducing the spin-flip term $J_\bot$; in contrast, $J_\perp$ favors the emergence of interlayer pairing superconductivity. We further demonstrate that the anisotropy of the interlayer spin-exchange directly governs the competition between these two phases, while the on-site interaction $U$ plays a complex role in tuning both the charge stripe and superconductivity. Our work identifies the key factors driving charge stripe formation, highlights the sensitivity of both the charge stripe and superconducting phases to interaction parameters, and thereby provides valuable insights into competing orders in strongly correlated systems.

cond-mat.supr-con↗

arXiv2025

Pairing Symmetry Crossover from $d$-wave to $s_{\pm}$-wave in a Bilayer Nickelate Driven by Hund's Coupling and Crystal Field Splitting

The pairing symmetry of the recently discovered bilayer nickelate superconductor La$_3$Ni$_2$O$_7$ is a subject of intense debate in condensed matter physics, with the two leading theoretical candidates being a sign-reversing $s_{\pm}$-wave and a $d$-wave state. To investigate its ground-state properties in the intermediate coupling regime which is critical for real materials, we construct a two-orbital bilayer Hubbard model and employ the constrained-path quantum Monte Carlo method for large-scale simulations. By systematically calculating ground-state pairing correlation functions across parameter spaces, we map its pairing symmetry phase diagram. We find that an increasing Hund's coupling selectively enhances the interlayer $s_{\pm}$-wave pairing while suppressing the intralayer $d$-wave pairing. Similarly, a larger crystal field splitting drives a transition from $d$-wave- to $s_{\pm}$-wave-dominant states. Further analysis reveals that the strength of the intralayer $d$-wave pairing is strongly correlated with the $(π, π)$ antiferromagnetic spin fluctuations, which are in turn effectively suppressed by a large crystal field splitting, thereby weakening the $d$-wave pairing channel. Additionally, the dominant pairing symmetry transition region roughly overlaps with the inversion of orbital occupancy response to Hubbard $U$, suggesting an intrinsic link between pairing competition and orbital physics. Our results indicate that, within the parameter regime relevant to the actual material, the $s_{\pm}$-wave is the most probable pairing symmetry.

cond-mat.str-el↗

arXiv2025

Dynamical structure factor and a new method to measure the pairing gap in two-dimensional attractive Fermi-Hubbard model

The measurement of the pairing gap plays an essential role in studying the physical properties of superconductors or superfluids. We develop a strategy for measure the pairing gap through the dynamical excitations. With the random phase approximation (RPA), the dynamical excitations of a two-dimensional attractive Fermi-Hubbard model are studied by calculating the dynamical structure factor. Two distinct collective modes are investigated: a Goldstone phonon mode at the transferred momentum ${\bf q}=\left[0,0\right]$ and a roton mode at ${\bf q}=\left[π,π\right]$. The roton mode demonstrates a sharp molecular peak in the low-energy regime. Remarkably, the area under the roton molecular peak scales with the square of the pairing gap, which persists even in three-dimensional and spin-orbit coupled (SOC) optical lattices. This result provides a potential strategy to measure the pairing gap of lattice systems experimentally by measuring the dynamical structure factor at ${\bf q}=\left[π,π\right]$.

cond-mat.quant-gas↗

arXiv2025

In-depth Investigation of Conduction Mechanism on Defect-induced Proton-conducting Electrolytes BaHfO$_3$

This study utilizes first-principles computational methods to comprehensively analyze the impact of A-site doping on the proton conduction properties of BaHfO$_3$. The goal is to offer theoretical support for the advancement of electrolyte materials for solid oxide fuel cells. Our research has uncovered that BaHfO$_3$ demonstrates promising potential for proton conduction, with a low proton migration barrier of $0.28$ eV, suggesting efficient proton conduction can be achieved at lower temperatures. Through A-site doping, particularly with low-valence-state ions and the introduction of Ba vacancies, we can effectively decrease the formation energy of oxygen vacancies ($ E_{\text{vac}} $), leading to an increase in proton concentration. Additionally, our study reveals that the primary mechanism for proton migration in BaHfO$_3$ is the Grotthuss mechanism rather than the vehicle mechanism. Examination of the changes in lattice parameters during proton migration indicates that while doping or vacancy control strategies do not alter the mode of H$^+$ migration, they do influence the migration pathway and barrier. These findings provide valuable insights into optimizing the proton conduction properties of BaHfO$_3$ through A-site doping and lay a solid theoretical foundation for the development of novel, highly efficient solid oxide fuel cell electrolyte materials.

cond-mat.mtrl-sci↗

arXiv2025

Breathing-Driven Metal-Insulator Transition in Correlated Kagome Systems

Inspired by the recent discovery of breathing kagome materials $\rm Nb_3Cl_8$ and $\rm Nb_3TeCl_7$, we have explored the influence of the breathing effect on the Hubbard model of the kagome lattice. Utilizing the determinant quantum Monte Carlo method, we first investigated the average sign problem in the breathing kagome lattice, which is significantly affected by both the breathing strength and the interaction strength. Secondly, we calculated the electronic kinetic energy, the direct current conductivity, and the electronic density of states at the Fermi level to determine the critical interaction strength for the metal-insulator transition. Our results indicate that the breathing effect, in conjunction with the interaction strength, drives the kagome system from a metal to an insulator. Finally, we evaluated the magnetic properties and constructed a phase diagram incorporating both transport and magnetic properties. The phase diagram reveals that as the interaction strength increases, the system transitions from a paramagnetic metal to a Mott insulator. Our research provides a theoretical guidance for utilizing the breathing effect to control the band gaps, conductivity, and magnetic properties of kagome materials with electronic interactions.

cond-mat.str-el↗

arXiv2025

Dual role of stripe phase on superconducting correlation in a bilayer square lattice

While the stripe phase has been observed not only in monolayer cuprates but also in bilayer cuprates, research on its behavior in bilayer cuprates has been limited. Using constrained path quantum Monte Carlo, we explore the effect of stripes on the bilayer square lattice. We find the system exhibits short-range antiferromagnetism, which is enhanced by stripes and is strongest when the electron density of the interstriped rows reaches half-filling. The hole doping concentration plays a crucial role in the interaction between stripes and superconductivity. The $d$-wave pairing is enhanced by stripe potential $V_0$ at the hole doping $δ_h=1/4$, whereas it is suppressed by stripe potential $V_0$ at the hole doping $δ_h=1/8$. We elucidate this phenomenon through an analysis of the magnetism of the interstriped rows. Furthermore, the effective $d$-wave pairing is stronger in the bilayer model compared to the monolayer model when stripes are introduced on the square lattice. Overall, our unbiased numerical simulations provide a further understanding of the crossed bilayer square lattice model.

cond-mat.str-el↗

arXiv2025

Comparison of superconducting pairing in doped cuprates and nickelates within an extended Hubbard model

Within a Hubbard model, we investigate the superconducting pairing behavior of infinite-layer nickelate $\mathrm{NdNiO_2}$ and cuprate superconductors by using the determinant quantum Monte Carlo method. Our focus is on comparing their dominant pairing symmetries. The results indicate that the $d_{x^2-y^2}$ pairing interaction is significantly enhanced at low temperatures in both doped nickelates and cuprates, whereas other typical pairing symmetries are effectively suppressed, highlighting the dominance of the $d_{x^2-y^2}$ pairing form. Additionally, we find that the effective pairing interaction for $d_{x^2-y^2}$ pairing in doped nickelates is slightly lower than that in doped cuprates, which may be attributed to the different degrees of Fermi surface warping caused by the third-nearest hopping $t''$. Further studies show that the hole doping and interaction strength have significant effects on the $d_{x^2-y^2}$ pairing interaction within the selected parameter range. The $d_{x^2-y^2}$ pairing interaction is notably weakened when the hole doping increases, whereas it is significantly enhanced with increasing Coulomb interaction strength $U$. This comparative analysis reveals the similarities and differences in the pairing behaviors of doped nickelates and cuprates, which may provide further insights into understanding the superconducting properties of these two classes of materials.

cond-mat.str-el↗

arXiv2024

Pairing correlation of the Kagome-lattice Hubbard model with the nearest-neighbor interaction

A recently discovered family of Kagome lattice materials, $\emph{A}\mathrm{V}_{3}\mathrm{Sb}_{5}$($\emph{A}$= $\mathrm{K,Rb,Cs}$), has attracted great interest, especially in the debate over its dominant superconducting pairing symmetry. To explore this issue, we study the superconducting pairing behavior within the Kagome-Hubbard model through the constrained path Monte Carlo method. It is found that doping around the Dirac point generates a dominant next-nearest-neighbour-$d$ pairing symmetry driven by on-site Coulomb interaction $U$. However, when considering the nearest-neighbor interaction $V$, it may induce nearest-neighbor-$p$ pairing to become the preferred pairing symmetry. Our results provide useful information to identify the dominant superconducting pairing symmetry in $\emph{A}\mathrm{V}_{3}\mathrm{Sb}_{5}$ family.

cond-mat.str-el↗

arXiv2024

Flat bands and superconductivity induced by periodic strain in monolayer graphene

Superconductivity in single-layer graphene has attracted considerable interest. Here, using the determinant quantum Monte Carlo method, we study transitions of superconductivity and magnetism in a monolayer graphene with a special periodic strain. Consistent with experiments, the deformation accumulates a series of flat bands, whose robustness under interaction is verified through electron localization in real space. During the reconstruction of the band structure, the superconductivity appears in flat band range with next-nearest neighbor $d+id$ pairing symmetry dominating other modes and is accompanied by ferromagnetism caused by symmetry breaking. We also demonstrate that the strain-induced symmetry breaking would accumulate an energy-gap antiferromagnetic insulating phase at half filling even under the limitation of not strong enough interaction, which shows its potential as a platform that exhibits strongly correlated phenomena.

cond-mat.str-el↗

arXiv2024

Supersolid Phase in the Diluted Holstein Model

The Holstein model on a square lattice at half-filling has a well-established finite temperature phase transition to an insulating state with long range charge density wave (CDW) order. Because this CDW formation suppresses pairing, a superconducting (SC) phase emerges only with doping. In this work, we study the effects of dilution of the local phonon degrees of freedom in the Holstein model while keeping the system at half filling. We find not only that the CDW remains present up to a dilution fraction $f \sim 0.15$, but also that long range pairing is stabilized with increasing $f$, resulting in a {\it supersolid} regime centered at $f \approx 0.10$, where long range diagonal and off-diagonal correlations coexist. Further dilution results in a purely SC phase, and ultimately in a normal metal. Our results provide a new route to the supersolid phase via the introduction of impurities at fixed positions which both increase quantum fluctuations and also are immune to the competing tendency to phase separation often observed in the doped case.

cond-mat.str-el↗

arXiv2024

Charge Stripe Manipulation of Superconducting Pairing Symmetry Transition

Charge stripes have been widely observed in many different types of unconventional superconductors, holding varying periods ($\mathcal{P}$) and intensities. However, a general understanding on the interplay between charge stripes and superconducting properties is still incomplete. Here, using large-scale unbiased numerical simulations on a general inhomogeneous Hubbard model, we discover that the charge-stripe period $\mathcal{P}$, which is variable in different real material systems, could dictate the pairing symmetries -- $d$ wave for $\mathcal{P} \ge 4$, $s$ and $d$ waves for $\mathcal{P} \le 3$. In the latter, tuning hole doping and charge-stripe amplitude can trigger a $d$-$s$ wave transition and magnetic-correlation shift, where the $d$-wave state converts to a pairing-density wave state, competing with the $s$ wave. These interesting phenomena arise from an unusual stripe-induced selection rule of pairing symmetries around on-stripe region and within inter-stripe region, giving rise to a critical point of $\mathcal{P}=3$ for the phase transition. In general, our findings offer new insights into the differences in the superconducting pairing mechanisms across many $\mathcal{P}$-dependent superconducting systems, highlighting the decisive role of charge stripe.

cond-mat.str-el↗

arXiv2024

Competition between $d$-wave and $d$+$is$-wave superconductivity in the Hubbard model on a checkerboard lattice

By employing determinant quantum Monte Carlo simulations, we investigate a checkerboard lattice with next-nearest-neighbor hopping $t'$ as the frustration-control parameter, which exhibits an energetically partial flat-band in the system. Our numerical simulation identifies the dominant pairing symmetry of the checkerboard lattice Hubbard model, and we reveal the competition between the $d$-wave and $d+is$ wave in the parameter space of electron filling $\avg{n}$ and frustration control parameter $t^{\prime}/t$. To ensure the reliability and accuracy of our results, we evaluate the sign problem. We also find that the spin susceptibility, the effective pairing interactions of different pairing symmetries and the superconducting instability are enhanced as the on-site Coulomb interaction increases, demonstrating that superconductivity is driven by strong electron--electron correlation. Our work provides a further understanding of pairing symmetry in the Hubbard model and improves prospects for exploring rich correlated behaviors in frustrated systems.

cond-mat.str-el↗

arXiv2024

Disorder-dependent superconducting pairing symmetry in doped graphene

Disorder and doping have profound effects on the intrinsic physical mechanisms of superconductivity. In this paper, we employed the determinant quantum Monte Carlo method to investigate the symmetry-allowed superconducting orders on the two-dimensional honeycomb lattice within the Hubbard model, using doped graphene as the carrier, focusing their response to bond disorder. Specifically, we calculated the pairing susceptibility and effective pairing interactions for the $d+id$ wave and extended $s$-wave pairings for different electron densities and disorder strengths. Our calculations show that at high electron densities, increased disorder strength may lead to a transform from $d+id$ wave dominance to extended $s$ wave dominance. However, at lower electron densities, neither of the two superconducting pairings appears under larger disorder strength. Our calculations may contribute to a further understanding of the superconducting behavior in doped materials affected by disorder.

cond-mat.supr-con↗

arXiv2024

Parameters dependent superconducting transition temperature in high temperature superconductors

Understanding the evolution of the superconducting transition temperature in relation to doping and interaction strengths is one of the most challenging problems in high temperature superconductivity. By refining determinant quantum Monte Carlo algorithm, we characterize the parameter dependence of the superconducting transition temperature within a bilayer Hubbard model, which is sign-problem-free at arbitrary filling. A striking feature of this model is its similarities to the bilayer nickelate-based superconductor $\mathrm{La}_{3}\mathrm{Ni}_{2}\mathrm{O}_{7}$, where superconductivity emerge from the bilayer $\mathrm{Ni}\mathrm{O}_{2}$ planes. We find that interlayer spin-exchange $J$ is critical to interlayer pairing, ant that on-site interaction $U$ contributes negatively to superconductivity at low doping levels but positively to it at high doping levels. Our findings identify the key parameter dependent superconducting transition temperature in nickelate-based superconductors and provide a new understanding of the high temperature superconductivity.

cond-mat.supr-con↗

arXiv2024

Magnetic fluctuation and dominant superconducting pairing symmetry near the tunable Van Hove singularity

We have investigated the magnetism and pairing correlations of the triangular lattice based on the Hubbard model using the determinant quantum Monte Carlo method and the constrained path Monte Carlo. The results show that the presence of the next-nearest-neighbor hopping integral $t^{\prime}$ introduces an additional energy scale to the system, and through $t^{\prime}$, one can regulate the shape of the density of states and thus the position of the van Hove singularity point. Increasing inverse temperature $β$ and on-site interaction $U$ favor the formation of ferromagnetic correlation in a rather large filling region, and the calculations for different lattice sizes show that the range of the ferromagnetic correlations is smaller than the smallest lattice simulated at the investigated temperatures. We study the different pairing correlations of the triangular lattice near several typical fillings and show that the $f$-wave pairing dominates the system in the filling region near the van Hove singularity point with a high density of states, where the ferromagnetic correlation is also enhanced. When the filling is close to half-filling, the pairing susceptibility with $f$ wave is suppressed and the pairing susceptibility of $f_n$ wave is enhanced, however, both the effective pairing interaction with $f$ wave and $f_n$ wave are negative, which indicates that neither $f$-wave nor $f_n$-wave superconductivity may exist. Finally, we find that the pairing channel of different symmetry in the system maybe closely related to the magnetic properties. Ferromagnetic fluctuation favors the formation of $f$-wave pairing, while antiferromagnetic fluctuation tends to promote $f_n$-wave pairing.

cond-mat.str-el↗

arXiv2024

Zigzag edge ferromagnetism of triangular-graphene-quantum-dot-like system

Here, the magnetic susceptibility of a triangular-graphene-quantum-dot-like system was examined by using the determinant quantum Monte Carlo method. We focused on three zigzag edge quantum dots or rings, namely, the triangular graphene quantum ring, bilayer triangular graphene quantum dot, and bilayer triangular graphene quantum ring. The triangular-graphene-quantum-dot-like system exhibited robust edge ferromagnetic behavior, which was independent of size, monolayer or bilayer, or dot or ring shape, according to the numerical results. At half filling, the edge magnetic susceptibility is increased by on-site interactions, especially in the low-temperature region. Spintronics systems may benefit from use of this system due to its robust edge ferromagnetic behavior.

cond-mat.mes-hall↗

arXiv2023

Different phase leads to different transport behavior in Pb$_9$Cu(PO$_4$)$_6$O compounds

The recent claimed room-temperature superconductivity in Cu-doped lead apatite at ambient pressure are under highly debate. To identify its physical origin, we studied the crystal structures, energy band structures, lattice dynamics and magnetic properties of the parent Pb$_{10}$(PO$_4$)$_6$O compound, in which two different phases of the LK-99 compound are analyzed in detail. Our results show that the Pb$_{10}$(PO$_4$)$_6$O compound is an indirect band gap semiconductor, where Cu doping at the 4$f$ site of Pb leads to a semiconducting to half-metallic transition. Two half-filled flat bands spanning the Fermi energy levels are present in the 4$f$-phase of LK-99, which are mainly formed by hybridization of the $d_{x^2-y^2}$ and $d_{zy}$ orbitals of Cu with the 2$p$ orbitals of O. In addition, 6$h$-phase of LK-99 always has spin polarity at the bottom of the conduction band and at the top of the valence band, making the material a bipolar magnetic semiconductor. Our results are basically consistent with the recent experimental transport properties of LK-99 posted on arXiv:2308.05778.

cond-mat.supr-con↗

arXiv2023

Doping driven metal-insulator transition in disordered graphene

Controlling the metal-insulator transition in graphene-based material is a crucial topic as it directly impacts its potential applications. Inspired by recent experiments, we study the effects of doping and bond disorder on metal-insulator transition in graphene within the Hubbard model on a honeycomb lattice. By using the determinant quantum Monte Carlo method, we first conduct tests on the value of under various parameters, such as electron density, on-site interactions, temperature, and lattice size, so as to select the appropriate parameters to alleviate the impact of the sign problem. Given the knowledge that bond disorder can lead to a mental-insulator transition, our study has revealed, after ruling out the influence of size effects, that the critical strength of disorder increases as the electron density decreases while decreasing as the on-site interactions increase. Furthermore, we compared our results with experimental data and concluded that, in actual graphene materials, the localization effect induced by doping plays a dominant role, resulting in an insulating phase.

cond-mat.str-el↗