arXiv subjects

Tianxing Ma

Publications and source records attributed to Tianxing Ma.

At least 55 records · Page 3Linked to original sources

arXiv2023

Strong ferromagnetic fluctuations in a doped checkerboard lattice

Using the determinant quantum Monte Carlo method, we study the magnetic susceptibility in the parameter space of the on-site interaction $U$, temperature $T$, electron filling $\avg{n}$, and the frustration control parameter $t^{\prime}$ within the Hubbard model on a two-dimensional checkerboard lattice. It is shown that the system exhibits stable and strong ferromagnetic fluctuations about the electron filling $\avg{n}\ge1.2$ for different $t^{\prime}$, and the ferromagnetic susceptibility is strongly enhanced by the increasing interaction and decreasing tempeture. We also discuss the sign problem to clarify which parameter region is accessible and reliable. Our findings not only demonstrate important implications for modulating magnetism in the checkerboard lattice, but will also provide a theoretical platform for a flat-band model that demonstrates a variety of physical properties.

cond-mat.str-el↗

arXiv2023

Evolution of magnetic correlation in an inhomogeneous square lattice

We explore the magnetic properties of a two-dimensional Hubbard model on an inhomogeneous square lattice, which provides a platform for tuning the bandwidth of the flat band. In its limit, this inhomogeneous square lattice turns into a Lieb lattice, and it exhibits abundant properties due to the flat band structure at the Fermi level. By using the determinant quantum Monte Carlo simulation, we calculate the spin susceptibility, double occupancy, magnetization, spin structure factor, and effective pairing interaction of the system. It is found that the antiferromagnetic correlation is suppressed by the inhomogeneous strength and that the ferromagnetic correlation is enhanced. Both the antiferromagnetic correlation and ferromagnetic correlation are enhanced as the interaction increases. It is also found that the effective $d$-wave pairing interaction is suppressed by the increasing inhomogeneity. In addition, we also study the thermodynamic properties of the inhomogeneous square lattice, and the calculation of specific heat provide good support for our point. Our intensive numerical results provide a rich magnetic phase diagram over both the inhomogeneity and interaction.

cond-mat.str-el↗

arXiv2023

Quantum Monte Carlo study of superconductivity in rhombohedral trilayer graphene under an electric field

By using the constrained-phase quantum Monte Carlo method, we performed a systematic study of the ground state of the half filled Hubbard model for a trilayer honeycomb lattice. We analyze the effect of the perpendicular electric field on the electronic structure, magnetic property and pairing correlations. It is found that the antiferromagnetism is suppressed by the perpendicular electric field, especially the long-range parts, and the dominant magnetic fluctuations are still antiferromagnetic. The electronic correlation drives a $d+id$ superconducting pairing to be dominant over other pairing patterns among various electric fields and interaction strengths. We also found that the $d+id$ pairing correlation is greatly enhanced as the on-site Coulomb interaction is increased. Our intensive numerical results may unveil the nature of the recently observed superconductivity in rhombohedral trilayer graphene under an electric field.

cond-mat.str-el↗

arXiv2022

Feature Weaken: Vicinal Data Augmentation for Classification

Deep learning usually relies on training large-scale data samples to achieve better performance. However, over-fitting based on training data always remains a problem. Scholars have proposed various strategies, such as feature dropping and feature mixing, to improve the generalization continuously. For the same purpose, we subversively propose a novel training method, Feature Weaken, which can be regarded as a data augmentation method. Feature Weaken constructs the vicinal data distribution with the same cosine similarity for model training by weakening features of the original samples. In especially, Feature Weaken changes the spatial distribution of samples, adjusts sample boundaries, and reduces the gradient optimization value of back-propagation. This work can not only improve the classification performance and generalization of the model, but also stabilize the model training and accelerate the model convergence. We conduct extensive experiments on classical deep convolution neural models with five common image classification datasets and the Bert model with four common text classification datasets. Compared with the classical models or the generalization improvement methods, such as Dropout, Mixup, Cutout, and CutMix, Feature Weaken shows good compatibility and performance. We also use adversarial samples to perform the robustness experiments, and the results show that Feature Weaken is effective in improving the robustness of the model.

cs.CV↗

arXiv2022

Antiferromagnetic fluctuations and a dominant $d_{xy}$-wave pairing symmetry in nickelate-based superconductors

Motivated by recent experimental studies on superconductivity found in nickelate-based materials, we study the temperature dependence of the spin correlation and the superconducting pairing interaction within an effective two-band Hubbard model by the quantum Monte Carlo method. Based on parameters extracted from first-principles calculations, our intensive numerical results reveal that the pairing with a $d_{xy}$-wave symmetry firmly dominates over other pairings at low temperature, which is mainly determined by the Ni 3$d$ orbital. It is also found that the effective pairing interaction is enhanced as the on-site interaction increases, demonstrating that the superconductivity is driven by strong electron-electron correlation. Even though the $(π,π)$ antiferromagnetic correlation could be enhanced by electronic interaction, there is no evidence for long-range antiferromagnetic order exhibited in nickelate-based superconductors. Moreover, our results offer possible evidence that the pure electron correlation may not account for the charge density wave state observed in nickelates.

cond-mat.supr-con↗

arXiv2022

The competition between antiferromagnetism and superconductivity in a doped Hubbard model with anisotropic interaction

The competition between antiferromagnetism and superconductivity is one of the central questions in the research of strong correlated systems. In this work, we utilize a double layer model containing Hubbard interaction and interlayer Heisenberg interaction to reveal their competitions. This model is free of sign problem at certain conditions, and we perform projector quantum Monte Carlo simulations to extract the ground state correlations of magnetism and superconductivity. Our results shows that the superconductivity emerges when the antiferromagnetism is suppressed by tuning the filling or the anisotropy of the interlayer Heisenberg interaction. This model can be seen as an analogue of unconventional superconductors and may help us to understand the transition from an antiferromagnetic insulator to a superconductor.

cond-mat.str-el↗

arXiv2022

An intermediate phase induced by dilution in a correlated Dirac Fermi system

Substituting magnetic ions with nonmagnetic ions is a new way to study dilution. Using determinant quantum Monte Carlo calculations, we investigate an interacting Dirac fermion model with the on-site Coulomb repulsion being randomly zero on a fraction $x$ of sites. Based on conductivity, density of states and antiferromagnetic structure factor, our results reveal a novel intermediate insulating phase induced by the competition between dilution and repulsion. With increasing doping level of nonmagnetic ions, this nonmagnetic intermediate phase is found to emerge from the zero-temperature quantum critical point separating a metallic and a Mott insulating phase, whose robustness is proven over a wide range of interactions. Under the premise of strongly correlated materials, we suggest that doping nonmagnetic ions can effectively convert the system back to the paramagnetic metallic phase. This result not only agrees with experiments on the effect of dilution on magnetic order but also provides a possible direction for studies focusing on the metal-insulator transition in honeycomb lattice like materials.

cond-mat.str-el↗

arXiv2022

Spin triplet superconducting pairing in doped MoS$_2$

Searching for triplet superconductivity has been pursued intensively in a broad field of material science and quantum information for decades. Nevertheless, these novel states remain rare. Within a simplified effective three-orbital model, we reveal a spin triplet pairing in doped MoS$_2$ by employing both the finite temperature determinant quantum Monte Carlo approach and the ground state constrained-phase quantum Monte Carlo method. In a wide filling region of $\avg{n}=0.60-0.80$ around charge neutrality $\avg{n}=2/3$, the $f$-wave pairing dominates over other symmetries. The pairing susceptibility strongly increases as the on-site Coulomb interaction increases, and it is insensitive to spin-orbit coupling.

cond-mat.supr-con↗

arXiv2022

Transport anisotropy and metal-insulator transition in striped Dirac fermion systems

Using the determinant quantum Monte Carlo method, we investigate the metal-insulator transitions induced by the stripe of charge density in an interacting two-dimensional Dirac fermion system. The stripe will introduce the transport anisotropy and insulating intermediate phase into the system, accompanied by the change of band structure and a peak of density of states around Fermi energy. In the case of strong correlation, stripe exhibits competition with Coulomb repulsion through closing the energy gap and disrupting the magnetic order, and finally drives the system in Mott insulating phase back to the metallic state. Our results may provide a feasible way to modify transport properties by setting charge stripes in experiments.

cond-mat.str-el↗

arXiv2022

Metal-insulator transition in the disordered Hubbard model of the Lieb lattice

Using the determinant quantum Monte Carlo method, we investigate the metal-insulator transition in the interacting disordered Hubbard model of a Lieb lattice, in which the system characterizes the flat band centered at the Fermi level. By choosing suitable electron densities, we ensure the weak interaction sign problem to improve the reliability of our results. It is found that disorder and on-site Coulomb repulsive interaction produce interesting effects that induce the metal-insulator transition which is impossible in the half-filled case. The density of states at the Fermi energy is still finite in the thermodynamic limit, suggesting that the system is an Anderson insulator rather than a Mott insulator. Moreover, this doping system is paramagnetic, unlike the half-filled system, which is ferrimagnetic.

cond-mat.str-el↗

arXiv2022

Josephson radiation patterns in underdamped topological Josephson junctions

Josephson radiation is a useful signature for detecting Majorana zero modes in topological superconductors. We study the Josephson radiation of the underdamped topological Josephson junction within the quantum resistively and capacitively shunted junction model. We show that the quantum dynamics of the Majorana two-level system induce oscillatory patterns in the Josephson emission spectra. With the Floquet method, we obtain analytical results for these oscillatory patterns and find that they are well described by Bessel functions. We perform numerical simulations to verify the analytical results and demonstrate that these Bessel radiation patterns exist for a wide range of junction parameters.

cond-mat.supr-con↗

arXiv2022

Magnetic phase transition in disordered interacting Dirac fermion systems via the Zeeman field

Using the determinant quantum Monte Carlo method, we investigate the antiferromagnetic phase transition that is induced by the Zeeman field in a disordered interacting two-dimensional Dirac fermion system. At a fixed interaction strength $U$, the antiferromagnetic correlation is enhanced as the magnetic filed increases, and when the magnetic field is larger than a $B_{c}(U)$, the antiferromagnetic correlation shall be suppressed by the increased magnetic field. The impact of Zeeman field $B$, Coulomb repulsion $U$ and disorder $Δ$ is not isolated. The intensity of magnetic field effect on the antiferromagnetic correlation shall be strongly suppressed by disorder. Differently, it will be promoted by weak interaction, but when $U$ becomes larger than $U_{c}=4.5$, the increased interaction will suppress the intensity of this effect, and here $U_{c}=4.5$ coincides with the critical strength inducing the metal-Mott insulator transition in clean system. Moreover, at a fixed magnetic field $B$, strong interaction shall suppress the antiferromagnetic phase rather than promote it.

cond-mat.str-el↗

arXiv2022

Enhancement of $d$-wave pairing in the striped phase with the nearest neighbour attraction

Recently, the experimental results by the angle-resolved photoemission spectroscopy suggested that an additional strong nearest neighbor attraction in the Hubbard model might be significant to describe the properties of doped cuprates more accurately. The stripe-ordered patterns, formed by the inhomogeneous distribution of spin, charge and pairing correlations in the CuO$_{2}$ planes, is a known feature of doped cuprates. In this work, the effect of the nearest neighbor attraction and the stripe phase are examined by using the constrained path quantum Monte Carlo method within the repulsive Hubbard model on two-dimensional square lattice. The ground state spin correlations along and cross the stripe regions, and the $d$-wave pairing correlation are calculated. It is found that the spin-spin correlation is the highest when the interstripe region is fairly close to half-filling, and $d$-wave superconducting correlation on neighboring sites could be enhanced in the presence of stripe pattern and strong nearest neighbor attraction, which reveals their crucial roles on superconductivity in the doped cuprates.

cond-mat.str-el↗

arXiv2022

Phase diagram of the Hubbard model on a honeycomb lattice: A cluster slave-spin study

The cluster slave-spin method is implemented to research the ground state properties of the honeycomb lattice Hubbard model with doping $δ$ and coupling $U$ being its parameters. At half-filling, a single direct and continuous phase transition between the semi-metal and antiferromagnetic (AFM) insulator is found at $U_{\text{AFM}}=2.43t$ that is in the Gross-Neveu-Yukawa universality class, where a relation between the staggered magnetization $M$ and the AFM energy gap $Δ_{\text{AFM}}$ is established as $M \propto Δ_{\text{AFM}}$, compared to $M \propto Δ_{\text{AFM}} ( \ln{Δ_{\text{AFM}}})^2$ in the square lattice case. A first-order semi-metal to the underlying paramagnetic (PM) insulator Mott transition is corroborated at $U_{\text{Mott}}=8.36t$, which is responsible for a broad crossover around $U_{c} = 5.4t$ between the weak- and strong-coupling regimes in the AFM state that increases with $δ$, in contrast to the square lattice case. In the doped system, the compressibility $κ$ near the van Hove singularity at $δ=1/4$ is suppressed substantially by the interaction before the semi-metal to AFM transition occurs, whereas $κ$ near the Dirac points is very close to the noninteracting one, indicating that the Dirac cone structure of the energy dispersion is rather robust. An overall phase diagram in the $U$-$δ$ plane is presented, consisting of four regimes: the AFM insulator at $δ=0$ for $U> U_{\text{AFM}}$, the AFM metal with compressibility $κ>0$ or $κ<0$, and the PM semi-metal, and the AFM metal with $κ<0$ only exists in an extremely small area near the phase boundary between the AFM and PM state.

cond-mat.str-el↗

arXiv2022

Doping-dependent metal-insulator transition in a disordered Hubbard model

We study the effect of disorder and doping on the metal-insulator transition in a repulsive Hubbard model on a square lattice using the determinant quantum Monte Carlo method. First, with the aim of making our results reliable, we compute the sign problem with various parameters such as temperature, disorder, on-site interactions, and lattice size. We show that in the presence of randomness in the hopping elements, the metal-insulator transition occurs and the critical disorder strength differs at different fillings. We also demonstrate that doping is a driving force behind the metal-insulator transition.

cond-mat.str-el↗

arXiv2021

Metal-Insulator transition in strained Graphene: A quantum Monte carlo study

Motivated by the possibility of a strain tuning effect on electronic properties of graphene, the semimetal-Mott insulator transition process on the uniaxial honeycomb lattice is numerically studied using Determinant Quantum Monte Carlo. As our simulations are based on the half-filled repulsive Hubbard model, the system is sign problem free. Herein, the temperature-dependent DC conductivity is used to characterize electronic transport properties. The data suggest that metallic is suppressed in the presence of strain. More interestingly, within the finite-size scaling study, a novel antiferromagnetic phase arises at around $U\sim U_{c}$. Therefore, a phase diagram generated by the competition between interactions and strain is established, which may help to expand the application of strain effect on graphene.

cond-mat.str-el↗

arXiv2021

Phase diagram of the Hubbard model on a square lattice: A cluster slave-spin study

We employ the cluster slave-spin method to investigate systematically the ground state properties of the Hubbard model on a square lattice with doping $δ$ and coupling strength $U$ being its parameters. In addition to a crossover reflected in the behavior of the antiferromagnetic gap $Δ_{\text{AFM}}$, this property can also be observed in the energetics of the cluster slave-spin Hamiltonian -- the antiferromagnetism at small $U$ is due to the potential energy gain while that in the strong coupling limit is driven by the kinetic energy gain, which is consistent with the results from the cluster dynamical mean-field theory calculation and the quantum Monte Carlo simulation. We find the interaction $U_{c}$ for the crossover in the AFM state, separating the weak- and strong- coupling regimes, almost remains unchanged upon doping, and it is smaller than the critical coupling strength $U_{\text{Mott}}$ for the first-order metal-insulator Mott transition in the half-filled paramagnetic state. At half-filling, a relationship between the staggered magnetization $M$ and $Δ_{\text{AFM}}$ is established in the small $U$ limit to nullify the Hartree-Fock theory, and a first-order Mott transition in the paramagnetic state is substantiated, which is characterized by discontinuities and hystereses at $U_{\text{Mott}}=10t$. Finally, an overall phase diagram in the $U$-$δ$ plane is presented, which is composed of four regimes: the antiferromagnetic insulator, the antiferromagnetic metal with the compressibility $κ>0$ or $κ<0$, and the paramagnetic metal, as well as three phase transitions: (i) From the antiferromagnetic metal to the paramagnetic metal, (ii) between the antiferromagnetic metal phases with positive and negative $κ$, and (iii) separating the antiferromagnetic insulating phase from the antiferromagnetic metal phase.

cond-mat.str-el↗

arXiv2021

Inducing a metal-insulator transition in disordered interacting Dirac fermion systems via an external magnetic field

We investigate metal-insulator transitions on an interacting two-dimensional Dirac fermion system using the determinant quantum Monte Carlo method. The interplay between Coulomb repulsion, disorder and magnetic fields, drives the otherwise semi-metallic regime to insulating phases exhibiting different characters. In particular, with the focus on the transport mechanisms, we uncover that their combination exhibits dichotomic effects. On the one hand, the critical Zeeman field $B_c$, responsible for triggering the band-insulating phase due to spin-polarization on the carriers, is largely reduced by the presence of the electronic interaction and quenched disorder. On the other hand, the insertion of a magnetic field induces a more effective localization of the fermions, facilitating the onset of Mott or Anderson insulating phases. Yet these occur at moderate values of $B$, and cannot be explained by the full spin-polarization of the electrons.

cond-mat.str-el↗