arXiv subjects

Can Shao

Publications and source records attributed to Can Shao.

17 recordsLinked to original sources

arXiv2026

Effect of reaction temperature on nascent carbonaceous particles from toluene shock-tube pyrolysis: Insights from FTIR and Raman spectroscopy

The transition from gaseous precursors to nascent solid particles and their subsequent structural maturation were investigated in single-pulse shock-tube experiments using ex situ Fourier-transform infrared (FTIR) and Raman spectroscopy of sampled products. A mixture of 2% toluene in argon was pyrolyzed at around 2.0 bar with temperature plateau times of 2.0 ms over the 1450-1800 K reaction temperature range. In situ laser extinction measurements indicate the onset of particle formation at 1570 K. At this temperature, Raman spectra exhibit emerging D and G bands, and transmission electron microscopy (TEM) reveals the disappearance of poorly defined structures, identifying 1570 K as the phase-transition reaction temperature. Approaching this reaction temperature, Raman spectra show a rapid disappearance of sp hybridized triple carbon bonds. At 1670 K reaction temperature, a maximum in primary particle diameter and a decrease in structural disorder inferred from Raman spectroscopy are observed, defining the ordering threshold. Deconvolution of the FTIR spectra enables separation of in ring double carbon bond stretching vibrations from isolated and ring-conjugated side-chain double carbon bond modes. The in-ring double carbon band is used to normalize aliphatic and aromatic C-H vibrations. FTIR analysis reveals ring-edge structures associated with electron-localization sites, including bay regions, five-membered ring defects, and benzylic positions, indicating a radical-rich environment below the phase-transition temperature. Between the phase-transition and ordering-threshold temperatures, K-regions and armchair structures associated with electron delocalization and thermal stability increase. The emergence of these electronic and structural characteristics highlights the critical role of radicals in soot inception and early structural ordering.

physics.chem-ph↗

arXiv2026

The antiferromagnetic Chern insulator phase in the Kane-Mele-Hubbard model

The emergence of the antiferromagnetic (AFM) Chern insulator (AFCI) phase in the Kane-Mele-Hubbard (KMH) model with a finite sublattice potential is investigated. The AFCI, characterized by AFM correlations coexisting with quantized Hall conductance, has long raised the question of whether it can exist in the KMH model that respects time-reversal symmetry (TRS). Using exact diagonalization, we analyze the excitation gap, anisotropic AFM correlations along the $z$ axis and in the $xy$ plane, and the fidelity susceptibility under twisted boundary conditions, all of which provide consistent evidence for the AFCI phase. In particular, our numerical evaluation on the (spin) Chern number reveals a breakdown of adiabatic continuity in the twist-angle space, indicating an instability toward TRS breaking driven by Hubbard-induced AFM perturbations. A modified computational scheme is further proposed, which yields a robust quantized Chern number $C=1$ within this phase.

cond-mat.str-el↗

arXiv2024

The single-particle spectral function of the extended Peierls-Hubbard model at half-filling and quarter-filling

By utilizing the twisted boundary conditions in the exact diagonalization method, we investigate the single-particle spectral function of the extended Peierls-Hubbard model at both half-filling and quarter filling. In one-dimensional (1D) interacting systems, the spin-charge separation can typically be identified in the single-particle spectral function by observing the distinct spinon and holon bands. At half filling, starting from the pure 1D Hubbard model with the on-site interaction $U=10$, we observe that the band structure indicative of the spin-charge separation gradually transitions to four individual bands as the Peierls instability $\delta$ increases. At $U=10$ and $\delta=0.2$ where the spin-charge separation is still observable, increasing the nearest-neighbor interaction $V$ can drive the system to a charge-density-wave (CDW) state when $V\gtrsim U/2$, without the obeservation of spinon and holon bands. At quarter-filling, on the other hand, the ground state of Peierls-Hubbard model manifests an antiferromagnetic Mott insulator in units of dimers. Increasing $U$ results in only a very small gap in the single-particle spectrum because even for $U=+\infty$, with the model transforming into a noninteracting half-filled dimerized tight-binding model, its gap determined by the Peierls instability $\delta$ remains small. Conversely, increasing $V$ can effectively open the single-particle gap and make the spinon and holon bands more prominent.

cond-mat.str-el↗

arXiv2024

Topological phase in the extended Haldane-Hubbard model with sublattice-dependent repulsion

We study the ground-state phase diagram of the half-filled extended Haldane-Hubbard model on the honeycomb lattice with sublattice-dependent on-site repulsion ($U_{\text{A/B}}$) using the exact diagonalization (ED) and mean-field (MF) methods. The resulting phase diagram shows that there is a topologically nontrivial phase with the Chern number $C=1$, emerging via the development of the imbalance between $U_{\text{A}}$ and $U_{\text{B}}$. In this phase, the antiferromagnetic correlations are observed in the ED calculation, in line with the finite antiferromagnetic order obtained by the MF method. The spontaneous symmetry breaking of SU(2) spin rotation in the phase is also identified in the MF level. Distinct from previous studies in which the exotic $C=1$ phase relies on the interplay between sublattice-dependent potentials and electronic interactions, our paper presents an alternative way by solely tuning the on-site interactions.

cond-mat.str-el↗

arXiv2024

Photoinduced phase switching from Mott insulator to metallic state in the quarter-filled Peierls-Hubbard model

Utilizing the exact diagonalization method, we investigate the one-dimensional Peierls-Hubbard model at quarter filling, where it manifests as an antiferromagnetic Mott insulator in units of dimers. By increasing the on-site Coulomb repulsion U, we observe a significant suppression of the Drude peak, based on a nonequilibrium linear response theory capable of capturing the zero-frequency (Drude) weight of the optical conductivity under periodic boundary conditions. However, after the ultrafast photoirradiation of this model with large U, we detect a distinct enhancement of the Drude peak, signifying the onset of a photoinduced insulator-metal transition. Comparing these dynamics with the half-filled Hubbard model and a noninteracting spinless half-filled Su-Schrieffer-Heeger model (corresponding to the quarter-filled Peierls-Hubbard model with infinite U), we propose a novel mechanism for the photoinduced metallic state: the empty-occupied and double-occupied dimers serve as the photoinduced charge carriers, akin to the holons and doublons in Hubbard model.

cond-mat.str-el↗

arXiv2024

Phase diagram of the interacting Haldane model with spin-dependent sublattice potentials

Using the exact-diagonalization (ED) and mean-field (MF) approaches, we investigate the ground-state phase diagram of the interacting Haldane model on the honeycomb lattice, incorporating spin-dependent sublattice potentials $\Delta_{\sigma,\alpha}$. Here $\alpha=\text{A}$,$\text{B}$ and $\sigma=\uparrow$,$\downarrow$ denote the sublattice and spin components, respectively. Setting $\Delta_{\sigma,\text{A}}=+\Delta$ ($-\Delta$) and $\Delta_{\sigma,\text{B}}$$=-\Delta$ ($+\Delta$) for $\sigma=\uparrow$ ($\downarrow$) results in the system favoring a spin ordered state. Conversely, introducing the nearest-neighbor Coulomb interaction can induce charge ordering in the system. Due to the competition between these factors, we observe that in both ED and MF approaches, an exotic state with Chern number $C=1$ survives amidst two locally ordered phases and a topologically ordered phase with $C=2$. In the ED method, various properties, such as the fidelity metric, the excitation gap and the structure factors, are employed to identify critical points. In the MF method, using a sufficiently large lattice size, we define the local order parameters and band gaps to characterize the phase transitions. The interacting Haldane model and the spin-dependent lattice potential may be experimentally realized in an ultracold atom gas, providing a potential means to detect this intriguing state.

cond-mat.str-el↗

arXiv2023

Time-resolved single-particle spectrum of the one-dimensional extended Hubbard model after interaction quenches

We investigate the non-equilibrium dynamics of the one-dimensional extended Hubbard model after interaction quenches. In strong-coupling regime with large on-site interaction, the ground states of this model with small and large nearest-neighbor interactions are in spin-density-wave and charge-density-wave phases, respectively. Combining twisted boundary conditions with the time-dependent Lanczos method, we obtain snapshots of the time-dependent single-particle spectrum after quenches. We find that for quench within the same phase, the single-particle spectrum becomes close to that of the quenched Hamiltonian immediately after the quench. While for quench across the critical point, the afterward evolution process depends mainly on the distribution of the initial state among the eigenstates of the quenched Hamiltonian. Our finding may serve as a way to detect the phase transition in ultracold atom systems with interactions.

cond-mat.str-el↗

arXiv2022

Phase transitions in the Haldane-Hubbard model with ionic potentials

By employing the exact-diagonalization method, we revisit the ground-state phase diagram of the Haldane-Hubbard model on the honeycomb lattice with staggered sublattice potentials. The phase diagram includes the band insulator, Mott insulator, and two Chern insulator phases with Chern numbers C=2 and C=1, respectively. The character of transitions between different phases is studied by analyzing the lower-lying energy levels, excitation gaps, structure factors, and fidelity metric. We find that the C=1 phase can be continuously deformed into the C=2 phase without a gap closure in the periodic boundary condition, while a further analysis on the Berry curvatures indicates that the excitation gap closes at the phase boundary in a twisted boundary condition, accompanied by the discontinuities of structure factors. All the other phase transitions are found to be first-order ones as expected.

cond-mat.str-el↗

arXiv2021

Photoinduced anomalous Hall effect in the interacting Haldane model: targeting topological states with pump pulses

We investigate the nonequilibrium dynamics of the spinless Haldane model with nearest-neighbor interactions on the honeycomb lattice by employing an unbiased numerical method. In this system, a first-order transition from the Chern insulator (CI) at weak coupling to the charge-density-wave (CDW) phase at strong coupling can be characterized by a level crossing of the lowest energy levels. Here we show that adiabatically following the eigenstates across this level crossing, their Chern numbers are preserved, leading to the identification of a topologically-nontrivial low-energy excited state in the CDW regime. By promoting a resonant energy excitation via an ultrafast circularly polarized pump pulse, we find that the system acquires a non-vanishing Hall response as a result of the large overlap enhancement between the time-dependent wave-function and the topologically non-trivial excited state. This is suggestive of a photoinduced topological phase transition via unitary dynamics, despite a proper definition of the Chern number remaining elusive for an out-of-equilibrium interacting system. We contrast these results with more common quench protocols, where such features are largely absent in the dynamics even if the post-quench Hamiltonian displays a topologically nontrivial ground state.

cond-mat.str-el↗

arXiv2021

High-harmonic generation approaching the quantum critical point of strongly correlated systems

By employing the exact diagonalization method, we investigate the high-harmonic generation (HHG) of the correlated systems under the strong laser irradiation. For the extended Hubbard model on a periodic chain, HHG close to the quantum critical point (QCP) is more significant compared to two neighboring gapped phases (i.e., charge-density-wave and spin-density wave states), especially in low-frequencies. We confirm that the systems in the vicinity of the QCP are supersensitive to the external field and more optical-transition channels via excited states are responsible for HHG. This feature holds the potential of obtaining high-efficiency harmonics by making use of materials approaching to QCP. Based on two-dimensional Haldane model, we further propose that the even- or odd-order components of generated harmonics can be promisingly regarded as spectral signals to distinguish the topologically ordered phases from locally ordered ones. Our findings in this work pave the way to achieve ultrafast light source from HHG in strongly correlated materials and to study quantum phase transition by nonlinear optics in strong laser fields.

cond-mat.str-el↗

arXiv2021

Interplay of local order and topology in the extended Haldane-Hubbard model

We investigate the ground-state phase diagram of the spinful extended Haldane-Hubbard model on the honeycomb lattice using an exact-diagonalization, mean-field variational approach, and further complement it with the infinite density matrix renormalization group, applied to an infinite honeycomb cylinder. This model, governed by both on-site and nearest-neighbor interactions, can result in two types of insulators with finite local order parameters, either with spin or charge ordering. Moreover, a third one, a topologically nontrivial insulator with nonlocal order, is also manifest. We test expectations of previous analyses in spinless versions asserting that once a local order parameter is formed, the topological characteristics of the ground state, associated with a finite Chern number, are no longer present, resulting in a topologically trivial wave function. Our study confirms this overall picture, and highlights how finite-size effects may result in misleading conclusions on the coexistence of finite local order parameters and nontrivial topology in this model.

cond-mat.str-el↗

arXiv2020

Analysis of time-resolved single-particle spectrum on the one-dimensional extended Hubbard model

We investigate the short-time evolution of the half filled one-dimensional extended Hubbard model in the strong-coupling regime, driven by a transient laser pump. Combining twisted boundary conditions with the time-dependent Lanczos technique, we obtain snapshots of the single-particle spectral function with high momentum resolution. The analysis of the oscillations of the spectral function shows that its characteristic frequencies are consistent with the magnitudes of the optical gap. Furthermore, we examine the time-evolving spectral structure in the charge-density-wave phase in detail and find that one of the bands in the single-particle spectrum originates from the photoinduced bond-order background.

cond-mat.str-el↗

arXiv2019

Photoinduced enhancement of bond-order in the one-dimensional extended Hubbard model

We investigate the real-time dynamics of the half-filled one-dimensional extended Hubbard model in the strong-coupling regime, when driven by a transient laser pulse. Starting from a wide regime displaying a charge-density wave in equilibrium, a robust photoinduced in-gap state appears in the optical conductivity, depending on the parameters of the pulse. Here, by tuning its conditions, we maximize the overlap of the time-evolving wavefunction with excited states displaying the elusive bond-ordered wave of this model. Finally, we make a clear connection between the emergence of this order and the formation of the aforementioned in-gap state, suggesting the potential observation of purely electronic (i.e., not associated with a Peierls instability) bond-ordered waves in experiments involving molecular crystals.

cond-mat.str-el↗

arXiv2019

Photoinduced charge carrier dynamics in Hubbard two-leg ladders and chains

The charge carrier dynamics of doped electronic correlated systems on ladders and chains, subject to ultrafast photoirradiation, is investigated using the time-dependent Lanczos method. The time-resolved optical conductivity and the temporal profiles of other relevant quantities, including the doublon number, the kinetic energy, and the interaction energy, are calculated. Two competitive factors that can influence the transient charge carrier dynamics are identified as the thermal effect and the charge effect. We demonstrate that the analysis of their interplay can provide an intuitive way to understand the numerical results and the recent optical pump-probe experiment on a two-leg ladder cuprate.

cond-mat.str-el↗

arXiv2016

Numerical method to compute optical conductivity based on pump-probe simulations

A numerical method to calculate optical conductivity based on a pump-probe setup is presented. Its validity and limits are tested and demonstrated via the concrete numerical simulations on the half-filled one-dimensional extended Hubbard model both in equilibrium and out of equilibrium. By employing either a step- or a Gaussian-like probing vector potential, it is found that in nonequilibrium, the method in the narrow-probe-pulse limit can be identified with variant types of linear response theory, which, in equilibrium, produce identical results. The observation reveals the underlying probe-pulse dependence of the optical conductivity calculations in nonequilibrium, which may have its applications in the theoretical analysis of ultrafast spectroscopy measurements.

cond-mat.str-el↗

arXiv2015

Superfluid response in heavy fermion superconductors

Motivated by recent London penetration depth measurement [H. Kim et al. Phys. Rev. Lett. \textbf{114}, 027003 (2015)] and novel composite pairing scenario [O. Erten, R. Flint and P. Coleman, Phys. Rev. Lett. \textbf{114}, 027002 (2015)] on Yb-doped heavy fermion superconductor CeCoIn$_{5}$, we revisit the issue of superfluid response in microscopic heavy fermion lattice model. However, it is found that in literature explicit expression of superfluid response function in heavy fermion superconductor is rare. In this paper, we make a contribution to this issue by investigating superfluid density response function in celebrated Kondo-Heisenberg model. To be specific, we derive corresponding formalism from an effective fermionic large-N mean-field pairing Hamiltonian, whose pairing interaction is assumed to originate from effective local antiferromagnetic exchange interaction. Interestingly, it is found that physically correct superfluid density formula can only be obtained if external electromagnetic field is directly coupled to heavy fermion quasi-particle. Such unique feature emphasizes the key role of Kondo-screening-renormalized quasi-particle for low-temperature/energy thermodynamics and transport behaviors. As an important application, the theoretical result is compared to experimental measurement in heavy fermion superconductor CeCoIn$_{5}$ and Yb-doped Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$, where the agreement is fairly good and the transition of pairing symmetry in the latter one is explained as a simple doping effect. In addition, the requisite formalism for the commonly encountered nonmagnetic impurity and non-local electrodynamic effect are developed.

cond-mat.supr-con↗

arXiv2015

Photoinduced in-gap excitations in the one-dimensional extended Hubbard model

We investigate the time evolution of optical conductivity in the half-filled one-dimensional extended Hubbard model driven by a transient laser pulse, by using the time-dependent Lanczos method. Photoinduced in-gap excitations exhibit a qualitatively different structure in the spin-density wave (SDW) in comparison to the charge-density-wave (CDW) phase. In the SDW, the origin of a low-energy in-gap excitation is attributed to the even-odd parity of the photoexcited states, while in the CDW an in-gap state is due to confined photogenerated carriers. The signature of the in-gap excitations can be identified as a characteristic oscillation in the time evolution of physical quantities.

cond-mat.str-el↗