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Doublon-holon dynamics is investigated in a pumped one-dimensional Hubbard model with a staggered on?site Coulomb interaction at half-filling. When the system parameters are set to be in the Mott insulating regime the equilibrium sublattice density of states exhibits several characteristic peaks, corresponding to the lower and upper Hubbard bands as well as hybridization bands. We study the linear absorption spectrum and find two main peaks characterizing the photon frequencies which excite the ground state to an excited state. For a system driven by a laser pulse with general intensity and frequency, both the energy absorption and the doublon-holon dynamics exhibit distinct behaviors as a function of laser amplitude and frequency. Single-photon processes are observed at low laser intensity where the energy is absorbed for resonance laser frequencies. For strong laser intensity multi-photon induced dynamics are observed in the system, which are confirmed by an evaluation of the Loschmidt amplitude. The contribution of multi-photon processes to site-specific double occupancy is also characterized by the generalized Loschmidt amplitude. The site-selective doublon-holon dynamics are observed in both the one and multi-photon processes and the site-selective behavior is explained within a quasiparticle picture. Our study suggests strategies to optically engineer the doublon-holon dynamics in one dimensional strongly correlated many-body systems., Comment: 10 pages, 7 figures

Using the time-dependent Lanczos method, we study the non-equilibrium dynamics of the one-dimensional ionic-mass imbalanced Hubbard chain driven by a quantum quench of the on-site Coulomb interaction, where the system is prepared in the ground state of the Hamiltonian with a different Hubbard interaction. A full exact diagonalization is adopted to study the zero temperature phase diagram in equilibrium, which is shown to be in good agreement with previous studies using density matrix renormalization group (DMRG). We then study the non-equilibrium quench dynamics of the spin and charge order parameters by fixing the initial and final Coulomb interaction while changing the quenching time protocols. The Lanczos method allows us to reach longer times following the quench than DMRG. Our study shows that the time evolution of the charge and spin order parameters strongly depend on the quenching time protocols. In particular, the effective temperature of the system will decrease monotonically as the quenching time is increased. Finally, by taking the final Coulomb interaction strength to be in the strong coupling regime, we find that the oscillation frequency of the charge order parameter increases monotonically with the Coulomb interaction. By contrast, the frequency of the spin order parameter decreases monotonically with increasing Coulomb interaction. We explain this result using an effective spin model in the strong coupling limit. Our study suggests strategies to engineer the relaxation behavior of interacting quantum many-particle systems., Comment: 9 pages, 6 figures

Using constrained-path quantum Monte Carlo method, we systematically study the Hubbard model on AA-stacked honeycomb lattices with electric field. Our simulation demonstrates a dominant chiral d+id wave pairing induced by the electric field at half filling. In particular, as the on-site Coulomb interaction increases, the effective pairing correlation of chiral d+id superconducting state exhibits increasing behavior. We attribute the electric field induced d+id superconductivity to an increased density of states near the Fermi energy and an suppressed antiferromagnetic spin correlation after turning on the electric field. Our results strongly suggest the AA-stacked graphene system with electric field is a good candidate for chiral d+id superconductors.

The potassium-doped p-terphenyl compounds were synthesized in recent experiments and the superconductivity with high transition temperatures were reported, but the atomic structure of potassium-doped p-terphenyl is unclear. In this paper, we studied the structural and electronic properties of potassium-doped p-terphenyl with various doping levels by the first-principles simulation. We first find out the low energy position of K atom in intralayer interstitial space of the molecular layer, then examine whether two rows of K atoms can be accommodated in this one space, at last the effect of the interlayer arrangement between adjacent two molecular layers on total energy is taken into account. Our results show that the doped K atoms prefer to stay at the bridge site of single C-C bond connected two phenyls instead of locating at the site above the phenyl ring, distinct from the situation of K-doped picene and phenanthrene. Among the possible structural phases of Kx-p-terphenyl, the K2-p-terphenyl phase with P212121 group symmetry is determined to be most appropriate, which is different from the one in recent report. The stable K 2 -p-terphenyl phase is semiconducting with an energy gap of 0.3 eV and the bands from the lowest unoccupied molecular orbitals are just fully filled by the electrons transferred from K atoms.

To understand the interplay between nematic fluctuation and superconductivity in iron-based superconductors, we performed a systematic study of the realistic two-orbital Hubbard model by using the constrained-path quantum Monte Carlo method. Our numerical results showed that the on-site nematic interaction induces a strong enhancement of nematic fluctuations at various momentums, especially at ($\pi$,$\pi$). Simultaneously, it was found that the on-site nematic interaction suppresses the ($\pi$,0)/(0,$\pi$) antiferromagnetic order and long-range electron pairing correlations for dominant pairing channels in iron-based superconductors. Our findings suggest that nematic fluctuation seems to compete with superconductivity in iron-based superconductors., Comment: 6 figures

We reveal an edge spin triplet $p-$wave superconducting pairing correlation in slightly doped zigzag graphene nanoribbons. By employing a method that combines random-phase approximation, the finite-temperature determinant quantum Monte Carlo approach, and the ground-state constrained-path quantum Monte Carlo method, it is shown that such a spin-triplet pairing is mediated by the ferromagnetic fluctuations caused by the flat band at the edge. The spin susceptibility and effective pairing interactions at the edge strongly increase as the on-site Coulomb interaction increases, indicating the importance of electron-electron correlations. It is also found that the doping-dependent ground-state $p$-wave pairing correlation bears some similarity to the famous superconducting dome in the phase diagram of a high-temperature superconductor, while the spin correlation at the edge is weakened as the system is doped away from half filling., Comment: Accepted for publication in Scientific Reports. The whole language is improved

By the first principles calculations and X-ray diffraction simulations, we perform a systematic study of multiple superconducting phases of potassium-doped picene [Nature {\bf464} 76, 2010]. The combination of optimized lattice parameters, formation energy, and simulated X-ray diffraction spectra indicates that the superconducting phase with transition temperature (T$_c$) of 7 K corresponds to a charge doped K$_2$picene, which is semiconducting with a 0.1 eV energy gap, while the superconducting phase with T$_c$=18 K corresponds to a metallic K$_3$picene. The distinct crystal and electronic structures of K$_2$picene and K$_3$picene provide a reasonable explanation of two superconducting phases in potassium-doped picene.

A systemically theoretical study has been presented to explored the crystal structures and electronic characteristics of polycyclic aromatic hydrocarbons (PAHs), such as solid phenanthrene, picene, 1,2;8,9-dibenzopentacene, and 7-phenacenes, since these PAHs exhibited the superconductivity when potassium doping into. For tripotassium-doped phenanthrene and picene, we demonstrate the K atomic positions to fit the experimental lattice parameters, and analyze the distinction between the stablest configuration and the fitted experimental one. Based on the first-principles calculations, for the first time, we predict the possible crystal configurations of pristine and tripotassium-doped 1,2;8,9-dibenzopentacene and 7-phenacenes, respectively. For these four PAHs, the electronic structures after doping are investigated in details. The results show that the electronic characters near the Fermi level are high sensitive to structure. Because of the change of the benzene rings arrangement, the 1,2;8,9-dibenzopentacene exhibits visibly different band structures from other three PAHs. In these metallic PAHs, two bands cross the Fermi level which results in the complicated multiband feature of Fermi surfaces. Fascinatingly, we find that the electronic states of potassium contribute to the Fermi surfaces especially for K-3$d$ electrons, which improves a way to understand this superconductivity. As a result, we suggest that the rigid-band picture is invalidated due to the hybridization between K atoms and PAH molecules as well as the rearrangement and distortion of PAH molecules., Comment: This paper has been withdrawn by the author due to some mistakes in the section of discussion

We systematically investigate the crystal structure of Ba-doped phenanthrene with various Ba doping levels by the first principle calculation method combining with the X-ray diffraction (XRD) spectra simulation. The optimized lattice parameters and the simulated XRD spectra of Ba$_2$phenanthrene are in good agreement with the experiment ones, and the strength difference of a few of XRD peaks can be explained by the existence of undoped phenanthrene in the experimental sample. Although the stoichiometry ratio of Ba atom and phenanthrene molecule is 1.5:1, the simulated XRD spectra, space group symmetry and the optimized lattice parameters of Ba$_{1.5}$phenanthrene are not consistent to the measured values. The sample of Ba-doped phenanthrene is deduced to be the mixture of Ba$_2$phenanthrene and some amount of undoped phenanthrene, instead of uniform Ba$1.5$phenanthrene. Our calculations indicate that Ba$_2$phenanthrene is a semiconductor with a small energy gap less than 0.05 eV. Our findings provide the fundamental information of crystal structure and electronic properties of Ba-doped phenanthrene superconductor.

By the first principles calculations based on the van der Waals density functional theory, we study the crystal structures and electronic properties of La-doped phenanthrene. Two stable atomic geometries of La$_1$phenanthrene are obtained by relaxation of atomic positions from various initial structures. The structure-\uppercase\expandafter{\romannumeral1} is a metal with two energy bands crossing the Fermi level, while the structure-\uppercase\expandafter{\romannumeral2} displays a semiconducting state with an energy gap of 0.15 eV, which has an energy gain of 0.42 eV per unit cell compared to the structure-\uppercase\expandafter{\romannumeral1}. The most striking feature of La$_1$phenanthrene is that La $5d$ electrons make a significant contribution to the total density of state (DOS) around the Fermi level, which is distinct from potassium doped phenanthrene and picene. Our findings provide an important foundation for the understanding of superconductivity in La-doped phenanthrene., Comment: Accepted by Journal of Chemical Physics

To address the issue of electron correlation driven superconductivity in graphene, we perform a systematic quantum Monte Carlo study of the pairing correlation in the t-U-V Hubbard model on a honeycomb lattice. For V=0 and close to half filling, we find that pairing with d+id symmetry dominates pairing with extended-s symmetry. However, as the system size or the on-site Coulomb interaction increases, the long-range part of the d+id pairing correlation decreases and tends to vanish in the thermodynamic limit. An inclusion of nearest-neighbor interaction V, either repulsive or attractive, has a small effect on the extended-s pairing correlation, but strongly suppresses the d+id pairing correlation., Comment: 5 pages, 5 figures

We systematically study magnetic correlations in graphene within Hubbard model on a honeycomb lattice by using quantum Monte Carlo simulations. In the filling region below the Van Hove singularity, the system shows a short-range ferromagnetic correlation, which is slightly strengthened by the on-site Coulomb interaction and markedly by the next-nearest-neighbor hopping integral. The ferromagnetic properties depend on the electron filling strongly, which may be manipulated by the electric gate. Due to its resultant controllability of ferromagnetism, graphene-based samples may facilitate the development of many applications., Comment: Published version in Applied Physics Letters

The frequency- and temperature-dependent optical conductivity of the copper oxide materials in the underdoped and optimal doped regimes are studied within the t-J model. The conductivity spectrum shows the unusual behavior at low energies and anomalous midinfrared peak in the low temperatures. However, this midinfrared peak is severely depressed with increasing temperatures, and vanishes at higher temperatures., Comment: 11 pages, Revtex, Two figures are not included, and can be obtained by request

The spin dynamics at the finite temperature for the t-J model in the underdoped and optimal doped regimes is studied within the fermion-spin theory. It is shown that the dynamical spin structure factor spectrum at the antiferromagnetic wave vector $Q=(\pi,\pi)$ are separated as low- and high-frequency parts, respectively, but the high-frequency part is suppressed in the dynamical susceptibility spectrum $\chi^{''}(Q,\omega)$, while the low-frequency part is the temperature dependent, which are in qualitative agreement with the experiments and numerical simulations., Comment: 3 pages, three figures are added

The spin response in the copper oxide materials at finite temperatures in the underdoped and optimal doped regimes is studied within the framework of the fermion-spin theory. The integrated dynamical spin structure factor is almost temperature independent, the integrated susceptibility shows the particularly universal behavior as $I(\omega, T) \propto arctan [ a_{1}\omega/T + a_{3}(\omega/T)^{3}]$, and the spin-lattice relaxation time is weakly temperature dependent, which are consistent with experiments and numerical simulations., Comment: 4 pages, four figures are added

Within the $t$-$J$ model, we study the doping dependence of antiferromagnetism in the copper oxide materials by considering quantum fluctuations of spinons in the random-phase-approximation. The staggered magnetization vanishes around doping $\delta=5%$ for a reasonable parameter value $t/J=5$, which is in agreement with the experiments on copper oxide materials., Comment: 14 pages

The charge dynamics of the copper oxide materials in the underdoped and optimal doped regimes is studied within the framework of the fermion-spin theory. The conductivity spectrum shows the non-Drude behavior at low energies and unusual midinfrared peak, and the resistivity exhibits a linear behavior in the temperature, which are consistent with experiments and numerical simulations., Comment: 10 pages, the figures are not included and can be air-mailed by request