Zhongbing Huang

Controllability of ferromagnetism in graphene

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.

Magnetic behavior of Fe(Se,Te) systems: First-principles calculations

The magnetic behaviors in Fe(Se,Te) systems have been investigated systematically using density functional calculations. At the experimental lattice parameters, the ground state is found to be in the double stripe magnetic phase for FeTe but in the single stripe magnetic phase for FeSe and FeSe0.5Te0.5, and there is no preference in the different easy axes of magnetization. Substitution of Se by Te enlarges the size of the Fermi surface in FeSe0.5Te0.5, resulting in a stronger nesting effect and thus enhancing the superconductivity. It is found that the double stripe order in FeTe1-xSex changes to the single stripe order when x> 0.18. Spiral calculations on FeSe0.5Te0.5 show that the lowest energy is at the commensurate point Q→= (0.5,0.5), accompanied by additional local minima at two incommensurate points near Q→= (0.5,0.5). This observation is consistent with the experimentally observed positions of low energy magnetic excitations. Geometry optimization calculations show that the tetragonal cell relaxe...

Magnetic instability and pair binding in aromatic hydrocarbon superconductors.

Understanding magnetism and electron correlation in many unconventional superconductors is essential to explore mechanism of superconductivity. In this work, we perform a systematic numerical study of the magnetic and pair binding properties in recently discovered polycyclic aromatic hydrocarbon (PAH) superconductors including alkali-metal-doped picene, coronene, phenanthrene, and dibenzopentacene. The π-electrons on the carbon atoms of a single molecule are modelled by the one-orbital Hubbard model, and the energy difference between carbon atoms with and without hydrogen bonds is taking into account. We demonstrate that the spin polarized ground state is realized for charged molecules in the physical parameter regions, which provides a reasonable explanation of local spins observed in PAHs. In alkali-metal-doped dibenzopentacene, our results show that electron correlation may produce an effective attraction between electrons for the charged molecule with one or three added electrons.

Pairing in graphene: A quantum Monte Carlo study

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.

Antiferromagnetism in Potassium-Doped Polycyclic Aromatic Hydrocarbons

First-principles density functional calculations are performed to investigate the magnetic characteristics in K-doped polycyclic aromatic hydrocarbons (PAHs) including phenanthrene, picene, 1,2:5,6-dibenzanthracene, 7-phenacene, and 1,2;8,9-dibenzopentacene. With the help of lowest energy crystal structures, the calculated total energies indicate that all five K-doped PAHs are stabilized in an antiferromagnetic ground state, with antiparallel spins between two molecular layers. The magnetic moment in these K-doped PAHs is increased with the increase of benzene rings number, while it is not sensitive to the arrangement of benzene rings. The enhancement of the magnetic moment is caused by a stronger spin splitting near the Fermi level and an increase of magnetic C atoms induced by K atoms with the increase of molecular size. Our results also indicate that the magnetism strongly depends on the crystal structure.

First-principles investigations on the magnetic property in tripotassium doped picene

First-principles calculations are performed to investigate the magnetic characteristics in the tripotassium doped picene, especially for the effects induced by the volume variations. When changing volume, both crystal lattice constants and atomic positions are optimized. For the system with the experimental crystal volume, the doped picene shows a weak antiferromagnetic instability. When the volume expands from this experimental crystal volume, the antiferromagnetic spin ordering becomes clear. The electronic structures show that the magnetism comes from the spin unbalance on the π orbitals of the C atoms. On the contrary, both ferromagnetic and antiferromagnetic spin orderings are strongly suppressed as the volume is reduced. Our results indicate that the magnetism is sensitive to the variation of volume or pressure in the tripotassium doped picene. No metal-insulator transition is observed for several considered volumes.

Van der Waals density functional study of the structural and electronic properties of La-doped phenanthrene

By the first principle 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 La₁phenanthrene are obtained by relaxation of atomic positions from various initial structures. The structure-I is a metal with two energy bands crossing the Fermi level, while the structure-II 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-I. The most striking feature of La₁phenanthrene is that La 5d electrons make a significant contribution to the total density of state 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.

Ba2phenanthrene is the main component in the Ba-doped phenanthrene superconductor

We have systematically investigated the crystal structure of Ba-doped phenanthrene with various Ba doping levels by the first-principles calculations combined with the X-ray diffraction (XRD) spectra simulations. Although the experimental stoichiometry ratio of Ba atom and phenanthrene molecule is 1.5:1, the simulated XRD spectra, space group symmetry and optimized lattice parameters of Ba1.5phenanthrene are not consistent with the experimental ones, while the results for Ba2phenanthrene are in good agreement with the measurements. The strength difference of a few XRD peaks can be explained by the existence of pristine phenanthrene. Our findings suggest that instead of uniform Ba1.5phenanthrene, there coexist Ba2phenanthrene and undoped phenanthrene in the superconducting sample. The electronic calculations indicate that Ba2phenanthrene is a semiconductor with a small energy gap less than 0.05 eV.

Pairing, charge, and spin correlations in the three-band Hubbard model

Using the Constrained Path Monte Carlo (CPMC) method, we simulated the two-dimensional, three-band Hubbard model to study pairing, charge, and spin correlations as a function of electron and hole doping and the Coulomb repulsion

Stability of the high-spin ground state in the Peierls-extended Hubbard model

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