Da-Yong Liu

Hydrostatic pressure induced three-dimensional Dirac semimetal in black phosphorus

Motivated by recent experimental observation of an hydrostatic pressure induced transition from semiconductor to semimetal in black phosphorus [Chen et al. in arXiv:1504.00125], we present the first principles calculation on the pressure effect of the electronic structures of black phosphorus. It is found that the band crossover and reversal at the Z point occur around the critical pressure Pc1=1.23 Gpa, and the band inversion evolves into 4 twofold-degenerate Dirac cones around the Z point, suggesting a 3D Dirac semimetal. With further increasing pressure the Dirac cones in the Gamma-Z line move toward the Gamma point and evolve into two hole-type Fermi pockets, and those in the Z-M lines move toward the M point and evolve into 2 hole-type Fermi pockets up to P=4.0 Gpa. It demonstrates clearly that the Lifshitz transition occurs at

Ground-state and finite-temperature properties of spin liquid phase in the J1–J2 honeycomb model

P_{c1}

Tunneling magnetoresistance of FePt/NaCl/FePt(001)

from semiconductor to 3D Dirac semimetal protected by the nonsymmorphic space symmetry of bulk. This suggests the bright perspective of black phosphorus for optoelectronic and electronic devices due to its easy modulation by pressure.

Band filling and correlation controlling electronic properties and magnetism in KxFe2−ySe2: a slave boson study

Abstract In this paper we analyze the groundstate and finite-temperature properties of a frustrated Heisenberg J 1 – J 2 model on a honeycomb lattice by employing the Schwinger boson technique. The phase diagram and spin gap as functions of J 2 / J 1 are presented, showing that the exotic spin liquid phase lies in 0.21 J 2 / J 1 0.43 . The temperature and magnetic-field dependences of specific heat, magnetic susceptibility and Knight shift are also presented. We find that the spin liquid state is robust with respect to an external magnetic field. These results provide clear information characterizing unusual properties of the exotic spin liquid phase for further experiments.

Interlayer magnetic-frustration–driven quantum spin disorder in the honeycomb compound In3Cu2VO9

We propose and theoretically investigate an interesting and potentially very attractive magnetic tunnel junction FePt/NaCl/FePt(001) for spintronics. It is attractive because the strain at the FePt/NaCl interface is relatively small and, as a result, spin injection from the FePt metal to the NaCl barrier is significant and thus a potentially large TMR ratio can be obtained. The electronic bands with the symmetry of L10 FePt cross the Fermi level for both the majority-spin and minority-spin channels, and the evanescent state with the symmetry dominates the electron transmission through the fcc NaCl barrier. Very respectable values of the tunnel magnetoresistance ratio are predicted. The microscopic physics of quantum transport in this system is systematically analyzed and understood.

Competition between crystal field splitting and Hund’s rule coupling in two-orbital magnetic metal-insulator transitions

We investigate the electronic and magnetic properties of K(x)Fe(2-y)Se2 materials at different band fillings utilizing the multi-orbital Kotliar-Ruckenstein slave boson mean-field approach. We find that the ground state of KFe2Se2 is a paramagnetic (PM) bad metal with intermediate correlation, in contrast with the previous antiferromagnetic (AFM) results obtained by the local density approximation. Our PM metallic ground state suggests that KFe2Se2 is the parent phase of superconducting K(x)Fe(2-y)Se2, supporting a recent scanning tunneling spectroscopy experiment. For pure Fe2+-based systems, the ground state is a striped AFM (SAFM) metal with a spin density wave gap partially opened near the Fermi level. In comparison, for Fe3+-based compounds, besides SAFM, a Néel AFM metal without orbital ordering is observed, and an orbital selective Mott phase (OSMP) accompanied by an intermediate-spin to high-spin transition is also found, giving a possible scenario of an OSMP in K(x)Fe(2-y)Se2. These results demonstrate that the band filling and correlation control the Fermi surface topology, electronic state and magnetism in K(x)Fe(2-y)Se2.

Magnetism and electronic structures of novel layered CaFeAs2 and Ca0.75(Pr/La)0.25FeAs2

In this paper we present the electronic and magnetic properties of the honeycomb compound In3Cu2VO9. We find that the band dispersions display Dirac cone features in the nonmagnetic phase, as observed in graphene. The parent phase is a charge transfer insulator with an energy gap of about 1.5 eV. Singly occupied electrons of copper ions contribute an spin, while vanadium ions show nonmagnetism. The planar nearest-neighbor, next-nearest-neighbor and interplane superexchange couplings of Cu spins are , and , suggesting a low-dimensional antiferromagnet rather than a spin liquid. We propose that the magnetic frustration along the c-axis leads to a quantum spin disorder in In3Cu2VO9, in accordance with recent experiments.

Influence of electronic correlations on orbital polarizations in the parent and doped iron pnictides

Abstract Competition between crystal field splitting and Hund’s rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb interaction, the system firstly transits from a paramagnetic (PM) metal to a Néel antiferromagnetic (AFM) Mott insulator, or to a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund’s rule coupling. The AFM Mott insulating, PM metallic and orbital insulating phases are not, partially and fully orbital polarized, respectively. For a small JH and a finite crystal field, the orbital insulator is robust. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund’s rule coupling tends to destroy it, driving the low-spin to high-spin transition.

Numerical optimization algorithm for rotationally invariant multi-orbital slave-boson method

The magnetic and electronic properties of the parent material CaFeAs2 of new superconductors are investigated using first-principles calculations. We predict that the ground state of CaFeAs2 is a spin-density-wave (SDW)-type striped antiferromagnet driven by Fermi surface nesting. The magnetic moment around each Fe atom is about 2.1 μB. We also present electronic and magnetic structures of electron-doped phase Ca0.75(Pr/La)0.25FeAs2, the SDW order was suppressed by La/Pr substitution. The As in arsenic layers is negative monovalent and acts as blocking layers enhancing two-dimensional character by increasing the spacing distance between the FeAs layers. This favors strong antiferromagnetic fluctuations mediated pairing, implying higher Tc in Ca0.75(Pr/La)0.25FeAs2 than Ca0.75(Pr/La)0.25Fe2As2.

Ferro-orbital order induced by electron-lattice coupling in orthorhombic iron pnictides

Orbital polarization and electronic correlation are two essential aspects in understanding the normal-state and superconducting properties of multi-orbital FeAs-based superconductors. In this paper, we present a systematic study on the orbital polarization of iron pnictides from weak to strong Coulomb correlations within the Kotliar-Ruckenstein slave boson approach. The magnetic phase diagram of the two-orbital model for LaFeAsO clearly shows that a striped antiferromagnetic metallic phase with orbital polarization exists over a wide doping range, in addition to the Slater-type insulator, Mott insulator and paramagnetic phases. A reversal of the orbital polarization occurs in the intermediate correlation regime in the absence of the crystal field splitting; however, a small crystal field splitting considerably enhances the orbital polarization, and stabilizes the xz-type orbital order. We argue that the ferro-orbital polarization is characteristic of a density wave, and leads to a pseudogap-like behavior in the density of states.