Jiangang He

Cornell University, Ithaca, New York, United States

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Publications (4)7.33 Total impact

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    ABSTRACT: Employing the random phase approximation we investigate the binding energy and Van der Waals (vdW) interlayer spacing between the two layers of bilayer transition metal dichalcogenides MoS2, MoSe2, WS2, and WSe2 for five different stacking patterns, and examine the stacking-induced modifications on the electronic and optical/excitonic properties within the GW approximation with a priori inclusion of spin-orbit coupling and by solving the two-particle Bethe-Salpeter equation. Our results show that for all cases, the most stable stacking order is the high symmetry AA' type, distinctive of the bulklike 2H symmetry, followed by the AB stacking fault, typical of the 3R polytypism, which is by only 5 meV/formula unit less stable. The conduction band minimum is always located in the midpoint between K and Γ, regardless of the stacking and chemical composition. All MX2 undergo an direct-to-indirect optical gap transition going from the monolayer to the bilayer regime. The stacking and the characteristic vdW interlayer distance mainly influence the valence band splitting at K and its relative energy with respect to Γ, as well as, the electron-hole binding energy and the values of the optical excitations.
    01/2014; 89(7).
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    Jiangang He, Cesare Franchini
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    ABSTRACT: The structure and ground state electronic structure of the recently synthesized SrPdO$_3$ perovskite [A. Galal {\em et al.}, J. Power Sources, {\bf 195}, 3806 (2010)] have been studied by means of screened hybrid functional and the GW approximation with the inclusion of electron-hole interaction within the test-charge/test-charge scheme. By conducting a structural search based on lattice dynamics and group theoretical method we identify the orthorhombic phase with $P_{nma}$ space group as the most stable crystal structure. The phase transition from the ideal cubic perovskite structure to the $P_{nma}$ one is explained in terms of the simultaneous stabilization of the antiferrodistortive phonon modes $R_4^+$ and $M_3^+$. Our results indicate that SrPdO$_3$ exhibits an insulating ground state, substantiated by a GW$_0$ gap of about 1.1 eV. Spin polarized calculations suggests that SrPdO$_3$ adopts a low spin state ($t_{2g}^{\uparrow\downarrow\uparrow\downarrow\uparrow\downarrow}e_g^0$), and is expected to exhibit spin excitations and spin state crossovers at finite temperature, analogous to the case of 3$d$ isoelectronic LaCoO$_3$. This would provide a new playground for the study of spin state transitions in 4$d$ oxides and new opportunity to design multifunctional materials based on 4$d$ $P_{nma}$ building block.
    12/2013; 89(4).
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    ABSTRACT: By means of hybrid density functional theory we investigate the evolution of the structural, electronic, and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO3 under pressure. We predict a transition from a low-pressure antiferromagnetic (AFM) insulator to a high-pressure ferromagnetic (FM) transport half metal (tHM), characterized by a large spin polarization (≈80–90%). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the Pnma orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S=2, mMn=3.7μB) to low-spin (S=1, mMn=1.7μB) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite.
    Physical Review B 02/2012; 85:915135. · 3.66 Impact Factor
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    ABSTRACT: The evolution of the magnetic ordering temperature of the 4d3 perovskites RTcO3 (R=Ca, Sr, Ba) and its relation with its electronic and structural properties has been studied by means of hybrid density functional theory and Monte Carlo simulations. When compared to the most widely studied 3d perovskites the large spatial extent of the 4d shells and their relatively strong hybridization with oxygen weaken the tendency to form Jahn-Teller like orbital ordering. This strengthens the superexchange interaction. The resulting insulating G-type antiferromagnetic ground state is characterized by large superexchange coupling constants (26–35 meV) and Neél temperatures (750–1200 K). These monotonically increase as a function of the R ionic radius due to the progressive enhancement of the volume and the associated decrease of the cooperative rotation of the TcO6 octahedra.
    Physical Review B 06/2011; 83:220402. · 3.66 Impact Factor