Miao Gao

The electron pairing of KxFe2−ySe2

We studied the pairing instabilities in KxFe2-ySe2 using a two-stage functional renormalization group (FRG) method. Our results suggest the leading and subleading pairing symmetries are nodeless d(x2-y2) and nodal extended s, respectively. In addition, despite having no Fermi surfaces we find the buried hole bands make important contributions to the final effective interaction. From the bandstructure, spin susceptibility and the FRG results we conclude that the low-energy effective interaction in KxFe2-ySe2 is well described by a J(1)-J(2) model with dominant nearest-neighbor antiferromagnetic interaction J(1) (at least as far as the superconducting pairing is concerned). In the end we briefly mention several obvious experiments to test whether the pairing symmetry is indeed d(x2-y2). Copyright (c) EPLA, 2011

Electronic Structures and Magnetic Order of Ordered-Fe-Vacancy Ternary Iron Selenides TlFe1.5Se2 and AFe(1.5)Se(2) (A = K, Rb, or Cs)

By the first-principles electronic structure calculations, we find that the ground state of the Fe-vacancies ordered TlFe1.5Se2 is a quasi-two-dimensional collinear antiferromagnetic semiconductor with an energy gap of 94 meV, in agreement with experimental measurements. This antiferromagnetic order is driven by the Se-bridged antiferromagnetic superexchange interactions between Fe moments. Similarly, we find that crystals AFe(1.5)Se(2) (A = K, Rb, or Cs) are also antiferromagnetic semiconductors but with a zero-gap semiconducting state or semimetallic state nearly degenerated with the ground states. Thus, rich physical properties and phase diagrams are expected.

Electronic and magnetic structures of the ternary iron selenides AFe(2)Se(2) ( A = Cs, Rb, K, or Tl)

We have studied the electronic and magnetic structures of the ternary iron selenides AFe(2)Se(2) (A = Cs, Rb, K, or Tl) using first-principles electronic structure calculations. We find that the ground state of these compounds is bicollinearly antiferromagnetically ordered, with the Fe moments having collinear antiferromagnetic order in each bipartite sublattice. This bicollinear antiferromagnetic order results from the superexchange interactions of Fe moments mediated by the Se 4p orbitals. We have also determined the density of states at the Fermi level, the specific heat coefficient, the Pauli susceptibility, and other related physical properties in both the nonmagnetic and bicollinear antiferromagnetic states for these compounds. The underlying mechanism is discussed according to the electronic structure analysis.

Ternary iron selenideK0.8Fe1.6Se2is an antiferromagnetic semiconductor

We have studied electronic and magnetic structures of K0.8+xFe1.6Se2 by performing the first-principles electronic structure calculations. The ground state of the Fe-vacancies ordered K0.8Fe1.6Se2 is found to be a quasi-two-dimensional blocked checkerboard antiferromagnetic (AFM) semiconductor with an energy gap of 594 meV and a large ordering magnetic moment of 3.37 mu B for each Fe atom, in excellent agreement with the neutron-scattering measurement. The underlying mechanism is the chemical-bonding-driven tetramer lattice distortion. K0.8+xFe1.6Se2 with finite x is a doped AFM semiconductor with low conducting carrier concentration, which is approximately proportional to the excess potassium content, consistent qualitatively with the infrared observation. Our study reveals the importance of the interplay between antiferromagnetism and superconductivity in these materials. This suggests that K0.8Fe1.6Se2, instead of KFe2Se2, should be regarded as a parent compound from which the superconductivity emerges upon electron or hole doping.

Effect of disorder with long-range correlation on transport in graphene nanoribbon

Transport in disordered armchair graphene nanoribbons (AGR) with long-range correlation between quantum wire contacts is investigated by a transfer matrix combined with Landauers formula. The metal-insulator transition is induced by disorder in neutral AGR. Therein, the conductance is one conductance quantum for the metallic phase and exponentially decays otherwise, when the length of AGR approaches infinity and far longer than its width. Similar to the case of long-range disorder, the conductance of neutral AGR first increases and then decreases while the conductance of doped AGR monotonically decreases, as the disorder strength increases. In the presence of strong disorder, the conductivity depends monotonically and non-monotonically on the aspect ratio for heavily doped and slightly doped AGR, respectively. For edge disordered graphene nanoribbon, the conductance increases with the disorder strength of long-range correlated disordered while no delocalization exists, since the edge disorder induces localization.

Surface structures of ternary iron arsenides AFe(2)As(2) (A = Ba, Sr, or Ca)

By the first-principles electronic structure calculations, we find that energetically the most favorable cleaved AFe2As2(001) surface (A=Ba, Sr, or Ca) is A-terminated with a ( √ 2 × √ 2)R45 or (1 × 2) order. The (1 × 2) ordered structure yields a (1 × 2) dimerized STM image, in agreement with the experimental observation. The A atoms are found to diffuse on the surface with a small energy barrier so that the cleaving process may destroy the A atoms ordering. At the very low temperatures this may result in an As-terminated surface with the A atoms in randomly assembling. The As-terminated BaFe2As2 surface in orthorhombic phase is ( √ 2× √ 2)R45 buckled, giving rise to a switchable ( √ 2× √ 2)R45 STM pattern upon varying the applied bias. No any reconstruction is found for the other As-terminated surfaces. There are surface states crossing or nearby the Fermi energy in the As-terminated and (1 × 2) A-terminated surfaces. A unified physical picture is thus established to help understand the cleaved AFe2As2(001) surfaces.

Prediction of phonon-mediated high-temperature superconductivity inLi3B4C2

The discovery of superconductivity in Magnesium Diborate (MgB

Spin wave excitations in AFe1.5Se2 (A?=?K, Tl): analytical study

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Pair-checkerboard antiferromagnetic order in β−Fe4Se5 with 5×5 -ordered Fe vacancies

) has stimulated great interest in the search of new superconductors with similar lattice structures. Unlike cuprate or iron-based superconductors, MgB

Ternary iron selenide K0.8Fe1.6Se2 is an antiferromagnetic semiconductor

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