Fengjie Ma

First-Principles Calculations of the Electronic Structure of Tetragonal alpha-FeTe and alpha-FeSe Crystals: Evidence for a Bicollinear Antiferromagnetic Order

Fengjie Ma, Wei Ji, Jiangping Hu, Zhong-Yi Lu, and Tao Xiang Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing 100190, China Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA Department of Physics, Renmin University of China, Beijing 100872, China and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China (Dated: September 29, 2008)

Arsenic-bridged antiferromagnetic superexchange interactions in LaFeAsO

From the first-principles calculations, we have studied the electronic and magnetic structures of LaFeAsO. It is found that a large magnetic moment of similar to 2.6 mu(B) is located around each Fe ion, embedded in the environment of itinerant electrons. In the ground state, these local Fe moments are in collinearly antiferromagnetic order, resulting from the interplay between the strong nearest- and next-nearest-neighbor superexchange antiferromagnetic interactions bridged by As atoms. The structure transition observed by the neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials.

Iron-based layered compound LaFeAsO is an antiferromagnetic semimetal

LaFeAsO is a parent compound of iron-based F-doped superconductors. An understanding of its ground phase and electronic structure is a precondition to understanding the underlying superconductivity mechanism. Our study shows that LaFeAsO is a quasi-two-dimensional antiferromagnetic semimetal with most carriers being electrons and with a magnetic moment of 2.3 mu(B) located around each Fe atom on the Fe-Fe square lattice. Physically, this is a commensurate antiferromagnetic spin-density wave due to the Fermi-surface nesting. The observed superconduction after the F doping happens on the Fe-Fe layer suggesting a new superconductivity mechanism mediated by spin fluctuations.

Electronic structures of ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe2As2 (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin-orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t-JH-J1-J2-type model [2008, arXiv: 0806.3526v2].

Pi junction to probe antiphase s-wave pairing in iron pnictide superconductors.

Josephson junctions between a FeAs-based superconductor with antiphase s-wave pairing and a conventional s-wave superconductor are studied. The translational invariance in a planar junction between a single crystal pnictide and an aluminum metal greatly enhances the relative weight of electron pockets in the pnictide to the critical current. In a wide doping region of the pnictide, a planar and a point contact junction have opposite phases, which can be used to design a trijunction ring with pi phase to probe the antiphase pairing.

Quantum Monte Carlo Calculations in Solids with Downfolded Hamiltonians

We present a combination of a downfolding many-body approach with auxiliary-field quantum Monte Carlo (AFQMC) calculations for extended systems. Many-body calculations operate on a simpler Hamiltonian which retains material-specific properties. The Hamiltonian is systematically improvable and allows one to dial, in principle, between the simplest model and the original Hamiltonian. As a by-product, pseudopotential errors are essentially eliminated using frozen orbitals constructed adaptively from the solid environment. The computational cost of the many-body calculation is dramatically reduced without sacrificing accuracy. Excellent accuracy is achieved for a range of solids, including semiconductors, ionic insulators, and metals. We apply the method to calculate the equation of state of cubic BN under ultrahigh pressure, and determine the spin gap in NiO, a challenging prototypical material with strong electron correlation effects.

Excited state calculations in solids by auxiliary-field quantum Monte Carlo

We present an approach for ab initio many-body calculations of excited states in solids. Using auxiliary-field quantum Monte Carlo, we introduce an orthogonalization constraint with virtual orbitals to prevent collapse of the stochastic Slater determinants in the imaginary-time propagation. Trial wave functions from density-functional calculations are used for the constraints. Detailed band structures can be calculated. Results for standard semiconductors are in good agreement with experiments; comparisons are also made with GW calculations and the connections and differences are discussed. For the challenging ZnO wurtzite structure, we obtain a fundamental band gap of 3.26(16) eV, consistent with experiments.

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.

Finite-size correction in many-body electronic structure calculations of magnetic systems

We extend the post-processing finite-size (FS) correction method, developed by Kwee, Zhang, and Krakauer [Phys. Rev. Lett. 100, 126404 (2008)], to spin polarized systems. The method estimates the FS effects in many-body electronic structure calculations of extended systems by a modified density functional theory (DFT) calculation, without having to repeat expensive many-body simulations. We construct a unified FS DFT exchange-correlation functional for spin unpolarized and fully spin polarized systems, under the local density approximation. The results are then interpolated to arbitrary spin polarizations. Generalization to other functional forms in DFT are discussed. The application of this FS correction method to several typical magnetic systems with varying supercell sizes demonstrates that it consistently removes most of the FS errors, leading to rapid convergence of the many-body results to the infinite size limit.

Auxiliary-field quantum Monte Carlo calculations with multiple-projector pseudopotentials

We have implemented recently developed multiple-projector pseudopotentials into the planewave based auxiliary-field quantum Monte Carlo (pw-AFQMC) method. Multiple-projector pseudopotentials can yield smaller planewave cut-offs while maintaining or improving transferability. This reduces the computational cost of pw-AFQMC, increasing its reach to larger and more complicated systems. We discuss the use of non-local pseudopotentials in the separable Kleinman-Bylander form, and the implementation in pw-AFQMC of the multiple-projector optimized norm-conserving pseudopotential ONCVPSP of Hamann. The accuracy of the method is first demonstrated by equation-of-state calculations of the ionic insulator NaCl and more strongly correlated metal Cu. The method is then applied to calibrate the accuracy of density functional theory (DFT) predictions of the phase stability of recently discovered high temperature and pressure superconducting sulfur hydride systems. We find that DFT results are in good agreement with pw-AFQMC, due to near cancellation of electron-electron correlation effects between different structures.