By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal alpha-FeTe is in a bicollinear antiferromagnetic order, in which the Fe local moments (approximately 2.5 microB) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This novel bicollinear order results from the interplay among the nearest, the next-nearest, and the next-next-nearest neighbor superexchange interactions, mediated by Te 5p band. In contrast, the ground state of alpha-FeSe is in a collinear antiferromagnetic order, similar to those in LaFeAsO and BaFe2As2. This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors.

We present numerical results for the equation of state of an infinite chain of hydrogen atoms. A variety of modern many-body methods are employed, with exhaustive cross-checks and validation. Approaches for reaching the continuous space limit and the thermodynamic limit are investigated, proposed, and tested. The detailed comparisons provide a benchmark for assessing the current state of the art in many-body computation, and for the development of new methods. The ground-state energy per atom in the linear chain is accurately determined versus bondlength, with a confidence bound given on all uncertainties.

From first-principles calculations, we have studied the electronic and magnetic structures of the ground state of LaOFeAs. The Fe spins are found to be collinear antiferromagnetic ordered, resulting from the interplay between the strong nearest and next-nearest neighbor superexchange antiferromagnetic interactions. The structure transition observed by neutron scattering is shown to be magnetically driven. Our study suggests that the antiferromagnetic fluctuation plays an important role in the Fe-based superconductors. This sheds light on the understanding of the pairing mechanism in these materials. PACS numbers: 74.25.Jb, 71.18.+y, 74.25.Ha, Recently an iron-based material LaOFeAs was reported to show superconductivity with a transition temperature T c ∼ 26K by partial substitution of O with F atoms [1]. Soon after, other families of Fe-As oxyarsenides with La replaced by Sm[2], Ce[3], Pr[4] and other rare earth elements were found superconducting with T c more than 50K. Like cuprates, these iron arsenides have a layered structure. The superconducting pairing is believed to happen in the iron-based FeAs layers. The high transition temperature and the preliminary band structure calculation suggests that the superconductivity in these Fe-arsenide superconductors is not mediated by electronphonon interaction. It is commonly believed that the understanding of electronic structures of the parent compound LaOFeAs is the key to determine the underlying mechanism to make it superconducting upon doping.The early band structure calculations suggested that the pure LaOFeAs compound is a nonmagnetic metal but with strong ferromagnetic or antiferromangtic (AFM) instability [5,6,7]. Later, it was found that the antiferromagnetically ordered state [8,9] has a lower energy than the nonmagnetic one, probably due to the Fermi surface nesting [8]. Dong et al.[10] predicted that the AFM state should form a collinear-striped structure by breaking the rotational symmetry. This collinear ordered AFM state has indeed been observed by the neutron scattering experiment [11,12]. Furthermore, the neutron scattering measurement found that there is a structure transition with a monoclinic lattice distortion at ∼ 150K and the collinear order is formed about 15∼20K below this transition temperature. Without this distortion, the square AFM order induced purely by the Fermi surface nesting is expected to be more stable since there are two orthogonal but equivalent nesting directions (π, π) and (π, −π), which can lower the energy of the ground state by keeping its rotational symmetry [8].In this paper, we report the theoretical result on the electronic and magnetic structures of the ground state of LaOFeAs obtained from first-principles band struc-ture calculations. We find that there are strong nearest and next-nearest neighbor superexchange interactions in this material (similar conclusion was obtained by Yildirim [13]). The nearest and next nearest neighbor superexchange interactions have almost the same amplitude within error of calculation....

No abstract

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe 2 As 2 (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 is described effectively by a t − J H − J 1 − J 2 -type model [1].

We present a systematic downfolding many-body approach 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 a frozen-core treatment. The computational cost of the many-body calculation is dramatically reduced without sacrificing accuracy. We use the auxiliary-field quantum Monte Carlo (AFQMC) method to solve the downfolded Hamiltonian. Excellent accuracy is achieved for a range of solids, including semiconductors, ionic insulators, and metals. We further test the method by determining the spin gap in NiO, a challenging prototypical material with strong electron correlation effects. This approach greatly extends the reach of general, ab initio many-body calculations in materials.PACS numbers: 02.70.Ss, 71.15.Nc Developing accurate and efficient computational approaches for quantum matter has been a long-standing challenge. Parameter-free, material-specific many-body calculations are needed where simpler methods, such as those based on density functional theory (DFT) [1] or perturbative approaches, break down. Examples range from strongly correlated materials, such as transition metal oxides, to bond-stretching or bond-breaking in otherwise moderately correlated systems. Quantum Monte Carlo (QMC) has become increasingly important in this regard [2][3][4][5][6][7][8][9][10]. However, systematic and routine applications of QMC in realistic materials still face major challenges. Here we present an approach which overcomes several of the obstacles and advances the capabilities of non-perturbative ground-state calculations in correlated materials in general.Our approach treats downfolded Hamiltonians expressed with respect to a truncated basis set of meanfield orbitals of the target system, using an auxiliaryfield quantum Monte Carlo (AFQMC) method [3,11,12]. This allows QMC calculations to be performed with a much simpler Hamiltonian while retaining materialspecific properties. The simplification, often with drastic reduction in computational cost, can extend the reach of ab initio computations to more complex materials. A large gain in statistical accuracy often results as well, because of the smaller range of energy scales (or many fewer degrees of freedom) which need to be sampled stochastically in the downfolded Hamiltonian.Two other key advantages follow as a result of this approach. First, by varying the cut-off that controls the truncation of the basis orbitals, one could in principle dial between the original full-basis Hamiltonian and the simplest model. QMC calculations can be performed at each stage. This allows a systematically improvable set of calculations that connect simple models to full materials specificity. Second, the approach introduces a new way for treating core electrons, which has been a critical issue in QMC. S...

No abstract

Ferroelectrics, which generate a switchable electric field across the solid–liquid interface, may provide a platform to control chemical reactions (physical properties) using physical fields (chemical stimuli). However, it is challenging to in-situ control such polarization-induced interfacial chemical structure and electric field. Here, we report that construction of chemical bonds at the surface of ferroelectric BiFeO3 in aqueous solution leads to a reversible bulk polarization switching. Combining piezoresponse (electrostatic) force microscopy, X-ray photoelectron spectroscopy, scanning transmission electron microscopy, first-principles calculations and phase-field simulations, we discover that the reversible polarization switching is ascribed to the sufficient formation of polarization-selective chemical bonds at its surface, which decreases the interfacial chemical energy. Therefore, the bulk electrostatic energy can be effectively tuned by H+/OH− concentration. This water-induced ferroelectric switching allows us to construct large-scale type-printing of polarization using green energy and opens up new opportunities for sensing, high-efficient catalysis, and data storage.