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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- J H- J1- J2-type model [2008, arXiv: 0806.3526v2]. [ABSTRACT FROM AUTHOR]

The original version of this Article did not acknowledge Qi Wang as an equally contributing author. This has now been corrected in the HTML and PDF versions of the Article. [ABSTRACT FROM AUTHOR]

Topological nodal-line semimetals are characterized by line-contact bulk band crossings and topological surface states. Breaking certain protecting symmetry turns this system into a Dirac semimetal or Weyl semimetal that hosts zero-dimensional isolated nodal points. Recent advances in band theory predicted a topological nodal-line semimetal state possessing a new type of nodal line in AlB2-type diborides. Here, we report an experimental realization of nodal-line fermions and associated surface states near the Fermi energy in ZrB2 by angle-resolved photoemission spectroscopy combined with first-principles calculations. The Dirac nodal lines in ZrB2 wind into two groups of nodal rings, which are linked together along the Γ-K direction. We further observe a distinct surface state connecting to each nodal line, indicative of the nontrivial topological nature of the bulk nodal lines. Therefore, our results provide convincing experimental evidence of nodal-line semimetal states in ZrB2 both in the bulk and on the surface, suggesting ZrB2 as a remarkable platform for discovering unique phenomena induced by nodal-line fermions. Topological materials: Drumhead surface states observed at last A new type of quantum state located at the surface of a topological material has been observed. A collaboration of researchers led by Shancai Wang from Renmin University of China used photoemission spectroscopy to visualise the band structure of ZrB2 to confirm its topological properties predicted by density functional theory calculations. They found strong evidence for two groups of nodal rings—topologically-protected crossings between two bands that are extended in a loop through momentum space—and the previously-unobserved "drumhead" surface states that fill out the loops. The surface states in ZrB2 lie close to the Fermi energy and are sharply distinct from the bulk bands, so this material is an excellent candidate for further investigations of the unique properties of nodal line semimetals. If you feel that the title is too narrow, it could be replaced with Topological materials: New type of surface state observed in ZrB2. [ABSTRACT FROM AUTHOR]

Perovskite-related materials have received increasing attention for their broad applications in photovoltaic solar cells and information technology due to their unique electrical and magnetic properties. Here we report three new antiperovskite chalco-halides: Ba3(FeS4)Cl, Ba3(FeS4)Br, and Ba3(FeSe4)Br. All of them were found to be good solar light absorbers. Remarkably, although the shortest Fe-Fe distance exceeds 6 Å, an unexpected anti-ferromagnetic phase transition near 100 K was observed in their magnetic susceptibility measurement. The corresponding complex magnetic structures were resolved by neutron diffraction experiments as well as investigated by first-principles electronic structure calculations. The spin-spin coupling between two neighboring Fe atoms along the b axis, which is realized by the Fe-S···S-Fe super-super exchange mechanism, was found to be responsible for this magnetic phase transition. [ABSTRACT FROM AUTHOR]

Exploring various topological states (TS) and topological phase transitions (TPT) has attracted great attention in condensed matter physics. However, so far, there is rarely a typical material system that can be used as a platform to study the TS and TPT as the system transforms from one-dimensional (1D) nanoribbons to two-dimensional (2D) sheet then to three-dimensional (3D) bulk. Here, we first propose that some typical TS in 1D, 2D, and 3D systems can be realized in a tight-binding (TB) model. Following the TB model and further based on first-principles electronic structure calculations, we demonstrate that the structurally stable (4,0) carbon nanotube derivatives are an ideal platform to explore the semiconductor/nodal-point semimetal states in 1D nanoribbons [1D-(4,0)-C 16 H 4 and 1D-(4,0)-C 32 H 4 ], nodal-ring semimetal state in 2D sheet [2D-(4,0)-C 16 ], and nodal-cage semimetal state in 3D bulk [3D-(4,0)-C 16 ]. Furthermore, we calculate the characteristic band structures and the edge/surface states of 2D-(4,0)-C 16 and 3D-(4,0)-C 16 to confirm their nontrivial topological properties. Our work not only provides new excellent 2D and 3D members for the topological carbon material family, but also serves as an ideal template for the study of TS and TPT with the change of system dimension. [Display omitted] [ABSTRACT FROM AUTHOR]

The magnetic and electronic properties of spinel oxide LiV2O4 have been systematically studied by using the spin-polarized first-principles electronic structure calculations. We find that a series of magnetic states, in which the ferromagnetic (FM) V4 tetrahedra are linked together through the corner-sharing antiferromagnetic (AFM) V4 tetrahedra, possess degenerate energies lower than those of other spin configurations. The large number of these energetically degenerated states being the magnetic ground state give rise to strong magnetic frustration as well as large magnetic entropy in LiV2O4. The corresponding band structure and density of states of such a typical magnetic state in this series, i.e., the ditetrahedron (DT) AFM state, demonstrate that LiV2O4 is in the vicinity of a metal–insulator transition. Further analysis suggests that the t2g and eg orbitals of the V atoms play different roles in the magnetic exchange interactions. Our calculations are consistent with previous experimental measurements and shed light on understanding the exotic magnetism and the heavy-fermion behavior of LiV2O4. [ABSTRACT FROM AUTHOR]

In kagome lattice, with the emergence of Dirac cones and flat band in electronic structure, it provides a versatile ground for exploring intriguing interplay among frustrated geometry, topology and correlation. However, such engaging interest is strongly limited by available kagome materials in nature. Here we report on a synthetic strategy of constructing kagome systems via self-intercalation of Fe atoms into the van der Waals gap of FeSe 2 via molecular beam epitaxy. Using low-temperature scanning tunneling microscopy, we unveil a kagome-like morphology upon intercalating a 2 × 2 ordered Fe atoms, resulting in a stoichiometry of Fe 5 Se 8 . Both the bias-dependent STM imaging and theoretical modeling calculations suggest that the kagome pattern mainly originates from slight but important reconstruction of topmost Se atoms, incurred by the nonequivalent subsurface Fe sites due to the intercalation. Our study demonstrates an alternative approach of constructing artificial kagome structures, which envisions to be tuned for exploring correlated quantum states.

Using first-principles calculations, we predict a stable two-dimensional atomically thin material MgN 4 . This material has a perfect intrinsic electron-hole compensation characteristic with high carrier mobility, making it a promising candidate material with extremely large magnetoresistance. As the magnetic field increases, the magnetoresistance of the monolayer MgN 4 will show a quadratic dependence on the strength of the magnetic field without saturation. Furthermore, nontrivial topological properties are also found in this material. In the absence of spin-orbit coupling, the monolayer MgN 4 belongs to a topological nodal-line material, in which the band crossings form a closed saddle-shape nodal-ring near the Fermi level in the Brillouin zone. Once the spin-orbit coupling is considered, a small local energy gap is opened along the nodal ring, resulting in a topological insulator defined on a curved Fermi surface with  2 = 1. The combination of two-dimensional single-atomic-layer thickness, an extremely large magnetoresistance effect, and topological non-trivial properties in the monolayer MgN 4 makes it an excellent platform for designing novel multi-functional devices.

Exploring quantum spin liquid (QSL) state has both fundamental scientific value and realistic application potential. Recently, α-RuCl 3 was experimentally observed to hold in-plane zigzag antiferromagnetic (AFM) order at low temperature, which was further proposed to be proximate to a Kitaev QSL ground state. We have studied the magnetic properties of α-RuCl 3 in the framework of electronic structure calculation based on density functional theory (DFT) with Hubbard U correction (DFT+U) and spin-orbit coupling. When the intra-orbital Hubbard interaction U and the inter-orbital Hund's coupling J adopt the commonly accepted values of U = 2.0 eV and J = 0.4 eV, the zigzag AFM order indeed owns the minimum energy, consistent with the experimental observation. More importantly, we find that compared with the ferromagnetic order in the previous theoretical studies, there exist a series of magnetic configurations energetically even closer to the zigzag AFM ground state. The further calculations and analysis indicate that these low-energy magnetic states are closely related to the electronic correlation effect of Ru 4d orbitals. By decreasing U and increasing J with just about 0.2 eV, they become energetically degenerate with the zigzag AFM order, inducing strong magnetic frustration and then yielding a state without long-range magnetic order but with nonzero local moments. Considering the facts that theoretically the pressure usually reduces the intra-orbital Hubbard interaction and meanwhile enhances the inter-orbital Hund's coupling, while experimentally the pressure drives α-RuCl 3 into a quantum disordered phase, our results provide a perspective to understand the exotic magnetic behaviors of α-RuCl 3 .

The seeking of room temperature ferromagnetic semiconductors, which take advantages of both the charge and spin degrees of freedom of electrons to realize a variety of functionalities in devices integrated with electronic, optical, and magnetic storage properties, has been a long-term goal of scientists and engineers. Here, by using the spin-polarized density functional theory calculations, we predict a new series of high temperature ferromagnetic semiconductors based on the melilite-type oxysulfide Sr 2 MnGe 2 S 6 O through hole (K) and electron (La) doping. Due to the lack of strong antiferromagnetic superexchange between Mn ions, the weak antiferromagnetic order in the parent compound Sr 2 MnGe 2 S 6 O can be suppressed easily by charge doping with either p-type or n-type carriers, giving rise to the expected ferromagnetic order. At a doping concentration of 25%, both the hole-doped and electron-doped compounds can achieve a Curie temperature (T c ) above 300 K. The underlying mechanism is analyzed. Our study provides an effective approach for exploring new types of high temperature ferromagnetic semiconductors., (Copyright © 2018 Science China Press. Published by Elsevier B.V. All rights reserved.)