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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.

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....

Owing to the unusual geometry of kagome lattices-lattices made of corner-sharing triangles-their electrons are useful for studying the physics of frustrated, correlated and topological quantum electronic states. In the presence of strong spin-orbit coupling, the magnetic and electronic structures of kagome lattices are further entangled, which can lead to hitherto unknown spin-orbit phenomena. Here we use a combination of vector-magnetic-field capability and scanning tunnelling microscopy to elucidate the spin-orbit nature of the kagome ferromagnet FeSn and explore the associated exotic correlated phenomena. We discover that a many-body electronic state from the kagome lattice couples strongly to the vector field with three-dimensional anisotropy, exhibiting a magnetization-driven giant nematic (two-fold-symmetric) energy shift. Probing the fermionic quasi-particle interference reveals consistent spontaneous nematicity-a clear indication of electron correlation-and vector magnetization is capable of altering this state, thus controlling the many-body electronic symmetry. These spin-driven giant electronic responses go well beyond Zeeman physics and point to the realization of an underlying correlated magnetic topological phase. The tunability of this kagome magnet reveals a strong interplay between an externally applied field, electronic excitations and nematicity, providing new ways of controlling spin-orbit properties and exploring emergent phenomena in topological or quantum materials.

Articles you may be interested inThe effect of ionization on the global minima of small and medium sized silicon and magnesium clusters Energies and spatial features for the rotationless bound states of He 3 + 4 ( Σ g + 2 ) : A cationic core from helium cluster ionization One-photon mass-analyzed threshold ionization spectroscopy of 1,3,5-trifluorobenzene: The Jahn-Teller effect and vibrational analysis for the molecular cation in the ground electronic state Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of hexafluorobenzene: The Jahn-Teller effect and vibrational analysis Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of p -, m -, and o -difluorobenzenes. Ionization energies and vibrational frequencies and structures of the cationsWe have performed a systematic ground state geometry search for the singly charged Si n cations in the medium-size range (nр20) using density functional theory in the local density approximation ͑LDA͒ and generalized gradient approximation ͑GGA͒. The structures resulting for nр18 generally follow the prolate ''stacked Si 9 tricapped trigonal prism'' pattern recently established for the lowest energy geometries of neutral silicon clusters in this size range. However, the global minima of Si n and Si n ϩ for nϭ6, 8, 11, 12, and 13 differ significantly in their details. For Si 19 and Si 20 neutrals and cations, GGA renders the prolate stacks practically isoenergetic with the near-spherical structures that are global minima in LDA. The mobilities in He gas evaluated for all lowest energy Si n ϩ geometries using the trajectory method agree with the experiment, except for nϭ18 where the second lowest isomer fits the measurements. The effect of gradient corrections for either the neutral or cationic clusters is subtle, but their inclusion proves to be critical for obtaining agreement with the mobility measurements in the nϭ15-20 range. We have also determined ionization potentials for our Si n neutral geometries and found that all experimental size-dependent trends are reproduced for nр19. This particularly supports our structural assignments for Si 9 , Si 11 , Si 12 , and Si 17 neutrals. The good overall agreement between the measured and calculated properties supports the elucidation of the ''prolate'' family of silicon clusters as stacks of trigonal prisms.

A main task in condensed-matter physics is to recognize, classify, and characterize phases of matter and the corresponding phase transitions, for which machine learning provides a new class of research tools due to the remarkable development in computing power and algorithms. Despite much exploration in this new field, usually different methods and techniques are needed for different scenarios. Here, we present SimCLP: a simple framework for contrastive learning phases of matter, which is inspired by the recent development in contrastive learning of visual representations. We demonstrate the success of this framework on several representative systems, including non-interacting and quantum many-body, conventional and topological. SimCLP is flexible and free of usual burdens such as manual feature engineering and prior knowledge. The only prerequisite is to prepare enough state configurations. Furthermore, it can generate representation vectors and labels and hence help tackle other problems. SimCLP therefore paves an alternative way to the development of a generic tool for identifying unexplored phase transitions.

The mechanism of high-temperature superconductivity in the iron-based superconductors remains an outstanding issue in condensed matter physics. The electronic structure plays an essential role in dictating superconductivity. Recent revelation of distinct electronic structure and high-temperature superconductivity in the single-layer FeSe/SrTiO3 films provides key information on the role of Fermi surface topology and interface in inducing or enhancing superconductivity. Here we report high-resolution angle-resolved photoemission measurements on the electronic structure and superconducting gap of an FeSe-based superconductor, (Li0.84Fe0.16)OHFe0.98Se, with a Tc at 41 K. We find that this single-phase bulk superconductor shows remarkably similar electronic behaviours to that of the superconducting single-layer FeSe/SrTiO3 films in terms of Fermi surface topology, band structure and the gap symmetry. These observations provide new insights in understanding high-temperature superconductivity in the single-layer FeSe/SrTiO3 films and the mechanism of superconductivity in the bulk iron-based superconductors.

Corresponding authors: W.J. (email: wji@ruc.edu.cn) and Z.Z. (email: zhong@nimte.ac.cn) † These authors contributed equally to this work.Diverse interlayer tunability of physical properties of two-dimensional layers mostly lies in the covalent-like quasi-bonding that is significant in electronic structures but rather weak for energetics. Such characteristics result in various stacking orders that are energetically comparable but may significantly differ in terms of electronic structures, e.g. magnetism. Inspired by several recent experiments showing interlayer antiferromagnetically coupled CrI3 bilayers, we carried out first-principles calculations for CrI3 bilayers. We found that the anti-ferromagnetic coupling results from a new stacking order with the C2/m space group symmetry, rather than the graphene-like one with 3 as previously believed. Moreover, we demonstrated that the intra-and interlayer couplings in CrI3 bilayer are governed by two different mechanisms, namely ferromagnetic super-exchange and direct-exchange interactions, which are largely decoupled because of their significant difference in strength at the strong-and weakinteraction limits. This allows the much weaker interlayer magnetic coupling to be more feasibly tuned by stacking orders solely. Given the fact that interlayer magnetic properties can be altered by changing crystal structure with different stacking orders, our work opens a new paradigm for tuning interlayer magnetic properties with the S2 freedom of stacking order in two dimensional layered materials.Introduction.-Magnetism in two dimensions has received growing attention since the two ferromagnetic monolayers, namely CrI3 [1] and Cr2Ge2Te6 [2], were successfully fabricated in 2017. The ferromagnetism in these two layers was believed to be stabilized by magnetic anisotropy as enhanced by spin-orbit coupling or external magnetic fields.Their Curie temperatures were up to ~50 K. Very recently, a room-temperature Tc were achieved in monolayer VSe2 [3] and MnSex [4], two members of the transition-metal dichalcogenides family. This shed considerable light on the search for high Tc ferromagnetic (FM) magnets. However, the tunability of magnetism has been emerging as a new challenge. The coupling strengths of two-dimensional (2D) materials are significantly different between intra-and inter-layer interactions. Such difference may offer diverse magnetic coupling mechanisms at strong and weak interacting limits. The interlayer magnetic coupling is of peculiar interest, as the effective coupling is relatively weak and confined within few atomic layers, which is much easier to model and more feasible to tune than strong and periodic couplings in three-dimension.Recent experiments demonstrated that the anti-ferromagnetic (AFM) interlayer order in bilayer CrI3 can be manipulated to a FM order by electric gating or reasonably large magnetic fields [5][6][7][8][9][10][11][12]. As a consequence, a magnetic tunnel junction with giant tunneling magnetoresistance values was achieved in bilayer CrI3 d...