Block spin ground state and three-dimensionality of (K,Tl)(y)Fe(1.6)Se2

The magnetic properties and electronic structure of (K,Tl)(y)Fe(1.6)Se(2) is studied using first-principles calculations. The ground state is checkerboard antiferromagnetically coupled blocks of the minimal Fe(4) squares, with a large block-spin moment ～11.2 μ(B). The magnetic interactions could be modeled with a simple spin model involving both the inter- and intrablock, as well as the nearest-neighbor and next-nearest-neighbor couplings. The calculations also suggest a metallic ground state except for y=0.8 where a band gap ～400-550 meV opens, showing an antiferromagnetic insulator ground state for (K,Tl)(0.8)Fe(1.6)Se(2). The electronic structure of the metallic (K,Tl)(y)Fe(1.6)Se(2) is highly three dimensional with unique Fermi surface structure and topology. These features indicate that the Fe-vacancy ordering is crucial to the physical properties of (K,Tl)(y)Fe(2-x)Se(2).     

First-principles study on the electronic structure and magnetism of layered oxyselenide La2Mn2Se2O3

The electronic structure and magnetism of layered oxyselenide La(2)Mn(2)Se(2)O(3) have been studied by using first-principles calculations within the generalized gradient approximation (GGA) and GGA + U methods. The G-type antiferromagnetic (AF) state is calculated to be the most stable phase among the various magnetic configurations of interest, irrespective of the choice of the functional used, which is in good agreement with the experiments. In contrast to La(2)Fe(2)Se(2)O(3) and La(2)Co(2)Se(2)O(3), in which the AF states show metallic behavior under the GGA method, we predict the ground state of La(2)Mn(2)Se(2)O(3) is a semiconductor with an indirect band gap of ～0.52 eV via the GGA calculations. This is closely related to a closed shell configuration and large exchange splitting (～3.5 eV) in the Mn 3d states. Moreover, the magnetic properties are also discussed in terms of the calculated Heisenberg spin exchange constants, suggesting that La(2)Mn(2)Se(2)O(3) is a strong two-dimensional magnetically frustrated system.     

Structural,electronic,and magnetic properties of vanadium atom-adsorbed MoSe_2 monolayer

Using the first-principles calculations, we study the structural, electronic, and magnetic properties of vanadium adsorbed MoSe_2 monolayer, and the magnetic couplings between the V adatoms at different adsorption concentrations. The calculations show that the V atom is chemically adsorbed on the MoSe_2 monolayer and prefers the location on the top of an Mo atom surrounded by three nearest-neighbor Se atoms. The interatomic electron transfer from the V to the nearestneighbor Se results in the polarized covalent bond with weak covalency, associated with the hybridizations of V with Se and Mo. The V adatom induces local impurity states in the middle of the band gap of pristine MoSe_2, and the peak of density of states right below the Fermi energy is associated with the V- dz~2 orbital. A single V adatom induces a magnetic moment of 5 μBthat mainly distributes on the V-3d and Mo-4d orbitals. The V adatom is in high-spin state, and its local magnetic moment is associated with the mid-gap impurity states that are mainly from the V-3d orbitals. In addition,the crystal field squashes a part of the V-4s electrons into the V-3d orbitals, which enhances the local magnetic moment.The magnetic ground states at different adsorption concentrations are calculated by generalized gradient approximations(GGA) and GGA+U with enhanced electron localization. In addition, the exchange integrals between the nearest-neighbor V adatoms at different adsorption concentrations are calculated by fitting the first-principle total energies of ferromagnetic(FM) and antiferromagnetic(AFM) states to the Heisenberg model. The calculations with GGA show that there is a transition from ferromagnetic to antiferromagnetic ground state with increasing the distance between the V adatoms. We propose an exchange mechanism based on the on-site exchange on Mo and the hybridization between Mo and V, to explain the strong ferromagnetic coupling at a short distance between the V adatoms. However, the ferromagnetic exchange mechanism is sensitive to both the increased inter-adatom distance at low concentration and the enhanced electron localization by GGA+U, which leads to antiferromagnetic ground state, where the antiferromagnetic superexchange is dominant.     

Reactive chemical doping of the Bi2Se3 topological insulator

Using angular resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator Bi(2)Se(3) as a function of water vapor exposure. We find that a surface reaction with water induces a band bending, which shifts the Dirac point deep into the occupied states and creates quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se abstraction, leaving positively charged vacancies at the surface. Because of the presence of water vapor, a similar effect takes place when Bi(2)Se(3) crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the Bi(2)Se(3) band structure.     

The synthesis, and crystal and magnetic structure of the iron selenide BaFe2Se3 with possible superconductivity at Tc = 11 K

We report on the synthesis of single crystals of BaFe(2)Se(3) and study their crystal and magnetic structures by means of synchrotron single-crystal x-ray and neutron powder diffraction. The crystal structure has orthorhombic symmetry and consists of double chains of FeSe(4) edge connected tetrahedra intercalated with barium. Below 240 K, long range spin-block checkerboard antiferromagnetic order is developed. The magnetic structure is similar to one observed in A(0.8)Fe(1.6)Se(2) (A = K, Rb or Cs) superconductors. The crystals exhibit a transition to the diamagnetic state with an onset transition temperature of T(c) ～ 11 K. Though we observe FeSe as an impurity phase (<0.8% mass fraction) it is not likely that the diamagnetism is attributable to the FeSe superconductor, which has T(c) ≈ 8.5 K.     

Exchange frequencies in the 2D Wigner crystal

Using path integral Monte Carlo we have calculated exchange frequencies as electrons undergo ring exchanges in a "clean" 2D Wigner crystal as a function of density. The results show agreement with WKB calculations at very low density, but show a more rapid increase with density near melting. Remarkably, the exchange Hamiltonian closely resembles the measured exchanges in 2D (3)He. Using the resulting multispin exchange model we find the spin Hamiltonian for r(s) < or = 175 +/- 10 is a frustrated antiferromagnetic; its likely ground state is a spin liquid. For lower density the ground state will be ferromagnetic.     

Self-trapping of single and paired electrons in Ge2Se3

We report the theoretical prediction of single and paired electron self-trapping in Ge(2)Se(3). In finite atomic cluster, density functional calculations, we show that excess single electrons in Ge(2)Se(3) are strongly localized around single germanium dimers. We also find that two electrons prefer to trap around the same germanium dimer, rupturing a neighboring Ge-Se bond. Localization is less robust in periodic, density functional calculations. While paired electron self-trapping is present, as shown by wavefunction localization around a distorted Ge-Ge dimer, single-electron trapping is not. This discrepancy appears to depend only on the boundary conditions and not on the exchange-correlation potential or basis set. For single- and paired-electron trapping, we report the adiabatic barriers to motion and we estimate hopping rates and freeze-in temperatures. For the single trapped electron, we also predict the (73)Ge and (77)Se hyperfine coupling constants.     

Raman amplification characteristics of As2Se3 photonic crystal fibers

We present the dispersion and Raman amplification characteristics of As2Se3 photonic crystal fibers (PCFs). We compare the gain characteristics with conventional As2Se3 fibers and find that the Raman gain efficiency in PCFs can be improved by a factor of more than 4. This allows us to either use a small length of the fiber or to use the low pump power to attain similar gain characteristics. Numerical simulations reveal that a peak gain of 10 dB can be achieved in a 1.1 m long PCF when it is pumped at 1.5 microm in wavelength with an input power of 500 mW.     

Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A=K,Rb,Cs,Tl) single crystals measured using neutron diffraction

Single-crystal neutron diffraction studies on superconductors A(2)Fe(4)Se(5), where A=Rb, Cs, (Tl, Rb), and (Tl, K) (T(c) ～ 30 K), uncover the same Fe vacancy ordered crystal structure and the same block antiferromagnetic order as in K(2)Fe(4)Se(5). The Fe order-disorder transition occurs at T(S)=500-578 K, and the antiferromagnetic transition at T(N) = 471-559 K with an ordered magnetic moment ～3.3μ(B)/Fe at 10 K. Thus, all recently discovered A intercalated iron selenide superconductors share the common crystalline and magnetic structure, which are very different from previous families of Fe-based superconductors, and constitute a distinct new 245 family.     

Theory of the interplay between the superconductivity and the blocked antiferromagnetic order in A(y)Fe(2-x)Se(2)

Based on an effective two-orbital tight-binding model, we examine the possible superconducting states in iron-vacancy-ordered A(y)Fe(2-x)Se(2). In the presence of ordered vacancies and blocked antiferromagnetic order, it is shown that the emergent SC pairing is the nodeless next-nearest-neighbor (NNN)-pairing due to the dominant antiferromagnetic (AFM) interaction between the inter-block NNN sites. In particular, we show that due to the ordered vacancies and the associated blocked AFM order, the interplay between the superconducting and AFM states results in three distinct states in the phase diagram as doping is varied. The divergent experimental observations can be accounted for by considering the different charge carrier concentrations in their respective compounds.     

Mesobonding (Se,Se) pairs in Si

Using symmetry, qualitative analyses of g-shift data for the (Se,Se)⁺ deep level in Si, and the general features of the electronic structure of Si, we find the mesobonding model of (Se,Se) substitutional pairs in Si to be preferred over the ordinary model -- without relying on any detailed model Hamiltonian.     

Topological Superconductivity in Cu(x)Bi(2)Se(3)

A topological superconductor (TSC) is characterized by the topologically protected gapless surface state that is essentially an Andreev bound state consisting of Majorana fermions. While a TSC has not yet been discovered, the doped topological insulator Cu(x)Bi(2)Se(3), which superconducts below ～3 K, has been predicted to possess a topological superconducting state. We report that the point-contact spectra on the cleaved surface of superconducting Cu(x)Bi(2)Se(3) present a zero-bias conductance peak (ZBCP) which signifies unconventional superconductivity. Theoretical considerations of all possible superconducting states help us conclude that this ZBCP is due to Majorana fermions and gives evidence for a topological superconductivity in Cu(x)Bi(2)Se(3). In addition, we found an unusual pseudogap that develops below ～20 K and coexists with the topological superconducting state.     

Phase stability and temperature effect in ScX (X=S,Se and Te) compounds

Based on first-principles calculations, the structural stability and temperature effect in ScX (X=S, Se and Te) compounds are studied with three typical structures of B1 (NaCl-type), hcp (NiAs-type) and β-hcp (inverse Li₂O₂-type). Their dynamic stability has been verified using phonon mode analysis and molecular dynamics simulations. From the total energy calculations, we find that the most stable ground state structures are B1 for ScS, and hcp for ScSe and ScTe, respectively. Moreover, structural stabilities at finite temperature are studied with the combination of phonon dependent dynamics analysis and first-principles calculations, which reveals a phase transition from hcp to B1 in ScSe around 230 K and a phase transition from hcp to β-hcp in ScTe around 460 K, in accordance with experimental findings. The energy barrier and pathway along the phase transformation from hcp to β-hcp ScTe are also calculated and analyzed by the solid-state nudged elastic band method.     

Block antiferromagnetism and possible ferroelectricity in KFe2Se2

Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe₂Se₃, was predicted to be multiferroic [Phys. Rev. Lett. 113 , 187204 (2014)] due to its unique one‐dimensional block‐type antiferromagnetism. Here, another iron‐selenide KFe₂Se₂, a parent state of iron‐based superconductor, is predicted to be multiferroic. Its two‐dimensional block‐type antiferromagnetism can generate a moderate electric dipole for each Fe–Se layer via the Fe–Se–Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe₂Se₂ may be switchable between antiferroelectric and ferroelectric phases.

Crystal structure of KFe₂Se₂. (a) Purple: K; green: Se; brown: Fe. Two Fe sheets in a minimum unit cell are indicated as A and B. (b) One Fe–Se layer with magnetism. Brown: spin up; blue: spin down. (c) A side view of Fe–Se bonds. The ionic displacements driven by exchange striction are indicated by arrows.     

Structural,electronic, and optical properties of bulk Cu2Se

By using first-principles calculations within the density functional theory and the many-body perturbation theory, we investigate the structural, electronic, and optical properties of bulk Cu₂Se with a recently discovered low-temperature layered configuration. We demonstrate that the effects of the van der Waals forces significantly modify the interlayer binding and distance in the layered Cu₂Se, while the band gap is invariant. Our density functional theory and post-processing GW calculations reveal that for the layered structure, GW correction remedies the serious band-gap underestimation of the density functional theory from 0.12 eV to 0.99 eV. By solving the Bethe–Salpeter equation, we find that the optical gap of the layered Cu₂Se is 0.86 eV, which is in close agreement with previous experimental observations. In addition, we show that the high-temperature fluorite structure has no band gap, even after GW correction, explaining that the band gap controversy among the theories stems from different structural models. This work may serve as an important guide in designing and evaluating photovoltaic devices using Cu₂Se-based materials.     

First-principles study on electronic,magnetic properties and optical absorption of vanadium doped rutile TiO2

The electronic, magnetic properties and optical absorption of vanadium (V) doped rutile TiO₂ have been studied by the generalized gradient approximation GGA and GGA+U (Hubbard coefficient) approach respectively. On the one hand, we consider the influence of vanadium with different doping concentration on the electronic structure. On the other hand, we study double V atoms doped TiO₂, mainly study four V-doped TiO₂ configurations, and find the magnetic ground states are ferromagnetic state. For the TiO₂@V-V1, TiO₂@V-V3 and TiO₂@V-V4 configurations without O ion as bridge between V-V atoms, there will have a metastable state of antiferromagnetic configurations, while, for the TiO₂@V-V2 configurations with an O ion as bridge between V-V atoms, due to the existence of superexchange between V-O-V, there will only exist the ground state of ferromagnetic state and there are no other metastable configurations. Furthermore, the optical properties of V-doped TiO₂ are calculated. The results show that the V-doped TiO₂ has strong infrared light absorption and visible light absorption.     

First-principles electronic-structure calculation on the spin distribution and the half-metallic properties of the mixed valence iron compound Ba2F2Fe1.5S3

The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba₂F₂Fe_1.5S₃. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Fe^III and Fe^II ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba₂F₂Fe_1.5S₃. The spin magnetic moment mainly comes from the Fe^III and Fe^II ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has half-metallic property.     

Incommensurate magnetic ordering in Cu2Te2O5X2 (X = Cl,Br) studied by neutron diffraction

We present the results of the first neutron powder and single crystal diffraction studies of the coupled spin tetrahedra systems Cu2Te2O5X2 (X = Cl,Br). Incommensurate antiferromagnetic order with the propagation vectors kCl approximately [0.150,0.422,1/2], kBr approximately [0.158,0.354,1/2] sets in below TN = 18 K for X = Cl and 11 K for X = Br. No simple collinear antiferromagnetic or ferromagnetic spin arrangements within Cu2+ tetrahedra fit these observations. Fitting the diffraction data to more complex but physically reasonable models with multiple helices leads to a moment of 0.67(1)microB/Cu2+ at 1.5 K for the Cl compound. The reason for such a complex ground state may be geometrical frustration of the spins due to the intratetrahedral and intertetrahedral couplings having similar strengths. In neither compound has any evidence for a structural transition accompanying the magnetic ordering been found.     

偶偶Ge和Se同位素核的形变HF态及负宇称带研究

将具有负宇称的fp轨道空间扩大到包含具有正宇称的1g_9/2轨道,采用修正的表面δ相互作用,对⁶⁴Ge,⁶⁶Ge,⁶⁸Ge,⁷⁰Se,⁷²Se和⁷⁴Se等6个偶偶核做了形变HF计算.得到了基态和一些激发态的解.同时,还用近似角动量投影形变Hartree-Fock(PDHF)方法,对⁶⁴Ge和⁷⁴Se进行了能谱计算,得到其正、负宇称带的解,计算结果与实验谱基本一致.     

Magnetic Properties of TlCo2Se2 Studied by Mössbauer Spectroscopy

Hyperfine Interactions - The quasi two-dimensional antiferromagnetic compound TlCo2Se2 has been studied by 57Fe Mössbauer spectroscopy. Small single crystals have been made with 2% Fe doping....