Hidden Fermi Surface in K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2–<Emphasis Type="Italic">y</Emphasis></Subscript>Se<Subscript>2</Subscript>: LDA + DMFT Study

In this paper we provide theoretical LDA + DMFT support of recent angle-resolved photoemission spectroscopy (ARPES) observation of the so-called hidden hole-like band and corresponding hidden Fermi surface sheet near Γ-point in the K_0.62Fe_1.7Se₂ compound. To some extent, this is a solution to the long-standing riddle of Fermi surface absence around Γ-point in the K_xFe_2–ySe₂ class of iron chalcogenide superconductors. In accordance with the experimental data, Fermi surface was found near the Γ-point within LDA + DMFT calculations. Based on the LDA + DMFT analysis in this paper it is shown that the largest of the experimental Fermi surface sheets is actually formed by a hybrid Fe-3d ( xy, xz, yz )quasiparticle band. It is also shown that the Fermi surface is not a simple circle as DFT-LDA predicts, but has (according to the LDA + DMFT) a more complicated “propeller”-like structure due to correlations and multiorbital nature of the K_xFe_2–ySe₂ materials. While the smallest experimental Fermi surface around Γ-point is in some sense fictitious, since it is formed by the summation of the intensities of the spectral function associated with “propeller” loupes and is not connected to any of quasiparticle bands.     

de Haas-van Alphen effect and the Fermi surface of PrNi5

The Fermi surface of PrNi₅ has been studied by the measurements of the de Haas-van Alphen (dHvA) effect at temperatures between 0.3 and in magnetic fields up to 12 T. Two dHvA frequencies have been obtained. The electronic structure of PrNi₅ was calculated using the full potential linearized augmented plane wave method. Five sheets of the Fermi surface and the multiple extremal cross sections were determined. First and second sheet have a hole-like structure. The agreement between theory and experiment is obtained by a small downward shift ( 0.1 eV) of the Fermi energy which is probably due to an underestimation of the role of 4 f electrons. Received 9 May 2000 and Received in final form 20 September 2000     

Self-consistent band structure for β-(BEDTTTF) 2I 3

Preliminary results of a self-consistent electronic band structure calculation for β-(BEDTTTF) ₂I ₃ are presented. The calculations are based on the local density functional approximation and the ASW-algorithm. Energy bands and the topology of the Fermi surface are determined and, by means of partial densities of states, the origin of the various states near the Fermi level is elucidated. The high value of the superconducting transition temperature is discussed qualitatively.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

Studies of the Fermi surface of V3Si by 2D ACAR measurements

We report two-dimensional angular correlation of annihilation radiation (2D ACAR) measurements using positrons in single crystals of V₃Si. Anisotropic structure commensurate with the reciprocal lattice is observed and attributed to the complex Fermi surface (FS). A simple geometrical model for the FS is developed and compared with folded ACAR distributions. The results of a 3D reconstruction are compared with recent band calculations.     

LDA and GGA investigations of some ground state properties of aluminium with the all electron MAPW method

Both in local-density approximation (LDA) and in generalised-gradient approximation (GGA) the electronic structure of Aluminium is evaluated by use of the modified augmented plane wave (MAPW) self-consistent scheme. The LDA based on the exchange correlation functional by Vosko, Wilk and Nusair gives the equilibrium lattice constant in good accord with its experimental value. The hole sheet of the Fermi surface, h₂, is well described by weakly distorted spheres with origin at and in the reciprocal lattice. Near and above the equilibrium lattice constant the electronic sheet, e₃, is found to be quite similar to the model originally proposed by Ashcroft. However, even moderate compressions induce a drastic variation.Received: 17 November 2003, Published online: 2 April 2004PACS: 71.18. + y Fermi surface: calculations and measurements; effective mass, g factor - 71.20.-b Electron density of states and band structure of crystalline solids     

Direct observation of the multisheet Fermi surface in the strongly correlated transition metal compound ZrZn2

The existence of flat areas of a Fermi surface (FS), predicted by electronic structure calculations and used in models of both magnetically mediated and phonon-mediated Fulde-Ferrell-Larkin-Ovchinnikov superconducting states, is reported in the paramagnetic phase of the ferromagnetic superconductor ZrZn2 using positron annihilation. The strongly mass-renormalized FS sheet, dominating the Fermi level density of states, is seen for the first time. The delocalization of the magnetization is studied using measured and calculated magnetic Compton profiles.     

Self-consistent semi-relativistic energy band structure of fcc and tetragonal Ni metal

The electronic structure of paramagnetic and ferromagnetic fcc Ni and tetragonal Ni has been determined by means of self-consistent semi-relativistic linear muffin-tin orbital (LMTO) energy band studies. The paramagnetic studies used the local density exchange-correlation potential of Hedin et al. whereas the ferromagnetic spin-polarized calculations used the local spin-density exchange-correlation potential of Gunnarson et al. The magnetic moments (0.54 μ_B), magnetization density, Fermi contact hyperfine fields and Fermi surface areas are found to agree with experiment and with other theoretical calculations, notably those of Wang and Callaway. For tetragonal Ni, distorted along the [111] direction similar to the local strain of Ni layers between Cu layers in modulated structures, the magnetic moment (0.46 μ_B) and resultant hyperfine fields are reduced substantially from that determined for fcc Ni.     

Band structure of palladium hydride by the augmented-plane-wave method

The band structure of palladium hydride (PdH) is calculated using the augmented-plane-wave method. Using these calculations the Fermi energy, Fermi surface, de Haas-van Alphen frequencies, and density of states have been determined. These results are compared with our previous calculations for Pd and with other reported results for PdH.     

Interchain coupling in Hg3−δAsF6

An electronic bandstructure of the incommensurate linear chain compound Hg_3?δAsF₆ has been calculated by considering various commensurate compounds approximating the real compound. From these calculations a Fermi surface has been determined in qualitative and even close quantitative agreement with experiment. The interchain coupling is found to be strong (?60mRy) but some corners of the Fermi surface are infinitely sharp because of the screw axis symmetry element of the space group with a concommitant vanishing of the structure constant. This work solves a previously posed dilemma.     

Geometry effects on the electronic properties of YBa2Cu3O7

We have performed electronic structure calculations for the high T _c compound YBa₂Cu₃O₇ with a 3% reduced unit-cell volume. The effect on band structure and Fermi surface is investigated. The predominant features are van Hoove singularities at the Fermi level. Keeping the experimental lattice parameters, nearly the same effects occur when replacing the local-density approximation by a generalized gradient correction scheme.     

Fermi surface of LaAg

Electronic structure, especially the Fermi surface, is calculated for the intermetallic rare-earth compound LaAg, known to show the structural phase transition when In is substituted for Ag, by a self-consistent fully-relativistic APW method with the exchange-correlation potential in a local-density approximation. The Fermi surface is found to consist of large hole and electron sheets as well as small hole and electron sheets. This result confirms well the theoretical prediction by Niksch et al. (1987). These Fermi surface sheets are found to explain the experimental results for the de Haas-van Alphen effect by Niksch et al. (1987) and Motoki et al. (1995) reasonably well. But, the frequency branches originating from the large hole sheet have been observed only partially. Local curvature of the large hole sheet is investigated as a possible origin of the disappearance of these frequency branches.     

De Haas-van Alphen effect in rhodium

The de Haas-van Alphen effect has been used to study the extremal areas and effective cyclotron masses on all five sheets of the Fermi surface of rhodium for the magnetic field in a (110)-plane. The measured extremal areas are in good agreement with relativistic-augmented-plane-wave calculations. The resulting deviations correspond to energy shifts of the calculated bands not exceeding 4 mRy. Several extremal orbits on the fifth band Γ-centered electron sheet have been observed. The mass enhancement determined from the ratio between the calculated and measured effective cyclotron masses is found to vary substantially over the different sheets of the Fermi surface. A rather isotropie factor of 1.40 is obtained for the sixth band Γ-centered electron sheet. For the third and fourth band hole pockets we obtained enhancement factors in the region 0.9–1.4.     

Topological phase transition in the trirutile-type MgBi2O6

Based on the first-principle calculations and k⋅p effective model analysis, we predicted a new topological semimetal (TSM) MgBi₂O₆. Without spin-orbit-coupling (SOC) and under the generalized-gradient-approximation (GGA), MgBi₂O₆ is a nodal-line semimetal. When the exchange-correlation energy was changed to HSE06, MgBi₂O₆ was trivial insulator in the equilibrium volume, but it became TSM under 7% hydrostatic tensile strain. MgBi₂O₆ might be an important platform to study the topological properties because of the two following advantages for measurements: (1) The nodal line, drumhead-liked surface state and Fermi Arc are very closely to the Fermi level; (2) The band structure is very “clean” (no other bulk bands except the related inverted conduction and valence bands around the Fermi level), which avoids the surface states been embedded into the bulk states.     

3C-SiC(001)-(2×1)表面原子与电子结构研究

采用广义梯度近似的密度泛函理论方法计算了3C-SiC（001）-（2×1）表面的原子及电子结构．计算结果表明,3C-SiC（001）-（2×1）表面为非对称性的Si二聚体模型,其二聚体的Si原子间键长为0.232 nm．电子结构的计算结果表明,在费米能级处有明显的态密度,因此3C-SiC（001）-（2×1）表面呈金属性．在带隙附近存在四个表面态带,一个位于费米能级附近,一个位于费米能级以上5 eV处,另外两个位于费米能级以下的价带中． 关键词： 碳化硅 密度泛函理论计算 原子结构 电子结构     

Novel macroscopic BC3 honeycomb sheet

We have successfully grown a uniform BC₃ honeycomb sheet with high crystalline quality over the entire macroscopic area of the NbB₂ (0001) surface by carbon-substituted technique in a boron honeycomb. This is the first macroscopic uniform sheet of single-crystal BC₃ with a monolayer thickness. So far, only BC₃ micro-crystals with radii of a few nanometres were produced. The properties of the sheet have been investigated by using XPS, AES, LEED, STM, ARUPS and HREELS. All the experimental data and the theoretical calculations, which are concerning with atomic structure, chemical concentration, and phonon structure, indicated that the uniform BC₃ honeycomb sheet was grown in an epitaxial manner to the substrate lattice.     

Structural and electronic properties and the fermi surface of the new non-centrosymmetric superconductors: 3.6 K CaIrSi<Subscript>3</Subscript> and 2.3 K CaPtSi<Subscript>3</Subscript>

Ab initio FLAPW-GGA calculations have been performed to investigate structural properties, electronic band structure, and Fermi surface topology of the newly discovered non-centrosymmetric superconductors: 3.6 K CaIrSi₃ and 2.3 K CaPtSi₃. As a result, the peculiarities of the crystal structure, electronic bands, total and site-projected l-decomposed densities of states, and the shape of the Fermi surface for CaIrSi₃ and CaPtSi₃ were obtained and analyzed.     

Doppler-shifted acoustic cyclotron resonance in tungsten

Doppler-shifted cyclotron resonances have been observed acoustically in high purity tungsten single crystals. The resulting Fermi surface area derivatives agree generally with those obtained from magnetomorphic size effect experiments and with calculations based on an empirical model fitted to de Haas-van Alphen data. Certain non-central orbit zones of the latter model are modified to fit both sets of results. The resulting correction to the hole octahedron sheet is minimal, the modified electron jack has a more spherical shape, and the necks are slightly further away from the zone center. The dimensions of the re-parameterised model agree better with the results obtained from other Fermi surface measurements.     

Band structure of (TMTSF)2X

The electronic structure of the organic conductors (TMTSF)₂X has been explored in terms of the tight binding band structures calculated for a sheet of TSF molecules. The Se 4d-orbitals appear to be critical in enhancing the interstack Se…Se interaction to the point that (TMTSF)₂X becomes pseudo two-dimensional. Based upon the present band structure study, it is discussed whether a normal metallic state or a spin density wave state provides the closed Fermi surface responsible for the Shubnikov-de Haas oscillations observed in (TMTSF)₂PF₆.     

Electronic structure of new LiFeAs high-T c superconductor

We present results of ab initio LDA calculations of electronic structure of “next generation” layered iron-pnictide high-T _c superconductor LiFeAs (T _c = 18 K). Obtained electronic structure of LiFeAs very similar to recently studied ReOFeAs (Re = La, Ce, Pr, Nd, Sm) and AFe₂As₂ (A = Ba, Sr) compounds. Namely close to the Fermi level its electronic properties are also determined mainly by Fe 3d-orbilats of FeAs₄ two-dimensional layers. Band dispersions of LiFeAs are very similar to the LaOFeAs and BaFe₂As₂ systems as well as the shape of the Fe-3d density of states and Fermi surface. The article is published in the original.