Absence of a holelike fermi surface for the iron-based K0.8F1.7Se2 superconductor revealed by angle-resolved photoemission spectroscopy

We have performed an angle-resolved photoemission spectroscopy study of the new iron-based superconductor K(0.8)Fe(1.7)Se(2) (T(c)～30 K). Clear band dispersion is observed with the overall bandwidth renormalized by a factor of 2.5 compared to our local density approximation calculations, indicating relatively strong correlation effects. Only an electronlike band crosses the Fermi energy, forming a nearly circular Fermi surface (FS) at M (π, 0). The holelike band at Γ sinks ～90 meV below the Fermi energy, with an indirect band gap of 30 meV, to the bottom of the electronlike band. The observed FS topology in this superconductor favors (π, π) inter-FS scattering between the electronlike FSs at the M points, in sharp contrast to other iron-based superconductors which favor (π, 0) inter-FS scattering between holelike and electronlike FSs.     

Fermi surface evolution and Luttinger theorem in Na(x)CoO2: a systematic photoemission study

We report a systematic angle-resolved photoemission study on Na(x)CoO2 for a wide range of Na concentrations (0.3 < or = x < or = 0.72). In all the metallic samples at different x, we observed (i) only a single holelike Fermi surface centered around gamma and (ii) its area changes with x according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The e'(g) band and the associated small Fermi surface pockets near the K points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.     

Doped Mott insulator as the origin of heavy-fermion behavior in LiV2O4

We investigate the electronic structure of LiV2O4, for which heavy-fermion behavior has been observed in various experiments, by the combination of the local density approximation and dynamical mean field theory. To obtain results at zero temperature, we employ the projective quantum Monte Carlo method as an impurity solver. Our results show that the strongly correlated a 1g band is a lightly doped Mott insulator which, at low temperatures, shows a sharp (heavy) quasiparticle peak just above the Fermi level, which is consistent with recent photoemission experiments by Shimoyamada et al. [Phys. Rev. Lett. 96, 026403 (2006)10.1103/PhysRevLett.96.026403].     

Electronic structure and Fermi surface of Bi2Sr2CaCu2O8

Applying angle-resolved photoemission spectroscopy of high angular and energy resolution we have measured the electronic structure of single crystals of Bi₂Sr₂CaCu₂O₈ near the Fermi energy. Along the high symmetry direction X a band is observed to disperse upwards and to cross the Fermi level, whose unoccupied part constitutes the hole-like carriers responsible for the superconductivity. From spectra along the M direction we find evidence for an electron pocket around the M point. The measured band widths appear to be drastically reduced compared with band structure calculations indicating strong electronic correlation effects. From the observation of Fermi-Dirac-like cut-offs in the spectra due to band crossings through the Fermi level we can confirm the existence and, in particular, the shape of the Fermi surface as calculated by band structure calculations.     

Weakly dispersive band near the fermi level of GaMnAs due to Mn interstitials

The nature of the weakly dispersive electronic band near the Fermi level observed in photoemission experiments on the diluted magnetic semiconductor GaMnAs is investigated theoretically. The combination of experimental features appears puzzling. We show that the formation of the band is closely related to the presence of the Mn interstitial impurities. The states forming the band have predominantly minority-spin Mn-3d character. The low experimental Mn-3d intensity is explained by the low content of the interstitial Mn impurities. The features of the band are robust with respect to the calculational technique [local density approximation (LDA), LDA + U].     

Evolution of the Fermi surface across a magnetic order-disorder transition in the two-dimensional Kondo lattice model: a dynamical cluster approach

We use the dynamical cluster approximation, with a quantum Monte Carlo cluster solver on clusters of up to 16 orbitals, to investigate the evolution of the Fermi surface across the magnetic order-disorder transition in the two-dimensional doped Kondo lattice model. In the paramagnetic phase, we observe the generic hybridized heavy-fermion band structure with large Luttinger volume. In the antiferromagnetic phase, the heavy-fermion band drops below the Fermi surface giving way to hole pockets centered around k=(pi/2,pi/2) and equivalent points. In this phase Kondo screening does not break down, but the topology of the resulting Fermi surface is that of a spin-density wave approximation in which the localized spins are frozen.     

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi-one-dimensional CuO chains by microprobe angle resolved photoemission spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (～40 meV), temperature dependence, and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high-temperature superconductors, indicating they have a common origin.     

Insight into the f-derived Fermi surface of the heavy-fermion compound YbRh2Si2

Angle-resolved photoelectron spectroscopy (ARPES) was used to study the Fermi surface of the heavy-fermion system YbRh(2)Si(2) at a temperature of about 10 K, i.e., a factor of 2 below the Kondo energy scale. We observed sharp structures with a well-defined topology, which were analyzed by comparing with results of band-structure calculations based on the local-density approximation (LDA). The observed bulk Fermi surface presents strong similarities with that expected for a trivalent Yb state, but is slightly larger, has a strong Yb-4f character, and deviates from the LDA results by a larger region without states around the Γ point. These properties are qualitatively explained in the framework of a simple f-d hybridization model. Our analysis highlights the importance of taking into account surface states and doing an appropriate projection along k(z) when comparing ARPES data with results from theoretical calculations.     

Momentum dependence of the electron-phonon coupling and self-energy effects in superconducting YBa2Cu3O7 within the local density approximation

Using the local density approximation and a realistic phonon spectrum we determine the momentum and frequency dependence of alpha(2)F(k,omega) in YBa(2)Cu(3)O(7) for the bonding, antibonding, and chain band. The resulting self-energy Sigma is rather small near the Fermi surface. For instance, for the antibonding band the maximum of ReSigma as a function of frequency is about 7 meV at the nodal point in the normal state and the ratio of bare and renormalized Fermi velocities is 1.18. These values are a factor of 3-5 too small compared to the experiment showing that only a small part of Sigma can be attributed to phonons. Furthermore, the frequency dependence of the renormalization factor Z(k,omega) is smooth and has no anomalies at the observed kink frequencies which means that phonons cannot produce well-pronounced kinks in stoichiometric YBa(2)Cu()3)O(7), at least, within the local density approximation.     

FRANZ-KELDYSH OSCILLATION FROM THE SPACE CHARGE REGION OF MBE GaAs FILMS

We have investigated doped MBE GaAs films using photoreflectance (PR) spec-troscopy. Special spectral structures have been observed in the vicinity of the funda-mental band gap, which are quite different from the Franz-Keldysh oscillation (FKO) from uniform electric fields under flatband modulations. Numerical analysis has been performed for FKO from electric fields in the space charge region under non-flatband modulations. Some typical FKO line shapes are illustrated. For moderately doped samples the calculated line shapes are basically consistent with experiments. The surface electric field and the Fermi level pinning have also been deduced from exper-iments.     

Electronic structure of La (0001) thin films on W (110) studied by photoemission spectroscopy and first principle calculations

Surface states that have a dz2 symmetry around the center of the surface Brillouin zone(BZ)have been regarded common in closely-packed surfaces of rare-earth metals.In this work,we report the electronic structure of dhcp La(0001)thin films by ultrahigh energy resolution angle-resolved photoemission spectroscopy(ARPES)and first principle calculations.Our first principle analysis is based on the many-body approach,therefore,density function theory(DFT)combined with dynamic mean-field theory(DMFT).The experimentally observed Fermi surface topology and band structure close to the Fermi energy qualitatively agree with first principle calculations when using a renormalization factor of between 2 and 3 for the DFT bands.Photon energy dependent ARPES measurements revealed clear kZ dependence for the hole-like band around the BZ center,previously regarded as a surface state.The obtained ARPES results and theoretical calculations suggest that the major bands of dhcp La(0001)near the Fermi level originate from the bulk La 5d orbits as opposed to originating from the surface states.     

Raising Bi-O bands above the Fermi energy level of hole-doped Bi2Sr2CaCu2O8+delta and other cuprate superconductors

The Fermi surface (FS) of Bi2Sr2CaCu2O8+delta (Bi2212) predicted by band theory displays Bi-related pockets around the (pi, 0) point, which have never been observed experimentally. We show that when the effects of hole doping either by substituting Pb for Bi or by adding excess O in Bi2212 are included, the Bi-O bands are lifted above the Fermi energy (E(F)) and the resulting first-principles FS is in remarkable accord with measurements. With decreasing hole doping the Bi-O bands drop below and the system self-dopes below a critical hole concentration. Computations on other Bi- as well as Tl- and Hg-based compounds indicate that lifting of the cation-derived band with hole doping is a general property of the electronic structures of the cuprates.     

Study on the electronic structure and Fermi surface of 3d-transition-metal disilisides CoSi2

We have investigated the electronic structure, the momentum density distribution ρ(p), and the Fermi surface FS of single crystals of the Pyrite-type 3d-transition-metal disilisides CoSi₂. The band structure calculations, the density of states DOS, and the FS, in vicinity of Fermi level, have been carried out using the full-potential linearized augmented plane wave FP-LAPW method within generalized gradient approximation GGA for exchange and correlation potential. The measurements have been performed via the 2D angular correlation of annihilation radiation ACAR experiments. ρ(p) has been reconstructed by using the Fourier transformation technique. The FS has been reconstructed within the first Brillion zone BZ through the Locks, Crisp, and West LCW folding procedures. The analysis confirmed that Si 3sp states hybrid with both Co 3d–t _2g and Co 3d–e _g states around Γ and X points, respectively. The dimensions of the FS of CoSi₂ have been compared to the present calculations as well as to the earlier results.     

Photofield emission spectroscopy of electronic states near the fermi level of 111 tungsten

The two closely situated electronic bands for the 111 tungsten direction between the Fermi level and the vacuum level have been observed using photofield emission spectroscopy and s-polarized light in the visible range. Measurements with p-polarized light reveal surface states below the Fermi level. Optical transitions have been found from the shoulders of the photocurrent characteristics. The observed excitations are in good agreement with the bulk band structure and the surface density of states of tungsten calculated by theory.     

Ab initio study of surface electronic structure of phosphorus donor impurity on Ge(111)-(2×1) surface

We present the results of numerical modeling of the electronic properties of the Ge(111)-(2 × 1) surface in the vicinity of a P donor impurity atom near the surface. We have shown that, in spite of well-established bulk donor impurity energy level position at the very bottom of the conduction band, the surface donor impurity might produce an energy level below the Fermi energy, depending on impurity atom local environment. It has been demonstrated that the impurity located in subsurface atomic layers is visible in scanning tunneling microscopy experiment. The quasi-one-dimensional character of the impurity image observed in scanning tunneling microscopy experiments is confirmed by our computer simulations.     

Fermi surface and electrical characteristics of molybdenum disilicide

Semirelativistic self-consistent calculations of the electronic structure of MoSi₂ are performed within the framework of the linearized augmented-plane-wave (APW) method in the local density functional approximation. The results of investigations of the band structure, the Fermi surface, and electrical characteristics (effective cyclotron masses, the conductivity anisotropy constant, the mean free path, and the coefficient γ of the heat capacity component linear in temperature) are reported. The Fermi surface consists of two sheets, namely, an electron sheet and a hole sheet. The extreme sectional areas of the Fermi surface agree well with the experimental data on the de Haas-van Alphen effect. The results of first-principles calculations need no additional correction.     

Theory of B(2)O and BeB(2) nanotubes: new semiconductors and metals in one dimension

We describe two new boron-based nanotubes: B(2)O and BeB(2). Both are isoelectronic to graphite, have reasonable curvature energies, and have already been made in their bulk planar forms. The lowest energy allotrope of planar single-layer B(2)O is a semiconductor with a moderate band gap. The local density approximation band gap of the corresponding (3,0) B(2)O nanotube [similar in size to (9,0) carbon nanotube tube] is direct and around 1.6 eV, within a range inaccessible to previous C or BN nanotubes. Single-layer BeB(2) has a fascinating structure: the Be atoms rest above the boron hexagonal faces, nearly coplanar to the boron sheet. The unusual K-point pi-pi(*) Fermi-level degeneracy of graphite survives, while a new nearly pointlike Fermi surface appears at the M point. As a result, BeB(2) nanotubes are uniformly metallic.     

Electronic structures of vacancies in Co_3Sn_2S_2

Co_3Sn_2S_2 has attracted a lot of attention for its multiple novel physical properties, including topological nontrivial surface states, anomalous Hall effect, and anomalous Nernst effect. Vacancies, which play important roles in functional materials, have attracted increasing research attention. In this paper, by using density functional theory calculations, we first obtain band structures and magnetic moments of Co_3Sn_2S_2 with exchange–correlation functionals at different levels. It is found that the generalized gradient approximation gives the positions of Weyl points consistent with experiments in bulk Co_3Sn_2S_2. We then investigate the electronic structures of defects on surfaces with S and Sn terminations which have been observed in experiments. The results show that the single sulfur vacancy on the S-terminated surface introduces localized bond states inside the bandgap near the Fermi level. For di-and tri-sulfur vacancies, the localized defect states hybridize with neighboring ones, forming bonding states as well as anti-bonding states. The Sn vacancy on the Sn-terminated surface also introduces localized bond states, which are merged with the valence bands. These results provide a reference for future experimental investigations of vacancies in Co_3Sn_2S_2.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Continuous transition from two- to one-dimensional states in Si(111)-( 5x2)-Au

A strong, gold-induced surface state is found on single-domain Si(111)-(5x2)-Au at low temperatures. Its band dispersion is one dimensional near the Fermi level E(F) and gradually becomes two dimensional towards the bottom of the band, thus providing a model for a continuous transition in dimensionality. A Peierls-like gap is observed in the one-dimensional portion of the band near E(F).