Pseudogap and Superconducting Gap in SmFeAs（O1-xFx） Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting SmOFeAs paxent compound and superconducting SmFeAs（O1-xFx） （x = 0.12, and 0.15） compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near IF, F with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.     

Multiple Nodeless Superconducting Gaps in (Ba0.6K0.4)Fe2As2 Superconductor from Angle-Resolved Photoemission Spectroscopy

High resolution angle-resolved photoemission measurements have been carried out to study the superconducting gap in the (Ba_0.6K_0.4)Fe₂As₂ superconductor with T_c=35 K. Two hole-like Fermi surface sheets around the Γ point exhibit different superconducting gaps. The inner Fermi surface sheet shows larger (10~12 meV) and slightly momentum-dependent gap while the outer one has smaller (7~8meV) and nearly isotropic gap. The lack of gap node in both Fermi surface sheets favours s-wave superconducting gap symmetry. Superconducting gap opening is also observed at the M(π,π) point. The two Fermi surface spots near the M point are gapped below T_c but the gap persists above T_c. The rich and detailed superconducting gap information will provide key insights and constraints in understanding pairing mechanism in the iron-based superconductors.     

Angle Integrated Photoemission Study of SmO0.85F0.15FeAs

The electronic structure of the new superconductor SmO_1-xF_xFeAs (x=0.15) is studied by angle-integrated photoemission spectroscopy. Our data show a sharp feature very close to the Fermi energy, and a relative flat distribution of the density of states between 0.5eV and 3eV binding energy, which agrees well with the band structure calculations considering an antiferromagnetic ground state. No noticeable gap opening is observed at 12K below thesuperconducting transition temperature, indicating the existence of large ungapped regions in the Brillouin zone.     

Enhanced Orbital Degeneracy in Momentum Space for LaOFeAs

The Fermi surfaces （FS） of LaOFeAs （in kz = 0 plane） consist of two hole-type circles around F point, which do not touch each other, and two electron-type co-centred ellipses around the M point, which are degenerate along the M - X line. By first-principles calculations, here we show that additional degeneracy exists for the two electron-type FS, and the crucial role ofF-doping and pressure is to enhance this orbital degeneracy. It is therefore suggested that the inter-orbital fluctuation is important to understand the unconventional superconductivity in these materials.     

Superconductivity in compressed potassium and rubidium

Calculations of the electron-phonon interaction in the alkali metals, potassium and rubidium, using the results of band theory and BCS theory-based techniques suggest that at high pressures K and Rb would be superconductors with transition temperatures approaching 10 K.     

Progress in the understanding of the normal state of the cuprates

Over the last years there has been increasing evidence that the normal state of the cuprates can not be described adequately with individual quasiparticles within Fermi-liquid theory. While the low-lying excitations in the superconducting state are nevertheless possibly of quasiparticle character, this character vanishes with the loss of superconducting phase coherence when going to the normal conducting state. Generally, this normal state is characterized by strong heterogeneity. In real space this manifests in charge and spin ordering, either static or dynamical, the so-called ‘stripes’. The spectral signatures of various models describing this unusual metallic normal state together with less exotic non-Fermi-liquid models, like the marginal Fermi liquid, will be compared to photoemission spectra with high angular and energy resolution and to results obtained by other methods. Received: 19 March 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/2093-7729, E-mail: christoph.janowitz@physik.hu-berlin.de     

Mn and Fe impurities in MgB2

Based on first principles calculations, we show that Mn impurities are magnetic in MgB₂ due to exchange-splitting of band. Thus, Mn impurities could act as strong magnetic scattering centres leading to pair-breaking effects in MgB₂. In contrast, we find Fe impurities in MgB₂ to be nearly non-magnetic.     

First-principles investigation of pressure-induced changes in structural and electronic properties of Y 2C3 superconductor

The structural and electronic properties of Y ₂C₃ superconductor under different external pressures were calculated by employing the first-principles method. This shows that the lattice constants as well as the lengths of C-C dimers decrease with the pressure. Results of band structure calculations indicate that the Fermi level advances to the bonding zone with an increase in pressure; meantime, the valence and conduction bands intersect more deeply with the Fermi level. Moreover, the Fermi level is found to shift from the valley bottom of the density of states (DOS) curve to the shoulder, which means an increase in N(E_F), and therefore the critical temperature, T_c. The calculations verify that the critical temperature is directly related to the electronic structure.     

Photoemission of molecular adsorbates

In photoemission the observed changes in band intensities as a function of light polarization, electron emission direction, and photon energy can be used to deduce information, for example, about geometry, two-dimensional band structures, intermolecular interactions and chemical reactivity of molecular adsorbates. To characterize the ion states of molecular adsorbates in the so-called inner valence electron region we discuss results from autoionization spectroscopy as a method that yields complementary information to photoemission in this spectral region. We use results on CO, N₂, and CO₂ adsorbates to discuss the various aspects of photoemission of molecular adsorbates.     

Electronic Structures of Wurtzite GaN with Ga and N Vacancies

The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GMV due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.     

Element Substitution Effect in Transition Metal Oxypnictide Re(O1-xFx)TAs (Re=rare earth, T=transition metal)

Different element substitution effects in transition metal oxypnictide Re(O_1-xF_x)TAs, with Re=La, Ce, Nd, Eu, Gd, Tm, T=Fe, Ni, Ru, are studied. Similarto the La- or Ce-based systems, we find that the pure NdOFeAs shows a strong resistivity anomaly near 145K, which is ascribed to the spin-density-wave instability. Electron doping by F increases T_c to about 50K. While in the case of Gd, Tc is reduced below 10K. The tetragonal ZrCuSiAs-type structure could not be formed for Eu or Tm substitution in our preparing process. For the Ni-based case, although both pure and F-doped LaONiAs are superconducting, no superconductivity is found when La is replaced by Ce in both the cases, instead a ferromagnetic ordering transition is likely to form at low temperature in the undoped sample. We also synthesize LaO_1-xF_xRuAs and CeO_1-xF_xRuAs compounds. The metallic behaviour is observed down to 4K.     

X-Ray Photoelectron Spectroscopy and Reflection High Energy Electron Diffraction of Epitaxial Growth SiC on Si（100） Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si on Si（100） surface at 850^o C is studied via x-ray photoelectron spectroscopy and reflection high energy electron diffraction （RHEED）. The C ls, O ls and Si 2p core-level spectra and the RHEED patterns indicate the formation of 3C-SiC.     

AuB2 in comparison to superconducting MgB2-an ab initio study

The noble metal diboride AuB₂, a potential candidate for superconductor, is studied by an ab initio method in comparison to the superconducting MgB₂. The results, described in terms of equilibrium lattice constants, bulk modulus, pressure derivative of bulk modulus and their in- and out-of-plane linear values, volume coefficient of T_c, density of states, band structure, show some similarity as well as dissimilarity between the behaviour of the two compounds. The implications for the behaviour are discussed.     

The Minimum Model for the Iron-Based Superconductors

A single band t-U-J₁-J₂ model is proposed as the minimum model to describe the superconductivity of the newly discovered iron-based superconductorsR(O_1-xF_x)FeAs and RO_1-xFeAs (R=La, Ce, Sm, Pr, Nd, Gd). With the mean-field approach, it is found that the pairing occurs in the d-wave channel. In the likely parameter region of the real materials, by lowering temperature, the system enters firstly the d_xy superconducting phase with D_4h-symmetryand then enters the time-reversal-symmetry-broken d^_xy+id_x²-y² superconducting phase with C_4h-symmetry.     

On the propensity of magnetism in 3d transition-metal-MgCNi3 alloys

The effects of disorder and incipient magnetism in MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ (T≡Fe$\mathrm{T}\equiv \mathrm{Fe}$, Co or Cu) alloys are studied using coherent-potential approximation and Ginzburg–Landau coefficients. The first-principles, local-density-functional-based calculations for substitutionally disordered Fe and Co impurities in the Ni sub-lattice of MgCNi₃, in low concentrations, show that incipient magnetism resides in these materials. The overestimation of the calculated magnetic properties points to the limitations of the local-density approximation. However, using a phenomenological approach based on Ginzburg–Landau coefficients and the fixed-spin moment method, we show that MgC(_Ni1-xTx)3$\mathrm{MgC}({\mathrm{Ni}}_{1-x}{\mathrm{T}}_x{)}_3$ alloys remain paramagnetic. At expanded volumes, we also find the possibility of a ferromagnetic state for MgC(_{Ni0.95Fe0.05)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.95}{\mathrm{Fe}}_{0.05}{)}_3$ and MgC(_{Ni0.90Co0.10)3}$\mathrm{MgC}({\mathrm{Ni}}_{0.90}{\mathrm{Co}}_{0.10}{)}_3$ alloys.     

Electronic Structure and Optical Properties of Semiconducting Orthorhombic BaSi2

Full potential linearized augmented plane wave （FPLAPW） method calculations are carried out for semiconducting orthorhombic BaSi2. The optical properties and the origin of the different optical transitions are investigated. Our calculated band gap of 1.0918eV is indirect, which is in good agreement with the experimental result. The bonds between Ba and Si are considered to be electrovalent bond. The anlsotropy in the imaginary part ε2（w） and real part εl（w） of the optical dielectric tensor are analysed. The contributions of various transition peaks are explained from the imagnary part of the dielectric function.     

Electronic structure of icosahedral alloys: The case of Al65Cu20Os15

Measurements with photoemission spectroscopy in the photon-energy range 35–130 eV have been used to determine the valence band of the stable icosahedral Al₆₅Cu₂₀Os₁₅. Resonant photoemission near the Os 5p 5d and 4f 5d transitions has been employed to show that the feature in the valence band with the maximum intensity at 1.5 (1) eV below the Fermi level is predominantly of the Os 5d character. This has been additionally verified by conducting the photoemission measurements in the constant-initial-state mode and by using the effect of the Cooper minimum in the photoionization cross section of the Os 5d orbitals. The valence band feature with the maximum intensity at 3.7 (1) eV below the Fermi level has been shown as being due mainly to the states of the Cu 3d character. The Os 5d and Cu 3d empirical partial density of states have been determined from the photoemission spectra. The decrease of intensity towards the Fermi level has been interpreted as being indicative of the presence of a theoretically predicted pseudogap around the Fermi level. It has been indicated, however, that the Fermi cut-off also contributes to the observed intensity decrease. It has been demonstrated that the energy resolution of the spectroscopic measurements performed so far on quasicrystals was not high enough to unambiguously determine the presence of such a pseudogap. No unusual features in the valence band of icosahedral Al₆₅Cu₂₀Os₁₅, which could be ascribed to its quasiperiodic nature, have been observed within the resolution of the experiment. High energy-resolution spectroscopic measurements were also shown to be essential to observe the theoretically predicted spikiness of the density of states in quasicrystals. A critical review of published spectroscopic data on the electronic structure of quasicrystals has also been presented.     

First-Principles Study of Defects in CuGaO2

Using the first-principles methods, we study the electronic structure, intrinsic and extrinsic defects doping in transparent conducting oxides CuGaO₂. Intrinsic defects, acceptor-type and donor-type extrinsic defects in their relevant charge state are considered. The calculation result show that copper vacancy and oxygen interstitial are the relevant defects in CuGaO₂. In addition, copper vacancy is the most efficient acceptor. Substituting Be for Ga is the prominent acceptor, and substituting Ca for Cu is the prominent donors in CuGaO₂. Our calculation results are expected to be a guide for preparing n-type and p-type materials in CuGaO₂.     

On the Fermi surface of some organic superconductors with sizeable critical temperatures

Recent studies of the Fermi surface of quasi-two-dimensional organic conductors comfort the idea that the presence of a van Hove anomaly near the Fermi level is a favourable factor for superconductivity. Received: 28 April 1998 / Revised and accepted: 13 May 1998     

Energy shift of photoemission spectra for organics-passivated CdSe nanoparticles: The final-state effect

Size-dependent energy shift of photoemission spectra with respect to bulk sample has been examined for colloidally prepared CdSe nanoparticles with a series of particle sizes. The core-level shifts are well described by a theoretical calculation based on a final-state effect model, whereas an additional initial-state effect due to quantum confinement is required to elucidate the valence-band edge shifts. The results indicate that the interaction between the photohole and the dielectric background in the final state has to be considered in photoemission measurements for organics-passivated nanoparticles. The calculated results in the literature appear to overestimate the initial-state effect compared to our experimental observation.