Some comments on the electronic properties of the surface space charge layer of copper phthalocyanine thin films

A simple analysis, using a theory of the surface space charge layer of semiconductors, of the published values of the work function φ and surface ionization energy Φ_s of copper phthalocyanine (CuPc) thin films was performed. Using a well known position of the Fermi level E_F within the band gap E_g the values of its absolute band bending eV_s and surface electron affinity X_s were determined. A small negative value of the absolute band bending eV_s = −0.17 ∓ 0.15 eV has been interpreted by the existence of the filled electronic surface states localized in the band gap below the Fermi level E_F. Such states were predicted theoretically for thin films and the crystalline surface of CuPc, and attributed to surface lattice defects of a high concentration.     

Electronic structure of new multiple band Pt-pnictide superconductors APt3P

We report LDA calculated band structure, densities of states and Fermi surfaces for recently discovered Pt-pnictide superconductors APt₃P (A = Ca, Sr, La), confirming their multiple band nature. Electronic structure is essentially three dimensional, in contrast to Fe pnictides and chalcogenides. LDA calculated Sommerfeld coefficient agrees rather well with experimental data, leaving little space for very strong coupling super-conductivity, suggested by experimental data on specific heat of SrPt₃P. Elementary estimates show, that the values of critical temperature can be explained by rather weak or moderately strong coupling, while the decrease in superconducting transition temperature T _c from Sr to La compound can be explained by corresponding decrease in total density of states at the Fermi level N(E _F). The shape of the density of states near the Fermi level suggests that in SrPt₃P electron doping (such as replacement Sr by La) decreases N(E _F) and T _c , while hole doping (e.g., partial replacement of Sr with K, Rb or Cs, if possible) would increase N(E _F) and possibly T _c .     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

Energy spectrum of charge carriers in Bi<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

Analysis of the quantum oscillations of magnetoresistance (the Shubnikov-de Haas effect) in Bi_{1 ? x} Sb _x alloys with an antimony content in the range 0.255 < x < 0.260 has revealed a Lifshitz electronic-topological transition, which quite possibly can be explained in terms of the existence of a saddle point in the energy spectrum of these compositions. Such a peculiarity comes into existence when the direct band gap at the L point of the Brillouin zone in the semiconductor region of the compounds with x > 0.04 becomes negative. This compel one to revise essentially all earlier calculations based on the previously obtained values of the band parameters. In order to check the agreement between the new values of the band parameters and the data on the density of states obtained from measurements of the thermopower in the classical limit of strong magnetic fields, theoretical calculations of the charge carrier concentration n and the density of states at the Fermi level ρ(E _F) have been performed for the case of negative values of the direct band gap at the L point E _gL. The calculations of the parameters n and ρ(E _F) have demonstrated that the change in E _gL and the corresponding correction of the band parameters ensure good agreement with the experimental data. According to these calculations, one electronic-topological transition occurs at an antimony content x ～ 0.165, when a saddle point appears in the energy spectrum. The second transition is associated with the transformation of the six ellipsoids of the Fermi surface into three dumbbell-like figures at antimony concentrations in the range 0.255 < x < 0.260.     

Fractal superconductivity near localization threshold

We develop a semi-quantitative theory of electron pairing and resulting superconductivity in bulk “poor conductors” in which Fermi energy E_F is located in the region of localized states not so far from the Anderson mobility edge E_c. We assume attractive interaction between electrons near the Fermi surface. We review the existing theories and experimental data and argue that a large class of disordered films is described by this model.Our theoretical analysis is based on analytical treatment of pairing correlations, described in the basis of the exact single-particle eigenstates of the 3D Anderson model, which we combine with numerical data on eigenfunction correlations. Fractal nature of critical wavefunction's correlations is shown to be crucial for the physics of these systems.We identify three distinct phases: ‘critical’ superconductive state formed at E_F = E_c, superconducting state with a strong pseudo-gap, realized due to pairing of weakly localized electrons and insulating state realized at E_F still deeper inside a localized band. The ‘critical’ superconducting phase is characterized by the enhancement of the transition temperature with respect to BCS result, by the inhomogeneous spatial distribution of superconductive order parameter and local density of states. The major new feature of the pseudo-gapped state is the presence of two independent energy scales: superconducting gap Δ, that is due to many-body correlations and a new “pseudo-gap” energy scale Δ_P which characterizes typical binding energy of localized electron pairs and leads to the insulating behavior of the resistivity as a function of temperature above superconductive T_c. Two gap nature of the pseudo-gapped superconductor is shown to lead to specific features seen in scanning tunneling spectroscopy and point-contact Andreev spectroscopy. We predict that pseudo-gapped superconducting state demonstrates anomalous behavior of the optical spectral weight. The insulating state is realized due to the presence of local pairing gap but without superconducting correlations; it is characterized by a hard insulating gap in the density of single electrons and by purely activated low-temperature resistivity ln R(T) ∼ 1/T.Based on these results we propose a new “pseudo-spin” scenario of superconductor-insulator transition and argue that it is realized in a particular class of disordered superconducting films. We conclude by the discussion of the experimental predictions of the theory and the theoretical issues that remain unsolved.     

First-principles study on the electronic structures of iron phthalocyanine monolayer

The electronic band structure and magnetic properties of iron phthalocyanine (FePc) monolayer were investigated by using the first-principles all-electron full-potential linearized augmented plane wave energy band method. It is found that the ferromagnetic FePc monolayer is energetically more stable than the paramagnetic one. The exchange interaction, which splits the majority and minority bands, influences strongly on the electronic structure near the Fermi level (E_F). Magnetic moment of the central Fe atom is calculated to 1.95 μ_B. The range of the positive polarization of Fe site is larger in the out-of-plane than in the in-plane direction. The FePc ligand remains paramagnetic. The presence of states at E_F indicates the metallic character of FePc monolayer both for the paramagnetic and ferromagnetic states. However, the large density of states at E_F of the majority spins in the ferromagnetic state is expected to cause a phase transition to insulating antiferromagnetic state from the metallic ferromagnetic one.     

Electronic properties of orthorhombic LiGaS<Subscript>2</Subscript> and LiGaSe<Subscript>2</Subscript>

We report theoretical calculations of the band structure and density of states for orthorhombic LiGaS₂ (LGS) and LiGaSe₂ (LGSe). These calculations are based on the full potential linear augmented plane wave (FP-LAPW) method within a framework of density functional theory. Our calculations show that these crystals have similar band structures. The valence band maximum (VBM) and the conduction band minimum (CBM) are located at Γ, resulting in a direct energy band gap. The VBM is dominated by S/Se-p and Li-p states, while the CBM is dominated by Ga-s, S/Se-p and small contributions of Li-p and Ga-p. From the partial density of states we find that Li-p hybridizes with Li-s below the Fermi energy (E _F), while Li-s/p hybridizes with Ga-p below and above E _F. Also, we note that S/Se-p hybridizes with Ga-s below and above E _F.     

Valence band contributions to photoluminescence excitation spectra of tightly bound holes in zincblende semiconductors

Photoluminescence excitation (PLE) spectra of deep acceptor states in ZnSe, for example the Cu-related luminescence band at ≈1.95 eV, contain a prominent excitation band at ≈3.25 eV. This band lies above the structure marking the lowest direct E_O band gap E_g by the spin-orbit splitting energy Δ of the valence bands at Γ. The higher energy feature is either absent or greatly de-emphasised in the PLE spectra of shallow acceptor states in ZnSe and of the oxygen iso-electronic trap in ZnTe, where the electron rather than the hole is tightly bound. However, a significant PLE component at E_g + Δ is observed for deep acceptor-like states in ZnTe, where Δ is ≈0.95 eV. Efficient PLE at E + Δ for luminescence from deep acceptor-like states is shown to be consistent with the extended wave-vector contributions to the bound state wave-functions of holes of binding energies ≈Δ.     

The valence electronic structure of conducting pyrrole polymers: Evidence against a linear metallic chain picture

A UPS study of various conducting polypyrrole films is presented. Most of the valence band features can be explained by states derived from the orbitals of the pyrrole monomer and the associated anion molecules. In close vicinity of the Fermi energy, a density of states is observed which decreases linearly towards E_F. The corresponding states are introduced by oligomer formation. The π-electronic density at E_F is reduced by at least two orders of magnitude compared to ordinary sp-metals. The UPS spectra are consistent with short conjugation lengths and large amounts of disorder, but the corresponding defect states can not directly be observed.     

Comparative photoemission study of the electronic properties of L-CVD SnO2 thin films

In this work we present the results of comparative XPS and PYS studies of electronic properties of the space charge layer of the L-CVD SnO₂ thin films after air exposure and subsequent UHV annealing at 400 °C, with a special emphasis on the interface Fermi level position.From the centre of gravity of binding energy of the main XPS Sn 3d_5/2 line the interface Fermi level position E_F − E_v in the band gap has been determined. It was in a good correlation with the value estimated from the offset of valence band region of the XPS spectrum, as well as from the photoemission yield spectroscopy (PYS) measurements. Moreover, from the valence band region of the XPS spectrum and PYS spectrum two different types of filled electronic band gap states of the L-CVD SnO₂ thin films have been derived, located at 6 and 3 eV with respect to the Fermi level.     

Near EF electronic structure of heavily boron-doped superconducting diamond

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T_c=7.2 K) in order to study the electronic structure near the Fermi level (E_F). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that E_F is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E_F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.     

Detection and study of Rb I Rydberg states

We describe the details of an experiment using an atomic beam of rubidium which allowed us to detect by field ionization techniques the np Rydberg states from n = 28 up to n = 78, to detect also ns and nd states using a Stark mixing, and for all of these detected states to check the classical law E_c = [16n^*4]^?1 concerning the critical ionizing electric field E_c. It turns out that for n as high as 65 this law is quite well verified.     

First-principles study of ferromagnetism in N doped TiO2 and TiO

N doped TiO is nonmagnetic, in which spin-split impurity states are not induced near the Fermi energy (E_F) by N dopant. N doped TiO₂ along with transition-metal (TM) doped TiO is magnetic, in which spin-split impurity states are induced across E_F. The magnetic moment is determined by the 3d4s electron configurations and the valence states of TM-dopant ions when they substitute Ti. Hence, the origin of ferromagnetism of N doped TiO₂ and TiO is not closely related to the width of the band gaps of host oxides, but would be crucially related to that if the dopant can induce spin-split impurity states near E_F.     

Effect of Cr on the electronic structure of Co3Al intermetallic compound: A first-principles study

The effect of doping with Cr on the electronic structure and magnetism of Co₃Al has been studied by density functional calculations. It has been found that the Cr atom has a strong site preference for the B-site in Co₃Al. With the substitution of Cr for Co, the total densities of states (DOS) change obviously: A DOS peak appears at E_F in the majority spin states and an energy gap is opened in the minority spin states. The effect of Cr in Co₃Al is mainly to push the antibonding peak of the Co (A,C) atoms high on the energy scale and to form the energy gap around E_F, and also to contribute to the large DOS peak at E_F in the majority spin direction. The calculations indicate a ferromagnetic alignment between the Co and Cr spin moments. The calculated total magnetic moment decreases and becomes closer to the Slater–Pauling curve with increasing Cr content. This is mainly due to the decrease of the Co (A,C) spin moments. At the same time, the moments of Co (B) and Cr (B) only change slightly.     

Temperature dependence of the energy separation between Γ and L minima in n-type GaSb

Energy separation ΔE_c between Λ and L minima of GaSb conduction band is deduced from temperature dependence of tunneling current in p−n junctions. ΔE_c is found to inceasing vx. temperature with a coefficient d(ΔE_c/dT) of about - 2.10^-4eV/dgK.     

Specific heat anomaly of single phase YBa2Cu3O7-δ at superconducting transition temperature

The specific heat of single phase YBa₂Cu₃O_7-δ has been measured using non-adiabatic method between 4.2K and 120K. There is a specific heat anomaly Δc at 90K (about 3.2% of total specific heat) approximately, due to superconducting transition. From the measured value of ΔC and transition temperature T_c, the electronic density of state at Fermi level N(E_F) and Sommerfeld parameter γ calculated are 2.55±0.30states/eV.Cu-atom and 2.77±0.30 mJ/mole.K², respectively. The experimental result of N(E_F) is consistent with that of the band calculation by Mattheiss. The Debye temperature above T_c in this material deduced from Debye function is about 340K. Below 20K, the relation C=γ'T+βT³ is satisfied. But the value of γ' is smaller. That means, most of the electrons have formed superconducting Cooper pairs which give no contribution to specific heat below 20K.     

Photoemission study of electronic structure evolution across the metal-insulator transition of heavily B-doped diamond

We studied the electronic structure evolution of heavily B-doped diamond films across the metal-insulator transition (MIT) using ultraviolet photoemission spectroscopy (UPS). From high-temperature UPS, through which electronic states near the Fermi level (E_F) up to ∼5k_BT can be observed (k_B is the Boltzmann constant and T the temperature), we observed the carrier concentration dependence of spectral shapes near E_F. Using another carrier concentration dependent UPS, we found that the change in energy position of sp-band of the diamond valence band, which corresponds to the shift of E_F, can be explained by the degenerate semiconductor model, indicating that the diamond valence band is responsible for the metallic states for samples with concentrations above MIT. We discuss a possible electronic structure evolution across MIT.     

Interference of ordinary and extraordinary waves in AgAsS2 crystals

The anisotropy of the near-bandgap absorption is investigated in AgAsS₂ crystals. The refraction indices, n^‖ and n^⊥ respectively for the E‖c and E⊥c polarizations as well as the spectral dependence of the refraction indices difference, Δn=n‖−n^⊥ are determined from the interference spectra of AgAsS₂ crystals. A transmission band with four maxima is observed in the transmission spectra of crystals placed between crossed polarizers. The optical parameters n, k, ε₁, and ε₂ for the E‖c and E⊥c polarizations are calculated from the reflection spectra by using the Kramers–Kronig relations.     

Defect-level analysis of semiconductors by a new differential evaluation ofn(1/T)-characteristics

A new method for the defect-level analysis of extrinsic semiconductors is described. Provided that the defect-level concentration is not too large and the temperature is not too low, the Fermi levelE _F is shifted with increasing temperature from a position near the conduction (or valence) band towards the middle of the forbidden gap monotonously. Thus majority carriers are emitted into the conduction (or valence) band from the defect levels successively. If for a small increment of the temperature the Fermi levelE _F is shifted by ΔE _F and the concentration of free majority carriers is increased by Δn, then the ratio Δn/ΔE _F is a measure of the defect-level concentration within ΔE _F . Furthermore we discuss how this analysis is influenced by additional defect levels outside the range over which the Fermi energy can be shifted by variation of the temperature.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.