Evolution of the band structure of quasiparticles with doping in copper oxides on the basis of a generalized tight-binding method

Two methods for stabilizing the two-hole ³ B _1g state as the ground state instead of the Zhang-Rice singlet are determined on the basis of an orthogonal cellular basis for a realistic multiband pd model of a CuO₂ layer and the dispersion relations for the valence band top in undoped and doped cases are calculated. In the undoped case, aside from the valence band, qualitatively corresponding to the experimental ARPES data for Sr₂CuO₂Cl₂ and the results obtained on the basis of the t-t′-J model, the calculations give a zero-dispersion virtual level at the valence band top itself. Because of the zero amplitude of transitions forming the virtual level the response corresponding to it is absent in the spectral density function. In consequence, the experimental ARPES data do not reproduce its presence in this antiferromagnetic undoped dielectric. A calculation of the doped case showed that the virtual level transforms into an impurity-type band and acquires dispersion on account of the nonzero occupation number of the two-hole states and therefore should be detected in ARPES experiments as a high-energy peak in the spectral density. The computed dispersion dependence for the valence band top is identical to the dispersion obtained by the Monte Carlo method, and the ARPES data for optimally doped Bi₂Sr₂CaCu₂O_8+δ samples. The data obtained also make it possible to explain the presence of an energy pseudogap at the symmetric X point of the Brillouin band of HTSC compounds.     

Strong impact of the electron-photon matrix element on angle-resolved photoelectron spectra of Sr 2 CuO 2 Cl 2

In recent years insight has been gained into the electronic structure of layered cuprates using angle-resolved photoelectron spectroscopy. In many of these studies it is assumed that the electron-photon matrix element follows the trends set by the atomic photoionization cross sections and does not influence lineshape, dispersion and the k-dependence of the spectral intensity. In this study using Sr₂CuO₂Cl₂ as an example it will be shown that the electron-photon matrix element can have a strong impact on both strength and shape of a feature in an angle-resolved photoelectron spectrum of a layered cuprate which can strongly affect information on character and the momentum-dependence of the energy and spectral weight of a state deduced from the spectra. The results of this study put an emphasis on the need to employ the whole parameter range of the ARPES method to get reliable information on the spectral function of cuprates for which purpose synchrotron radiation is an uniquely suited tool. Received 10 January 2000 and Received in final form 15 January 2001     

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott-Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La³⁺,Y³⁺)₂CuO₄ and doped T′-(La³⁺,Ce⁴⁺)₂CuO₄ thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu-O bond distance displacement. The broadened Cu-O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.     

Observation of p- and d-like surface states on CuCl(100) by angle-resolved photoemission

In the course of a systematic ultraviolet photoemission study of the electronic band structure of CuCl, we have identified two occupied surface states on CuCl(100), situated at 0.25 and 3.0 eV below valence band maximum in normal emission spectra. They essentially show pure p- and d-like orbital symmetry, respectively. We interpret them as a chlorine p_x,y-like occupied antibonding resonance and a copper Γ₁₂-derived state split off from the bulk orbitals by the surface potential. We also present critical point energies along Γ-X and Γ-L.     

Photoemission studies on electron doped cuprate Pr0.85LaCe0.15CuO4: Revisiting the chemical pressure effect

We performed angle resolved photoemission spectroscopy (ARPES) experiments on electron doped cuprates Pr_0.85LaCe_0.15CuO₄ (PLCCO) and Nd_1.85Ce_0.15CuO₄ (NCCO). Critical temperatures (T_c) of PLCCO and NCCO are similar but PLCCO has weaker Fermi surface curvature than NCCO. As the ionic radius of Pr and La is larger than that of Nd, this result is inconsistent with the earlier view that chemical pressure determines the Fermi surface curvature. On the other hand, anti-ferromagnetic (AFM) band renormalization effect in PLCCO is larger than that in NCCO, which implies AFM is stronger in PLCCO. This is consistent not only with the view that AFM is correlated with t′/t but also with recent inelastic neutron scattering results. Therefore, we suggest that the chemical pressure effect is not the only factor that determines the Fermi surface topology.     

Polarization dependence of angle-resolved photoemission spectroscopy of graphite

We have used variable polarization synchrotron radiation to map the valence band electronic structure of graphite by angle-resolved photoemission spectroscopy (ARPES). The experimental results with two orthogonal linear polarization of light signifies the contribution of either even or odd symmetry with respect to the crystal mirror plane towards the photoemission intensity. The σ₁ and σ₂ valence bands show odd reflection symmetry while the π valence band shows even symmetry with respect to the mirror plane. The measured ARPES spectrum using left and right circular polarized lights shows asymmetry in intensity around M point of the Brillouin zone, which ultimately mimicking different partial wave character of σ₁ and σ₃ bands.     

Matrix element and strong electron correlation effects in ARPES from cuprates

We discuss selected results from our recent work concerning the angle-resolved photoemission (ARPES) spectra from the cuprates. Our focus is on developing an understanding of the effects of the ARPES matrix element and those of strong electron correlations in analyzing photointensities. With simulations on Bi₂Sr₂CaCu₂O_8+δ (Bi2212), we show that the ARPES matrix element possesses remarkable selectivity properties, such that by tuning the photon energy and polarization, emission from the bonding or the antibonding states can be enhanced. Moreover, at low photon energies (below 25 eV), the Fermi surface (FS) emission is dominated by transitions from just the O-atoms in the CuO₂ planes. In connection with strong correlation effects, we consider the evolution with doping of the FS of Nd_2−xCe_xCuO_4±δ (NCCO) in terms of the t−t′−U Hubbard model Hamiltonian. We thus delineate how the FS evolves on electron doping from the insulating state in NCCO. The Mott pseudogap is found to collapse around optimal doping suggesting the existence of an associated quantum critical point.     

Photoemission via bloch states and evanescent band gap states for Cu(110)

Normal emission photoelectron spectra from Cu(110) using polarized synchrotron radiation (hv < 35 eV) can be explained with a direct transition model using realistic final state bands. Prominent surface photoemission via evanescent final states is observed in the large X′₅-X₁ conduction band gap. Accurate valence band eigenvalues at K and X have been determined.     

Manifestations of the pseudogap and superconducting gap of cuprates in photoemission

The degree to which the interpretation of the existence of a pseudogap and a superconducting gap in cuprates on the basis t-t′-U the Hubbard-model corresponds to the data obtained from the photoemission spectra is discussed. The pseudogap in the model is interpreted as the work function of electrons from the insulating parts of the Brillouin zone boundary. On this basis one can explain the angle dependence of the gap measured in the photoemission spectra and its evolution on changes in doping and temperature. In particular, an explanation is found for the decline in the ratio of the angle derivative of the gap near the site, v _Δ = (1/2)dΔ(?)/d?, to the maximum value of the gap, Δ_max, with decreasing doping. That behavior and the different temperature dependence of the gap Δ(?) for different angles are due to the presence of two contributions to Δ with different anisotropies—from the pseudogap and from the superconducting gap. The calculation of the spectral functions confirms the sharp Fermi boundary observed in the direction and the smeared edge of the distribution along the path Γ(0, 0)-M(π, 0)-Y(π, π).     

Spatial dependence of the dichroism of photoemission of Fe<Subscript>1/4</Subscript>TiTe<Subscript>2</Subscript> upon excitation with circularly polarized radiation

This paper reports on the results of the experimental investigation into the angular dependence of the photoemission of the valence band (the angle-resolved photoelectron spectrum) for the Fe_1/4TiTe₂ single crystal upon excitation with circularly polarized radiation. The angle-resolved photoelectron spectrum is studied in the vicinity of the Γ point of the Brillouin zone in the two directions ΓK and ΓM with the highest symmetry. The measurements are performed in noncoplanar geometry. The circular dichroism in the photoemission angular distribution is observed over the entire ranges of energies and angles. The magnitude of the dichroism depends on the choice of the band, the binding energy, and the polar angle and varies from 0 at the Γ point to 20%.     

New class of T′-structure cuprate superconductors

High-temperature superconductivity has been discovered in La_2−xBa_xCuO₄ [J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. [1]], a compound that derives from the undoped La₂CuO₄ crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2], [3] and [4]]. The undoped material La₂CuO₄ is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5]] of the form La_2−xCe_xCuO_4+y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6] and [7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO₂-plane. It is well known that for La_2−xCe_xCuO_4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La_2−xRE_xCuO₄ can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La_2−xRE_xCuO₄, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.     

Polarons and bipolarons in oxides

The physical properties of Fröhlich polarons and bipolarons are reviewed in relation to the recent experimental study of the mid-infrared absorption spectra of high- Tc cuprate superconductors, in particular, of Nd₂CuO_4-y , YBa₂Cu₃O_6+x and La₂CuO_4+x . The spectral region studied is of particular relevance for the open question of the presence of polarons and bipolarons in these materials, since it coincides with the frequency region where polaron and bipolaron absorption is to be expected. In our view, the optical absorption spectra as observed in the doped cuprates are a manifestation of the polaron type relaxed excited final states.     

Influence of the Diagonal and Off-Diagonal Electron–Phonon Interactions on the Formation of Local Polarons and Their Band Structure in Materials with Strong Electron Correlations

For systems with strong electron correlations and strong electron–phonon interaction, we analyze the electron–phonon interaction in local variables. The effects of the mutual influence of electron–electron and electron–phonon interactions that determine the structure of local Hubbard polarons are described. Using a system containing copper–oxygen layers as an example, we consider the competition between the diagonal and off-diagonal interactions of electrons with the breathing mode as the polaron band structure is formed within a corrected formulation of the polaron version of the generalized tight-binding method. The band structure of Hubbard polarons is shown to depend strongly on the temperature due to the excitation of Franck–Condon resonances. For an undoped La₂CuO₄ compound we have described the evolution of the band structure and the spectral function from the hole dispersion in an antiferromagnetic insulator at low temperatures with the valence band maximum at point (π/2, π/2) to the spectrum with the maximum at point (π, π) typical for the paramagnetic phase. The polaron line width at the valence band top and its temperature dependence agree qualitatively with angle-resolved photoemission spectroscopy for undoped cuprates.     

Variational approach to strong correlation in the photoemission of electron-doped superconductors

We use a variational approach with strictly strong-correlated constraint to gain insight into low-energy states of t-t^′-t^″-J model in the electron-doped regime. Compared with the recent results on the electron-doped cuprates obtained by angle-resolved photoemission spectroscopy (ARPES), we show that based on the long-range ordered antiferromagnetic metallic state prohibiting vacant sites, our results lead to qualitatively similar trends in ARPES spectra and Fermi surface topology. Additionally, the results about the evolution of the energy gap and spectral weight as a function of doping will be discussed.     

Method of envelope functions and intervalley Γ-X z interaction of states in (001) III–V semiconductor heterostructures

The kp method is used to analyze the problem of intervalley Γ-X _z interaction of conduction-band states in the (001) lattice-matched III–V semiconductor heterostructures. A convenient basis for expansion of the wave function is systematically selected and a multiband system of equations is derived for the envelope functions which is then reduced to a system of three equations for three valleys Γ₁, X ₁, and X ₃) by using a unitary transformation. Intervalley Γ-X _z mixing is described by short-range potentials localized at heterojunctions. The expressions for the parameters determining the Γ-X _z mixing strength explicitly contain the chemical-composition profile of the structure since mixing is naturally stronger for abrupt heterojunctions than for structures with a continuously varying chemical composition. It is shown that direct Γ_{1?X 1} interaction of comparable strength to the Γ₁?X ₃ interaction exists. This must be taken into account when interpreting tunnel and optical experiments since the X ₁ valley is substantially lower in energy than the X ₃ valley.     

Direct angle resolved photoemission spectroscopy and superconductivity of strained high-T c films

Since 1997 we systematically perform direct angle resolved photoemission spectroscopy (ARPES) on in-situ grown thin (<30 nm) cuprate films. Specifically, we probe low-energy electronic structure and properties of high-T _c superconductors (HTSC) under different degrees of epitaxial (compressive vs. tensile) strain. In overdoped and underdoped in-plane compressed (the strain is induced by the choice of substrate) ≈15 nm thin La_{2 − x} Sr _x CuO₄ (LSCO) films we almost double T _c to 40 K, from 20 K and 24 K, respectively. Yet the Fermi surface (FS) remains essentially two-dimensional. In contrast, ARPES data under tensile strain exhibit the dispersion that is three-dimensional, yet T _c drastically decreases. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO₂ plane, enhances the two-dimensional character of the dispersion and increases T _c, while the tensile strain acts in the opposite direction and the resulting dispersion is three-dimensional. We have established the shape of the FS for both cases, and all our data are consistent with other ongoing studies, like EXAFS. As the actual lattice of cuprates is like a ‘Napoleon-cake’, i.e. rigid CuO₂ planes alternating with softer ‘reservoir’, that distort differently under strain, our data rule out all oversimplified two-dimensional (rigid lattice) mean field models. The work is still in progress on optimized La-doped Bi-2201 films with enhanced T _c.      

Shift of a Γ-X junction due to hydrostatic pressure

Hydrostatic pressure causes a shift of a Γ-X junction in the direction opposite to the band-gap gradient. The shift is of the order of 1μ/kbar in the samples investigated. From the shift and the band-gap gradient, the intervalley pressure coefficient (?/?P) (E_X–E_Γ) is estimated to be — 9·0^-6 eV/bar for the composition of Al_xGa_1-xAs corresponding to the Γ-X crossover.     

Effect of pressure on the electronic structure of cuprates with strongly correlated electrons

For cuprates of the n and p types, the effect of pressure on the electronic structure of a CuO₂ layer is studied. In the calculations performed, a generalized tight-binding method is used taking into account the influence of strong electronic correlations on the electronic structure of cuprates. The results obtained demonstrate the unusual effect of pressure on the nature of quasiparticle states at the top of the valence band in p-type cuprates. As the pressure increases, the hole states in these materials cease to be Zhang-Rice singlets and become combined singlet-triplet states.     

Electronic Structure of FeSe Monolayer Superconductors: Shallow Bands and Correlations

Electronic spectra of typical single FeSe layer superconductor—FeSe monolayer film on SrTiO₃ substrate (FeSe/STO) obtained from ARPES data reveal several puzzles: what is the origin of shallow and the so called “replica” bands near the M-point and why the hole-like Fermi surfaces near the Γ-point are absent. Our extensive LDA+DMFT calculations show that correlation effects on Fe-3d states can almost quantitatively reproduce rather complicated band structure, which is observed in ARPES, in close vicinity of the Fermi level for FeSe/STO. Rather unusual shallow electron-like bands around the M-point in the Brillouin zone are well reproduced. Detailed analysis of the theoretical and experimental quasiparticle bands with respect to their origin and orbital composition is performed. It is shown that for FeSe/STO system the LDA calculated Fe-3d _xy band, renormalized by electronic correlations within DMFT gives the quasiparticle band almost exactly in the energy region of the experimentally observed “replica” quasiparticle band at the Mpoint. However, correlation effects alone are apparently insufficient to eliminate the hole-like Fermi surfaces around the Γ-point, which are not observed in most ARPES experiments. The Fermi surfaces remain here even if Coulomb and/or Hund interaction strengths are increased while overall agreement with ARPES worsens. Increase of number of electrons also does not lead to vanishing of this Fermi surface and makes agreement of LDA+DMFT results with ARPES data much worse. We also present some simple estimates of “forward scattering” electron-optical phonon interaction at FeSe/STO interface, showing that it is apparently irrelevant for the formation of “replica” band in this system and significant increase of superconducting T _c .     

Possible propagation of the Zhang-Rice singlet as a probable Cooper channel in the CuO2 planes

The issue of how superconductivity originate in the CuO₂ planes believed to be crucial to understanding the high Tc superconducting cuprates is still an going debate. In the wake of recent experimental observations of the Zhang-Rice singlet (ZRS), its formation and propagation need to be revisited especially by using a simple approach almost at a phenomenological level. Within a highly simplified correlated variational approach (HSCVA) in this Letter, a new formation of the ZRS as constituting the ground state of a single-band t-J model of the CuO₂ planes is developed. This formation is then used to demonstrate how the ZRS can be propagated as a probable Cooper channel in the CuO₂ planes.