Undoped Cuprates Research Articles

Analysis of the valence-band photoemission spectrum ofSr2CuO2Cl2along the high-symmetry directions

Band structure calculations have been used to identify the different bands contributing to the polarisation-dependent photoemission spectra of the undoped model cuprate Sr$_2$CuO$_2$Cl$_2$ at the high-symmetry points of the CuO$_2$ plane $\Gamma$, $(\pi/a,0)$ and $(\pi/a,\pi/a)$ and along the high-symmetry directions $\Gamma - (\pi/a,\pi/a)$ and $\Gamma - (\pi/a,0)$. Results from calculations within the local density approximation (LDA) have been compared with calculations taking into account the strong electron correlations by LDA+U, with the result that the experimental order of energy levels at the high-symmetry points is better described by the LDA+U calculation than by the simple LDA. All the main peaks in the photoemission spectra at the high symmetry points could be assigned to different Cu 3$d$ and O 2$p$ orbitals which we have classified according to their point symmetries. The dispersions along the high-symmetry directions were compared with an 11-band tight-binding model which was fitted both to the LDA+U band structure calculation and the angle-resolved photoemission data. The mean field treatment successfully describes the oxygen derived bands but shows discrepancies for the copper ones.

The electronic structure of one- and two-dimensional cuprates from high-energy spectroscopy

We report studies of the electronic structure and elementary excitations of undoped, one- and two-dimensional cuprates. Using high-energy spectroscopies such as X-ray absorption, photoemission and electron energy-loss spectroscopy, important information regarding the charge distribution, hole dynamics and dielectric response has been obtained. The comparison of the experimental data with suitable theoretical models sets constraints on the parameters entering into the model calculations.

Theory of Spin Excitations in Undoped and Underdoped Cuprates

We consider the magnetic properties of highTccuprates from a gauge theory point of view, with emphasis on the underdoped regime. Underdoped cuprates possess certain antiferromagnetic correlations, as evidenced, for example, by different temperature dependence of the Cu and O site NMR relaxation rates, that are not captured well by slave boson mean field theories of thet-Jmodel. We show that the inclusion of gauge fluctuations will remedy the deficiencies of the mean field theories. As a concrete illustration of the gauge-fluctuation restoration of the antiferromangetic correlation and the feasibility of the 1/Nperturbation theory, the Heisenberg spin chain is analyzed in terms of a 1+1D U(1) gauge theory with massless Dirac fermions. The 1/N-perturbative treatment of the same gauge theory in 2+1D (which can be motivated from the mean fieldπ-flux phase of the Heisenberg model) leads to a dynamical mass generation corresponding to an antiferromagnetic ordering. On the other hand, it is argued that in a similar gauge theory with an additional coupling to a Bose (holon) field, symmetry breaking does not occur, but antiferromagnetic correlations are enhanced, which is the situation in the underdoped cuprates.

Indirect-exchange coupling and high- TC superconductivity: analysis of TC as a function of pressure in Hg 1− xAu xBa 2Ca 2Cu 3O y. Comparison with other Hg-cuprates

Results of recent experiments by Acha et al. [Phys. Rev. B 57 (1998) R5630] on the effect of pressure on critical temperatures of four samples of the mercury cuprate Hg 1− x Au x Ba 2Ca 2Cu 3O y , at different Au doping levels x, are analyzed in the framework of indirect-exchange Cooper pair formation via diamagnetic oxygen anions, developed earlier. It is found that all experimental results are quantitatively explained on this basis, and that partial substitution of Hg by Au must imply a hole doping process in the CuO 2 layers. In particular, values of d T C/d P at P=0 are all positive, in the range of doping investigated, in spite of the four systems being hole-overdoped. We also show that pressure dependence of a proximity effect as suggested in the Reference quoted for the undoped cuprate cannot be the cause of the large increase in critical temperature observed with this system.

Spin-polaron damping in the spin-fermion model for cuprate superconductors

A self-consistent, spin rotational invariant Green's function procedure has been developed to calculate the spectral function of carrier excitations in the spin-fermion model for the CuO2 plane. We start from the mean field description of a spin polaron in the Mori-Zwanzig projection method. In order to determine the spin polaron lifetime in the self-consistent Born approximation, the self-energy is expressed by an irreducible Green's function. Both, spin polaron and bare hole spectral functions are calculated. The numerical results show a well pronounced quasiparticle peak near the bottom of the dispersion at (pi/2,pi/2), the absence of the quasiparticle at the Gamma-point, a rather large damping away from the minimum and an asymmetry of the spectral function with respect to the antiferromagnetic Brillouin zone. These findings are in qualitative agreement with photoemission data for undoped cuprates. The direct oxygen-oxygen hopping is responsible for a more isotropic minimum at (pi/2,pi/2).

Temperature dependence of the mid-infrared absorptions in undoped cuprate Sr 2CuO 2Cl 2

We have studied temperature dependence of three mid-infrared absorption peaks of Sr 2CuO 2Cl 2, observed in transmission measurement; a lower-energy sharp peak and two higher-energy broad peaks. As temperature increases, the first one shows position redshift and width broadening. We relate the behaviors to Lorenzana and Sawatzky's model [J. Lorenzana, G.A. Sawatzky, Phys. Rev. Lett. 74, 1995, p. 1867] and attribute the peak to the phonon-assisted bimagnon excitation. We examine the temperature dependence of the peak width and suggest magnon scattering mechanism coupled with magnetic disorder in CuO 2 plane, in order to explain the anomalous broadening of the width around the Neel temperature.

Cu-O network-dependent core-hole screening in low-dimensional cuprate systems: A high-resolution x-ray photoemission study

We present an experimental study of the dynamics of holes in the valence bands of 0, 1, and 2 dimensional undoped model cuprates, as expressed via the screening of a Cu 2p core hole. The response depends strongly upon the dimensionality and the details of the Cu-O-Cu network geometry and clearly goes beyond the present theoretical state-of-the-art description within the three-band d-p model.

Single-particle properties of a two-dimensional Fermi liquid at finite frequencies and temperatures

We review the leading momentum, frequency, and temperature dependences of the single-particle self-energy and the corresponding term in the entropy of a two-dimensional Fermi liquid (FL) with a free-particle spectrum. We calculate the corrections to these leading dependences for the paramagnon model and the electron gas and find that the leading dependences are limited to regions of energy and temperature which decrease with decreasing number density of fermions. This can make it difficult to identify the frequency- and temperature-dependent characteristics of a FL ground state in experimental quantities in low-density systems even when complications of band structure and other degrees of freedom are absent. This is an important consideration when the normal-state properties of the undoped cuprate superconductors are analyzed.

Superexchange Coupling and Spin Susceptibility Spectral Weight in Undoped Monolayer Cuprates

A systematic inelastic neutron scattering study of the superexchange interaction in three different undoped monolayer cuprates (La_2CuO_4, Nd_2CuO_4 and Pr_2CuO_4) has been performed using conventional triple axis technique. We deduce the in-plane antiferromagnetic (AF) superexchange coupling $J$ which actually presents no simple relation versus crystallographic parameters. The absolute spectral weight of the spin susceptibility has been obtained and it is found to be smaller than expected even when quantum corrections of the AF ground state are taken into account.

An equation to calculate oxygen content relating to copper oxidation level of doped cuprates

A new equation relating oxygen contenty to copper valence 2+p has been derived and proposed. Compared to the previous equation, there is a corrected function in the new equation. The corrected function depends on dopant contentx and copper oxidation levelp. As a metalM is substituted for the metal of the sample, the corrected function has a different form, and the function is discussed in detail. For each case, taking high-T c cuprate for example, itsp is determined andy calculated by the new and previous equations. Changes in the corrected function are shown in the figures. All the discussions show that it is necessary to use the new equation to calculatey if we are to get accurate experiment results. The new equation can be widely applied to calculate the oxygen content of multicomponent doped or undoped cuprates, and the exact stoichiometries and superconducting or impurity phases of samples need not be known.

Resistivity and Hall effect of metallic oxygen-deficient YBa2Cu3Ox films in the normal state.

We present a systematic study of normal-state transport properties in a series of $c$-axis-oriented $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{x}$ (YBCO) epitaxial thin films and Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$/Pr${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ (YBCO/PrBCO) superlattices. The hole doping level in the YBCO films is varied from the optimum-doped metallic down to the underdoped insulating regime by changes in the oxygen content $x$. We find that the magnitude of the resistivity $\ensuremath{\rho}$ and Hall coefficient ${R}_{H}$ increases monotonically with decreasing $x$ and that their respective temperature dependences undergo marked changes. The ${R}_{H}(T)$ behavior is reminescent of the Hall effect behavior in heavy fermion metals, taking into account a difference in temperature by a factor of 100. The Hall angle ${cot\ensuremath{\theta}}_{H}=\frac{\ensuremath{\rho}}{{R}_{H}B}$ shows a quadraticlike temperature dependence, with systematic deviations at high and low doping levels. Transport measurements in YBCO/PrBCO superlattices, with the YBCO layers in the two-dimensional regime, indicate that the deviations of a ${T}^{2}$ dependence of the Hall angle are intrinsic and not related to the dimensionality of the system. A method of analyzing the transport data is presented, revealing a striking scaling behavior of the respective properties. A comparison with reported transport data in the literature suggests that the observed scaling behavior may be universal for underdoped cuprates. Furthermore, we show that reported NMR Knight shift data for oxygen-deficient YBCO samples can also be mapped on a single scaling curve, by using the same scaling parameter derived from our transport measurements. This finding strongly indicates that the dominant scattering mechanism in these materials is of magnetic origin. Going one step further, we present a qualitative analysis of the conductivity under the assumption that we may use the expression for a two-dimensional quantum liquid and that the inelastic scattering length may be replaced by the magnetic correlation length $\ensuremath{\xi}(T)$. Expressions for the latter are taken from reported calculations for undoped and doped cuprates. A good qualitative agreement is obtained between the calculated and experimentally observed temperature-dependent conductivity.

CuOCu in-plane angle distribution in layered superconductors probed by EXAFS

Accurate Cu K-edge EXAFS spectra of doped and undoped lanthanum cuprates, collected in the temperature range 10–300 K, are analyzed taking proper account of the strong multiple-scattering (MS) signal found in these layered compounds (GNXAS method). A double-electron excitation [1s3p] channel has been evidenced and considered in the data treatment. CuO and Cu(La,Sr/Ba) distance distributions have been measured, as well as the distribution of the CuOCu in-plane quasi-linear three-body configurations. In the latter case, the strong MS signal allowed an accurate study of the bond-angle distribution, associated with the buckling of the CuO 2 planes. A detailed analysis of the statistical errors on the structural parameters has been performed, showing the reliability of the short-range structural refinement. The results are found to be consistent with previous diffraction and infrared data. Different features of the CuOCu distribution in superconducting and non-superconducting materials are discussed.

Symmetry of hole states in La 2− xSr xCuO 4 + δ and La 2− xSr xNiO 4 + δ

We present results from polarization-dependent X-ray absorption spectroscopy (XAS) of the O1s absorption edges of single-crystalline La 2− x Sr x CuO 4 + δ and La 2− x Sr x NiO 4 + δ. From these measurements we derive the differences in the unoccupied electronic structure close to the Fermi level between the undoped cuprate and nickelate. In case of the doped nickelate, we present evidence for (a) the strongly bound Zhang-Rice character of the carriers and (b) their binding to crystal field excitons.

Ingap state in doped and undoped cuprates

The dispersive electronic state emerging upon carrier-doping in cuprates, i.e., the so-called ingap state, is derived in the three-band and one-band Hubbard models by means of the exact diagonalization of finite-size clusters. The dispersive state near the charge transfer gap at half filling undergoes a strongly momentum-dependent transfer of the spectral weight by carrier doping, and evolves into the new ingap state at the edge of the gap with a free-electron-like dispersion characteristic of a large Fermi surface consistent with Luttinger's sum rule. It is shown that the extended Hubbard model with strong nearest-neighbor repulsive interaction, on the other hand, forms a small Fermi surface by carrier doping and provides a counter-example of the doped cuprates.

Spin-spin correlation functions for the square-lattice Heisenberg antiferromagnet at zero temperature.

We have calculated the dynamical transverse and longitudinal spin-correlation functions for the two-dimensional Heisenberg antiferromagnet at zero temperature, by the Dyson-Maleev spin-wave theory to second order in perturbation theory. The transverse correlation function is characterized by a dominant one-magnon peak and a broad three-magnon continuum. For spin 1/2 the contribution of the three-magnon excitations is small but not negligible, and might be detected in highly sensitive neutron-scattering experiments in the undoped layered cuprates. We have also computed the transverse equal-time correlations and compared the results with recent series-expansion estimates. The good agreement between the two formalisms reinforces the validity of spin-wave theory. The longitudinal structure factor, to leading order, displays a two-peak structure similar to that obtained by the Schwinger-boson mean-field formalism. The magnon interaction reduces the second peak, in some cases substantially. We discuss how umklapp processes affect the multiple-magnon excitations. We have finally computed the staggered magnetization and transverse susceptibility corrected to second order. For spin 1/2 we find m=0.3069\ifmmode\pm\else\textpm\fi{}0.00020 for the staggered magnetization, and ${\mathit{Z}}_{\mathrm{\ensuremath{\chi}}}$=0.4844\ifmmode\pm\else\textpm\fi{}0.00010 for the susceptibility renormalization constant, in agreement with the results obtained by other techniques.

Electronic structure of HgBa 2CuO 4

Local density approximation calculations of the electronic structure of HgBa 2CuO 4 are reported along with a number of properties identifiable from it. For the stoichiometric material, the only band crossing the Fermi energy is the Cu-O derived antibonding state characteristic of high- T c cuprates; this band is half-filled. Thus, the stoichiometric material is expected to be a Mott insulator, as are the other undoped cuprates, and the excess oxygen reported in the as-synthesized material is no doubt essential for its superconductivity. Electric field gradients are calculated and compared with those in other cuprates. The position wave-function has maximum weight in the rather large holes in the Hg layer. However, unlike the majority of high- T c materials, there is substantial weight throughout the unit cell, including the Cu-O layer region.

Magnetic critical properties of the undoped cuprates in a quasi-two-dimensional nonlinear-sigma-model approach.

A quasi-two-dimensional (2D) nonlinear sigma model (NLSM) is used to model the antiferromagnetism of the insulating layered undoped cuprate materials. As Chakravarty, Halperin, and Nelson (CHN) did for their 2D NLSM, we make a renormalization-group analysis of the quasi-2D NLSM. In contrast with CHN's 2D model, the quasi-2D NLSM yields a nonzero Ne\ifmmode\acute\else\textasciiacute\fi{}el temperature so that our theoretical Ne\ifmmode\acute\else\textasciiacute\fi{}el temperatures can be fitted completely with the experimental ones. Our theoretical correlation lengths are fitted very well with the experimental ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ data of the materials within 220 K above ${\mathit{T}}_{\mathit{N}}$.

A quasi-2D nonlinear spin wave theory for the insulating undoped cuprates

We propose a nonlinear spin wave theory of anisotropic 3D (quasi-2D) quantum Heisenberg antiferromagnets, which is equivalent to or reduces to known spin wave theories of isotropy parameter δ takes the values 0 or 1, respectively. With δ increasing from zero also the Néel temperature T N increases from zero. Applied to the antiferromagnetism of the layer-like cuprate La 2CuO 4 it describes quite satisfactorily the existing experimental data of the Néel temperature, correlation length, and staggered magnetization of the material if J = 1034 K and δ = 4×10 −5.

ChemInform Abstract: Synthesis and Structural Aspects of Doped and Undoped High Tc Bismuth Cuprates

ChemInform Abstract: Synthesis and Structural Aspects of Doped and Undoped High Tc Bismuth Cuprates