Modification of electron spectrum and properties of HTSC during doping

A mechanism for the influence of doping on the electron spectrum of La_2–xSr_xCuO₄ and Nd_2–xCe_xCuO₄ is proposed. In the framework of simple model taking into account the local nature of HTSC electron properties, the appearance of midgap states and the existence of critical concentration values (0.15 and 0.25) are explained. The effect of doping-induced changes of electron structure on transport and superconducting properties of La_2–xSr_xCuO₄ and Nd_2–xCe_xCuO₄ is considered. The dependence of the Hall resistance on doping concentration,x, is discussed. It is shown that in Nd_2–xCe_xCuO₄ the pair level of negativeU-centers arises at the top of the valence band only atx=0.15, and the dielectric-metal transition takes place. The superconductivity in Nd_2–xCe_xCuO₄ occurs in the hole-conductive phase.     

Substitution effects on the electronic anomaly and superconductivity in La2−xBaxCuO4 and La2−xSrxCuO4 with x ∼ 1/8

The effects of substitution of Y or Bi on the low-temperature structural phase transition, electronic anomaly and superconductivity have been studied in La_2-y-xY_yBa_xCuO La_2-y-xBi_yBa_xCuO₄ and La_1.9-y-xTb_0.1Bi_ySr_xCuO₄ with x = 1/8. Moreover, an electronic anomaly, which is analogous to those observed in La_2–xBa_xCuO₄ and La_1.6-xNd_0.4Sr_xCuO₄ with x 1/8, has been found below about 60 K in the thermoelectric power measurements for the Zn-substituted La_2–xSr_xCu_1–yZn_yO₄ with x 0.115 and y = 0.01 – 0.02. These experimental results support the conclusions by Tranquada et al. that static order of the stripe correlations of holes and spins, owing to pinning by the low-temperature structure or impurities (Zn in this case), is the origin of the electronic anomaly, leading to the suppression of superconductivity.     

Pair breaking as a probe of d-wave high-temperature superconductivity

A unified picture is obtained of the Cooper pair-breaking data by Cu-site Zn and Ni in Nd_2–z Ce _z CuO₄, La_2–SrCuO₄, Bi₂Sr₂CaCu₂O₈, Bi_1.8Pb_0.2Sr₂Ca₂Cu₃O₁₀, YBa₂Cu₃O₇, and YBa₂Cu₄O₈. The data are generally inconsistent with spin-fluctuation d-wave pairing mechanisms of superconductivity and with all two-dimensional cuprate-plane models. The data are consistent with superconductivity in the charge reservoirs.     

Domain growth in partially melted bulk Nd1+xBa2−x Cu3O7−δ

Domain growth of bulk partially melted Nd_1+xBa_2–xCu₃O_7– in air was found to be severely limited by liquid-phase segregation and losses. The use of a reduced oxygen pressure greatly improved the stoichiometry control, leading to the formation of well-developed domains and enhanced superconducting properties. Direct nucleation of Nd_1+xBa_2–xCu₃O_7– from the liquid phase by dissolution of Nd₂BaCuO₅ and a spiral growth mechanism in three perpendicular directions of the platelets stacked in the domains, are suggested.     

Study of lanthanum-deficient compositions in the systems La-Cu-O and La-Sr-Cu-O

The effect of varying lanthanum composition upon the formation of the nonsuperconducting La_1.67Sr_0.33Cu₂O_5– and the stoichiometry variation tolerance of the ternary system La_2–xSr_xCuO₄ (with x=0 and 0.15) were investigated using a variety of techniques. Results indicate that total conversion to La₂CuO₄ from freeze-dried precursors with an LaCu = 2.001 ratio is achieved after sintering (1253 K, 12 h) and annealing in oxygen (773 K, 6 h). Small variations in the stoichiometry of solids La_2–xSr_xCuO₄ with x=0 and 0.15 were detected. Lanthanum-deficient composition in the ternary system induces the formation of the non-superconducting phase whose stoichiometry is better represented by La_1.60Sr_0.40Cu₂O_5–. The temperature-programmed reduction profiles of the cuprates can be associated with the formation of more or less stable intermediates.     

Structural commonalities of high-temperature superconductors

It is shown that all of the presently known high-temperature superconductors belong to one structural type with a basic cell of approximate 4/mmm,D _4h symmetry in tetragonal compounds andmmm,D _2h symmetry in orthorhombic compounds. In the Ba(Pb1–xBi _x )O₃ and YBa₂Cu₃O_7– compounds the basic cell is the unit cell and in the (La_1–x Sr _x )₂CuO₄, BiSrCaCuO and TIBaCaCuO compounds it is half the unit cell. The features of more than one copper oxide layer per basic cell, oxygen-free cation layers (e.g., Ca, Y) between CuO₂ layers, and slightly puckered CuO₂ planes seem to favor higher transition temperatures. The feasibility of several proposed or potentially superconducting structures are analyzed by this scheme.     

Phononic and spin excitations in Y1?xPrxBa2Cu3O6.9 crystals

The Y_1–x Pr _x Ba₂Cu₃O_6.9 system has been investigated by quantitative-Raman measurements. The magnetic response of the antiferromagnetic ordered Cu spins in the CuO₂ plane has been observed via the two-magnon-Raman scattering. The in-plane exchange energy is determined to 780 cm^–1 in the system withx=1. Phonon behavior as well as magnetic scattering indicate that the band structure of the CuO₂ planes in PrBa₂Cu₃O_6.9 is similar to that in insulating YBa₂Cu₃O₆.     

Transport anomalies in the Zn-substituted La2−x Sr x Cu1−y Zn y O4 with x ∼ 0.115; Possibility of the Pinning of CDW and SDW by Zn

Anomalies in the thermoelectric power and the Hall coefficient, which are analogous to those observed in La _2–x Ba _x CuO ₄ with x 1/8, have been observed in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x = 0.115 and y = 0 – 0.025 and with x = 0.15 and y 0.02. Based on the so-called stripe model, these are explained as being due to the formation of the static order of the stripe-patterned CDW and SDW on account of the pinning by Zn. Accordingly, the long-standing problem on the conspicuous suppression of superconductivity in the Zn-substituted La _2–x Sr _x Cu _1–y Zn _y O ₄ with x 0.115 is likely to be attributed to this static order.     

Microstructure Effects on the Superconducting Properties of YBa2Cu3 – x M x O7 – δ (M = Ag, Hg)

The properties of YBa₂Cu_{3 – x} Ag _x O_{7 –} and YBa₂Cu_{3 – x} Hg _x O_{7 –} (0 < x 0.5) solid solutions were studied. The solute concentration is shown to have a significant effect on the superconducting transition temperature, density, and grain size of the solid solutions. The difference in the composition dependences of the properties of the solid solutions is interpreted in terms of the crystal-chemical behavior of the Ag and Hg ions.     

μSR Study of Magnetic Order in LTO and LTT Phase of La2−x M xCu1−y Zn y O4 (M=Sr, Ba)

In order to investigate the origin of so-called 1/8 anomaly in La(Sr,Ba)-214, we have performed SR experiments for the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structure phases in La_2–x Sr _x Cu_1–y Zn _y O₄ (LSCO) and La_2–x Ba _x Cu_1–y Zn _y O₄ (LBCO), and decided the magnetic order temperature T _m where -spin coherent rotation starts. 1% Zn-substitution depresses magnetic order in the LTT phase of LBCO while it induces or enhances magnetic order in the LTO phase of LSCO. With doping Tm rises first and keeps constant around x=1/8 and decreases in both Zn-substituted LSCO and LBCO. This implies that doped holes are never uniformly distributed on the CuO₂ plane, suggesting the segregation of spin and hole around x=1/8.     

Superconductors consisting of CuO2 and RuO2 layers

We have synthesized compounds which may be viewed as stacking of pairs of CuO₂ layers alternating with RuO₂ layers. There are two structural variants, RuSr₂GdCu₂O_8- and RuSr₂Gd_1+xCe_1–xCu₂O_10–, which are denoted shortly 1212 and 1222 structures. In their stacking sequence, they differ solely by the sheets separating the CuO₂ bilayers. The synthesis of these phases is accompanied by the formation of small amounts of SrRuO₃ impurities, which appear to have a much more devastating effect on the superconducting properties of the 1212 phase as compared to the 1222 phase. We present an investigation of the influence of various synthesis procedures on the appearance of impurity phases and on superconductivity in the 1212 system.     

Infinite-layer,T′-Phase,and 1-D-ladder copper oxide compounds

Results of recent research on electron-doped infinite-layer compounds (e.g., Sr_1–xLa_xCuO₂), T-phase compounds (e.g., Nd_2–xCe_xCuO₄), and spin-1/2 quasi-1-D ladder compounds (e.g., Sr₁₄Cu₂₄O₄₁) are presented, including structural and magnetic measurements. Studies of steric effects indicate that superconductivity disappears in both electron-doped systems for values of the in-plane lattice constant below a critical value, a_cr 3.92 Å. Attempts to hole dope both the infinite-layer and T phases are described. For the quasi-1-D ladder compound Sr₁₄Cu₂₄O₄₁, the anisotropic susceptibility of this system and its small gap are discussed. Collaborative studies of these systems using SR, NMR, and other spectroscopies are reviewed.This research was supported by NSF Grant DMR-9158089 and Welch Foundation Grant F-1191.     

Effect of Oxygen-Deficiency of CuO2 Plane on the Structure, Magnetism, and Transport Properties

Polycrystalline samples La_{2 – x} Sr _x CuO_{4 –} (0.06 x 1.0) and the corresponding vacuum annealed samples were investigated systematically by means of XRD, infrared (IR) spectra, ESR, and resistivity. It is found that the doping of Sr and the oxygen vacancies in CuO₂ planes show different effects on the lattice parameters a and c, and the changes of a and c of La_{2 – x} Sr _x CuO_{4 –} samples (x 0.2) rest with the cooperation of the two factors. The two high frequency IR vibration modes at 500 and 689 cm^–1, which are referred to the vibrations of apical oxygen and planar oxygen, are analyzed in detail. The disappearance of the mode at 689 cm^–1 is interpreted in terms of the screened effect of charge carriers in CuO₂ planes. ESR results indicate that oxygen deficiency of CuO₂ layers not only decreases carrier concentration, but also destroys the antiferromagnetic spin correlation or spin fluctuation and results in localized Cu²⁺ spins. The effect of localized Cu²⁺ spins (or spin scattering) on the superconductivity and transport properties is discussed in the text.     

Annealing Effects in (Hg,Cr)Sr2CuO4+δ: Transport and X-Ray Absorption Studies

Superconducting (Hg_1–x Cr _x )Sr₂CuO₄₊, x 0.36, samples of 1201-type, synthesized in partial vacuum, show T _c ^onset of 58 K, T _c (R = 0) 52 K. It is found that T _c is not affected by the subsequent Ar or O₂ annealing treatment. Remarkably, the annealed samples show significant improvement in the diamagnetic signal. Hg L₃-edge measurements on 1201 samples show divalent state of mercury. The Cu K-edge spectra for the samples, after taking due account of the impurity phases, show noticeable modifications in the split main peak features on Ar or O₂ annealing, suggesting changes in the crystal field asymmetry. In the superconducting (SC) samples, a weak but distinct signature of Cu¹⁺ is seen. The Cu¹⁺ feature is absent in all the non-SC 1201 samples as well as in the impurity phase SrCuO₂ and Sr₂CuO₃ samples. An attempt is made to explain the observed Cu¹⁺ feature in SC (Hg,Cr)-1201 in the light of the available reports.     

The Study of Hole Distribution in High-Tc Tl- and Hg-based 1212 Type Cuprates via High-Resolution X-Ray Absorption Near Edge Structure

High resolution O K-edge and Cu L₃-edge X-ray absorption near-edge-structure (XANES) spectra of the high-T_c cuprates of (Tl_0.5Pb_0.5)Sr₂(Ca_1–xY_x)Cu₂O₇ (Tl-1212) and (Hg_0.5Pb_0.5)Sr₂(Ca_1–xY_x)Cu₂O₇ (Hg-1212) in powder form were measured using a bulk sensitive total-fluorescence-yield technique. Near the O 1s edge, the pre-edge peak with maxima at 528.3 eV is ascribed to the transitions to O 2p holes located in the CuO₂ planes. The intensity of this pre-edge peak increases with increasing doping level of Ca²⁺ into the Y³⁺ sites in Tl-1212 and Hg-1212. In the Cu L-edge absorption spectra, high-energy shoulders at around 933 eV are attributed to the transitions to the Cu(2p_3/2)^–13d¹⁰L states in the CuO₂ layers, where L denotes the O 2p ligand hole. The behavior of these shoulders in Tl-1212 and Hg-1212 correlates with that of the pre-edge peak at 528.3 eV in the O K-edge absorption spectra. The results can lead us to understand the hole distribution in high-T_c cuprates which will give a direction to find new high-T_c materials.     

Investigation of two-well potential of the apex oxygen in YBa2Cu2O7?x crystals by Raman spectroscopy

The dependence of oxygen modes intensity on pump power and temperature in YBa₂Cu₃O_7–x single crystals with different oxygen contents (0<x<1) has been measured. When YBa₂Cu₃O_6.5 crystals are heated, the peak intensity of the 500-cm^–1 band (apex oxygen) decreases, that of the 590-cm^–1 band (defect mode) increases, and the peak intensity of the 330-cm^–1 band (plain oxygen) remains unchanged. The resonant dependence of the apex oxygen, and the intensity of the defect modes have also been measured. In order to explain the experimental results we suppose that the apex oxygen ion is in a double-well potential.     

High oxygen pressure synthesis of new copper oxide superconductors

The unique coordination of the copper ions in +1, +2, and +3 oxidation states, which are stable in a range of oxygen partial pressure. 10^–6<P(O₂)< 10³ atm, makes possible the formation of a wide variety of distinct structures. By controlling the oxygen pressure during the synthesis and annealing, the distribution of metal and oxygen ions can be modified on an atomic scale to optimize the structural and electronic properties. We present several examples of compounds for which the critical structural elements for superconductivity, i.e., the perfectly ordered CuO₂ planes, have been obtained and doped with holes by means of high oxygen pressure,P(O₂)>1 atm. These materials, La₂CuO₄₊ , La_2–x Ca _x CuO₄, and YSr_2–x La _x Cu₃O₇₊ , offer a unique opportunity to study the relationship between superconductivity and structural, magnetic, and chemical properties.     

Thermal conductivity and phonon scattering mechanisms in La#x2212;x#x2212;x#x2212;xMxCuO4

The thermal conductivity of high density La_2–xM_xCuO₄ (M=Ba, Sr) sintered materials was measured between 15K and 150K for the various concentrations of Ba and the phonon thermal conductivity was analyzed comparing with that of Nd_2–xCe_xCuO₄. The pretty large value for pure La₂CuO₄ was drastically diminished by substituting La by a small amount of Ba atoms especially at low temperatures. It was found that a new type of the phonon scattering center such as a two-level tunneling must be taken account of in order to explain the observed reduction.