Spin dynamics in the LTT phase of La-Sr cuprates as revealed by ESR

We present the results of the ESR study on Gd spin probes of the spin dynamics of the CuO₂ planes in the low temperature tetragonal (LTT) phase of La_2–xSr_xCuO₄ which is stabilized below 110 – 130 K by doping this compound by Eu. Instead of the Korringa like behaviour occurred in the orthorhombic (LTO) phase at higher temperatures, Gd³⁺ ESR linewidth H broadens considerably at low temperatures in the LTT phase. Maximal broadening of H is observed for the samples with suppressed superconductivity. The analysis of the data shows that the spin correlation length in the LTT phase rapidly increases at low temperatures and for non superconducting samples may exceed 100 lattice constants. Correspondingly, the frequency of the antiferromagnetic spin fluctuations decreases down to 10¹⁰ – 10¹¹sec^–1. It is argued that such extremely slow antiferromagnetic spin dynamics is a characteristic property of the magnetism in the LTT structural phase of La_2–xSr_xCu0₄.     

Spatial modulation of low-frequency spin fluctuations in hole-doped La2CuO4

Systematic neutron scattering measurements have been performed on the Sr-doped La_{2-x}Sr_xCuO₄ to study the doping dependence of spatially modulated dynamical spin correlations or so-called incommensurate spin fluctuations. The modulated spin correlations appear beyond x ≈ 0.05 which is close to the lower boundary of the superconducting phase. First evidence was observed for the linear relation between the degree of spatial modulation or the incommensurability Σ(x) and the maximumT _c atx. We present a universal curve for Σ(x) by adding data from other La₂CuO₂ systems such as oxygen-doped superconductors, oxygen-reduced or Zn-substituted nonsuperconductors, and La_1.6-xNd_0.4Sr_xCuO₄.     

Magnetic order and evolution of the electronic state aroundx=0.12 in La2?xBaxCuO4 and La2?xSrxCuO4

Muon spin rotation ( ⁺SR) measurement provides clear evidence of the antiferromagnetic order of Cu moments below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the change of the electronic state coupled with the lattice instability is relevant to the local suppression of SC and freezing of spin fluctuations of the Cu moment.     

Characteristic features of low-temperature structural phase transitions in La x-2Sr x CuO4

Features of low-temperature structural phase transitions in La_{2-x}Sr_xCuO₄ with x = 0.115 and 0.15 have been examined by transmission electron microscopy. There are 100-type forbidden spots due to thePccn/LTT phase as well as fundamental spots due to the LTO phase in the electron-diffraction patterns of La_x-2Sr_xCuO₄ with x = 0.115 and 0.15 at 12 K. As for 100-type forbidden dark field images at 12 K, a bright-line contrast due to thePccn/LTT phase exists only along the twin boundary in the LTO phase for x = 0.15. In the case of x = 0.115, a bright-spot contrast due to thePccn/LTT phase also appears partially inside the LTO domains, in addition to the bright-line contrast along the twin boundary. That is, the low-temperature transition in x = 0.115 is characterized by the appearance of thePccn/LTT phase with the spotty shape inside the LTO domain.     

Is the tetragonal overdoped phase (La, Sr)2CuO4 superconductive?

Comparative studies of Meissner and shielding processes have been performed on variously sized samples of the solid solution (La_1–x Sr _x )₂CuO₄: (polycrystalline, powder, and single-crystalline) under various magnetic fields in the range 0.03–10 Oe. Considering the kinetics of the two processes, this systematic approach has indicated that the observed Meissner signal can be significantly low with the sample still being wholly superconductive. Based on this comparative study, the tetragonal region of (La_1–x Sr _x )₂CuO₄ withx greater than 0.105 has been concluded to be superconductive, suggesting the nonessential involvement of the buckling of the Cu-O-Cu-O bond in the CuO₂ layer in the manifestation of superconductivity.     

Influence of doping on the electronic structure of (La, Sr)2CuO4

High-statistics (>4 × 10⁸ counts), room-temperature measurements of the electron-positron momentum density of La_2–x Sr _x CuO₄ have been performed for samples with Sr concentrations of x=0.0, 0.1, 0.13, and 0.2. These spectra have been analyzed in conjunction with theoretical calculations of the electron-positron momentum density. The metallic samples show features consistent with the presence of a Fermi surface, but its evolution with increasing Sr concentration does not follow the predictions of band theory. These results may indicate the effects of electron-electron correlation on the electron momentum distribution in the Cu-O plane.     

The Underdoped Region of the Phase Diagram of YBa2Cu3O6+x

Here we present a reviewed phase diagram of the high-T _c superconducting YBa₂Cu₃O_{6+ x} compound, finely mapped in the strongly underdoped region (0 < x < 0.5), from the pure antiferromagnetic state to the superconducting regime. The Neèl and spin freezing temperatures have been measured by μSR experiments while the hole density per Cu atom in the CuO₂ planes has been determined from the resistive T _c and from Seebeck coefficients at 290 K. The phase diagram is discussed in comparison to those of La_{2− x} Sr _x CuO₄ and Y_{1− x} Ca _x Ba₂Cu₃O₆ cuprate systems.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Microscopic Charge Inhomogeneities in Underdoped La2?xSrxCuO4: Local Structural Evidence

We present local structural evidence for the existence of charge inhomogeneities at low temperature in underdoped and optimally doped La_2–x Sr _x CuO₄. The inhomogeneities disappear for x 0.2. The evidence for the charge inhomogeneities comes from an anomalous increase in the inplane Cu–O bond length distribution in the underdoped samples as well as evidence for CuO₆ octahedral tilt inhomogeneities in the intermediate range structure. Preliminary analysis of the temperature dependence of this phenomenon indicates that the inhomogeneities set in at temperatures in the range 60 K < T _co < 130 K, which depends on doping.     

Substitution site and photoluminescence spectra of Eu-substituted SrTiO3 prepared by Pechini method

Eu³⁺-substituted SrTiO₃ (Sr_1 − xEu_xTiO₃) without second phase has been successfully prepared by Pechini method. X-ray diffraction patterns have suggested that solid-solubility limit would be 1-2 mol% due to large ionic radius difference between Eu³⁺ and Sr²⁺. Photoluminescence spectra of Sr_1 − xEu_xTiO₃ have shown the intense ⁵D₀-⁷F₁ transition higher than ⁵D₀-⁷F₂ transition, which indicates that Eu³⁺ was substituted for Sr²⁺ site.     

Optical Conductivity of La1.875Ba0.04Sr0.085CuO4

The in-plane optical conductivity and dc resistivity of La_1.875Ba_0.125–YSr _Y CuO₄ with y = 0.085 was measured from 30 to 20,000 cm^–1 and from 295 to 8 K. A strong extra-Drude absorption at finite frequency is detected both in the normal and superconducting state, and remains unchanged when the free carriers condensate. The present results confirm and extend recent infrared data on La_2–xSr_xCuO₄.     

Point contact spectroscopy in oriented La2?xSrxCuO4 superconductors; energy gap and Fermi velocity

Point contact measurements in oriented La_2–xSr_xCuO₄ samples were performed using metal tips. The current-voltage curves measured along the CuO plane direction (ab) are characteristic of the Andreev reflection phenomenon. The superconducting energy gap in theab plane is determined, _ab = 6± 1 meV, with a possibility for the existence of a lower subgap in the plane. A lower limit for the Fermi velocity in the CuO planes is also set by the measurements;V _F6×10⁷ cm/sec, which is significantly higher than the average velocity obtained by band calculations. The results are discussed in the context of different models for superconductivity in the layered oxides. In particular, we raise the possibility of an anisotropic gap parameter which may indicate a nonstandards-wave pairing in La_2–xSr_xCuO₄. A comparison with previous results obtained on YBa₂Cu₂O_7– is made.     

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.     

Dynamic Inhomogeneities in the La2CuO4-Based Superconductors

Polarization-dependent X-ray-absorption fine-structure (XAFS) measurements on the local structure of the La₂CuO₄-based high-T _c superconductors La_2–x Sr _x CuO₄, La_2–x Ba _x CuO₄, and La_1.6–x Sr _x Nd_0.4CuO₄ find, among others, orientation disorder induced in the Cu–O₂ planes by doping Sr, Ba, and alloying Nd atoms, all such atoms residing in La-sites. The orientation disorder is of two types: mostly static-buckling disorder, and dynamic disordering of the tilt angles of the Cu–O₆ octahedra correlated in nanoscale regions, with respect to neighboring nanoscale regions. Buckling disorder in the Cu–O₂ planes has the greatest detrimental effect on T _c and conductivity for such foreign atoms.     

On the oxygen isotope shift in La2?xSrxCuO4

The anomalous oxygen isotope shift observed in the high-temperature superconductors La_2–xSr_xCuO₄ and La_2–xBa_xCuO₄ is discussed within the model of superconductivity in which the apex oxygen vibrations play the major role in the pairing mechanism. We show that the dependence of both the critical temperature and its isotope shifts on the doping level is consistently described within this model. The anomaly occurring in the isotope shift between x = 0.11 andx=0.15 is traced back to the oxygen mass exponent associated with the apex oxygen frequency.     

Critical Behavior in the Low-Energy Spin Fluctuations in High-Tc Superconductors

We study the spin fluctuations of pure and Zn-substituted La_{2 – x} Sr _x CuO₄ using the muon spin relaxation technique. Superconductivity is found to coexist with low-frequency spin fluctuations over a large region of the superconducting phase diagram. The characteristic temperature of spin fluctuations detected by SR decreases with increasing x and vanishes above a critical doping x _c 0.19. This value of x _c coincides with the doping at which the normal state pseudogap extrapolates to zero. These results are discussed in terms of a quantum transition that separates the superconducting phase diagram of high-temperature superconductors into two distinct ground states.     

Stability of the low-temperature tetragonal phase in La2?xBaxCuO4 and related compounds

In La_2–x Ba _x CuO₄ (LBCO), the structural transition to a low-temperature tetragonal phase below 60 K and suppression of superconductivity are observed when the carrier density isp 1 /8 per copper. The replacement by divalent ions smaller than Ba²⁺ suppresses the static deformation of the lattice. We have found that the variationsT _d2 and superconducting transition temperatureT _c are quantitatively characterized by the averaged ionic radius at the La site or lattice parameters. This aspect of substitution could be regarded as the effect of chemical pressure, since similar variations have been reported on applying hydrostatic pressure. In La_2–x–y Nd _y (Ba, Sr) _x CuO₄,T _d2 increases with increasingy in a wide range ofp whileT _c is suppressed only at p l /8. The structural transition atT _d2 here should be ascribed mainly to the crystallochemical origin.     

The localized holes properties in LTO and LTT phases of lanthanum-stronthium cuprates

By analyzing the¹³⁹La-NQR we have studied the magnetic properties of localized holes for both the orthorhombic (LTO) phase of pure as well as for the low temperature tetragonal (LTT) phase of Eu doped (La,Sr)₂CuO₄. Correlated to the different superconducting properties of these phases we find pronounced differences in the magnetic characteristics estimated from the nuclear spin relaxation data. They are discussed in terms of variation of localized holes properties depending on the low temperature symmetry of the structure.     

Experimental determination of the spatial distribution of electron defects in La2 ? xSrxCuO4 and Nd2 ? xCexCuO4 crystal lattices

The data of M?ssbauer emission spectroscopy on ⁶⁷Cu(⁶⁷Zn) and ⁶⁷Ga(⁶⁷Zn) isotopes show that holes appearing as a result of the Sr²⁺ substitution for La³⁺ in the La_{2 − x} Sr _x CuO₄ crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce⁴⁺ substitution for Nd³⁺ in the Nd_{2 − x} Ce _x CuO₄ crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La_{2 − x} Sr _x CuO₄ and Nd_{2 − x} Ce _x CuO₄ crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).