Magnetic order in LaEuSrCuO studied [2pt] by Fe Mössbauer spectroscopy

⁵⁷Fe M?ssbauer effect studies of La_1.65Eu_0.20Sr_0.15CuO₄ doped with 0.5 at% ⁵⁷Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La_2-xSr_xCuO₄. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La_2-xSr_xCuO₄ is independent of the RE moment. The direction of the ⁵⁷Fe magnetic moment in the magnetically ordered state is within the CuO₂ plane, while it has been found to be parallel to the c-axis in Nd doped La_2-xSr_xCuO₄. Received: 23 June 1998 / Accepted: 14 July 1998     

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.     

Study of carrier doping across the parent Mott insulator La2CuO4

Ce substituted La₂CuO₄ single crystals are investigated to try doping electrons into the parent Mott insulator. Transport properties of slightly Ce substituted La₂CuO₄ show that carriers are still holes activated from an impurity level of which activation energy is the same as the parent La₂CuO₄.     

On the interpretation of the phase diagram of La<Subscript>2−x</Subscript>M<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

The difference between the phase diagrams of La_2?x(Ba,Sr)_xCuO₄ and Nd_2?xCe_xCuO₄ is discussed. It is proposed that the discrepancy of x-values corresponding to the transition from antiferromagnetic dielectric state to conducting one (respectively, x≈0.06 in La_2?x(Ba,Sr)_xCuO₄ and x≈0.125 in Nd_2?xCe_xCuO₄) results from non-homogeneous doping of La_2?x(Ba,Sr)_xCuO₄ over the range 0.06<x<0.125, when localized holes are added to each second CuO₂ layer. Therefore the actual phase diagram of the “holedoped” superconductor La_2?x(Ba,Sr)_xCuO₄ coincides with one for the “electron-doped” superconductor Nd_2?xCe_xCuO₄. It is shown that all features observed in La_2?x(Ba,Sr)_xCuO₄ around x=0.125 are clarified on this basis.     

119Sn and57Fe Mössbauer study of La2−x Sr x CuO4

Mössbauer absorption measurements were carried out for¹¹⁹Sn and⁵⁷Fe impurities in La₂CuO₄, La_1.85Sr_0.15CuO₄ and La_1.6Sr_0.4CuO₄. For all cases,⁵⁷Fe spectra at 4.2 K show magnetic hyperfine splittings. On the other hand, a magnetic hyperfine field at Sn nucleus is observed only in the case of La_1.85Sr_0.15CuO₄. The results indicate that the Cu magnetic spins are collectively fluctuating in the superconducting sample.     

Magnetic order in La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125

We present μSR experiments on La_2-x Ba_x CuO₄ and La_1.6-x Nd_0.4 Sr_x CuO₄ for x=0.125. Both of these materials order magnetically with T_N\approx30\ K, while a superconducting sample of La_1.4 Nd_0.4 Sr_0.2 CuO₄ showed no evidence for static copper moments. Our results support the conclusion that the so‐called “1/8” anomaly in La_2-x Ba_x CuO₄ is a result of static (pinned) charge segregation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comprehensive study of high-Tc interface superconductivity

Using ALL-MBE technique, we have synthesized different heterostructures consisting of an insulator La₂CuO₄ (I) and a metal La_1.56Sr_0.44CuO₄ (M) layer neither of which is superconducting by itself. The M-I bilayers were superconducting with a critical temperature T_c≈30-36 K. This highly robust phenomenon is confined within 1-2 nm from the interface and is primarily caused by the redistribution of doped holes across the interface. In this paper, we present a comprehensive study of the interface superconductivity by a range of experimental techniques including transport measurements of superconducting properties.     

μSR study of internal magnetic fields in superconducting La1.9Sr0.1CuO4 in the mK region

Muon-spin-rotation (μSR) experiments were performed on superconducting La_1.90Sr_0.10CuO₄ in zero external field. Below 2 K, a fast depolarization of the muon signal is observed, indicating that internal magnetic fields are present at the muon site. The average magnitude of the magnetic field in La_1.90Sr_0.10CuO₄ is in the order of 3 mT and thus approximately 14 times smaller than the corresponding field in antiferromagnetic La₂CuO₄. The present data support theoretical models of superconductivity in which magnetic correlations are responsible for pairing.     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

The possible origin of incommensurate spin textures in HTSC

An model of La_2-xSr_xCuO₄ explaining the features of incommensurate spin textures without any assumption of stripe formation is proposed. The foundations of this model are the mechanism of negative-U center formation proposed earlier and the concept of specific ordering of doped ions. It is shown that within the framework of the proposed model the features of “stripe” textures of La_2-xSr_xCuO₄ reflect exclusively the geometrical relations existing in a square lattice and the competition between different types of doped hole ordering.     

Location of superconductivity in La2-βSrβCuO4

Evidence is presented that the superconductivity of La_{2 ?β} Sr_β CuO₄ and La₂CuO4_+δ occurs via holes in the LaO planes, rather than in the CuO₂ planes.     

Effect of annealing conditions on high temperature superconductivity in La2−xSrxCuO4

Abstract

Besides investigating a number of oxides of the series La_2?xM_xCuO₄ (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La_1.8Sr_0.2CuO₄. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper.     

Low-temperature synthesis of T′-La2CuO4 using CaH2 as reductant

We have successfully synthesized polycrystalline bulk samples of La₂CuO₄ with the Nd₂CuO₄-type (T′) structure at low temperatures. First of all, La₂CuO₄ with the K₂NiF₄-type (T) structure was prepared by the conventional solid-state reaction. Secondly, La₂CuO_3.5+δ with the Sr₂CuO₃-type structure was obtained by heating mixed powder of T-La₂CuO₄ and CaH₂ at 175 °C in an evacuated Pyrex tube. Thirdly, T′-La₂CuO_4+δ was prepared by heating La₂CuO_3.5+δ at 250 °C in air. Finally, T′-La₂CuO_4+δ was reduced using CaH₂ to remove excess oxygen. The powder X-ray diffraction pattern has revealed that the product is of the single phase of T′-La₂CuO₄, but no superconductivity has been observed at T > 2 K in the magnetic susceptibility measurement. The residual excess oxygen may suppress the appearance of superconductivity.     

Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La_1.87Sr_0.13CuO₄ are directly measured with the electric field polarized perpendicular to the CuO planes (E∥c) covering the frequency range 10-40 cm⁻¹ and temperatures 5-300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper-oxygen planes of La_1.87Sr_0.13CuO₄. A strongly enhanced low-frequency (below 15 cm⁻¹) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.     

X-ray Fluorescence Spectroscopic Studies of High TeSuperconductors

The Cu-Lβ/Cu-Lα x-ray emission intensity ratio of the following compounds have been found to show an interesting consistency: Cu, Cu₂O, CuO, La_1.85Ba_.15CuO₄ (vacuum annealed), La_1.85Ba_.15CuO₄ (air annealed, T_C = 26K), La_1.85Sr_.15CuO₄ (T_C = 35K), YBa₂Cu₃O_7-Δ (T_C = 91K), ErBa₂Cu₃O_7-Δ (T_C = 81K), and La₂CuO₄. The interesting consistency is that all of the last six compounds, all superconductors or superconductor related, have the same Lβ/Lα intensity ratio, namely, 0.33±0.01. The Cu-Lα,β spectra of CuO, air and vacuum annealed La_1.85 Ba_.15CuO₄ and YBa₂Cu₃O_7-Δ are presented. Also presented are O-K x-ray fluorescence spectra of air and vacuum annealed La_1.85 Ba_.15CuO₄, and of “new” and “old” YBa₂Cu₃O_7-Δwhich seem to show a correspondence between spectral sharpness and high T_C behavior.     

Are the high temperature superconductors real metals?

A comparison of the experimental band structure of oxygen on Cu(110) and the theoretical band structure of the prototype material for the high temperature superconductors La₂CuO₄ is performed. This comparison suggests, that the theoretical band structure describes the occupied electronic states of La₂CuO₄ adequately.     

Intercalation and staging behavior in super-oxygenated La2CuO4 + δ

A high temperature electrochemical oxidation process has been used to produce large single crystals of La₂CuO_{4 + δ} suitable for neutron scattering experiments. Below room temperature the oxygen-rich phases have structural superlattice scattering peaks which indicate new periodicities ranging from 2 to 6.6 layers perpendicular to the copper oxide planes. A model structure originally proposed for La₂NiO_{4 + δ} can account for the superlattice peaks as a result of anti-phase domain boundaries between different tilt directions of the CuO₆ octahedra. Within this model, the changes in CuO₆ tilt directions are induced by segregated layers of interstitial oxygen which order in a manner similar to intercalants in graphite. This structural model thus clarifies previous work and establishes La₂CuO_{4 + δ} as a unique lamellar superconducting system with annealed disorder.     

On the La2CuO4-Nd2CuO4 system: Influence of oxygen pressure on the phase diagram,oxygen stoichiometry and transport properties of the T/O and T′ type solid solutions

Abstract

Two types of solid solutions exist in the La₂CuO₄-Nd₂CuO₄ system, with respectively the T/O and T′ type structure of La₂CuO₄ and Nd₂CuO₄. When synthesized in air at high temperature and normal pressure, both solid solutions exhibit semi-conducting properties. A treatment under oxygen pressure causes a slight change in the unit cell parameters of the T/O type solid solution, a significant shift of its upper limit towards higher neodynium contents and a small increase of the average oxidation state of copper. All these modifications result from the insertion of some oxygen species in the T/O type-structure and induce a superconducting behaviour.     

Microhardness of the compounds La2CuO4 and La1.85Sr0.15CuO4 at temperatures from 81 to 292 K

The microhardness of La₂CuO₄ and La_1.85Sr_0.15CuO₄ crystals is measured in the temperature range 81–292 K, and the influence of various factors on it is investigated. The thermal activation parameters of the plastic deformation process in the vicinity of the indenter impression are estimated. The change of the phase state of the compound La_1.85Sr_0.15CuO₄ at the temperature T ₀=180 K and the appearance of domain (twin) boundaries formed in the ferroelastic tetraortho transition are not seen in the temperature dependence of the microhardness. The results of the study are compared with previously published results for YBa₂CuO_7−x crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 493–497 (March 1998)