Influence of Sr doping on the free carrier absorption in high-T c superconductors La2−x SrxCuO4

The reflectance spectra of polycrystalline La_2–x Sr _x CuO₄ samples were investigated in the energy range between 50 meV and 4 eV in dependence of the Sr content. The spectra are attributed to free carrier absorption of the Drude type, superimposed by optical phonon excitations below 0.1 eV and intrinsic absorption above 1 eV. From the influence of Sr doping onto the plasma energy it is deduced that La_2–x Sr _x CuO₄ is ap-type conductor with a maximum carrier concentration of 2.0×10²¹ cm^–3 forx=0.15. The results are interpreted in terms of a Hubbard model with an empty upper and ap-doped lower Hubbard band with a width of 1.9 eV.     

A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2?x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O^2?→Cu²⁺+O^?, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2?x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.     

Soft phonon behavior and magnetism at the low temperature structural phase transition of La1.65Nd0.35CuO4

We report a neutron scattering investigation of a La_1.65Nd_0.35CuO₄ single crystal which undergoes a second order structural transition with decreasing temperature from a high temperature orthorhombic phase (space group Bmab) to a distinct low temperature orthorhombic phase (space group Pccn) atT _s =76 K. The transition is induced by the softening of the phonon mode that involves rotations of the tilt axis of the CuO₆ octahedra. Antiferromagnetic long range order is established belowT _N =316 K, and a hysteretic reorientation to a noncollinear spin structure occurs belowT _s . The low energy spin dynamics of La_1.65Nd_0.35CuO₄ aboveT _s are closely similar to those of stoichiometric La₂CuO₄; at 100 K the gaps for in-plane and out-of-plane polarized spin waves are 2.3 meV and 5 meV, respectively. Out-of-plane polarized magnons are little affected by the structural transition, but the gap for in-plane polarized magnons increases markedly belowT _s .     

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.     

Investigations of magnetic excitations in a La2CuO4 single crystal

A detailed neutron scattering study is carried out of magnetic excitations in pure La₂CuO₄ in the low energy region up to 15 meV, at temperatures in the vicinity of the transition temperatureT _N to the paramagnetic state. In particular the question of existence of propagating spin waves in this energy region is addressed. The experimental results show that the energy region studied (up to 15 meV) is characterized by aquasielastic peak of the width of order 4 meV (at 300 K). Thus the magnetic correlation function has relaxational behavior in the studied energy region, no indications for propagating spin waves are obtained. The measured quasielastic width agrees well with existing theories. Polarized neutrons and polarization analysis are also used to identify the magnetic origin of the observed scattering.     

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.     

XPS studies of several high temperature superconductors: Y-Ba-Cu-O and La-Ba-Cu-O systems

Copper 2P core level X-ray photoelectron spectra on several high T_c superconductors La_1.85Ba_0.15CuO₄, Y_1.2Ba_0.8CuO_4−δ and Y_0.4B_0.6CuO_4−δ oxides have been measured. The results indicate the coexistence of Cu²⁺ and Cu³⁺ in these compounds.     

Mössbauer studies of57Co-doped layered perovskites

Mössbauer spectra of⁵⁷Co-doped polycrystalline or single-crystal samples of layered perovskites La₂Mo₄ (M=Cu, Co, Ni) and R₂CuO₄ (R=Nd, Eu, Gd) were recorded at room temperature and below. Of the samples studied, only La₂CuO₄ shows a widely separated doublet at room temperature and a single clearly resolved sextet well belowT _N.     

Theory for the Gaussian Component of the 63Cu Nuclear Spin-Echo Decay Rate 1/T 2G in La2−x Sr x CuO4

We report calculations of the Gaussian component of the ⁶³Cu nuclear spin-echo decay rate 1/T _2G employing the theory for spin susceptibility as derived within the t-J model starting from carrier-free La₂CuO₄ and right up to optimally doped superconducting layered copper oxide La_2?x Sr _x CuO₄. The theory reproduces the temperature and doping behavior of the experimental data for 1/T _2G. A quantitative agreement with experimental 1/T _2G data in doped La_2?x Sr _x CuO₄ compounds is obtained with account of both the spin–spin and “fermion”–“fermion” correlations.     

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La₂CuO₄ are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La₂CuO₄ lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D _2h ¹⁸ ) into the tetragonal phase (space group D _4h ¹⁷ ) under hydrostatic compression of an La_2−x Sr_xCuO₄ crystal is modeled. The (P,x) structural phase diagram for La_2−x Sr_xCuO₄ is constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 1096–1102 (June 1999)     

Dc and Ac conductivity of La2CuO4+δ

The complex conductivity of La₂CuO_4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence σ～v ^s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La₂CuO_4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process.     

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.     

Theoretical explanations to the EPR g factors for Er3+ ion in La2−xSrxCuO4 superconductor

The electron paramagnetic resonance (EPR) g factors g_∥ and g_⊥ for Er³⁺ ion in La_2−xSr_xCuO₄ superconductor are theoretically explained by using the perturbation formulas of g factors for a 4f¹¹ ion in tetragonal symmetry. In these formulas, the contributions to the g factors arising from the second-order perturbation terms and the admixture of different states are included. The related crystal field parameters are calculated from the superposition model and the local structural parameters of the impurity Er³⁺ occupying the host La³⁺ site, and the superposition model parameters used in this work are comparable with those for similar tetragonal Er³⁺ centers in some zircon compounds in the previous work. The theoretical studies on g factors of Er³⁺ ion in this work would be of some use to experimentalists doing EPR on La_2−xSr_xCuO₄ (or other superconductors) with Er dopants.     

Experimental observation of Bose condensation in high-temperature superconductors

Emission Mössbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn) and ⁶⁷Ga (⁶⁷Zn) isotopes was used to show that for the superconductors Nd_1.85Ce_0.15CuO₄, La_1.85 Sr_0.15CuO₄, and Tl₂Ba₂ CaCuO₈ in the temperature range T > T _c the temperature dependence of the center of gravity S of the Mössbauer spectrum is determined by the second-order Doppler shift, while in the range T < T _c the Bose condensation of Cooper pairs influences the value of S (here T _c is the superconducting transition temperature). The spatial nonuniformity produced in the electron density by a Bose condensate of Cooper pairs was observed for La_1.85 Sr_0.15CuO₄.     

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     

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.     

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.     

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.