Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method ( ⁺SR) is applied on La_2–x Sr _x CuO₄ around forx=0.12 at which the high-T _c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105x0.120. The magnetic phase boundary is very similar to the one of the La_2–x Ba _x CuO₄ in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO₂ plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T _c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.     

57Fe and119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4

⁵⁷Fe- and¹¹⁹Sn Mössbauer effect studies on 0.5 at %⁵⁷Fe (or¹¹⁹Sn) dopednon-superconducting La_1.25Nd_0.6Sr_0.15CuO₄ in the temperature region 300 K T4.2 K reveal an onset of local magnetic ordering occurring atT _m 32 K for both⁵⁷Fe—as well as¹¹⁹Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field ofB _thf 11.0(5)T at the¹¹⁹Sn nuclei. Since such a large field is neither present in antiferromagnetic La₂CuO₄, nor in superconducting La_1.85Sr_0.15CuO₄ or overdoped La_2–xSr_xCuO₄ (x=0.4), the occurrence of such local spin correlations seems to be a signature of the non-superconducting low temperature tetragonal phase of La_1.25Nd_0.6Sr_0.15CuO₄.     

Electrostatic lattice coefficients and binding energy of orthorhombic La2-x Sr x CuO4

Three valency models for orthorhombic La_2-x Sr _x CuO₄ were investigated for increasing Sr concentrationsx (0x0.21): 1. Cu²⁺Cu³⁺, 2. apex O^2–O^– and 3. in-plane O^2–O^–. All calculations were done by using structural parameters valid for the temperature range from 10 to 22 K. We thereby calculated the electrostatic interaction energy which, next to ionization potentials and electron affinities, comprises a major of the binding energyE _B of crystals. Second-order effects were accounted for by calculating the strength of ionic dipole moments induced by crystal electric fields at relevant lattice sites. Their largest strengths are comparable to the dipole moment of the water molecule. Three out of five dipoles in La_2-x Sr _x CuO₄ vanish during the transition from the orthorhombic to the tetragonal phase. The binding energy differences between the different models suggest that the system is in a state of model 1. However, the differences are very small, being in the order of 0.3 to 0.76 eV atx=0.13.     

Elastic and inelastic neutron scattering studies on the tetragonal to orthorhombic phase transition of La2−x Sr x CuO4±δ

The phase transition from the tetragonal to the orthorhombic structure in La_2–x Sr _x CuO₄₊ has been studied combining elastic and inelastic neutron scattering and specific heat measurements on the same well characterized crystals. This analysis focusses on the comparison between undoped and doped compounds. The proportionality between the orderparameter and the orthorhombic strain, predicted by the Landau theory, can be confirmed in La_2–x Sr _x CuO₄₊ in a wide concentration and temperature range. We observe different values for the critical exponent for doped superconducting and undoped insulating crystals. The temperature dependent study of the soft modes on a metallic superconducting crystal reveals an extremely large temperature range where this mode is strongly anharmonic. The comparison with previously published data shows that the frequencies of the soft modes at low temperatures decrease with doping, i.e. with increasing charge carrier concentration.     

Competition of magnetic order and superconductivity investigated by positive muon spin rotation in La2−x M x CuO4 (M = Ba,Sr) aroundx=0.12

Muon spin rotation (SR) is applied to La_2–x Ba _x CuO₄ aroundx=0.12 where the superconductivity (SC) is suppressed remarkably. The magnetic ordering of Cu-moments appears below 35 K in the narrow range ofx where a lattice instability from the low temperature orthorhombic (LTO) to the low-temperature tetragonal (LTT) structure exists. The present study suggests strongly that the magnetic ordering of Cu-moments is an important factor in the suppression of high-T _c superconductivity aroundx=0.12 in La_2–x Ba _x CuO₄. Similar results are obtained for the La_2–x Sr _x CuO₄ and La_2–x–y Sr _x Nd _y CuO₄ systems.     

μSR and NMR investigations on electronic and magnetic state aroundx=0.12 in La2−x Sr x CuO4 and La2−x Ba x ;CuO4

The muon spin rotation ( ⁺SR) and NMR measurements provides clear evidences of the antiferromagnetic order of Cu-moment 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 freezing of spin fluctuations of Cu-moment is relevant to the local suppression of SC under an change of the electronic state coupled with the lattice instability.     

Coexistence of superconductivity and magnetism in single crystalline highT c superconductor La2−x Sr x CuO4 revealed by the muon spin relaxation method

The positive muon (⁺) spin relaxation method under zero external field is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2–x Sr _x CuO₄ (0.10<x<0.15). A series of well characterized crystals with nearly complete flux exclusion were found to exhibit a magnetic ordering with transition temperature depending onx with a peak atx=0.11. Possible explanations are given.     

Superconductivity above 100 K in multi-phase Y−Ba−Cu−O

We report the synthesis, characterization and superconducting temperature of high-temperature superconducting Y_0.3Ba_0.7CuO₃, Y_0.4Ba_0.6CuO₃ and Y_1.2Ba_0.8CuO₄. The volume fraction of the superconducting phase is estimated to be of the order of 10%.HighT _c superconductivity withT _c -onsets up to 125 K (midpoint 102 K, zero resistance: 90 K) is observed in multi-phase Y–Ba–Cu–O oxygen defect compounds with the nominal composition of Y_1.2Ba_0.8CuO_4–y (0y0.4).     

Pb1−x Fe x films prepared by vapor condensation

ZF, LF and TF SR experiments with antiferromagnetic (AF) ceramical samples La_2–x Sr _x CuO_4–y have been performed in the temperature range 10–300 K. Zero field muon spin polarization functions obtained below the Neel temperature clearly show a nonzero initial precession phase-–0.35 rad. We propose an explanation based on existence of the dynamical magnetic fields on the muon.We thank Drs. A.G. Chistov and A.M. Brjazkalo from RSC Kurchatov Institute for the preparation the #2 La₂CuO_4–y sample.     

Mössbauer studies on the series La1-xSrxFeO3

Mössbauer spectra have been recorded at 4.2 and 300 K on the series La_1–x Sr _x FeO₃, wherex varies from 0 to 1.0 in steps of 0.1. Neutron diffraction experiments have shown that the crystal structure is orthorhombic for 0x<0.3, rhombohedral for 0.4x0.7, and cubic for 0.8<x1.0. Mössbauer spectra at 4.2 K are composed of magnetic sextet components arising from different charge states of iron ions. In the orthorhombic and rhombohedral phases, the charge states Fe³⁺ and Fe⁵⁺ coexist. In the cubic phase, iron is present as Fe³⁺ and Fe⁴⁺ states. At 300 K, the samples are magnetically ordered in the range 0 x0.3 and the coexistence of Fe³⁺ and Fe⁵⁺ remains. For samples 0.4x1.0, the samples are paramagnetic. Fits to these spectra require two components, one corresponding to an Fe⁴⁺ state, the other being best described as an Fe³⁺ ion forx0.7 but forx>0.7 having a mean charge state which increases to 3.5 forx=1.0.     

Reflection Spectra of Single Crystals of La2CuO4−x ‐Type Metal‐Oxide Compounds

We have investigated the reflection spectra in polarized light at 80 and 295 K in an energy region of 1.0–4.8 eV for the single crystals of ternary and quaternary copperoxide systems with the participation of rareearth elements (La₂CuO_4–x , LaCuO₄, Pr₂CuO_4–x , GdBa₂Cu₃O_7–x , and LaSrCaCuO).     

Temperature dependence of (μ−O) paramagnetic shift in highT c superconductor La2−x Sr x CuO4

In order to investigate the electronic structure around oxygen in superconducting La_2–x Sr _x CuO₄ (x=0.10 and 0.14), precise measurements were carried out on the paramagnetic Knight shifts in the spin rotation frequency of the negative muon bound to the oxygen with reference to antiferromagnetic La_2–x Sr _x CuO₄ (x=0.05). Remarkable temperature as well as crystalline-axis dependence was observed for the highT _c case with a divergence character towardsT _c. Possible mechanisms to explain the observed results are given.     

Structural chemistry,electrical resistivity,magnetization and specific heat in La1.85Sr0.15Cu1−xZn x O4

Samples from the oxide series of solid solutions La_1.85Sr_0.15Cu₁–xZn _x O₄, quenched from 1100°C, have been characterized by X-ray powder analysis. We report on the structural chemistry and on measurements of the electrical resistivity, magnetization, ac susceptibility and specific heat of these samples. Zinc was found to statistically replace the copper atoms in the body centered tetragonal K₂NiF₄-type of structure up to values ofx smaller than 0.15. At higher zinc-concentrations (0.15<x<0.45) the crystal structure transformes into the orthorhombically distorted K₂NiF₄-type, known for the low-temperature form of Sr-and Zn-free La₂CuO₄. Limited solid solutions are encountered when Cu is substituted by Cd and Hg. Whilst the Zn-free sample La_1.85Sr_0.15CuO₄ was superconducting with a sharp onset at 35 K, substitution of Cu by Zn by as little as 6 mole percent was sufficient to suppress superconductivity belowT _c<4.5 K. Isoelectronic substitution of Cu by (Ni_0.5Zn_0.5) was found to cause an even larger degradation ofT _c than equivalent amounts of Zn. At the superconducting transition the discontinuity of the specific heat C _p is observed in agreement with recent publications, but is found to decrease with Zn substitution. A linear term inC _p(T) is resolved at low temperatures which significantly rises upon Cu/Zn substitution.     

Structure and superconductivity of La1.8Sr0.2CuO4 and multi-phase Y1.2Ba0.8CuO4 under high pressure

Superconducting La_1.8Sr_0.2CuO₄ has been investigated under pressure. Its resistiveT _c -midpoint was found to be 37 K at ambient pressure. It inductively determinedT _c is 33 K monotonically shifting up to 38.4 K at 19.2 kbar. The initial linear increase is 0.295 K/kbar. X-ray investigations in a diamond anvil revealed no phase transformation of the K₂NiF₄ structure up to about 50 kbar at 300 K, 60 K and 15 K. The bulk modulus of La_1.8Sr_0.2CuO₄ was 1070 kbar at 300 K and 1500 kbar at 15 K. Thec/a ratio decreases and shows pronounced differences between room temperature and low temperature data under pressure. Multi-phase Y–Ba–Cu-Oxide with resistiveT _c -onset of 90 K was investigated under pressure, too. The inductive transition starts at 70 K and is completed at 20 K. The superconducting volume fraction was estimated to be about 2% at 10 K.     

Calcium doping effects in57Fe : La2−x Ca x CuO4−y oxides

La_2–x Ca _x Cu_0.99Fe_0.01O_4–y samples with 0.00 x 0.30 were studied by X-ray diffraction and Mössbauer spectroscopy after heat treatment under different atmospheres (air, oxygen and helium). The variation of the relative population and the hyperfine interactions of the sites as well as the change in the lattice parameters were obtained.On leave from National Center for Scientific Research, PO Box 6990, Havana, Cuba.     

Nernst effect of stripe ordering La<Subscript>1.8−x</Subscript>Eu<Subscript>0.2</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

We investigate the transport properties of La_1.8−xEu_0.2Sr_xCuO₄ (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La_1.8−xEu_0.2Sr_xCuO₄ and superconducting La_2−xSr_xCuO₄ at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.     

XES studies of La-Sr-Cu-O and Y-Ba-Cu-O ceramics

X-ray emission spectroscopic studies of superconducting ceramics characterised by the following compositions: La_2.19Sr_0.07Cu_1.05O_x, Y_0.9Ba_1.8Cu_3.3O_y and Y_0.9Ba_2.0Cu_3.1O_y, as well as of a semiconducting ceramic, Y_1.9Ba_1.0Cu_1.1O_z, have been performed using spectrometers of an electron probe microanalyser. Concentrations of the superconducting phases and semiconducting one exceeded 80% of the whole sample compositions. The CuL spectra detected for superconducting samples revealed a satellite maximum connected with Cu³⁺ ions lying at+1.4 eV in the high energy side of the spectrum. Mean valence of copper ions in the La–Sr–Cu–O sample has been estimated to be +2.16 and for the Y_0.9Ba_2.0Cu_3.1O_y, sample above+2.1. It was confirmed that the binding energy of 3d electrons taking part in the electronic transition involving this satellite maximum and the main one is equal to 3.5 eV. The satellite at+2.6 eV is connected with states close to the Fermi level, and the satellite caused by theM _2,3 M _4,5 M _4,5 super Coster-Kronig transition at about 4.2 eV has been established. Analysis of the LaL₄ spectrum showed evidence for weak bonding between the lanthanum ions and oxygen ions.     

Charge ordering,superconductivity, and stripes in doped La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The special features of the phase diagrams of La_2?xSr_xCuO₄ are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La_2?xSr_xCuO₄. According to this model, the main characteristics of the experimental phase diagrams of La_2?xSr_xCuO₄ only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.     

Charge Carrier Dynamics in Zn-Doped Cuprates

We have measured the static magnetic susceptibility, the resistivity and muon spin relaxation (⁺SR) spectra of Zn and Sr doped La₂CuO₂. Our data show that non-magnetic Zn impurities lead to an increase of the Néel temperature T _N in weakly hole doped compounds. This increase of T _N correlates with an increase of the resistivity. The analysis of our data strongly suggests that the hole mobility is the most important source for the strong suppression of long range antiferromagnetic order in La_2–x Sr _x CuO₄.