CuO4 Superconductor Research Articles

Application of spark plasma sintering to the manufacture of La2-xSrxCuO4 superconducting ceramics

To explore the possibility of improving the critical current density (Jc) of polycrystalline La1.84Sr0.16CuO4 (LSCO) superconductor ceramics, high density La1.84Sr0.16CuO4 samples have been prepared by means of spark plasma sintering (SPS). Structural and superconducting properties of samples made by different variations of the process have been measured for systematic comparison. The density of the SPS samples can reach up to 98 % of the theoretical density of LSCO, but the critical temperature (Tc) and critical current density (Jc) are suppressed by the process. Subsequent annealing has been applied to improve the properties of the SPS samples. After treatment at 1050 °C, in spite of partial decomposition that was observed on the surface of the samples, the Jc of LSCO was significantly improved. Detailed data as well as assumptions for explaining these results are provided. The SPS technique, coupled with a suitable post-annealing treatment is thereby appearing as a possible methodology to improve the critical current density of polycrystalline high temperature cuprate superconductors.

Effect of Trapped Magnetic Flux on Neutron Scattering in La1.85Sr0.15CuO4 Superconductor

The superconducting La1.85Sr0.15CuO4 ceramics has been studied by small angle neutron scattering, magnetization measurements, and scanning electron microscopy. Experiments have shown that the intensity of the magnetic scattering is 2–3 times higher in the field cooled regime than in the zero-field cooled regime. Additional magnetic heterogeneities due to closure of the trapped magnetic flux cause the excess scattering in the field cooled regime. The isotropic nature of the scattering is associated with the absence of a preferred direction of the Abrikosov vortices, which is caused by the random orientation of the ab planes of the anisotropic superconductor granules.

A Procedure to Protect Granular and Porous Samples From Oxygen Contamination in High-Resolution Magnetization Measurements: Application to Granular High- <inline-formula> <tex-math notation="LaTeX">$T_{c}$</tex-math> </inline-formula> Cuprate Superconductors

Due to the paraantiferromagnetic transition of oxygen at 45 K, the contamination with this element may deeply affect measurements performed with high-resolution magnetometers around this temperature. We have studied this spurious effect by measuring with a commercial SQUID-based magnetometer (Quantum Design, model MPMS-XL) the magnetization of granular La1.85Sr0.15CuO4 (LSCO) superconductors by using different sample holders. Our results demonstrate the crucial role played by the O2 physisorption in the extended surface of the LSCO grains, an effect that may be deeply mitigated by encapsulating the grains into an epoxy resin (EPO-TEK 301). This procedure may be also useful for high-precision magnetization measurements at low temperatures in other granular or porous materials.

Effect of sintering temperature on the nature of weak links and flux pinning mechanism in La1.85Sr0.15CuO4 superconductor

AC and DC susceptibility study is carried out to investigate the granular nature of La1.85Sr0.15CuO4 superconductor. Presence of weak links and flux pinning phenomena are two important implications of granular type II high temperature superconductors. Weak links manifested as Superconductor-Insulator-Superconductor (SIS) / Superconductor-Normal metal-superconductor (SNS) Josephson Junctions are investigated from the temperature and field dependence study of AC susceptibility. On the other hand, DC susceptibility measurement is used to study the flux pinning mechanism. Mechanism of flux pinning is dependent on the nature and size of the pinning centres as well as on the microstructure wavelength. Thus, the nature of grain boundaries plays an important role in determining the nature of pinning mechanism of flux lines. In the present work, effect of sintering temperature on the nature of weak links and flux pinning mechanism in the bulk polycrystalline sample of La1.85Sr0.15CuO4 superconductor is studied.

The Correlation between the Energy Gap and the Pseudogap Temperature in Cuprates: The YCBCZO and LSHCO Case

The paper analyzes the influence of the hole density, the out-of-plane or in-plane disorder, and the isotopic oxygen mass on the zero temperature energy gap (2Δ(0))Y1-xCaxBa2Cu1-yZny3O7-δ(YCBCZO) andLa1.96-xSrxHo0.04CuO4(LSHCO) superconductors. It has been found that the energy gap is visibly correlated with the value of the pseudogap temperature (T⋆). On the other hand, no correlation between 2Δ(0) and the critical temperature (TC) has been found. The above results mean that the value of the dimensionless ratio2Δ0/kBTCcan vary very strongly together with the chemical composition, while the parameter2Δ(0)/kBT⋆does not change significantly. In the paper, the analytical formula which binds the zero temperature energy gap and the pseudogap temperature has been also presented.

Upper critical field in electron-Doped Nd1.86Ce0.14CuO4 superconductor

Results from studying the temperature dependence of resistivity of single-crystal films of Nd1.86Ce0.14CuO4 (underdoped region) in magnetic fields of up to 90 kOe in the temperature range of 1.8–40 K are presented. The temperature dependence of the upper critical field obtained via the resistive method is analyzed using the impurity superconductor model with two types of carriers.

New Results on the Anomalous Precursor Diamagnetism in the Underdoped La1.9Sr0.1CuO4 Superconductor

The new results summarized here, including a brief comparison with the paraconductivity, further suggest that the anomalous precursor (above Tc) diamagnetism recently observed in the underdoped La1.9Sr0.1CuO4 superconductor could be attributed to the presence, in addition to the conventional superconducting pair fluctuations, of Tc-inhomogeneities with long characteristic lengths associated with chemical disorder.

Oxygen Isotope Effect in Cuprates Results from Polaron-induced Superconductivity

The planar oxygen isotope effect coefficient measured as a function of hole doping in the Pr- and La-doped YBa2Cu3O7 (YBCO) and the Ni-doped La1.85Sr0.15CuO4 (LSCO) superconductors quantitatively and qualitatively follows the form originally proposed by Kresin and Wolf [Phys. Rev. B 49, 3652 (1994)], which was derived for polarons perpendicular to the superconducting planes. Interestingly, the inverse oxygen isotope effect coefficient at the pseudogap temperature also obeys the same formula. These findings allow the conclusion that the superconductivity in YBCO and LSCO results from polarons or rather bipolarons in the CuO2 plane. The original formula, proposed for the perpendicular direction only, is obviously more generally valid and accounts for the superconductivity in the CuO2 planes.

Influence of Grain Size on the Superconductivity of La1.85Sr0.15CuO4

We study the influence of varying grain size on superconductivity of bulk La1.85Sr0.15CuO4 superconductor. The samples are synthesized by a sol-gel method. The grain size is varied by sintering the samples at various temperatures between 700 °C to 1050 °C. The samples are characterized by X-Ray Diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), resistivity and magnetization measurements. The electrical resistivity measurements revealed considerable lowering of the superconducting transition temperature (\(T_{\mathrm{c}}^{R=0}\)) and broadening of the transition width (ΔTc) with decreasing grain size though the onset of transition temperature (\(T_{\mathrm{c}}^{\mathrm{onset}}\)) changes only marginally. The magnetic measurements carried out are consistent with each other and scale well with the grain size. Critical current density has been calculated from the magnetization hysteresis, assuming that supercurrents flow throughout the sample as a whole and within the individual grains as well. The observed results have been discussed on the basis of inter- and intra-granular boundary characteristics of high-temperature superconductors (HTSc). It is found that lowering of grain size deteriorates the superconducting properties in general.

In-plane anisotropic vortex pinning and relaxation in a stripe-ordered La1.45Nd0.4Sr0.15CuO4 superconductor

The angular dependence of the ac susceptibility for the magnetic field rotated in the CuO2 plane of a stripe-ordered La1.45Nd0.4Sr0.15CuO4 single crystal shows an unusual fourfold symmetry below the superconducting transition temperature (Tc). The real part of the ac susceptibility reaches its maximum for the magnetic field parallel to the stripes, and the in-plane anisotropy of the susceptibility is enhanced with increasing temperature, which indicates that the anisotropy can be ascribed to the vortex pinning of the fourfold symmetric static stripe phase. The in-plane dc magnetic relaxation rate also shows the fourfold symmetry and proves the existence of the anisotropic vortex pinning effect.

Magnetic cobalt-ion substitution effect in electron-doped La1.89Ce0.11CuO4 superconductor

We investigate the in-plane transport and magnetic properties of La1.89Ce0.11(Cu1−xCox)O4 thin films with x=0.01–0.05. With increasing Co concentration, Tc is obviously suppressed and a crossover from superconductor to insulator at low temperature is observed, and the negative Hall coefficient is enhanced. All the Co-doped samples exhibit long-range ferromagnetic ordering even at the room temperature by magnetic measurements.

Temperature dependent local Cu-O displacements from underdoped to overdoped La-Sr-Cu-O superconductor

We have studied doping evolution of the temperature dependent local Cu-O displacements in the La2-x Sr x CuO4 superconductor by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. While temperature dependent Debye-Waller factor of the Cu-O bonds, measuring the local Cu-O displacements, shows an anomalous increase at low temperature for the underdoped single crystals, we do not find such a dependence for the case of the overdoped system. The results, which are discussed in the light of recent angle resolved photoemission measurements, provide an evidence for some important correlation between the doping dependent electron-lattice interaction, the charge inhomogeneities and the local Cu-O displacements in the copper oxide superconductors.

Spatial distribution of Bose condensate in high-temperature superconductors,determined by emission Mössbauer spectroscopy

It has been shown that temperature dependences of the gravity centres of67Cu(67Zn) and67Ga(67Zn) emission Mössbauer spectra of Nd1.85Ce0.15CuO4,La1.85Sr0.15CuO4 and Tl2Ba2Ca2CuO8 superconductors are described by thesecond-order Doppler shift in the temperature range T ≥Tc, andBose condensation of Cooper pairs should be taken into account at T ≤Tc (Tc isthe superconducting transition temperature). The spatial nonuniformity of theelectron density created by the Bose condensate has been observed inLa1.85Sr0.15CuO4.

Field and temperature dependences of the specific heat of the La1.85Sr0.15CuO4 superconductor

This paper reports on the temperature and field dependences of the specific heat of high-quality La1.85Sr0.15CuO4 single crystals carried out at low temperatures in magnetic fields of up to 8 T for two magnetic field orientations, namely, along the [100] and [110] crystallographic axes. The field dependence of the electronic density of states (DOS) was found to be anisotropic for different magnetic field orientations in the a–b plane, with the electronic density being the lowest along the a axis (for H ∥ [100]) and maximum for the field inclined at 45° to the a axis (for H ∥ [110]). Electronic specific heat in a magnetic field was observed to depend linearly on temperature T and nonlinearly on the magnetic field H: CDOS=bTH1/2. In a zero field, the electronic specific heat grows quadratically with temperature as CDOS=αT2. Estimation of the maximum superconducting gap width from the experimentally determined values of the α coefficient of T2 and of the electronic DOS in the normal state yields Δ 0=300 K. The observed features indicate that La1.85Sr0.15CuO4 is a superconductor with d symmetry of the order parameter.

Effect of doping concentration on normal state resistivity of La2 −x (Ba, Sr) x CuO4 superconductors

Based on free electron layered electron gas model of quasi two dimensional CuO2 layers in La(Ba/Sr)CuO superconductors a model potentialV(q) is developed earlier with the electron-electron and electron-phonon interactions. The model approach facilitates the dielectric functions and the dispersion relations of 2D acoustic phonon and plasmon modes. We have then worked out the coupling strength (γ) linking electrons to the 2D acoustic phonon mode (ħω_) from the residue at the pole ofV(q). Furthermore, the scattering time (τ e−ph) during electron-phonon interaction (EPI) for this simplified system is also estimated. The contribution to the normal state in plane resistivity due to EPI is then evaluated. Finally, the variations ofτ andρ is studied with the doping concentration (x) and temperature (T) and the results obtained by us show reasonably good agreement with the available experimental data.

Structural and morphological evolution from a freeze-dried precursor to the La1.85Sr0.15 CuO4 superconductor during thermal processing

The crystalline phases and morphology of solids with a composition of La1.85Sr0.15CuO4 were studied in terms of thermal processing conditions of freeze-dried acetates. Samples were characterized by X-ray diffraction, scanning electron microscopy and iodometric titration techniques. Results indicate that a full development of the superconducting phase only occurs after pressing sintering and annealing under oxygen of the powders treated at T f=1253 K for 8 h. The structural evolution observed in this case involves a solid-state reaction between La1.67Sr0.33Cu2O5−δ, La(OH)3 and CuO. The existence of transient liquid phase at temperatures around 600 K generates local composition fluctations (lanthanum deficiency) where the non-superconducting ternary compound La1.67Sr0.33Cu2O5−δ develops. The selection of high heating rates (50 K min−1) below 773 K minimizes the deleterious effect of transient melting and, at the same time, ensures a high reproducibility in the morphology of the final material obtained.

Anomalous behavior of the Nd1.84Ce0.16CuO4 superconductor

The as-prepared electron-doped system Nd2−xCexCuO4 (where x≈0.16) is not superconducting. It becomes superconducting only after removal of a minuscule amount of extraneous oxygen (0.02 of O per unit formula). Mossbauer effect studies were carried out for oxygenated and deoxygenated Nd2−xCexCu(57Co)O4 with x=0.14, 0.16, and 0.18. The spectra show evidence of oxygen anions attaching to the probe57Co in apical positions, to form 5- and 6-coordinated species. A conventional procedure for deoxygenation brings about little change in the Mossbauer spectra both above and below the optimal superconducting concentration; however, for x=0.16, a dramatic change is observed—a major fraction of the magnetically split five-coordinated species manifests itself as a paramagnetically relaxed doublet upon deoxygenation, which costitutes a microscopic measure of the superconducting volume fraction. This apparently anomalous behavior at x≈0.16, where the extraneous oxygen is more readily desorbed, may be related to an electronic and/or local structural change in the CuO2 plane.

Study of doping effects on transition temperature of La2 −x (Ca, Na, K) x CuO4 superconductors

We have investigated doping effects on the transition temperature (T c ) of La2 −x M x CuO4 (M=Ca, Na and K) by incorporating the effects of two-dimensional (2D) acoustic plasmons in the framework of the strong coupling theory. The contributions from 2D acoustic plasmon mechanism toT c have been obtained from a Fourier-transformed effective potential, which has been earlier found to be successful in predicting the composition dependence ofT c in La2 −x (Ba, Sr) x CuO4. The results obtained by us on the variation of transition temperature with composition (x) in La2−x M x CuO4 superconductors are in reasonably good agreement with the available experimental data. This success has led to the conclusion that 2D acoustic plasmons are adequate to explain the pairing mechanism and the variation ofT c with composition (x) in cuprate superconductors.

Point contact spectroscopy in oriented La2?x Sr x CuO4 superconductors; energy gap and Fermi velocity

Point contact measurements in oriented La2−xSrxCuO4 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, Δab = 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;VF≥6×107 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 La2−xSrxCuO4. A comparison with previous results obtained on YBa2Cu2O7−δ is made.

Electric field gradient at copper sites and distribution of the conductivity electrons in the Nd1.85Ce0.15CuO4 superconductor

Abstract The parameters of the electric field gradient (EFG) tensor at the copper site of the Nd1.85Ce0.15CuO4 lattice were determined by means of the 67Cu (67Zn) emission Mossbauer spectroscopy. The data obtained suggest that the substitution of Ce4+ for Nd3+ ions involves the formation of charge compensating electrons distributed among the copper sites.