Silver addition in La2CuO4 ceramics

The effects of Ag addition on the microstructural and electrical properties of sintered La₂CuO₄ have been studied. Unexpectedly, Ag-doped sample shows strong limitation of grain growth. The resistivity behavior of the variable range hopping was observed in the low temperature range (13–41 K) and the ordinary thermal activation process was observed in the high temperature range (41–90 K). A model was proposed to deduce the accurate transition temperature and transition range. The experimental results indicate that the addition of Ag not only increases the hole density but also creates sufficient disorder in the sample.     

ESR Studies of Doped La2CuO4

ESR of Gd spin probes in La _2–x–y Eu _y Sr _x CuO ₄ has been employed to study the Cu spin dynamics and its relation to stripe order of spins and charges in the low temperature tetragonal phase of this compound. In the magnetically ordered hole doped samples (0 < x 0.02) a strong enhancement of magnetic fluctuations on the ESR frequency scale is observed in this structural phase. At larger hole dopings (0.08 x 0.17) a drastic decrease of the Cu spin fluctuation frequency, competing with bulk superconductivity, is apparent. For the elevated hole concentrations the obtained results are consistent with the stripe model, whereas in the low hole doping regime the data give no evidence for the stripe phase formation.     

Thermal conductivity of La2CuO4, La2NiO4, and Nd2CuO4 in the semiconducting and metallic phases

Thermal conductivity of polycrystalline La₂CuO₄, La₂NiO₄, and Nd₂CuO₄ was measured in the temperature range 300–1000 K. No anomaly in thermal conductivity has been observed during the semiconductor-to-metal transition of La₂NiO₄ or in the metallic phases of La₂CuO₄ and La₂NiO₄. A change of slope has been found, however, in the thermal conductivity of La₂CuO₄ at the crystallographic transition. The thermal conductivity of the oxides is mainly phononic in this temperature range.     

Charge Stripes and Superconductivity in La2−x Ba x CuO4 and La1.6−x Nd0.4Sr x CuO4

We report x-ray scattering study of charge density wave (stripe) order in La_2−x Ba _x CuO₄ (LBCO) and La_1.6−x Nd_0.4Sr _x CuO₄ (LNSCO). The doping dependence of the charge stripe ordering temperature and the low-temperature correlation length is compared with the bulk superconducting transition temperature. We find that the charge stripe ordering temperature and correlation length are quite different in these two families of cuprates, while there seems to be inverse correlation between the bulk superconducting temperature and the charge stripe correlation length.     

Competing Striped Structures in La2CuO4+y

High temperature superconductivity emerges in unique materials, like cuprates, that belong to the class of heterostructures at atomic limit, made of a superlattice of superconducting atomic layers intercalated by spacer layers. The physical properties of a strongly correlated electronic system, emerge from the competition between different phases with a resulting inhomogeneity from nanoscale to micron scale. Here, we focus on the spatial arrangements of two types of structural defects in the cuprate La₂CuO_4+y : (i) the local lattice distortions in the CuO₂ active layers and (ii) the lattice distortions around the charged chemical dopants in the spacer layers. We use a new advanced microscopy method: scanning nano X-ray diffraction (nXRD). We show here that local lattice distortions form incommensurate nanoscale ripples spatially anticorrelated with puddles of self-organized chemical dopants in the spacer layers.     

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.     

Phonon dispersion relations and densities of states in La2CuO4 and La2NiO4

Rigid-ion model calculations of phonon dispersion relations, densities of states and partial densities of states of the highT _c superconductor La₂CuO₄ and its isostructural compound La₂NiO₄ have been carried out both in the tetragonal and orthorhombic phases of La₂CuO₄ and the tetragonal phase of La₂NiO₄. The calculations are in fair agreement with reported experiments. The computed phonon dispersion in the tetragonal phase of La₂CuO₄ reproduces the soft mode behaviour for the lowest Σ₄ TO branch which is found to harden in the orthorhombic phase, consistent with experimental data.     

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.     

Preparation and characterization of alkaline-earth substituted superconducting La2CuO4

Metallic-oxygen deficient compounds in ternary systems MOz.sbnd;La₂O₃CuO with M = Ca, Sr, Ba have been prepared by coprecipitation and subsequent low-temperature solid-state reaction, as well as by high-temperature reaction from the corresponding oxides and carbonates. Substitution of trivalent La by divalent alka-line-earth ions results in sharp drops in resistivity in the 30 – 40 K range, first discovered for the Ba-La-Cu-O system. Room-temperature X-ray powder diffraction shows the presence of a perovskite-like layer-type phase of K₂NiF₄-type to be responsible for the high T_c super-conductivity. For single-phase samples of La₂CuO₄:M composition, the resistivity drop and especially the onset of the diamagnetic susceptibility are enhanced, and occur near the composition where an orthorhombic-to-tetragonal structural phase transition is detected by X-ray diffraction.     

Anomalous structure and phonon dynamics in La1.85Sr0.15CuO4

We performed X-ray diffraction and neutron scattering experiments on high quality single crystals of La_1.85Sr_0.15CuO₄. We found a forbidden (1,0,0) peak, which develops below the HTT-LTO structural transition temperature, and an anomalous behavior of acoustic phonon at (2,2,0). These results indicate that the structural anomaly has a strong relation with an superconductivity.     

First Principles Investigation of Local Distortions in Doped La2CuO4

We present the results of first-principles cluster calculations of local distortions in doped La₂CuO₄. Local deformations of the CuO₆ octahedra associated with the presence of an additional extrinsic hole were determined using the density-functional (DF) method on various cluster models. The results indicate that a localized hole induces a contraction of the CuO(apical) distance from 2.40 to 2.28 Å. The contracted distance is in very good agreement with the anomalous, short CuO(apical) distance of 2.3 Å, observed recently by polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) in La_1.85Sr_0.15CuO₄ and associated with the hole-rich stripe region.     

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−x Ba x CuO4 and La2−x Sr x CuO4

Muon spin rotation (μ ⁺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.     

Phase Separation in La2CuO4+δ by electrical transport measurements

The dc conductivity and the high-frequency capacitance of a La₂CuO_4+δ single crystal are determined by impedance spectroscopy in the frequency range 20 Hz to 1 MHz at liquid helium temperature. Using isochronal annealing, the effect of the phase separation on these quantitites is studied for E parallel and perpendicular to the CuO₂ planes as well as for different oxygen doping levels,δ, of the sample. The results are consistent with the model of a diffusion-controlled formation of a metallic—and belowT _c superconducting—percolation network in an insulating background.     

Stripe correlation of tilts and superconductivity in La2CuO4+Σ

In single crystal La₂CuO_4+Σ,Σ ≈ 0.015, c-axis interstitial oxygen ordering (staging) in the oxygen-rich phase of the phase-separated sample was observed through satellite peaks in the neutron scattering which first appear at 250 K. If the sample is cooled through the temperature region around 210 K over a period of several hours, these satellite peaks show splitting in the a*-direction. This first-order phase transition involves a periodic one-dimensional modulation of the in-plane order. Microscopic modeling leads to a picture where the excess oxygen, and therefore also the doped holes, order in stripes along the a-direction in addition to staging along the c-direction. This ordering in stripes can be regarded as a continuation of the phase separation but now on a local scale, leading to a local increase in oxygen and hole density. This increase can account for the increase of the superconductingT _c of 2.5 K observed in our sample and also for many previously reported annealing experiments.     

Ultrafast Spectroscopy of La2−x Sr x CuO4 Single Crystals

We present measurements of the ultrafast optical response of La_2–x Sr _x CuO₄ single crystals at two different doping levels (x=0.15, 0.07) as a function of temperature. Owing to the 40-fs time resolution of our experiment, we are able to measure both the build-up and the decay of the ultrafast signal. We show for the first time that the temperature dependence of the transient reflectance amplitude tracks the thermal depletion of the superfluid density. At low temperature we identify a new regime characterized by an extremely long, temperature-dependent rise-time (3 ps). The doping dependence of the relaxation rate suggests that this is sensitive to the scattering of quasiparticles by low-energy spin excitations.     

Complex Phase Separation in Oxygen-Doped Cuprates La2CuO4+ y Superconductors

The multiple-phase separation with coexistence of two different superconducting phases in the phase diagram of oxygen-doped La₂CuO_{4+ y} cuprates is interpreted with a modified Van der Waals scheme. The model includes the tendency of intercalated oxygen ions to form clusters at preferential densities. We obtain the coexistence and competition of two different superconducting phases with different T _c and interstitial oxygen content y, in agreement with experiments.     

Activated behavior of the phase separation in La2CuO4+δ

Investigation of the recovery of superconductivity by controlled annealing and equlibration experiments in La₂CuO_4+δ samples quenched from room temperature to temperatures below the phase-separation temperature provides a tool to investigate time constants and activation energies governing phase separation in La₂CuO_4+δ . A series of different annealing experiments on ceramic samples of La₂CuO_4+δ (0.01≤δ≤0.04) is analyzed quantitatively and activation energies determining the phase formation are extracted. We identify two separate activated processes between 150 and 250 K with activation energies of 0.36 (2) and 0.46 (3) eV which we attribute to activated interstititial oxygen atom or vacancy migration.