Is the tetragonal overdoped phase (La, Sr)2CuO4 superconductive?

Comparative studies of Meissner and shielding processes have been performed on variously sized samples of the solid solution (La_1–x Sr _x )₂CuO₄: (polycrystalline, powder, and single-crystalline) under various magnetic fields in the range 0.03–10 Oe. Considering the kinetics of the two processes, this systematic approach has indicated that the observed Meissner signal can be significantly low with the sample still being wholly superconductive. Based on this comparative study, the tetragonal region of (La_1–x Sr _x )₂CuO₄ withx greater than 0.105 has been concluded to be superconductive, suggesting the nonessential involvement of the buckling of the Cu-O-Cu-O bond in the CuO₂ layer in the manifestation of superconductivity.     

On the oxygen isotope shift in La2?xSrxCuO4

The anomalous oxygen isotope shift observed in the high-temperature superconductors La_2–xSr_xCuO₄ and La_2–xBa_xCuO₄ is discussed within the model of superconductivity in which the apex oxygen vibrations play the major role in the pairing mechanism. We show that the dependence of both the critical temperature and its isotope shifts on the doping level is consistently described within this model. The anomaly occurring in the isotope shift between x = 0.11 andx=0.15 is traced back to the oxygen mass exponent associated with the apex oxygen frequency.     

Magnetism of the LTT phase of Eu-doped La2-x Sr x CuO4

The ESR signal of Gd spin probes (0.5 at.%) as well as the static normal state susceptibility of Eu (J(Eu³⁺) = 0) doped La_2-x-ySr_xEu_yCuO₄ reveal pronounced changes of the Cu magnetism at the structural transition from the orthorhombic to the low-temperature tetragonal phase for all nonsuperconducting compositions. Both a jumplike decrease ofx as well as the ESR data show an increase of the in-plane magnetic correlation length in the LTT phase. From the Gd³⁺ ESR linewidth we find that for specific Eu and Sr concentrations in the LTT phase the correlation length increases up to more than 100 lattice constants and the fluctuation frequency of the CuO₂ spin system slows down to ~10¹⁰-10¹¹ s^-1 However, there is no static order above T~8 K in contrast to the LTT phase of Nd-doped La_{2-x}Sr_xCuO₄ with pinned stripe correlations.     

Influence of Lattice Parameter Scaling on Local Electronic and Magnetic Properties in La2CuO4

A large amount of static and dynamic NMR experiments have been carried out to determine the spin properties in high temperature superconductors cuprates (HTSC). In the majority of cuprates the magnetic shift at the planar copper nucleus is independent of the temperature when the field is applied perpendicular to the CuO₂ planes and does not exhibit the expected reduction of the spin susceptibility in the superconducting state. This has been explained by a coincidental cancellation of on-site and transferred hyperfine fields at the copper nucleus, which would lead to a vanishing spin shift. To examine this explanation we determine the hyperfine fields by ab initio cluster calculations for La₂CuO₄ within the framework of spin-polarized density functional theory. In particular we investigate the sensitivity of the hyperfine fields on the lattice constants.     

Characteristic features of low-temperature structural phase transitions in La x-2Sr x CuO4

Features of low-temperature structural phase transitions in La_{2-x}Sr_xCuO₄ with x = 0.115 and 0.15 have been examined by transmission electron microscopy. There are 100-type forbidden spots due to thePccn/LTT phase as well as fundamental spots due to the LTO phase in the electron-diffraction patterns of La_x-2Sr_xCuO₄ with x = 0.115 and 0.15 at 12 K. As for 100-type forbidden dark field images at 12 K, a bright-line contrast due to thePccn/LTT phase exists only along the twin boundary in the LTO phase for x = 0.15. In the case of x = 0.115, a bright-spot contrast due to thePccn/LTT phase also appears partially inside the LTO domains, in addition to the bright-line contrast along the twin boundary. That is, the low-temperature transition in x = 0.115 is characterized by the appearance of thePccn/LTT phase with the spotty shape inside the LTO domain.     

Structure studies of La1.85Sr0.15CuO4 doped with mg at high doping level

Ceramic cuprates of La_1.85Sr_0.15Cu_1-yMg_yO₄ with 0.00≤y≤0.30 are synthesized. The resistivity dependence of temperature is obtained up to room temperature. The metal-insulator transition at the range of higher doping level (y> 0.10) is observed. There is no impurity phase detected in the entire Mg-doped region. The structure of these Mg-doped samples is characterized by X-ray diffraction studies. The atomic structural parameters are obtained by Rietveld refinements for the Mg-doped samples with y≤0.30. No phase transition is detected within the accuracy of our experiments. The Mg doping reduce the local Jahn-Teller distortion of the CuO₆ octahedra. Some meaningful bond distances are determined according to the refined results. According to the variations of the lattice parameters with y, the whole doping range could be divided into two regions: low doping level (LDL) and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO₆ octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and high doping level (HDL). The bond length between the two apical oxygen atoms in the CuO₆ octahedra for the Mg-doped samples increases with increasing content of Mg in the LDL, and decreases in the HDL.     

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.     

Band structure of Bi2Sr2CuO6: Strong effects due to structural modulation

The electronic structure of idealized tetragonal Bi₂Sr₂CuO₆ is compared with that of an orthorhombic 2×2 model that includes atomic displacements due to the structural modulation. The electronic structure, particularly that related to Bi and O(3) atoms, is strongly modified near the Fermi energy. In particular the shape and topology of the Bi-O(3) related Fermi surfaces near the ¯M point are changed. With the modulation, bands occur near the Fermi energy over a significant part of the zone near this point. A method is proposed to distinguish between lattice distortions and antiferromagnetic correlations as the cause of recently observed shadow Fermi surfaces.     

Theoretical evidence forc-axis phonon-plasmon coupling and phonon renormalization in La2CuO4

A suitable decomposition of the electronic density response and of the electron-phonon interaction into local and nonlocal contributions is extended to the nonadiabatic regime taking dynamical screening into account. It is shown for La₂CuO₄ that the already large (nonlocal) adiabatic electron-phonon coupling is further strongly enhanced for certain phonons along the (0, 0, 1) direction. In this region of wave-vector space we also have a low-lying plasmon mode which mixes strongly with the phonons of appropriate symmetry leading to an anomalous renormalization for certain phonons. The coexistence of phononlike and plasmonlike resonances in the same frequency range is argued to be a specific feature of the layered high-temperature superconductors.     

Microstructure and properties of (Sr, Ca)CuO2/(Sr, Ca)RuO3 multilayers

Epitaxial multilayer thin films of infinite-layer (Sr, Ca)CuO₂ and perovskite (Sr, Ca)RuO₃ have been prepared on (100) SrTiO₃ substrates by multitarget rf magnetron sputtering. X-ray diffraction analyses revealed that the multilayer structure of (Sr, Ca)CuO₃/(Sr, Ca)RuO₃ was successfully fabricated with a minimum layer thickness of 20 Å. Transmission electron microscopy measurements of the multilayers indicated that there was no dislocation which normally exists in single-layer films with an infinite-layer structure. Resistivities of multilayer films at room temperature ranged from 1 to 10 m cm and showed semiconductor-like dependence against the temperature.     

Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La2CuO4.1 Superconductor

Structural properties of La₂CuO_4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transitions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo-induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Vibronic states in La{2-x}BaxCuO4BaxCuO4

The temperature dependences of the resistivity R(T) and the thermoelectric power α(itT) under different hydrostatic pressuresP for the system La_{2-x}BavCuO₄, 0.11 ≤ x < -1.6, are reported and interpreted as evidence for strong electron coupling to oxygen vibrations along the CuO-Cu bond axes of the CuO₂ sheets. The sensitivity of the transport properties to the bending Φ of the (180‡-Φ) Cu-O-Cu bond angle and their insensitivity to long-range magnetic order indicate that the segregation into hole-rich and magnetic stripe domains is driven primarily by electron-lattice interactions and not by electron-electron interactions alone.     

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.     

Synthesis and characterization of novel nanostructured fishbone-like Cu(OH)2 and CuO from Cu4SO4(OH)6

The Cu₄SO₄(OH)₆ was synthesized by a simple hydrothermal reaction with a yield of ~ 90%. Using Cu₄SO₄(OH)₆ as the starting material, novel fishbone-like Cu(OH)₂ was produced by a direct reaction of Cu₄SO₄(OH)₆ with NaOH solution. The Cu(OH)₂ consists of many needle-like nanorods parallel to each other and perpendicular to the direction of backbone, forming fishbone-like structure. Using the fishbone-like Cu(OH)₂ as the sacrificial precursor, CuO with similar size and morphology was obtained through a simple heat treatment. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, X-ray photoelectron spectroscopy, BET nitrogen adsorption, and UV-Vis absorption spectroscopy were employed to characterize the as-prepared samples. The conversion of the Cu₄SO₄(OH)₆ to the fishbone-like Cu(OH)₂ was visualized by time-dependent SEM images. A mechanism was also proposed based on the observed results.     

Salt-assistant combustion synthesis of nanocrystalline Nd2(Zr1 − xSnx)2O7 (0 ≤ x ≤ 1) solid solutions

Nanocrystalline Nd₂(Zr_{1 − x}Sn_x)₂O₇ series solid solutions were prepared by a convenient salt-assisted combustion process using glycine as fuel. The samples were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The results showed the Zr ion can be partially replaced by Sn ion. The partial substituted products were still single-phase solid solutions and the crystal form remained unchanged. TEM images reveal that the products are composed of well-dispersed square-shaped nanocrystals. The method provides a convenient and low-cost route for the synthesis of nanostructures of oxide materials.     

Dielectric and electric studies of the [N(CH3)4][N(C2H5)4]ZnCl4 compound at low temperature

The [N(CH₃)₄][N(C₂H₅)₄]ZnCl₄ compound was prepared and characterized by electrical technique. The temperature dependence of the dielectric permittivity shows that this compound is ferroelectric below T = 268 K. The two semi-circles observed in the complex impedance identify the presence of the grain interior and grain boundary contributions to the electrical response in the material. The equivalent circuit is modeled by a combination series of two parallel R_P–CPE circuits. The frequency dependent conductivity is interpreted in term of Jonscher's law. The modulus plots can be characterized by the empirical Kohlrausch–Williams–Watts (K.W.W.) function: ?(t) = exp [(−t/τ)^β]. The temperature dependence of the alternative current conductivity (σ_p), direct current conductivity (σ_dc) and the relaxation frequency (f_p) confirm the presence of the ferroelectric–paraelectric phase transition.     

Resistive upper critical field of single crystals of Tl2Ba2CuO6

We have measured the resistive upper critical field of overdoped single crystals of Tl₂Ba₂CuO₆ from the zero-field transition temperatureT _c (approximately 20 K) to temperatures as low as 12 mK, corresponding to less than 0.001T _c. In sharp contrast to the predictions of standard theories of superconductivity, the critical field is found to rise steeply with positive curvature as the temperature is reduced, and no sign of saturation is observed down to the lowest temperatures reached.     

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.