L. G. Mamsurova

Peculiarities in the low-temperature specific heat related to nanoscale structural inhomogeneity in fine-crystalline YBa 2 Cu 3 O 6.93 high- T c superconductors

A comparative study of the low-temperature specific heat for two types of YBa2Cu3Oy high-Tc superconductor samples is performed within the temperature range of 2−10 K. The samples of the first type are fine-crystalline optimally doped ones with different degrees of nanoscale structural inhomogeneity. The second type includes coarse-crystalline equilibrium samples with different hole doping levels. A similarity in the behavior of different contributions to the specific heat for structurally inhomogeneous and underdoped samples is revealed. The samples of both types exhibit a metal-like contribution linear in temperature to the specific heat ~γT, which is not characteristic of the superconducting phase. It is found that this contribution moderately grows with the decrease in the oxygen content, whereas with the increase in the structural inhomogeneity, such growth of the linear contribution (γT) becomes anomalously large. This leads to the conclusion about the coexistence of metallic and superconducting states in the bulk of the samples under study. Such common feature of electron systems could be related to the formation of the pseudogap regime. It is demonstrated that this regime suppresses just the superconducting states, leaving intact the metallic ones.

Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa2Cu3O6.93 high-Tc superconductor

The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature Tc have been measured for a series of fine-crystalline YBa2Cu3Oy samples having nearly optimum values of y = 6.93 ± 0.3 and Tc = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field Hc are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in Hc(T) and the jump of the specific heat ΔC/Tc. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-Tc superconductors.

Oxygen defects relaxation in high-temperature YBa2Cu3O7 superconductors

The temperature dependences of normal state static magnet susceptibility χ(T) for high-temperature YBa2Cu3O7 superconductors (Tc ≤ T ≤ 400 K) measured for the same sample before and after 22 years of storage are compared. It is shown that during longime storage of a maximally doped sample, its electronic system is more in equilibrium than it was in the initial state, while a similar underdoped sample decomposed completely. Comparing the χ(T) curves for YBa2Cu3O7, we draw conclusions as to the nature of the Curie contribution and the fluctuation-induced contribution to the magnetic susceptibility of the investigated sample, and the experimental procedure for determining Tc.

The oxygen content and its ordering in the chain planes in fine-grained HTSC YBa2Cu3Oy

A comparative analysis of the results of the X-ray and Mösbauer studies of the high-temperature superconductor (HTSC) YBa2Cu3Oy and YBa2Cu3 − x57FexOy (x = 0.015, Tc ≈ 91.5 K) samples with different average grain sizes in the micron and submicron ranges has been performed. The regularities in the change in the lattice parameter c and in the degree of occupation of different oxygen sites in the CuOδ chain planes taking place at the decrease in have been studied. The quantitative interrelation between the parameter c and the oxygen content δ in the CuOδ planes exceeding the amount of the mobile oxygen due to the interplane oxygen redistribution is established.

Suppression of the superconducting gap near d -wave nodes caused by the structural disorder in fine-crystalline YBa 2 Cu 3 O y high- T c superconductors

The low-temperature (2 K ≤ T ≤ 10 K) specific heat of the series of fine-crystalline samples of YBa2Cu3Oy high-Tc superconductor optimally doped with oxygen and having different degrees of nanoscale structural inhomogeneity has been studied at the applied magnetic field H = 8 T. The result are compared to those obtained for the equilibrium polycrystalline samples with different oxygen contents y. Information on the quasiparticle excitations in the magnetic field near d-wave nodes of the gap function is obtained. The changes introduced by the structural inhomogeneity to the nodal gap slope (νΔ) in the k-space, which is a key parameter of high-Tc superconductors, have also been studied. It is found that νΔ increases with the degree of structural disorder, but the superconducting transition temperature remains nearly the same (Tc = (91.5 ± 0.5) K). It is shown that this is possible if superconductivity is suppressed not only at the nodal point but also near it (owing to the structural disorder). In this case, the density of states in such parts of the Fermi surface is nonzero even at zero temperature, promoting the existence of the linear in temperature contribution to the specific heat (~γ(0)T), which is characteristic of metals.

Paramagnetism of copper–oxygen chains in high-temperature YBa2Cu3O6 + δ superconductors

Experimental evidence for the existence of chain paramagnetic Curie-type contributions to the temperature dependence of static magnetic susceptibility χ(T) in the normal (nonsuperconducting) state is obtained for a series of pure YBa2Cu3O6 + δ high temperature superconductors with different oxygen contents (0.6 < δ ≤ 1). It is shown that the chain contribution is obvious on χ(T) curves only in the T < 150K range of temperatures, grows along with the number of oxygen vacancies in Cu1–O4 chains, and depends on the ordering of these vacancies.

Suppression of hole doping in fine-grained HTSC YBa2Cu3Oy due to structural disordering

It was proven experimentally that the structural disordering inherent to fine-grained high-temper- ature YBa2Cu3Oy superconductors (with an average grain size of 〈D〉 < 2 μm) leads to a reduction of the level of hole doping and the creation of features inherent to the pseudogap state (antiferromagnetic correlations and the lowered density of states at the Fermi level) even in samples with optimum oxygen content y ≈ 6.92.

The influence of interplane oxygen redistribution on the magnetization of fine-grained HTSC YBa2Cu3Oy

The magnetization of the fine-grained high-temperature superconductor (HTSC) YBa2Cu3Oy is experimentally investigated at T < Tc. A distinctive feature of this material is the increased oxygen content in CuOδ planes. The magnetization decrease with an increase in δ is revealed. This correlation indicates that during interplane oxygen redistribution, which is characteristic of fine-grained samples, the oxygen content in the chain planes increases due to its reduction in the superconducting CuO2 planes.

Magnetic susceptibility of fine crystalline high-temperature YBa2Cu3O6.93 superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (Tc ≤ T ≤ 400 K) and specific heat C(T) near temperature Tc of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa2Cu3Oy superconductor, having y and Tc close to optimal but differing in the degree of nanoscale structural disordering. It is shown that under the influence of structural disordering, there is enhancement of anomalous pseudogap behavior of the studied characteristics and a significant increase in the width of the pseudogap.

Manifestation of pseudogap features in structurally inhomogeneous optimally doped HTSC YBa2Cu3O6.92

Magnetization M(H,T) in magnetic fields H up to 90 kOe and at temperatures 2 K ≤ T < Tc (where Tc is the superconducting transition temperature), along with magnetic susceptibility χ(T) in the normal state Tc < T < 400 K for optimally oxygen-doped samples of YBa2Cu3O6.92 with varying degrees of defects in the crystal structure, are studied to determine the influence of structural inhomogeneity on the electron systems characteristics of cuprate superconductors. It is shown that the existence of structural inhomogeneity of samples leads to the manifestation of peculiarities appropriate to pseudogap regime in their properties.