Enhancement of pseudogap anomalies induced by nanoscale structural inhomogeneity in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> 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 T _c have been measured for a series of fine-crystalline YBa₂Cu₃O _y samples having nearly optimum values of y = 6.93 ± 0.3 and T _c = (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 H _c 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 H _c(T) and the jump of the specific heat ΔC/T _c. 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-T _c superconductors.     

The influence of interplane oxygen redistribution on the magnetization of fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The magnetization of the fine-grained high-temperature superconductor (HTSC) YBa₂Cu₃O _y is experimentally investigated at T < T _c . A distinctive feature of this material is the increased oxygen content in CuO_δ 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.     

Disordering effects in the atomic structure of fine-crystalline HTSC YBa2Cu3O y

The atomic structure of YBa₂Cu₃O _y fine-crystalline HTSC samples with various average crystallite sizes ??D?? ranging from 0.4 to 2 ??m and an oxygen concentration y close to the optimal value for superconductivity (y ?? 6.93) is investigated by the neutron diffraction technique. We have found some effects associated with the redistribution of cations and oxygen atoms and with variations in the positions of atomic layers in the unit cell, which are not observed in macrocrystalline samples. In all probability, these effects appear due to nonequilibrium conditions of synthesis required for obtaining this compound in the fine-crystalline state. The results have made it possible to explain the peculiar physical properties of fine-crystalline YBa₂Cu₃O _y samples (in particular, the coexistence of high superconducting transition temperatures T _c and noticeably lower values of magnetization in strong magnetic fields for T < T _c ). It is shown that a nanoscale structural inhomogeneity exists in fine-crystalline YBa₂Cu₃O _y samples with the optimal oxygen content and changes the fundamental superconducting parameters, viz., the magnetic field penetration depth and the coherence length.     

Positron annihilation studies in high-Tc superconductors RBa2Cu3Oy,R: La,Nd, Sm,Eu, Gd,Dy, Ho,Y, Er

Positron lifetime and Doppler broadening of the annihilation line measurements have been performed at room temperature in high-T_c superconductors RBa₂Cu₃O_y, where R: La, Nd, Sm, Eu, Gd, Dy, Ho, Y, Er and 6.9 < y ≤ 7. Doppler broadening of the annihilation line measurements have also been performed in high-T_c superconducting samples R RBa₂Cu₃O_y, where R: Nd, Sm, Eu, Ho, Y, Er as a function of temperature between 14 K and 293 K. It was observed that the positron lifetime and the S parameter values at room temperature have no obvious trend in their variation from the yttrium substitution by a rare-earth element. It was also observed that the temperature dependence of the positron annihilation parameters is similar in the high-T_c superconducting samples.     

Peculiarities in the low-temperature specific heat related to nanoscale structural inhomogeneity in fine-crystalline YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> superconductors

A comparative study of the low-temperature specific heat for two types of YBa₂Cu₃O _y high-T _c 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.     

A new superconducting oxide GdBaCuO with Tc (ρ=0) above 92K

A new superconducting oxide Gd_xBa_1−xCuO_3−y with T_c (ρ=0) above 92K has been successfully synthesized by the ceramic technology. The samples with composition of x=0.5, 0.625 and 0.9 are all observed with high T_c superconductivity. The highest T_c for samples measured is observed in Gd_0.5Ba_0.5CuO_3−y which has a resistive midpoint of 95K with a width of 1.5K, and a zero-resistance temperature of 92.5K. The results of a.c. magnetic susceptibility measurement are also reported.     

Resistivity and magnetic susceptibility of superconducting and non-superconducting ErBa2Cu3O7−y

The oxide ErBa₂Cu₃O_7−y has been obtained in the orthorhombic and the tetragonal forms by suitable heat treatments. The orthorhombic form is metallic and superconducting with T_c=88 K while the tetragonal form has high resistance and is non-superconducting. Magnetic studies on both types of compounds reveal that the rare earth ion moment is very close to its free ion value independent of the structure. In the tetragonal compound, deviations in the susceptibility from the Curie-Weiss behavior are noted at low temperatures which may be due to the effect of crystalline electric fields.     

Dependence of the magnetic and superconducting characteristics of fine-grained HTSCs YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> on the degree of structural disorder and type of oxygen vacancy ordering in chain planes

The characteristics determining different contributions to the magnetic susceptibility at T > T _C (Pauli susceptibility, coherence length at T = 0, and Curie constant) as functions of the degree of structural disorder have been analyzed for high-temperature superconducting YBa₂Cu₃O _y samples ( y ≈ 6.92, T _C ≈ 92 K) with micrometer and submicron average grain sizes D _av. It is shown that the decrease in these characteristics, which is observed in fine-grained samples with a decrease in D _av, occurs in various ways, depending on the number and type of oxygen vacancy ordering in chain planes.     

Heat transport in the insulating phases of Sm1+x Ba2-x Cu3Oy high temperature superconductors

Thermal conductivity (κ) of two series of high-temperature superconductors with general formula Sm_1+x Ba_2-x Cu₃O_y have been measured. Both series begin with the same sample of T _c = 90.4 K and extend to nonsuperconducting phases. The first series is of 123 cation stoichiometry and variable oxygen content y, the second is a series of solid solutions with variable x. The temperature dependences of κ are very similar for superconducting partner samples from both series (i.e. with the same T _c), whereas the nonsuperconducting samples reveal dramatic differences. We propose to attribute the huge increase of κ and the change of its temperature characteristics of insulating oxygen deficient sample to some additional heat carriers, supposedly of magnetic origin. Absence of this additional heat transport mechanism for insulating solid solution allows to treat it as a proper reference for estimation of phonon contribution in the superconducting 123 compounds.     

Magnetic susceptibility of fine crystalline high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6.93</Subscript> superconductors. The role of nanoscale structural disordering

Static magnetic susceptibility χ(T) in the normal state (T_c ≤ T ≤ 400 K) and specific heat C(T) near temperature T_c of the transition to the superconducting state are experimentally studied for a series of fine crystalline samples of high-temperature YBa₂Cu₃O_y superconductor, having y and T_c 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.     

Metastable phases in YBCO films created by short-term annealings

The effect of short-term low-temperature annealing in air and in vacuum on the properties of HTSC films of YBCO is studied. It is shown that, under certain conditions of preparation of initial samples, a transition from the HTSC phase with the superconducting transition temperature T_c=90 K to a phase with T_c=60 K occurs without a noticeable change in the oxygen content. It is found that, as a result of short-term annealings, a transition from the HTSC phase with T_c=60 K to the phase with T_c=90 K can occur only through the vacuum annealing stage, which converts the sample into the superconducting state. Short-term annealings lead to multiple reversible “switching” of the films from one phase to another. The obtained results are of practical interest, since the proposed method can be used to quickly obtain superconducting YBCO films in various phase states. It is shown, in addition, that the annealing procedure makes it possible not only to increase the oxygen concentration but also to produce a structural rearrangement of a YBCO film.     

The impact of Y substitution on the 110 K high Tc phase in a Bi (Pb):2223 superconductor

The structural and superconducting properties of Bi_1.7Pb_0.3Sr₂Ca_2−xY_xCu₃O_y superconducting samples are investigated by X-ray diffraction (XRD), resistivity and thermoelectric power (TEP) measurements. XRD results reveal that the volume percentage of the 2223 high T_c phase decreases with an increase in Y content. The replacement of the Ca²⁺ ion by the Y³⁺ ion does not influence the tetragonal structure of the pure Bi (Pb): 2223 system and the lattice parameters vary with Y content. The results of resistivity indicate that the critical temperatures T_c of the samples decrease monotonically with an increase in Y content. Further, the critical concentration of Y to completely suppress superconductivity in the Y-doped Bi (Pb):2223 system is higher (0.60) than that reported (0.20) for the other rare-earth elements. On the other hand, the values of TEP at room temperature are found to be negative for Y=0.00 and 0.10 samples, and it changed to positive with further increase in Y content. The hole-carrier concentration per Cu ion (P) is deduced by using two different ways: the first in terms of T_c values in the superconducting state and the other in terms of TEP values in the normal state. Interestingly, it is found that the values of P deduced from the formal way are not consistent with the reported parabolic behavior for superconducting systems in the under-doped region, and consequently disagree with the general roles of substitution. However, the vice versa is recorded for the values of P deduced from the latter way. The results are discussed in terms of the possible reasons for the suppression of superconductivity in the considered system.     

Effects of Zn on the grain boundary properties of La2−xSrxCu1−yZnyO4 superconductors

The properties of the grain boundaries (GBs) are of significant importance in high-T_c cuprates. Most large scale applications of cuprate superconductors involve usage of sintered compounds. The critical current density and the ability to trap high magnetic flux inside the sample depend largely on the quality of the GBs. Zn has the ability to pin vortices but it also degrades superconductivity. In this study we have investigated the effect of Zn impurity on the intergrain coupling properties in high-quality La_2−xSr_xCu_1−yZn_yO₄ sintered samples with different hole concentrations, p (≡x), over a wide range of Zn contents (y) using field-dependent AC susceptibility (ACS) measurements. The ACS results enabled us to determine the superconducting transition temperature T_c, and the temperature T_gcp, at which the randomly oriented superconducting grains become coupled as a function of hole and disorder contents. We have analyzed the behavior of the GBs from the systematic evolution of the values of T_gcp(p, y), T_c(p, y), and from the contribution to the field-dependent ACS signal coming from the intergrain shielding current. Zn suppresses both T_c and T_gcp in a similar fashion. The hole content and the carrier localization due to Zn substitution seem to have significant effect on the coupling properties of the GBs. We have discussed the possible implications of these findings in detail in this article.     

掺Ba对Bi系2212相超导电性的影响

研究了掺Ｂａ对Bi₂Sr_2-xBa_xCaCu₂O_y（０≤ｘ≤０．１５，０．３）单晶和多晶样品超导电性的影响，结果表明，有少量Ｂａ^２＋离子进入了超导相，且有固溶度极限．对于２２１２相单晶，ｃ轴参数和Ｔ_c均随Ｂａ含量增加而增加；对于慢冷多晶样品，掺Ｂａ可明显提高Ｔ_c；然而对于淬大多晶样品，Ｔ_c没有明显变化，用掺Ｂａ 关键词：     

Superconducting properties of Ca-doped MoSr2YCu2O8−δ

We synthesized samples with nominal compositions MoSr₂YCu₂O_8?δ and MoSr₂Y_0.8Ca_0.2Cu₂O_y and investigated their superconducting properties. It was established that the Ca-doping increases the T_c of samples, prepared at different conditions. It was found that this doping does not destroy the antiferromagnetic state of the undoped Mo-1212 but significantly decreases the remnant magnetization. The effect of the inhomogeneous Ca-distribution on the superconducting properties of the doped samples was discussed.     

Superconductivity of Bi2 (Sr0.9Ca0.1)2 CuO y single crystals

Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y single crystals were grown by the self-flux method. The temperature dependence of resistance and ac susceptibility measurements showed that the as-grown crystal is semiconductive between 4.2 K and 300 K. However, after annealing in air for more than 20 days the crystal became superconducting with a T _c of 66 K obtained from the R-T curve. The ac susceptibility data also showed a strong superconducting transition at 71 K. X-ray diffraction indicated that the structure of the as-grown and the annealed crystal both are orthorhombic. The T _c of 71 K of the Bi₂ (Sr_0.9Ca_0.1)₂ CuO _y crystal may be due to the increase of oxygen content in the crystal after the long-time annealing in air. It is suggested that the superconductivity of the Ca-doped 2201 phase of the Bi-based system, which up to now was found to have a T _c=7–22 K, is similar to the 2201 phase of the Tl-based system.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

YBa2Cu1-xCox(Cu1-yZny)2Oz体系的Co,Zn共同掺杂效应

本文报道通过对YBa₂Cu_1-xCo_x(Cu_1-yZn_y)₂O_z(0≤x,y≤0.1)体系晶体结构、氧含量、正常态电阻-温度关系、Hall效应以及超导临界温度等的综合测量,发现随着Co和Zn含量的增加,体系经历了从正交结构的超导金属向四方结构的非超导半导体的转变,超导临界温度T_c和载流子浓度n_h均迅速下降,Co 关键词：     

Increase in Tc and spin glass transition in La0.9Ca0.1Mn1−yCryO3 (y=0, 0.1 and 0.2)

We report on the structural, frequency dependent ac susceptibility, dc magnetization and magnetoresistance (MR) measurements on polycrystalline samples of La_0.9Ca_0.1Mn_1−yCr_yO₃ (y=0, 0.1 and 0.2) prepared by sol-gel technique. For y=0, a paramagnetic to ferromagnetic transition is observed at T_c=136 K. Both for y=0.1 and 0.2, T_c increases from 136 to 180 K. For y=0, the imaginary part of the ac susceptibility shows a broad transition at T_f<T_c which does not depend very much on the frequency. However, for y=0.1 and 0.2, the frequency dependence resembles that of a spin glass. Though all the three samples show a semi-conducting behavior between 300 and 5 K, a negative MR is observed corresponding to T_c and T_f. The value of MR decreases for the Cr substituted samples.     

Characteristic fields of a Bi2Sr2CaCu2Oy crystal

The field dependences of the transverse resistance of a Bi₂Sr₂CaCu₂O_y (BSCCO) layered superconducting single crystal with T _c0≥92 K are studied in a perpendicular (H⊥(ab)) pulsed magnetic field up to 50 T in a wide temperature range, 4.2–300 K. The temperature dependences of the characteristic fields identified with the “irreversibility curve” and the field corresponding to the nucleation of the superconducting phase are determined. The results obtained for the latter field are compared with the theoretical dependences.