Effects of Carbonate Defects on the Superconductivity of HgBa2CuO4+δ

Carbon contents inHgBa ₂ CuO ₄₊ andstarting materials (HgO, BaO, and CuO) for preparing it weredetermined by infrared spectroscopy to examine influences ofCO ₃ ^2- ions substituted for[HgO ₂ ] ^2– groups on itssuperconducting properties. Measurements of DC magnetic susceptibilitiesand resistivities revealed that superconducting transition in anoxygen-annealed sample with a minimum C content of 0.88 mol% wasvery sharp with T _c 's of 98.0 K (negativemagnetization) and 96.5 K (zero resistance),respectively. They are the highest T _c 's thathave ever been attained inHgBa ₂ CuO ₄₊ including single crystals. Structural data obtained by neutronpowder diffraction showed our sample to have high quality and a value of 0.122(5).     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO_4+δ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7?δ. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7?δ samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7?δ, dominate superconducting properties of bulk HgBa₂CuO_4+δ. With respect to YBa₂Cu₃O_7?δ, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Effect of Ti Doping on Structural and Superconducting Property of YBa2Cu3O7−y High T c Superconductor

Dimensional fluctuations of superconducting order parameters in YBa₂(Cu_1?x Ti _x )₃O_7?y (x=0.01, 0.02, 0.04, 0.05) have been analyzed. SEM micrographs reveal the reduced grain size and the formation of TiO₂ nanowires covering over the grains of YBCO matrix. XRD graphs show the unchanged orthorhombic structure. With the increase of TiO₂%, it is found that the superconducting transition temperatures determined from standard four-probe method decrease gradually. Excess conductivity fluctuation analysis using the Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T _c . 2D to 3D crossover temperature, i.e., the Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Ti incorporation in YBCO matrix. The decrease in the Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of 3D region with increase in Ti%.     

Current Noise in HgBa2CuO4+δ High Tc Superconductor

Experimental results of current noise in magnetic fields up to 1 kG after zero field cooling and field cooling are reported in bulk HgBa₂CuO₄₊ prepared with vapor–solid reaction in a two-temperature zone reactor. Noise measurements performed in Hg-based superconductor materials are compared with previously studied superconductors, in particular, YBa₂Cu₃O_7–. A maximum in the noise is observed at the same resistance, approximately one order of magnitude lower than the normal state value and independently of the applied field. This result is consistent with experiments in bulk YBa₂Cu₃O_7– samples and with percolative model simulations of lattice junctions in which the maximum is obtained at the same value of the concentration of superconducting junctions and, thus, at the same macroscopic resistance. Maximum intensity is shown to increase with magnetic field up to 1 kG, with noise values slightly higher in the field cooling procedure. Our results attest that weak-link effects, as in YBa₂Cu₃O_7–, dominate superconducting properties of bulk HgBa₂CuO₄₊. With respect to YBa₂Cu₃O_7–, no saturation at high magnetic field in both noise and magnetoresistance measurements is observed.     

Annealing Effects in (Hg,Cr)Sr2CuO4+δ: Transport and X-Ray Absorption Studies

Superconducting (Hg_1–x Cr _x )Sr₂CuO₄₊, x 0.36, samples of 1201-type, synthesized in partial vacuum, show T _c ^onset of 58 K, T _c (R = 0) 52 K. It is found that T _c is not affected by the subsequent Ar or O₂ annealing treatment. Remarkably, the annealed samples show significant improvement in the diamagnetic signal. Hg L₃-edge measurements on 1201 samples show divalent state of mercury. The Cu K-edge spectra for the samples, after taking due account of the impurity phases, show noticeable modifications in the split main peak features on Ar or O₂ annealing, suggesting changes in the crystal field asymmetry. In the superconducting (SC) samples, a weak but distinct signature of Cu¹⁺ is seen. The Cu¹⁺ feature is absent in all the non-SC 1201 samples as well as in the impurity phase SrCuO₂ and Sr₂CuO₃ samples. An attempt is made to explain the observed Cu¹⁺ feature in SC (Hg,Cr)-1201 in the light of the available reports.     

c-axis Superfluid Response and Quasiparticle Conductivity in Bi2Sr2CaCu2O8+δ and Bi2Sr2CuO6+δ

Josephson plasma resonance for underdoped Bi ₂ Sr ₂ CaCu ₂ O ₈₊ and Bi ₂ Sr ₂ CuO ₆₊ have been measured by sweeping the microwave, frequency continuously. The resonance enables us to determine the superfluid density and quasiparticle conductivity in the c-axis accurately. We show that the. superfluid response and the low energy excitations out of the condensate in the c-axis of these materials are very different from those in the ordinary Josephson multilayer tunnel junctions. The Josephson coupling energy in single layer Bi ₂ Sr ₂ CuO ₆₊ is more than 5000 times smaller than is predicted in interlayer tunneling model.     

Optical Study of the Pseudogap State in Single Layer Bi2Sr2CuO6+δ

We present infrared reflectivity measurements of the ab-plane response of Pb- and La-doped Bi ₂ Sr ₂ CuO ₆₊ single crystals. A suppression of scattering rate is observed for La-doped samples below and above T _c , suggestive of a pseudogap. Compared with double-layer Bi ₂ Sr ₂ CaCu ₂ O ₈₊ , the suppression is weak and appears at lower frequencies. This may reflect a weak pairing interaction and/or smaller gap amplitude.     

Pressure effects of resistivity along thec axis in La2−x Sr x CuO4

Out-of-plane resistivity p_c(T) of optimally-doped high-T_c superconductor La_2–xSr_xCuO₄ has been investigated under hydrostatic pressure up to 8GPa. We found a drastic decrease in _c which reaches 50% at 8GPa. The large change of _c is understood as due to a small variation of the lattice parameter c. In comparison with a change of p_c(T) estimated from a change of c due to thermal contraction, we conclude that the well known temperature-linear dependence of _c is apparent metallic one; essentially _c is nonmetallic over the whole temperature range, and can be described in terms of tunneling or quantum hopping conduction between CuO₂ layers. Such tunneling conduction is enhanced by the scattering in the CuO₂ plane at the oxygen defects which affect the in-plane resistivity only very weakly.     

Impurity Resonance States in Bi2Sr1.6La0.4CuO6+δ

Using scanning tunneling spectroscopy (STS) at 4.2?K, we observed impurity resonance in single crystals of Zn doped Bi₂Sr_1.6La_0.4CuO_6+?? (T _C =28?K) (Bi2201). Many impurity resonances characterized by the peak structure near E _F were observed. We found that the number of the resonances was less than that of the doped impurities (0.45?% determined by inductively coupled plasma optical emission spectrometry). The resonance peaks were observed only in the region having smaller gap less than ??_cut??26?meV. This ??_cut was about half of that reported in Bi₂Sr₂CaCu₂O_8+?? .     

XRD and PES Studies of the Bi2Sr2Ca1−xPrxCu2O8+δ System

The Bi₂Sr₂CaCu₂O_8+δ system samples doped with Pr on Ca sites were synthesized. Resistivity measurements, x-ray powder diffraction, and photoemission experiments were performed. The experiment results show that Pr ions entered the lattice and chemical environment plays a major role in the shift of core-level spectra and valence-band spectra.     

Independent Control of Low-Energy Resonant States and Polaron States by the Zn-Doping and the Structural Transition in La2−x Sr x CuO4 and La2−x Ba x CuO4 (x=0.11)

The superconductivity depression mechanisms at x≈1/8 in La_2−x Ba _x CuO₄ and Zn substitution in La_2−x Sr _x CuO₄ were investigated by Raman scattering. About 80% of low-energy electronic states are two-dimensional at x≈1/8 and form the Fermi arc around (π/2,π/2). The low-energy states are composed of the resonant peak relating to the insulator–metal transition and the polaron states of the B _3u phonons. Zn substitution depresses the resonant peak, while the LTT structure depresses the polaron states. The superconductivity is suppressed if one of them is reduced.     

Structural, thermal, and electrical properties of La0.7Sr0.3Mn1−y FeyO3±δ

The crystal structure, thermal expansion, and electrical conductivity of the doped manganites La_0.7Sr_0.3Mn_1?y Fe_yO_3±δ (y = 0, 0.1, 0.2, 0.3) are studied. The manganites are shown to crystallize in rhombohedral symmetry (sp. gr. $R\bar 3c$ ). Their lattice parameters are determined, and their thermal expansion coefficients are measured in the range 600–1223 K in air. The conductivity of the manganites is measured in broad ranges of oxygen partial pressures (p _{O 2} = 0.1 to 10^?15 MPa) and temperatures (T = 1073–1273 K). The effects of Fe content, temperature, and p _{O 2} on the dominant defect species and the nature of electrical transport in La_0.7Sr_0.3Mn_1?y Fe_yO_3±δ are analyzed.     

Superconducting Density of States from the Magnetic Penetration Depth of Electron-Doped Cuprates La2−x Ce x CuO4−y and Pr2−x Ce x CuO4−y

From measurements of the magnetic penetration depth, (T), from 1.6 K to T _c in films of electron-doped cuprates La_2–x Ce _x CuO_4–y and Pr_2–x Ce _x CuO_4–y we obtain the normalized density of states, N _s(E) at T=0 by using a simple model. In this framework, the flat behavior of ^–2(T) at low T implies N _s(E) is small, possibly gapped, at low energies. The upward curvature in ^–2(T) near T _c seen in overdoped films implies that superfluid comes from an anomalously small energy band within about 3k _B T _c of the Fermi surface.     

Flux Flow Resistance and Transmittivity in Overdoped Tl2Ba2CuO6+δ

We have measured the surface resistance and transmittivity of overdoped Tl ₂ Ba ₂ CuO ₆₊ in magnetic field up to 7 T in order to investigate the unusual dc resistive transition. Introduction of columnar defects largely shifts the irreversibility line to higher field, showing that the resistive transition is caused by the melting of the vortex lattice. It is found that flux flow resistance of this compound is unusual as compared with that expected from the dc resistivity.     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

Impact of High Pressure and High Temperature on Annealed Oxygen-deficient CoSrO<Subscript>3−<Emphasis Type="Italic">δ</Emphasis></Subscript> Perovskites: Structural and Magnetization Studies

In this brief article, structural, magnetization studies of oxygen deficient perovskite cobalt oxides CoSrO_3?δ synthesized in two different ways are reported. The structural refinements (JANA2006) of X-ray powder diffraction data indicate that the sample prepared under ambient pressure (stage-1) shows a hexagonal structure with P6₃/mmc (194) as a possible space group. The stage-1 sample subsequently sintered at 1450?°C for 1–2 h under high-pressure 6 GPa conditions (stage-2) crystallizes in tetragonal symmetry with I4/mcm (140) space group having a=5.444(7) Å and c=7.68(2) Å. While the stage-1 sample exhibits a paramagnetic nature in the magnetic susceptibility, M(T), measurements, interestingly the sample annealed under high-pressure conditions shows ferromagnetism in the magnetic susceptibility, M(T) and field dependence magnetization, M(H) measurements. The high-pressure annealed sample shows hysteresis opening with a quite large coercive field of 7.3 kOe at 1.8 K. The temperature dependence of the inverse molar susceptibility curves exhibits linear behavior in the high-temperature regime and could be fitted to the Curie–Weiss expression, χ(T)=C/(T?θ _W). The experimental values of θ _W and p _eff obtained from the linear region of the inverse molar magnetic susceptibility curves are found to be: ?210.7(5) K and 2.38(2) μ _B/Co for stage-1 and 260.2(7) K and 1.87(3) μ _B/Co for stage-2 samples, respectively. A negative sign of θ _W indicates rather strong antiferromagnetic correlations in the stage-1 sample. Apart from these results, the structural parameters reported by various groups for the strontium-based perovskite cobalt oxides are also presented in the form of literature collections.     

The Energy Gap in the (Hg1−x Sn x )Ba2Ca2Cu3O8+y Superconductor

The experimental data show that the increase of tin in the (Hg_1?x Sn _x )Ba₂Ca₂Cu₃O_8+y superconductor results in a visible increase in the pseudogap temperature (T ^?) with a slight decrease in the value of the critical temperature (T _C). Based on the above results, the dependence of the average amplitude of the energy gap on the temperature (G(T)) has been determined for x∈〈0,0.25〉. On this basis, it has been found that the dimensionless ratio R _G ≡G(0)/k _B T _C increases with the increasing tin content from the value of 3.48 to the value of 4.88. Next, for x=0.25 and T∈〈0,T ^?〉, the dependence of the energy gap on the wave vector has been calculated in the surrounding of the Fermi level. It has been shown that in the anti-nodal region the energy gap is very weakly dependent on the temperature. On the other hand, in the nodal region its values strongly decrease together with the increasing temperature.     

Pressure and Grain-Size Dependences of the Chiral-Glass Transition in Superconductive Ceramics of YBa2Cu3O7−y

Ceramic high-T _c superconductors may be viewed as a weakly coupled random Josephson network containing -junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa₂Cu₃O_7–y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T _c1 and furthermore among the grains at T _c2 (<T _c1). The increase in T _c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T _c2 also increases with increase of pressure, however, the sharpness of the transition at T _c2 does not change. In spite of difference in T _c2's, the increasing rates of T _c2 by pressure are nearly the same.     

In-Plane Anisotropy of the Upper Critical Field of La2−x Sr x CuO4 in the Underdoped and Overdoped Regimes

For La_2–x Sr _x CuO₄ single-crystals in the underdoped and overdoped regimes, we have measured the electrical resistivity along the c-axis, _c , under constant magnetic fields, changing the field direction in the ab-plane. At x=0.08, 0.12 and 0.16 in the underdoped and almost optimally-doped regimes, clear anisotropy of _c with the fourfold symmetry in the ab-plane has been observed. This symmetry is regarded as the fourfold symmetry of the upper critical field, H _c2, and explained as being mainly due to the anisotropy of the superconducting energy gap owing to the d_x ^{2-y 2} pairing. The magnitude of the fourfold component of H _c2, H _c2/H _c2, is largest in x=0.08 among the three crystals. At x=0.20 in the overdoped regime, no clear fourfold symmetry has been observed. It appears that the fourfold symmetry is easy to observe in the underdoped regime rather than in the overdoped regime.     

Magnetization and Re LIII-Edge Studies of (Hg,Re)Sr2CuO4 + δ System

Chemically stable nominal compositions of (Hg_1-x Re _x )Sr₂CuO_{4 +} (x = 0.05, 0.10, and 0.20), designated as (Hg,Re)-1201 phase, were synthesized at 920°C in partial vacuum. The samples with x = 0.05 and 0.10 are superconducting (SC) with T _c ^onset 40 and 54 K, respectively. On the other hand, for the composition with x = 0.20, the sample is nonsuperconducting down to 5 K. The sample with composition (Hg_0.9Re_0.1)Sr₂CuO_{4 +} on cooling, below 10 K in dc-field of 4 kOe, shows a dramatic decrease (reversal) in diamagnetic signal. Such a reduction in diamagnetic response is also reflected by the sample's supercurrent induced magnetic hysteresis curve generated at 9 K. In the light of the available reports and our Re L_III-edge measurements, we attribute this effect to a field enhanced paramagnetic contribution probably arising mainly from mixed valent (6⁺, 7⁺ dominant) Re ions. The critical current density (J _c), calculated using Bean's model, for (Hg_0.9Re_0.1)Sr₂CuO_{4 +} is 6 × 10³ A/cm² (4.5 K, 5 kOe).