Xuefeng Sun

Pr-doping effect on the structure and superconductivity of Bi2Sr2Ca1-xPrxCu2Oy single crystals

Abstract Effect of Pr-doping on the structure and superconductivity of Bi2212 system was studied for Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O y single crystals with x =0–0.78. The a - and b -axis lengths increase with increasing Pr concentration, while the c -axis length and the period of modulation decrease with Pr-doping. These can be explained by the higher valence of Pr ion substituting at Ca site, the extra oxygen in BiO bilayers and the lattice mismatch between perovskite layer and BiO bilayer. In addition, the changes in lattice parameters are more dramatic in the Pr-rich region than those in the Ca-rich region, which can be attributed to the difficulty in the intercalation of extra oxygen in the BiO bilayer at high-doping level. The superconducting transition temperature increases slightly at first and then drops gradually upon Pr doping and the complete suppression of superconductivity occurs when the Pr content reaches 0.60. The superconducting volume faction also decreases with Pr content and is proportional to 1− ax 2 .

Growth of Bi2Sr2Ca1−xPrxCu2Oy single crystals from Bi-rich melts

Abstract Bi 2 Sr 2 Ca 1− x Pr x Cu 2 O y single crystals were successfully grown by a self-flux method from Bi-rich melts for the first time. For starting composition x s =0–0.6, all the crystals possess the pure Bi2212 phase with the actual Pr content varying from x =0 to 0.78 correspondingly. However, the grown crystals are of the Bi2201 and the Bi2222 phase for starting composition x s =0.7–0.9 and x s =1.0, respectively, with evident difference between the nominal and actual composition of crystals. The superconducting transition temperature of Bi2212 crystals increases slightly at first and then drops gradually upon Pr doping and the complete suppression of superconductivity occurs when the actual Pr content reaches 0.60.

Correlation between Tc and ns/m* in Bi2Sr2CaCu2O8+δ single crystals

Abstract The reversible magnetization measurements with applied field parallel to the c -axis were carried out for a series of Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) single crystals with different oxygen content varying from underdoped to overdoped states, from which the doping dependence of penetration depth and n s / m * can be obtained. It was firstly found in this system that with increasing oxygen content, the value of n s / m * increases in the underdoped region and reaches a maximum at the optimal doping, then decreases in the overdoped region. The observed deviation from the linear relationship of T c ∝ n s / m * in the underdoped region can be regarded to be due to the structural distortion and the disorder scattering, while the decrease of n s / m * in the overdoped region is suggested to be due to the phase-separation related pair breaking.

Variations of normal state resistivity and Cu2+ localized spin moment in the single crystal Bi2Sr2 CaCU2O8 + x

Abstract The effects of oxygen content on the anisotropic resistivity and Cu 2+ localized spin moments have been studied for the Bi 2 Sr 2 CaCu 2 O 8 + x single crystals. It was found that the in-plane and out-of-plane resistivities ( ϱ ab and ϱ c ), as well as the electron spin resonance (ESR) signals decrease with increasing oxygen contents from underdoping to overdoping states. We believe that the in-plane resistivity upturn in an underdoped region is closely related to the carrier localization, and the scattering of spin fluctuations plays an important role in the charge transport properties. On the origin of c-axis resistivity, we should take into account the scattering in the `barrier layer between CuO 2 ‘cells’ and the scattering or fluctuations in the planes.

Effects of oxygen doping and structural distortion on the superconductivity of single crystals

Effects of oxygen doping and structural distortion on the superconductivity of (Bi2212) were studied by ac susceptibility and x-ray diffraction measurements for a group of high-quality single crystals. The crystals were annealed at 600 and under different oxygen pressures ranging from to 6080 Torr. It was found that the c-axis length showed a plateau at - Torr and decreased with increasing oxygen pressure above Torr, which strongly suggested that two stages of oxygen diffusion may be involved. The full width at half maximum (FWHM) of the 00 main reflection, which reflects structural distortion in the c direction, showed a peak around the optimized oxygen doping. This implies that the structural modification may be another factor that affects apart from oxygen content. On the other hand, the period of incommensurate modulation along the b direction, s, was found to have nothing to do with the oxygen content or superconductivity. Relationships between superconductivity, oxygen content and microstructure of the Bi2212 single crystal are discussed.

Growth of superconducting Bi2Sr2Ca(Cu1-xMnx)2Oy single crystals

Abstract High-quality Bi 2 Sr 2 Ca(Cu 1− x Mn x ) 2 O y single crystals were successfully grown by the self-flux method for the first time. For starting composition x =0−10%, all the crystals possess the pure Bi2212 phase with evident difference between the nominal and actual Mn concentration. The c -axis lattice parameter decreases with increasing Mn content for both as-grown and air-annealed single crystals. The superconducting transition temperature decreases dramatically upon Mn doping, with the T c suppression rate larger than those of other 3 d transition elements doped Bi2212 reported previously.

New experimental evidence of the structural modification in single crystal Bi2Sr2CaCu2Oy by X-ray diffraction observation

Abstract The variations of the high angle 00 38 peak-shape by means of X-ray l scans of the 00 l fundamental reflections were investigated in detail for a highly oriented Bi 2 Sr 2 CaCu 2 O y (Bi2212) crystal with sufficiently small intrinsic mosaicity and the same crystal annealed in air at 250, 300, 400, 600, and 750°C for 20 h in consequence. For the first time, we observed a new additional reflection almost overlapped original 00 l fundamental reflection at annealing temperature below 400°C by X-ray diffraction measurement, which shows that there coexisted two sets of lattice periodicity in the c -direction of the annealed crystal. The new additional reflection appeared at 250°C and disappeared at 400°C. Its intensity was increased at 300°C. The measurements of the AC susceptibility, c -axis parameter and full width at half maximum (FWHM) of the 00 38 peaks showed that the new additional reflection was associated with the oxygen diffusion in CuO 2 planes and the changes of strain field. The results provide the new experimental evidence that the structural distortion is more sensitive to the oxygen diffusion in CuO 2 planes than to that in Bi–O layers.

A growth and annealing experiment concerning large crystals of with only a single domain

Annealing experiments in air have been performed on large crystals of with only a single domain. which were grown by a directional solidification method and are free from intergrowth defects, twins and, more importantly, showed sufficiently small mosaicity. It is found that, after annealing at 450 - C, the crystals showed a very narrow diamagnetic transition width (< 1.5 K). Above this annealing temperature range, the annealed crystals, just like the as-grown crystals, showed a much broader diamagnetic transition. To explain these typical changes of with the annealing temperature, a selected crystal was progressively annealed in air at 500 and C for 4 and 8 h and full structural characterization by x-ray diffraction of the crystal as grown and after each annealing was conducted. Apart from variations in the cell parameters, it is evident that the crystal becomes highly oriented, corresponding to the extreme narrowness of .

Time effect of annealing on Bi2Sr2CaCu2Oy single crystals

Abstract Highly-oriented as-grown Bi 2 Sr 2 CaCu 2 O y single crystals have been air-annealed at 500 or 750°C for different times. The thermal treatments improved the superconductivity of the crystals relative to the as-grown T c and ΔT c distinctly in several minutes. The results have been explained by oxygen configuration and it is suggested that oxygen out-diffusion in the crystals is rather rapid at the beginning of the annealing.

Thermopower of Bi2Sr2Ca1-xRxCu2Oy (R = Gd, Pr) single crystals

The temperature and carrier concentration dependence of the thermopower (TEP) S in Bi2Sr2Ca1-xRxCu2Oy (R = Gd, Pr) single crystals were carefully studied. Upon Gd or Pr doping, the superconductivity changes systematically from overdoping to underdoping with the TEP increasing monotonically. S(T)/S(Tscale) against T/Tscale for all the underdoped crystals can be well scaled into a universal curve, where Tscale is the temperature at which S begins to deviate from T-linear behaviour. This scaling behaviour is assumed to correlate with the opening of the pseudogap. Furthermore, the temperature dependence of the TEP is analysed and discussed in terms of two different two-band models and the Nagaosa-Lee model.