Optimally-doped Samples Research Articles

STRUCTURAL INSTABILITY AND SUPERCONDUCTIVITY OF THE (Nd, Gd, Ce)2CuO4 SYSTEM

The influence of partial substitution of Nd by Gd and Ce on the Nd 2 CuO 4n-type superconductor was studied by using X-ray diffraction measurements. For x = 0.15 Ce the maximum value of critical transition temperature Tc was obtained. The influence of Gd concentration on the lattice parameters and unit-cell volume was studied in the optimal doped sample (x = 0.15 Ce ) and in the samples with 0≤x≤0.2 Ce . The relation between structural parameters and critical transition temperature was evidenced. The transition from the structural normal phase to the distortion phase was evidenced above y = 0.75 Gd in the optimal Ce -doped sample.

Short range charge/orbital ordering inLa1−xSrxMn1−zBzO3 (B = Cu,Zn) manganites

We have measured the reflectivity spectra ofLa1−xSrxMn1−zBzO3 (B = Cu,Zn; ; ) manganites over wide frequency(100–4000 cm−1) andtemperature (80–300 K) ranges. Besides the previously observed infrared active modes or mode pairs at about160 cm−1 (external mode),350 cm−1 (bond bendingmode) and 590 cm−1 (bond stretching mode), we have clearly observed two additional phonon modes at about 645 and720 cm−1 below thetemperature T1 (T1<TC), which coincides with the phase transition temperature when the system transforms fromferromagnetic metallic into a ferromagnetic insulator state. This transition is related to theformation of short range charge/orbitally ordered domains. The temperatureT1 of the phase transition is dependent on the doping concentration and for optimallydoped samples we have found that . Electrical resistivity and magnetization measurements versus temperature and magneticfield support the short range charge/orbital ordering scenario.

Quantized Vortices and Diamagnetic Precursor to the Meissner State in High-Tc Superconductors

We report the magnetic imaging for underdoped and optimally-dopedLa2−xSrxCuO4 (LSCO) thin films on single substrates and nearly optimallydoped YBa2Cu3O7−x (YBCO) thin films on tricrystal substrates in the temperature range both below and above Tc using scanning SQUID microscopy. Below Tc, clear integer- and half-integer quantized vortices were observable. Above Tc, however, the inhomogeneous diamagnetic domains appeared. The local diamagnetic domains that led to the Meissner state were found in the broad temperature range for underdoped samples and in the narrow limited temperature range for optimally-doped samples. The results provide evidence that local diamagnetic domains are closely related to the pseudogap state. The continuous connection of the domain state above Tc with the state of a half-integer vortex at the tricritical point in the YBCO film below Tc also indicates that the diamagnetic domains are also closely related to the occurrence of dx2-y2-wave superconductivity.

Effect of oxygen redistribution in Bi-based high- Tc superconductors on their normal and superconducting properties

Bi-based high- T c superconductors of three types, Bi 2Sr 2CuO 6.13 (2201), Bi 2Sr 2CaCu 2O 8.24 (2212), and Bi 1.9Pb 0.3Sr 1.8Ca 2Cu 3O 10+ x (2223), has been investigated. The temperature dependence of resistivity ρ( T) has been measured during slow heating of samples (∼2 K/h) up to 600 K in air. The maximum of ρ( T) at T max>400 K is observed for all samples. T c increases for overdoped samples (2201 and 2212) and does not change in the optimal doped sample (2223) after cooling from T max, and hole concentration decrease for all samples. ρ and T c return to initial states after the heating of samples up to 600 K. Powder X-ray diffraction is carried out during slow heating of Bi based samples.

Annealing effect on the irreversibility line in (Cu,C)Ba 2Ca 2Cu 3O y

Annealing effects on irreversibility fields ( H irr) and physical properties in high-quality (Cu,C)Ba 2Ca 2Cu 3O y ((Cu,C)-1223) samples have been investigated. The T c of (Cu,C)-1223 increased from 70 K (as-prepared) to 120 K by annealing up to 550 °C in a nitrogen gas. The decomposition temperature (≃550°C) and the annealing effect on T c of (Cu,C)-1223 were found to be different from that in C-free samples, which may be due to stability of carbonates in the (Cu,C)-site. The slightly overdoped sample ( T c =111 K ) showed high H irr (=9.4 T) at 77 K rather than the optimal-doped sample ( T c =120 K ). This indicates that control of doping level as well as T c is important to enhance the H irr at 77 K.

Nonuniversal power law of the Hall scattering rate in a single-layer cuprateBi2Sr2−xLaxCuO6

In-plane resistivity \rho_{ab}, Hall coefficient, and magnetoresistance (MR) are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals with various carrier concentrations, from underdope to overdope. Our crystals show the highest T_c (33 K) and the smallest residual resistivity ever reported for Bi-2201 at optimum doping. It is found that the temperature dependence of the Hall angle obeys a power law T^n with n systematically decreasing with increasing doping, which questions the universality of the Fermi-liquid-like T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of the optimally-doped sample changes as T^{1.7}, not as T^2, while \rho_{ab} shows a good T-linear behavior. The systematics of the MR indicates an increasing role of spin scattering in underdoped samples.

Effect of Bi–Sr replacement and oxygen doping on vortex-matter phase transitions in Bi 2+ xSr 2− xCaCu 2O 8+ δ

To investigate the effect of oxygen doping level with a wide range on vortex-matter phase transitions, magnetization for H‖ c was measured for Bi 2+ x Sr 2− x CaCu 2O 8+ δ single crystal with x=0.05–0.25 and various doping level δ. Optimum condition against δ for the superconducting transition temperature was found to depend on x, which is reasonably explained from the difference in valences between Bi and Sr. The second-peak of the low temperature magnetization curve was observed in all the samples, ranging from ∼120 Oe for the mostly underdoped sample to ∼1300 Oe for the most overdoped one. For the optimally-doped samples the second-peak was observed at ∼400 Oe almost independent of x. Vortex-melt transition field, H m, was well evaluated for the as-grown samples and for the slightly overdoped ones. The shift of the melting line, H m( T), with x and δ can be explained from the change in anisotropy and penetration depth. Near the superconducting transition temperature vortex transition is attributed to the transition due to the decoupling of pancake vortices, while at lower temperatures to the melting transition with both the electromagnetic- and the Josephson-coupling between vortices.

Low-temperature IR-reflectivity of superconducting Bi-2212 single crystals with different oxygen contents

We report on low temperature infrared reflectivity measurements of Bi2Sr2CaCu2O8+δ single crystals (Bi-2212) with two different oxygen contents having Tc values of 91 K for an optimally-doped crystal and 73 K for an over-doped crystal. The reflectivity of the optimally-doped sample at low temperature (10 K) was near unity in the range between 150 cm-1 and 400 cm-1 and for the over-doped sample noticeably smaller. The results for the optimally-doped sample are similar to results of other IR-reflectivity measurements on Bi-2212 single crystals. For the over-doped crystal we additionally determined the absorptivity in the range of 20 cm-1 and 140 cm-1 by photothermal interference spectroscopy.

Crossover from the first-order vortex phase transition to the peak effect in Bi 2Sr 2CaCu 2O y having different oxygen contents

We have measured the magnetization and the heat capacity of over-doped (OVD) and optimally doped (OPD) Bi 2Sr 2CaCu 2O y single crystals in the mixed state under magnetic fields applied along the c-axis. In both samples, a distinct magnetization step is observed in the magnetization curve at high temperatures and is found to change to a kink at low temperatures. At still lower temperatures, the peak effect dominates the magnetization curve and the kink vanishes. The origin of the step and the kink in the magnetization curve is considered to be the first-order phase transition in a quasi-three-dimensional vortex system although the heat capacity shows little anomalies at the transition because the latent heat is small. The first-order phase transition line of each sample is drawn on the H- T plane and it is found that the slope of the line of the OVD sample is steeper than that of the OPD sample. The entropy jump at the transition of the OVD sample is estimated to be larger than that of the OPD one at all temperatures and magnetic fields studied. This behavior is contradictory to the simple vortex-lattice melting picture.