X.B. Xu

Crossover from high-temperature vortex-liquid to low-temperature vortex-glass behavior in single phase HgBa2Ca2Cu3O8+δ superconductor

Abstract The ρ – T curves in our single phase HgBa 2 Ca 2 Cu 3 O 8+ δ superconductor were measured as a function of temperature and magnetic field, ρ = ρ 0 exp(− U eff / κ B T ). It can be transformed to another form d(ln ρ )/d(1/ T )=− U eff + T d U eff /d T , then this becomes a plot of the activation energy U eff as a function of temperature. Our data plotted in these ways show a clear crossover from high-temperature two-dimensional vortex-liquid to a critical region associated with the low-temperature three-dimensional vortex-glass phase transition. The critical exponents v ( z −1)=3.9±1.9 in this system are little different with previous measurements in BSCCO and YBCO systems.

Scaling of the vortex–liquid resistivity in high temperature superconductors

By considering the competition of thermal activation and effective pinning, a general description of temperature dependent resistivity under magnetic fields has been deduced in high temperature superconductors. The temperature dependent resistivity can be expressed as ρ = ρn(1−x)s, with the energy ratio x = Ueff(T,B)/kBT. This model is a consistent description of vortex glass theory. In the experiments of both single-phase Hg-1223 and Tl-2212 thin film superconductors, the temperature dependent resistivity under various magnetic fields can be scaled onto a straight line by plotting lnρ versus ln((T− Tg)/(Tc− Tg)) with the slope s. This scaling behaviour is striking evidence of the proposed model.

Dynamical hysteresis V–I curves of superconductors

We study the dynamical hysteresis of transport V –I characteristics based on the flux creep model characterized by flux diffusion speed v = v0(j/jc)|j/jc|n. It is shown that there are two sorts of hysteresis loop in the V –I curve: clockwise and anticlockwise. The clockwise loop can be observed if v is lower by far than the speed of the applied current va (, where d is the sample thickness): . This formula predicts that it is very difficult to observe the V –I loop in a thin film because of its small d. Furthermore, the clockwise loop could always be observed if the critical current of the sample increases with time, whereas the anticlockwise loop could only be explained by the critical current decreasing with time, no matter which mechanism is responsible for the critical currents change. The numerical results could be used to understand experiments on a variety of samples.

A significant drop in resistance around 200 K in superconducting Hg-1223 and discussion

Tholence et al. and Yao et al. observed a significant drop in resistance around 250 K and 230 K respectively on superconducting Hg-based compounds. Yao et al. believed that this drop in resistance is due to the presence of the free-state liquid mercury or Hg-Au alloys in the sample. We report here the electrical resistivity measurements on single phase (as determined by X-ray powder technique) Hg-1223, which has shown a partial or total drop in resistivity around 200 K but with no significant observation of diamagnetic signal. On the other hand, an internal friction peak around 203 K was observed in our internal friction experiment. We are sure that the drop in resistance for our Hg-1223 sample around 200 K is not due to the presence of liquid mercury or Hg–Au alloys in the sample since we did not use the gold foil or silver foil to enwrap our sample. We rather believe that the drop in resistivity is due to superconductivity or other mechanisms.

Pseudogap Phenomenon in Single Phase Hg-1223 Cuprate Superconductors

AbstractThe single phase Hg-1223 cuprate superconductors have been successfully fabricated. The postannealing treatment under oxygen (O2) gas and argon (Ar) gas with different annealing time was performed in order to obtain various hole density samples. These samples were used to study the pseudogap phenomenon in the normal phase. With the plot of ln[1/ρ(T) − 1/ρN(T)] against 1/T, three characteristic temperatures T*, TS, and TF were obtained, which describe the role of the pseudogap phenomenon. The magnitudes of the pseudogap were extracted from the slope of the stated plots. It is found that the three characteristic temperatures T*, TS, and TF change with the hole density linearly. The relationship between T* and TC goes within T* = T*op+Γ0 (1−TC/TmaxC)

Four-fold symmetry of resistivity and flux pinning energy in Ba0.67K0.33BiO3+δ single crystal

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Observation of flux flow based on vortex-glass scaling analysis for single phase HgBa2Ca2Cu3O8+δ superconductor

, where T*op and TCmax are the pseudogap opening temperature and the critical temperature for the optimal sample. It is also found that the pseudogap phenomenon only exists below the optimal regime in the hole doping phase diagram.

Surface barrier and bulk pinning in MTG YBaCuO

The phase transition from vortex liquid to vortex slush has been confirmed in resistance measurements for YBCO crystal superconductors, on the basis of scaling analysis. The temperature dependence of the resistivity under various magnetic fields collapses onto two branches in the scaling behaviour, associated with the vortex slush and the vortex liquid states. The lower branch, for temperatures below the transition point, has a negative curvature, while the upper one shows a plateau above the transition point. The critical exponents are estimated from the scaling result. The phase diagram in the H–T plane is presented and compared with previous reports.