H F Chu

Reproducible low-voltage resistive switching in a low-initial-resistance Pr0.7Ca0.3MnO3 junction

Current?voltage (I?V) characteristics are investigated in a low-initial-resistance Ag/Pr0.7Ca0.3MnO3/Pt sandwich structure. It is found that the junction can show stable low and high resistance states in ?0.3?V voltage sweeping cycles. The set and reset voltage values are, respectively, +0.1?V and ?0.2?V, which are very low as compared with those reported previously. Furthermore, the I?V curves in both resistance states exhibit rather linear behaviour, without any signature of metal/insulator interface effects. This implies that the Schottky interface mechanism might not be an indispensable factor for the colossal electroresistance effect. The origin of low switching voltages is attributed to the reduced effective distance for electric field action due to the sufficient oxygen content of the PCMO layer. The underlying physics is discussed in terms of the filament network model together with the field-induced oxygen vacancy motion model.

Enhancement of critical current density and flux pinning in oxygen ion-irradiated MgB2 thin films

The effects of 300?keV O2+ ion irradiation on the temperature and magnetic field dependence of the critical current density (JC) in high quality epitaxial MgB2 thin films have been studied. Upon irradiation, the critical temperature TC drops gradually and ultimately to less than 2?K and the residual resistivity increases from about 1 to 125????cm with increasing doses. At a moderate radiation level of 1 ? 1013?cm?2, JC is enhanced and its performance in magnetic fields is significantly improved over a certain range of fields applied along the c-axis. The scaling behavior of reduced pinning force with reduced field reveals that the flux pinning mechanism evolves from grain-boundary-like pinning in the pristine and lightly irradiated films, to point-like pinning in moderately irradiated films, and again to grain-boundary-like pinning in heavily irradiated films. It is then concluded that point-like pinning centers, which can be modified by the damage level, have been introduced after oxygen ion irradiation.

Superconductivity, Hall properties and disorder effect in Pr1−xCaxBa2Cu3O7−δ (0.4≤x≤0.6) epitaxial thin films

Superconducting Pr1?xCaxBa2Cu3O7???(PCBCO) thin films with high Ca concentration (0.4?x?0.6) have been prepared. As characterized by x-ray diffraction, good quality c-axis and a-axis oriented epitaxial thin films are grown on SrTiO3 substrates by changing the substrate temperature. However, for each Ca concentration, higher zero resistance temperature?(TC0) is generally observed in samples with mixed orientations grown at moderate temperatures. The best TC0 are 26.5?K, 34.1?K and 16.8?K for x = 0.4, 0.5 and 0.6 samples, respectively. Hall measurements reveal that the hole mobility decreases as x increases, while the Hall number nH reaches a maximum as x approaches 0.5, then reduces dramatically. It appears that the T2 dependence of cot?H is still feasible in this system, and the effect of Ca substitution on transport properties in PCBCO is a result of the competition between carrier introduction and carrier localization.