Hysteresis In High Tc Superconductor YBCO Ceramic Under Laser Irradiation

Effects of light illumination on high-temperature superconductors have been a hot topic in recent years.For the high superconducting transition temperature of YBa₂Cu₃O_7-??T_c of YBa₂Cu₃O₇ is 93 K?,the high critical magnetic fields and high critical density of the non-ideal type II superconductor,the safe and simple preparation process,and the cost advantages,scientists' enthusiasm for the response of YBCO material under light illumination keeps increasing.In this paper,oxygen-rich YBa₂Cu₃O_6.96 ceramic sample prepared with the traditional solid state reaction method is employed for research.X-ray diffraction and Field Emission Scanning Electron Microscopy were employed for analyzing the crystal structure and the surface morphology,respectively.Then the resistance and I-V characteristic curves were measured by the standard four-point technique using a vibration sample magnetometer?VersaLab,Quantum Design?with a quartz crystal window.We study the continuous purple laser??=405 nm?effects on the oxygen-rich YBa₂Cu₃O_6.96 ceramic sample and main results as follows:Firstly,we study the light effects on the resistance of the oxygen-rich YBa₂Cu₃O_6.96 ceramic sample.With laser irradiation,the normal state resistance increases linearly with increasing laser intensity.Light illumination destroys the superconductivity and the onset T_c shifts to a lower temperature,which become more obvious with larger laser intensities.The zero resistance superconducting transition temperature reduces and consequently the temperature range of superconducting transition is widened by laser irradiation.An anomalous peak is observed around the superconducting transition during cooling of the laser irradiated sample.The resistance peak temperature T_peak is independent of laser intensity with laser beam directed at the same position.The resistance peak height ?R?the resistance difference before and after jump?increases with laser increasing intensity.The resistance anomalous peak appears under laser illumination during cooling only if the sample is cooled down to a temperature below T_peak.Then the resistance increases with temperature with no peak but hysteresis during heating.In addition,a larger value is obtained and a lower superconducting transition temperature during heating.The resistance comes to the same value as that during cooling when the sample is warmed up to 256 K.The hysteresis still exists in the superconductor till it comes to about 265 K,when the hysteresis begins to disappear gradually.There is a disappearing process of the hysteresis in YBCO ceramic,or in another words,the evolution process of the resistance peak.The hysteresis disappears at enough high temperature and then an anomalous peak appears during a new cooling measurement.The upper limit temperature for the hysteresis is 275 K above which the effect of the anomalous peak on the resistance is eliminated completely.Secondly,we study the photovoltaic properties and the hysteresis in it for the oxygen-rich YBa₂Cu₃O_6.96 ceramic through measuring the I-V characteristic curves with laser illumination and without light.There exists a potential in the interface of YBCO ceramic and Ag electrode,which separate photo-generated e-h pairs created when the superconductor is irradiated with laser for its very small energy gap?Eg?,then an open circuit voltage V_oc is produced.The potential points from the metal electrode to superconductor at 300 K,while it reverses at 50 K.Then a negative V_oc is obtained at 300 K and a positive V_oc is observed when laser beam is directed around anode electrodes.An anomalous peak is also observed in the V_oc-T curve at the same temperature as in differential resistance during cooling.The obvious correlation between the temperature dependence of V_oc and that of dV/dI suggest that the anomalous feature observed in the same temperature range for two different physical properties have the same origin.While huge hysteresis and more evidently photovoltaic effect are observed during heating,the absolute value of V_oc keeps lager until about 276 K,above which the value of V_oc obtained from cooling and heating becomes essentially indistinguishable.The critical temperature 276 K obtained here is fairly close to that found in resistance hysteresis study.It can be concluded that there is an intrinsic hysteresis in the oxygen-rich YBa₂Cu₃O_6.96 ceramic through the systematic research on the electrical properties and photovoltaic properties for the sample with purple laser employed.Experimental results indicate that the thermal hysteresis in the superconductor YBCO is intrinsic,which can be amplified rather than caused by the laser irradiation.Our observations provide new ideas for the further explore of the mechanism of YBCO superconductor.The rapid and notable response to the light illumination for the resistance of YBCO superconductor near the superconducting transition temperature paves the way to its application in the field of photosensitive device.