Photovoltaic Properties Of High Tc Superconductor YBCO And SmOFFeAs

Since the 1990’s, many researchers studied the electrical, magnetic and superconductivity properties of high Tc superconductor YBa2Cu3O7-?(YBCO). High Tc superconductive materials YBCO have been widely used in areas such as strong magnets and electric power due to its excellent physical properties. Recently, experimental results show that the physical properties of YBCO can be obviously improved when the sample is illuminated by laser. In 2004, the photovoltaic properties of the YBa2Cu3O7-δ/SrNb0.05Ti0.95O3(YBCO/NSTO) junction were investigated by Asakura and the open-circuit voltage(Voc) was observed to be 0.8 V when the sample was illuminated by continuous ultraviolet(UV) light or pulsed x-ray photoemission spectroscopy. The researching leads to the possible application of high Tc superconductors in areas such as photo-electronic devices and solar cells. In this thesis, we study the photovoltaic effect of high Tc superconductor YBCO(thin film and ceramic samples) and SmOFFeAs ceramic, and the main results as follows:Firstly, we study the photovoltaic properties of YBCO/NSTO heterojunction. Obvious photovoltaic effect can be observed when the YBCO/NSTO junction is illuminated by simulated sun light: the open-circuit voltage Voc of 1.065 V can be reached. To reveal the role of YBCO in the photovoltaic effect of YBCO/NSTO junction, the open-circuit voltage Voc and short current density Jsc under the simulated solar illumination were measured after the junction was annealed in different oxygen partial pressure. The annealing furnace should be purged for high vacuum before annealing, and then vented with high purity oxygen, to reduce impurities at the surface of the sample and avoid the S-shape I-V curves. The experimental results show that the photovoltaic effect of the YBCO/NSTO heterojunction is very sensitive to the oxygen content of YBCO: oxygen-rich YBCO leads to a Sckottky junction, while p-n junction is obtained by oxygen-deficit YBCO. These two types of heterojunction give rather different photovoltaic behavior. Temperature dependence of photovoltaic effect of YBCO/NSTO junction shows that open-circuit voltage decreases while short-circuit current increases with increasing temperature.Secondly, we study the origin photovoltaic effect in YBCO ceramic. Remarkable photovoltaic effect in YBa2Cu3O6.96(YBCO) ceramic was observed when the area around the electrode was laser illuminated. When YBCO is in normal state, I-V curves move upwards parallel to the original one with increasing laser intensity irradiating at the cathode leads. A negative short-circuit current(Isc) when V=0 and a positive open-circuit voltage(Voc) at which the current is zero are obtained. Opposite polarity of Voc and Isc are observed when the laser beam points to the anode electrode. Move the laser spot to the middle of the two voltage electrode or far away from any of the electrodes, no photovoltaic effect can be observed. These observations indicate that there should be a potential in the interface of YBCO ceramic and Ag electrode, even with an ohm contact between the YBCO sample and Ag paste. Our results strongly suggest that the interface potential points from Ag electrode to YBCO sample when YBCO is in normal state, and this potential directs from superconductor to metal electrode as YBCO is superconducting. By studying the temperature, laser intensity and magnetic field dependence of the photovoltaic effect, we reveal an intrinsic interface potential at YBCO/Ag heterojunction and establish a new method for investigating the proximity effect of superconductor-metal interface. Our findings may shed further light on the mechanism of high Tc superconductive material.At selected temperatures from 100 to 300 K, the current-voltage curves of YBCO/Ag system under illumination of fixed laser intensity rotate around a point C. The output voltage Vc remains relatively constant at variation of temperature with bias current Ic, indicating zero temperature coefficient of resistivity(TCR). Laser induced TCR may pave the way to new applications of photon-electronic devices.Finally, we study the photovoltaic effect in n-type iron based high Tc superconductor SmOFFeAs/Ag paste system. Distinct photovoltaic effect was observed when the SmOFFeAs sample was illuminated by laser. Negative Voc is obtained when the laser beam points to the cathode electrode, no matter the superconductor is in normal or superconducting state. Voc decreases linearly with increasing temperature at fixed laser intensity, while at fixed temperature, Voc increases with increasing laser intensity when SmOFFeAs sample is in normal state. The experimental data indicate clearly that there always exists an electrical potential pointing from SmOFFeAs sample to Ag electrode in the interface of the heterojunction no matter what state the SmOFFeAs sample is. In contrast to that found in YBCO-Ag system, larger photovoltaic effect was observed in SmOFFeAs system. To study the evolution process of the laser induced voltage, it is beneficial to use a SmOFFeAs/Ag heterojunction which may give a relatively high accuracy.The same polarity of the interface potential is always observed in any superconductor/metal system, when the superconductor is in superconductive state, regardless which particular superconductor is involved. It is evident that this potential must arise from the superconductivity and is, very likely, a result of the Proximity effect. In normal state, opposite polarity of the open-circuit voltage is obtained for the two types of superconductor when the same electrode is illuminated by laser. The interface potential is closely related to the charge density, type and the mobility of carriers when the superconductor is in normal state. Measurement of photovoltaic properties provides rich information about the nature of Proximity effect and the transportation behavior of charge carriers in the materials involved.So far, there are two pieces of important work about the superconductor-metal interface(SN): one is negative interfacial energy proposed by A. A. Abrikosov in 1952; and the other is the Proximity effect. Here we have proved, for the first time, the exist of an intrinsic interface potential between the superconductor and normal metal and at the same time solved the problem of origin of the photovoltaic effect in superconductors. Our findings provide a new method for studying Proximity effect and pave the way to the superconductivity mechanism of high Tc superconductors.