本論文利用量測交流磁化率及電阻的方式，探討不同孔徑的超導奈米複合材料的特性。
本實驗中的超導奈米複合材料為將第一類超導金屬錫，嵌入蛋白石(OPAL)及多孔隙玻璃中，蛋白石的孔隙直徑為100nm與50nm，多孔隙玻璃為18-20nm，金屬顆粒會在材料內部形成三維網狀結構。經由改變不同外加磁場量測電阻以及交流磁化率，可以了解不同孔徑的樣本，其漩渦態的行為及得到H-T相圖。
經由H-T相圖以及活化能(activation energy)的計算，發現在H-T相圖中其Hc2(T)線，其曲率隨著磁場增加會由正曲率轉變為負曲率，曲率轉換的磁場為HCO，我們發現活化能隨著外加磁場增加而減少，並且當外加磁場大於HCO活化能會急速降低。而在不同孔徑下的樣品中，電阻與交流磁化率的量測結果相當一致。而在不同孔徑的樣品中所計算出的活化能降低的趨勢以及HCO/ Hc2(0)，差異相當明顯。由此我們驗證了在超導奈米複合材料中，孔徑的大小對漩渦的行為，有著相當大的影響。

Extend ABSTRACT
The studies of superconducting nanocomposites by measuring Ac susceptibility and Electrical Properties
ZONG-JIE WU
Lieh-Jeng Chang
Department of Physics National Cheng Kung University
SUMMARY
In this thesis, we report detailed studies of superconductivity in two kind nanocomposites (OPAL and porous glass) by measuring resistance and AC susceptibility. The two nanocomposites has large different in voids size. Inside OPAL there are two different voids 50nm and 100nm.The porous glass which we use has 18-20nm porous size. The metal Tin were embedded into this two nanocomposites which formed a three-dimensional metal grain network. The two different grain size sample ,by measuring ac susceptibility amplitude dependence we found they are same linking type .The result of resistance and AC susceptibility are quite similar in the same sample. But this two grain size sample exhibit very different result. In H-T phase diagram a crossover of the upper critical field line HC2(T) with a crossover field HCO was observed in two sample, in this two sample the HCO and HC2(0),also the ratio of HCO / HC2(0) are quite different ,then we calculate activation energy Ua the behavior versus applied magnetic field also quite different , in the end we find the different grain size will effect the vortices behavior

Key word: superconducting-nanocomposites, vortex, resistance, AC susceptibility, activation energy
INTRODUCTION
In many superconductor include many high temperature superconductor, the vortex behavior has been study for so many years, recently the nanotechnology is become very popular and the making of nano-composites is very successes, we combine the superconductor and the nano-composites making a superconducting nanocomposites and study their vortex behavior. Our sample is provided by professor E. V. Charnaya and her group from Faculty of Physics, St. Petersburg State University. The sample is Sn embed into opal and porous glass and we use our ppms (Physical Property Measurement System) and the extend resistance unit and the ACMS (AC susceptibility measurement system ) unit to measure the sample and calculate the activation energy, the porous glass’s porous size is 18-20nm the opal has two different voids size 50nm and 100nm, metal will become a three dimension network. With this three dimension network of metal we observed the different behavior for the H-T diagram which in normal type II superconductor the HC2 with temperature is negative curvature, in our measurement result the H-T diagram’s low magnetic field area is positive curvature with a crossover field HCO in opal is 600Oe and porous glass is 6000Oe which is different with the normal superconductor, in the result of activation energy we observed a fast decay when the field is large then HCO this means the vortex phase transition. In the end we observed with decrease porous size the HC2(0) and the HCO will increase, in porous glass the activation energy shows a vortex transition when the magnetic field large than the HCO.
MATERIALS AND METHODS
Our sample is sn in opal and sn in porous glass, the opal is made by SiO2 sphere which diameter near 250nm with Hexagonal Close Packed (hcp) the voids inside is 50nm and 100nm, porous glass is making by the Borosilicate glasses with acid leaching move out the sodium borate part, remain the silica part, making voids, with control the ratio of silica and sodium borate we can control the void’s size in our experiment the size of voids is 18-20 nm.
In the AC susceptibility measurement we use the ACMS unit making several measurement with the zero DC magnetic field we measure the amplitude dependence and frequency dependence, the aptitude dependence applied a AC magnetic field which the frequency is 500Hz, the range of amplitude is 0.1 Oe to 3 Oe, then we apply a DC magnetic field to measure the frequency dependent the DC magnetic field range for opal is 0 to 1200Oe, amplitude is 1 Oe, frequency is 20Hz to 5000 Hz, in porous glass the DC magnetic field is from 0 Oe to 12000 Oe.
The resistance measurement is in applied DC magnetic field with four point probe measurement, the applied DC current source with pulse time 1 sec. The range of applied DC magnetic field is 0 Oe to 1200 Oe in opal measurement, the range of porous glass is 0 Oe to 22000 Oe.
The result of measurement will plot a H-T phase diagram and calculate activation energy to plot H-Ua (activation energy ) figure.

RESULT AND DISCUSSION
In our measurement the result of AC susceptibility first we check the amplitude dependent find the maximum of the imaginary part of AC susceptibility with the temperature TP, then plot the H-T diagram with TP and amplitude by fitting we find the result shows metal grain is linked by strong coupling Josephson junctions in opal and porous glass, then the frequency dependent part we choice TC which is superconductor onset temperature with correlation magnetic field to plot the H-T diagram , we find the curvature with a negative curvature in low magnetic field and positive curvature in high magnetic field and a crossover field (HCO opal 600Oe, porous glass 6000 Oe)this means a vortex phase transition when field higher than Hco. And we use measurement frequency with Tp to calculate the activation energy it’s also shows a vortex phase transition with field higher than Hco.
In resistance measurement we use the measurement to fitting the onset temperature TC to plot H-T diagram the result is very similar with AC susceptibility the HCO is 600 Oe in opal with 6000 Oe in porous glass. The activation energy of resistance is calculated by slope of phase transition the result is quite same with AC susceptibility also shows a vortex phase transition with field higher than Hco.
CONCLUSION
In our experiment we report a measurement of Sn nano-composites use AC susceptibility and resistance. The result of H-T diagram is shows a different curvature with normal superconductor which imply a vortex phase transition with the curvature change, the key point is HCO a crossover field when magnetic field large then HCO the flux will into the superconductor cause a vortex phase transition, in the activation energy the result shows a rapidly decay with magnetic field large then HCO it means the flux is come over the surface barrier into the superconductor which is the same result as we observed in H-T diagram with this two result we find the structure of nano-composites will affect the vortex behavior.

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