博士 / 國立清華大學 / 物理系 / 103 / We have used a home-made Scanning SQUID Microscope (SSM) to study vortices in type II superconducting thin films with primary focus on Nb and FeSe0.3Te0.7. SSM is a powerful tool to detect small magnetic field and flux distributions. The SQUID chip used in our SSM system typically has a loaded flux noise floor of 1-3 ´ 10-5 Φ0/ÖHz with a pickup loop close to a circular loop of 10 mm in diameter, situated inside a low-temperature m-metal cup. Thus the spatial resolution of SSM is limited to half of the loop size, i.e. 5 mm. The maximum scan range is about 1 mm ´ 1 mm controlled by an X-Y stage with precision stepping motors. Mapping a single vortex in superconducting film with our SSM does not reveal any useful information about the London penetration depth l, because it is typically much smaller than our spatial resolution. However, we have found out that the measured vortex peak flux value can be used to deduce the penetration depth provided the pickup loop to the film distance is known. Using this method, we have determined the effective penetration depth for our 150 nm thick FeSe0.3Te0.7 film to be about 0.88 mm, which is about twice the reported valued for single crystal samples of similar compositions. This is hardly surprising since our film has much shorter carrier mean free path than single crystals.
We have also studied the vortex dynamics with the SSM, even though the response time of the SSM is longer than the time required for studying the vortex dynamics in real time. It is made possible by using the repeated perturbing-then-watching method. We used pulsed ramp currents to drive the vortices in a Nb film patterned as a meander line. With this method we can clearly distinguish the strong pinning sites from the weaker ones. We have also found that the strong pinned vortices are hardly moved by increasing the amplitude of the pulsed ramp currents until a critical value is reached. Then all strongly pinned individual vortices disappear, and instead, several bundled vortices of opposite polarity appear at certain locations. Further increasing the amplitude of the pulsed ramp current does not alter the pattern any more. Applying a reversed pulsed ramp current of the critical value, the bundled vortices are dispersed into individual ones. More such pulses will result in bundled vortices with opposite polarities comparing to the original one. This surprising observation requires further studies to reveal its cause.
| Identifer | oai:union.ndltd.org:TW/103NTHU5198046 |
| Date | January 2015 |
| Creators | Lin, Huiting, 林慧婷 |
| Contributors | Chi, Cheng Chung, Chen, Jeng Chung, 齊正中, 陳正中 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 83 |
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