我們利用自製的掃描式超導量子干涉顯微鏡系統(SSM)來研究量子磁通渦旋在鈮與鐵硒碲之第二類超導薄膜中的行為。SSM是一個在探測微小磁場與磁通分布方面非常強大的工具，它被廣大的運用在各個方面，例如描繪第二類磁通渦旋分布、銅基超導對對稱性的驗證及其他…。我們所使用的掃描晶片基本上擁有磁通雜訊約1-3 ´ 10-5 Φ0/ÖHz，而收集磁力線的集磁迴圈直徑約10微米，因此空間解析度大約為5微米；最大掃描範圍為1毫米乘1毫米。本以為我們的系統是無法取得足夠關於滲透深度的量測，但我們發現最大的磁通渦旋值與集磁迴圈的距離有一定的關係；利用此方法，我們得到150奈米厚的鐵硒碲薄膜的等效滲透深度約為0.88微米，此為相似成份單晶樣品的兩倍左右，這個差異大致上是來自於我們的薄膜樣品有較短的平均自由徑。
雖然SSM的反應時間較長，但我們利用重複干擾然後量測的方式使SSM仍可以做到動態磁通渦旋的研究。我們利用三角脈衝電流驅動在鈮膜上的磁通渦旋，我們可以藉由此方式分辨何處為釘札較強的區域，此區域內的磁通漩渦較難以三角脈衝電流驅動直到有夠大的電流強度；而隨後即出現的磁通渦旋聚集現象，會聚集在特定位置，極性會隨電流方向改變而變化，這種令人驚訝的發現需要有更深入的研究才能推論出形成的原因。

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.

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