沒有用清晰的語言寫，節(jié)約時(shí)間，以后的專欄也都以速記為主。

一般測量得到的超導(dǎo)零場冷和場冷信號(hào)都長這樣，下面那根線是零場冷的，上面那根是場冷。

進(jìn)入超導(dǎo)態(tài)后，再加場（ZFC, Zero Field Cooling)；和在正常態(tài)下加場，再進(jìn)入超導(dǎo)（FC,?Field Cooling)。這兩種的抗磁信號(hào)不一致，說明樣品不完全是超導(dǎo)體，而是有雜質(zhì)的。

要注意這里加的外場，小于超導(dǎo)臨界磁場，對(duì)于typeII的超導(dǎo)體，小于下臨界場，所以超導(dǎo)體如果進(jìn)入超導(dǎo)態(tài)的話，一定是邁斯納態(tài)，而不是混合態(tài)。

看下面隨手畫的簡圖，F(xiàn)C信號(hào)是覆蓋SC composition2的，但是轉(zhuǎn)變溫度卻只反映SC而屏蔽了SC composition2。

屏蔽百分比（Shielding fraction）-->零場冷（ZFC）

邁斯納百分比（Meissner fraction）-->場冷（FC）

Question

How do we measure Miessner fraction and Shielding fraction for superconductors?

I have attached a schematic can anyone please tell me if I am right?

一些回答如下：

All Answers (4)

Grover Lamar Larkins

Florida International University

Dinesh,

What you are describing is a magnetization measurement result. The demagnetizing factor depends upon the shape of the sample in a strong manner. I suggest that you do the measurement on a reproducible and controlled shape of sample and then measure the demagnetization of the unknown and compare it with that of a known superconductor taken well below Tc using the same sample shape and apparatus.

This will give you a better feel than attempting to do things in this manner as ZFC and FC are subject to the consideration of just how far from Hc1 you are and is the material a type II or type I material (I include weakly linked heterogenous grain boundary connected materials as type II, this is not strictly correct but behaviorly similar enough).

One other concern is the sample size -- it must be significantly larger than the penetration depth.....

Hope this helps,

Grover Larkins

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O. F. de Lima

University of Campinas

Dear Dinesh,

I agree with the observations of G. Larkins, about the relevance of demagnetizing factor, measuring down to temperatures well below Tc and having sample size much larger then the penetration depth.? However, a good estimate of Meissner fraction can also be made using the usual type of magnetization measurements (ZFC and FC), paying attention to the following: ?1) Use a smallest possible value of applied field (e.g. H ~ 5 or 10 Oe), this will ensure staying below Hc1 at? T << Tc;? 2) Take measurements down to temperatures well below Tc, ensuring that a full Meissner state is reached. A good indication for that is to get a practically horizontal line of M vs. T in this region. The ordinate value of this line can be associated with 100% shielding for ZFC measurement and with the Meissner fraction for FC measurement (as shown in your sketch);? 3) To minimize errors due to demagnetizing factor try to use samples having their length parallel to H at least three times larger than to the other transversal dimensions.

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Grover Lamar Larkins

Florida International University

Dear Dinesh,

O.F. de Lima is correct, I would only add that the sample thickness must be at least 10 times the penetration depth.

Of course, if the sample is unknown, the FC/ZFC method is of limited utility as the penetration depth and, whether or not, it is a heterogenously connected set of weak links is also unknown as is Hc1. For very thin samples (small in any dimension in relationship with the penetration depth IN THAT DIMENSION's direction)? Hc1 becomes depressed and may actually go to zero, despite the field strength used to measure it. That is always worth noting when dealing with very small sample volumes with uncertain (and for the High Tc materials, LARGE) penetration depths. The penetration depth for Nb is?39nm and for YBCO it is 150nm in the ab plane and 800nm in the c direction. Note the difference; if your sample size was nbased on the Nb penetration depth you would be totally out of luck using that assumption in a mixed YBCO impure sample to get the SC volume.....

All the best,

Grover

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Jean Louis Tholence

French National Centre for Scientific Research

The Meissner effect is obtained by coolig in a dc field as small as possible.

It is 100% for very good superconductors (type 1 without defect, or type 2 without pinning). In fact, the field can be trapped by defects during the cooling, which results into a reduced meissner effect.

The shielding effect,is measured after zero field cooling, and applying a very small field (<Hc1). It's larger (or equal) to the Meissner effect, since the ?shielding currents are the most effective. In a ceramics of HTSC, the shielding effect can be 100% (in very small ac fields) and the Meissner effect 0 (all the flux is trapped during cooling)!