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Weak current measurement report for superconducting material by lock-in amplifier


Samples of superconducting material applied by a weak alternating voltage can occur a weak current through itself, which can be measured by LIA in order to avoid the measurement error from thermoelectric EMF effect.

For most of superconducting experiment, researchers need put superconducting samples into a ultra-low environment temperature and measures its resistance at that condition. Unfortunately, for superconducting material, a weak current applied on the material can generate a lot of Joule heat which will rise the measurement temperature and cause a big result error. Thus, the applied current should be very weak and the voltage will be very low because of low resistance of superconducting material. And if the applied current is alternating, it can erase the thermoelectric EMF effect. 

One of the most important structure of superconducting material is Josephson junction, which consists of two superconductors coupled by a weak link, which can consist of a thin insulating barrier (known as a superconductor-insulator-superconductors junction, or S-I-S). Fig.1 shows the current-voltage characteristics of Josephson junction. 

Fig.1 I/V characteristics of Josephson junction

The measurement platform shows in Fig.2. An AC current with a DC current bias applies to the sample while LIA measures the AC voltage between the sample.

Superconducting Quantum Interference Device(SQUID) sensor consists of two Josephson junctions as Fig.3. The I/V characteristics of SQUID sensor is like Fig.4 where I is highly sensitive to the magnetic field in the SQUID ring, which is used to measure the changes of the magnetic field of human brain or the earth.

 

Fig.2 Measurement platform diagram

 

Fig.3 The structure of SQUID sensor

Fig.4 I/V characteristics of SQUID sensor where I changes with different magnetic fields in it. 


Based on the perfect diamagnetism of superconductor, which is called Meissner effect, magnetic field line can not go through the inside of superconductor. The typical method to confirm the Meissner state is to make a mutual induction platform as Fig.5.

The platform consists of two primary coils and two secondary coils in opposite direction. When there is no superconductor, the signals in two secondary coils are the same amplitude and a different direction so that no signal inputs LIA. When there is a superconductor, this equilibrium state breaks down and a signal will input LIA. But when the perfect diamagnetism takes effect, the equilibrium state will be back again.

Fig.5 Mutual induction platform to confirm Meissner effect.


This phenomenon can confirm the Meissner effect of superconductors. And LIA can catch this small change of current.