# Determining Tc value of various superconductors

• leright
In summary, the conversation discusses the process of determining the critical temperature (Tc) values for two superconductors by plotting resistance vs. thermocouple voltage. It also addresses the difficulty in accurately determining the Tc value due to the lack of a well-defined "knee" in the plot. Possible methods for determining Tc and the associated uncertainties are mentioned, such as choosing the location where resistivity flattens out or using a combination of resistivity and susceptibility measurements. The conversation also touches on the differences between superconductors of Type I and Type II and the importance of measuring the drop in susceptibility in determining superconductivity. Finally, the use of SQUID (Superconducting Quantum Interference Device) for measuring magnetic susceptibility is
leright
I am plotting resistance vs. thermocouple voltage (which is to be used to determine the temperature from a conversion chart) for two superconductors. I need to determine where to take the Tc value. This is difficult to determine since the "knee" is not well defined. Also, how should I guess my uncertainty in my Tc value? Are there typical criteria that physicists go by when doing these measurements?

Thanks.

leright said:
I am plotting resistance vs. thermocouple voltage (which is to be used to determine the temperature from a conversion chart) for two superconductors. I need to determine where to take the Tc value. This is difficult to determine since the "knee" is not well defined. Also, how should I guess my uncertainty in my Tc value? Are there typical criteria that physicists go by when doing these measurements?

Thanks.

It depends on how "accurate" you want it. What most people do is to show such a curve and then indicate where they would consider to be the value of Tc. Usually such a thing varies by, at most a +/- 5K.

If you want to be conservative, then pick the location where the resistivity flattens out at zero at the bottom of the inflection. Or you can pick the middle of the inflection and write it down with an uncertainty equal to the spread of the transition.

As long as you make it clear how you picked the value of Tc, it shouldn't be that big of an issue where you chose it. Of course, if you want to have a more accurate determination, you should accompany the resistivity measurement with the susceptibility measurement. A value consistent with both measurements is looked upon with a greater degree of certainty than simply from one.

Zz.

Do other methods to measure critical temperature exist? If yes, does the thermal dependence of resistence plot remain the most accurate one? However, is the measurement accuracy (whatever method used) sensitive to superconductor's kind (soft or hard), or what really matter is only the specific material?

armandowww said:
Do other methods to measure critical temperature exist? If yes, does the thermal dependence of resistence plot remain the most accurate one? However, is the measurement accuracy (whatever method used) sensitive to superconductor's kind (soft or hard), or what really matter is only the specific material?

Er... say what?

What is "thermal dependence of resistance plot"?

For most cases, the susceptibility measurement is considered to be the "slam-bam" evidence of superconductivity, with resistivity measurement a close second. Other supporting evidence would come from tunneling measurement that indicates for formation of an energy gap to the spectral-weight pile-up in optical conductivity measurement.

Zz.

With the thermal dependence of resistance plot I just mean the function R(Tc)... ;-) Thank you.
But in superconductors of II kind, experimentally speaking, does Hc1 give such a relevant corresponding point on the magnetization chart?

armandowww said:
With the thermal dependence of resistance plot I just mean the function R(Tc)... ;-) Thank you.

That's temperature dependence of the Resistivity, i.e. R(T), not R(Tc). That is what I said earlier about the resistivity measurement.

But in superconductors of II kind, experimentally speaking, does Hc1 give such a relevant corresponding point on the magnetization chart?

Yes it does. The drop in susceptibility is present no matter if it is Type I or Type II. The Meissner effect kicks in even if there are flux penetration.

Zz.

You misunderstand me about "thermal"... What about your "kicks in"? I thought the size effects may affect superconductors of Type II. How about the susceptibility measurement technique?
Thanks

armandowww said:
You misunderstand me about "thermal"... What about your "kicks in"? I thought the size effects may affect superconductors of Type II. How about the susceptibility measurement technique?
Thanks

I have no idea what you just asked here. Are you asking what is a susceptibility measurement? Or are you not understanding how the drop in susceptibility occurs?

Zz.

The diamagnetic behaviour of the sample below Tc makes the internal field vanishing. How CAN this effect be measured? So, if you prefeer, what is a susceptibility measurement? Thank you

armandowww said:
The diamagnetic behaviour of the sample below Tc makes the internal field vanishing. How CAN this effect be measured? So, if you prefeer, what is a susceptibility measurement? Thank you

You attach a SQUID to the sample you're measuring, and stick it into a weak external field. Then you measure the susceptibility from the SQUID as you lower the temp. That's the standard technique of measuring any magnetic susceptibility.

Zz.

What is the most appropriate, dc-SQUID or rf-SQUID?

armandowww said:
What is the most appropriate, dc-SQUID or rf-SQUID?

Depends on what you mean by "appropriate".

A DC SQUID (or any type of pickup coil, really), is easier to set up and measure. Often, this is the type of "good enough" results that most people want. And it is quick.

An AC SQUID requires more work, more setup, and a more accurate measurement procedure for there to be any benefit to the technique.

Zz.

## 1. What is the Tc value in superconductors?

The Tc value, or critical temperature, is the temperature at which a material transitions from a normal, resistive state to a superconducting state. It is a characteristic property of superconductors and varies depending on the material.

## 2. How is the Tc value determined in superconductors?

The Tc value can be determined through various experimental techniques, such as electrical resistivity measurements, magnetic susceptibility measurements, and specific heat measurements. These techniques involve subjecting the material to different temperatures and observing its behavior.

## 3. What factors affect the Tc value in superconductors?

The Tc value is influenced by several factors, including the chemical composition and structure of the material, external pressure, and magnetic field strength. The presence of impurities or defects can also affect the Tc value.

## 4. Why is determining the Tc value important in superconductors?

The Tc value is a crucial parameter in understanding and utilizing superconductors. It allows scientists to identify and study materials with potential applications in various fields, such as energy transmission, medical imaging, and quantum computing.

## 5. How has the determination of Tc values in superconductors evolved over time?

The study of superconductivity and the determination of Tc values have advanced significantly since the discovery of superconductors in the early 20th century. With the development of new experimental techniques and advanced materials, scientists have been able to achieve higher Tc values and explore the potential applications of superconductors further.

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