8/23/02 clc

 

SQUID

 

            A SQUID is a Superconducting Quantum Interference Device. The SQUID used in this experiment is a commercial one made by STAR Cryoelectronics. It is made of a high Tc material (YBCO) which goes superconducting at 90 K, above the liquid nitrogen temperature of 77 K. Thus one can easily make it superconducting.  In this experiment you will observe how a SQUID works. You will measure the VI characteristic curve for the device, and measure how the V across the device varies with the magnetic flux through it. You will see that this V oscillates with the flux, with a period equal to the quantum of flux h/2e, the “fluxon”. This is an extremely small quantity of flux, which makes the SQUID a very sensitive probe of magnetic flux. Many of the applications of the device are based on this behavior. You will also measure the resistance of the SQUID as a function of temperature in order to observe the transition from a normal conductor to a superconductor.

 

Your apparatus includes:

 

 

Some references: Principles of Superconductive Devices and Circuits, Van Duzer and Turner, QC 612 .S8 V36

 

You will be working with liquid nitrogen in your dewar. Be very careful not to spill it on you- it can cause severe burns. Please ask your instructor to provide you with a filled dewar, which should easily last you for the whole laboratory period.

 

The physics explanation and detailed explanation of specific experiments is given in the Mr. SQUID manual. You should be sure to read the following sections:

Introduction , p1-4. Read this before you begin.

Getting started…, pp 10-18. This is quite operational. Use it as an instruction manual as you perform the experiments. Note that some physical properties of the SQUID which might be useful are given on  pp 19-20, even though you are not asked to do the measurement of section 3.8.

An introduction to superconductivity and squids, pp. 27-35. You should read through this before starting, then read it more seriously as the experiment progresses.

 

You are of course to read other sections as well, but the above ones are required reading.

 

Do the following experiments:

  1. Measure the VI curve for the superconducting SQUID. Do this with the internal m metal shield in place. Calculate the critical current. Explain in your writeup what you are seeing.
  2. Measure the Vf curve for the SQUID under the same conditions. Explain in your writeup exactly why you are seeing what you see.
  3. Warm up the SQUID and remove the m metal internal shield. Repeat the above measurements using the external m metal shield and the external coil to zero out the earth’s field. Be sure to have the external shield and the coil in place and adjusted before you cool down the probe again.  The m metal shield also cancels the earth’s field, so you are canceling it twice. However, the SQUID is so sensitive that it can still see outside influences even through the m metal shield. Vary the field outside the shield, using the external coil, and calculate how much the field changes at the site of the SQUID for a change in external field of about 0.3 Gauss (roughly the earth’s field). Do some experiments to show yourselves how sensitive the SQUID is to changes in the magnetic environment.
  4. Measure the resistance of the SQUID as a function of temperature. This experiment is described in section 7.1 of the Mr. SQUID manual, pp 55-61.
  5. Discuss with your instructor other things you can do with the device.