### Re: Can a magnet ever touch the surface of a perfect superconductor?

Date: Fri Mar 19 08:34:12 2004
Posted By: Guy Beadie, Staff, Optical sciences, Naval Research Lab
Area of science: Physics
ID: 1078595057.Ph
Message:
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Hello, Craig.

The type of question you're asking is extremely valuable to the pursuit
of all sciences: what happens when we push a current idea to the extreme?
Most ideas in science are applicable only if certain conditions are met,
and trying to find the boundaries of these conditions is an important part
of discovery.

In your particular case, the question can be restated: what are the
boundaries of the Meissner Effect?

It turns out that superconductivity has a phase diagram, much like water
does.  [I'll explain what a phase diagram is in just a minute.]  For a
superconductor, applying too much magnetic field will eventually ruin the
effect that causes superconductivity, thus destroying the Meissner
Effect.  So, as you bring a magnet closer and closer to a superconductor,
eventually the magnetic field will be strong enough to destroy the
superconductivity and wipe out the repulsive force.

That's really the answer.  To understand it fully, we would want to do
two things: 1) describe what we mean by a phase diagram, and 2) explain
why the phase diagram looks like it does.  The first item isn't difficult
to explain, so I'll go ahead and do that here.  If you're interested in
why a superconductor phase diagram looks the way it does, we'll have to
save that for a later question (which is another way of saying that I
don't know :) )

A phase diagram outlines the boundaries of specific phases that a
material can achieve.  Water, for example, can exist as in solid, liquid,
or gas phases.  Under what conditions in temperature and pressure will
water take a particular form?  That's what a phase diagram will tell us.

We know from everyday experience that water will boil into a gas if it
gets too hot, and will freeze into ice if it gets too cold.  At a normal,
everyday atmospheric pressure, water freezes at about 0 degrees C and
boils at about 100 degrees C.

This changes, however, if you change the pressure of the air around
water.  Up in the mountains, where the atmospheric pressure is lower,
water boils at a much lower temperature - this actually makes cooking
difficult for mountain climbers.

A phase diagram outlines careful measurements which outline as closely
as possible the actual boundaries of the phases.  It will track the change
in the temperature at which water boils (and freezes) as a function of
pressure.

This is important to your question because superconductivity is another
phase of matter for some materials.  There are even different phases
within superconductivity!  Therefore, there are boundaries beyond which
the same material will stop being superconductors.  Temperature is an
important parameter for superconductors, just like for water.

It turns out that magnetic fields are important, too.  Just as changing
the pressure around water will change boiling points, changing the
magnetic field changes the maximum temperature at which a superconductor
will stay a superconductor.  For each temperature, there is a "critical
magnetic field" beyond which a material cannot stay superconducting.  The
colder the superconductor is, the higher the critical field, but even if
you cool the superconductor to near zero degrees, there will still be a
field strength that can stop the superconductivity.

Good job, and keep asking questions!

- Guy

PS Turns out there aren't that many easily-found examples of
superconductor phase diagrams.  Here's one:
http://www-
unix.mcs.anl.gov/superconductivity/phase.html

For an introduction to superconductors, you can look at:
http://superconductors.org/

or
http://www.ame
ricanmagnetics.com/tutorial/basics.html

```

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