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

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