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for superconductors we have a phase transition diagram. according to that phase diagram in a temperature below Tc, we can only increase the applied magnetic field to make a superconductor a normal. but when i ask it from a prof. he said that we can not do that just by increasing the applied magnetic field because the applied magnetic field does not increase the velocity and so the copper pairs can not be broken. so is his reasoning right or wrong?

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    $\begingroup$ You can definitely kill the superconductivity by applying magnetic field. Your professor's reasoning does not make any sense at all. $\endgroup$ – Meng Cheng May 10 '15 at 6:32
  • $\begingroup$ forgot about him as being a prof. he says that the change from superconductor to normal phase is due to the difference in the kinetic energy of the cooper pairs. so as the magnetic field can not increase the velocity, it can not (alone) break the superconductivity... $\endgroup$ – P.A.M May 10 '15 at 7:48
  • $\begingroup$ you know, the question here come is that why should increasing the magnetic field increase the kinetic energy of the super electrons? we say that magnetic field can not do work. and just changes the direction of the charged particle. but here we see that increasing the magnetic field increases the kinetic energy, as far as i know. $\endgroup$ – P.A.M May 10 '15 at 7:54
  • $\begingroup$ Comments deleted here and on an answer below. Keep it impersonal and focussed on the physics, folks. $\endgroup$ – dmckee --- ex-moderator kitten May 10 '15 at 20:16
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Superconductors have both a critical temperature, at which they transition to the normal phase, and a critical applied magnetic field value. Once the applied magnetic field is at the critical value, a transition to normal occurs, regardless of the fact that the superconductor is below its critical temperature.

The critical value of the applied magnetic field varies with temperature, from a maximum at absolute zero temperature, to lower values as the critical temperature for superconductivity is approached.

If an applied magnetic field is above the critical value, the Meissner effect no longer functions to exclude the magnetic field from the superconductor. A magnetic field stronger than the critical value induces a high electric current that breaks superconductivity. The energy of electron interaction which forms Cooper pairs is quite weak, and can be broken either by high temperature or by a strong magnetic field.

See this summary: http://hyperphysics.phy-astr.gsu.edu/hbase/solids/scbc.html.

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  • $\begingroup$ you know, the question here come is that why should increasing the magnetic field increase the kinetic energy of the super electrons? we say that magnetic field can not do work. and just changes the direction of the charged particle. but here we see that increasing the magnetic field increases the kinetic energy, as far as i know. $\endgroup$ – P.A.M May 10 '15 at 7:53
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    $\begingroup$ @P.A.M. While it's true that a magnetic field can not change a particle's kinetic energy, a magnetic field can change the direction of the particle's velocity by exerting a perpendicular force. This accelerates the particle without changing its speed. As the Cooper pairs have a very small binding energy, a strong enough applied magnetic field will exert enough torque on the electron spins to break up the pairs. Here's an explanation: socratic.org/questions/… $\endgroup$ – Ernie May 10 '15 at 13:12
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    $\begingroup$ @P.A.M. Scroll down to the bottom of this link, and see "Spin Alignment vs Electron Pairs": hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html $\endgroup$ – Ernie May 10 '15 at 13:14
  • $\begingroup$ although your answer did not explain the problem but it helped a lot. thanks. :) $\endgroup$ – P.A.M May 16 '15 at 5:11

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