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You are right, electrons in vacuum can carry a current without resistivity. However, superconductivity is not only zero resistance, but something more. Superconductivity is defined by zero resistivity and by the presence of the Meissner effect, i.e., the expulsion of magnetic fields from the system. The zero resistivity of electrons in vacuum is usually ...

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EDIT: My previous answer was wrong, I think. The theory is explained here. It seems like in a perfectly symmetric system there is no a priori way to say in which direction the current will flow. Instead it is some kind of symmetry breaking. So think of it as "turning the voltage down to zero" (i.e. $V\to0^+$), but observing that a current remains. EDIT2: ...

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I would disagree here. The coherence length is the length scale where the electrons stay in their coherent, superconducting state. This gets important on boundaries of a superconductor (i.e. the proximity effect) or at vortices of a type II superconductor in the mixed phase. In both examples, you can measure the coherence length. This is done by fitting the ...

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A few less technical details than the ones you can find in Meng Cheng answer on this page are perhaps welcome. For practical applications, we are not interested in Majorana modes, since they are a simple mathematical rewriting of the fermionic creation and annihilation operators. Say differently, to any creation $c^{\dagger}$ and annihilation $c$ operators ...

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You should forget about the name "Majorana fermions", although people have been using it (unfortunately) a lot in the literature on topological superconductivity. A much better terminology is "Majorana zero modes". "Majorana fermions" is more appropriate when you have propagating modes like chiral edge modes of a 2D topological superconductor, but not for ...

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Consider a metal sheet placed in a uniform electrical field normal to the sheet surface. The electrons will be "dragged" by the electrical field and form an excess of negative charges on one side of the metal sheet and an excess of positive charges on the other side of the metal sheet. The excess charges produce an electrical field that cancels the applied ...

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Current going through a superconductor (or otherwise) will form a magnetic field. The potential energy of the magnetic field depends on its size, and the permeability of its surrounding environment. The current will be divided between the two superconductors such that the total magnetic field energy is minimal. Actually, this effect is observable using ...

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