Superconductivity induced by a magnetic field

Exchange-correlation effects in magnetic-field-induced superconductivity

“…This allows the JPE to take place at fields above the value of H c2 expected in the absence of magnetic ions. [7][8][9]13 Again, we see that the magnetic atoms suppress both, paramagnetic (Pauli) and diamagnetic (Meissner) pair breaking, and again the orbital cancellation is not simply due to their exchange interaction with the conduction electron orbits.…”

“…The JPE has very recently been observed in the organic conductor λ − (BET S) 2 F eCl 4 , for magnetic fields ranging from 18 to 41 T. 5,6 Before that, it had already been seen in pseudoternary chalcogenides of the form Sn x Eu y M o z S 8 , from 4 − 23 T, [7][8][9][10] and there is strong experimental evidence that the JPE is also at work in chalcogenides of the form P b x Eu y M o z S 8 (at up to 70 T), 10,11 in transition-metal systems (M o 1−x M n x Ga 4 at up to 7.6 T), 12,13 in heavy-fermion superconductors (CeP b 3 at up to 15 T 14 and CeCu 2 Si 2 at about 2 T 15,16 ), and perhaps even in high-T c cuprates (Gd 1−x P r x Ba 2 Cu 3 O 7−δ , near 9 T). 17 It is crucially important to recognize that in the standard JPE scenario 2,4 it is only the action of the external magnetic field on the electron spins that is compensated by the internal (exchange) field, not that on the currents.…”

“…By integrating (3) with (9) one obtains (8), up to a physically irrelevant gauge term. Although (9) can be shown to be an identity under certain conditions, 26,27 all one needs for the present purpose is that it holds to within an accuracy δ, given by the magnitude of the imperfection in the cancellations (3) and ( 8).…”

“…Some consequences of Eq. (8): First, note that the details of the system do not enter the arguments leading to (8) and (9), which are rather general and not tied to particular features of the system's electronic structure. This generality may explain why the JPE could be observed in the physically very different systems listed above.…”

“…2,4 Third, the mechanism for spin compensation is located at the magnetic ions, whose exchange-correlation magnetic field cancels the external one. Relation (9) implies that the origin for compensation of the induced currents must be located at the same place. The search for a physical explanation of current compensation can thus concentrate on the magnetic ions and, in particular, their correlations.…”