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A superconductor in a magnetic field has surface currents that prevent the magnetic field from penetrating its interior. These currents carry kinetic energy and mechanical momentum. When the temperature is raised and the system becomes normal the currents disappear. Where do the kinetic energy and mechanical momentum of the currents go, and how? Here we propose that the answer to this question reveals a key necessary condition for materials to be superconductors, that is not part of conventional BCS-London theory: superconducting materials need to have hole carriers. [ABSTRACT FROM AUTHOR]

Superconductors have often been described as “giant atoms”. The simplest description of atoms that heralded their quantum understanding was proposed by Bohr in 1913. The Bohr atom starts from some simple assumptions and deduces that the angular momentum of the electron in Bohr orbits is quantized in integer units of . This remarkable result, which does not appear to be implicit in the assumptions of the model, can be interpreted as a “theoretical proof” of the model's validity to describe physical reality at some level. Similarly we point out here that from some simple assumptions it can be deduced that electrons in superconductors reside in mesoscopic orbits with orbital angular momentum . This implies that both in superconductors and in ferromagnets the long-range order results from elementary units of identical angular momentum. Similarly to the case of the Bohr atom we propose that this remarkable result is compelling evidence that this physics, which is not part of conventional BCS theory, describes physical reality at some level and heralds a qualitatively new understanding of superconductors. [ABSTRACT FROM AUTHOR]

It is generally believed that superconductivity can occur in materials irrespective of whether the charge carriers in the material are electrons or holes. Here we point out that Josephson tunneling would not occur between superconductors with charge carriers of opposite sign. Consequently, observation of Josephson tunneling between two superconductors implies that their charge carriers have the same sign. We propose that this has profound implications for the understanding of superconductivity and in particular is consistent with the theory of hole superconductivity. [ABSTRACT FROM AUTHOR]