Unconventional giant "magnetoresistance" in bosonic semiconducting diamond nanorings.

The emergence of superconductivity in doped insulators such as cuprates and pnictides coincides with their doping-driven insulator-metal transitions. Above the critical doping threshold, a metallic state sets in at high temperatures, while superconductivity sets in at low temperatures. A hitherto unanswered question is whether the formation of Cooper pairs in a well-established metal will inevitably transform the host material into a superconductor, as manifested by a resistance drop. Here, this question is addressed by investigating the electrical transport in nanoscale rings (full loops) and half loops manufactured from heavily boron-doped diamond. It is shown that in contrast to the half-loops exhibiting a metal-superconductor transition, the diamond nanorings demonstrate a sharp resistance increase up to 430% and a giant negative "magnetoresistance" below the superconducting transition temperature of the starting material. The finding of the unconventional giant negative "magnetoresistance", as distinct from existing categories of magnetoresistance, i.e., the conventional giant magnetoresistance in magnetic multilayers, the colossal magnetoresistance in perovskites, and the geometric magnetoresistance in semiconductor-metal hybrids, reveals the transformation of the diamond nanorings from metals to bosonic semiconductors upon the formation of Cooper pairs. Diamond nanorings like these could be used to manipulate the motion of Cooper pairs in superconducting quantum devices. This article is protected by copyright. All rights reserved.