Order in Disorder—Magnetoresistance Anisotropy of Superconducting Boron Doped Diamond

Two decades after its discovery, superconductivity in heavily boron-doped diamond (HBDD) presents unresolved fundamental questions regarding its origins and properties [<b>1</b>]. Superconducting HBDD is also of potential interest for heterogeneous quantum technologies that exploit the transduction of quantum information between Josephson junction-based qubits and spin-based quantum defect qubits. We report electrical magnetotransport measurements of homoepitaxial HBDD films in the transition regime from the normal to the superconducting state. Although these single-crystal films bear the hallmarks of inhomogeneous granular superconductivity, the dependence of electrical resistivity on temperature (T), magnetic field vector (<b>H</b>), and current density (<b>J</b>) reveals a surprising anisotropy, accompanied by the emergence of a spontaneous transverse voltage below T_c, onset at H=0, known as the 'Hall anomaly’ , previously observed in other quasi-2D [<b>2</b>] and high T_c superconductors [<b>3</b>]. This transport anisotropy further shows three phases with distinct symmetries depending on the relative position in T-H phase space, which is closely followed by the temperature dependence of Hall anomaly. Similar exotic transport behavior has been reported in polycrystalline HBDD that can be modeled by a disordered array of Josephson junctions [<b>4</b>]. However, based on transmission electron microscopy and atomic force microscopy, our single crystal films lack any obvious structural disorder. This indicates that the hidden anisotropic order observed in disordered superconducting diamond might be related to the symmetry of the order parameter itself. Understanding the source of the magnetoresistance anisotropy and Hall anomaly in homoepitaxial HBDD can give us insights into the origins of its superconductivity and may help achieve higher T_c beyond the BCS limit as theoretically predicted [<b>5</b>].<br/><br/>*Supported by the U.S. Department of Energy Office of Science National Quantum Information Science Research Centers (Q-NEXT), the University of Chicago, and Penn State Materials Research Institute.<br/><br/>1. Wakita, T. <i>et al.</i> Physics of heavily doped diamond: Electronic states and superconductivity. <i>Physics and Chemistry of Carbon-Based Materials: Basics and Applications</i> :65-96 (2019)<br/>2. Segal, A., <i>et al</i>. "Inhomogeneity and transverse voltage in superconductors." <i>Phys. Rev. B</i> <b>83.9</b>: 094531 (2011).<br/>3. Vašek, P. "Transverse voltage in high-Tc superconductors in zero magnetic fields." <i>Physica C: Supercond. & its app.</i> <b>364</b>: 194-196 (2001).<br/>4. Zhang, Gufei, <i>et al</i>. "Global and local superconductivity in boron-doped granular diamond." <i>Adv. Mater.</i> <b>26.13</b>: 2034-2040 (2014).<br/>5. Shirakawa, Tomonori,<i> et al</i>. "Theoretical study on superconductivity in boron-doped diamond." J. Phys. Soc. Jpn. <b>76.1</b>: 014711 (2007).