2, North Carolina State University, Raleigh, North Carolina, United States
3, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The discovery of Q-carbon has drawn a lot of attention in the past two years due to its interesting physical properties. Q-carbon is synthesized by rapid quenching (~1010 K/s) of highly undercooled carbon melt and is constituted of ~80% sp3 and ~20% sp2 hybridized carbon. In the present study, we present a correlation of electronic structure of Q-carbon with the ferromagnetism and superconductivity properties. In contrast to the other diamagnetic derivatives of carbon, such as graphite, it is shown that Q-carbon nanostructures exhibit room temperature ferromagnetism with finite coercivity. Using electron energy-loss spectroscopy (EELS), we demonstrate that the C K-edge of Q-carbon consists of a sharp π* peak and a broad σ* peak. On comparing the C K-edge of amorphous Q-carbon with various diamond-like-carbon (DLC) films having a different sp3-sp2 ratio, it is found that π* peak intensity is exceptionally high in spite of having just ~20% sp2 content. This increase in the intensity corresponds to the increased unpaired spin electron density in Q-carbon due to the highly non-equilibrium synthesis route and gives rise to the room temperature ferromagnetism. Q-carbon, due to this dramatic increase in unpaired spin electron density, also exhibits the extraordinary Hall Effect characteristics.
Using EELS, we also demonstrate the correlation between superconductivity and the role of B doping in Q-carbon. We show that the nanosecond laser melting and rapid quenching of C results in strongly bonded unique superconducting phase of B-doped Q-carbon. This results into a type II superconductivity in B-doped Q-carbon with a transition temperature of 36.0±0.5 K. The EELS results show that we can achieve a homogeneously distributed B doping in Q-carbon as high as 17.0±1.0 at% with the employed synthesis process. An essential conduction for superconductivity in B-doped C is that B stays in sp3 hybridized state with carbon. We quantify that ~60% B atoms bond with sp3 hybridized C and contribute in the superconducting state of B-doped Q-carbon. With monochromated low-loss EELS and Raman spectroscopy, we demonstrate a higher electronic density of states near the Fermi energy level, which leads us to achieve remarkably high superconductivity transition temperature in B-doped Q-carbon. With this study, we present an insight on the role of electronic structure in achieving high-temperature superconductivity.
 Bhaumik, A; Sachan, R; Narayan, J. High-Temperature Superconductivity in Boron-Doped Q Carbon. ACS Nano 2017, DOI: 10.1021/acsnano.7b01294.