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Ritesh Sachan1 2 Jordan Hachtel3 Anagh Bhaumik2 Siddarth Gupta2 Juan Carlos Idrobo3 Jagdish Narayan2

1, Army Research Office, Research Triangle Park, North Carolina, United States
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.[1] 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.
[1] Bhaumik, A; Sachan, R; Narayan, J. High-Temperature Superconductivity in Boron-Doped Q Carbon. ACS Nano 2017, DOI: 10.1021/acsnano.7b01294.

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