Structure–property correlations in phase-pure B-doped Q-carbon high-temperature superconductor with a record Tc = 55 K

Abstract

Here, we report the detailed structure–property correlations in phase-pure B-doped Q-carbon high-temperature superconductor having a superconducting transition temperature (T_c) of 55 K. This superconducting phase is a result of nanosecond laser melting and subsequent quenching of a highly super undercooled state of molten B-doped C. The temperature-dependent resistivity in different magnetic fields and magnetic susceptibility measurements indicate a type-II Bardeen–Cooper–Schrieffer superconductivity in B-doped Q-carbon thin films. The magnetic measurements indicate that the upper and lower critical fields follow H_c2(0)[1 − (T/T_c)^1.77] and H_c1(0)[1 − (T/T_c)^1.19] temperature dependence, respectively. The structure–property characterization of B-doped Q-carbon indicates a high density of electronic states near the Fermi-level and large electron–phonon coupling. These factors are responsible for s-wave bulk type superconductivity with enhanced T_c in B-doped Q-carbon. The time-dependent magnetic moment measurements indicate that B-doped Q-carbon thin films follow the Anderson–Kim logarithmic decay model having high values of pinning potential at low temperatures. The crossover from the two-dimensional to the three-dimensional nature of Cooper pair transport at T/T_c = 1.02 also indicates a high value of electron–phonon coupling which is also calculated using the McMillan formula. The superconducting region in B-doped Q-carbon is enclosed by T_c = 55.0 K, J_c = 5.0 × 10⁸ A cm⁻², and H_c2 = 9.75 T superconducting parameters. The high values of critical current density and pinning potential also indicate that B-doped Q-carbon can be used for persistent mode of operation in MRI and NMR applications. The Cooper pairs which are responsible for the high-temperature superconductivity are formed when B exists in the sp³ sites of C. The electron energy loss spectroscopy and Raman spectroscopy indicate a 75% sp³ bonded C and 70% sp³ bonded B in the superconducting phase of B-doped Q-carbon which has 27 at% B and rest C. The dimensional fluctuation and magnetic relaxation measurements in B-doped Q-carbon indicate its practical applications in frictionless motors and high-speed electronics. This discovery of high-temperature superconductivity in strongly-bonded and light-weight materials using non-equilibrium synthesis will provide the pathway to achieve room-temperature superconductivity.