Manipulation of Spin-States in All-Organic Di-Radicals Molecular Junctions

Graphene is believed to be an optimal material for a promising new generation of nanoscale devices using spin as information carrier, storage and processing [<b>1</b>]. During the last years, atomically precise synthesis of poly-aromatic hydrocarbons (PAH) has been developed [<b>2</b>-<b>3</b>] allowing new possibilities to engineer their transport properties and controll the separations between the spins. The intrinsic pi-paramagnetism emerging spontaneously as radical states in open-shell graphene structures give rise to a more delocalized, mobile and isotropic magnetism than electrons in <i>d-</i>orbitals. Nonetheless, graphene-based nanomaterials also emerge as an ideal solution to combine excellent spin transport properties with large diffusion length and long coherence time needed for quantum spintronic applications with energy scales compatible with room-temperature operations.<br/>We report on manipulation of spin-spin interactions for PAH di-radical molecules [<b>4</b>] in a mechanically controlled break junction (MCBJ) and in 3-terminal device (electro-migration break junction EMBJ) at low temperature. Magnetic fingerprints of the molecules manifest themselves by zero-bias peaks in the differential conductance spectra attributed to a Kondo resonance arising from hybridization of one of the radical electrons with the metallic electrodes and spin-flip inelastic electron tunnelling spectroscopy (IETS) steps originating from the singlet-triplet gap of the free electrons [<b>5</b>]. Varying the distance between the electrodes in the MCBJ affects the coupling strength and varying the gate voltage in the EMBJ affects the energy level alignment with the Fermi level of the electrodes. Thus, by mechanical manipulation and electrostatic gating the spin signature of the molecule can be adjusted and controlled. This study provides new insight on the interplay between magnetic fingerprints of all-organic molecules embedded in solid-state devices.<br/><br/><u>References:</u><br/>[<b>1</b>] Hirohata, A., Hillebrands B. <i>et al.,</i> J. Magn. Magn. Mater. 166711 (2020).<br/>[<b>2</b>] Pavliček, N., Mistry, A., Majzik, Z. <i>et al., </i>Nature Nanotech <b>12, </b>308–311 (2017).<br/>[<b>3</b>] Zhang, X., Xu, Z., Si, W. <i>et al., </i>Nat Commun <b>8, </b>15073 (2017).<br/>[<b>4</b>] Baum, T. Y., Fernández, S., Peña, D., van der Zant, H. S. J, Nano Lett. <b>22</b>, <b>20</b>, 8086–8092 (2022).<br/>[<b>5</b>] Ternes, M., New J. Phys. <b>17</b> 063016 (2015).<br/><br/>This project received funding from the European Union Horizon 2020 research and innovation program under grant agreement N°863098 – SPRING. The molecules used were synthesized by the group of Diego Peña from Universidade de Santiago de Compostella.