Proximity-Induced Shiba States in a Molecular Junction

Superconductors containing magnetic impurities exhibit intriguing phenomena derived from the competition between Cooper pairing and Kondo screening. At the heart of this competition are the Yu-Shiba-Rusinov (Shiba) states which arise from the pair breaking effects a magnetic impurity has on a superconducting host. Hybrid superconductor-molecular junctions offer unique access to these states but the added complexity in fabricating such devices has kept their exploration to a minimum. Here, we report on the successful integration of a model spin $1/2$ impurity, in the form of a neutral and stable all organic radical molecule, in proximity-induced superconducting break junctions. Our measurements reveal excitations which are characteristic of a spin-induced Shiba state due to the radical’s unpaired spin strongly coupled to a superconductor. By virtue of a variable molecule-electrode coupling, we access both the singlet and doublet ground states of the hybrid system which give rise to the doublet and singlet Shiba excited states, respectively. Our results show that Shiba states are a robust feature of the interaction between a paramagnetic impurity and a proximity-induced superconductor where the excited state is mediated by correlated electron-hole (Andreev) pairs instead of Cooper pairs.

Figure 1

Proximity induced interaction with an all organic radical molecule and device design. (a) Schematic representation of a radical molecule coupled to a proximity induced superconducting gold electrode. The red circle represents the unpaired spin at the center of the molecule. (b) False-colored scanning electron microscopy (SEM) image of a lithographically identical junction with superconducting electrodes colored blue and gold nanowire colored gold. The inset shows a SEM image of an electromigrated junction after measurement, scale bar 50 nm.

Figure 2

Characterization and calculations of a molecular junction in the superconducting state. (a) Low temperature (100 mK) measurement of the differential conductance ($dI/d{V}_b$) as a function of voltage bias ($V_b$) of a radical molecular junction at zero magnetic field (superconducting state) and 200 mT (normal state). The blue dashed curve shows the same measurement at 1.2 K ($B=0\mathrm{T}$). The inset shows the magnetic field dependence of the Shiba peaks. (b) Theoretical calculations of the modeled system taking into account the proximity-induced DOS of the leads. (c) Schematic representation of the probing of the Shiba excited state as a result of the coupling of the radical molecule with the proximity-induced superconducting Au electrode.

Figure 3

Ground state spectroscopy of the hybrid radical system. (a) The differential conductance ($dI/d{V}_b$) as a function of voltage bias ($V_b$) of a radical molecular junction at zero magnetic field (superconducting state) and 200 mT (normal state) in the doublet ground state regime ($k_B{T}_K<{\delta }_{\mathrm{avg}}$). Inset shows calculations for this regime ($\delta =9k_B{T}_K$). (b) Color plot of the differential conductance ($dI/d{V}_b$) as a function of voltage bias and magnetic field for the junction in panel (a). (c) Excitation picture of the ground and excited states in the doublet ground state regime. (d) The differential conductance ($dI/d{V}_b$) as a function of voltage bias ($V_b$) of a radical molecular junction at zero magnetic field (superconducting state) and 2 T (normal state) in the singlet ground state regime ($k_B{T}_K>{\delta }_{\mathrm{avg}}$). Inset shows calculations for this regime ($\delta =0.5k_B{T}_K$). (e) Color plot of the differential conductance ($dI/d{V}_b$) as a function of voltage bias and magnetic field for the junction in panel (d). (f) Excitation picture of the ground and excited states in the singlet ground state regime.