Spectroscopy of <i>d</i>-wave superconductors using DNA as a probing tip

Andreev reflection in DNA molecules terminated by a<i>d</i>-wave superconductor is investigated for demonstrating advantages in using DNA as the probe for the spectroscopy of the superconductor. DNA molecules are incorporated in the simulations using a two-leg ladder model with a simplification as constructed by homopolymers. The increase of the Andreev reflection probability at zero bias originating from the midgap surface states of<i>d</i>-wave superconductors appears even when the DNA molecule is coupled strongly. The zero-bias peak is enhanced by orders of magnitude when the coupling is weakened. The one-dimensional transport in DNA strands gives rise to the remarkable sensitivity in the spectroscopy, where the changes of the reflection probability caused by the midgap states are also in orders of magnitude when the voltage bias is varied and when the orientation of the<i>d</i>-wave symmetry is inclined with respect to the superconductor surface. The quantum interference of the transport between the two strands in DNA modifies the zero-bias increase. A narrow dip occurs at zero bias with a plateau sandwiched by two peaks in the immediate vicinity of the zero bias. The characteristics of these transmission resonances are dependent on the parameters describing the model molecules, and so the width of the resonance peaks, for instance, enables us to evaluate the strength of the inter-strand coupling.