Publication

Tunnelling dynamics between superconducting bound states at the atomic limit

Abstract

A magnetic impurity is placed on the tip of a scanning tunnelling microscope, allowing direct tunnelling between two Yu-Shiba-Rusinov bound states. This technique can probe and enhance the impurity state lifetime. There is a limit to the miniaturization of every process, and for charge transport this is realized by the coupling of two single discrete energy levels at the atomic scale. In superconductors, Yu-Shiba-Rusinov (YSR) states are such levels. Here, we place a magnetic impurity on the tip of a scanning tunnelling microscope (YSR-STM) and use it to demonstrate sequential tunnelling of electrons between parity-protected YSR states on the tip and in the sample. Using this Shiba-Shiba tunnelling technique we probe the YSR lifetime, which we can enhance by reducing the relaxation of the excited YSR state to the intrinsic channel. Our work offers a way to characterize and manipulate coupled superconducting bound states, such as Andreev levels, YSR states or Majorana bound states at the atomic limit.

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Majorana fermion
A Majorana fermion (maɪə'rɑːnə), also referred to as a Majorana particle, is a fermion that is its own antiparticle. They were hypothesised by Ettore Majorana in 1937. The term is sometimes used in opposition to a Dirac fermion, which describes fermions that are not their own antiparticles. With the exception of neutrinos, all of the Standard Model fermions are known to behave as Dirac fermions at low energy (lower than the electroweak symmetry breaking temperature), and none are Majorana fermions.
Energy level
A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The term is commonly used for the energy levels of the electrons in atoms, ions, or molecules, which are bound by the electric field of the nucleus, but can also refer to energy levels of nuclei or vibrational or rotational energy levels in molecules.
Scanning tunneling microscope
A scanning tunneling microscope (STM) is a type of microscope used for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer, then at IBM Zürich, the Nobel Prize in Physics in 1986. STM senses the surface by using an extremely sharp conducting tip that can distinguish features smaller than 0.1 nm with a 0.01 nm (10 pm) depth resolution. This means that individual atoms can routinely be imaged and manipulated.
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