The development of superconducting devices was greatly stimulated after the acceptance of the basic theory of superconductivity proposed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer – BCS theory. The basic idea is that the electron waves in the superconducting state no longer act independently, as in Bloch’s model. Instead, they are paired together at the critical temperature – the so-called Cooper pairs – so that their wave functions act as one unit as they interact with the crystal lattice. All the electron pairs move together in one collective motion, so that if any single electron is scattered by the lattice it is pulled back into the flow by its partner, and if any pair of electrons is somehow scattered off track, it is pulled back into the collective flow by all the other pairs. Since there is no scattering or inelastic collisions, there is no resistance, and the material becomes a superconductor. Consequently, superconducting materials provide an ideal platform for testing coherent dynamics of many-body states and exploring their potential as qubits.
Pairing effects lead to a many-body ground state formed by a condensate of Cooper pairs, protected from quasi-particle excitations by a pairing energy gap. The excitation of this superconducting ground state can be achieved by electrons, correlated pairs in Josephson currents, or microwave photons. In bulk superconductors, these excitations populate a continuum of bogoliubons (Bogoliubov quasiparticles) and admix with other states that quickly quench their quantum coherence.
Many of the properties of superconductors related to quantum coherence are revealed when the superconducting state is forced to vary in space. Among the earliest examples is Andreev reflection of an electron into a retro-reflected hole at a normal-superconducting interface. In regions of strong inhomogeneity multiple Andreev reflection leads to the formation of sub-gap states, Andreev bound states, with excitation energies below the superconducting gap. 1.
Sub-gap quasiparticle excitations can live long in a coherent state, allowing detection and manipulation of their quantum nature with high fidelity. Thus, sub-gap Andreev bound states in a proximitized link between two superconductors host addressable doublet quasiparticles and singlet pair-breaking excitations that can store quantum information.
Population of these excited states follows parity-conserving rules: bogoliubons states are odd in fermion parity and can be excited by adding or removing a fermion to the even-parity BCS ground state. Pair excitations involve the creation of two correlated bogoliubons into an excited state and, thus, have even-parity. Therefore, they are accessible by the absorption of one microwave photon or the addition of two particles with opposite spin.
Now, a team of researchers shows 2 that tunnelling electrons can also excite a superconducting pair-breaking transition in the presence of magnetic impurities, which is hidden for electrons on bare superconductors.
Combining scanning tunneling spectroscopy with theoretical modelling, the researchers map the excitation spectrum of a Fe-porphyrin molecule on a Au/V(100) proximitized surface into a manifold of many-body excitations and follow their behaviour across a parity-changing transition. They find that pair excitations emerge in the tunneling spectra as peaks outside the gap in the strong interaction regime, scaling with the pair correlation.
This novel route for addressing pair excitations on a proximitized superconductor unravels the quantum nature of magnetic impurities on superconductors, and prove that pair excitations are parity detectors for magnetic impurities.
Author: César Tomé López is a science writer and the editor of Mapping Ignorance
Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.
- J. A. Sauls (2018) Andreev Bound States and Their Signatures Phil. Tran. Roy. Soc. A doi: 10.1098/rsta.2018.0140 ↩
- Stefano Trivini, Jon Ortuzar, Katerina Vaxevani, Jingchen Li, F. Sebastian Bergeret, Miguel A. Cazalilla, and Jose Ignacio Pascual (2023) Cooper Pair Excitation Mediated by a Molecular Quantum Spin on a Superconducting Proximitized Gold Film Phys. Rev. Let. doi: 10.1103/PhysRevLett.130.136004 ↩