Studies of Superconducting BDD films and their use in Novel Superconducting Devices

Once the boron doping in diamond exceeds a density of about 1020 cm-3, the material will show metal-like behaviour in electrical conductivity. Moreover, upon cooling these highly boron-doped diamond samples below around 10 K they exhibit superconductivity, that follows the standard Bardeen-Cooper-Schrieffer (BCS) theory with the current being carried via pairs of coupled electrons called Cooper pairs. This means that heavily boron-doped diamond (BDD) may be used in superconducting devices, such as sensors, detectors or quantum circuits, which consume much less power, have high sensitivity, and operate very rapidly.

Things become even more interesting when polycrystalline diamond is doped to superconducting levels, as the material then behaves as what's known as a 'granular superconductor'. Each crystallite in the film is a perfect superconductor, but they are joined to their neighbours via grain boundaries with lower conductivity, which act as 'weak links' or Josephson junctions. Thus, a granular highly BDD superconductor can be considered as being a 3D network of superconducting grains connected via multiple Josephson junctions. The electrical behaviour of such systems can be rather strange, leading to unusual or even bizarre phenomena.

Superconductor–bosonic insulator transition Schematic illustration of weak disorder–tuned superconductor–bosonic insulator transition in 3D BDD systems. (a) A disorder-free 3D system undergoes the metal-superconductor transition directly (blue curve), while in many cases a small degree of disorder delays the onset of global superconductivity by tuning the system into a bosonic insulator first. A sequence of transitions [metal–bosonic insulator–superconductor, labeled from (1) to (4) in the figure] gives rise to the anomalous narrow ρ(T) peak just preceding the onset of the global superconducting state ρ=0 at lower temperature (upper curve). (b) Schematic representation of the metal–bosonic insulator–superconductor transition with panels numbered in accordance with panel (a). Disorder is depicted by spatial variation of the potential U(ρ); quasiparticles quasiparticle ; Cooper pairs Cooper pair; Gi, Gj, and Gk grains on the percolation path; phase locking between bosonic islands = wave curves.

For over 12 years we have been collaborating with Dr Gufei Zhang (KU Leuven, then the University of Southern Denmark, then Harbin Institute of Technology, and now at the Slovak Academy of Sciences) to study the behaviour of such systems. We have identified the following unusual interesting phenomena in these polycrysalline diamond films:

Diamond Superconducting Quantum Devices

These various discoveries have led to a number of suggestions for use of BDD in novel superconducting devices. For example, one of the most challenging problems faced by the development of quantum computers is the limited integration density of qubits, because the more qubits you put onto a quantum chip, the more vulnerable a qubit becomes, due to the influence from other qubits. The long-range Yu-Shiba-Rusinov states, which we found in ferromagnetic superconducting diamond, can be used to implement topologically protected qubits with an extraordinarily high robustness surpassing that of other types of qubits. This may be crucial for the development of a range of quantum technologies, such as spin qubits. Recently, our discovery of giant negative magnetoresistance in a BDD ring structure suggests applications for magnetometry, as well as cavities to trap Cooper pairs for potential applications in other quantum devices, e.g., artificial atoms for charge qubits.

 Superconducting Diamond Ring Structure
Superconducting Diamond Ring Structure. When the superconducting current meets the ring, it can choose to go clockwise or anticlockwise around the ring. It turns out that this choice is quantised. Such structures might be used to fabricate superconducting quantum switches.

Example papers

Many of these papers can also be seen on Dr Zhang's website.