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Junior Research Fellow (St. John's College)

MEng (Bioengineering) University of Naples Federico II
MSc (Nanoscience) Arizona State University
PhD University of Cambridge

Superconductivity, Magnetism and Spintronics

My research concerns the investigation of unconventional superconducting states emerging at the interface between superconductors and other magnetic or non-magnetic materials such as graphene.

This research involves the fabrication of thin film multilayer heterostructures through a variety of physical vapour deposition and nanofabrication techniques including pulsed laser deposition, magnetron sputtering and mechanical exfoliation. The physical properties of the as-grown heterostructures are characterised using a combination of spectroscopic techniques like scanning tunnelling microscopy (down to 300 mK) and low-energy muon spectroscopy.

Superconducting Spintronics

In spintronics, information is processed via the spin carried by electrons in addition to their charge. The spin of an electron can be compared to a bar magnet where the ‘north pole’ can point either ‘up’ or ‘down’: these two distinguishable spin states can be used to encode the 1 and 0 bits of spintronic-based logic.

Although logic processing based on electrons’ spins is faster than the charge-based equivalent used in semiconductor transistors, the input currents required by spintronics devices are much larger than the operational current of semiconductor devices. The integration of superconductors (S), which are materials with virtually no dissipation, in spintronics systems cannot offer a solution to this problem, since their Cooper pairs of electrons in a conventional S are in an antiparallel spin-state (spin-singlet state) meaning that they carry charge but they do not carry a net spin.

Our group have recently shown, however, that coupling a S material to a magnetically inhomogeneous ferromagnet (F), spin-singlet pairs can be converted into spin-triplet pairs, which are fully-polarized and therefore exploitable in energy-efficient spintronics devices. The aim of this research is to fabricate devices that can perform logic and memory operations in the superconducting regime and therefore be applied for the development of novel quantum computing technologies.

Spectroscopy measurements of superconducting states

Unconventional superconducting states can emerge at the interface of a conventional S with another F or non-F material. Using a combination of low-temperature scanning tunnelling microscopy and low-energy muon spectroscopy, we have demonstrated direct evidence for the generation of these states, and we are currently characterizing the unconventional physical phenomena associated with them. For example, due to the formation of spin-triplet pairs, we have shown that S/F systems can exhibit an inverse (paramagnetic) Meissner response meaning that, in contrast with what is considered a hallmark signature of superconductivity (i.e. a diamagnetic Meissner effect), they can attract other than expel magnetic flux.

2D heterostructures

This research aims at exploiting the diverse properties of 2D materials ranging from semiconducting to metallic through to superconducting and magnetic, and the novel physics emerging from their interaction in 2D heterostructures to fabricate energy-efficient spintronics devices with new functionalities.

One of our most recent studies show that, an unconventional superconducting order parameter (p-wave) characterized by Cooper pairs in a spin-aligned state can be triggered in graphene in proximity contact with a high-temperature metal-oxide superconductor. Based on this result, we plan to apply graphene and other 2D materials for the fabrication of spintronics devices where Cooper pairs are intrinsically in a spin-aligned state (p-wave), without any need for a ferromagnetic layer to achieve this pairing configuration, and working at temperatures much higher than liquid helium temperature (4.2 K).

(Top) Low-energy muon spectroscopy in a Au/Ho/Nb multilayer thin film. (Bottom) dI/dVversus V tunnelling spectra measured at 4.2 K on single-layer graphene / Pr1.85Ce0.15CuO4.  
  • A Di Bernardo, Y Kalcheim, M Barbone, M Amado, D De Fazio, U Sassi, A Ott, J Linder, AC Ferrari, O Millo, JWA Robinson, “Evidence for unconventional superconductivity triggered in graphene via proximity coupling with a high-temperature superconductor”, Nature Communications 8, 14024 (2017).
  • JA Ouassou, A Di Bernardo, JWA Robinson, J Linder, “Electric control of superconducting transition through a spin-orbit coupled interface”, Nature Scientific Reports 6, 29312 (2016).
  • A Di Bernardo, Z Salman, XL Wang, M Amado, M Egilmez, MG Flokstra, A Suter, SL Lee, JH Zhao, T Prokscha, E Morenzoni, MG Blamire, J Linder, JWA Robinson, “Intrinsic paramagnetic Meissner effect due to s-wave odd frequency superconductivity”, Physical Review X 5, 041021 (2015).
  • A Di Bernardo, S Diesch, Y Gu, J Linder, E Scheer, MG Blamire, JWA Robinson, “Signature of magnetic-dependent gapless odd frequency states at superconductor/ferromagnet interfaces”, Nature Communications 6, 8053 (2015).
  • XL Wang, A Di Bernardo, N Banerjee, A Wells, FS Bergeret, MG Blamire, JWA Robinson, "Giant triplet proximity effect in superconducting pseudo spin-valves with engineered anisotropy", Physical Review B Rapid 89, 140508 (R) (2014).