Near Critical Superconducting States

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The understanding of the near-critical current-carrying superconducting state is a fully fundamental subject, however it is intimately related to the applications of superconductors in single particle (photon) detectors. Superconducting single particle detector (SSPD) technology has emerged as a building block for numerous applications, including quantum communication, optical quantum computing or space-to-ground communications [1].

Such devices are made of nano-patterned ultrathin superconducting films (in our case – 3-5nm-thick NbN elaborated on sapphire substrate by our German collaborators from KIT, Karlsruhe). The detector is a long (about 10-100 micron) folded superconducting nanowire (typically 10-100nm wide) (see figure); the wire is biased by a super-current who’s intensity is just below the critical current value, that keeps the wire very close to the transition to the normal (resistive) state. When an incident high-energy particle (photon, electron etc.) hits the wire and gets absorbed, it locally destroys already weakened superconductivity and creates a resistive region, generating a measurable voltage drop across the detector [2].

While such ultra-sensitive detectors become widely used, the microscopic picture of the particle-to-signal conversion is far from being understood. How the presence of strong supercurrents before the particle absorption modifies the superconducting properties of the wire? Are there “preferential locations” where the conversion takes place? How the film structure, intrinsic and extrinsic inhomogeneities [3], wire edges and bends affect the detector efficiency? Are there vortices, and do they influence the detection process?

We try to address the above cited open questions, using the new ultrahigh vacuum low-temperature Scanning Tunneling Microscopy / Scanning Tunneling Spectroscopy / Atomic Force Microscope (STM/STS/AFM) equipment recently installed at LPEM-ESPCI and unique in France. For the first time, it allows one for studying both the distribution of supercurrents and vortices when biasing the superconducting nanowire and give relevant information to understand the conversion process. This study may potentially have a strong impact since it could give ideas for optimizing design and improving efficiency of SSPDs. The nanofabrication is carried out using electron beam lithography at ESPCI and using clean room facility at Ecole Normale Supérieure de Paris (ENS-Paris). Then the real current-biased device will studied at low temperatures with the STM/STS/AFM equipment. As a second step, the device will be triggered using a local current pulse produced by STM tip or by a photon delivered by a laser connected to an optical fiber, and its local and global responses will be analyzed.

Some References
[1] Nature Photon. 3 696–705 (2009)
[2] Supercond. Sci. Technol. (2012)
[3] C. Carbillet PhD Thesis, Paris. (2014)

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