■ Scanning Tunneling microscopy of superconducting single photon detectors

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Description of the PhD project

Superconducting single photon 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 and 10 nm-thick YBCO on STO susbtrate). The detector is a long ( 10-100 micron) folded superconducting nanowire ( 10-100nm wide); the wire is biased by a current whose intensity is just below the critical current value. When an incident high-energy particle (photon, electron etc.) hits the wire and is absorbed, it locally destroys superconductivity and creates a resistive region, generating a measurable voltage pulse 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? Some of these open questions will be addressed during this project, 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 Paris and unique in France. For the first time, it will allow both the distribution of supercurrents and vortices to be studied under current biasing, providing relevant information to understand the conversion process. The two expected outcomes of this study are the following: clear direction for optimizing design and improving efficiency of low Tc SSPDs and better understanding of superconducting properties in High Tc nanowire to make the first high Tc SSPDs working upon 40K. This latest point would be a significant achievement since it would reduce the cryogenic operating cost of commercially available SSPDs detectors that requires temperatures below 4K.
Nanofabrication will be carried out using electron beam lithography at ESPCI Paris and using clean room facility inside the consortium “Paris centre-centrale de proximité”. Then the device under current-biased device will studied at low temperature with the STM/STS/AFM equipment. In a second step, the device response 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 such as studying how pinned vortices move after a detection event. In a third step, we will investigate the effect of magnetic impurities on superconducting states and the formation of the hotspot.

[1] Nat. Photon. 3 696–705 (2009) [2] SUST (2012) [3] Carbillet PhD Thesis, Paris. (2014) SUST (2018) [4]

Keywords

Superconductors, single photons detectors, Atomic force microscope, Scanning tunneling microscope, spectroscopy, superconducting to insulating transition.

Research unit

UMR7636 Physics & Materials

Description of the research Unit/subunit

The project will take place in the LPEM (UMR8213) in both QuantumSpec (D. Roditchev, S. Pons, S. Vlaic) and Phasme Group (C. Feuillet-Palma, J. Lesueur and N. Bergeal) .
The QuantumSpec group is headed by D. Roditchev, who recently moved from INSP to the LPEM in ESPCI-Paris, an assistant professor S. Vlaic and a CNRS researcher S. Pons.
D. Roditchev is internationally recognized for his expertise in building and operating low temperature STM and has built the 300mK apparatus used in INSP. He is a worldwide expert recognized internationally in superconductivity.
The Phasme group has been studying superconductors at a meso and nanoscale for many years, from High-Tc cuprates to 2-DEG at oxide interfaces, from basic research to applications. The group made essential contributions to understand the fundamental properties of superconducting cuprates, and at the same time, developed a new technology to make HTc nanowires and Josephson junctions which are used in various functional circuits (SQUID, SQUIFS, THz mixers and single photon detectors).
The complementarity of the teams brings the full span of skills necessary for the successful completion of the project.

Name of the supervisor
Dimitri Roditchev (dimitri.roditchev (arobase) espci.fr)

Name of the co supervisor
Chéryl Feuillet-Palma (cheryl.palma (arobase) espci.fr)

3i Aspects of the proposal

This project aims at studying the fundamental issues related to the study of the parameters affecting the global and local phase coherence of disordered superconducting thin films and nanowires to give clues to the microscopic mechanism below the formation of the hotspot. It will allow improving this available low Tc technology and give better understanding of superconducting properties in High Tc nanowire to make the first high Tc SSPDs working upon 40K.
The project is interdisciplinary as it involves several fields such as materials physics (with ultra-thin film deposition whose thickness is closed to the superconducting to insulating transition) , nano-fabrication (to make device compatible with the AFM-STM equipment), low temperature transport measurements under photon illumination and local spectroscopy by AFM-STM technique.
The project will involve several foreign collaborators mostly in European institutions. On one hand, the QuantumSpec group has a long-standing collaboration with IMS-KIT (Institute of Micro- and Nanoelectronic Systems, Karlsruhe Institute of Technology) in particular they supply high quality NbN ultra-thin films. This collaboration has been strengthened by the ANR SUPERTRIPES grants for both collaborators. On the other hand the PHASME group has established a long-standing collaboration with a group of theoreticians at La Sapienza University in Roma (Italy). Around ten co-authored scientific articles have been produced over the past years through this fruitful collaboration.

Expected Profile of the candidate

We are looking for motivated applicants with an excellent academic background in particular in quantum physics and solid state physics. The Phd applicant should be motivated by experimental physic and able to work independently. He/She should have good communication skills and be proficient in spoken and written English.
Although not mandatory, previous experiences in the following fields will be positively considered:
scanning tunneling microscope
atomic force microscope
ultra-high vacuum
electronic transport measurements
cryogenic systems operation
micro and nanofabrication

Important dates

Call for applications : from July 16th to September 17th 2018
Eligibility check results : Late September
3i Committee evaluation results : Late October
Interviews from the shortlisted candidates with the Selection Committee : Mid-December (week of December 10th)
Final results : Late December





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