Research activity report
Ph.D. period of Francesco Vischi
My Ph.D. involves two institutions, that are, the Università di Pisa and the NEST laboratory, Pisa. The research activity took place at NEST, specifically in the Superconducting Quantum ELectronics (SQEL) group, guided by dr. Francesco Giazotto.
Three main topics can be discerned within my research. The first topic is a theoretical study of the equilibrium thermodynamics of a hybrid superconductor/normal metal junction, which results are included in the published articles PRB 96, 245311 (2017), Sci. Rep. 9, 3238 (2019), Entropy 21, 1005 (2019). In these works, we discuss the microscopical statistical mechanism underlying the Maxwell relation between the supercurrent and the entropy of the junction. Then, we exploit these findings to build up a complete thermodynamic description based on thermodynamic processes and cycles involving supercurrent, temperature, condensate phase, and entropy.
The second result of the Ph.D. activity is a theoretical study of the Peltier refrigeration of a superconductor/graphene device and its bolometric characteristics. We have demonstrated that the low electron-phonon coupling and the low heat capacity in graphene allow reaching low base temperatures and fast thermal response. These results are available in the preprint arXiv:1906.10988 .
Finally, the third topic of the Ph.D. activity is experimentally focused on field-effect in superconducting constriction-junctions. In detail, we have nano-fabricated and characterized a superconducting interferometer based on two gated constriction-junction. We have demonstrated an unconventional relationship between the switching current and the gating field. A phenomenological theoretical model simulates the experimental results and suggests the existence of a coupling between phase and gating field. These results are published in Nano Letters 19, 6263 (2019).
During the three years of Ph.D., I have attended the following courses at Università di Pisa: ‘statistical data analysis’ by prof. Giovanni Punzi, ‘many-body physics’ by prof. Marilù Chiofalo, and
‘micro-electronics’ by prof. Luca Fanucci. In ‘statistical data analysis,’ we treated fundamentals of statistics and studied advanced mathematical tools for statistical inference and properties of statistical estimators. In ‘many-body physics,’ we studied the formalism of the correlation functions and how these can be calculated from a microscopical matter using perturbative methods to interacting statistical systems. Relatedly, we applied the Feynman diagrams to these systems and explained how to represent (and calculate) correlation functions. For the exam, I presented a seminar concerning the Matsubara Green function approach to the BCS theory and introducing its usage for strong-coupling superconductors. In ‘micro-electronics,’ we learned to program in VHDL and to implement digital circuits on an FPGA, and for the exam, I implemented a converter from a pulse-width-modulation signal to a digital serial signal. As external activities, I have attended two schools. The first is ‘Novel frontiers in superconducting electronics,’ in Pozzuoli, December 2016, where different topics of superconducting electronics were treated: superconducting computing, caloritronics, topology in superconducting systems, bolometry. The second school is ‘Nanotechnology meets quantum information,’ in Donostia-San Sebastian, July
2019, where the seminars were focused on quantum computation with superconductors, involving spintronics, cavity QED, topological quantum computing.
Published works
➢ “Thermodynamics of a phase-driven proximity Josephson junction” by F. Vischi, M. Carrega, A. Braggio, P. Virtanen, F. Giazotto
Entropy 21, 1005 (2019)
➢ “Electron cooling with graphene-insulator-superconductor tunnel junctions and applications to fast bolometry”
by F. Vischi, M. Carrega, A. Braggio, F. Paolucci, F. Bianco, S. Roddaro, F. Giazotto preprint on arXiv:1906.10988 (2019)
➢ “Field-effect controllable metallic Josephson interferometer”
by F. Paolucci, F. Vischi, G. de Simoni, C. Guarcello, P. Solinas, F. Giazotto Nano Letters 19, 6263 (2019)
➢ “Thermodynamic cycles in Josephson junctions”
by F. Vischi, M. Carrega, P. Virtanen, E. Strambini, A. Braggio, F. Giazotto Scientific Reports 9, 3238 (2019)
➢ “Quasiparticle entropy in superconductor/normal metal/superconductor proximity junctions in the diffusive limit”
by P. Virtanen, F. Vischi, E. Strambini, M. Carrega, F. Giazotto Physical Review B 96, 245311 (2017)
