Josephson junctions are often introduced as “simple” nonlinear elements whose supercurrent depends on the phase difference between two superconductors. In many modern materials and hybrid platforms, however, the Josephson effect becomes a sensitive probe of symmetry breaking and emergent quantum phenomena. My research in this area focuses on unconventional and anomalous Josephson physics arising from spin–orbit coupling (SOC), magnetism, and topological electronic states—and on how these effects can be engineered into functional superconducting devices (memories, phase batteries, diodes, interferometers) that are relevant for quantum technologies.
In conventional junctions the current–phase relation is (approximately) I(φ)=Icsinφ, implying zero supercurrent at zero phase bias. When inversion and time-reversal symmetries are jointly broken—for instance by combining SOC with magnetism—the junction can acquire an intrinsic phase shift and support a finite supercurrent even at φ=0. This is the physics of φ₀-junctions and, more broadly, of magneto-electric Josephson effects.
A key theme of my work is to understand:
how φ₀ behavior emerges microscopically at hybrid interfaces,
how it appears in experimentally accessible observables (switching statistics, interferometry, rf-SQUID readout),
and how it can be exploited for non-volatile superconducting functionalities.
Hybrid junctions with magnetic elements are compelling because they couple superconducting phase dynamics to magnetic degrees of freedom. This opens routes to devices where information is stored in bistable magnetic configurations and read out through superconducting transport.
In this direction, I have investigated ferromagnetic anomalous Josephson junctions, with emphasis on two complementary aspects:
Device concepts
The anomalous phase shift can act as a built-in “phase source”, enabling elements such as phase batteries and cryogenic memory units where the stored state is robust and readable through a superconducting circuit.
Stochastic robustness and switching statistics
Real devices operate in noisy environments. I study how thermal fluctuations and stochasticity affect magnetization reversal, switching-current distributions, and overall stability—connecting microscopic symmetry breaking to measurable statistical signatures.
This blend—symmetry-based design plus stochastic modeling—often provides a direct bridge between theory and experimentally testable predictions.
Another active line is the Josephson diode effect, i.e., non-reciprocal supercurrent transport where the critical current differs for opposite bias directions. Diode behavior can arise from the same “ingredients” (SOC + broken symmetries) but typically requires asymmetry and nontrivial phase dynamics. I work on identifying minimal design principles and on quantifying diode performance in realistic junction geometries, including nanostructures where Rashba SOC can be engineered.
This topic is attractive both fundamentally—because it clarifies which symmetry breakings are required for non-reciprocity—and technologically, since superconducting diodes are promising for dissipationless rectification, on-chip isolation, and novel circuit building blocks.
Topological superconductivity and topological bands can imprint distinctive signatures on Josephson transport. My approach is pragmatic: rather than treating “topology” as an abstract label, I focus on operational diagnostics in junction experiments, such as:
fractional Shapiro steps under microwave irradiation,
interferometric fingerprints in junctions built from oxide nanochannels or other confined platforms,
and phase-diagram mapping that distinguishes trivial from non-trivial regimes.
The underlying goal is to provide clear, testable hallmarks that help experimentalists interpret data and guide device design.
Methodologically, I combine:
effective models of SOC-driven magneto-electric coupling and symmetry breaking,
Josephson circuit modeling (including interferometers and rf-SQUID readout),
nonlinear dynamics and stochastic analysis when thermal fluctuations or switching processes are central,
and close interaction with experimentally relevant observables (critical currents, Shapiro maps, interferometric patterns, switching distributions).
I particularly enjoy collaborations where theory can (i) explain an existing experimental anomaly, or (ii) propose a clean discriminating experiment between competing mechanisms (SOC-driven φ₀ shift vs. geometric asymmetry vs. magnetic texture effects, etc.).
These are well-scoped projects that work for MSc/PhD theses and/or joint theory–experiment collaborations:
Design rules for φ₀ junctions in hybrid stacks
Identify minimal symmetry requirements and derive tunability knobs (gate, field orientation, interface engineering). Deliverables: predicted phase shifts, interferometric response, stability windows.
Switching statistics as a diagnostic of anomalous Josephson physics
Use switching-current and switching-time distributions to separate φ₀ behavior from conventional junction asymmetries. Deliverables: fitting/inference pipeline + proposed measurement protocols.
Josephson diode optimization in SOC nanostructures
Quantify diode efficiency versus geometry, disorder, and phase dynamics; propose architectures maximizing non-reciprocity while maintaining coherence.
Topological fingerprints via Shapiro spectroscopy
Model fractional Shapiro steps and their robustness; propose parameter regimes where the signatures remain visible under realistic noise and finite-temperature constraints.
If you’re an experimental group with a specific platform (nanowires, oxide nanochannels, magnetic insulators, etc.), I’m happy to tailor the modeling to your geometry and measurement constraints.
E. Strambini, A. Iorio, O. Durante, R. Citro, C. Sanz-Fernández, C. Guarcello, I. V. Tokatly, A. Braggio, M. Rocci, N. Ligato, V. Zannier, L. Sorba, F. S. Bergeret, F. Giazotto, A Josephson phase battery, Nature Nanotechnology 15, 656 (2020).
C. Guarcello, F. S. Bergeret, A cryogenic memory element based on an anomalous Josephson junction, Physical Review Applied 13, 034012 (2020).
C. Guarcello, R. Citro, O. Durante, F. S. Bergeret, A. Iorio, C. Sanz-Fernández, E. Strambini, F. Giazotto, A. Braggio, rf-SQUID measurements of anomalous Josephson effect, Physical Review Research 2, 023165 (2020).
C. Guarcello, F. S. Bergeret, Thermal noise effects on the magnetization switching of a ferromagnetic anomalous Josephson junction, Chaos, Solitons & Fractals 142, 110384 (2021).
C. Guarcello, L. Chirolli, M. T. Mercaldo, F. Giazotto, M. Cuoco, Frustration-driven Josephson phase dynamics, Physical Review B 105, 134503 (2022).
G. Singh, C. Guarcello, E. Lesne, D. Winkler, T. Claeson, T. Bauch, F. Lombardi, A. Caviglia, R. Citro, M. Cuoco, A. Kalaboukhov, Gate-tunable pairing channels in superconducting non-centrosymmetric oxides nanowires, npj Quantum Materials 7, 2 (2022).
L. Chirolli, M. T. Mercaldo, C. Guarcello, F. Giazotto, M. Cuoco, Colossal orbital-Edelstein effect in non-centrosymmetric superconductors, Physical Review Letters 128, 217703 (2022).
A. Maiellaro, J. Settino, C. Guarcello, F. Romeo, R. Citro, Hallmarks of non-trivial topology in Josephson junctions based on oxide nanochannels, Physical Review B Letters 107, L201405 (2023).
C. Guarcello, S. Bergeret, R. Citro, Switching current distributions in ferromagnetic anomalous Josephson junctions, Applied Physics Letters 123, 152602 (Editor’s Pick) (2023).
C. Guarcello, A. Maiellaro, J. Settino, I. Gaiardoni, M. Trama, F. Romeo, R. Citro, Probing Topological Superconductivity of oxide nanojunctions using fractional Shapiro steps, Chaos, Solitons & Fractals 189, 115596 (2024).
A. Maiellaro, M. Trama, J. Settino, C. Guarcello, F. Romeo, R. Citro, Engineered Josephson diode effect in kinked Rashba nanochannels, SciPost Physics 17, 101 (2024).
C. Guarcello, S. Pagano, G. Filatrella, Diode-effect efficiency in asymmetric inline long Josephson junctions, Applied Physics Letters 124, 162601 (2024).
C. Guarcello, R. Citro, Progresses on topological phenomena, time-driven phase transitions, and unconventional superconductivity, EPL 132, 60003 (2021).
M. Trama, I. Gaiardoni, C. Guarcello, J. I. Facio, A. Maiellaro, F. Romeo, R. Citro, J. van den Brink, Non-linear anomalous Edelstein response at altermagnetic interfaces, Physical Review B 112, 184404 (2025).
A. Maiellaro, F. Romeo, M. Trama, I. Gaiardoni, J. Settino, C. Guarcello, N. Bergeal, M. Bibes, R. Citro, Theory of charge-to-spin conversion under quantum confinement, Physical Review Research 7, 043100 (2025).
I. Gaiardoni, M. Trama, A. Maiellaro, C. Guarcello, F. Romeo, R. Citro, Edelstein effect in isotropic and anisotropic Rashba models, Condensed Matter 10, 15 (2025).