arXiv subjects

Marta Perego

Publications and source records attributed to Marta Perego.

9 recordsLinked to original sources

arXiv2026

Proximity-induced superconductivity in a bilayer graphene quantum point contact

We report the realization of a gate-defined quantum point contact (QPC) in bilayer graphene proximitized by a single aluminum superconducting electrode. Superconducting correlations induced in the ballistic channel enhance the conductance plateaus beyond their normal-state values. In addition, we observe a pronounced above-gap conductance anomaly which serves as a spectroscopic signature of the loss of superconductivity and the associated collapse of the Andreev excess current. By reconstructing the nonlinear current-voltage characteristics, we find that the magnitude of the excess current increases as successive QPC modes are populated. Additionally, we find that the switching current associated with the loss of superconductivity follows the underlying mode structure of the QPC, exhibiting discrete levels consistent with a heat dissipation-driven transition. These results demonstrate that the one-dimensional transport modes of the QPC govern both the equilibrium proximity effect and the non-equilibrium dynamics of the hybrid system.

cond-mat.mes-hall↗

arXiv2026

Vortex Dynamics in Magic-Angle Twisted Graphene

We use a gate-defined Josephson junction (JJ) device made from twisted-layer graphene for studying vortex dynamics in two dimensions. The JJ sensor signals the presence of individual vortices in the superconducting leads nearby the junction through shifts in the Fraunhofer interference pattern of the magnetic-field-dependent critical current $I_c(B)$ across the junction. Rapid vortex fluctuations manifest as telegraph-type noise in time traces of the junction voltage $V(t)$. Measurements of $I_c(B)$ and $V(t)$ are interpreted in terms of multi-vortex processes where fast vortex fluctuations in the leads are modulated by quasi-stationary vortices trapped in the leads. The different timescales associated with these processes allow for their disentangling and quantitative analysis. Tracking the temperature dependence of the vortex-dynamical rates between $T = 7$ mK and $T = 120$ mK, we find that the creep type vortex motion is thermally activated above $T \approx 100$ mK, while the saturation of rates below $T \approx 80$ mK is suggestive of a sharp transition to macroscopic quantum tunneling of vortices.

cond-mat.supr-con↗

arXiv2026

In-plane and out-of-plane magnetic field driven Josephson diode effect in magic-angle twisted four-layer graphene

The superconducting diode effect offers a powerful probe into the fundamental symmetries of quantum materials. Recent studies on twisted graphene diodes have predominantly focused on bilayer or trilayer systems under out-of-plane magnetic fields. Here, we demonstrate both out-of-plane and in-plane driven Josephson diode effects in a magic-angle twisted four-layer graphene junction, i.e., an even number of layers. We observe the emergence of a diode effect at zero out-of-plane field, tuned by an increasing in-plane magnetic field. This result points to the presence of strong in-plane orbital coupling, which is highly sensitive to the specific layer parity of the structure. Our findings provide experimental insights into the symmetry-breaking mechanisms of even-layer twisted graphene, establishing in-plane magnetic fields as a vital tool for unravelling their microscopic properties.

cond-mat.mes-hall↗

arXiv2026

Monolithic hybrid quantum dot devices in superconducting twisted bilayer graphene

Gate-tunable superconductivity in magic-angle twisted bilayer graphene (MATBG) has enabled the realization of superconducting devices, such as Josephson junctions, within a single crystal. This interface-free platform provides a reconfigurable and scalable architecture that overcomes limitations of conventional superconducting-semiconducting systems. Incorporating single-electron control enables access to regimes in which flat-band superconductivity competes with strong Coulomb repulsion, providing a platform for studying correlated physics phenomena. Here, we report a new class of quantum devices that combines electrostatic confinement with tunable superconductivity in a monolithic MATBG architecture. Within a single device, we demonstrate two complementary hybrid systems: superconducting islands and proximitized quantum dots. The superconducting island exhibits $2e$-periodic transport, indicating a well-defined gap protected against quasiparticle poisoning. The proximitized quantum dot hosts subgap Andreev states together with a strongly parity-modulated supercurrent.

cond-mat.mes-hall↗

arXiv2026

Pearl-Vortex Tunneling in Magic-Angle Twisted Graphene

Twisted graphene provides a tunable platform for studying superconductivity in two dimensions. In the presence of electric currents and magnetic fields, vortices determine the phenomenological properties of the material. Related studies usually address bulk properties averaging over ensembles of vortices. Here, we employ a gate-defined Josephson junction as a single-vortex sensor, enabling direct access to individual vortex dynamical events. Our measurements reveal that, at elevated temperatures (T > 100 mK), vortices enter the superconducting leads via classical thermal activation over energy barriers. At lower temperatures (T < 90 mK), we observe macroscopic quantum tunneling through these barriers. The data are consistent with a sharp, first-order type quantum-to-classical transition. From our measurements, we extract vortex entry and exit energy barriers on the order of a few Kelvin and estimate the barrier thickness to be approximately 100 nm, corresponding to about one tenth of the device width.

cond-mat.supr-con↗

arXiv2024

Experimental detection of vortices in magic-angle graphene

The tunability of superconducting magic-angle twisted-layer graphene films elevates this material system to a promising candidate for superconducting electronics. We implement a gate-tuned Josephson junction in a magic-angle twisted four-layer graphene film. Field-dependent measurements of the critical current show a Fraunhofer-like pattern that differs from the standard pattern with characteristics typical for a weak transverse screener. We observe sudden shifts associated with vortices jumping into and out of the leads. By tuning the leads to the edge of the superconducting dome, we observe fast switching between superconducting and normal states, an effect associated with vortex dynamics. Time-dependent measurements provide us with the vortex energy scale and an estimate for the London penetration depth, in agreement with recent kinetic inductance measurements on twisted graphene films. Our results prove the utility of our junction as a sensor for vortex detection, allowing us to extract fundamental properties of the 2D superconductor.

cond-mat.mes-hall↗

arXiv2024

Quasiparticle and superfluid dynamics in Magic-Angle Graphene

Magic-Angle Twisted Bilayer Graphene shows a wide range of correlated phases which are electrostatically tunable. Despite a growing knowledge of the material, there is yet no consensus on the microscopic mechanisms driving its superconducting phase. In particular, elucidating the symmetry and formation mechanism of the superconducting phase may provide key insights for the understanding of unconventional, strongly coupled and topological superconductivity. A major obstacle to progress in this direction is that key thermodynamic properties, such as specific heat, electron-phonon coupling and superfluid stiffness, are extremely challenging to measure due to the 2D nature of the material and its relatively low energy scales. Here, we use a gate-defined, radio frequency-biased, Josephson junction to probe the electronic dynamics of magic-angle twisted bilayer graphene (MATBG). We reveal both the electronic quasiparticle dynamics, driven by their thermalization through phonon scattering, as well as the condensate dynamics, driven by the inertia of Cooper pairs. From these properties we recover the evolution of thermalization rates, and the superfluid stiffness across the phase diagram. Our findings favor an anisotropic or nodal pairing state and allow to estimate the strength of electron-phonon coupling. These results contribute to understanding the underlying mechanisms of superconductivity in MATBG while establishing an easy-to-implement method for characterizing thermal and superfluid properties of superconducting 2D materials.

cond-mat.supr-con↗

arXiv2023

Gate-defined superconducting channel in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MATBG) combines in one single material different phases like insulating, metallic and superconducting. These phases and their in-situ tunability make MATBG an important platform for the fabrication of superconducting devices. We realize a split gate-defined geometry which enables us to tune the width of a superconducting channel formed in MATBG. We observe a smooth transition from superconductivity to highly resistive transport by progressively reducing the channel width using the split gates or by reducing the density in the channel. Using the gate-defined constriction, we control the flow of the supercurrent, either guiding it through the constriction or throughout the whole device or even blocking its passage completely. This serves as a foundation for developing quantum constriction devices like superconducting quantum point contacts, quantum dots, and Cooper-pair boxes in MATBG.

cond-mat.mes-hall↗

arXiv2023

Tunable quantum interferometer for correlated moir\'e electrons

Magic-angle twisted bilayer graphene (MATBG) can host an intriguing variety of gate-tunable correlated states, including superconducting and correlated insulator states. Junction-based superconducting devices, such as Josephson junctions and SQUIDs, have been introduced recently and enable the exploration of the charge, spin, and orbital nature of superconductivity and the coherence of moir\'e electrons in MATBG. However, complementary fundamental coherence effects - in particular, the Little-Parks effect in a superconducting and the Aharonov-Bohm effect in a normal conducting ring - remained to be observed. Here, we report the observation of both these phenomena in a single gate-defined ring device where we can embed a superconducting or normal conducting ring in a correlated or band insulator. We directly observe the Little-Parks effect in the superconducting phase diagram as a function of density and magnetic field, confirming the effective charge of $2e$. By measuring the Aharonov-Bohm effect, we find that in our device, the coherence length of normal conducting moir\'e electrons exceeds a few microns at 50 mK. Surprisingly, we also identify a regime characterized by $h/e$-periodic oscillations but with superconductor-like nonlinear transport. Taken together, these experiments establish a novel device platform in MATBG, and more generally in tunable 2D materials, to unravel the nature of superconductivity and other correlated quantum states in these materials.

cond-mat.mes-hall↗