arXiv subjects

Peter Koopmann

Publications and source records attributed to Peter Koopmann.

2 recordsLinked to original sources

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

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↗