Superconducting magic-angle twisted trilayer graphene hosts competing magnetic order and moir\'e inhomogeneities

TL;DR

This study provides direct experimental evidence of in-plane magnetic order near superconductivity in magic-angle twisted trilayer graphene, revealing competing magnetic and superconducting phases influenced by moiré inhomogeneities.

Contribution

It demonstrates the coexistence of magnetic order and superconductivity in MATTG through electrical transport measurements, highlighting the role of moiré inhomogeneity and magnetic fluctuations.

Findings

Evidence of in-plane magnetic order near superconducting state

Hysteresis in magnetoresistance indicating magnetic order

Broadened BKT transition due to moiré inhomogeneity

Abstract

The microscopic mechanism of superconductivity in the magic-angle twisted graphene family, including magic-angle twisted trilayer graphene (MATTG), is poorly understood. Properties of MATTG, like Pauli limit violation, suggest unconventional superconductivity. Theoretical studies propose proximal magnetic states in the phase diagram, but direct experimental evidence is lacking. We show direct evidence for an in-plane magnetic order proximal to the superconducting state using two complementary electrical transport measurements. First, we probe the superconducting phase by using statistically significant switching events from superconducting to the dissipative state of MATTG. The system behaves like a network of Josephson junctions due to lattice relaxation-induced moir\'e inhomogeneity in the system. We observe non-monotonic and hysteretic responses in the switching distributions as a function of temperature and in-plane magnetic field. Second, in normal regions doped slightly away from the superconducting regime, we observe hysteresis in magnetoresistance with an in-plane magnetic field; showing evidence for in-plane magnetic order that vanishes $\sim$900 mK. Additionally, we show a broadened Berezinskii-Kosterlitz-Thouless transition due to relaxation-induced moir\'e inhomogeneity. We find superfluid stiffness $J_{\mathrm{s}}$$\sim$0.15 K with strong temperature dependence. Theoretically, the magnetic and superconducting order arising from the magnetic order's fluctuations have been proposed - we show direct evidence for both. Our observation that the hysteretic magnetoresistance is sensitive to the in-plane field may constrain possible intervalley-coherent magnetic orders and the resulting superconductivity that arises from its fluctuations.