Magnetic Josephson Junctions and Superconducting Diodes in Magic Angle Twisted Bilayer Graphene

Zero-field superconducting diode effect in small-twist-angle trilayer graphene

“…The critical current of a superconductor can be different in opposite directions when both time-reversal and inversion symmetry are absent. By breaking these symmetries with an applied magnetic field [1] or by construction of a magnetic tunnel junction [2], this nonreciprocity, which creates a superconducting diode, has recently been demonstrated experimentally and attracted significant attention [3][4][5]. Here we report an intrinsic superconducting diode effect which is present at zero external magnetic field in a heterostructure consisting of twisted trilayer graphene (tTLG) and tungsten diselenide (WSe 2 ).…”

“…The uniform response across the entire density range is consistent with a highly uniform sample. At the same time, it suggests that the superconducting diode effect is intrinsic to the superconducting phase, which is distinct from the diode effect in a magnetic tunnel junction [2] and Josephson networks [31]. As such, we have realized a junctionfree superconductor film in the tTLG/WSe 2 heterostruc-ture, which operates as a supercurrent diode and achieves one-way transport of electrical charge without power consumption.…”

Anomalous Hall effect at half filling in twisted bilayer graphene

“…However, in this case the proximity-induced SOC in the tBLG couples the spin and valley degrees of freedom, and a generalized Hund's term explicitly favors the VP-QAH state at ν = 2. A separate report of a gate-tunable Josephson junction tBLG device (without WSe 2 ) revealed evidence for some form of magnetic ordering at ν = −2 [30]. Compressibility measurements performed in a large parallel magnetic field in other devices have also raised the possibility of magnetic ordering at half filling [31].…”

Pairing symmetry of twisted bilayer graphene: a phenomenological synthesis

“…While it is natural to assume that the SC is obtained by doping holes into the CI (with this scenario apparently being born out in the STM study of [23]), this does not necessarily always need to be the case, and it is possible that different types of CIs (likely all with the same type of flavor polarization) are present in different samples that superconduct at ν = −2 − δ. This possibility is suggested by the fact that in [22] the gap was seen to possibly close between the SC and insulator, and by the Josephson study of [70], which found indications of a correlated insulator that strongly breaks time reversal being present in a de-vice which also superconducts. All of this discussion goes to show that flavor polarization -rather than the existence of a correlated insulator -is the more fundamental phenomenon to take into account when trying to address the nature of the SC, and indeed only flavor polarization has played a role in shaping some aspects of our phenomenological analysis.…”