Yuval Ronen Lab

Twisted trilayer graphene (TTG) provides a tunable moiré platform to study correlated phases emerging from flat-band physics. Here, we investigate the interplay between superconductivity and spontaneous orbital magnetism (OM) in alternating TTG devices with intermediate twist angles (1.381.44°). Using electrostatically defined Josephson junctions, we demonstrate that OM, stabilized near the charge neutrality point (CNP), competes with gate-induced superconductivity. The OM phase is characterized by sharp jumps in Hall resistance, current-induced bistability, and a CurieBloch temperature dependence, indicating broken time-reversal symmetry. Additionally, nonreciprocal Josephson transport─manifested as asymmetric Fraunhofer patterns and a superconducting diode effect─provides independent evidence of an orbital magnetic state confined to the weak link. The observed critical temperature hierarchy, where superconductivity dominates over OM at higher carrier densities and displacement fields, reveals a tunable competition between two broken-symmetry ground states. Our findings establish alternating TTG Josephson devices as a minimal and versatile platform to probe the coexistence of magnetism and superconductivity in engineered moiré systems.

Even-denominator fractional quantum Hall states are promising candidates for fault-tolerant quantum computing due to their underlying non-Abelian topological order. However, the topological order of these states remains hotly debated. Here, we report transport measurements on ultra-clean bilayer graphene heterostructures, where we observed four quarter-filled states and their corresponding Levin-Halperin daughter states, constraining their topological order. Moreover, we complete the sequence of half-filled plateaus by detecting states at ν=−32 and ν=12 whose daughters suggest an alternating sequence of non-Abelian orders. This pattern suggests a universal origin supporting their use in identifying topological order at even-denominator fillings, though further confirmation is needed via direct measurements. The observed quarter- and half-filled states appear in N = 0 and N = 1 Landau levels, respectively, and thus highlight a competition between interactions favoring paired states of either four- or two-flux composite fermions. Additionally, we observe several next-generation quantum Hall states that require strong interactions between composite fermions.

Topological and crystalline orders of electrons both benefit from enhanced Coulomb interactions in partially filled Landau levels. In bilayer graphene (BLG), the competition between fractional quantum Hall liquids and electronic crystals can be tuned electrostatically. Applying a displacement field leads to Landau-level crossings, where the interaction potential is strongly modified due to changes in the orbital wave functions. Here, we leverage this control to investigate phase transitions between topological and crystalline orders at constant filling factors in the lowest Landau level of BLG. Using transport measurements in high-quality hBN-encapsulated devices, we study transitions as a function of displacement field near crossings of N=0 and N=1 orbitals. The enhanced Landau-level mixing near the crossing stabilizes electronic crystals at all fractional fillings, including a resistive state at ν=13 and a reentrant integer quantum Hall state at ν=73. On the N=0 side, the activation energies of the crystal and fractional quantum Hall liquid vanish smoothly and symmetrically at the transition, while the N=1 transitions out of the crystal appear discontinuous. Additionally, we observe quantized plateaus forming near the crystal transition at half filling of the N=0 levels, suggesting a paired composite fermion state stabilized by Landau level mixing.