arXiv subjects

Ruiheng Su

Publications and source records attributed to Ruiheng Su.

6 recordsLinked to original sources

arXiv2026

Signatures of a ferro-Josephson effect in twisted graphene

When a spin-polarized current is driven across a magnetic domain wall, the resulting spin-transfer torque may, beyond a critical threshold, set the wall's moments into precession. This precession modulates the Berry curvature experienced by electrons traversing the wall, producing an electromotive force that is topological in nature and proportional to the precession frequency, mapping precisely onto the DC Josephson effect and leading to the name ferro-Josephson effect. We report signatures consistent with this effect in a twisted graphene van der Waals heterostructure, where spin and valley textures are linked by exchange, Hund's coupling, and spin-orbit interactions. Tuned to fillings where the isospin degeneracy is spontaneously broken, the samples develop a sharp peak in the longitudinal resistance within a fraction of a millitesla of $B_\parallel=0$---a peak that disappears as the current is reduced toward zero. In differential resistance the feature resolves into sharp resonances that disperse with $B_\parallel$ on microtesla and picoampere scales. We argue that these arise from the current-driven precession of spin-domain-wall moments, in competition with the in-plane anisotropy set by a minuscule applied field, and that they establish nonlinear transport as a sensitive probe of isospin domain-wall dynamics at energy scales far below $k_BT$.

cond-mat.mes-hall↗

arXiv2026

Metastable magnetic domains and the anomalous $B_\parallel=0$ resistance peak in twisted double bilayer graphene

In graphene moir\'es, valley polarization gives rise to orbital magnetism, manifested as an anomalous Hall effect and resulting in Barkhausen jumps in longitudinal resistance when changing domain configurations modify quasiparticle scattering. Beyond a simple picture of polarized domains, however, spin and valley textures within and between the domains are less well understood, as is the effect of these textures on transport. In the valley-polarized quarter-metal state of twisted double bilayer graphene, a sharp and metastable peak in longitudinal resistance often appears at zero in-plane magnetic field, whose microscopic origin has yet to be identified. Here, we show that this peak depends on the configuration of domains of orbital magnetism, which is itself set by the gate-voltage trajectory used to enter the ordered state and by the magnetic field --- particularly the in-plane component --- present during that trajectory. The sensitivity of the effect to in-plane magnetic field components points to spin, linked to valley polarization through spin-orbit coupling, as the key degree of freedom in both the domain formation and the resistance peak.

cond-mat.mes-hall↗

arXiv2024

Topological electronic crystals in twisted bilayer-trilayer graphene

In a dilute two-dimensional electron gas, Coulomb interactions can stabilize the formation of a Wigner crystal. Although Wigner crystals are topologically trivial, it has been predicted that electrons in a partially-filled band can break continuous translational symmetry and time-reversal symmetry spontaneously to form a form of topological electron crystal known as an anomalous Hall crystal. Here, we report the observation of a generalized version of the anomalous Hall crystal in twisted bilayer-trilayer graphene, whose formation is driven by the moire potential. The crystal forms at a band filling factor of one electron per four moiré unit cells ($ν=1/4$) and quadruples the unit-cell area, coinciding with an integer quantum anomalous Hall effect. The Chern number of the state is exceptionally tunable, and can be switched reversibly between $+1$ and $-1$ by electric and magnetic fields. Several other topological electronic crystals arise in a modest magnetic field, originating from $ν=1/3$, $1/2$, $2/3$, and $3/2$. The quantum geometry of the folded bands is likely very different from that of the original parent band, enabling possible future discoveries of correlation-driven topological phenomena

cond-mat.mes-hall↗

arXiv2024

Interplay of electronic crystals with integer and fractional Chern insulators in moiré pentalayer graphene

The rapid development of moiré quantum matter has recently led to the remarkable discovery of the fractional quantum anomalous Hall effect, and sparked predictions of other novel correlation-driven topological states. Here, we investigate the interplay of electronic crystals with integer and fractional Chern insulators in a moiré lattice of rhomobohedral pentalayer graphene (RPG) aligned with hexagonal boron nitride. At a doping of one electron per moiré unit cell, we see a correlated insulator with a Chern number that can be tuned between $C=0$ and $+1$ by an electric displacement field, accompanied by an array of other such insulators formed at fractional band fillings, $ν$. Collectively, these states likely correspond to trivial and topological electronic crystals, some of which spontaneously break the discrete translational symmetry of the moiré lattice. Upon applying a modest magnetic field, a narrow region forms around $ν=2/3$ in which transport measurements imply the emergence of a fractional Chern insulator, along with hints of weaker states at other fractional $ν$. In the same sample, we also see a unique sequence of incipient Chern insulators arising over a broad range of incommensurate band filling near two holes per moiré unit cell. Our results establish moiré RPG as a fertile platform for studying the competition and potential intertwining of electronic crystallization and topological charge fractionalization.

cond-mat.mes-hall↗

arXiv2024

Topological flat bands in a family of multilayer graphene moiré lattices

Moiré materials host a wealth of intertwined correlated and topological states of matter, all arising from flat electronic bands with nontrivial quantum geometry. A prominent example is the family of alternating-twist magic-angle graphene stacks, which exhibit symmetry-broken states at rational fillings of the moiré band and superconductivity close to half filling. Here, we introduce a second family of twisted graphene multilayers made up of twisted sheets of $M$- and $N$-layer Bernal-stacked graphene flakes. Calculations indicate that applying an electric displacement field isolates a flat and topological moiré conduction band that is primarily localized to a single graphene sheet below the moiré interface. Phenomenologically, the result is a striking similarity in the hierarchies of symmetry-broken phases across this family of twisted graphene multilayers. Our results show that this family of structures offers promising new opportunities for the discovery of exotic new correlated and topological phenomena, enabled by using the layer number to fine tune the flat moiré band and its screening environment.

cond-mat.mes-hall↗

arXiv2022

Superconductivity in Twisted Double Bilayer Graphene Stabilized by WSe$_2$

Superconductivity has been previously observed in magic-angle twisted stacks of monolayer graphene but conspicuously not in twisted stacks of bilayer graphene, although both systems host topological flat bands and symmetry-broken states. Here, we report the discovery of superconductivity in twisted double bilayer graphene (TDBG) in proximity to WSe$_2$. Samples with twist angles 1.24$^\circ$ and 1.37$^\circ$ superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized states near van Hove singularities and next to regions with broken isospin symmetry, showing the correlation between a high density of states and the emergence of superconductivity in TDBG while revealing a possible role for isospin fluctuations in the pairing.

cond-mat.supr-con↗