Perpendicular electric field drives Chern transitions and layer polarization changes in Hofstadter bands.

Moiré superlattices engineer band properties and enable observation of fractal energy spectra of Hofstadter butterfly. Recently, correlated-electron physics hosted by flat bands in small-angle moiré systems has been at the foreground. However, the implications of moiré band topology within the single-particle framework are little explored experimentally. An outstanding problem is understanding the effect of band topology on Hofstadter physics, which does not require electron correlations. Our work experimentally studies Chern state switching in the Hofstadter regime using twisted double bilayer graphene (TDBG), which offers electric field tunable topological bands, unlike twisted bilayer graphene. Here we show that the nontrivial topology reflects in the Hofstadter spectra, in particular, by displaying a cascade of Hofstadter gaps that switch their Chern numbers sequentially while varying the perpendicular electric field. Our experiments together with theoretical calculations suggest a crucial role of charge polarization changing concomitantly with topological transitions in this system. Layer polarization is likely to play an important role in the topological states in few-layer twisted systems. Moreover, our work establishes TDBG as a novel Hofstadter platform with nontrivial magnetoelectric coupling.

Dynamics of Interfacial Bubble Controls Adhesion Mechanics in Van der Waals Heterostructure.

Two-dimensional van der Waals heterostructures (vdWH) can result in novel functionality that crucially depends on interfacial structure and disorder. Bubbles at the vdWH interface can modify the interfacial structure. We probe the dynamics of a bubble at the interface of a graphene-hBN vdWH by using it as the drumhead of a NEMS device because nanomechanical devices are exquisite sensors. For drums with different interfacial bubbles, we measure the evolution of the resonant frequency and spatial mode shape as a function of electrostatic pulling. We show that the hysteretic detachment of layers of vdWH is triggered by the growth of large bubbles. The bubble growth takes place due to the concentration of stress resembling the initiation of fracture. The small bubbles at the heterostructure interface do not result in delamination as they are smaller than a critical fracture length. We provide insight into frictional dynamics and interfacial fracture of vdWH.

Bulk valley transport and Berry curvature spreading at the edge of flat bands.

2D materials based superlattices have emerged as a promising platform to modulate band structure and its symmetries. In particular, moiré periodicity in twisted graphene systems produces flat Chern bands. The recent observation of anomalous Hall effect (AHE) and orbital magnetism in twisted bilayer graphene has been associated with spontaneous symmetry breaking of such Chern bands. However, the valley Hall state as a precursor of AHE state, when time-reversal symmetry is still protected, has not been observed. Our work probes this precursor state using the valley Hall effect. We show that broken inversion symmetry in twisted double bilayer graphene (TDBG) facilitates the generation of bulk valley current by reporting experimental evidence of nonlocal transport in a nearly flat band system. Despite the spread of Berry curvature hotspots and reduced quasiparticle velocities of the carriers in these flat bands, we observe large nonlocal voltage several micrometers away from the charge current path - this persists when the Fermi energy lies inside a gap with large Berry curvature. The high sensitivity of the nonlocal voltage to gate tunable carrier density and gap modulating perpendicular electric field makes TDBG an attractive platform for valley-twistronics based on flat bands.

Fat embolism syndrome: Experience from an Australian trauma centre.

BACKGROUND: Fat Embolism Syndrome (FES) is a rare condition where circulating fat emboli can lead to life threatening multisystem dysfunction. Diagnosis of FES occurs mainly by exclusion. The aim of this study was to describe the injury, event details, and factors associated with the diagnosis of FES following trauma presenting to a Level 1 Trauma Centre in Melbourne, Australia. METHODS: Medical records of all patients with a complication of FES between 2006 and 2018 were retrospectively reviewed. Demographics, injury factors, hospital event details, radiological procedures and length of stay were collected. RESULTS: Thirty six patients, with median age of 26 years (IQR 19, 42) and median Injury Severity Score (ISS) of 18.5 (IQR 10, 27) were diagnosed with FES. Other associated factors included male gender (86%) and having one or more long bone fractures (97%). Of the major FES diagnostic criteria, 94% of patients experienced hypoxia, 36% had mental status changes, whereas only 11% had petechiae. Computed Tomography Pulmonary Angiogram (CTPA) was performed on 19 patients (53%) with all being negative for pulmonary embolus. Most demonstrated ground glass opacity (58%), and FES was reported as the likely cause of patient presentation in 53% of cases. CONCLUSION: FES following trauma was rare, with its diagnosis based on clinical factors including long bone fracture and hypoxia, and the exclusion of other respiratory diagnoses. CTPA was frequently used since 2008 and further research is warranted to determine if this radiological procedure can assist with more definitive diagnosis of FES.