No abstract

We use temperature-dependent resistivity in small-angle twisted double bilayer graphene to measure bandwidths and gaps of the bands. This electron-hole asymmetric system has one set of non-dispersing bands that splits into two flat bands with the electric field -distinct from the twisted bilayer system. With electric field, the gap between two emergent flat bands increases monotonically and bandwidth is tuned from 1 meV to 15 meV. These two flat bands with gap result in a series of thermally induced insulator to metal transitions -we use a model, at charge neutrality, to measure the bandwidth using only transport measurements. Having two flat bands with tunable gap and bandwidth offers an opportunity to probe the emergence of correlations. arXiv:2001.09916v1 [cond-mat.mes-hall]

Single and twinned plates of 2D layered BiI3 have been found to be piezoelectric and can be used as a nanoscale pressure sensor.

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.

Metal induced decrease of crystallization temperature of sol-gel derived titanium dioxide (TiO 2 ) thin films is reported. It is shown that the Au induced onset of crystallization occurs at a temperature of 250 C as compared to 400 C when it is deposited directly on the same substrate. The crystallization process is probed using X-ray diffraction and confirmed by Raman spectroscopy. The onset of crystallization is evidenced by the appearance of the diffraction peak from the (101) plane of anatase TiO 2 and the peak due to A 1g + B 1g Raman mode at 515 cm À1 . Polarized optical microscopy and Raman imaging indicated that the spatial spread of crystallization across the surface of the film increases with the increase in temperature. Unipolar resistive switching is demonstrated by fabricating an Au/TiO 2 /Au stack, which shows formation at 9 V, set voltage of 0.5 V and reset voltage of 3.3 V. The maximum set-reset resistance ratio achieved was 10 5 . The mechanism of resistive switching is established by a correlation with photoluminescence spectra which indicates the presence of defects that aid in the switching process.