Superconductivity in twisted double bilayer graphene stabilized by WSe2.

Identifying the essential components of superconductivity in graphene-based systems remains a critical problem in two-dimensional materials research. This field is connected to the mysteries that underpin investigations of unconventional superconductivity in condensed-matter physics. Superconductivity has been 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 WSe2. Samples with twist angles 1.24° and 1.37° superconduct in small pockets of the gate-tuned phase diagram within the valence and conduction band, respectively. Superconductivity emerges from unpolarized phases near van Hove singularities and next to regions with broken isospin symmetry. Our results show 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.

Spontaneous time-reversal symmetry breaking in twisted double bilayer graphene.

Twisted double bilayer graphene (tDBG) comprises two Bernal-stacked bilayer graphene sheets with a twist between them. Gate voltages applied to top and back gates of a tDBG device tune both the flatness and topology of the electronic bands, enabling an unusual level of experimental control. Metallic states with broken spin and valley symmetries have been observed in tDBG devices with twist angles in the range 1.2-1.3°, but the topologies and order parameters of these states have remained unclear. We report the observation of an anomalous Hall effect in the correlated metal state of tDBG, with hysteresis loops spanning hundreds of mT in out-of-plane magnetic field (B⊥) that demonstrate spontaneously broken time-reversal symmetry. The B⊥ hysteresis persists for in-plane fields up to several Tesla, suggesting valley (orbital) ferromagnetism. At the same time, the resistivity is strongly affected by even mT-scale values of in-plane magnetic field, pointing to spin-valley coupling or to a direct orbital coupling between in-plane field and the valley degree of freedom.

Thickness dependent transition from the 1T' to Weyl semimetal phase in ultrathin MoTe2: electrical transport, noise and Raman studies.

Bulk 1T'-MoTe2 shows a structural phase transition from the 1T' to Weyl semimetallic (WSM) Td phase at ∼240 K. This phase transition and transport properties in the two phases have not been investigated on ultra-thin crystals. Here we report electrical transport, 1/f noise and Raman studies on ultra-thin 1T'-MoTe2 (∼5 to 16 nm thick) field-effect transistor (FETs) devices as a function of temperature. The electrical resistivities for a thickness of 16 nm and 11 nm show maxima at temperatures of 208 K and 178 K, respectively, making a transition from the semiconducting to semi-metallic phase, hitherto not observed in bulk samples. Raman frequencies and linewidths for an 11 nm thick crystal show a change around 178 K, attributed to the additional contribution to the phonon self-energy due to the enhanced electron-phonon interaction in the WSM phase. Furthermore, the resistivity at low temperature shows an upturn below 20 K along with the maximum in the power spectral density of the low frequency 1/f noise. The latter rules out the metal-insulator transition (MIT) being responsible for the upturn of resistivity below 20 K. The low temperature resistivity follows ρ∝ 1/T, changing to ρ∝T with increasing temperature supports electron-electron interaction physics at electron-hole symmetric Weyl nodes below 20 K. These observations will pave the way to unravel the properties of the WSM state in layered ultra-thin van der Waals materials.

Tunability of 1/f Noise at Multiple Dirac Cones in hBN Encapsulated Graphene Devices.

The emergence of multiple Dirac cones in hexagonal boron nitride (hBN)-graphene heterostructures is particularly attractive because it offers potentially better landscape for higher and versatile transport properties than the primary Dirac cone. However, the transport coefficients of the cloned Dirac cones is yet not fully characterized and many open questions, including the evolution of charge dynamics and impurity scattering responsible for them, have remained unexplored. Noise measurements, having the potential to address these questions, have not been performed to date in dual-gated hBN-graphene-hBN devices. Here, we present the low-frequency 1/f noise measurements at multiple Dirac cones in hBN encapsulated single and bilayer graphene in dual-gated geometry. Our results reveal that the low-frequency noise in graphene can be tuned by more than two-orders of magnitude by changing carrier concentration as well as by modifying the band structure in bilayer graphene. We find that the noise is surprisingly suppressed at the cloned Dirac cone compared to the primary Dirac cone in single layer graphene device, while it is strongly enhanced for the bilayer graphene with band gap opening. The results are explained with the calculation of dielectric function using tight-binding model. Our results also indicate that the 1/f noise indeed follows the Hooge's empirical formula in hBN-protected devices in dual-gated geometry. We also present for the first time the noise data in bipolar regime of a graphene device.

Probing 2D black phosphorus by quantum capacitance measurements.

Two-dimensional materials and their heterostructures have emerged as a new class of materials, not only for fundamental physics but also for electronic and optoelectronic applications. Black phosphorus (BP) is a relatively new addition to this class of materials. Its strong in-plane anisotropy makes BP a unique material for making conceptually new types of electronic devices. However, the global density of states (DOS) of BP in device geometry has not been measured experimentally. Here, we report the quantum capacitance measurements together with the conductance measurements on an hBN-protected few-layer BP (∼six layers) in a dual-gated field effect transistor (FET) geometry. The measured DOS from our quantum capacitance is compared with density functional theory (DFT). Our results reveal that the transport gap for quantum capacitance is smaller than that in conductance measurements due to the presence of localized states near the band edge. The presence of localized states is confirmed by the variable range hopping seen in our temperature dependence conductivity. A large asymmetry is observed between the electron and hole side. This asymmetric nature is attributed to the anisotropic band dispersion of BP. Our measurements establish the uniqueness of quantum capacitance in probing the localized states near the band edge, hitherto not seen in conductance measurements.