Multi-valley superconductivity in ion-gated MoS2_2 layers

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of cor- relations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS2, de- velop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K’ points of the Brillouin Zone. However, experimental and theoretical results suggest that a multi-valley Fermi surface (FS) is associated with the SC state, involving 6 elec- tron pockets at the Q/Q’ points. Here, we perform low-temperature transport measurements in ion-gated MoS2 flakes. We show that a fully multi-valley FS is associated with the SC onset. The Q/Q’ valleys fill for doping& 2 · 1013cm−2, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q1 and Q2. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors.We acknowledge funding from EU Graphene Flagship, ERC Grant Hetero2D, EPSRC Grant Nos. EP/509K01711X/1, EP/K017144/1, EP/N010345/1, EP/M507799/ 5101, an

Nanomechanical probing of the layer/substrate interface of an exfoliated InSe sheet on sapphire

Van der Waals (vdW) layered crystals and heterostructures have attracted substantial interest for potential applications in a wide range of emerging technologies. An important, but often overlooked, consideration in the development of implementable devices is phonon transport through the structure interfaces. Here we report on the interface properties of exfoliated InSe on a sapphire substrate. We use a picosecond acoustic technique to probe the phonon resonances in the InSe vdW layered crystal. Analysis of the nanomechanics indicates that the InSe is mechanically decoupled from the substrate and thus presents an elastically imperfect interface. A high degree of phonon isolation at the interface points toward applications in thermoelectric devices, or the inclusion of an acoustic transition layer in device design. These findings demonstrate basic properties of layered structures and so illustrate the usefulness of nanomechanical probing in nanolayer/nanolayer or nanolayer/substrate interface tuning in vdW heterostructures

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103cm2V-1s-1and 104cm2V-1s-1at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.EU, EPSRC. The Royal Societ

Multi-Valley Superconductivity in Ion-Gated MoS2 Layers

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of correlations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS2, develop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K′ points of the Brillouin Zone. However, experimental and theoretical results suggest that a multivalley Fermi surface (FS) is associated with the SC state, involving six electron pockets at Q/Q′. Here, we perform low-temperature transport measurements in ion-gated MoS2 flakes. We show that a fully multivalley FS is associated with the SC onset. The Q/Q′ valleys fill for doping ≳ 2 × 10^13 cm^–2, and the SC transition does not appear until the Fermi level crosses both spin–orbit split sub-bands Q 1 and Q 2. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors

Multi-valley superconductivity in ion-gated MoS2_2 layers

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of cor- relations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS2, de- velop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K’ points of the Brillouin Zone. However, experimental and theoretical results suggest that a multi-valley Fermi surface (FS) is associated with the SC state, involving 6 elec- tron pockets at the Q/Q’ points. Here, we perform low-temperature transport measurements in ion-gated MoS2 flakes. We show that a fully multi-valley FS is associated with the SC onset. The Q/Q’ valleys fill for doping& 2 · 1013cm−2, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q1 and Q2. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors

Gate tuneable ultrafast charge transfer in graphene/MoS2 heterostructures

We report ultrafast pump-probe measurements on a graphene/MoS2 heterostructure and demonstrate sub picosecond exciton dissociation and charge transfer from MoS2 to graphene, one order of magnitude faster than in type II two-dimensional heterostructures. The process can be controlled by applying an external gate and shifting the Fermi level of graphene. For pump-probe measurements we excite the gate controlled graphene/MoS2 heterostructure at 400 nm, well above the MoS2 bandgap, and probe the normalized differential transmission changes (ΔT/T) of the MoS2 first exciton (A exciton) at 660nm with time resolution~200fs. In this configuration, MoS2 acts as the absorbing material for visible wavelengths while graphene is the electron scavenger

Gate tuneable ultrafast charge transfer in graphene/MoS<inf>2</inf>heterostructures

Gate tuneable ultrafast charge transfer in graphene/MoS2heterostructure