Denis A. Bandurin

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 103 cm2 V-1 s-1 and 104 cm2 V-1 s-1 at 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 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayers 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.

Edge currents shunt the insulating bulk in gapped graphene

An energy gap can be opened in the spectrum of graphene reaching values as large as 0.2 eV in the case of bilayers. However, such gaps rarely lead to the highly insulating state expected at low temperatures. This long-standing puzzle is usually explained by charge inhomogeneity. Here we revisit the issue by investigating proximity-induced superconductivity in gapped graphene and comparing normal-state measurements in the Hall bar and Corbino geometries. We find that the supercurrent at the charge neutrality point in gapped graphene propagates along narrow channels near the edges. This observation is corroborated by using the edgeless Corbino geometry in which case resistivity at the neutrality point increases exponentially with increasing the gap, as expected for an ordinary semiconductor. In contrast, resistivity in the Hall bar geometry saturates to values of about a few resistance quanta. We attribute the metallic-like edge conductance to a nontrivial topology of gapped Dirac spectra.

Field emission spectroscopy evidence for dual-barrier electron tunnelling in nanographite

Nanocarbon films with upstanding flake-like graphite crystallites of nanometre thickness were fabricated by carbon condensation from a methane–hydrogen gas mixture activated by a direct-current discharge. The nanographite (NG) crystallites are composed of a few graphene layers. The adjacent atomic layers are connected partially at the edges of the crystallites to form strongly curved graphene structures. The extraordinary field emission (FE) properties were revealed for the NG films with an average current density of a few mA/cm2, reproducibly obtained at a macroscopic applied field of about 1 V/μm. The integral FE current–voltage curves and electron spectra (FEES) of NG cathodes with multiple emitters were measured in a triode configuration. Most remarkably, above a threshold field, two peaks were revealed in FEES with different field-dependent shifts to lower energies. This behaviour evidences electron emission through a dual potential barrier, corresponding to carbon–carbon heterostructure formed as a ...

Field emission spectroscopy of nanographite films

Nanocarbon films with high aspect ratio graphite crystallites were fabricated by plasma-enhanced chemical vapor deposition. In order to reveal the origin of their extraordinary field emission properties, integral current-voltage curves and electron spectra from two nanographite cathodes with different morphology were measured in triode configuration. An average current density of about 1 mA/cm2 was reproducibly obtained at an applied field of 1.2-1.8 V/μm due to high field enhancement. Most remarkably, above a threshold field the electron spectra of both samples revealed two peaks with different field-dependent shift to lower energy. The role of sp3 surface states for these results will be discussed at the conference.

Fluidity onset in graphene

Viscous electron fluids have emerged recently as a new paradigm of strongly-correlated electron transport in solids. Here we report on a direct observation of the transition to this long-sought-for state of matter in a high-mobility electron system in graphene. Unexpectedly, the electron flow is found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a wide temperature range, showing signatures of viscous flows only at relatively high temperatures. The transition between the two regimes is characterized by a sharp maximum of negative resistance, probed in proximity to the current injector. The resistance decreases as the system goes deeper into the hydrodynamic regime. In a perfect darkness-before-daybreak manner, the interaction-dominated negative response is strongest at the transition to the quasiballistic regime. Our work provides the first demonstration of how the viscous fluid behavior emerges in an interacting electron system.Experimental demonstrations of materials supporting electron fluids have been elusive so far. Here, the authors investigate nonlocal transport in bilayer graphene across the ballistic-to-hydrodynamic crossover, and identify a sharp maximum of negative resistance at the transition between the two regimes.

Homogeneous low-voltage field emission from nanographite films for cold cathode applications

Systematic field emission studies on PECVD grown nanocarbon films with vertically aligned thin graphite crystallites are reported. Field emission current maps of such nanographite cathodes with about 6 μm resolution revealed a number density of at least 7×104/cm2 well-distributed nearly homogeneous emission sites at field levels of 1-2 V/μm. Local measurements reproducibly yielded FN-like current-voltage curves with a β-factor of about 1000 up to currents of 10 μA. SEM images of high-current processed spots revealed the onset of cathode destruction. Short-term current fluctuations were reduced to ±10 % after one hour. Luminescent screen images confirmed the FE homogeneity over the whole cathode. Therefore, a maximum current density of about 0.2 A/cm2 can be achieved at voltages below 500 V. The application potential of these FE cathodes will be discussed.