Ballistic transport occurs whenever electrons propagate without collisions deflecting their trajectory. It is normally observed in conductors with a negligible concentration of impurities, at low temperature, to avoid electron-phonon scattering. Here, we use suspended bilayer graphene devices to reveal a new regime, in which ballistic transport is not limited by scattering with phonons or impurities, but by electron-hole collisions. The phenomenon manifests itself in a negative four-terminal resistance that becomes visible when the density of holes (electrons) is suppressed by gate-shifting the Fermi level in the conduction (valence) band, above the thermal energy. For smaller densities transport is diffusive, and the measured conductivity is reproduced quantitatively, with no fitting parameters, by including electron-hole scattering as the only process causing velocity relaxation. Experiments on a trilayer device show that the phenomenon is robust and that transport at charge neutrality is governed by the same physics. Our results provide a textbook illustration of a transport regime that had not been observed previously and clarify the nature of conduction through charge-neutral graphene under conditions in which carrier density inhomogeneity is immaterial. They also demonstrate that transport can be limited by a fully electronic mechanism, originating from the same microscopic processes that govern the physics of Dirac-like plasmas.Ever since Sharvin's pioneering work 1 , the occurrence of ballistic transport in metallic conductors has been exploited to investigate the electronic properties of solids. In the quasi-classical regime, for instance, magnetic focusing experiments have allowed probing electron-dynamics and the shape of the Fermi surface in ultra-pure crystals of different metals 2 , in semiconducting heterostructures 3,4 , and -more recently-in graphene-based systems 5,6 . In the quantum regime, when the electron wavelength and the conductor size are comparable, ballistic motion normally leads to conductance quantization and allows probing transport through individual quantum channels 7,8 . In all cases, the observation of ballistic transport requires the elastic mean free path determined by collisions of electrons with impurities to be longer than the system size, and the rate of inelastic processes such as phonon scattering to be sufficiently small. Electron-electron collisions are less detrimental, as they only slowly de-collimate a focused beam of ballistic electrons influencing their trajectories gradually, with effects that usually become relevant at rather high temperature 6,9 .

Suspended Bernal-stacked graphene multilayers up to an unexpectedly large thickness exhibit a broken-symmetry ground state, whose origin remains to be understood. Here we show that a finitetemperature second order phase transition occurs in multilayers whose critical temperature Tc increases from 12 K in bilayers to 100 K in heptalayers. A comparison of the data to a phenomenological model inspired by a mean field approach suggests that the transition is associated with the appearance of a self-consistent valley-and spin-dependent staggered potential changing sign from one layer to the next, appearing at Tc and increasing upon cooling. The systematic evolution with thickness of several measured quantities imposes constraints on any microscopic theory aiming to analyze the nature of electronic correlations in this system.

We perform magnetotransport experiments on VI 3 multilayers to investigate the relation between ferromagnetism in bulk and in exfoliated layers. The magnetoconductance measured on field-effect transistors and tunnel barriers shows that the Curie temperature of exfoliated multilayers is T C = 57 K, larger than in bulk (T C,bulk = 50 K). Below T ≈ 40 K, we observe an unusual evolution of the tunneling magnetoconductance, analogous to the phenomenology observed in bulk. Comparing the magnetoconductance measured for fields applied in-or out-of-plane corroborates the analogy, allows us to determine that the orientation of the easy-axis in multilayers is similar to that in bulk, and suggests that the in-plane component of the magnetization points in different directions in different layers. Besides establishing that the magnetic state of bulk and multilayers are similar, our experiments illustrate the complementarity of magnetotransport and magneto-optical measurements to probe magnetism in 2D materials.

We report the electron doping of single-layer graphene (SLG) grown by chemical vapor deposition (CVD) by means of dissociative hydrogen adsorption. The transfer characteristic showed n-type doping behavior similar to that of mechanically exfoliated graphene. Furthermore, we studied the thermoelectric power (TEP) of CVD-grown SLG before and after exposure to high-pressure H 2 molecules. From the TEP results, which indicate the intrinsic electrical properties, we observed that the CVD-grown SLG is n-type doped without degradation of the quality after hydrogen adsorption. Finally, the electron doping was also verified by Raman spectroscopy. V C 2015 AIP Publishing LLC.

We report the experimental investigation of transport through bilayer graphene (BLG)/chromium trihalide (CrX3; X=Cl, Br, I) van der Waals interfaces. In all cases, a large charge transfer from BLG to CrX3 takes place (reaching densities in excess of 10 13 cm −2 ), and generates an electric field perpendicular to the interface that opens a band gap in BLG. We determine the gap from the activation energy of the conductivity and find excellent agreement with the latest theory accounting for the contribution of the sigma bands to the BLG dielectric susceptibility. We further show that for BLG/CrCl3 and BLG/CrB3 the band gap can be extracted from the gate voltage dependence of the low-temperature conductivity, and use this finding to refine the gap dependence on magnetic field. Our results allow a quantitative comparison of the electronic properties of BLG with theoretical predictions and indicate the presence of correlations in electrons occupying CrX3 conduction band.