Two-dimensional materials, such as graphene and monolayer hexagonal BN (h-BN), are attractive for demonstrating fundamental physics in materials and potential applications in next-generation electronics. Atomic sheets containing hybridized bonds involving elements B, N and C over wide compositional ranges could result in new materials with properties complementary to those of graphene and h-BN, enabling a rich variety of electronic structures, properties and applications. Here we report the synthesis and characterization of large-area atomic layers of h-BNC material, consisting of hybridized, randomly distributed domains of h-BN and C phases with compositions ranging from pure BN to pure graphene. Our studies reveal that their structural features and bandgap are distinct from those of graphene, doped graphene and h-BN. This new form of hybrid h-BNC material enables the development of bandgap-engineered applications in electronics and optics and properties that are distinct from those of graphene and h-BN.

We investigate by electrical transport the field-induced superconducting state (FISC) in the organic conductor λ-(BETS)2FeCl4. Below 4 K, antiferromagnetic-insulator, metallic, and eventually superconducting (FISC) ground states are observed with increasing in-plane magnetic field. The FISC state survives between 18 and 41 T, and can be interpreted in terms of the Jaccarino-Peter effect, where the external magnetic field compensates the exchange field of aligned Fe 3+ ions. We further argue that the Fe 3+ moments are essential to stabilize the resulting singlet, two-dimensional superconducting state Superconductivity is usually destroyed by diamagnetic currents induced in the presence of strong magnetic fields. This effect has orbital character and prevails in most conventional "s-wave" superconductors that involve singlet state of the Cooper pairs. In addition, superconductivity can also be suppressed by the Pauli pair breaking mechanism: here the external field destroys the spinsinglet state of the Cooper pair, imposing the so-called Clogston-Chandrasekhar paramagnetic limit [1,2]. Nevertheless, and despite these well known physical limitations, S. Uji et al. [3] have recently reported the observation of a magnetic-field induced superconducting phase (FISC) in the quasi-two-dimensional organic conductor λ-(BETS) 2 FeCl 4 for fields exceeding 18 tesla, applied parallel to the conducting layers. This is particularly remarkable since this compound, at zero field, is an antiferromagnetic insulator (AI) below T p ∼ = 8.5K [4]. The AI state is suppressed by the application of magnetic fields above 10 tesla at low temperatures [5].The present work was motivated by the apparent increase in the critical temperature of the FISC above 18 T with increasing magnetic field (Ref. [3]). Here, for instance, in the case of spin-triplet superconductivity, there would be in principle, no limit on the upper critical field. The presence of Fe 3+ magnetic moments, which coexist with the FISC state, adds further appeal to the triplet state model. To clarify the nature of the FISC, we have studied the λ-(BETS) 2 FeCl 4 compound at low temperatures in steady, tilted magnetic fields up to 42 tesla. Our main result is the observation of reentrance towards the metallic state at a temperature-dependent critical field. We obtain a temperature-magnetic field phase diagram for the FISC state, which we interpret in terms of the Jacarino-Peter (JP) field compensation effect [6]. This implies that the Cooper pairs condense into a spin-singlet state. We argue further that the Fe 3+ magnetic state is indeed necessary to stabilize the singlet superconducting state by suppression of diamagnetic currents in the associated in-plane high magnetic fields.λ-(BETS) 2 FeCl 4 (where BETS stands for Bis(ethylenedithio)tetraselenafulvalene) crystallizes in a triclinic unit cell. The BETS planar molecules are stacked along the crystallographic a-axis, and constitute conducting planes parallel to the a-c plane. These conducting layers alternate along the b-axis...

The study of two-dimensional (2D) electronic systems is of great fundamental significance in physics. Atomic layers containing hybridized domains of graphene and hexagonal boron nitride (h-BNC) constitute a new kind of disordered 2D electronic system. Magnetoelectric transport measurements performed at low temperature in vapor phase synthesized h-BNC atomic layers show a clear and anomalous transition from an insulating to a metallic behavior upon cooling. The observed insulator to metal transition can be modulated by electron and hole doping and by the application of an external magnetic field. These results supported by ab initio calculations suggest that this transition in h-BNC has distinctly different characteristics when compared to other 2D electron systems and is the result of the coexistence between two distinct mechanisms, namely, percolation through metallic graphene networks and hopping conduction between edge states on randomly distributed insulating h-BN domains.

Shubnikov-de Haas oscillations for two well-defined frequencies, corresponding, respectively, to areas of 0.8 and 1.36% of the first Brillouin zone, were observed in single crystals of Na 0:3 CoO 2 . The existence of Na superstructures in Na 0:3 CoO 2 , coupled with this observation, suggests the possibility that the periods are due to the reconstruction of the large Fermi surface around the ÿ point. An alternative interpretation in terms of the long sought-after " 0 g pockets is also considered but found to be incompatible with existing specific heat data. DOI: 10.1103/PhysRevLett.97.126401 PACS numbers: 71.18.+y, 71.30.+h, 72.15.Gd A number of theoretical treatments have suggested that the nature of the superconducting pairing mechanism in hydrated Na x CoO 2 is unconventional and that it probably corresponds to a spin-triplet state [1][2][3][4]. Nevertheless, the experimental situation remains unclear with heat capacity experiments in the superconducting state suggesting that the electronic contribution can either be described in terms of an order parameter having nodal lines [5], a hypothesis supported by muon spin resonance ( SR) experiments [6], or simply in terms of inhomogeneity in the Na content [7]. Measurements of the 59 Co nuclear magnetic resonance Knight shift supports either spin-triplet [8] or singlet pairing [9]. At the same time SR experiments [10] find no indication of static moments in the superconducting state, implying that time reversal symmetry is not broken.For conventional superconductivity, as well as for most unconventional superconductivity scenarios, the pairing mechanism and consequently the superconducting transition temperature critically depend on the electronic structure near the Fermi level. An accurate description of the Fermi surface (FS) is therefore critical for the superconductivity of the hydrated Na 0:3 CoO 2 whose FS size and precise shape still is a central but unsettled issue. Localdensity approximation (LDA) calculations [11] for the unhydrated NaCo 2 O 4 compound indicate that two-bands, the A 1g and one of the " 0 g bands, cross the Fermi level creating, respectively, a large hexagonal Fermi surface around the ÿ point of the Brillouin zone (BZ) and six small elliptical pockets of holes near the K point.This Fermi surface geometry is the starting point for several of the proposed theories of unconventionally mediated superconductivity [1], where the existence of small nearly perfectly elliptical hole pockets resulting from the " 0 g band is essential. [3,4] However, angle-resolved photoemission (ARPES) on Na x CoO 2 (for 0:3 x 0:72) reveals only a single FS centered around the ÿ point whose area changes with x according to the Luttinger theorem [12,13], while the " 0 g band and the associated FS pockets were found to sink below the Fermi energy independently of the doping level or temperature [13]. This discrepancy between ARPES and LDA calculations were claimed to result either from strong electronic correlations [14 -16] or Na disorder [17].For over half a c...