Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene

Bilayer graphene has attracted considerable interest due to the important role played by many-body effects, particularly at low energies. Here we report local compressibility measurements of a suspended graphene bilayer. We find that the energy gaps at filling factors v = 4 do not vanish at low fields, but instead merge into an incompressible region near the charge neutrality point at zero electric and magnetic field. These results indicate the existence of a zero-field ordered state and are consistent with the formation of either an anomalous quantum Hall state or a nematic phase with broken rotational symmetry. At higher fields, we measure the intrinsic energy gaps of broken-symmetry states at v = 0, 1 and 2, and find that they scale linearly with magnetic field, yet another manifestation of the strong Coulomb interactions in bilayer graphene.Comment: 9 pages, including 4 figures and supplementary material

Transconductance fluctuations as a probe for interaction induced quantum Hall states in graphene

Transport measurements normally provide a macroscopic, averaged view of the sample, so that disorder prevents the observation of fragile interaction induced states. Here, we demonstrate that transconductance fluctuations in a graphene field effect transistor reflect charge localization phenomena on the nanometer scale due to the formation of a dot network which forms near incompressible quantum states. These fluctuations give access to fragile broken-symmetry and fractional quantum Hall states even though these states remain hidden in conventional magnetotransport quantities.Comment: 6 pages, 3 figure

Ferroelectric and anomalous quantum Hall states in bare rhombohedral trilayer graphene

Nontrivial interacting phases can emerge in elementary materials. As a prime example, continuing advances in device quality have facilitated the observation of a variety of spontaneous quantum Hall-like states, a cascade of Stoner-like magnets, and an unconventional superconductor in bilayer graphene. Its natural extension, rhombohedral trilayer graphene is predicted to be even more susceptible to interactions given its even flatter low-energy bands and larger winding number. Theoretically, five spontaneous quantum Hall phases have been proposed to be candidate ground states. Here, we provide transport evidence for observing four of the five competing ordered states in interaction-maximized, dually-gated, rhombohedral trilayer graphene. In particular, at vanishing but finite magnetic fields, two states with Chern numbers 3 and 6 can be stabilized at elevated and low electric fields, respectively, and both exhibit clear magnetic hysteresis. We also reveal that the quantum Hall ferromagnets of the zeroth Landau level are ferroelectrics with spontaneous layer polarizations even at zero electric field, as evidenced by electric hysteresis. Our findings exemplify the possible birth of rich interacting electron physics in a simple elementary material

What can the activation energy tell about the energetics at grain boundaries in polycrystalline organic films?

Charge-carrier transport at the semiconductor-gate dielectric interface in organic field-effect transistors is critically dependent on the degree of disorder in the typically semi-crystalline semiconductor layer. Charge trapping can occur at the interface as well as in the current-carrying semiconductor layer itself. A detailed and systematic understanding of the role of grain boundaries between crystallites and how to avoid their potentially detrimental effects is still an important focus of research in the organic electronics community. A typical macroscopic measurement technique to extract information about the energetics of the grain boundaries is an activation energy measurement. Here, we compare detailed experiments on the energetic properties of monolayer thin films implemented in organic field-effect transistors, having controlled numbers of grain boundaries within the channel region to kinetic Monte-Carlo simulations of charge-carrier transport to elucidate the influence of grain boundaries on the extracted activation energies. Two important findings are: 1) whereas the energy at the grain boundary does not change with the number of grain boundaries in a thin film, both the measured and simulated activation energy increases with the number of grain boundaries. 2) In simulations where both energy barriers and valleys are present at the grain boundaries there is no systematic relation between the number of grain boundaries and extracted activation energies. We conclude, that a macroscopic measurement of the activation energy can serve as general quality indicator of the thin film, but does not allow microscopic conclusions about the energy landscape of the thin film

Electronic decoupling of an epitaxial graphene monolayer by gold intercalation

The application of graphene in electronic devices requires large scale epitaxial growth. The presence of the substrate, however, usually reduces the charge carrier mobility considerably. We show that it is possible to decouple the partially sp3-hybridized first graphitic layer formed on the Si-terminated face of silicon carbide from the substrate by gold intercalation, leading to a completely sp2-hybridized graphene layer with improved electronic properties.Comment: 7 pages, 4 figures, 1 tabl

Anisotropic Strain Induced Soliton Movement Changes Stacking Order and Bandstructure of Graphene Multilayers

The crystal structure of solid-state matter greatly affects its electronic properties. For example in multilayer graphene, precise knowledge of the lateral layer arrangement is crucial, since the most stable configurations, Bernal and rhombohedral stacking, exhibit very different electronic properties. Nevertheless, both stacking orders can coexist within one flake, separated by a strain soliton that can host topologically protected states. Clearly, accessing the transport properties of the two stackings and the soliton is of high interest. However, the stacking orders can transform into one another and therefore, the seemingly trivial question how reliable electrical contact can be made to either stacking order can a priori not be answered easily. Here, we show that manufacturing metal contacts to multilayer graphene can move solitons by several $\mu$m, unidirectionally enlarging Bernal domains due to arising mechanical strain. Furthermore, we also find that during dry transfer of multilayer graphene onto hexagonal Boron Nitride, such a transformation can happen. Using density functional theory modeling, we corroborate that anisotropic deformations of the multilayer graphene lattice decrease the rhombohedral stacking stability. Finally, we have devised systematics to avoid soliton movement, and how to reliably realize contacts to both stacking configurations

Highly Efficient and Scalable Separation of Semiconducting Carbon Nanotubes via Weak Field Centrifugation

The identification of scalable processes that transfer random mixtures of single-walled carbon nanotubes (SWCNTs) into fractions featuring a high content of semiconducting species is crucial for future application of SWCNTs in high-performance electronics. Herein we demonstrate a highly efficient and simple separation method that relies on selective interactions between tailor-made amphiphilic polymers and semiconducting SWCNTs in the presence of low viscosity separation media. High purity individualized semiconducting SWCNTs or even self-organized semiconducting sheets are separated from an as-produced SWCNT dispersion via a single weak field centrifugation run. Absorption and Raman spectroscopy are applied to verify the high purity of the obtained SWCNTs. Furthermore SWCNT - network field-effect transistors were fabricated, which exhibit high ON/OFF ratios (10$^{5}$) and field-effect mobilities (17 cm$^{2}$/Vs). In addition to demonstrating the feasibility of high purity separation by a novel low complexity process, our method can be readily transferred to large scale production

Interaction-driven (quasi-) insulating ground states of gapped electron-doped bilayer graphene

Bernal bilayer graphene has recently been discovered to exhibit a wide range of unique ordered phases resulting from interaction-driven effects and encompassing spin and valley magnetism, correlated insulators, correlated metals, and superconductivity. This letter reports on a novel family of correlated phases characterized by spin and valley ordering, observed in electron-doped bilayer graphene. The novel correlated phases demonstrate an intriguing non-linear current-bias behavior at ultralow currents that is sensitive to the onset of the phases and is accompanied by an insulating temperature dependence, providing strong evidence for the presence of unconventional charge carrying degrees of freedom originating from ordering. These characteristics cannot be solely attributed to any of the previously reported phases, and are qualitatively different from the behavior seen previously on the hole-doped side. Instead, our observations align with the presence of charge- or spin-density-waves state that open a gap on a portion of the Fermi surface or fully gapped Wigner crystals. The resulting new phases, quasi-insulators in which part of the Fermi surface remains intact or valley-polarized and valley-unpolarized Wigner crystals, coexist with previously known Stoner phases, resulting in an exceptionally intricate phase diagram