Bilayer graphene bears an eightfold degeneracy due to spin, valley, and layer symmetry, allowing for a wealth of broken symmetry states induced by magnetic or electric fields, by strain, or even spontaneously by interaction. We study the electrical transport in clean current annealed suspended bilayer graphene. We find two kinds of devices. In bilayers of type B1 the eightfold zero-energy Landau level is partially lifted above a threshold field revealing an insulating ν=0 quantum-Hall state at the charge neutrality point. In bilayers of type B2 the Landau level lifting is full and a gap appears in the differential conductance even at zero magnetic field, suggesting an insulating spontaneously broken symmetry state. Unlike B1, the minimum conductance in B2 is not exponentially suppressed, but remains finite with a value G is < or approximately equall to e(2)/h even in a large magnetic field. We suggest that this phase of B2 is insulating in the bulk and bound by compressible edge states.

We show experimentally that in nanometer scaled superconductor/normal metal hybrid devices and in a small window of contact resistances, crossed Andreev reflection (CAR) can dominate the nonlocal transport for all energies below the superconducting gap. Besides crossed Andreev reflection, elastic cotunneling (EC) and nonlocal charge imbalance can be identified as competing subgap transport mechanisms in temperature dependent four-terminal nonlocal measurements. We demonstrate a systematic change of the nonlocal resistance vs. bias characteristics with increasing contact resistances, which can be varied in the fabrication process. For samples with higher contact resistances, CAR is weakened relative to EC in the midgap regime, possibly due to dynamical Coulomb blockade. Gaining control of crossed Andreev reflection is an important step towards the realization of a solid state entangler.Quantum mechanically entangled pairs of particles are a major building block of quantum computation and information processing. A natural source of entangled electrons is a BCS-type superconductor where the Cooper pairs (CP) form spin singlet states. The two electrons of a Cooper pair can be spatially separated into two different metallic leads in a nonlocal process called crossed Andreev reflection (CAR) [1, 2, 3,4].At temperatures (T ) well below the superconducting transition temperature, T c , and for bias potentials below the superconducting energy gap ∆, electrons from a normal metal contact (N) can enter the superconductor (S) only as Cooper pairs by a process known as Andreev reflection (AR). In this local process a hole is reflected into the same N to conserve momentum. If two normal metal contacts, N1 and N2, are spatially separated by less than the coherence length ξ, the two electrons forming a CP can originate from different normal contacts, see Fig. 1(a). This process opens an additional nonlocal conduction path known as CAR. An inverse process was proposed as the basis of a solid-state entangler: the electrons of a CP are split between the two leads while retaining their entanglement from the superconductor. However, this method of creating entangled particles can be accompanied by two additional processes that lead to correlated signals on N1 and N2, but not to entanglement. In the first, a single electron from N1 can reach the other contact N2 by elastic cotunneling (EC) [5, 6,7], see Fig. 1(b). In the second, called nonlocal charge imbalance (CI), electrical charge can be transferred to the second contact by the diffusion of quasi-particles generated by finite temperatures or finite bias.Recently, the relative strength of these subgap processes was the subject of extensive theoretical work. Standard BCS theory predicts that to lowest order in the tunneling rates, CAR and EC exactly cancel in normal metal/insulator/superconductor (NIS) systems at low T and bias [6]. This cancelation can be lifted for higher transmissions [8], by spin-active interfaces [9] or ferromagnetic contacts [7], by disorder, or by electron-e...

We show nonlinear transport experiments on clean, suspended bilayer graphene that reveal a gap in the density of states. Looking at the evolution of the gap in magnetic fields of different orientation, we find that the groundstate is a spin-ordered phase. Of the three possible gapped groundstates that are predicted by theory for equal charge distribution between the layers, we can therefore exclude the quantum anomalous Hall phase, leaving the layer antiferromagnet and the quantum spin Hall phase as the only possible gapped groundstates for bilayer graphene.

In this Letter we demonstrate that Permalloy (Py), a widely used Ni/Fe alloy, forms contacts to carbon nanotubes (CNTs) that meet the requirements for the injection and detection of spin-polarized currents in carbon-based spintronic devices. We establish the material quality and magnetization properties of Py strips in the shape of suitable electrical contacts and find a sharp magnetization switching tunable by geometry in the anisotropic magnetoresistance (AMR) of a single strip at cryogenic temperatures. In addition, we show that Py contacts couple strongly to CNTs, comparable to Pd contacts, thereby forming CNT quantum dots at low temperatures. These results form the basis for a Py-based CNT spin-valve exhibiting very sharp resistance switchings in the tunneling magnetoresistance, which directly correspond to the magnetization reversals in the individual contacts observed in AMR experiments.Comment: 3 page

We present non-linear transport measurements on suspended, current annealed bilayer graphene devices. Using a multi-terminal geometry we demonstrate that devices tend to be inhomogeneous and host two different electronic phases next to each other. Both of these phases show gap-like features of different magnitude in non-linear transport at low charge carrier densities, as already observed in previous studies. Here, we investigate the magnetic field dependence and find that both features grow with increasing field, the smaller one with 0.6 meV/T, the larger one with a 5-10 times higher field dependence. We attribute the larger of the two gaps to an interaction induced broken symmetry state and the smaller one to localization in the more disordered parts of the device.