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.

A fast all-electrical activation and control mechanism for biomolecular motor-powered nanoactuators has been developed. Rapid and reversible on–off control of actomyosin biomolecular motors was experimentally demonstrated using in vitro motility assays. The results show that the motility of the actin filaments can be cycled repeatedly by electrically controlled thermal activation in the temperature range from 10°C to 50°C without functional loss. The fast response of the filaments upon rapid temperature switching suggests that thermal activation provides an effective method for turning actomyosin-powered nanoactuators on and off.

Crossed Andreev reflection (CAR) in metallic nanostructures, a possible basis for solid-state electron entangler devices, is usually investigated by detecting non-local voltages in multi-terminal superconductor/normal metal devices. This task is difficult because other subgap processes may mask the effects of CAR. One of these processes is the generation of charge imbalance (CI) and the diffusion of non-equilibrium quasi-particles in the superconductor. Here we demonstrate a characteristic dependence of non-local CI on a magnetic field applied parallel to the superconducting wire, which can be understood by a generalization of the standard description of CI to non-local experiments. These results can be used to distinguish CAR and CI and to extract CI relaxation times in superconducting nanostructures. In addition, we investigate the dependence of non-local CI on the resistance of the injector and detector contacts and demonstrate a quantitative agreement with a recent theory using only material and junction characteristics extracted from separate direct measurements.

We report on the observation of a non-local voltage in a ballistic (quasi) one-dimensional conductor, realized by a single-wall carbon nanotube with four contacts. The contacts divide the tube into three quantum dots which we control by the back-gate voltage Vg. We measure a large oscillating non-local voltage V nl as a function of Vg. Though a classical resistor model can account for a non-local voltage including change of sign, it fails to describe the magnitude properly. The large amplitude of V nl is due to quantum interference effects and can be understood within the scattering-approach of electron transport.PACS numbers: 73.23.Ad,73.63.Fg,73.63.Nm,73.63.Kv,72.80.Rj The recent realization of the spin field-effect transistor in carbon nanotube (CNT) devices [1] demonstrated the ability to control spin transport in a quantum dot (QD) [2]. However, additional effects, such as the anomalous magnetoresistance, can contribute to the observed signal in spin-valves [3,4,5,6]. It seems clear, that despite a number of large responses seen in CNT-based devices [1,7,8,9], one needs to go beyond two terminal structures by realizing multi-terminal devices where non-local measurements are feasible [10]. The non-local measurement in spin-valve devices has been pioneered by Johnson and Silsbee [11] in metallic spin-valves and was further applied to various other systems [12,13,14]. This technique separates spin from charge effects. Recent application of the non-local spin technique in CNTs [10] showed the feasibility and yet tremendous challenge of performing such measurements in low dimensional mesoscopic systems. The hallmark of these measurements is that a positive voltage is measured when the magnetization of the injector and detector electrodes are parallel and a negative only when they are antiparallel. However, it has been reported recently that the four-probe resistance with non-magnetic probes in CNTs can be negative due to interference effects [15]. This suggests that the measurement of the non-local spin transport in mesoscopic systems like CNTs with ferromagnetic contacts should be strongly influenced by quantum interference effects.We report here on measurements of a large non-local voltage V nl in multi-terminal CNT devices (Fig. 1a) in the quantum-dot (QD) regime which changes sign and magnitude as the back-gate voltage is swept. We show that V nl cannot be explained by a classical resistor model. Instead, a quantum approach is required. We also show that in these devices, which have relative transparent contacts with resistances in the range of 10 − 100 kΩ, the magnitude of the oscillating V nl greatly exceeds any nonlocal spin signal.Our devices consist of single-wall CNTs grown by chemical vapor deposition (CVD) and contacted with four probes as shown in Fig 1a. Two middle electrodes are ferromagnetic (F) made of PdNi(20nm)/ Co(25nm)/ Pd(10nm) tri-layer, whereas the two outer probes are normal (N) Pd(40nm) electrodes. A PdNi alloy with 30% Pd is used, because it makes stable contacts to the CNT [1], whi...