Jelena Trbovic

Spontaneously gapped ground state in suspended bilayer graphene

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.

All-electrical switching and control mechanism for actomyosin-powered nanoactuators

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.

Permalloy-based carbon nanotube spin-valve

In this paper 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.

Contact resistance dependence of crossed Andreev reflection

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 CAR, elastic cotunneling (EC) and nonlocal charge imbalance (CI) 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 CAR is an important step towards the realization of a solid state entangler.

Zeeman splitting in ferromagnetic Schottky barrier contacts based on doped EuS

Schottky barrier contacts of nonstoichiometric EuS, a ferromagnetic semiconductor, have been fabricated on Si (100) substrate and the current–voltage (I–V) characteristics are investigated at temperatures 150–5.0K. The electrical transport across such Schottky contacts is found to be dominated by thermionic emission at high temperatures, while at low temperatures and low biases, electron tunneling is dominant. The lower bound estimate of the Schottky barrier heights was obtained by analyzing the high-bias current–voltage characteristics. A decrease in barrier height of 0.26±0.06eV was deduced from the I–V characteristics as the temperature decreases below the ferromagnetic ordering temperature (TC) of the EuS. The variation of the barrier height below TC is the result of a spontaneous Zeeman splitting of the conduction band, and its temperature dependence resembles that of the spontaneous moment in EuS. The results point to the plausibility of using doped EuS as a spin injector and detector.

Superconductivity-enhanced conductance fluctuations in few-layer graphene.

We investigate the mesoscopic disorder induced rms conductance variance delta G in short few-layer graphene (FLG) flakes contacted by two superconducting (S) Ti/Al contacts. By sweeping the back-gate voltage, we observe pronounced conductance fluctuations superimposed on a linear background of the two-terminal conductance G. The linear gate voltage induced response can be modelled by a set of interlayer and intralayer capacitances. delta G depends on temperature T and source-drain voltage V(sd). delta G increases with decreasing T and |V(sd)|. When lowering |V(sd)|, a pronounced cross-over at a voltage corresponding to the superconducting energy gap Delta is observed. For [Formula: see text] the fluctuations are markedly enhanced. Expressed in the conductance variance G(GS) of one graphene-superconductor (G-S) interface, values of 0.58 e(2)/h are obtained at the base temperature of 230 mK. The conductance variance in the sub-gap region is larger by up to a factor of 1.4-1.8 compared to the normal state. The observed strong enhancement is due to phase coherent charge transfer caused by Andreev reflection at the G-S interface.

Spin symmetry of the bilayer graphene ground state

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 ground state is a spin-ordered phase. Of the three possible gapped ground states 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 ground states for bilayer graphene

Magnetic field and contact resistance dependence of non-local charge imbalance

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.

Large oscillating non-local voltage in multi-terminal single wall carbon nanotube devices

We report on the observation of a nonlocal 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

Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy

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