F. Freitag

Nanolithography and manipulation of graphene using an atomic force microscope

We use an atomic force microscope (AFM) to manipulate graphene films on a nanoscopic length scale. By means of local anodic oxidation with an AFM we are able to structure isolating trenches into single-layer and few-layer graphene flakes, opening the possibility of tabletop graphene based device fabrication. Trench sizes of less than 30 nm in width are attainable with this technique. Besides oxidation we also show the influence of mechanical peeling and scratching with an AFM of few layer graphene sheets placed on different substrates.

From one electron to one hole: quasiparticle counting in graphene quantum dots determined by electrochemical and plasma etching.

Graphene is considered to be a promising material for future electronics. The envisaged transistor applications often rely on precision cutting of graphene sheets with nanometer accuracy. In this letter we demonstrate graphene-based quantum dots created by using atomic force microscopy (AFM) with tip-assisted electrochemical etching. This lithography technique provides resolution of about 20 nm, which can probably be further improved by employing sharper tips and better humidity control. The behavior of our smallest dots in magnetic field has allowed us to identify the charge neutrality point and distinguish the states with one electron, no charge and one hole left inside the quantum dot.

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.

Giant fluctuations and gate control of the g-factor in InAs nanowire quantum dots

We study the g-factor of discrete electron states in InAs nanowire based quantum dots. The g values are determined from the magnetic field splitting of the zero bias anomaly due to the spin 1/2 Kondo effect. Unlike to previous studies based on 2DEG quantum dots, the g-factors of neighboring electron states show a surprisingly large fluctuation: g can scatter between 2 and 18. Furthermore electric gate tunability of the g-factor is demonstrated.

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.

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.

Conductance fluctuations in graphene devices with superconducting contacts in different charge density regimes

Conductions fluctuations (CF) are studied in single layer graphene devices with superconducting source and drain contacts made from aluminium. The CF are found to be enhanced by superconductivity by a factor of 1.4-2. This (near) doubling of the CF indicates that the phase coherence length is l(phi)greater than or similar to L/2. As compared to previous work, we find a relatively weak dependence of the CF on the gate voltage, and hence on the carrier density. We also demonstrate that whether the CF are larger or smaller at the charge neutrality point (CNP) can be strongly dependent on the series resistance R(C), which needs to be subtracted