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Director: Pertti Hakonen
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LT » Nano Group » Josephson junctions » Junction interfaces

Junction interfaces

Tunnel junctions in normal metals are basic components of mesoscopic single electronics. Their study dates back to the 80'ies when the lithographic techniques became mature enough to produce well-defined tunnel junctions with a capacitance below 1 fF. Typically, they are made of aluminum but we have shown titanium is also one alternative.

We have studied normal tunnel junctions with well defined resistive environment. The resistive environment (3 - 300 kΩ) is made of a thin film chrome resistor with dimensions ~ 6 x 100 x 20000 nm³. These resistors behave as lumped elements at small bias voltages but, at higher voltages, they behave more as RC-transmission lines.

We have used these results, in particular, in analyzing the behavior of multiwalled carbon nanotubes. The metal-nanotube interface often behaves as a tunnel junction and, consequently, the environmental analysis can be employed to deduce the parameters of the nanotube as a transmission line [1].

Inverse proximity effect in superconductors near ferromagnetic material

We have studied the electronic density of states in a mesoscopic aluminum wire near a transparent interface with a ferromagnetic nickel metal. In our tunnel spectroscopy experiment, a substantial density of states is observed at sub-gap energies close to a ferromagnet. We compare our data with detailed calculations based on the Usadel equation, where the effect of the ferromagnet is treated as an effective boundary condition. We achieve an excellent agreement with theory when non-ideal quality of the interface is taken into account.

The proximity effect across a normal metal - superconductor interface can be investigated using a tunnel junction to probe the tunneling density of states (DOS) on either side of the interface. Below is a SEM-image of a sample in which the DOS is measured near an Al-Ni interface on the aluminum side using a copper probe behind an AlOx barrier.

Normalized differential conductance of a tunnel probe, when properties of S are affected by the proximity of a ferromagnet. The circles are the experimental data at a distance of 180 nm the interface, and that for the bulk Al (measured at T = 100 mK). The solid curves are the best fits to theory, with alpha = 7.5, EC / Delta = 1.5, rb = 3. The gap of Al was Δ = 0.22 mV.

Titanium single-electron transistor fabricated by electron-beam lithography

We have fabricated non-superconducting mesoscopic tunnel junctions by oxidation of Ti. The fabrication process uses conventional electron-beam lithography and shadow deposition through an organic resist mask. Superconductivity in Ti is suppressed by performing the deposition under a suitable background pressure.

We have employed this method to make single-electron transistors which operated at T < 0.4 K and had a moderate charge noise of 2.5 x 10^-3 e/√Hz at 10 Hz. Based on nonlinearities in the current-voltage characteristics at higher voltages, we deduce the oxide barrier height of approximately 110 mV. The non-superconducting Ti junctions can be useful in several applications.

IV-characteristics of Ti SET in the Coulomb blockade regime, with two extreme gate voltages giving the largest and the smallest width of the Coulomb gap. Inset: image of a sample similar to the measured one. The tunnel junctions are marked by arrows.

RC environment

Tunneling in a normal tunnel junction depends crucially on the electromangetic environment that the tunneling electron feels.

We have investigated asymptotic behavior of normal tunnel junctions at voltages where even the best ohmic environments start to look like RC transmission lines. In the experiments, this is manifested by an exceedingly slow approach to the linear behavior above the Coulomb gap. As expected on the basis of P(E)-theory, better fits are obtained using 1/√V- than 1/V-dependence for the asymptote. These results agree with the horizon picture if the frequency-dependent phase velocity is employed instead of the speed of light in order to determine the extent of the surroundings seen by the junction.

Reduced current vs. voltage for a sample with tunneling resistance 11.1 kΩ in an environment modelled by RC-transmission line.

Related publications

• Titanium single-electron transistor fabricated by electron-beam lithography

M.A. Sillanpää and P.J. Hakonen

Physica E 15, 41 (2002)

• Inverse proximity effect in superconductors near ferromagnetic material

M.A. Sillanpää, T.T. Heikkilä, R.K. Lindell, and P.J. Hakonen

Europhys. Lett. 56, 590 (2001)