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Quantum transport in Cu nanowires

Investigating the spin-dependent quantum transport in nanowires made from Cu

In this project we have investigated the spin-dependent quantum transport in nanowires made from macroscopic Cu wires. These wires were made using a break junction method. Two wires are brought into contact and form an electrical contact. At the same time physical bonds are also made between the two wires, but because there are relatively few of them they are weak and easily broken. However, as the wires separate the physical contact undergoes a necking process getting thinner and longer (see figure below) until a nanowire is formed. The experiments have been performed in air so while the Cu wires used to make the wires are pure (99.99%+) the nanowires will have been oxidized. pull-nanowires (28K)

quantum1

The figure illustrates the process of creating a nanowire, a contact is stretched and a necking process occurs and so a nanowire is formed. The figure shows two kinds of atoms Cu (orange) and O (blue), oxygen will be present since the experiments have been performed in air.

 

 

 

The figure below shows some of the data gathered. (a) and (b) are conductance histograms taken at 0 and 5mT respectively. From these we observe two features:

1. That the conduction is quantized - this is more easily seen in (c) which shows the conductance against time.

2. There is a significant effect of applying a magnetic field.

It is highly tempting to interpret this as a sign that we have lifted the spin degeneracy in the nanowire; however, there are other mechanisms that can cause fractional order peaks independent of an applied magnetic field. What we can say is that this is a very significant effect, especially when one considers that bulk Cu is non-magnetic.

quantum2The figure illustrates the process of creating a nanowire, a contact is stretched and a necking process occurs and so a nanowire is formed. The figure shows two kinds of atoms Cu (orange) and O (blue), oxygen will be present since the experiments have been performed in air.

 

This figure shows some of the data gathered during the experiment. (a) & (b) are conductance histograms taken at 0 and 5mT respectively. (c) shows a conductance vs. time curve this clearly shows steps in units of e2/h.

As mentioned above these experiments were done in air so it is almost certain that the nanowires will not be made of pure Cu, but will be oxidized to CuO. To test this theory calculations were done by Professor Wu at the University of California, Irvine. These confirmed that pure Cu would not cause these effects but that CuO chains could. This project is ongoing and we aim to experimentally test the predictions of Professor Wu and to clarify the role of chemically induced effects by adsorbates on the quantum transport.

Some references to this work can be found in the following articles:

 

* D. M. Gillingham, I. Linington, and J. A. C. Bland. Journal of Physics Condensed Matter, 14(29):L567, 2002.

* D. M. Gillingham, I. Linington, C. Muller, and J. A. C. Bland. Journal of Applied Physics, 93(10):7388, 2003.

* D. M. Gillingham, C. Muller, and J. A. C. Bland. Journal of Physics- Condensed Matter, 15(19):L291, 2003.

* D. M. Gillingham, C. Muller, and J. A. C. Bland. Journal of Applied Physics, 95(11):6995, 2004.

* J. Hong, R. Q. Wu, D. M. Gillingham, C. Muller, and J. A. C. Bland. to be published.