Skip to main content

Quantum Transport in Nanoconstrictions made of no magnetic metals

Final Activity Report Summary - QTNMMM (Quantum Transport in Nanoconstrictions made of no magnetic metals)

Electrical conductance, G, is the reciprocal of the resistance and is a measure of how easily electricity flows along a certain path through an electrical element. When the electrical element size is shrunk down to several atoms in lateral dimensions, so that they become comparable to the Fermi wavelength, the electrons flowing through find themselves confined and the transport regime is called ballistic. Under these conditions, the conductance across that nanoconstriction becomes quantised and the conductance quantum unit is defined as 2e2/h (the so called GO) and corresponds to 12.9kO-1.

The main aim of our project was to investigate the striking spin-polarisation effect found in quantized conductance of quantum point contacts (QPC) made of Cu, which is a diamagnetic novel metal in bulk state. Those QPC, which are constrictions of a few atom in diameter formed between bulk-like leads, were originally created by tapping macroscopic Cu wires together, using the so-called mechanical controllable break junction (MCBJ) technique. To overcome different disadvantages that this MCBJ technique presented, preventing of gain a further understanding of the physics underlying that phenomenal spin-polarisation effect, we have developed a new approach to the production of mechanically stable, more reliable, highly reproducible and durable in time metallic QPC by combining an optical lithographic technique and an electrochemical method. By the later, the gap between the so lithographed electrodes onto a template is refilled and eventually a nanoconstriction is formed at the precise moment they come into touch. QPCs of different sizes and therefore controllable quantized conductance values were systematically produced at will.

We have run measurements of conductance versus applied magnetic field, Ha, either along or transversal to the current flow in ambient conditions on Cu QPCs of different lateral sizes, i. e. showing different values for the conductance ranging from 10GO down to 1GO. Our results clearly reveal no trace of field-dependent quantized conductance on steady and unstrained QPCs produced by electrodepositing Cu. By contrast, only for Ha transversal to the current flow configuration and when the QPCs are strained (under elongation stress) a phenomenal spin-polarisation effect is observed regardless the initial quantized conductance value for the QPC under study. At the switching field, which value has been established to be around 16-20 Oe, an abrupt step in G can be seen. The step-height depends on G(H=0), being always a value close to G(H=0)/2. For the same experimental conditions, but now for Ha along the current flow, the conductance remains unchanged up to our maximum experimental field, 1 kOe.

On the other hand, if the same type of experiment is carried out under special ambient conditions, i.e. running G versus Ha measurements applying stress to the QPCs, for a low content in oxygen concentration environment as that is the case for QPCs immersed in de-aired and distilled water, no change in the quantized conductance versus Ha has been observed regardless the field configuration up to 1 kOe.

This unique result is strong evidence that points to the formation of CuO nano-chains at the QPC rather than just Cu ones. In principle, no field-induced spin-polarisation phenomenon would be seen in QPCs made of a diamagnetic novel metal as Cu, for which no spin-polarisation is possible for the conductions electrons having s character. Consequently, a spin-filtering effect in Cu QPC needs the presence of oxygen to take place.