Final Report: Novel Nanowires as Probes of Electron Coherence and Correlations in Restricted Geometries

Abstract

This is a final summary report of the research conducted under DEFG03-01ER4594G, which was a research program using metal nanostructures to examine quantum coherence of electrons in normal and ferromagnetic metals. This program was the PI'S first federal research grant, and by augmenting with other funds (Packard Foundation), this grant supported two graduate students during its duration. In normal metal nanostructures, quantum coherence was assessed by two independent techniques: weak localization magnetoresistance, and time-dependent universal conductance fluctuations (TDUCF noise). This work found that, in AuPd nanowires, the coherence information inferred from these two techniques were quantitatively consistent, even in the presence of magnetic impurity and phonon scattering. This confirmed theoretical expectations. However, in Ag and Au wires, the two techniques disagree, with noise measurements indicating a lower coherence length at low temperatures than weak localiztion. We have a candidate explanation for this, and are finishing these experiments. This work shows that subtleties remain in our understanding of coherence processes even in normal metals, particularly those involving the tunneling two-level systems that produce low frequency noise; this has relevance for quantum information processing implementations using metal devices. We have also studied time-dependent universal conductance fluctuations in ferromagnetic metals for the first time. The TDUCF in ferromagnetic nanowires show that the Cooperon channel of coherent processes is suppressed in these correlated materials. Furthermore, the surprisingly steep temperature dependence of the noise suggests that decoherence in these systems is through a different process than in normal metals. We are finishing nieasurements of "magnetofingerprint" conductance fluctuations in ferromagnetic metals to examine this unusual temperature dependence with an independent technique. This program has produced three papers (one Phys. Rev. B Rapid Communication, one PRB Brief Report, and a longer PRB article), with two more in preparation; it 1 1 s also resulted in six APS contributed talks by students, and two invited seminars by the PI. Over the last twenty years, tremendous progress has been made in understanding the role of quantum coherence in the dynamics of electrons in solids. Because the experimental consequences of quantum coherence are most readily detected in mesoscale systems, this discipline has come to be known as 'Lmesoscopics". The improved understanding of the role of quantum coherence in solid state systems has been driven by the development and widespread availability of sophisticated patterning and fabrication techniques with submicron resolution.Here I summarize the results of our research efforts using state-of-the-art nanostructures as tools to address fundamental, unresolved issues related to quantum coherence of electrons in conducting systems. The background and common methods of probing coherence physics are explained below in Sect...