Advanced Nanofabrication for Quantum Material Exploration

Quantum materials (QMs) exhibit a diverse range of macroscopic phenomena, from unconventional and high temperature superconductivity, to exotic types of magnetism and topological states of matter. These novel functionalities have the potential to become part of radically novel technologies when such materials are nanofabricated into devices. A key defining feature of any correlated QM is its dimensionality. The effective dimensionality of the electronic ground state profoundly impacts the physical properties and can give rise to exotic phenomena that are prohibited in higher dimensions. A dramatic example is the phenomenon of spin-charge separation when charge is confined within a single conducting chain. Isolated, two-dimensional layers can also exhibit novel physics, due to enhanced correlation effects, as well as novel phases. Bulk materials with pure one- (1D) or two-dimensional (2D) electronic states, however, are rare. Most bulk QMs reside in the crossover region between these two extremes of dimensionality as invariably there exists finite coupling between adjacent chains or planes. Despite the remarkable expansion and refinement of experimental and theoretical toolboxes in recent decades, the intrinsic nature of the electronic state of these so-called quasi-1D or quasi-2D materials is still largely unknown. As a result, the unique properties of many low-dimensional QMs, remain poorly understood.

Given the vast range of functionalities accessible in QMs of differing dimensionality, having the ability to continuously tune the dimensionality of a chosen QM and thereby induce or destroy a particular ground state would open up a whole new playground for explorative research. In this proposal, we seek to develop such capability through the creation of a new experimental platform that exploits the power and versatility of Focused Ion Beam microscopes (FIBs). The FIB is an advanced nanofabrication tool that bypasses the limitations of other lithography techniques, since it directly images and etches material without a mask or template, and is therefore capable of sculpting small and irregular crystals into a desired architecture. FIBs can achieve spot sizes with resolution approaching 10 nm, as well as deposit material on the sub-micron scale. With such an instrument, we will be able to restrict charge and heat current to flow along specific crystallographic directions, as well as modify the size and geometry of what are often irregularly-shaped samples (and thus inaccessible to traditional lithographic techniques). We will also be able to make multiple electrical and/or thermal contacts to a single sample of limited dimensions, enabling multiple properties to be measured simultaneously, thereby greatly reducing the challenge of sample-to-sample variability. Even for larger samples, accessing nanoscale fabrication gives advantages: e.g. to generate high current densities in a superconductor. Finally, in many QMs, nanoscale defects may present novel physics locally, and isolating these defects for specific study can be a key step in understanding their macroscopic properties.

Historically, FIBs have primarily used a Ga source for the ion beam, which can give problems with Ga contamination. Recently, a new generation of Plasma FIBs (PFIBs) has been developed that have revolutionised the capabilities and versatility of this technique. Remarkably, up to 60 times more material can be removed by a PFIB compared to a traditional Ga FIB in an equivalent time. Crucially for this proposal, the removal of Ga as a contaminant will enable much more effective etching of the QMs that form a central part of the work packages outlined in this research programme. The programme itself will be performed on a PFIB recently purchased by the University through an EPSRC strategic equipment bid and will provide a transformational research platform with the potential to significantly enhance the world-leading status of QM research at Bristol.