Tuning the electronic properties of SrTiO3 with ionic liquid gating

Strontium titanate (SrTiO3) can be doped from an electronically insulating to semi-conducting state for a relatively low concentration of dopants. Subsequent cooling to sufficiently low temperatures sees SrTiO3 exhibit a zero resistance superconducting state, which persists even for very dilute doped samples and is at odds with currently accepted mechanisms of how superconductivity emerges.

A channel of highly mobile electrons called a two dimensional electron gas (2DEG) is seen at SrTiO3 interfaces prepared with a certain crystal structure, called a heterostructure, which is otherwise unseen in bulk SrTiO3. Many common methods to dope SrTiO3 face limitations in the extent to which low and high concentrations of electrons can be accumulated at this interface. This project uses an alternative doping approach called ionic liquid (IL) gating to accumulate electrons to extremes of low and high concentration with a large degree of tuneability, thereby surpassing other doping methods.

This degree of tuneability is promising with regards to developing an alternative to conventional computer microprocessors called 'Mottronics', wherein current signals are either impeded or relayed through the microprocessor depending on whether the Mott transistor is in an 'on' metallic or an 'off' insulating state. IL gating can be used to accumulate very large concentrations of electrons for relatively small applied voltages, which enables a large current flow through computer chips for relatively low turn-on voltages, thereby reducing power consumption and excess heating.

From a fundamental physics perspective, IL gating can very precisely tune the increments of doping at the critical points where the superconducting state disappears, which may offer further insight into the mechanism behind which superconductivity emerges in SrTiO3. Exotic electronic phenomena has previously been observed as the concentration of electrons is further lowered in IL gated SrTiO3, and may be the only method that can provide such low concentrations at low temperatures to verify theoretical predictions of a Bose Einstein condensate existing in zirconium-doped SrTiO3.

This project will use IL's specifically synthesised to increase the accumulation of electrons at the interfacial 2DEG in both undoped and niobium- and zirconium- doped SrTiO3 heterostructure devices to investigate improvements in the mobility of electrons, to identify exotic electronic phenomena around and below the emergence of the superconducting state for lower electron concentrations, and to look for signatures of Bose Einstein behaviour.

Cleanroom fabrication will be used to fabricate SrTiO3 heterostructure devices, and improvements in the electron mobility will be investigated by placing a thin, 2D layer of hexagonal boron nitride as a barrier between the IL and the substrate. Following fabrication, transfer characteristic measurements will be conducted in a cryostat to assess device performance at low temperatures, and high gate voltages will be applied to maximise accumulation for each IL. Magnetic field measurements in a dilution refrigerator will then be conducted to determine the extent of electron accumulation along the channel, to investigate changes in the transition temperature to superconducting state, and to identify the emergence of exotic electronic phenomena such as the Kondo effect and anomalous Hall behaviour under the influence of a magnetic field.

This project will also explore the electronic properties of other materials including KTaO3, UO2, and chalcogenide 2D materials on SrTiO3, to see if the methods of electronic doping developed in the first stage of the project can then be applied to other materials to induce exotic electronic properties.

The Institute of Physics has estimated that physics-dependent businesses directly contribute 8.5% to the UK's economic output, employ more than a million people and generated exports amounting to more than £100bn in 2009. They go on to say: "It is important for businesses to have access to a range of highly skilled (and motivated) individuals capable of thinking 'outside of the box', particularly physics-trained postgraduates due to the highly numerate, analytical and problemsolving skills that are acquired during their training." If funded, the graduates of this CDT will have such skills and motivation. We would hope that this would significantly contribute towards satisfying the UK's need for trained scientists, particularly in the field of condensed matter physics. The impact would go further than this. By working more closely with industry and other partner organisations, we would reshape the conventional PhD programme to improve the experience for all.

In addition to the training aspect of the CDT there would be an important research impact. The Universities of Bristol and Bath have many world-leading researchers across the condensed matter field. By working with the high-quality students that we hope to recruit into the programme we will produce significant cutting edge research in condensed matter. The research would bear on some of the grand challenges facing condensed matter physics such as: understanding the emergence of new phenomena far from equilibrium; the nanoscale design of functional materials such as graphene; and harnessing quantum Physics for new technologies. Ultimately, this would contribute to improvements in many technologies, for example, energy or data storage technology.