Periodic Reporting for period 4 - HiTIMe (High Frequency Topological Insulator devices for Metrology)
Periodo di rendicontazione: 2021-11-01 al 2022-10-31
In this project we study and exploit the properties of 3D topological insulator (TI) materials incorporated into high frequency devices. The main driver of the project is the prospect of using a TI nanoribbon to create a topologically protected single-electron charge pump that can be used as a metrological quantum current standard, or in other words to lay the technological foundations for a TI-based device that can realize the SI Ampere.
An accurate charge pump that can operate at temperatures and magnetic fields achievable using affordable table-top systems would be of immediate use in the realization of the Ampere. The technological development in this project will lay the groundwork for charge pumping in TI nanoribbons, as well as for other devices that exploit the unique properties of TI for high-frequency applications including sensing, precision measurement and topologically protected quantum computation.
Materials science has always been intertwined with the development of new electronic devices and new innovations are rapidly adopted by industry and the research community if it is shown that they enable novel functionality or economic benefits. Topological insulators is an example of a new class of quantum materials that is on the cusp of finding applications in electronic devices. Focus so far has mostly been on improving our understanding of the many fascinating properties of TI materials, but it is now becoming clear that they possess electronic properties that make them interesting for a wide range of applications.
An accurate charge pump that can operate at temperatures and magnetic fields achievable using affordable table-top systems would be of immediate use in the realization of the Ampere. The technological development in this project will lay the groundwork for charge pumping in TI nanoribbons, as well as for other devices that exploit the unique properties of TI for high-frequency applications including sensing, precision measurement and topologically protected quantum computation.
Materials science has always been intertwined with the development of new electronic devices and new innovations are rapidly adopted by industry and the research community if it is shown that they enable novel functionality or economic benefits. Topological insulators is an example of a new class of quantum materials that is on the cusp of finding applications in electronic devices. Focus so far has mostly been on improving our understanding of the many fascinating properties of TI materials, but it is now becoming clear that they possess electronic properties that make them interesting for a wide range of applications.
The initial work has been focused on setting up all the required infrastructure to carry out the research within the consortium, implement the project management structure and create a framework for public outreach.
WP1 has seen the development of novel high quality materials: work on both charge doping, for suppression of TINR bulk conductance (giving access to the TI surface states alone), and magnetic doping to control the TI surface state gap has continued. Nanoribbons with Fe doping up to 8% have been achieved. We are working to improve the homogeneity of the doping.
In WP2 we have made good progress with the development of measurement tools to more rapidly characterise a large number of TINRs; Initial results shows that sSNOM can be used to detect the carrier concentration and potentially also signatures of the topological surface states. We have developed theory to understand the sSNOM response for multilayer material stacks. In parallel we have set up both cryogenic 2-port microwave nanoscale imaging capabilities and achieved atomic resolution tunneling spectroscopy imaging of nanoribbons synthesised by partners using the 4-probe STM system at NPL.
In WP3 we have:
1) demonstrated gating of a single TINR through the Dirac point on STO substrates. We are also using hBN flakes to decouple the nanoribbons from the substrate and to avoid the formation of a 2DEG at the bottom interface. This is a crucial step towards the required control of TINR properties for implementing useful devices not influenced neither by the bulk nor by a trivial 2DEG.
2) realised tunable resonators for wireless interrogation of TINRs. They have also been used to characterise the topological surface states. A patent for the novel tunable resonator design has been filed. The measurements have shown that one needs to reduce the diameter of the nanoribbon or to suspend it to be able to detect the specific ESR resonances. Theory has been developed to help the extraction of the TINR properties from the resonator measurements.
3) theoretically explored a nanoribbon geometrical model which can potentially be used to form a quantum dot. It consists of locally reducing the nanoribbon width to form the quantum dot. We are implementing this geometry in real devices.
4) first realisation of the topological insulator quantum dot
In summary, during third period of the project we are progressing towards the realisation of the final device.
WP1 has seen the development of novel high quality materials: work on both charge doping, for suppression of TINR bulk conductance (giving access to the TI surface states alone), and magnetic doping to control the TI surface state gap has continued. Nanoribbons with Fe doping up to 8% have been achieved. We are working to improve the homogeneity of the doping.
In WP2 we have made good progress with the development of measurement tools to more rapidly characterise a large number of TINRs; Initial results shows that sSNOM can be used to detect the carrier concentration and potentially also signatures of the topological surface states. We have developed theory to understand the sSNOM response for multilayer material stacks. In parallel we have set up both cryogenic 2-port microwave nanoscale imaging capabilities and achieved atomic resolution tunneling spectroscopy imaging of nanoribbons synthesised by partners using the 4-probe STM system at NPL.
In WP3 we have:
1) demonstrated gating of a single TINR through the Dirac point on STO substrates. We are also using hBN flakes to decouple the nanoribbons from the substrate and to avoid the formation of a 2DEG at the bottom interface. This is a crucial step towards the required control of TINR properties for implementing useful devices not influenced neither by the bulk nor by a trivial 2DEG.
2) realised tunable resonators for wireless interrogation of TINRs. They have also been used to characterise the topological surface states. A patent for the novel tunable resonator design has been filed. The measurements have shown that one needs to reduce the diameter of the nanoribbon or to suspend it to be able to detect the specific ESR resonances. Theory has been developed to help the extraction of the TINR properties from the resonator measurements.
3) theoretically explored a nanoribbon geometrical model which can potentially be used to form a quantum dot. It consists of locally reducing the nanoribbon width to form the quantum dot. We are implementing this geometry in real devices.
4) first realisation of the topological insulator quantum dot
In summary, during third period of the project we are progressing towards the realisation of the final device.
Progress beyond state of the art: So far, the major leap forward is a new understanding of the impact the substrate has on the doping/charge accumulation of the TI surfaces and novel device implementations that reduce such effects. One potential solution has also been found where the 2DEG formation is avoided by using an hBN flake to decouple the nanoribbon from the oxide substrate. Several other tasks in the project have also lead to new results which can be found in the published papers, and the ones to be submitted. Further scientific impact has been delivered through 10 conference presentations throughout the reporting period.
We expect that until the end of the project we have built solid theoretical and experimental foundations that will allow us to realise novel devices, showing enhanced control of single charge carriers in topological materials and novel methods to characterise them.
Preliminary results on the realisation of the quantum dot are promising
Socio-economic impact: The project has now employed 6 new personnel out of which 3 are full time PhD students. Training and working in the leading scientific environments provided by the partner organisations will lead to highly skilled personnel and future European scientific leadership.
We expect that until the end of the project we have built solid theoretical and experimental foundations that will allow us to realise novel devices, showing enhanced control of single charge carriers in topological materials and novel methods to characterise them.
Preliminary results on the realisation of the quantum dot are promising
Socio-economic impact: The project has now employed 6 new personnel out of which 3 are full time PhD students. Training and working in the leading scientific environments provided by the partner organisations will lead to highly skilled personnel and future European scientific leadership.