Periodic Reporting for period 3 - IMAGINE (Non-Invasive Imaging of Nanoscale Electronic Transport)
Periodo di rendicontazione: 2022-10-01 al 2024-03-31
Electronic transport in nanostructures and thin films shows a rich variety of physical effects that have been fundamental to the development of modern electronics and communication devices. The standard method for investigating electronic transport – resistance measurements – does not provide any information on the nanoscale current distribution in such structures. The lack of spatial information is unfortunate, because the current distribution plays a key role in many intriguing physical phenomena. Having a technique at hand that could simply look at nanoscale current flow would be immensely valuable.
The goal of this project is to implement a sensitive magnetic microscopy technique that can directly image the current distribution in nanostructures with sub-50-nm spatial resolution. Our approach is based on the recent technique of scanning diamond magnetometry, a scanned-probe method that utilizes a single spin in a diamond tip as a high-resolution sensor of magnetic field. Scanning diamond magnetometry exploits quantum metrology to achieve very high sensitivities, and has recently enabled a breakthrough in the passive analysis of magnetic surfaces. Our project has three overall objectives: (i) Lay the instrumental and conceptual groundwork required for imaging tiny (nanoampere) current variations in two-dimensional conductors. (ii) Demonstrate imaging of a variety of mesoscopic transport features on a well-established model system: Mono- and bilayer graphene. (iii) Explore the potential of our technique for probing electronic properties beyond transport, like phase transitions and photoexcitation.
Together, our experiments are designed to establish a powerful new technology for imaging current distributions non-invasively and with nanometer spatial resolution. This capability will provide the unique opportunity for directly looking at electronic transport in nanostructures, with a potentially transformative impact on condensed matter physics, materials science and electrical engineering.
The goal of this project is to implement a sensitive magnetic microscopy technique that can directly image the current distribution in nanostructures with sub-50-nm spatial resolution. Our approach is based on the recent technique of scanning diamond magnetometry, a scanned-probe method that utilizes a single spin in a diamond tip as a high-resolution sensor of magnetic field. Scanning diamond magnetometry exploits quantum metrology to achieve very high sensitivities, and has recently enabled a breakthrough in the passive analysis of magnetic surfaces. Our project has three overall objectives: (i) Lay the instrumental and conceptual groundwork required for imaging tiny (nanoampere) current variations in two-dimensional conductors. (ii) Demonstrate imaging of a variety of mesoscopic transport features on a well-established model system: Mono- and bilayer graphene. (iii) Explore the potential of our technique for probing electronic properties beyond transport, like phase transitions and photoexcitation.
Together, our experiments are designed to establish a powerful new technology for imaging current distributions non-invasively and with nanometer spatial resolution. This capability will provide the unique opportunity for directly looking at electronic transport in nanostructures, with a potentially transformative impact on condensed matter physics, materials science and electrical engineering.
Work during the first 36 months of the project has focused on setting up a scanning magnetometer operating at sub-Kelvin temperatures, on fabricating a first generation of graphene devices, and on exploring initial questions of nanoscale transport in graphene at room temperature. Two important results could be obtained: First, we demonstrated scanning NV magnetometry at temperatures down to 350 mK (arXiv:2203.15527; accepted at Appl. Phys. Lett). Although mainly a technical achievement, this is an important prerequisite for follow-up transport imaging experiments on graphene bilayers and other materials systems. In addition, the result demonstrates the general feasibility of sub-Kelvin NV magnetometry in spite of issues like NV charge stability, laser and microwave heating. Second, we demonstrated sensitive (~5 nT) imaging of current flow in bilayer graphene devices (arXiv:2201.06934; accepted at Phys. Rev. Applied). Beyond fulfilling a technical milestone, the experiment gave first insights into transport in bilayer graphene at room temperature: The absence of any hydrodynamics (as opposed to monolayer graphene), and current focusing by so-called “bubbles” in the encapsulating hexagonal boron nitride. Finally, in an unrelated and exploratory side project, the current density in antiferromagnetic memory cell was studied and linked to magnetic re-orientation (arXiv:1912.05287).
Several technical milestones were achieved that go beyond the state-of-the-art at the beginning of the project. The first milestone is the demonstrated scanning NV magnetometry at temperatures down to 350 mK (arXiv:2203.15527); previously, the lowest reported temperatures were around 2K. This progress was enabled by installing a dilution refrigerator with a dedicated microscopy insert, and the development of efficient microwave delivery and low-power quantum readout protocols. The second milestone is the scanning NV magnetometry of AC currents down to below 300 nA amperes, with a corresponding sensitivity of 20 nA/um (arXiv:2201.06934). To our knowledge, this is the best sensitivity reported for scanning NV magnetometry in terms of current imaging, and on par with state-of-the-art scanning superconducting quantum interference devices.
In the second part of the project we will shift the focus from technical demonstrations to the physics of nanoscale transport, including features of hydrodynamic and ballistic transport, effects of magnetic field, and influence of device edges. Further, we will use some of the knowledge gained to study magnetotransport in the context of spintronic devices.
In the second part of the project we will shift the focus from technical demonstrations to the physics of nanoscale transport, including features of hydrodynamic and ballistic transport, effects of magnetic field, and influence of device edges. Further, we will use some of the knowledge gained to study magnetotransport in the context of spintronic devices.