Periodic Reporting for period 4 - IMAGINE (Non-Invasive Imaging of Nanoscale Electronic Transport)
Période du rapport: 2024-04-01 au 2024-09-30
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 was to implement a sensitive magnetic microscopy technique that can directly image the current distribution in nanostructures with sub-50-nm spatial resolution. Our approach was based on the recent technique of scanning diamond magnetometry, a scanned-probe method that utilizes a single spin in a diamond tip – a single nitrogen-vacancy (NV) center – 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 had three overall objectives: (i) Build up the instrumentation and develop the sensing concepts 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, photoexcitation or magnetotransport.
Together, our experiments have established a powerful new technology for imaging current distributions non-invasively and with nanometer spatial resolution. Our instrument provides the unique opportunity for directly looking at electronic transport in nanostructures, down to very low (sub-Kelvin) temperatures. This capability has been demonstrated on graphene and superconducting devices, two areas of strong fundamental and technological interest. The instrument design has indicated new directions for diamond-based quantum sensing technology that will be exploited in the future.
The goal of this project was to implement a sensitive magnetic microscopy technique that can directly image the current distribution in nanostructures with sub-50-nm spatial resolution. Our approach was based on the recent technique of scanning diamond magnetometry, a scanned-probe method that utilizes a single spin in a diamond tip – a single nitrogen-vacancy (NV) center – 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 had three overall objectives: (i) Build up the instrumentation and develop the sensing concepts 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, photoexcitation or magnetotransport.
Together, our experiments have established a powerful new technology for imaging current distributions non-invasively and with nanometer spatial resolution. Our instrument provides the unique opportunity for directly looking at electronic transport in nanostructures, down to very low (sub-Kelvin) temperatures. This capability has been demonstrated on graphene and superconducting devices, two areas of strong fundamental and technological interest. The instrument design has indicated new directions for diamond-based quantum sensing technology that will be exploited in the future.
Work during the first half of the project has focused on setting up a scanning magnetometer operating at sub-Kelvin temperatures. This build-up has resulted in a one-of-its-kind instrument that, to the best of our knowledge, is the only scanning diamond magnetometer system world-wide operating at such low temperatures. The successful operation was demonstrated by imaging the Meissner screening from superconducting aluminum micro-discs. These experiments also verified that NV centers remain stable at those temperatures and retain their high sensitivity.
In a second part, several key demonstrations were made by imaging several prominent samples. The first demonstration included the imaging of current flow in bilayer and monolayer graphene devices; in the latter, the formation of current whirlpools could be observed, which are a hallmark of hydrodynamic transport. Notably, this observation could already be made at room temperature, which is rare for these kinds of transport experiments. A second demonstration included the imaging of gate-controlled suppression of superconductivity (a “superconducting transistor”) via the Meissner screening. While many resistance-based experiments have been conducted, our instrument allowed for the first direct observation of this notable effect by imaging.
The instrumentation and capabilities realized with the ERC grant will on the one hand enable many more imaging studies of nanoscale electronic transport and superconductivity. Further, we will use some of the knowledge gained to study magnetotransport in the context of spintronics. On the other hand, we expect that some of the technology developed can be exploited in the future to build more compact, more stable diamond NV sensors, especially for cryogenic experiments.
In a second part, several key demonstrations were made by imaging several prominent samples. The first demonstration included the imaging of current flow in bilayer and monolayer graphene devices; in the latter, the formation of current whirlpools could be observed, which are a hallmark of hydrodynamic transport. Notably, this observation could already be made at room temperature, which is rare for these kinds of transport experiments. A second demonstration included the imaging of gate-controlled suppression of superconductivity (a “superconducting transistor”) via the Meissner screening. While many resistance-based experiments have been conducted, our instrument allowed for the first direct observation of this notable effect by imaging.
The instrumentation and capabilities realized with the ERC grant will on the one hand enable many more imaging studies of nanoscale electronic transport and superconductivity. Further, we will use some of the knowledge gained to study magnetotransport in the context of spintronics. On the other hand, we expect that some of the technology developed can be exploited in the future to build more compact, more stable diamond NV sensors, especially for cryogenic experiments.
Both technical and experimental 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; 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 technical milestone is the scanning NV magnetometry of AC currents down to below 300 nA amperes, with a corresponding sensitivity of 20 nA/um. 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.
On the experimental side, two key demonstrations were made that advanced our physical understanding of charge transport in graphene resp. Superconducting devices. The observation of current whirlpools in monolayer graphene demonstrated that this theoretically predicted and indirectly measured (through negative resistance) effect indeed exists and, somewhat surprisingly, can be readily seen at room temperature. Likewise, the observation that superconductivity can be suppressed by a gate voltage – evidenced via the Meissner effect – had only been observed in resistance-based transport measurements.
On the experimental side, two key demonstrations were made that advanced our physical understanding of charge transport in graphene resp. Superconducting devices. The observation of current whirlpools in monolayer graphene demonstrated that this theoretically predicted and indirectly measured (through negative resistance) effect indeed exists and, somewhat surprisingly, can be readily seen at room temperature. Likewise, the observation that superconductivity can be suppressed by a gate voltage – evidenced via the Meissner effect – had only been observed in resistance-based transport measurements.