Imaging electrons at the nanoscale

At the mesoscale, electrons move in fascinating ways. They can zip through a device like a bullet without scattering, they can flow freely along edges of an otherwise insulating material, or they can refract and interfere much like waves of light. The study of mesoscopic physics crucially relies on designing clever device geometries and inferring the electron behavior inside a device based on how electrical currents flow into various electrical contacts. This can be a powerful approach, but often leaves us wondering if our inferences are correct, or even if there is some underlying complexity yet to be discovered.  Thus, our group has a history of and is actively pursuing research in imaging electrons at the nanoscale.

Fig. 1 Branching trajectories out of a quantum point contact in GaAs showcasing narrow, branching trajectories with no obvious scattering.

Our primary approach is referred to scanned gate microscopy (SGM) where we navigate nanometer-scale needles near a transport device, and by applying an electrical potential, locally deflect the trajectory of electrons near the tip and thus change the overall resistance we measure. By systematically scanning the tip near the device surface, we can take pictures of how electrons flow, and uncover information on how they bounce, deflect or interfere in interesting model systems.

Fig. 2 a) Image of our scanning probe system which is built into a closed-cycle dilution refrigerator. We developed and built custom vibration isolation system to filter out the mechanical noise in the fridge. b) we image surfaces with sharpened probes attached to “tuning forks”. We utilize HEMT amplifiers to read out vibrations of the tuning fork to improve our imaging capability.  c) Schematic of a tip scanning over a sample of interest. d) electron “pong paddle” gate that is sharpened in a focused ion beam. e) false-colored coaxial tip for making sharper SGM images.

Presently, we are expanding on this technique, carving the sharp needles into linear “electron pong paddles” to act as mirrors, and we are making “coaxial tips” to overcome the inherent blurriness of traditional SGM.  We are also working on building a highly stable imaging system in a new style of cryogenic refrigerator that reaches low temperatures without wasting liquid helium. This system is "cryogen-free" in the sense that no liquid helium is used for cooling, and instead a closed cycle cryocooler is used to reach 3 K, where a traditional dilution circuit then cools the sample to base temperature. Building scanning probe microscope in a system with a cryocooler is challenging because there are significantly larger mechanical vibrations introduced by the cryocooler as compared to a liquid helium bath. This makes performing scanning probe measurements difficult because one typically needs a very low vibration environment to maintain small displacements between the tip and sample. In our system, the vibration isolation stage shown in Figure 2 serves to dampen the mechanical vibrations from the fridge on the tuning fork-mounted tip.

Another thrust is our collaboration with the electron physics group at NIST, where we spatially and spectroscopically probe electronic phases in graphene using scanning tunneling microscopy (STM). This technique uses electrons that quantum mechanically tunnel from the surface of a material of interest (graphene in our case) onto an electrically biased, atomically sharp tip, and can be used to locally measure the density of electron states in the material – a critical and difficult-to-measure parameter that helps us understand electron behavior in a material. Given the high quality of our graphene devices, the DGG group and our collaborators are uniquely positioned to observe novel graphene physics, like quantum hall stripes [1].

For more information on STM and scanning probe measurements in general, please see Designing Advanced Scanning Probe Microscopy Instruments (http://www.nist.gov/cnst/epg/advanced_microscopy_instruments.cfm) from NIST.

1. Wang, H., Sheng, D. N., Sheng, L. & Haldane, F. D. M. Broken-symmetry states of dirac fermions in graphene with a partially filled high landau level. Phys. Rev. Lett. 100, 1–4 (2008).