Electron Hydrodynamics

What do I work on?

Over the years, many experimental and theoretical works have studied various types of motions of electrons in materials, and my research focuses on the fundamental reasons that give rise to motion of electrons in various materials and under different conditions. Movement of electron in a material is called transport. Till now, we knew that electrons in most metals move just like balls in a Pinball machine, colliding with the impurities present in the metals and losing their momentum to those impurities, giving rise to resistance in a metal. This type of transport is called diffusive transport. If the impurities in the metal are less or not present, the electrons collide with the walls of the metal and lose their momentum there, leading to ballistic transport. What we observed is that there is a third type of transport possible, where electrons start interacting with other electrons and start flowing collectively, behaving just like water. This type of transport is called hydrodynamic transport. 

My research deals with understanding the conditions under which the fluid properties of electrons occur and thus my field of research is electron hydrodynamics. This is very analogous to water flowing in pipes, and thus in my research I use already worked out mathematical equations in fluid dynamics and modify them for the material system at hand. This hydrodynamic regime of transport requires very pure material where the length of electron-electron collisions is much smaller than the mean free path of the system (which occurs in a particular temperature range). For our material (GaAs/AlGaAs) which also shows metallic behavior, this condition is satisfied for the entire range of temperatures measured (4K to 30 K). Once we understood these conditions, we used this interesting property of electrons to discover this new phenomenon of electron pumping and create the world’s first electron pump which works just like a water pump and which further validated that electrons can indeed flow like water. In fact, electrons actually form a very viscous fluid more like honey or motor oil. This behavior has also been seen in other two-dimensional materials like Graphene, PdCo02, Dirac materials, Weyl semi metals etc. 

Why is this research important?

Though the electrons form a viscous honey-like fluid, quite counterintuitively, once becoming a fluid, their ease of flow or conductance increases (electrons in cooperation can achieve what a single electron cannot). That means, using this phenomenon, we can increase the electrical current flow in a material, which is really useful especially in a nanoscale research. Secondly, this unique property of electrons opens up many exciting avenues for creating faster and more efficient electronic devices. More importantly, understanding such behavior of electrons is important for fundamental science and research because it would a lot of difference electrons are moving like balls in a pinball machine or rather swim in a viscous but very mobile fluid, creating whirlpools. 

How do I actually fabricate tiny devices to study?

Our collaborators at Purdue university (Manfra group) grow this heterostructure using Molecular Beam Epitaxy technique. We, then use the lithography techniques of photolithography and electron beam lithography to fabricate mesoscopic structures on this heterostructure. After fabrication process is done, we use low frequency standard lock-in measurement technique to perform measurements on our fabricated device at various temperatures and magnetic fields.