Quantum Materials

The 2016 Nobel prize in physics was awarded for the development of topological concepts in condensed matte physics. A lot of this helps us discover new quantum materials, and classify their electronic states systematically. However, what can be a practical use for these concepts, especially from a device perspective?  The conventional argument is enhanced mobility - after all, topological properties that limit backscattering increases their ON current. However, for most electronic switches, the challenge is the OFF current, which is exponentially sensitive to transmission, as opposed to ON current. We argue that topology, specifically how relevant degrees of freedom wind around the Fermi surface (spins, pseudospins, magnetization) determines the OFF current across tunnel barriers as well, due to added conservation rules enforced by symmetry.

Take a look at the following classes of materials, and their winding indices. Much like topology distinguishes shape differences between a donut and a bun, the rate of winding of the arrows here separates the materials into different indices or Chern numbers.  

In short, quantum topology controls transport properties (lifetime, mobility, barrier transmission) across barriers, because of the constraints imposed by conservation of energy, momentum, and Chern number. Topology depends on symmetries inherent in the system. Barriers and symmetries can be gated. We posit therefore that topological properties allow us to realize unconventional <i>classical computing paradigms</i> using quantum properties. 

  • Graphene transistors using Dirac fermion 'optics'

    Utilizing the 'graphene-ness' of graphene

    The pseudospin structure of graphene's dimer orbitals causes electrons to collimate across a P-N junction, the core of any electronic switch The resulting unconventional electron-optics can be exploited to build a Klein tunnel transistor that creates a gate tunable transmission gap in bulk graphene, making its current saturate and turning it off as needed. Bilayer graphene has twice the winding number, giviing rise to the electronic equivalent of Brewster angles and leading to anti-Klein tunneling. A combination of Klein-antiKlein tunneling can thus create a high mobility high bandgap electronic metamaterial. 

  • Switching magnets using Topological Insulators

    A gate tunable spin Hall device

    Spin-momentum locking at the surface of a topological insulator like Bi2Se3 or SmB6 means that PN junction leads to a highly spin polarized transmission, and large charge-spin conversion in reflection. The build-up of spins can be used to torque a magnet with a gate tunable spin Hall coefficient.


  • Spintronics with Topological Semi-metals

    This will take a Weyl

    The surface states of a Weyl Semi-metal like TaAs have broken Fermi arcs that connect through their bulk Dirac point. Their spin textures manifest through local and non-local spin voltage measurements. We are trying to understand how their topological Chern numbers influence tunneling across gate tunable heterointerfaces.


  • Ultrasmall Skyrmions

    Physics of size, speed and lifetime

    Skyrmions are chiral vortex like structures in thin magnetic films that can be as small as tens of nanometers and driven at high speeds around a few hundred meters per second with modest currrents. We study the fundamental physics of their size-speed-stability relation, ways to drive and self-focus them using engineered compensation points, and possible applications such as their ability to convert time into coordinate and thereby encode temporal memory.