Weyl semimetals (WSMs) are three-dimensional topological materials, which host fermions that follow relativistic motion, similar to graphene in two dimensions. Properties of Weyl semimetals promise their exciting potential to be a platform for quantum technologies. Measurements on WSMs reveal arc-like segments in Fermi surfaces (called Fermi arc, FA), resulting from their non-trivial topological order. These states at the FA are surface states, protected by the topological order in the bulk, and are predicted to give non-local responses in transport and correlations.
In this present work, we study interface states and quantum effects of two spatially separated WSM slabs, which are only interacting through Coulomb repulsion. Electron-hole pairs can be formed between the layers due to quantum fluctuations. A resonant coherent superposition of such electron-hole pairs is the collective mode that is called an exciton, which also follow bosonic statistics. Such coherence implies a state with broken symmetries, and one expects related Goldstone modes as excitations. These modes are responsible for dissipationless transport.
To explore the effects of interaction, we use random phase approximation and calculate the self energies in finite temperature self consistently, which shows the condensate has phase coherence near each of the Fermi arcs. Moreover, different classes of self energies try to overwhelm each other. This competition depends on the Coulomb matrix element.
The direct experimental evidence of the formation of excitons would be coherence experiments, i.e., a double-slit experiment on a beam of excitons will show interference patterns, but as the excitons live in between the two WSM surfaces, this experiment seems implausible. One can perform a Coulomb drag experiment; the exciton condensates effect will be visible in drag resistivity. Excitons are also responsible for dissipation less transport. These modes’ properties and origins can be tuned, giving rise to control of many-body excitations, which may have important applications in quantum devices.
I am also involved in some other research projects, which I briefly mention here.
We are numerically studying the thermal Hall conductivity of a quantum spin liquid in honeycomb lattice in the presence of time-reversal symmetry breaking, chiral, three-spin terms.
We are calculating coulomb drag between the interface two WSMs. The resistivity calculations can show the presence of excitons in between the interface.
We are trying to design electrical circuits that mimic higher-order topological insulators.
I have contributed to the following project: Mohapatra, Shubhajyoti, Ritajit Kundu, Ashutosh Dubey, Debasis Dutta, and Avinash Singh.“Role of orbital off-diagonal spin and charge condensates in a three orbital model for Ca2RuO4–Coulomb renormalized spin-orbit coupling, orbital moment, and tunable magnetic order." arXiv preprint arXiv:2007.06978 (2020).
Ritajit Kundu,
Department of Physics, IITK,
PMRF cycle May-2019