Quantum Materials

We generate enormous electronic data, which has been forecasted to grow to a rate of >100 billion terabytes/year by 2025. It has also been forecasted that this "tsunami of data" will be responsible for 3.2% of all carbon emissions and consume 20% of global electricity by 2030 if a paradigm shift in computing technology is not observed. To handle the large volume of data and address the associated issues, there is a growing interest in quantum computing and quantum information processing technologies, as it promises to efficiently solve problems that would overwhelm the best supercomputers in the world. Conventional approaches encode quantum information, often known as the "qubit", into an ion, photon, or superconducting circuit, which relies on quantum coherence and is fragile against noise. The noise from the environment and various parts of circuitry is inevitable, and state-of-the-art quantum computing approaches rely on quantum error correction techniques, just like a bit correction was needed for classical computers in the era of vacuum tubes. Conventional techniques of quantum error correction often entangle a grid of >1000 ancillary qubits to safeguard a single logical qubit, which creates challenges at the system level and limits the scalability. Even after employing a rigorous quantum error correction technique, the recent demonstrations of quantum supremacy exhibited a degraded signal-to-noise ratio.

I am interested in studying a new kind of quantum information processing enabled by non-Abelian quasiparticle excitations emerging from the interactions inside matter, known as the topological quantum computation. The quasiparticle's properties may fluctuate due to noise from the environment and the circuits; however, topological quantum information processing does not encode information into the quasiparticles. The information is encoded in a collective topological property of the system described by the order in which we swap positions of the quasiparticles. Such encoding is quite robust against fluctuation and promises a 10⁴x reduction in the error rate compared to conventional approaches. There have been many ongoing works to realize such quasiparticles on magnetic topological materials and superconductor junctions and in quantum spin liquid phases in materials with strong anisotropic interactions. I have developed a quantum transport modeling framework for these materials, which reproduced many experimental observations, evaluated the parameter spaces to observe the quantum phases of materials, and predicted new phenomena within such quantum phases.

Magnetic Topological Insulators

I have been exploring diverse spin-orbit materials, including topological insulators, to enable an efficient SOT MRAM. My previous research on topological insulators involved materials with protected time-reversal symmetry. All the observed phenomena can be explained using a semiclassical model. Recently, magnetic topological insulators have developed a growing interest, where the time-reversal symmetry is broken using magnetic impurities with an out-of-plane magnetization. Such magnetic topological insulators exhibit new quantum phenomena related to unique chiral edge conduction: (1) quantum anomalous Hall effect, where the transverse Hall conductance is quantized to q²/h while the longitudinal resistance is negligible, and (2) axion insulator state, where the transverse Hall conductance is 0 while the longitudinal resistance is infinitely large. To understand these phenomena, we need quantum-transport modeling.

I have developed a quantum-transport framework for such materials using the non-equilibrium Green's function (NEGF) method that captured the physics of chiral edge conduction, reproduced the experimentally observed quantum phenomena, and provided insights on why such phenomena are restricted to low temperatures. The magnetic impurities create a gap around the Dirac point in the linear E-k of the topological surface states and induce a chiral edge conducting channel where the chirality is determined by the magnetization direction of the impurities. When two interfaces (top and bottom) have the same chirality, the phases add up and provide a quantized Hall conductance and a large longitudinal conductance; however, when the two interfaces have opposite chirality, the phases cancel out and provide a zero Hall conductance a large longitudinal resistance. These phenomena are observable when the Fermi level lies within the magnetic gap. The contacts used for measurements can broaden the energy spectrum higher than the magnetic gap to destroy the phenomena, which is one of the major reasons we need the low temperature to observe these phenomena. Besides, the low curie temperature of the magnetic impurities is another reason for low-temperature experiments.

The axion insulator and quantum anomalous Hall insulator states are interesting to exhibit a unique magnetoelectric effect and axion electrodynamics within a condensed matter system. Thus, these materials are of great interest for new quantum phenomena and quantum devices for efficient quantum information processing and communication.

These materials are interesting to explore the non-Abelian quasiparticle excitations when made a junction with superconducting wires, which can enable a new kind of topological quantum computation. However, a demonstration in such materials is a topic of active debate; hence, there is a lot of scopes to explore this topic.

Spin liquid and quantum phases of materials

I have been exploring 4d and 5d oxides and halides with honeycomb lattice structures, which can exhibit novel quantum magnetism and spin liquid phases due to an interplay between isotropic and anisotropic exchange interactions, described by the Heisenberg-Kitaev spin model.