Columbia Researchers Find Candidate Fractional Topological Insulator

Columbia researchers have potentially confirmed the existence of a fractional topological insulator, a quantum state predicted theoretically fifteen years ago. The team, led by Xiaoyang Zhu, Howard Family Professor of Nanoscience at Columbia, discovered the candidate material within a specially constructed moiré superlattice, achieving this formation by doping with four-thirds holes per moiré unit cell. This doping process appears to create two copies of two-thirds fractional states, specifically fractional Chern insulators, with opposite spin polarizations. “This is likely the first experimental evidence for this quantum phase,” said Zhu, describing the discovery published in Physical Review X. Spin-polarized modes preserving time-reversal symmetry and helical edge modes are characteristics of the material and may render edge currents robust to imperfections and external disturbances.

Moiré Superlattices Reveal Candidate Fractional Topological Insulator

Researchers at Columbia, building on last year’s discovery of a dozen new quantum states within these structures, are now focusing on this elusive phase, probing its properties with advanced spectroscopic techniques. This configuration results in a non-magnetic ground state. Spin-polarized, helical edge modes, which describe counterpropagating “spin up” and “spin down” channels that preserve time-reversal symmetry, are characteristics of this potential insulator. The topological nature of this state may render these edge currents robust to imperfections in the material and to external disturbances, a highly desirable trait for quantum computing applications. Gillian Minarik, co-first author of the study published in Physical Review X, explained how the team adapted their pump-probe spectroscopy to utilize circularly polarized light, allowing them to investigate magnetism within the moiré superlattices and observe the emergent properties driven by enhanced electronic interactions.

Initial observations confirmed zero net magnetization, consistent with the expected cancellation of spins in the oppositely polarized Chern insulators; however, the application of a minute magnetic field unexpectedly induced magnetism in the sample, a fleeting but definitive signal. While transport measurements are still needed to confirm the presence of helical edge currents, the team is optimistic. Minarik added, “If this is true, the binding of two FCI copies with opposite chiralities provides the first evidence for pairing of fractional charges in the first Chern band.” This observation suggests the possibility of realizing more exotic topological states stabilized through pairing.

Circularly Polarized Spectroscopy Detects ⁴⁄₃ Fractional State & Magnetization

The search for fractional topological insulators, a theoretically predicted quantum state, has long challenged condensed matter physicists; for fifteen years, experimental confirmation remained elusive until last year’s work at Columbia potentially bridged the gap between theory and observation. Researchers there employed a refined pump-probe spectroscopy technique utilizing circularly polarized light to investigate the magnetic properties of moiré superlattices, materials exhibiting emergent behaviors due to strong electron interactions. These lattices, created by stacking and twisting layers of material, are known to host a variety of quantum states, and the Columbia team focused on a configuration achieved through doping with four-thirds holes per moiré unit cell.

This transition occurred within a narrow range of twist angles, between 3.7 and 3.9 degrees, aligning with theoretical predictions for a fractional topological insulator comprised of oppositely spinning Chern insulators. The material exhibits spin-polarized, helical edge modes, counterpropagating channels of “spin up” and “spin down” electrons, which may prove particularly valuable for quantum computing.