First-principles Design Advances Excitonic Insulator Research, Addressing a 60-Year Challenge

For over six decades, physicists have pursued the elusive excitonic insulator, a unique state of matter where interactions between electrons spontaneously create excitons, bound pairs of electrons and holes, that condense into the ground state. Now, H. W. Qu, H. T. Liu, Y. C. Li, and colleagues present a comprehensive review of this challenging field, charting its history and outlining recent progress in the computational design of these materials. Their work focuses on utilising first-principles calculations to predict and understand excitonic behaviour, particularly through a screening process guided by the concept of ‘dark excitons’. This approach not only expands the possibilities for finding excitonic instability in a wider range of semiconductors, but also predicts the existence of entirely new states of matter, including half-excitonic and spin-triplet insulators, bringing the realisation of a true excitonic insulator closer than ever before.

Excitonic Condensates in Two-Dimensional Materials

Research into two-dimensional materials and their stacked structures, known as heterostructures, reveals fascinating behavior of excitons, bound pairs of electrons and holes. Scientists are investigating how these excitons can condense into a new state of matter and the unique properties that emerge. These materials, including molybdenum disulfide, tungsten disulfide, and hexagonal boron nitride, exhibit strong exciton binding energies and tunable optical characteristics, making them ideal for this research. A key focus involves creating Moiré patterns by stacking these materials with a slight twist.

These patterns generate periodic landscapes that dramatically alter the electronic and optical properties, leading to novel exciton behavior. Researchers are discovering how interactions between excitons influence their properties and promote the formation of these condensates, aiming to understand and control the optical absorption, emission, and transport properties of these materials, exploring quantum phenomena like Bose-Einstein condensation, superfluidity, and topological states. Investigations demonstrate that exciton properties, such as energy and lifetime, can be tuned by controlling the twist angle, stacking order, and composition of the heterostructures. Evidence suggests that excitons can move through the material, a crucial characteristic for potential applications in optoelectronic devices. The surrounding environment, specifically the use of hexagonal boron nitride, also influences exciton properties, highlighting a strong connection between the electronic band structure of these materials and their optical characteristics. This research addresses challenges in achieving stable exciton condensation and seeks to explore the full potential of these materials, with significant implications for fundamental physics, materials science, and the development of next-generation optoelectronic devices, including high-efficiency solar cells, light-emitting diodes, photodetectors, and quantum devices.

Bethe-Salpeter Equation Identifies Excitonic Insulators

Scientists are pursuing the theoretical excitonic insulator, a unique state of matter where excitons condense to become the ground state. Identifying materials exhibiting this behavior remains challenging due to the lack of easily recognizable experimental signatures and difficulties in distinguishing excitonic mechanisms from competing phenomena. To overcome these hurdles, researchers developed a computational approach based on the Bethe-Salpeter equation (BSE) to quantify key parameters determining excitonic insulator formation. The method begins by solving equations to determine single-electron energies and wavefunctions.

A crucial step involves approximating the exchange-correlation potential, where standard methods underestimate band gaps. Scientists employ the GW approximation, a non-local and energy-dependent potential, to improve accuracy. To address band gap issues, researchers sometimes apply a “scissor operator,” a technique that rigidly shifts the conduction band away from the valence band, providing a computationally efficient alternative to fully self-consistent calculations. The team refined their design strategy with the “dark-exciton rule,” recognizing that larger band gaps lead to weaker screening, stronger exciton binding, and ultimately, a higher probability of excitonic insulator formation, leveraging quantum confinement to increase exciton binding energy and decrease exciton radius.

Dark Excitons Stabilize Insulating State in Manganese Compounds

Scientists have achieved a breakthrough in understanding excitonic insulators, materials where excitons condense to become the ground state, altering the material’s properties. This work details how specific materials can exhibit this unusual behavior, opening avenues for novel states of matter and potential device applications. Calculations reveal that in manganese compounds, the lowest-energy exciton appears at 0. 24 electron volts and exhibits a “dark” characteristic, meaning its behavior is decoupled from the electric field. Experiments demonstrate that as an electric field increases from 0 to 0.

2 volts per angstrom, the exciton energy decreases by only 0. 05 electron volts, a change significantly smaller than that observed in the material’s bandgap. This decoupling allows the excitonic instability, and thus the excitonic insulator state, to emerge when the electric field reaches approximately 0. 15 volts per angstrom, as indicated by a negative transition energy. Further research identifies a unique class of topological excitonic insulators, materials combining the properties of topological insulators and excitonic insulators.

Calculations show that spontaneous exciton condensation increases the bulk gap, potentially enabling higher operating temperatures for these materials. Importantly, the excitonic instability in these materials is less sensitive to defects, enhancing their stability. First-principles calculations confirm that these materials exhibit negative transition energies, indicating the onset of excitonic instability. The team also investigated magnetic excitonic insulators, identifying a new state of matter termed the “half-excitonic insulator. ” Calculations demonstrate that monolayers of 1T-NiCl2, NiBr2, CoCl2, and CoBr2 exhibit this behavior, where spontaneous exciton condensation occurs in only one spin channel. This arises from a spin-resolved analysis of the transition energy, revealing that one spin channel has a negative transition energy while the other remains positive.

Predicting Novel Excitonic Insulator States of Matter

This research presents significant advances in the long-standing theoretical pursuit of excitonic insulators, materials where interactions between electrons and ‘holes’ drive an insulating state. Scientists have refined computational methods, specifically employing first-principles calculations, to predict and design potential excitonic insulators in a wider range of materials, including those with indirect band gaps and even strongly correlated electrons. This work not only expands the possibilities for realizing excitonic instability but also predicts the existence of novel states of matter, such as half-excitonic insulators and spin-triplet.