Orbital textures and charge density waves in transition metal dichalcogenides (1409.7341v2)

Abstract: Low-dimensional electron systems, as realized naturally in graphene or created artificially at the interfaces of heterostructures, exhibit a variety of fascinating quantum phenomena with great prospects for future applications. Once electrons are confined to low dimensions, they also tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves; a state of matter that currently attracts enormous attention because of its relation to various unconventional electronic properties. In this study we reveal a remarkable and surprising feature of charge density waves (CDWs), namely their intimate relation to orbital order. For the prototypical material 1T-TaS2 we not only show that the CDW within the two-dimensional TaS2-layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow to manipulate salient features of the electronic structure. Indeed, these orbital effects enable to switch the properties of 1T-TaS2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultra-fast devices based on layered transition metal dichalcogenides.

• The paper elucidates the impact of orbital textures on the stability and dynamics of charge density waves in TMDs, providing evidence from DFT studies.
• It demonstrates that stacking order dramatically influences electronic transitions, causing semiconductor-to-metal changes and tuning bandgaps near 200 meV.
• The work introduces 'orbitronics', a concept that harnesses orbital ordering for rapid electronic state switching, promising advances for nanoscale device applications.

Orbital Textures and Charge Density Waves: An In-depth Study of Transition Metal Dichalcogenides

The paper "Orbital textures and charge density waves in transition metal dichalcogenides" presents a comprehensive examination of the interplay between orbital ordering and charge density waves (CDWs) in layered materials, specifically focusing on 1T-TaS₂. This research elucidates the intricate dynamics that govern the formation and modulation of CDWs and introduces a novel concept termed "orbitronics," which holds potential implications for future electronic applications.

The paper's primary focus is on the prototypical transition metal dichalcogenide (TMD) 2H-TaS₂, renowned for its multifaceted electronic phase diagram, which includes several forms of CDW phases and pressure-induced superconductivity. Notably, the research highlights the newly discovered orbital textures which are intimately related to CDWs. These textures not only involve complex orbital patterns within the two-dimensional TaS₂ layers but also demonstrate how metastable stackings can significantly alter electronic properties, ranging from metallic to semiconducting, with bandgaps on the order of 200 meV.

Key findings in the research include:

1. Orbital Textures and CDW Dynamics: The research presents concrete evidence of interconnected orbital textures associated with CDWs, underscoring their significant role in stabilizing or destabilizing various electronic states. This intricate relationship offers explanations for experimental observations, such as the rapid suppression of CDWs under external pressure and the CDW-order alterations in the nearly commensurate phase.
2. Stacking-Dependent Electronic Properties: The paper demonstrates that electronic structures are highly sensitive to the stacking order of layers, introducing the concept of a semiconductor-to-metal transition driven by varying orbital orientations. The research utilizes density functional theory (DFT) to calculate band structures for different stacking configurations, revealing that these variations can lead to either pronounced or diminished electron flow within layers.
3. Potential for Orbitronics in Device Applications: One of the novel implications of this research is the introduction of "orbitronics," where exploitation of orbital arrangements can facilitate rapid switching between electronic states. This potential for ultrafast manipulation of electronic properties could pave the way for miniaturized and high-speed devices based on TMDs, opening new avenues for technological advancements.

The implications of this work are profound, as it not only enhances our understanding of low-dimensional electron systems but also lays the groundwork for the development of advanced materials and devices. By leveraging orbital ordering and CDW manipulation, the paper suggests a new frontier for creating customizable electronic properties at the nanoscale.

Future research could explore the precise mechanisms enabling such rapid changes in electronic states, determine the role of external stimuli (e.g., pressure, temperature, and electric fields) on orbital textures, and further investigate how these principles might be applied to a broader spectrum of TMDs and similar materials.

In summary, the insights into orbital textures in TMDs offer a promising vista for tailoring material properties through fine control of electronic interactions, which might catalyze the next generation of electronic, photonic, and quantum devices. The rich interplay between CDWs and orbital order in these materials underscores a pivotal area of research with significant technological potential.