A metallic mosaic phase and the origin of Mott insulating state in 1T-TaS2 (1507.01312v1)

Abstract: Electron-electron and electron-phonon interactions are two major driving forces that stabilize various charge-ordered phases of matter. The intricate interplay between the two give rises to a peculiar charge density wave (CDW) state, which is also known as a Mott insulator, as the ground state of layered compound 1T-TaS2. The delicate balance also makes it possible to use external perturbations to create and manipulate novel phases in this material. Here, we study a mosaic CDW phase induced by voltage pulses from the tip of a scanning tunneling microscope (STM), and find that the new phase exhibit electronic structures that are entirely different from the Mott ground state of 1T-TaS2 at low temperatures. The mosaic phase consists of nanometer-sized domains characterized by well-defined phase shifts of the CDW order parameter in the topmost layer, and by altered stacking relative to the layer underneath. We discover that the nature of the new phases is dictated by the stacking order, and our results shed fresh light on the origin of the Mott phase in this layered compound.

• The paper reveals that specific stacking orders in 1T-TaS2 trigger a mosaic metallic phase that deviates from standard Mott insulating behavior.
• It uses scanning tunneling microscopy to map nanometer-scale phase shifts and domain structures within the CCDW framework.
• The study suggests that manipulating external factors like strain and electric fields could enable tunable electronic properties in layered materials.

An Analytical Examination of the Mosaic Metallic and Mott Insulating Phases in 1T-TaS<sub\>2</sub>

The paper of charge-ordered phases of materials remains a prominent field of inquiry within condensed matter physics, particularly due to the role of electron-electron and electron-phonon interactions in stabilizing these phases. The paper under consideration explores the complex interplay of these interactions in the context of the layered transition metal dichalcogenide, 1T-TaS<sub\>2</sub>. More specifically, it explores the puzzling coexistence of metallic and Mott insulating states within this material, a subject of intense debate and interest.

In evaluating the charge density wave (CDW) phenomena in 1T-TaS<sub\>2</sub>, the paper draws attention to the unique attributes of the Mott insulating (MI) state, which does not conform to typical Mott behavior as it emerges within a commensurate CDW (CCDW) phase without apparent magnetic ordering. The research identifies the intricate role of stacking order in dictating electron localization and mobility, presenting experimental evidence obtained via scanning tunneling microscopy (STM) of a new mosaic CDW phase that deviates fundamentally in electronic structure from the parent MI state.

This novel CDW state is characterized by nanometer-sized domains with precise phase shifts and altered interlayer stacking compared to the conventional MI ground state. Such findings imply that the stacking order is a decisive factor not only for the emergence of the metallic mosaic (MM) phase but also for the initial formation of the MI state within the CCDW phase. This phase is metastable and reverts to the MI state upon thermal cycling, akin to transitions induced by ultra-fast laser and current excitations in previous experiments.

A significant result of this investigation is the association of different stacking configurations with variances in electronic behaviors, posing the hypothesis that particular stacking arrangements could lead to increased interlayer hopping amplitudes, effectively broadening the bandwidth relative to on-site Coulomb repulsion. Consequently, this facilitates the transition from the Mott insulating to metallic states. This assertion is supported by the presence of domain wall networks traversing adjacent layers, implying a complex interlayer coupling mechanism that is not yet fully understood.

The implications of these findings are profound for the theoretical understanding of strongly correlated electron systems. They shed light on longstanding ambiguities concerning the origins of the MI state in 1T-TaS<sub\>2</sub>, emphasizing the necessity to consider interlayer interactions in analyses involving both experimental and computational models of charge density waves and related phenomena.

Furthermore, the paper suggests that manipulation of external parameters, such as electric fields and mechanical strain, could offer novel pathways for controlling the electronic phase behaviors in layered materials. This could, in turn, be extrapolated to design materials with tunable electronic properties for applications in electronic devices and quantum materials, especially in the context of phase-change memory and resistive switching.

In conclusion, the insights provided by this research open new avenues for the exploration of phase transitions in low-dimensional materials. Future investigations are warranted to elucidate the precise nature of interlayer coupling and the potential applications of phase-switching phenomena in material science. The integration of theoretical advancements with these experimental observations promises to significantly enhance our comprehension of charge-ordered systems and their technological potential.