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The electronic evolution of doped Mott insulators has been extensively studied for decades in search of exotic physical phases. The proposed Mott insulator 1T-TaS2 provides an intriguing platform to study the electronic evolution via doping. Here we apply scanning tunneling microscopy (STM) to study the evolution in Ti-doped 1T-TaS2 at different doping levels. The doping Ti atom locally perturbs the electronic and spin state inside the doped star of David and induces a clover-shaped orbital texture at low-doping levels (x < 0.01). The insulator to metal transition occurs around a critical point x = 0.01, in which small metallic and large insulating domains coexist. The clover-shaped orbital texture emerges at a broader energy range, revealing a competition with the electron correlation. It transforms to a disorder-induced Anderson insulating behavior as doping increases. We directly visualize the trapped electrons in dI/dV conductance maps. The comprehensive study of the series of Ti-doped 1T-TaS2 deepens our understanding of the electronic state evolution in a doped strong-correlated system.

Tuning the electronic properties of a matter is of fundamental interest in scientific research as well as in applications. Recently, the Mott insulator-metal transition has been reported in a pristine layered transition metal dichalcogenides 1T -TaS 2 , with the transition triggered by an optical excitation, a gate controlled intercalation, or a voltage pulse. However, the sudden insulatormetal transition hinders an exploration of how the transition evolves. Here, we report the strain as a possible new tuning parameter to induce Mott gap collapse in 1T -TaS 2 . In a strain-rich area, we find a mosaic state with distinct electronic density of states within different domains. In a corrugated surface, we further observe and analyze a smooth evolution from a Mott gap state to a metallic state. Our results shed new lights on the understanding of the insulator-metal transition and promote a controllable strain engineering on the design of switching devices in the future. * yiyin@zju.edu.cn 1 arXiv:2005.13311v2 [cond-mat.str-el] 28 May 2020

Quasiparticle interference (QPI) of the electronic states has been widely applied in scanning tunneling microscopy to analyze the electronic band structure of materials. Single-defect-induced QPI reveals defectdependent interaction between a single atomic defect and electronic states, which deserves special attention. Due to the weak signal of single-defect-induced QPI, the signal-to-noise ratio is relatively low in a standard twodimensional QPI measurement. In this paper, we introduce a projective quasiparticle interference (PQPI) method in which a one-dimensional measurement is taken along high-symmetry directions centered on a specified defect. We apply the PQPI method to the topological nodal-line semimetal ZrSiS. We focus on two special types of atomic defects that scatter the surface and bulk electronic bands. With an enhanced signal-to-noise ratio in PQPI, the energy dispersions are clearly resolved along high-symmetry directions. We discuss the defect-dependent scattering of bulk bands with the nonsymmorphic symmetry-enforced selection rules. Furthermore, an energy shift of the surface floating band is observed, and a branch of energy dispersion (q 6) is resolved. This PQPI method can be applied to other complex materials to explore defect-dependent interactions in the future.

In this article, we review the recent progress of the scanning tunneling microscopy studies of 1T-TaS2 and 1T-TaSe2 for bulk single crystals and molecular beam epitaxy monolayer films. We focus on how to understand the Mott insulating state in the whole set of materials, even when the stacking order takes effect. Based on this understanding, we discuss tuning the Mott insulator to a metallic state with different techniques, with Mott physics information revealed from the tuning process. The Kondo physics and quantum spin liquid state of 1T-TaS2 and 1T-TaSe2 are further discussed. This good platform of strong correlation must bring more intriguing phenomenon and physics in the future.