Visualizing the evolution from Mott insulator to Anderson insulator in Ti-doped 1T-TaS2

Abstract

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.

Understanding the Mott insulating state in 1T-TaS2 and 1T-TaSe2

“…Another point is that the Mott state in 1T-TaS 2 /1T-TaSe 2 resides in the periodic CDW superlattice instead of the atomic lattice. This Mott state is then very susceptible to different external perturbations, as shown in previous experiments of transport, scanning tunneling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES) [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57]. Controllable or natural tuning of the Mott insulating state could mutually corroborate information of the strong correlation effect.…”

“…The STM measurements further give the doping effect and the phase transition from the microscopic level [49]. Figure 8c shows the typical dI/dV spectrum of undoped sample, x = 0.004, and x = 0.01 sample.…”

“…However, they can not explain why the MBE grown monolayer 1T-TaS 2 and 1T-TaSe 2 exhibit a large-gap insulating state [31,32,35,36]. It is also hard to explain some previous experiments about Mott-insulatormetal transition in which the large insulating gap can be effectively tuned by doping, strain, or intercalation [40,41,[43][44][45][46][47][48][49][50][51]. A systematic STM study about the stacking order effect has been implemented in bulk 1T-TaS 2 and 1T-TaSe 2 [58,59].…”

“…For example, the substitution of S with Se gradually suppresses the CCDW phase until x = 0.8 [40][41][42], while the substitution of Ta with Fe or Ti can quickly suppress the CCDW phase with low doping of x = 0.01 [49,50].…”

“…The doped system is relatively stable due to the similar radii of Ti and Ta atoms. A combination of STM, transport, and DFT calculations has been applied to systematically investigate the Ti-doped 1T-Ti x Ta 1−x S 2 at a wide range of doping levels from x = 0 to x = 1 [49,50]. Figure 8a shows the phase diagram of 1T-Ti x Ta 1−x S 2 derived from the transport measurements [50].…”