Chiral charge density waves induced by Ti-doping in 1T-TaS2

Abstract

We investigate the Ti-doping effect on the charge density wave (CDW) of 1T-TaS2 by combining scanning tunneling microscopy (STM) measurements and first-principle calculations.Although the Ti-doping induced phase evolution seems regular with increasing of the doping concentration (x), an unexpected chiral CDW phase is observed in the sample with x = 0.08, in which Ti atoms almost fully occupy the central Ta atoms in the CDW clusters. The emergence of the chiral CDW is proposed to be from the doping-enhanced orbital order. Only when x = 0.08, the possible long-range orbital order can trigger the chiral CDW phase. Compared with other 3d-elements doped 1T-TaS2, the Ti-doping retains the electronic flat band and the corresponding CDW phase, which is a prerequisite for the emergence of chirality. We expect that introducing elements with a strong orbital character may induce a chiral charge order in a broad class of CDW systems. The present results open up another avenue for further exploring the chiral CDW materials.

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.…”

“…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].…”

Raman Optical Activity of 1T-TaS2

“… 41 In contrast to 1T-TiSe 2 , chirality has not been observed for pristine 1T-TaS 2 . 42 , 43 However, recent reports show that chiral CDWs can be induced in 1T-TaS 2 by Ti doping 43 and in the case of the light-induced hidden state. 33 Our results showing an insusceptibility of the polarization properties upon sample rotation and the observed Raman optical activity indicate that the studied sample is chiral.…”