Local resilience of the 1T−TiSe2 charge density wave to Ti self-doping

Abstract

In Ti-intercalated self-doped 1T -TiSe2 crystals, the charge density wave (CDW) superstructure induces two nonequivalent sites for Ti dopants. Recently, it has been shown that increasing Ti doping dramatically influences the CDW by breaking it into phase-shifted domains. Here, we report scanning tunneling microscopy and spectroscopy experiments that reveal a dopant-site dependence of the CDW gap. Supported by density functional theory, we demonstrate that the loss of the longrange phase coherence introduces an imbalance in the intercalated-Ti site distribution and restrains the CDW gap closure. This local resilient behavior of the 1T -TiSe2 CDW reveals a novel mechanism between CDW and defects in mutual influence. The quasi-two-dimensional transition-metal dichalcogenide (TMDC) 1T -TiSe 2 has been largely studied over many years with the desire to understand the mechanisms lying behind its many interesting properties related to its phase transitions. Below T CDW ≈ 200 K, 1T -TiSe 2 exhibits a commensurate CDW phase with a 2x2x2 modulation and a weak periodic lattice distortion (PLD) [1]. Upon Cu intercalation [2], and under pressure [3], it can also host superconductivity that has been proposed to emerge in incommensurate CDW domain walls therefore reflecting the complex 1T -TiSe 2 phase diagram [4,5].Doping has shown to be an important tuning parameter of these collective mechanisms [1,2,6]. In particular, intercalation of Ti dopants, known to occur depending on the crystal growth temperature [1], leads to electrondonor impurity states close to the Fermi energy [7], enhances the Coulomb screening, and tends to reduce longrange electronic correlations. In a recent scanning tunneling microscopy (STM) study of Ti self-doped 1T -TiSe 2 crystals, it has been further reported that for sufficient dopant concentration, the CDW breaks up in randomly phase-shifted domains with subsisting commensurate 2x2 charge modulation separated by atomically sharp phase slips [6]. This first observation of short-range phase coherent CDW nanodomains induced by Ti-doping not only provides new insight about the microscopic nature of the 1T -TiSe 2 CDW, but is also of great concern for the understanding of the interplay between dopants and novel electronic phases of TMDCs, in general [2,4,[8][9][10][11][12][13].In this paper, we report STM and scanning tunneling spectroscopy (STS) experiments that allow site-specified probing of the local density of states (LDOS) close to the Fermi level (E F ). We demonstrate that the loss of the long-range phase coherence is a local resilient behavior of the CDW to self-doping. Together with density functional theory (DFT) calculations, our observations show that the CDW locally adapts to the random distribution of the intercalated-Ti atoms existing in two nonequivalent PLD-related conformations. This CDW-induced Ti-conformation imbalance not only explains the CDW's domain formation in Ti self-doped 1T -TiSe 2 , but also reveals a novel CDW-impurities cooperative mechanism.

Semimetal-to-semiconductor transition and charge-density-wave suppression in 1T−TiSe2−xSx single crystals

“…Figure 1 . Fig 1(e) confirms the S substitution identification at V bias = +0.6 V. Also, even if at lower V bias the CDW modulation tends to hide the topographic depletions associated with S substitutions of non-displaced Se atoms of the PLD [11], it is still possible to identify them as weakened CDW maxima [see black arrows on Fig 1(f)].…”

Emergent superconductivity upon disordering a charge density wave ground state

“…A qualitatively similar phase diagram has been found, not only using Cu-intercalation as the tuning parameter, but also by applying pressure on 1T-TiSe 2 [34,35], self doping with Ti [36] and intercalating with Pd [37]. The formation of domains of CDW and the nucleation of superconductivity within arXiv:1806.06568v1 [cond-mat.supr-con] 18 Jun 2018 the domain walls is not special just for 1T-TiSe 2 , but is found for several other TMDs.…”

Local Real-Space View of the Achiral 1T−TiSe2 2×2×2Hildebrand1, Jaouen2, Mottas3 et al. 2018Phys. Rev. Lett.Self Cite400270View full textAdd to dashboardCite

“…Therefore, the electronic reconstruction associated with the commensurate 2 × 2 × 2 CDW of 1T -TiSe 2 can be easily tracked in the topmost Se layer up to atomic resolution [10,11]. This local approach has indeed allowed to propose the chirality of the CDW [8,12] and to observe the formation of phaseshifted CDW domains induced by intercalation of Ti or Cu atoms [13][14][15][16]. However, the three-dimensional (3D) character of the CDW as well as the periodic lattice distortion (PLD) accompanying the phase transition can, in principle, not be directly probed, the displacement amplitude induced by the PLD [17,18] being well below the usual spatial resolution of STM [19].Yet, being able to obtain information about the 3D character of the CDW as well as getting insight about the PLD in real-space is of fundamental interest in the context of domain formation or CDW dimensional crossover [20], but also with respect to the question of chirality which strictly signifies a loss of inversion symmetry between two adjacent Se-Ti-Se sandwiches.Here, the electronic signature of defects induced by the presence of interstitial Ti atoms in-between Se-Ti-Se sandwiches is exploited for probing the symmetry of the PLD.…”

“…Therefore, the electronic reconstruction associated with the commensurate 2 × 2 × 2 CDW of 1T -TiSe 2 can be easily tracked in the topmost Se layer up to atomic resolution [10,11]. This local approach has indeed allowed to propose the chirality of the CDW [8,12] and to observe the formation of phaseshifted CDW domains induced by intercalation of Ti or Cu atoms [13][14][15][16]. However, the three-dimensional (3D) character of the CDW as well as the periodic lattice distortion (PLD) accompanying the phase transition can, in principle, not be directly probed, the displacement amplitude induced by the PLD [17,18] being well below the usual spatial resolution of STM [19].…”

“…For more precisions about the PLDinduced inequivalent conformations of single-atom defects see Refs. [11,15].…”