No abstract

In the transition metal dichalcogenide IrTe 2 , low-temperature charge-ordered phase transitions involving Ir dimers lead to the occurrence of stripe phases of different periodicities, and nearly degenerate energies. Bulksensitive measurements have shown that, upon cooling, IrTe 2 undergoes two such first-order transitions to (5 × 1 × 5) and (8 × 1 × 8) reconstructed phases at T c 1 ∼ 280 K and T c 2 ∼ 180 K, respectively. Here, using surface sensitive probes of the electronic structure of IrTe 2 , we reveal the first-order phase transition at T c 3 = 165 K to the (6 × 1) stripes phase, previously proposed to be the surface ground state. This is achieved by combining x-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy, which give access to the evolution of stripe domains and a particular surface state, the energy of which is dependent on the Ir dimer length. By performing measurements over a full thermal cycle, we also report the complete hysteresis of all these phases.

Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional transition metal dichalcogenides remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe2, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy data reveal the strain-stabilized phase has a 6 × 1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory calculations, we show that strain induces an Ir to Te charge transfer resulting in strongly weakened inter-layer Te bonds and a reshaped energetic landscape favoring the 6×1 phase. Our results highlight the potential to exploit strain-engineered properties in layered materials, particularly in the context of tuning inter-layer behavior.

The transition metal dichalcogenide 1T-TiSe_{2}-two-dimensional layered material undergoing a commensurate 2×2×2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below ≈200  K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allows us to go beyond the usual spatial resolution of STM and to intimately probe the three-dimensional character of the PLD. Furthermore, the inversion-symmetric achiral nature of the CDW in the z direction is revealed, contradicting the claimed existence of helical CDW stacking and associated chiral order. This study paves the way to a simultaneous real-space probing of both charge and structural reconstructions in CDW compounds.

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.