Visualizing anisotropic propagation of stripe domain walls in staircaselike transitions ofIrTe2

Abstract

We present a scanning tunneling microscopy (STM) study of the domain evolution across two first-order phase transitions of stripe modulations in IrTe 2 that occur at T C ≈ 275 K and T S ≈ 180 K, respectively. Phase coexistence of the hexagonal (1 × 1) structure and the (5 × 1) stripe modulation is observed at T C , while various (p × 1) modulations (p = 3n + 2 with 2 ≤ n ∈ N) are observed below T S . Using STM atomic resolution, we observe anisotropic propagation of domain boundaries along different directions, indicating significantly different kinetic energy barriers. These results are consistently explained by a theoretical analysis of the energy barrier for domain wall propagation as obtained by density functional theory. Individual switching processes observed by STM indicate that the wide temperature range of the transition from the (5 × 1) stripes to the (6 × 1)-ordered ground state is probably caused by the numerically limited subset of switching processes that are allowed between a given initial and the final state. The observations on IrTe 2 are discussed in terms of a "harmless staircase" with a finite number of first-order transitions between commensurate phases and within a "dynamical freezing" scenario.

Examining the surface phase diagram of IrTe2 with photoemission

“…Therefore, phases of different stripe periodicities appear with increasing dimer densities as temperature decreases. Their complex kinetics, involving dimers breaking and reassembling, has been studied by Mauerer et al [12]. Interestingly, a very recent study predicts the correct T c 1 by considering the entropy of the (5 × 1 × 5) phase [33].…”

“…At T c 2 ∼ 180 K, a second phase transition follows and the charge-ordering wave vector of this new low-temperature phase is (8 × 1 × 8) in the bulk of IrTe 2 . This has stimulated many scanning tunneling microscopy (STM) studies, which evidenced additional ordering patterns and revealed a surface (6 × 1) periodicity proposed to be the ground-state reconstruction [12,16,25,28]. In addition, a detailed low-energy electron diffraction (LEED) and STM study observed the coexistence of the surface (8 × 1) and (6 × 1) phases over a wide temperature range [29].…”

“…In this context, the surface of IrTe 2 offers an exciting platform for studying ordered phases in a quasi-2D material with large spin-orbit coupling on the transition metal site. A complex succession of chargeordered phases involving the creation of Ir dimers [11][12][13] has been observed in IrTe 2 at low temperature, which gives way to superconductivity for thin samples [14], after rapid cooling [15], or with Pt substitution [7].…”

“…It was later realized that the (6 × 1 × 6) phase can also appear in the bulk at very low temperature, if the cooling rate is higher than 4 K/min [30]. Numerous angle-resolved photoelectron spectroscopy (ARPES) investigations have exposed large changes in the spectral weight of electronic states up to 2 eV below the Fermi level (E F ) at low temperatures, leading to severe band broadening [12,16,25,28,29,31]. In parallel, x-ray photoemission spectroscopy (XPS) studies have identified a large splitting of the Ir 4 f core levels across the charge-ordered phase transitions into Ir +3 and Ir +4 mixed valence states [16,32].…”

Surface phases of the transition-metal dichalcogenide IrTe2

“…Therefore, with the bulk 5×1×5 phase, all that can be seen is a 5×1 surface phase. The surface properties of IrTe 2 have been investigated using STM, which probes the local charge density but can infer the structural symmetry [17,18,[31][32][33]. In contrast to the bulk, the fully dimerized 6×1 modulation [ Fig.…”