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Register a new userHierarchical stripe phases in IrTe2 driven by competition between Ir dimerization and Te bonding
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Layered 5d transition metal dichalcogenide (TMD) IrTe2 is distinguished from the traditional TMDs (such as NbSe2) by the existence of multiple CDW-like stripe phases and superconductivity at low temperatures. Despite of intensive studies, there is still no consensus on the physical origin of the stripe phases or even the ground state modulation for this 5d material. Here, we present atomic-scale evidence from scanning tunneling microscopy and spectroscopy (STM/STS), that the ground state of IrTe2 is a q=1/6 stripe phase, identical to that of the Se-doped compound. Furthermore, our data suggest that the multiple transitions and stripe phases are driven by the intralayer Ir-Ir dimerization that competes against the interlayer Te-Te bonding. The competition results in a unified phase diagram with a series of hierarchical modulated stripe phases, strikingly similar to the renowned devils staircase phenomena.
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Pressure-dependent transport measurements of Ir$_{1-x}$Pt$_x$Te$_2$ are reported. With increasing pressure, the structural phase transition at high temperatures is enhanced while its superconducting transition at low temperatures is suppressed. These pressure effects make Ir$_{1-x}$Pt$_x$Te$_2$ distinct from other studied $T$X$_2$ systems exhibiting a charge density wave (CDW) state, in which pressure usually suppresses the CDW state and enhances the superconducting state. The results reveal that the emergence of superconductivity competes with the stabilization of the low temperature monoclinic phase in Ir$_{1-x}$Pt$_x$Te$_2$.
The crystal structure of layered metal IrTe2 is determined using single-crystal x-ray diffraction. At T=220 K, it exhibits Ir and Te dimers forming a valence-bond crystal. Electronic structure calculations reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the Ir and Te dimers, which exhibit a signature of covalent bonding character development. Evidence for significant charge disproportionation among the dimerized and non-dimerized Ir (charge order) is also presented.
The coupled electronic-structural modulations of the ligand states in IrTe$_2$ have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive pre-edge structures are observed at the Te-$M_{4,5}$ (3$d$ $rightarrow$ 5$p$) absorption edge, indicating the presence of a Te 5$p$-Ir 5$d$ covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3$d$ $rightarrow$ 5$p$ resonance at the $Q!=!(1/5,0,-1/5)$ superlattice reflection is observed below the structural transition temperature $T_ssim$ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5$p$-Ir 5$d$ bond-density in driving the stripe-like order in IrTe$_2$, and uncovers its coupling with the charge and/or orbital order at the Ir sites. The similarity between these findings and the charge-ordering phenomenology observed in the high-T$_c$ superconducting cuprates suggests that the iridates may harbor similar exotic phases.
Single-crystal x-ray diffraction studies with synchrotron radiation on the honeycomb iridate $alpha$-Li$_{2}$IrO$_{3}$ reveal a pressure-induced structural phase transition with symmetry lowering from monoclinic to triclinic at a critical pressure of $P_{c}$ = 3.8 GPa. According to the evolution of the lattice parameters with pressure, the transition mainly affects the $ab$ plane and thereby the Ir hexagon network, leading to the formation of Ir--Ir dimers. These observations are independently predicted and corroborated by our textit{ab initio} density functional theory calculations where we find that the appearance of Ir--Ir dimers at finite pressure is a consequence of a subtle interplay between magnetism, correlation, spin-orbit coupling, and covalent bonding. Our results further suggest that at $P_{c}$ the system undergoes a magnetic collapse. Finally we provide a general picture of competing interactions for the honeycomb lattices $A_{2}$$M$O$_{3}$ with $A$= Li, Na and $M$ = Ir, Ru.
Building on the growing evidence based on NMR, magnetization, neutron scattering, ESR, and specific heat that, under pressure, SrCu$_2$(BO$_3$)$_2$ has an intermediate phase between the dimer and the Neel phase, we study the competition between two candidate phases in the context of a minimal model that includes two types of intra- and inter-dimer interactions without enlarging the unit cell. We show that the empty plaquette phase of the Shastry-Sutherland model is quickly replaced by a quasi-1D full plaquette phase when intra- and/or inter-dimer couplings take different values, and that this full plaquette phase is in much better agreement with available experimental data than the empty plaquette one.
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