Bond order and the role of ligand states in stripe-modulatedIrTe2

Abstract

The coupled electronic-structural modulations of the ligand states in IrTe2 have been studied by x-ray absorption spectroscopy (XAS) and resonant elastic x-ray scattering (REXS). Distinctive preedge structures are observed at the Te-M4,5 (3d → 5p) absorption edge, indicating the presence of a Te 5p -Ir 5d covalent state near the Fermi level. An enhancement of the REXS signal near the Te 3d → 5p resonance at the Q = (1/5, 0, −1/5) superlattice reflection is observed below the structural transition temperature Ts ∼ 280 K. The analysis of the energy-dependent REXS lineshape reveals the key role played by the spatial modulation of the covalent Te 5p -Ir 5d bond-density in driving the stripe-like order in IrTe2, 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-Tc superconducting cuprates suggests that the iridates may harbor similar exotic phases.PACS numbers: 71.45. Lr, 78.70.Ck, 78.70Dm, 71.20.Be Transition-metal compounds exhibit surprisingly rich electronic and magnetic properties due to the partially filled d orbitals. The fundamental properties of the electronic structure of transition-metal compounds can be described within the Zaanen-Sawatzky-Allen (ZSA) scheme. This differentiates between the Mott-Hubbard regime (U < ∆) and the charge-transfer regime (∆ < U ), depending on the relative balance of the on-site Coulomb interaction U between the d electrons and the chargetransfer energy ∆ between the ligand states and the transition-metal d states [1]. When ∆ approaches zero, the ligand states are almost degenerate in energy with the transition-metal d levels. As a result, the ligand states may participate in those spin, charge, and/or orbital ordering phenomena that are peculiar to the correlated nature of the d-orbitals. As an example of such phenomenology, ordering of the oxygen 2p holes is realized in the stripe-ordered phase of layered cuprates [2-6], or in the ladder-type Cu oxides [7].Very recently, a first-order structural transition was discovered in the 5d transition-metal chalcogenide IrTe 2 at T s ∼ 280 K. This attracted great interest due to the concomitant discovery of superconductivity in the Ptand Pd-substituted or intercalated compounds [8, 9]. Clarifying the origin of the structural phase transition might be a critical step towards the understanding of superconductivity itself; however, to date several mechanisms have been debated, with a universal consensus still lacking. The phase transition is accompanied by the emergence of a superstructure lattice modulation in electron diffraction [9] -with wavevector Q = (1/5, 0, −1/5) as expressed in reciprocal lattice units in tetragonal notation -which is here illustrated in Fig. 1. The main elements are the Ir-Ir dimerization along the a axis with period 5a, and the consequent distortion of the triangular Ir sublattice in the a − b plane, conflating to an overall trigonal-to-triclinic symmetry reduction. The IrIr dimeriza...

Surface phases of the transition-metal dichalcogenide IrTe2

“…As stated previously, this is unexpected for a weakly bounded layered material [23][24][25]. Such ordering turns the high-temperature trigonal phase into the lowtemperature triclinic phase, with new unit cell vectors orientated in entirely different directions (the new c-axis is not perpendicular to a and b) [6,[26][27][28]. Further cooling causes the second structural transition at ~ 180 K, forming an 8×1×8 structural modulation with additional dimers in the unit cell [21,29].…”

“…However, STM does not directly probe the atomic structure. To date, the only surface structural measurement is via low energy electron diffraction (LEED), which showed a 5×1 reconstruction at 200 K and 1×1 character at 300 K [28]. Here, we report the temperature dependence of IrTe [34].…”

Broken translational and rotational symmetry via charge stripe order in underdoped YBa 2 Cu 3 O 6+ y

“…The sensitivity of the experimental signal to these fluctuating quantities is material-dependent: in the cuprates it has been shown that the resonant scattering is predominantly controlled by variations in the energy shifts (i.e., a scalar field) [30,31], whereas in a different material, e.g. IrTe 2 , the RXS signal (at the Te-M absorption edge) was shown to be primarily associated to a combination of valence modulations and atomic displacements [46]. In presence of valence modulations or energy shifts (i.e., scalar operators), the fluctuating quantity is proportional to modulations in the electronic density ρ, i.e.…”

Phase separation at the dimer-superconductor transition in Ir1−xRhxTe2

“…tuations of this state possibly mediate superconductivity [29,46], tacitly implying similarity to unconventional SCs. Since most studies focus on the origin and mechanism of the phase transition [32,42,[45][46][47][48][49][50][51], the question whether the suppression of the low-T phase with chemical doping proceeds in a first or second order (QCP-like) manner is still not resolved.…”