Coupling between colossal charge density wave ordering and magnetism in Ho2Ir3Si5

Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu, Er and Ho) compounds that exhibit a colossal first-order charge density wave (CDW) transition where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir-Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from non-magnetic Lu to magnetic Er or Ho lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K and TCDWHo = 90 K. Out of the three compounds, Ho2Ir3Si5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er2Ir3Si5, there appears to be a coupling between CDW and magnetism such that the Ho3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1 K, whereas AFM order is suppressed and only the CDW is present in our highly ordered single-crystal. First-principles calculations predict Ho2Ir3Si5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.