title: Charge-density-wave ordering in three-dimensional metallic compounds
Creators: name: Ramakrishnan, Sitaram
Publisher: name: University of Bayreuth
Year: 2020
Subjects: subject: Charge-density-wave
subject: Aperiodic crystallography
subject: Superspace
subject: 530
subject: 540

description: The Charge-density-wave (CDW) is a static modulation of the density of conduction electrons which is accompanied by a periodic distortion of the lattice. Although CDW mechanisms have been established in one-dimensional (1D) and two-dimensional (2D) systems, the driving force behind the CDW remains an enigma for three-dimensional (3D) systems. This thesis reports on two 3D systems, CuV2S4 and Er2Ir3Si5, with the purpose of explaining the mechanism for the formation of the CDW. Detailed investigations are presented of phase transitions of the compounds CuV2S4 and Er2Ir3Si5, using physical property measurements of single-crystals and single-crystal X-ray diffraction (SXRD). Another compound, Ni{0.89}V{2.11}Se4 with Ni/V disorder is also presented in the thesis. Earlier studies report that CuV2S4 undergoes an incommensurate CDW phase transition at 90 K and a second phase transition at 50 K. Upon the analysis of the SXRD data below 90 K, we observe incommensurate superlattice reflections at positions q = (σ, σ, 0), with σ = 3/4+δ. Moreover, there is a distortion of the lattice, where the symmetry lowers from cubic Fd-3m to orthorhombic Imm2, which is in agreement with the previous work. Below 50 K, the symmetry remained orthorhombic Imm2, however, we found the nature of the 50 K phase transition to be a lock-in transition towards a threefold superstructure. The lock-in transition occurs only on annealed crystals. As-grown (without annealing) crystals, on other hand, suffer from lattice defects, and as a result, they do not undergo the 50 K phase transition. Instead, the σ component of the modulation wave vector q decreases and passes the rational value of 3/4, but never reaches 2/3. From the analysis of the SXRD data, we have established a superspace model for the crystal structure of the CDW phase suggesting that the formation of extended 3D clusters of Vanadium atoms is at the origin of the CDW. At room temperature, R2Ir3Si5 (R = Lu, Er, Ho) is orthorhombic Ibam. A previous study by Electron diffraction (ED) of Lu2Ir3Si5 revealed the presence of incommensurate superlattice reflections at q = (-σ, 2σ, σ), with σ = 0.23~0.25, associated with a CDW phase transition below 140 K. From studies of the physical properties (2 to 300 K) of a single-crystal of Er2Ir3Si5 we have concluded the CDW in the material is a first-order phase transition. The analysis of the SXRD data below 150 K, shows the presence of incommensurate superlattice reflections at positions q = (1/4-δ, 1/2-δ, 1/4-δ) accompanied by a strong monoclinic distortion of the lattice. However, we find that triclinic symmetry I-1 provides a better fit to the model compared to monoclinic symmetry. Our analysis of the crystal structure shows that the CDW resides on the zigzag chains of Iridium atoms. What makes this CDW unusual is that, firstly, it is an incommensurate first-order transition accompanied by a monoclinic lattice distortion, and secondly, from the magnetic susceptibility measurements, we observe that there is a strong coupling between the CDW and magnetism, as the Er^{3+} moments are influenced by the CDW. We also show that in the high-quality single-crystal, the magnetic ordering of the compound is suppressed to at least 0.1 K. However, previous studies report that in a polycrystal of Er2Ir3Si5 the antiferromagnetic ordering is observed around 2.1 K. This seems to suggest that disorder in the polycrystal brings back the antiferromagnetism at the expense of the CDW transition. According to the literature, polycrystalline NiV2Se4 is reported to be a CDW system at 165 K. We sought to investigate the CDW in the material, however, the attempts to synthesize single crystals of NiV2Se4 led to Ni deficient material with Ni/V site disorder, resulting in Ni{0.89}V{2.11}Se4. By studying temperature-dependent structural and bulk properties of Ni{0.89}V{2.11}Se4, we report a possible Non-Fermi-liquid (NFL) to a Fermi-liquid (FL) transition at ambient pressure. The electrical resistivity shows metallic behavior with a broad anomaly around 150-200 K. ρ(T) is found to exhibit an anomalous T^{3/2} dependence which is a strong indication of NFL, and below 15 K it exhibits a T^2 dependence down to 1.5 K, meaning that the FL behaviour is recovered below 15 K. From the analysis of the SXRD data at 100 K, we observed no superlattice reflections and no change to the structure as it remained monoclinic I2/m, indicating the absence of a CDW phase transition. The presence of magnetic fluctuations and quenched disorder on the Ni/V sites could be the cause of NFL to FL transition, given that stoichiometric NiV2Se4 is claimed to be a non-magnetic CDW system. Synthesis of a stoichiometric NiV2Se4 without the disorder is necessary to investigate the possibility of a CDW phase transition, and it is being undertaken.