Infrared properties of three low-dimensional materials

Abstract

<p>This thesis includes far-infrared measurements of three new and very different materials, the spin-Peierls compound CuGeO3 , the quasi-two-dimensional organic superconductor κ-(ET)2 Cu[N(CN)2 ]Br, and the high-temperature superconductor HgBa2 Ca2 Cu 3 O8-δ . All three have reduced dimensionality and exhibit properties which are both of great interest to solid state physicists and ideal for study using far-infrared techniques. CuGeO3 is the first inorganic spin-Peierls compound to be discovered. It has a simple structure, is easily doped with impurities, and can be grown in the form of very large single crystals. These properties make it far easier to study than its organic predecessors. Like other spin-Peierls compounds, CuGeO3 consists of chains of large tightly bound blocks that can move as units, in this case GeO4 tetrahedra, and the spin-Peierls distortion can be described in terms of a displacement of these units. This thesis discusses a newly discovered local libration of the GeO4 tetrahedra near impurity atoms in doped CuGeO3 . This unusual mode is highly anharmonic and causes striking features in the far-infrared spectra. The superconductor κ-(ET)2 Cu[N(CN)2 ]Br belongs to a family of organic superconductors that, like the much studied high-temperature superconductors, consist of metallic sheets that are relatively isolated from one another. Conduction within the sheets is coherent, but the nature of the transport between the sheets is poorly understood. In this thesis, the first ever interplane far-infrared spectra for this material are reported. No evidence of coherent transport is found down to the lowest frequencies suggesting that in this respect this family of organic superconductors is similar to the high-temperature superconductors. Many internal modes of the ET molecule are also identified in the spectra. HgBa2 Ca2 Cu3 O8-δ has the highest superconducting transition temperature ever measured, but single crystals large enough for infrared and far-infrared studies have only recently become available. This thesis presents in-plane infrared measurements on this material that show the presence of a pseudogap much like that seen in other high temperature superconductors except that it is considerably larger in size. In fact, its pseudogap is larger than those of the other materials by the same factor by which its transition temperature is higher. This suggests a possible relationship between pseudogap size and transition temperature which is discussed in light of other results in the literature. All three of these materials are either quasi-one-dimensional or quasi-two-dimensional in nature and demonstrate both the diversity of the phenomena found in solid state systems and the utility of far-infrared spectroscopy in their study.</p>