Raman Scattering Study On Structure And Electron-phonon Coupling Of Novel Quantum Materials

Quantum materials have several degrees of freedom:charge,spin,orbital,lattice and topological nature,which are coupled together and determine the materials' properties.Raman scattering spectroscopy is a powerf'ul tool to study the crystal symmetry,structural phase transitions,electron-phonon coupling and various collective excitations.Thus,it plays an important role in the study of the crystal structure and electron-phonon coupling in quantum materials.The thesis is organized as f'ollows.Chapter 1 is an introduction of quantum materials including superconductors,charge-density-wave and topological mate-rials.In Chapter 2,we illustrate the fundamental Raman theory and the experi-ment setup.From Chapter 3 to Chapter 5,we present Raman scattering studies of structures and electron-phonon interactions in several quantum materials:1.Anomalous increase of an Ag phonon' linewidth with temperature decreas-ing in quasi-1D superconductor Ta4Pd3Te16.We observe 28 out of 33 Raman ac-tive modes,with frequencies in good accordance with first-principles calculations.Although most of the phonons observed vary only slightly with temperature and do not exhibit any asymmetric profile that would suggest strong electron-phonon coupling,the linewidth of the Ag phonon mode at 89.9 cm-1 shows an unconven-tional increase with temperature decreasing,which is possibly due to a CDW transition or the emergence of CDW fluctuations below a temperature estimated to fall in the 140-200 K range.2.Electron-phonon coupling in thin films of LiTi2O4 spinel oxide supercon-ductor.We detected four out of five Raman active modes predicted by calculation-s.Three T2g modes show a Fano lineshape from 5 K to 295 K,which suggests an electron-phonon coupling in LiTi2O4.Interestingly,the electron-phonon coupling shows an anomaly across the negative to positive magnetoresistance transition at 50 K,which may be due to the onset of other competing orders.The strength of the electron-phonon interaction estimated from the Allen's formula and the observed lineshape parameters suggests that the three T2g modes at 0 momentum are not sufficient to drive superconductivity.3.Structural phase transition and CDW collective mode in Na2Ti2As20.We identified a symmetry breaking structural transition at around Ts = 150 K,which matches a large bump in the electrical resistivity.Combined with first-principles calculations,our polarization-dependent measurements suggest a charge instability driven lattice distortion along one of the Ti-O bonds that breaks the fourfold symmetry and more than doubles the unit cell.We also detected the CDW amplitude mode at around 70.2 cm-1 with Ag symmetry.4.Lattice instability in the non-symmorphic topological insulator KHgSb with hourglass-like electronic dispersion.While we observe one of two calculated Raman active E2g phonons predicted at room temperature using space group P63/mmc,an additional A1g peak appears at 99.5 cm-1 upon cooling below T*= 150 K,which suggests a lattice distortion.Several weak peaks associated with two-phonon excitations emerge with this lattice instability.We also show that the sample is very sensitive to high temperature and high laser power,conditions under which it quickly decomposes,leading to the formation of Sb.Theoretical calculations suggest that structure with space group P63mc is more stable than the presumed P63/mmc structure and preserves the glide plane symmetry necessary to the formation of hourglass fermions.Chapter 6 is the summary and prospects.