Investigation Of The Quantum Material Selenides By Angle-resolved Photoemission Spectroscopy And Neutron Scattering

With the continuous increasing of the energy demand and the rapid decline of the non-renewable resources,energy crisis has been the gridlock of the economic development.Functional quantum materials,such as thermoelectric materials and topological materials,have aroused world-wide interests in the last few years.Thermoelectric materials possess the ability to convert the waste heat into useful electricity,and the application of the topological materials could significantly improve the dissipation issue of the electronic device.These two materials could provide very effective solutions to address the energy crisis issue.In this dissertation,various experimental techniques have been exploited,for instance,neutron scattering/diffraction,Raman and angle-resolved photoemission spectroscopy,to have systematic investigations on thermoelectric material SnSe system and topological nodal line semimetal ZrSiSe.The following is a list of our results:We have studied how electronic structures and phonon dynamics evolve with temperatures in Na-doped SnSe.Our data indicate that the effective mass has a weak temperature dependence along the ?-Z direction at low temperatures.The significant shift of the chemical potential with temperature is associated with the thermal excitation of holes in this hole-doped material.However,the increase of the band gap with temperature indicates that the change in electronic structures around the top of the valence bands and the bottom of the conduction bands also needs to be taken into account to quantitatively depict electronic properties around the Fermi level.The INS data reveal that the low-lying TO1 and TA phonon modes contribute to the decline of the lattice thermal conductivity with increasing temperatures,while the T02 mode does not.Moreover,our INS data suggest that extrinsic factors,such as deficiencies or grain boundaries,have a strong effect on the reduction of the lattice thermal conductivity,which could effectively improve the performance of the thermoelectric in Na-doped SnSe.Our findings provide valuable information for a better understanding of the high thermoelectric performance of Na-doped SnSe,which could be helpful for the further enhancement of ZT in SnSe.In particular,the remarkable variations of electronic structures and phonon dynamics with temperature suggest that it is inadequate to extrapolate the thermoelectric performance by merely adopting the low-temperature band structures or phonon dynamics as the input parameters,and their evolution with temperature has to be further investigated to quantitatively account for the high thermoelectric performance of hole-doped SnSe.We performed the systematic investigation of the anharmonicity effect indoped SnSe system by neutron powder diffraction,neutron total scattering,inelastic neutron scattering and Raman scattering measurements.We found that the first PDF peak evolves from symmetric Gaussian-like to asymmetric non-Gaussian-like shape with elevating the temperature.Our neutron diffraction results indicate that the atom off-centering is not occurred at high temperatures,which means the asymmetry of the PDF is not related to the atom off-centering.Furthermore,our first-principle calculations show that the PDF peaks corresponding to d1 and d2 become asymmetrical as elevating the temperature,which suggests the presence of anharmonicity in SnSe system.By means of INS and Raman scattering,the zone-centered optical modes are observed to exhibit the evident softening and broadening.The anharmonicity effect is believed to be presented by using the anharmoncity as well as thermal expansion model,and the phonon scattering is dominated by three-phonon process.Moreover,the lattice anharmonicity is extremely evident at high temperatures.We found that B3g and Ag3 exhibit strong softening which is due to the strong anharmonicity effect of these two modes.Such finding suggests that the higher optical modes are also accountable for the decline of the lattice thermal conductivity with increasing the temperature.Our angle-resolved photoemission spectroscopy results reveal that the electronic structures of ZrSiSe display non-rigid band shift induced by gas adsorption.The surface states derived from different orbital characters move towards higher binding energies with different amount.Importantly,gas adsorption acts as the role of perturbation,leading to the non-degenerate of the bulk bands along the X-M direction though the non-symmorphic symmetry at the X point is preserved.Our study shed lights on a new insight to modulate the electron structures in the surface region with the possibility of realizing novel quantum states on the surface of WHM materials.