A comprehensive ARPES study on the anomalous transport properties and topological character of ZrTe5

The pentatellurides, HfTe5 and ZrTe5, have attracted the attention of the scientific community since their discovery. The reason descends from the anomalous transport properties of these materials, showing a peak in the resistivity at a characteristic temperature T* and a sudden sign-change of its large thermopower at the same temperature. The possible application of HfTe5 and ZrTe5, as promising thermoelectric materials, has driven intense studies about the possible origin of their transport anomalies. However, a clear answer to these problems is still lacking. ZrTe5 has recently received a renewed interest, with a particular focus on its possible topological character and its proximity to a topological phase transition, for it could be a promising candidate for spintronics applications. However, these issues are opening an intense debate. The scope of this thesis is the address the questions concerning the topological character and the anomalous transport properties of ZrTe5 by means of Angle Resolved Photoelectron Spectroscopy (ARPES). In particular, I have studied in details the electronic band structure of ZrTe5 as a function of the temperature. These experiments allowed me to link the anomalous resistivity behaviour to a rigid band shift observed by varying the temperature across T*.. In addition, by means of time resolved ARPES (Tr-ARPES), I have proven the capability to manipulate, on ultrafast time scales (hundreds of fs), the electronic structure of ZrTe5, reproducing the findings at equilibrium, eventually leading to an ultrafast optical control of its transport properties via an impulsive modification of the electronic temperature. Finally, I have focused my attention on the topological character of ZrTe5. Thanks to a multi-technique approach, by using ultra violet ARPES, soft X-ray ARPES, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), density functional theory (DFT) calculations, X-ray diffraction (XRD) and spin resolved ARPES I have demonstrated that this material is a strong topological insulator. For all these reasons the studies reported in this thesis can be possibly be viewed as an important advancement for understanding of the topological nature and the transport properties of ZrTe5.