Temperature-Induced Lifshitz Transition in Weak Topological Insulator Candidates ZrTe$_{5}$ and HfTe$_{5}$

The transition metal pentatellurides ZrTe$_{5}$ and HfTe$_{5}$ have attracted consideration attention since the 70s, due to their unusual transport properties like resistivity peak at \textasciitilde 135K/\textasciitilde 65K and the sign change of the Hall coefficient and thermopower. The origin of the most peculiar transport properties remains controversial. Lately, ZrTe$_{5}$ and HfTe$_{5}$ have ignited renewed interest because it is predicted that single-layer ZrTe$_{5}$ and HfTe$_{5}$ are two-dimensional topological insulators and there is possibly a topological phase transition in bulk ZrTe$_{5}$ and HfTe$_{5}$. In this talk we will present temperature dependent Fermi surface and band structure of ZrTe$_{5}$ and HfTe$_{5}$, by using our super-high resolution angle-resolved photoemission system equipped with the VUV laser and the time-of-flight (TOF) electron energy analyzer. Upon cooling down, we found a gradual transition from hole-like band into electron-like band around the Brillouin zone center. Such an electron state transition forms the underlying physics for the abnormal transport properties. We will also discuss on the topological nature of ZrTe$_{5}$ and HfTe$_{5¡£}$