We use ultra-high resolution, tunable, VUV laser-based, angle-resolved photoemission spectroscopy (ARPES) and temperature and field dependent resistivity and thermoelectric power (TEP) measurements to study the electronic properties of WTe2, a compou
nd that manifests exceptionally large, temperature dependent magnetoresistance. The temperature dependence of the TEP shows a change of slope at T=175 K and the Kohler rule breaks down above 70-140 K range. The Fermi surface consists of two electron pockets and two pairs of hole pockets along the X-Gamma-X direction. Upon increase of temperature from 40K, the hole pockets gradually sink below the chemical potential. Like BaFe2As2, WTe2 has clear and substantial changes in its Fermi surface driven by modest changes in temperature. In WTe2, this leads to a rare example of temperature induced Lifshitz transition, associated with the complete disappearance of the hole pockets. These dramatic changes of the electronic structure naturally explain unusual features of the transport data.
We have developed an angle-resolved photoemission spectrometer with tunable VUV laser as a photon source. The photon source is based on the fourth harmonic generation of a near IR beam from a Ti:sapphire laser pumped by a CW green laser and tunable b
etween 5.3eV and 7eV. The most important part of the set-up is a compact, vacuum enclosed fourth harmonic generator based on KBBF crystals, grown hydrothermally in the US. This source can deliver a photon flux of over 10^14 photons/s. We demonstrate that this energy range is sufficient to measure the kz dispersion in an iron arsenic high temperature superconductor, which was previously only possible at synchrotron facilities.
