Electrical Properties In Bi-biased Topological Insulators And WTe₂ Family

Quantum materials,such as topological electronic materials and two-dimensional materials,have attracted tremendous attention in condensed matter physics and ma-terial science studies.Topological electronic materials,such as topological insulators and topological semimetals,can be perfect platforms for searching non-trivial quasi-particle excitation in solid state energy band structure.There is a bright future for two-dimensional materials in electronic device applications.The studies on quantum materials are filled with opportunities as well as challenges.In this thesis,the author manipulates and studies the electrical properties of two typical quantum material sys-tems,Bi-based three-dimensional topological insulators and WTe2 compound family,try to provide reliable detailed experimental results and physics mechanisms for quan-tum materials and condensed matter physics researches.Bi-based three dimensional topological insulators are a class of Z2 topological in-sulators with a single Dirac cone on their time reversal symmetry protected topological surface states.The author grew high quality intrinsic topological insulators BiSbTeSe2 and Sn-Bi1.1Sb0.9Te2S by melting method.It was found that the transport signature of topological surface states can be detected in bulk crystals.The electrical band gap of bulk is about 30meV and 100meV,respectively.The two dimensional resistivity is around kΩ under low temperature.The angle-resolved photoemission spectroscopy and quantum oscillation experiments discovered perfect electronic band structures of topological surface states.In BiSbTeSe2 and Sn-Bi1.1Sb0.9Te2S devices,we observed quantum Hall effect from the topological surface states,made the transport of real Dirac Fermions possible.WTe2 is a new quantum material with several novel properties.The most attractive property of WTe2 is the non-saturating parabolic magnetoresistance.The author grew high quality WTe2 single crystals by chemical vapor transport method.By using mobil-ity spectra obtained from the magnetotransport data,we demonstrated an electron-hole balance mechanism in this material,with an uncertainty of 2%,explained the magne-toresistance phenomenon successfully.We also found a small linear magnetoresistance signature in this material.The author and collaborators broke the electron-hole balance by high pressure technique and manipulated the magnetoresistance.With the burst of density of states under pressure,we successfully observed the electrical superconduc-tivity in WTe2,and obtained the dome-shaped superconductivity phase transition dia-gram.Superconductivity appears at a pressure of 2.5GPa,reaching a maximum critical temperature of 7K at around 16.8GPa.The sister compound of WTe2 has new properties.By doping Mo,we can obtain its sister compound MoxW1-xTe2.This system has been predicted to be type-Ⅱ Weyl semimetal by theory.The author and collaborators studied MoxW1-xTe2 by pump-probe angle-resolved photoemission spectroscopy and revealed the type-Ⅱ Weyl state firstly in experiment.We found that certain Weyl points are at the Fermi level when x=0.25.When x～0.07,a topological phase transition occurs.The system changes from topological trivial state in WTe2 to type-Ⅱ Weyl state in MoxW1-xTe2.We ob-tained the Fermi surface evolution by quantum oscillations under pulsed magnetic field.We also found the parabolic non-saturating magnetoresistance persists after Mo dop-ing,the electron-hole balance is not broken.The author also studied the anisotropic magnetoresistance in this system.