Transport Measurements Of Emergent Topological Semimetal And 2D Quantum Materials

The finding of new classes of quantum materials with exotic physical properties,which al-ways bring great opportunities both for fundamental research and device applications,play an important role in condensed matter physics.Quantum charge transport measurements are the most fundamental and widely utilized techniques for exploring the physical properties of novel quantum materials in condensed matters.In this thesis,by using quantum electric and thermo-electric transport,in complementary with other state-of-the-art characterization techniques,we get insights into the physical properties of two emergent quantum materials,namely topological Weyl semimetal NbP and two-dimensional thermoelectric material SnSe.We prove that the ul-trahigh charge carrier mobility in NbP is protected by helicity,a fascinating properties of Weyl quasiparticle excitations associated with 3D massless energy bands without spin degeneracy.For SnSe,we show unambiguous evidences on the defect origin of the hole doping in the bulk,and we reveal that the extraordinary thermoelectric figure of merit of SnSe is rooted in pudding-mold shaped valence bands with multi-valley contributions to the thermal transport.Dirac,Weyl and Majorana fermions are the only three relativistic fermions in high energy physics in the framework of special relativity with Poincare symmetry.However,except Dirac fermion,the other two fundamental fermions have never been discovered in experiments.Quasi-particle excitations in condensed matter systems,which obey Dirac,Weyl,or Majorana equation,shed new light on experimental studies on these relativistic fermions.Theory and first principle calculations predict that in some crystals,the collective mode of quasiparticles can simulate the behavior of these relativistic high-energy fermions.Using quantum transport,Seebeck and Nernst coefficient measurements,we prove the existence of Weyl fermions in topological semimetal NbP.For massless Weyl fermions,the helicity associated with the Weyl nodes require strict parallel or anti-parallel alignment between the orientation of spin and momentum.Such helicity quantum number protects Weyl fermions from backscattering in NbP,leading to astonishing ultra-high charge carrier mobility of 107cm2V-1s-1.Since such helicity protection is rooted in time reversal symmetry,we find that the Weyl fermions in NbP are very sensitive to magnetic impurities.Non-magnetic impurities like Zn or Cu have barely effect on the transport properties,while magnetic impurities like Fe or Cr drastically suppress the mobility of carriers in NbP.Our study prove the protection mechanism of helicity in Weyl semimetal for the first time.Applying temperature gradient will generate thermal voltage in a conductive crystal with free charge carriers,a phenomenon called Seebeck effect.Using Seebeck effect,we can directly convert thermal energy into electric voltage,or use its reverse-effect to make solid-state Peltier coolers.In general,we use thermoelectric figure of merit(ZT=s2?T)to evaluate the perfor-mance of thermoelectric material.To maximize ZT,a material is required to have large Seebeck coefficient(S),high electric conductivity(?),and low thermal conductivity(k),which is normal-ly challenging to be achieved in one material system.Recently,SnSe is discovered to have ZT as large as 2.6 at 973K along b-axis.However,a comprehensive understanding of the electron-ic structure and most critically,the self-hole doping mechanism in SnSe are still absent.By us-ing quantum oscillation and complementary angle resolved photoemission spectroscopy(ARPES)measurements,we discover a unique pudding-mold shaped valence band with quasi-linear energy dispersion in SnSe,which plays a decisive role in producing the high electric conductivity and large Seebeck coefficient.Equally important,we prove that p-type doping in SnSe is extrinsically controlled by the local phase segregation of SnSe2 micro-domains via interfacial charge trans-ferring.Angle-dependent quantum oscillations reveal 3-dimensional Fermi surface with unusual interlayer coupling strength in p-SnSe,in which individual monolayers are interwoven by peculiar point dislocation defects.Our results suggest that defect engineering may provide versatile routes in improving the thermoelectric performance of the SnSe family.