Studies On Topological Transport Properties Of Tellurium

Topological materials have been widely concerned due to the non-trivial band topology,and moreover the as-induced fascinating physical properties and superior performances.In recent years,topological semimetals have gradually become a new research focus after the discovery of graphene and topological insulators.As a novel topological quantum state,Weyl semimetals are well known for their unique features,such as three-dimensional linear dispersion,Fermi arc surface state,chiral anomaly,etc.Such characteristics originate from the existence of Weyl fermions in Weyl semimetals.In a nutshell,a Weyl point is just a two-fold band degeneracy,and therefore can in principle exist in any material satisfying the symmetry requirements.Until now,Weyl physics has been generally regarded as unique to semimetals.Nevertheless,considering the high tunability and compatibility with modern electronic industry,semiconductors are more suitable for device applications than semimetals.Therefore,it is promising to extend Weyl physics to semiconductor systems,which will promote the development of high-performance topological electronic devices.Tellurium(Te)is a narrow gap semiconductor possessing strong spin-orbit coupling,and it lacks space inversion symmetry due to the characteristic chiral crystal structure.Therefore,Te has been considered as a potential topological material.This thesis investigates the topological transport properties of Te single crystals through systematic magneto-transport measurements.Typical Weyl signatures are demonstrated,including the negative magnetoresistance in parallel magnetic fields,the planar Hall effect,as well as the intriguing logarithmically periodic quantum oscillations.Our discovery reveals the topological nature of Te.In the first chapter,we focus on the background knowledge about Weyl semimetals,transport signatures of the chiral anomaly,log-periodic quantum oscillations and Te.First,the topological band structure and several essential features of Weyl semimetals are introduced.We mainly concentrate on the transport signatures of the chiral anomaly,such as the negative longitudinal magnetoresistance and planar Hall effect.Next,the experimental discoveries and theoretical explanations of log-periodic oscillations are specifically addressed.The relevant background knowledge includes quantum limit,discrete scale invariance,supercritical collapse,Efimov physics,and etc.Finally,we introduce the basic characters and transport properties of Te,mainly focusing on the theoretical and experimental progresses on the topological transport properties.In the second chapter,we explore the basic transport properties of Te.High-quality bulk Te single crystals are successfully grown via physical vapor deposition.Through magneto-transport measurements,the basic transport properties of Te are systematically characterized,including the temperature dependence of resistance,transverse magnetoresistance,Hall resistance,and etc.The results pave the way for the investigation of the topological transport properties of Te.In the third chapter,the topological transport properties of Te are systematically investigated.Via magneto-transport measurements,the topological transport signatures of Weyl physics,e.g.,the negative longitudinal magnetoresistance and the planar Hall effect,are observed.The relations between the topological transport properties and several factors,including temperature,angle and magnetic field,are analyzed in detail and ultimately ascribed to the chiral anomaly.The simultaneous occurrence of the negative longitudinal magnetoresistance and planar Hall effect in Te constitutes a smoking gun evidence for the chiral anomaly,and also reflects the significant influence of Weyl fermions on the electrical transport.Via first principles calculations,our theoretical collaborators discover new Weyl points located near the valence band maximum of Te,arising from the crossing of two spin-splitting bands.We propose that Te combines the advantages of semiconducting characters and topological band structures.Therefore,Te is a new topological material with potential application values,which can be refered to as "Weyl semiconductor".Our study on the topological transport properties of Te successfully extends the Weyl physics to semiconductor systems.In the fourth chapter,the log-periodic quantum oscillations in Te are explored by magneto-transport measurements at low temperatures.Both the magnetoresistance and Hall resistance are found to exhibit oscillation behaviors in the quantum limit,which cannot be explained by the conventional Shubnikov-de Haas oscillations.High magnetic field facilities are utilized to perform transport measurements in a large magnetic field range.Intriguingly,the peculiar log-periodic oscillations in both the magnetoresistance and Hall data are revealed beyond the quantum limit.Our discovery reflects the topological nature of Te and provides a new platform for the exploration of discrete scale invariance in condensed matter physics.In the fifth chapter,we investigate the basic and topological transport properties of two-dimensional Te flakes.Via a simple low-temperature physical vapor deposition method,high-quality single-crystalline Te flakes are successfully fabricated.A low field negative magnetoresistance is observed under perpendicular fields,which can be attributed to the two-dimensional weak localization effect.More significantly,large non-saturated negative magnetoresistance in parallel fields are clearly demonstrated over a wide temperature range and ascribed to the chiral anomaly.Note that the details of the as-observed negative longitudinal magnetoresistance differ distinctly from those in Te bulk crystals,indicating the quantum confinement modification of Weyl physics.It is believed that the dimension reduction of Weyl semiconductors will stimulate the realization of high-tunable and easy-integrated topological electronic devices in the future.