| Over the last decade,topological quantum materials,originated from quantum Hall effect,have attracted tremendous attention in diverse scientific communities,due to their unique electronic band structures and exotic physical properties.Since the experimental discovery of two-dimensional(2D)graphene at 2004,a series of topological quantum materials,including topological insulator,Dirac/Weyl semimetal,Type-II Dirac/Weyl semimetal and Dirac node-line semimetal,have been predicted and explored in the recent years.These topological quantum materials,because of their rich features in band structures,can serve as perfect platforms for exploring exotic quasi-particle excitations(such as Weyl fermion)predicted by quantum field theory.Taking advantage of these strange particles could lead to develop a new generation of electronic components.Up to now,a great many features of topological materials have been discovered by angle resolved photoemission spectroscopy(ARPES)and scanning tunneling microscopy(STM).To explore potential device applications,it's necessary to characterize the electric transport properties of topological quantum materials in-depth,since the current electronic devices are mostly based on the processing of the electrical signals.However,transport studies of topological states are relatively much less reported due to the facts that the transport properties of the Dirac fermions are shielded by the plenty of trivial Schrodinger fermions at the Fermi energy level,which makes the characterization of electrical properties of topological quantum materials quite challenging.This difficulty can be solved through two routes:on one hand,we can manipulate the Fermi energy level of the crystals to make it close to Dirac/Weyl nodes,which will enhance the effects of Berry phase;on the other hand,it is feasible to look for new topological materials with dominant Dirac/Weyl fermions at Fermi level.In this thesis,we study three families of the binary compounds crystals:topological insulator Zr/HfTes crystals,type-II Weyl semimetals Mo/WTe2,and WP2 crystals.We prepared a series of high quality single crystals using chemical vapor transport(CVT)method and adjusted the Fermi energy level by doping or thermal annealing.The structures,compositions,and transport properties of these materials were systematically characterized and analyzed by both expertimetal and theoretical methods.We have observed several novel transport evidences of Dirac/Weyl fermions in these materials.Main work and conclusions are summarized as follow:1)Based on the structures,compositions,and transport characterizations of ZrTes,HfTe5 and their doping crystals,it is reasonably assumed that the main physical origin of the resistivity anomaly(135 K for ZrTes,70 K for HfTe5)is attributed to electron-hole perfect compensation and the lower effect of carrier concentration is realized around the anomalous peak.The Shubnikov-de Haas(SdH)effect was measured on the high quality ZrTes crystals at low temperature.SdH oscillation up to eight-order was observed in raw data under maximum 9 T magnetic field.The Landau level fan diagram and phase analysis of SdH substantiates that nontrivial ?-Berry phase is observed in the c-plane SdH oscillation,but not in the b-plane one.The data strongly suggests that bulk ZrTe5 at low temperature(-2 K)belongs to a weak topological insulator,rather than Dirac semimetal and strong topological insulator reported previously.By STM cooperated with the other group,we observed a bulk band gap of?80 meV with topological edge states at the atomic steps of ZrTes crystals,consistent to our transport results.2)By means of X-ray diffraction and Raman spectroscopy characterizations,we map out the temperature and composition dependent phase diagram of Mo1-xWxTe2 crystal system,establishing Td phase as a topological nontrivial phase.There are temperature and composition dependant topological phase transitions in Mo1-xWxTe2 system.The detailed magnetoresistance(MR)characterizations of Td-MoTe2 crystals reveal anisotropic MR behaviors,which are strongly correlated to specific features of Fermi surface and the correlation effect in Td-MoTe2.Analysis of MR behaviors of Td-WTe2 crystals suggests that both electron-hole concentration and carrier mobility play the crucial role on the extremely large MR in Td-WTe2 material.Particularly,Td-WTe1.98 crystals were prepared via thermal annealing process,which is electron-doped crystal with its Fermi level close to the Weyl node.In these Fermi-level delicately adjusted WTei.98 crystals,we observed the significant anisotropic Adler-Bell-Jackiw(ABJ)anomaly.Quantitatively,CW,a coefficient representing intensity of ABJ anomaly,along a-and b-axis of WTei.98 are 0.030 and 0.051 T-2 at 2 K,respectively.Boltzmann electrical transport study and first-principles calculation reveal that observed ansotropic ABJ both along a-and b-axis in WTe1.98 is attributed to electrical transport in the quasi-classical regime.We also found that temperature-sensitive ABJ anomaly is due to topological phase transition from type-II Weyl semimetal to trivial semimetal phases under thermal agitation,verifing by first-principles calculation using experimentally determined lattice parameters at different temperatures.3)We successfully synthesized highly crystalline a-WP2 and ?-WP2 crystals by CVT with different temperature gradient,respectively.The hole-doped P-WP2.11 crystals were obtained by controlling growing conditions in CVT growth procedure.Crystals a-WP2 and P-WP2 all exhibit extreme large and highly anisotropic MR in magneto-transport measurements.The noteworthy MR effect can be attributed to the perfect electron-hole compensation and the open orbital in Fermi surface,while the carrier mobility has a dominated effect on the extremely large MR effect.It's particularly significant that we observed the giant chiral magnetic effect evidenced by:"negative" resistivity and corresponding voltage-current curves lying the second-fourth quadrant(anomalous Ohm's law)in type-II Weyl semimetal WP2.11 crystals under following conditions:the misaligned angle between electrical field(E)and magnetic field(B)is smaller than 20°,temperature<30 K and externally applied E<50 mA.Using density of state(DOS)theoretical calculation,we can find that the Fermi level in WP2.11 is shifted very close to the energy of the Weyl points.Phenomenologically,based on macroscopic electrodynamics with Chern-Simon-Maxwell equation,the main features of our experimental results can be described by larger current density j? generated by axion field in Weyl semimetals under E//B,and its effect is called as a chiral electrical battery.These works demonstrate unique physical properties of topological quantum materials from the point of view of transport behaviors,substantially broadening and deepening our understanding of the physics of topological quantum materials,which paves the way to develop electronic devices based on topological quantum materials. |
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