The Search Of Kondo Weyl Semimetals And Pressure-induced Topological Phase Transitions

The search of topological materials is one of the most intriguing project in the condensed matter physics.After the discovery of topological insulators,various topological materials with comprehensive properties have been found.In this thesis,I present two important aspects of the research of topological materials,including the discovery of Kondo Weyl semimetal candidates,and the pressure-induced topological quantum phase transitions.1.The search of Kondo Weyl semimetal candidates has attracted lots of attentions in recent years.In these exotic materials,many unique phenomena have been induced due to the interplay between band topology and strong electronic correlations.However,to find the proper material candidates is of great challenge.Due to the strong electronic correlations,the power methods in weakly correlated systems,such as band structure calculation or ARPES measurements,are no longer ideal research tools.Thus to search for the new Kondo Weyl semimetals,what we need is not only the former experience in weakly correlated systems,but also the construction of new methods and schemes,which needs the strong collaboration between theorists and experimentalists.Our studies are mainly on two materials,CeSb and YbPtBi.CeSb is a typical Kondo magnetic semimetal.Via angle dependent magnetoresistance measurements,we found that after the appearance of field induced ferromagnetic ordered state,a strong longitudinal negative MR is observed,which suggests the existence of chiral anomaly in this material.The band structure calculation results confirmed that due to the breaking of time-reversal symmetry in ferromagnetic state,a pair of Weyl nodes emerged below the Fermi level,which is the source of the chiral anomaly effect.These results suggest that CeSb is a possible Kondo Weyl semimetal candidate.On the other hand,in the studies of YbPtBi,we found that Weyl nodes are strongly modified by the band hybridization effect,which induced various changes of the physical properties of this material.At high temperature before the band hybridization happens,the negative longitudinal MR is observed,which is resulted from the chiral anomaly effect.With lowering temperature,the band hybridization effect starts to modify the band structure of this sample,and the heavy quasiparticles start to form,hence the effect Fermi velocity drops rapidly.Due to this drop,the strength of chiral anomaly effect,which strongly depends on the Fermi velocity,deceases rapidly as well.In the meantime,however,the heat capacity coefficient of the sample shows a strong T3 behavior,indicates a strong nodal excitation,which originated from the Weyl nodes in the modified band structure.And the topological and anomalous Hall effect observed within the whole temperature range suggest that the position of the Weyl nodes are almost not altered by the band hybridization.These results suggest that YbPtBi is an ideal Kondo Weyl semimetal candidate,which provides a suitable platform for the study of the interplay between strong electronic correlations and Weyl physics.2.Topological phase transition is another interesting topic in topological material research.In the previous studies,people usually substitute the elements in the current materials to tune the strength of spin-orbit coupling,which leads to the realization of topological quantum phase transition.Recently,some theoretical and experimental researchs show that by applying external pressure,the band topology of the materials can be changed.In this thesis,we mainly present the results of magnetoresistance and Hall resistivity measurements of PbTe thin film and ZrTes under pressure.In PbTe thin film samples,we found that by applying pressure,the Fermi surface will be reconstructed,which results in the change of band topology.For ZrTes,with the combination of experimental measurements and theoretical calculations,we constructed the complete pressure-topology phase diagram.In the meantime,based on this phase diagram,we successfully explained the deviation and contradiction in the previous reports.These results are not only meaningful in understanding ZrTes,but also helpful in explaining the general sample dependence problem in the research of topological semimetals.