| In this dissertation,I study the physical aspects of topological semimetals,a set of three-dimensional materials whose low-energy excitations are explained by Dirac quasi-exotic particles.The unconventional characteristics of these systems,ranging from fundamental physics to technological applications,have acquired significant research activity in recent years.The concern in Dirac quasi-exotic particles systems lies in part in the relativistic nature of their electronic degrees of freedom,causing these materials ideal class for examining and exploring fundamental physics phenomena.Furthermore,a single species of Weyl fermions displays chiral anomaly that gives rise to a possibility of controlling the valley polarization.The most prominent effects in Dirac or Weyl materials are negative longitudinal magneto resistivity(NLMR)and strong anomalous Hall conductivity which originate from the chiral anomaly.The chiral anomaly was first experimentally confirmed in Weyl semimetals TaAs.This dissertation is devoted to the research on the charge transport properties of Dirac systems(ZrSiS,NdSb,TaAs)hosting an extremely large magneto resistance(XMR),quantum oscillations,high mobility,and largely suppression of thermal conductivity at ultralow temperature.These materials are highly demanded for fabrication of the magnetoresistance devices.I also discuss the effective spin-1/2 triangular lattice antiferromagnet YbMgGaO4 at the ultralow temperature AC susceptibility,thermal conductivity,and magnetic torque measurements.I also describe research results of quantum spin liquid candidate YbMgGaO4 of great interest in physics.In chapter one,I describe the progress of research about the charge transport properties of the Dirac and Weyl semimetals.The thesis consists of four chapters;the major contents of each chapter are as follows:In chapter two,we report the results of electrical resistivity,thermal conductivity,and thermal Hall conductivity of ZrSiS single crystals at low temperature.Recently,ZrSiS has drawn intensive research attention due to exotic physical properties originating from non-trivial surface states and its bulk Fermi surface,and due to its simple crystal structure that is easy to grow.However,such efforts have been explored in studies of even broader physical phenomena,such as highly conductive,XMR,chiral anomaly,and nontrivial Berry phase.The thermal conductivity(?xx/T)is dramatically decreased in magnetic fields,having a good correlation with the large positive transverse magneto resistance(TMR).The most impressive result is that the thermal Hall conductivity(?xy/T)shows sharp peaks at the extremely low field which reveal the high mobility of the electrons.The detailed study of ZrSiS,which gives a fundamental importance platform to study the physics of topological semimetals and can be generalized to other topological phases of matter.In chapter three,we have measured resistivity and thermal conductivity at ultralow temperatures and in high magnetic fields for antiferromagnet Dirac semimetal NdSb by using single crystal samples.Due to highly conductive,perfect compensation between electron and hole pockets,extremely large magneto resistance,and with the exotic electronic structures,the antiferromagnet Dirac semimetal NdSb offer fertile ground for the exploration of the charge transport behavior of quasi-exotic particles.Here,it is found that thermal conductivity dramatically is decreased by more than two orders of magnitude and displays clear quantum oscillations at the high field.It is found that zero-field ?(T)exhibits a T4 behavior which was far lacking to explain by all the conventional heat-transport mechanisms through diffusion of propagating electrons.This unusual behavior of thermal conductivity causes a strong violation of the Weidemann-Franz law.In chapter four,we studied the effective spin-1/2 triangular lattice antiferromagnet YbMgGaO4 by revisiting the ultralow temperature AC susceptibility,thermal conductivity and new magnetic torque measurements.One recent focus on quantum spin liquid(QSL)studies is about how disorder/randomness in a QSL candidate affects its true magnetic ground state.The ultimate question is whether the QSL survives through the disorder or the disorder leads to a "spin-liquid-like" state,such as the proposed random-singlet(RS)state.This question puts many studied QSL candidates under scrutiny since they inevitably have chemical disorder.YbMgGaO4,a triangular lattice antiferromagnet with effective spin-1/2 Yb3+ions,is an ideal system since it shows no long-range magnetic ordering and with Mg/Ga site disorder.However,despite the intensive studies,this question remains unsolved for YbMgGaO4.Either the QSL or the RS state has plenty of supports.Here,through ultralow temperature thermal conductivity and magnetic torque measurements,we observed a residual ?0/T term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO4.These observations confirm that a QSL state with itinerant excitations and quantum spin fluctuations survives through the disorder in YbMgGaO4 and therefore settles down its controversy. |
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