Several New Dirac Materials:Synthesis,Transport And Magnetic Properties

The Dirac materials have generated immense interest in the last decade,owning to their massless Dirac particles in the low-energy excitations.Novel physical properties have always been observed in these materials,like large magnetoresistance(MR),ultrahigh mobility,quantum hall effect(QHE)and so on.Especially,these behaviors can even been detected at room temperature,which can be harnessed for possible applications.The work will focus on three aspects in this article:(1)High quality EuMn0.8Sb2 single crystal was synthesized and its transport and magnetic properties were systematically studied.It was found that it has large magnetoresistance and giant magnetoelectric effects;(2)High quality SrAgAs single crystal was synthesized and its magnetoresistance and SdH oscillation were studied systematically.It demonstrates the existence of two-dimensional Dirac fermions in SrAgAs is proved;(3)The high quality EuAg4As2 single crystal was synthesized,and its transport and magnetic properties were systematically studied to reveal the spin flip phase transition of Eu magnetic moment at low temperature and the related novel magnetoresistance.The research results are listed as following.(1)In this article,the structure,transport and magnetic properties were studied in details for EuMn0.8Sb2 crystals with the orthorhombic structure and Mn deficiencies.It was found that the temperature dependence of the In-plane resistivity Pbc and inter-plane resistivity pa exhibits a metallic behavior in the whole measuring temperature range,different from that in the crystals EuMnSb2 without Mn deficiencies.A large positive magnetoresistance(MR)(127%at?2 K and 25%at?300 K,in 9T field)was observed,which can be ascribed to the combination of semiclassical MR and quantum limit MR of Dirac electrons.We also observed the high mobility(5050cm2/Vs and 1450cm2/Vs)of the carriers and large magnetothermopower effect at low temperatures,and two magnetic transitions emerging at?24K and?10K,respectively,corresponding to the antiferromagnetic ordering and canted arrangement of the Eu moments.Our findings shed new light on the intrinsic properties of EuMn0.8Sb2 and demonstrate the existence of Dirac fermions(2)We successfully synthesized SrAgAs single crystal by self-flux method.Its structure and chemical composition were confirmed by X-ray diffraction and EDX respectively.Then we measured its transport properties systematically.We found that its magnetoresistive at low temperature will show obvious SdH oscillation under the 9T magnetic field.The fast Fourier transform result shows that the oscillation frequency is 92.6T.Through L-K formula fitting,we get that its carrier effective mass is 0.088me and has Berry phase 0.92?.At the same time our hall measurement results show that the carrier type is hole,at T=2 K concentration of carrier is 4.6×1019cm-3 and get the mobility of up to 1067cm2/Vs.All these results support the existence of Dirac electrons in the SrAgAs carrier.Finally,we also measured the angular magnetoresistance and found that the angular magnetoresistance behavior shows that SrAgAs has a two-dimensional Fermi surface.With these measurements,we have shown that SrAgAs has a quasi-two-dimensional Dirac fermion.By analyzing the resistivity data of the external magnetic field at different angles to the current,it is found that SrAgAs has a two-dimensional Fermi surface(3)We successfully synthesized EuAg4As2 single crystal by self-flux method.Its structure and chemical composition were confirmed by X-ray diffraction and EDX respectively.On the basis of obtaining high quality single crystal samples,we measured the systematic transport and magnetic properties.The measurement of magnetic properties shows that EuAg4As2 at low field(?0H = 0.1T)experiences two magnetic phase transitions(short-range antiferromagnetic order and long-range antiferromagnetic order)at T=15K and T=9K with the decrease of temperature.And the phase transition temperature decreases with the increase of magnetic field Magnetic resistance(MR)measurement at low temperature shows that the magnetoresistance first increases and then decreases with the increase of magnetic field,and even negative magnetoresistance appears.Finally,we propose the phase diagram of EuAn4As,according to the magnetic properties and transport properties measured at different magnetic fields and temperatures.