Electron Transport Properties Of Magnetic Semiconductor HgCr₂Se₄

The anomalous Hall effect is one of the most fundamental electron transport properties.Thanks to the research efforts of more than a century,three mechanisms,including intrinsic,skew scattering and side jump,have been established.At the beginning of 21 century,Qian Niu et al.revealed that the intrinsic mechanism of the anomalous Hall effect is related to the Berry curvature of the band structure of the material.The anomalous Hall effect observed previously in various materials is explained with the transport theory under the single particle picture.It remains unclear whether many-body interactions between electrons can have observable influence on the anomalous Hall conductivity.Magnetic semiconductor is a kind of material in which charge and spin of electron can be manipulated simultaneously.HgCr₂Se₄ is a type of magnetic semiconductor with spinel structure,which can maintain metallic even when the electron density is as low as 10¹⁵ cm^-3,providing a unique platform to explore the anomalous Hall effect and many-body interactions.It was also predicted in 2011that HgCr₂Se₄ is a magnetic Weyl semimetal.However,this prediction has not been confirmed to date.In this thesis,we use high quality n-type HgCr₂Se₄ single crystal samples to study the transport properties of HgCr₂Se₄ in detail,including magnetoresistance?MR?and anomalous Hall?AH?effect.The main results are summarized as follows:1.“W”shaped MR of n-HgCr₂Se₄.The“W”shaped MR refers to the magnetoresistance with negative slopes at low magnetic fields and positive slopes at high magnetic fields.It has been observed in n-type samples with carrier densities spanning over three orders of magnitude(10¹⁵-10¹⁸ cm^-3).The negative MR appears far beyond the unsaturated region of magnetization,and extends to a wider magnetic field range as the density decreases.At temperatures below 2 K,the shape of negative MR remains nearly unchanged,while at T>2 K,both the amplitude and magnetic field range of negative MR become larger with increasing temperature.In addition,the negative MR shows a very weak anisotropy in titled magnetic fields.Based on these characteristics one can rule out the contributions from usual mechanisms such as spin scattering related to magnetic domains,weak localization effect,and chiral anomaly of Weyl fermions.We propose a spin scattering mechanism based on local spin texture to explain the negative MR observed in HgCr₂Se₄.The weak anisotropy of the negative MR,AH conductivities far below theoretical values,and the observed quantum correction to the longitudinal conductivity,jointly suggest that HgCr₂Se₄ is unlikely to be Weyl semimetal.Our experiment suggests the ferromagnetic ground state of HgCr₂Se₄ is a half-metallic ferromagnetic semiconductor instead.2. The quantum corrections to AH effect due to many-body interactions between electrons have been studied for about 30 years.It is predicted in theory that electron-electron interaction?EEI?will not cause any corrections to AH conductivity.We observed a square-root?T?-type temperature dependence of the longitudinal resistivity?conductivity?,ordinary Hall resistivity and AH resistivity?conductivity?in moderately disordered n-HgCr₂Se₄ samples with temperature varied over two orders of magnitude?0.02-2 K?.The relative change in ordinary Hall resistivity is two times than that in longitudinal resistivity.The corrections to longitudinal and ordinary Hall components can be described quantitatively by the quantum correction theory of EEI developed by Altshuler et al..The relative change in AH resistivity?conductivity?is nearly two orders of magnitude larger than that in longitudinal resistivity?conductivity?,and becomes more obviously with increasing disorder strength.The quantum correction amplitude of the AH effect in HgCr₂Se₄ is much larger than that observed in conventional ferromagnetic metals or semiconductors.Such giant correction effect is inconsistent with the existing theory.Moreover,an unconventional scaling relation between AH conductivity and momentum relaxation time is observed with a set of samples with carrier densities spanning more than one order of magnitude.Our experiment shows that the giant quantum corrections to the AH effect observed in n-HgCr₂Se₄ at low temperatures cannot be attributed to the weak localization effect,and the contributions of EEI are worthy of further theoretical treatments.