The Research On The Extremely Large Magneto-Resistance (XMR) In Non-magnetic Semimetals

Since the British physicist T.Thomson discovered the magnetoresistance effect,materials with magnetoresistance have been widely used in magnetic storage,spin valves,various magnet-ic sensors,GPS navigation and many other fields;The discovery of the giant magnetoresistance effect in magnetic multilayer films has led to a qualitative leap in magnetic storage technology;so searching new materials with giant magnetoresistance is an important direction in condensedmatter physics and materials science.In the past decade,based on the theoretical study of the topological properties of energy bands,many new topological quantum states have been discovered.Different from traditional classification,materials can be classified according to different topological properties,revealing the origin of material physical properties.Topological materials can be divided into topological insulators,topological semimetals,and topological superconductors.The main features of thetopological material energy band are:in the momentum space,the valence band and the conduc-tion band cross,energy and momentum act linearly,the intersection point is called Dirac point,and the so-called Dirac cone will be formed in the three-dimensional momentum space.Because the topological material has such a peculiar energy band structure,it exhibits many strange transport properties,such as:extremely magnetoresistance,quantum spin Hall effect,anomalous Hall effect,negative magnetoresistance,weak anti-localization effect,high mobility of carriers,topologically non-trivial Berry phase,etc.However,in the last two years,people have discovered a series of sim-ilar singular transport properties in some topologically trivial semimetals,such as:not only have a great magnetoresistance effect,and the magnetoresistance shows a parabolic relationship with the magnetic field,the relationship between resistivity and temperature at low temperatures presents a platform,compensation effects of electrons and holes present at low temperatures,mobility of carrier is ultrahigh,etc.The discovery of these materials provides a new choice for people looking for new types of giant magnetoresistive materials,and provides a new idea for the study of the microscopic mechanism of their magnetoresistance.This dissertation is divided into five chapters.Firstly,it summarizes the topological material-s,magnetoresistance effect and the singular transport properties found in typical transition metal phosphorus compounds.The single-crystal growth technology,physical property measurement and energy band theory calculation method introduced in this paper are introduced systematically.For three types of semimetals with different electronic structures:Weyl semimetal TaP,topolog-ically trivial semimetal ?-WP₂,and CaAl₂Si₂ with corrugated double hexagonal lattice structure,based on the successful acquisition of high-quality single-crystal samples,a series of innovative re-search results have been obtained through systematic research of its crystal structure,energy band structure,and transport properties under magnetic fields?these results were reported for the first time in the world?.The summary is as follows:?1?TaP has a non-centrosymmetry tetragonal structure and was predicted to be Weyl semimet-al.After successfully growing single-crystal of TaP in the world for the first time,we measured its magnetic transport properties.It was found that when the magnetic field is zero,the tempera-ture dependence of resistivity displayed a metallic behavior.After the magnetic field is applied,at low temperatures,the resistivity will appear upwards and finally reach saturation.It is usually considered to be due to field-induced metal-insulator phase transitions.At low temperatures,the sample has a extremely magnetoresistance and shows no signs of saturation within the maximum magnetic field used for our measurements.There is a strong SdH oscillation at low temperatures.It can be concluded that the sample has a non-trivial Berry phase.When the direction of the current and the direction of the magnetic field are parallel,a negative magnetoresistance due to a chirality anomaly will occur,and no saturation occurs at 9 T.The Hall effect confirms that our sample has two kinds of carriers.The concentration of electrons and hole carriers at the low temperature is nearly equal,that is,the compensation effect occurs.Through the fitting of the data we found that carrier mobility is very high.From the above results,it can be determined that the TaP sample is Weyl semimetal.?2??-phase WP₂ has a centrosymmetry monoclinic structure,the space group is C2/m.After successfully growing single-crystal of ?-WP₂,we performed magnetic transport properties mea-surement and calculation.The band calculations show that it is a topologically trivial semimetal,which contrasts to the recently discovered robust type-II Weyl semimetal phase in ?-WP₂.The magnetic transport properties measurement show that ?-WP₂ exhibits almost all the characteristics of XMR materials:the near-quadratic field dependence of MR,a field-induced up-turn in resistivi?ty following by a plateau at low temperature,which can be understood by the compensation effect,and high mobility of carriers confirmed by our Hall effect measurements.It was also found that the normalized MRs under different magnetic fields has the same temperature dependence in a-WP₂,the Kohler scaling law can describe the MR data in a wide temperature range,and there is no ob-vious change in the anisotropic parameter ? value with temperature.The resistance polar diagram has a peanut shape when field is rotated in ac plane,which can be understood by the anisotropy of Fermi surface.These results indicate that both field-induced-gap proposed for magnetoresistive behavior in Bismuth crystals and graphite,and temperature-induced Lifshitz transition are not the origin of up-turn in resistivity in the ?-WP₂ semimetal.Our fndings establish ?-WP₂ as a new reference material for exploring the XMR phenomena.?3?CaAl₂Si₂ has a La₂O₃-type crystal structure,with a space group of P3m1.The[AlSi]^2- ion layer is a double-layer corrugated structure similar to the double-layered graphene after defor mation.They form a two-dimensional crystal structure through Ca²⁺ ions.Based on the successful growth of CaAl₂Si₂ high-quality single crystals,we systematically measured the resistance in the magnetic field,Hall resistivity,and anisotropy magnetoresistance.The results show that at zero magnetic field,the resistance-temperature relationship exhibits metal behavior;the applied mag-netic field leads to a sharp increase in resistance and saturation at low temperatures,showing sig-nificant magnetoresistance effects.When the magnetic field and the current are parallel,the sample exhibits a significant temperature-dependent negative magnetoresistance effect,and exhibits weak anti-localization behavior in the low field.Hall measurements confirm that there are two types of carriers,electron and hole,in CaAl₂Si₂,and that holes dominate at high temperatures.At low temperatures,their carrier concentration is low and not equal,and does not exhibit the so-called compensation effect,and the mobility of carriers is high.In addition,a non-zero Berry phase is ob-tained by SdH effect at a low temperature.Combined with the DFT calculation results,we believe that CaAl₂Si₂ is a new type of Weyl semimetal.In addition,in the last chapter of this dissertation some directions for further research are put forward.