Growth,Spin Dynamics And Heterostructure Construction Of High-Quality Wte₂ Thin Films

Since the 21st century,people have gradually entered the era of high-speed developing information technology.At present,the preparation of electronic devices with higher speed,smaller size,lower power consumption,higher fault tolerance and other excellent performance has become the key issue to research.However,with the decrease of device size,the sharp increase of various negative quantum effects and thermal effects,the development of electronic devices relying on traditional materials has experienced a bottleneck period that is difficult to break through.In order to break the deadlock,people have to try to find the new degree of freedom:spin(besides the charge)property while topological new materials have attracted global attention in recent years,hoping to prepare a new generation of spintronic devices.In this paper,we start with a typical topological new material WTe₂,which belongs to type II Weyl semimetal.It has a layered structure with a twist on the W Chain along the a-axis,which leads to the broken inversion symmetry.The special band structure and the strong spin orbit coupling(SOC)of WTe₂ bring many exotic physical phenomena:the high mobility,large unsaturated magnetoresistance and the modulated superconductivity of bulk material;the unexpected ferroelectricity in the few layers;the topological insulation and quantum spin Hall Effect in the monolayer,etc.These excellent properties of WTe₂ make it have attractive potential application on the future electric transport and spin electronics devices.Here,we start from the fabrication and the transport of WTe₂ to the ultrafast spin dynamics of it.Moreover,we try to prepare the related heterostructure based on WTe₂ and Y₃Fe₅O₁₂film,one of the hottest magnetic insulator materials recently.From preparation to transport measurement,from single structure to heterostructure,we comprehensively research WTe₂ films,perfectly corresponding to“from basic science to practical application”.We follow the steps of era and make a significant contribution to future devices based on Weyl semimtals.This paper can be summarized as the following three aspects:(1)Based on pulsed laser deposition and post annealing technology,large-area high-quality singlecrystal T_d-phase WTe₂ thin films were successfully grown.The influence of annealing time on the film quality was studied in detail by the structure and transport measurements.By annealing at 700 ^oC for 72 hours,the large-area WTe₂ film with Sd H quantum oscillation was obtained for the first time.The Berry phase of this oscillation is?,while the mobility is?1400 cm²V^-1S^-1 and the carrier concentration is?10¹⁸ cm^-3.Compared with our previous work,the mobility is doubled but the carrier concentration is reduced by an order.(2)We also use TR-MOKE technology to study the ultrafast spin-valley dynamics of our single crystal WTe₂ films.The whole relaxation process consists of two sub processes:the intraband polarization reversal process of 0.8 ps and the following defect scattering process of 5 ps.The former corresponds to the spin splitting by the strong SOC at the bottom of conduction band,while the latter comes from the Te vacancy in the film.In addition,we also find that the relaxation process exhibits dual-anisotropy,including an approximate six-fold symmetry and another two-fold symmetry.This work is the first comprehensive study of ultrafast spin-valley dynamics in single crystal WTe₂ films,which may play an important role in application of high-speed devices related to WTe₂ material.(3)Finally,we try to combine WTe₂ thin film with YIG thin film to prepare heterostructure.Compared with two other YIG-based heterostructures,we research potential effects in WTe₂/YIG heterostructure,including spin pump effect and Abnormal Hall Effect(AHE).We prepared WTe₂/YIG heterostructures by two methods,including wet transfer and directly epitaxially growing WTe₂films on the surface of YIG films.The wet transfer process has been detailedly study.However,by MBE and PLD technology,directly epitaxially growing WTe₂thin films on YIG surface is still at an original stage.