Electrical Transport Properities in Dirac Materials-Topological Insulator and Graphene Thin Films

Author:Wang Wen Jie

Supervisor:li zhi qing gao kuang hong


Degree Year:2017





In recent years,the emerging Dirac material has a dirac cone with linear dispersion relationship,giving the system massless,near the speed of light features.Among them,graphene and topological insulators have the widest energy spectrum range and are extensively studied.This excellent property also imparts novel physical phenomena and electronic properties to this material,such as semi-integer,fractional and fractal quantum Hall effect in graphene;topological quantum states in topological insulators,promoting future quantum computation and Moore’s Law Extension.Therefore,these two types of materials have a high value on both basic and applied research.However,there are still some problems to be solved in the basic research.Our work is organized around the following issues:Question 1:Is there a classical linear magnetoresistance(LMR)phenomenon in graphene?The LMR shows a phenomenon that the magnetoresistance increases linearly with the increase of the magnetic field.Although linear magnetoresistance satisfying the quantum model in graphene has been reported,the actual material is more susceptible to disorder and the higher carrier concentration leads to dissatisfying the quantum limit condition.Our work investigated the electrical transport properties of epitaxial single-layer graphene on SiC substrates and found that the system has a pronounced linear magnetoresistance in the temperature range of 2-300 K.At the same time,the carrier concentration in the system is 1013 cm-2,which is two orders of magnitude higher than the critical value required by the quantum limit condition.Therefore,it does not fit the quantum model.However,we find that linear magnetoresistance and mobility have the same trend with temperature,and the critical magnetic field also has the same trend with the inversed mobility,which is obviously in line with the mobility-dominated linear magnetoresistance effect.Moreover,when changing the mobility in the sample,we found that the linear magnetoresistance decreases linearly with the decrease of the mobility,which realizes the regulation of the linear magnetoresistance in graphene samples.This work is the first time experimently reporting the phenomenon of the classic linear magnetoresistance in graphene.Question 2:Is there a linear magnetoresistance in the topological insulator dominated by mobility disorder width ?μ?Compared with graphene,the analysis of linear magnetoresistance in topological insulators is more complicated,which includes topological surface states and nontrivial bulk state.A series of Bi2Se3 thin films with different thicknesses were prepared by magnetron sputtering.The results show that there is an obvious linear magnetoresistance in the temperature range of 2-60 K.The carrier concentration of 1015 cm-2 is four orders of magnitude higher than that of the critical carrier density,thus,excluding the quantum model.However,the linear magnetoresistance in the system does not change with the change of mobility,which may be due to the fact that the mobility of the system is too small and three orders of magnitude lower than that of graphene.In the system of μ<?μ,the linear magnetoresistance is dominated by the mobility disorder width ?μ.At the same time,the experimental results show that MR and 1kF l have a consistent trend with temperature,and by decreasing the value of 1kF l,we find that the magnetoresistance decreases linearly.Thus, ?μ dominated linear magnetoresistance can be analyzed by 1kF l in place of experimentally unobtained ?μ.In addition,the results of linear magnetoresistance measurements in parallel field and vertical field show that the topological surface states and bulk state all contribute to this phenomenon.This work is also the first time experimently reporting the mobility disorder ?μ dominated linear magnetoresistance in topological insulator.Question 3:Study on the change of transport channels in topological insulator system.Although the number of transport channels experimentally reported is obtained through the means of the weak anti-localization(WAL)effect and the analysis of electron-electron interaction(EEI).However,none of these experiments have systematically validated EEI as effective as the WAL effect.We fabricated a series of Bi2Se3 thin films with thickness from 6 nm to 108 nm.Transport measurements show that the magnetoresistance at low temperature shows a clear WAL effect.From the WAL effect analysis,we can see thataincreases from less than 0.5 to 1 as the film thickness increases from 6 nm to 108nm.It reflects that as the film thickness increases,the system changes from one topological transport channel to two decoupled independent topological transport channels.At the same time,the resistance of different thicknesses films showed an increase with the decrease of logarithmic temperature at lower temperature,revealing obvious electron-electron interaction.Through the analysis of multi-channel electron-electron interaction,we found that the transport channel n changes from 1 to 3 as the film thickness increases from 6 nm to 108 nm,revealing that the system changes from one transport channel comprising the direct coupling surface states and bulk state to two decoupled independent topological transport channels and independent bulk channel.Thus,our work confirms the equivalence of these two approaches,and the electron-electron interaction can analyze the transport channels in the same way as the analysis of weak anti-localization effect.Question 4:Anomalous transport peaks in element-doped topological insulators.Researching comprehensive experimental reports,we found that RT curves in the topological insulator film often show an abnormal transport peak.And it is particularly evident in the element-doped topological insulator,but the physical mechanism is not clear.Our experiments prepared a series of time and thickness dependent Bi1.92Cr0.08Se3thin films and found that the two series of thin films have abnormal transport peak in the RT curves in the metal-insulator transition region,and this phenomenon is also common in other materials.After a comprehensive analysis,this may be related to varied valence defects.At the same time,it is found that the time can act as the thickness to tune transport channel in topological insulator.Moreover,the magnetic measurements of the Bi1.92Cr0.08Se3 thin film reveal the helical-spin structure of its surface state.