Transport Properties Of Topological Insulators Induced By Superconducting Proximity And Magnetic Doping

Topological quantum computing is a type of quantum computing method with high fault tolerance and thus has significant application potentials.The realization of topological quantum computation requires anyons with non-abelian statistics.Majorana fermions(MFs),as one kind of anyons,are predicted by theory to exist in topological insulators with proximity-induced superconductivity.In this thesis,we fabricate a series of Josephson junctions based on Bi₂Te₃ and(Bi,Sb)₂Te₃ topological insulator(TI)thin films and measure the Nb-induced superconducting proximity effect.We control the critical current(I_c)in Josephson junctions by tuning the gate voltage(V_g)to change the carrier density and type in TIs.We found that the resistance reaches maximum at the charge neutral point(CNP),whereas the corresponding I_c is minimum at this point.When the Fermi level is tuned away from CNP by changing V_g,the resistance decreases and I_cincreases.In addition,a PN junction can be formed near the interface between the TI and Nb electrodes when the sample is in the hole-doped regime.The transport measurements in variable temperatures show that the Josephson junctions mainly carry superconducting current through ballistic channels,but at low temperature the superconducting current transported by diffusive channels may also occur.The PN junction can block diffusive superconducting current,which helps improve the composition of the 4?-periodic current that is considered as the hallmark of MFs.The presence of chiral MFs has also been predicted on quantum anomalous Hall(QAH)insulators with proximity-induced superconductivity.The QAH effect can be realized in magnetic topological insulator(MTI)thin films,however,the realization of chiral MFs in the hybrid system of a MTI thin film coupled with a superconductor requires the precise control of the transition between quantized Hall plateaus related to magnetic structure of the MTI.We carry out transport experiments on a Cr and V co-doped(Bi,Sb)₂Te₃ magnetic topological insulator thin film.We investigate the magnetic moments flipping process and its physical mechanism by studying the influence of temperature and V_g on the maximum slope of Hall resistivity near the coercive field(H_c).We find that with lowering temperature,the magnetic moments flipping rate increases first,then decreases,and then increases again.Through data fitting,we suppose that the possible physical process inludes both domain reversal and domain wall propagation at H_c of MTI.From 13 to 16 K,the main behavior near H_c is the reversal of superparamagnetic small domains,and the magnetic moments flipping rate increases with the decrease of temperature.In the vicinity of 2?13 K,the domain reversal is dominant and the domain wall propagation is secondary.As the temperature drops,the interaction between small domains becomes stronger and the domain wall propagation process increases,while the magnetic anisotropy becomes stronger and the domain reversal becomes difficult,thus the magnetic moments flipping rate decreases.From 2 K to the lowest temperature,the behavior near H_c is dominated by domain wall propagation,while domain reversal is secondary.With the decrease of temperature,the magnetic moments flipping rate increases.In addition,when V_g is tuned away from CNP,the effect of RKKY becomes stronger with the increase of carrier density,leading to the weakening of the saturated magnetization and consequently the reduction of magnetic moments flipping rate.In this paper,we achieve the manipulation of superconducting current in TI proximitized with a superconductor,and deepen the understanding of the effect of magnetism on the transport phenomenon in MTI.These results provide useful clues for finding MFs in TI-based system in the future.