Transportation And Superconducting Proximity Effect In Topological Insulator Bi₂Se₃nanowires

Topology is the study of a system’s invariants began to rise in mathematics,and was generally introduced into physics to classify different systems without spontaneous symmetry breaking.At the same time,people began to realizes the importance of electronic wave functions instead of considering only the energy spectrum of a system.Bloch bundles over the Brillouin zone,where fibres are the space of states with the same momentum k,together with Halmiltonians acting on fibres can give out the topology of a system.Insulators with time reversal symmetry can be classified by Z2 invariant,with Z2= 0 being conventional insulator,and Z2= 1 being topological insulator.In Z2= 1 topological insulator,who has inverse band structure,once the Krammers’ pairs are protected by time reversal symmetry,there will always be a surface state.We used VLS method successfully grew 3D topological insulator Bi₂Se₃nanowires（BSNWs）on Si/SiO2 substrates with 20 nm gold nano-particles acted as catalyst.Then we used standard electron beam lithography and electron beam evaporation to fabricate electrodes with Ohmic contact or tunneling contact on Bi₂Se₃ nanowires.The resistance of BSNWs ranges from 1 k? to 20 k?,much greater than the resistance in Bi₂Se₃nano-plates（< 1 k? or even only a few tens of ?）,indicating that the contribution from bulk state can be suppressed.Thus the surface state can be dominant in transportation experiments.The contact resistance in Ohmic contact electrodes is always much smaller than the resistance in BSNWs,guaranteeing that what we measure comes mainly from the nanowires instead of the interface,and the formation of proximity effect.On the other side,we observed Coulomb blockades in tunneling junctions,which is necessary in measuring density of states and in spin injection/detection.When a Au-BSNW-Au device was cold down to 10 mK,we observed hysteretic loops in magnetoresistance.These hysteretic loops generally vanished with the increasing of temperature or applied bias current,indicating that the spins in BSNWs tend to be aligned to the same direction,forming magnetic ordered states.In a changing magnetic field,these magnetic ordered states were exited to a metastable state,and if the magnetic field was held,they would decade back to the stable state.We conclude that spin domains can form due to spin-orbit coupling and quantization due to size effect of BSNWs.But we failed to find out what breaks time reversal symmetry,and we need more experiments to investigate the magnetic ordered states in BSNWs.We introduced superconductivity into BSNWs by replacing Au leads to Al leads and cold the Al-BSNW-Al devices to low temperature（10 mK,for example）.When a small magnetic field was applied,the critical current of the Al-BSNW-Al junction increased with the increasing of field.At the same time,the residual resistance in the junction due to thermal fluctuations generally decreased below the measurement limit.Both of these two phenomena indicates that proximity effect can be enhanced by applied magnetic field in Al-BSNW-Al junctions,showing an unconventional behavior.When the applied magnetic field exceeded the field in which the resistance in Al leads began to rise,the enhancement stopped and the induced superconductivity generally suppressed to zero.If we lift the critical fields of the superconductor leads,we believe that the enhancement will keep up to higher magnetic fields.Then we can find more interesting phenomena,and it will be possible for us to find the nature of enhancement.It is possible to observe magnetic field enhanced proximity effect and hysteretic loop in the same device in proper temperatures.Then we try to explain what we found by referring works in superconductor-ferromagnet junctions and proximity effect in materials with strong spin-orbit coupling.In these systems,the induced superconductivity is a mixture of singlet component and triplet component.Singlet component is suppressed by magnetic field,while the triplet increases,since magnetic field increases the spin-polarization degree,helping the formation of triplet component.How ever,we still need to confirm the cooper pair symmetry by using ferromagnetic electrodes and so on.We also investigated the ac Josephson effect in Al-BSNW-Al junctions.In zero magnetic field,all Shapiro steps can be observed,expect that the first zero node of the 0th Shapiro step split into two.In magnetic field,due to the enhancement of proximity effect,the steps were more and more clear.But the positions of Shapiro steps were influenced by Josephson current.They generally moved towards the zero bias current region.High steps moved more,resulting overlapping of Shapiros steps.Such overlapping made two steps became one,showing a similar behavior to so-called fractional Joseohson effect due to the contribution of Majorana bound states.If more experiments results support the existence of Majorana bound states,we can use this system to realize topological computation.Otherwise double Shapiro steps can come from trivial mechanisms.Finally,we tried to bring the measurement frequency from DC to MHz.On one hand,Shot noise can provide the scattering information in devices.On the other hand,we need to realize time domain measurement and control in the high frequency region.Then we altered the measurement cables,increased the signal frequency,added low temperature amplifiers and finally realized Shot noise measurement with a low noise background.On BSNW,we observed a low noise state in low bias current,low temperature region,implying that electrons are protected from scattering due to topological protected surface states.In the main text,I firstly introduce backgrounds of this field,and then introduce main experimental techniques we use.Then I will show the experimental result in Au-BSNW-Au and Al-BSNW-Al devices.Finally,I will introduce the work we done in Shot noise measurement.