Majorana Zero Modes In Josephson Devices Based On Topological Insulators

In this thesis,I will first introduce the basic theoretical concepts and then the major experimental progresses on MZM(Majorana zero mode)researches.MZM is a quasiparticle state in condensed matter physics.It is its own antiparticle state,obeying non-abelian statistics.The quantum systems composed of MZMs are believed to be insensitive to local disturbance,thus plausible to be used to construct fault-tolerant topological quantum computer,in solving the decoherence problem that other quantum computation schemes are facing.Because of these intriguing properties and their significant potential for applications,the research on MZM has become one of the hottest topics in condensed matter physicsMZM locates at the defects of TSCs(topological superconductors).The basic physical phenomena related to MZMs include ZBCP(zero bias conductance peak)and 4?-period current-phase relation.By now,ZBCP including its spin-selective Andreev reflections and the quantization of peak height have been observed in a number of hybrid systems,and fractional Josephson effect as signature of 4?-period current-phase relation has also been observed in TI(topological insulator)-based Josephson junctions These phenomena are regarded as important signature for MZM.However,whether the systems presenting these phenomena obey non-abelian statistics or not remains to be further exploredThe helical electron states on the surface of 3D(three dimensional)TIs can be induced to form topological superconductivity via proximity effect in s-wave superconductor-TI hybrid structures.According to an important theory proposed by Fu and Kane in 2008,when a Josephson single junction on the surface of TIs reaches ?phase difference,the minigap in the junction closes and the MZM exists.Furthermore,over extended regions in 2D phase space,the minigap at the center of a Josephson trijunction closes and the MZM appears.Such Josephson trijunctions could serve as the building blocks for 2D scalable topological quantum circuits,allowing the verification of non-Abelian statistics and the further exploration of universal quantum computationIn this thesis work,we experimentally explored the Fu-Kane theory proposed in 2008,with special attention paid to find the evidence of MZM from the perspective of phase-sensitive detections.We constructed Josephson trijunctions via superconducting proximity effect on the surface of 3D TIs,and fabricated local superconducting coils for controlling the phase differences of the single Josephson junctions,through which we were able to map out the states of the trijunctions over the entire 2D phase space We also developed a method to extract the information of minigap in the junctions from contact resistance measurement,with the use of the BTK(Blonder-Tinkham-Klapwijk)theory.Complete minigap closing was observed in single Josephson junctions when the phase difference reached ?,and minigap closing was observed at the center of the trijunctions over extended regions in 2D phase space.These phenomena are consistent with the Fu-Kane theory proposed more than ten years ago.These work would shed some light on using Josephson trijunctions as the basic building blocks for constructing topological quantum computersOn the other hand,we also generalized the contact-resistance-measurement method for extracting the minigap to the circumstance with RF(radio frequency)irradiation We found that with the increase of RF power,the measured minigap becomes broadened and extends to higher energies,in a way similar to the RF power dependence of the outer border of the Shapiro step region.We showed that the corresponding data of contact resistance under RF irradiation can be well interpreted by using the RS J model(resistively shunted Josephson junction model)and the BTK theory.Our findings could be useful when using the contact-resistance-measurement method to study Majorana-related physics in TI-based Josephson junctions under RF irradiation,especially when fractional Josephson effect occurs.