Final Report Summary - TOPOTRONICS (Topological Josephson devices as a novel platform for creating and controlling non-Abelian anyons)
A three-dimensional topological insulator is insulating in the bulk and has topological surface states. At these surface states the electron spin is locked to the momentum in a helical manner. The same spin-momentum locking exists in the bulk of a topological Dirac semimetal. Once superconductivity is induced into these states, quasiparticles can be created with special properties. It has been the aim of this project to find and manipulate Majorana bound states in topological Josephson junctions.
In order to do so one first needs good materials. In the field of topological insulators it has turned out to be a challenge to have properly insulating bulk properties due to defects. Within the TOPOtronics project we have tackled this problem by thin film deposition methods. By applying a surplus of Te during growth we were able to reduce the number of Te vacancies in Bi2Te3. In-situ characterization showed the absence of bulk conduction as long as the material is not exposed to ambient conditions. For further integration into devices, a capping layer technology was developed.
Topological Josephson contacts have been made, both with the mentioned thin film material, as well as by using exfoliated flakes of single crystals. Five important findings have come out of the research:
1. Gate-tunable Josephson effects have been realized. This allows to study the critical current as function of the normal state gate tunable properties such as the carrier density of bulk and surface states. Correlating these effects has helped to disentangle the contributions of supercurrent through trivial bulk states as well as contributions of the topological surface states.
2. Shot noise was observed at superconducting contacts to the topological insulator materials with a Fano factor reduction to 0.22 with respect to the 1/3 that is normally measured for diffusive normal metal contacts. This observation was published in Phys. Rev. Lett., but it remains to be explained theoretically.
3. Indications for Majorana bound states have been observed when using the topological interlayer material Bi1-xSbx. In order to have as few Andreev bound states as possible in the junction the wavelength should be as large as possible, which is realized Bi1-xSbx sensitively as function of doping. For x=3% we observe about 10% contribution of Majorana modes to the supercurrent in the device. This work is published in Nature Materials. As an interesting side effect we showed phase sensitive proof of zero-to-pi transitions in these junctions, due a an externally applied magnetic field.
Also in other systems, Majorana modes have been observed. First of all in another Dirac semimetal, namely Cd3As2 (published in Phys. Rev. Lett.), and secondly in topological insulator based Josephson junctions.
4. The Majorana bound states in all of these systems can only be observed at high frequency. At longer time scales, quasiparticle poisoning destroys the 4pi periodic current-phase relation. This finding puts an upper limit to the time scales at which future braiding experiments can take place.
In order to do so one first needs good materials. In the field of topological insulators it has turned out to be a challenge to have properly insulating bulk properties due to defects. Within the TOPOtronics project we have tackled this problem by thin film deposition methods. By applying a surplus of Te during growth we were able to reduce the number of Te vacancies in Bi2Te3. In-situ characterization showed the absence of bulk conduction as long as the material is not exposed to ambient conditions. For further integration into devices, a capping layer technology was developed.
Topological Josephson contacts have been made, both with the mentioned thin film material, as well as by using exfoliated flakes of single crystals. Five important findings have come out of the research:
1. Gate-tunable Josephson effects have been realized. This allows to study the critical current as function of the normal state gate tunable properties such as the carrier density of bulk and surface states. Correlating these effects has helped to disentangle the contributions of supercurrent through trivial bulk states as well as contributions of the topological surface states.
2. Shot noise was observed at superconducting contacts to the topological insulator materials with a Fano factor reduction to 0.22 with respect to the 1/3 that is normally measured for diffusive normal metal contacts. This observation was published in Phys. Rev. Lett., but it remains to be explained theoretically.
3. Indications for Majorana bound states have been observed when using the topological interlayer material Bi1-xSbx. In order to have as few Andreev bound states as possible in the junction the wavelength should be as large as possible, which is realized Bi1-xSbx sensitively as function of doping. For x=3% we observe about 10% contribution of Majorana modes to the supercurrent in the device. This work is published in Nature Materials. As an interesting side effect we showed phase sensitive proof of zero-to-pi transitions in these junctions, due a an externally applied magnetic field.
Also in other systems, Majorana modes have been observed. First of all in another Dirac semimetal, namely Cd3As2 (published in Phys. Rev. Lett.), and secondly in topological insulator based Josephson junctions.
4. The Majorana bound states in all of these systems can only be observed at high frequency. At longer time scales, quasiparticle poisoning destroys the 4pi periodic current-phase relation. This finding puts an upper limit to the time scales at which future braiding experiments can take place.