Final Report Summary - MAGNETOP (Probing the effect of Time Reversal Symmetry breaking by the application of a local magnetic field in topological insulators.)
This project was aimed to push beyond the exisiting understanding of scattering phenomena (intrinsic and caused by a local magnetic field) in 2 and 3-dimensional topological insulators (2D-TI and 3D-TI respectively). This knowledge would then be used to explore the effect of confinement in these materials, predicted to generate a variety of new exotic electronic phenomena.
In the case of 2D-TI, the Quantum Spin Hall edge states of HgTe quantum wells present the expected dissipation-less transmission. However, this happens only for distances in the order of the micron, and backscattering has been reported to occur over longer distances. The mechanisms responsible for this scattering remain still undetermined. In this context, the objectives of this project included: to identify and understand such scattering mechanisms in absence of magnetism, to study the effect of a small magnetic field and make a distinction between scattering induced by potential fluctuations and the one created by the presence of a local magnetic field. Then, we proposed the possibility of using magnetism to open a gap in the helical edge states and in this way, confine Dirac fermions to short stretches of the edge with the aim of creating a quantum dot.
For the first year of the project, we successfully overcome the limitations for the study of transport and scattering on the QSH states in HgTe samples by Scanning Probe techniques. By collaborating with different growers, nominally the group of Prof. Molenkamp in Wuerzburg (Germany) and the semiconductor company Teledyne, we improved the quality and quantity of available material and its suitability for the proposed experiments. The sample fabrication process and the existing experimental Scanning Probe setup at Stanford were also revised and upgraded.
During her stay at Prof. Goldhaber-Gordon’s laboratory, the research fellow performed Scanning Gate Microscopy. The new sample growth and fabrication methods allow to fully tuning the material from the conductive bulk to the gap, where the edge states dominate the transport without requiring a top gate. These results will help to understand not only the scattering in the QSH states, but also how these states interact with carriers in the bulk as this becomes conductive.
During the return phase, the fellow has successfully transferred the knowledge acquired during the outgoing period and implemented the necessary upgrades to perform Scanning Gate measurements in the existing commercial Attocube setup in CIC Nanogune and developed high-aspect ration Tuning-Fork probes. Further experiments in HgTe quantum wells are being performed at Prof. Pascual´s laboratory in CIC Nanogune, San Sebastian. From our results we expect to characterize the effect of a small magnetic field in the scattering at the samples edge and related to the predicted lift of topological protection.
In parallel, we explored the effect of confining carriers to a Fabry-Perot like cavity in HgTe quantum wells. Bipolar junctions were created combining the effect of the backgate and implementing narrow top gates. The resulting quantum interference can be detected in the transport measurements. These results should provide a better insight in the role of confinement in the edge states and in the interaction of those states with bulk carriers in the absence and presence of a small magnetic field. Furthermore, in presence of high fields, the interplay between Quantum Hall and Quantum Spin Hall can be studied. The results of this work performed at Stanford University, have been analyzed and prepared for publication over the return phase.
A number of collaborations were established with surrounding groups at Stanford working on different SPM techniques. The fellow has produced samples suitable to be studied by Microwave Impedance Microscopy at the laboratory of Prof. Z.X. Shen. The results obtained by this technique provide a new insight in some properties of the edge states. Unexpected edge conduction up to high magnetic field was observed that challenges the existing predictions for this material. These results have been recently published in Nature Communications. Specific samples were also produced for Prof. Moler’s lab for further current imaging and for persistent current studies, which are currently work in progress.
Finally, the proposed research in 3DTI topological insulators for the return phase, has been substituted by the study of local properties in hybrid materials susceptible to present the so-called topological superconductivity due to the change of return host institutions and the evolution of the research field.
In summary, in spite of small deviations from the initial plan due to the need for high quality material and the remaining open questions in the field, the project has produced relevant results that should shred light into the properties of the Quantum Spin Hall edge states in HgTe quantum wells. In addition, the fellow has been involved in a number of promising collaboration with surrounding groups at the Stanford Physics and Material Science departments. The knowledge acquired regarding novel SPM techniques during the outgoing phase, has been successfully transferred to the return host, were the Scanning Gate technique has been implemented. Furthermore, collaboration between the host institutions has been established. Finally, new projects regarding the study of topological properties of matter has been initiated at the return host institution, where the fellow will continue her career for the next years.
In the case of 2D-TI, the Quantum Spin Hall edge states of HgTe quantum wells present the expected dissipation-less transmission. However, this happens only for distances in the order of the micron, and backscattering has been reported to occur over longer distances. The mechanisms responsible for this scattering remain still undetermined. In this context, the objectives of this project included: to identify and understand such scattering mechanisms in absence of magnetism, to study the effect of a small magnetic field and make a distinction between scattering induced by potential fluctuations and the one created by the presence of a local magnetic field. Then, we proposed the possibility of using magnetism to open a gap in the helical edge states and in this way, confine Dirac fermions to short stretches of the edge with the aim of creating a quantum dot.
For the first year of the project, we successfully overcome the limitations for the study of transport and scattering on the QSH states in HgTe samples by Scanning Probe techniques. By collaborating with different growers, nominally the group of Prof. Molenkamp in Wuerzburg (Germany) and the semiconductor company Teledyne, we improved the quality and quantity of available material and its suitability for the proposed experiments. The sample fabrication process and the existing experimental Scanning Probe setup at Stanford were also revised and upgraded.
During her stay at Prof. Goldhaber-Gordon’s laboratory, the research fellow performed Scanning Gate Microscopy. The new sample growth and fabrication methods allow to fully tuning the material from the conductive bulk to the gap, where the edge states dominate the transport without requiring a top gate. These results will help to understand not only the scattering in the QSH states, but also how these states interact with carriers in the bulk as this becomes conductive.
During the return phase, the fellow has successfully transferred the knowledge acquired during the outgoing period and implemented the necessary upgrades to perform Scanning Gate measurements in the existing commercial Attocube setup in CIC Nanogune and developed high-aspect ration Tuning-Fork probes. Further experiments in HgTe quantum wells are being performed at Prof. Pascual´s laboratory in CIC Nanogune, San Sebastian. From our results we expect to characterize the effect of a small magnetic field in the scattering at the samples edge and related to the predicted lift of topological protection.
In parallel, we explored the effect of confining carriers to a Fabry-Perot like cavity in HgTe quantum wells. Bipolar junctions were created combining the effect of the backgate and implementing narrow top gates. The resulting quantum interference can be detected in the transport measurements. These results should provide a better insight in the role of confinement in the edge states and in the interaction of those states with bulk carriers in the absence and presence of a small magnetic field. Furthermore, in presence of high fields, the interplay between Quantum Hall and Quantum Spin Hall can be studied. The results of this work performed at Stanford University, have been analyzed and prepared for publication over the return phase.
A number of collaborations were established with surrounding groups at Stanford working on different SPM techniques. The fellow has produced samples suitable to be studied by Microwave Impedance Microscopy at the laboratory of Prof. Z.X. Shen. The results obtained by this technique provide a new insight in some properties of the edge states. Unexpected edge conduction up to high magnetic field was observed that challenges the existing predictions for this material. These results have been recently published in Nature Communications. Specific samples were also produced for Prof. Moler’s lab for further current imaging and for persistent current studies, which are currently work in progress.
Finally, the proposed research in 3DTI topological insulators for the return phase, has been substituted by the study of local properties in hybrid materials susceptible to present the so-called topological superconductivity due to the change of return host institutions and the evolution of the research field.
In summary, in spite of small deviations from the initial plan due to the need for high quality material and the remaining open questions in the field, the project has produced relevant results that should shred light into the properties of the Quantum Spin Hall edge states in HgTe quantum wells. In addition, the fellow has been involved in a number of promising collaboration with surrounding groups at the Stanford Physics and Material Science departments. The knowledge acquired regarding novel SPM techniques during the outgoing phase, has been successfully transferred to the return host, were the Scanning Gate technique has been implemented. Furthermore, collaboration between the host institutions has been established. Finally, new projects regarding the study of topological properties of matter has been initiated at the return host institution, where the fellow will continue her career for the next years.