Topological states in atomic structures probed by scanning tunneling microscopy
Final Report Abstract
This research fellowship at the California Insitute of Technology in the group of Stevan Nadj-Perge was to investigate in local electronic states of 2-dimensional materials via scanning tunneling microscopy and scanning tunneling spectroscopy. The proposed idea was to create heterostructures out of 2-dimensional materials, like NbSe2 and WTe2. Induced superconductivity, gating effects and topological states were supposed to be realized with these heterostructures. Indeed, we did observed induced superconductivity but only for NbSe2/graphene heterostructures. Also, it was not possible for me to achieve monolayer WTe2. At the same time we became aware of the magic angle data from twisted bilayer graphene which includes superconductivity, the topology from graphene and insulated correlated states. We decided to investigate in twisted bilayer graphene at the magic angle. We successfully created twisted bilayer graphene close to the magic angle and at the magic angle. Our samples differ from the ones that are used for transport measurements because our samples aren’t encapsulated. We have a graphite back gate and we have BN as an insulator between the back gate and the twisted bilayer. In the topography we see a superlattice called moiré lattice. We find that the area with the most density of states corresponds to the AA sites that is when the carbon atoms are directly on top of each other. Spectroscopically we see the van Hove singularities which emerge from the flat bands that are created in twisted bilayer graphene due to interlayer hybridization. The van Hove singularities shift with the twist angle of the twisted bilayer graphene. The distance between the peaks of the van Hove singularities is getting less with reduced twist angle. With the back gate we shifted the fermi level and investigated in the emerging effects. One of the key observation that we saw with spatially resolved maps of the density of states is that near charge neutrality the flat bands split further apart than away from charge neutrality. The splitting is explained within the tenband model and an introduced symmetry preserved short range interaction strength EC. Our collaborators solved the model self-consistently within the mean-field theory and assumed a preserved symmetry for the four flavors of spin and valley. The broken C3 symmetry reproduces the observed splitting near charge neutrality. Overall the model reproduces the decreased peak intensities around the charge neutrality point. Additionally, the model predicts that the C3 symmetry breaking is most pronounced on bridges between AA sites. Our spatially resolved maps of the density of states confirm this prediction. From the experimental analysis I found that strain isn’t the main effect for this sample as the effect is very low compared to the interaction strength. It is more likely that a nematic ground state exists for the twisted bilayer near the charge neutrality point. Another hint towards this ground state is the suppression of the density of states for certain areas at 0 bias voltage. From an analysis of the electron charge on one moiré site and the back gate range in which the flat bands cross the fermi energy we estimate the filling factors. We find that the suppression of the density of states is related to half filling and sometimes even quarter and three-quarter filling. The model of a coulomb gap fails to explain all the features that we observe. The next step would be to measure devices for the STM also by using transport measurements. The STM at Caltech in the Nadj-Perge group is designed for these types of measurements and it should be the next goal.
Publications
- Imaging Electron Correlations in Twisted Bilayer Graphene near the Magic Angle; Nature Physics
Y. Choi, J. Kemmer, Y. Peng, A. Thomson, H. Arora, R. Polski, Y. Zhang, H. Ren, J. Alicea, G. Rafael, F. von Oppen, K. Watanabe, T. Taniguchi, S. Nadj-Perge
(See online at https://doi.org/10.1038/s41567-019-0606-5) - Indirect Chiral Magnetic Exchange through Dzyaloshinskii-Moriya - Enhanced RKKY Interactions in Manganese Oxide Chains on Ir(100); Nature Communications 10, 2610 (2019)
M. Schmitt, P. Moras, G. Bihlmayer, R. Cotsakis, M. Vogt, J. Kemmer, A. Belabbes, P. Sheverdyaeva, A. K. Kundu, C. Carbone, S. Blügel, M. Bode
(See online at https://doi.org/10.1038/s41467-019-10515-3)
