Related publications

(1) B. Jäck et al., Science 364, 1255-1259 (2019)
(2) B. Jäck, et al., PNAS 117, 16214-16218 (2020)
(3) B. Jäck et al., Nature Reviews Physics 3, 541-554 (2021)
(4) J. Zheng, …, B. Jäck, arXiv: arXiv:2503.19032 (2025)
(5) E.J. König, …, and B. Jäck, PRL 125, 267206 (2020)

The geometric phase of the electronic wave function engenders topologically nontrivial electronic states that can give rise to novel phenomena, such as quantized Hall, anomalous Hall, and spin Hall effects. When Coulomb and magnetic exchange interactions between topological electronic states are present, novel ground states that are characterized by topological order parameters with fractional charge and spin excitations and exotic quantum statistics can arise. Scanning tunneling microscopy (STM) with its ability to visualize electronic wave functions at atomic length scales is an extremely powerful tool to examine the quasiparticle excitations of topological quantum materials. High-resolution spectroscopy at the atomic scale further provides unique insights into the nature of the topological electronic states. In the past we used this method to demonstrate the existence of a Majorana zero mode in the topological edge state of bismuth (1) and to visualize backscattering of helical quasiparticles induced by magnetic atoms (2). We also discussed the unique abilities of STM to distinguish topological Majorana zero modes from trivial quasiparticle states (3). More recently, we visualized the topological quasiparticle excitation of a loop current order parameter using single atom magnetic sensors (4). On the theoretical side, we proposed inelastic tunneling spectroscopy with the STM as a powerful tool to detect the charge neutral fractional spin excitations of a quantum spin liquid state (5).

Related publications

(1) Y. Xie, B. Lian, B. Jäck et al., Nature 572, 101–105 (2019)
(2) Y Jia,…, B. Jäck, et al., Nature Physics 18, 87-93 (2022)
(3) B. Jäck, et al., Phys. Rev. Res. 3, 013022 (2021)
(4) C. Chen, …, B. Jäck, Phys. Rev. Res. 5, 043269 (2023)
(5) C. Chen, …, B. Jäck, arXiv: arXiv:2409.06933 (2024)

Coulomb interactions between charge carriers can profoundly influence material properties and give rise to novel quantum states, such as superconductivity, magnetism, correlated insulating states, nematic order, and quantum critical phenomena. When Coulomb interactions dominate over the charge carriers’ kinetic energy, particularly robust new quantum states, so called strongly correlated states, can emerge. The ability to examine the low-energy electronic structure and visualize the underlying wave functions at atomic length scales enables STM to generate unique insights into the nature of strongly correlated quantum materials. In the past, we have used this method to present spectroscopic evidence for strong correlations in the flat bands of magic angle twisted bilayer graphene (1) and for an excitonic insulator state in monolayer WTe2 (2). We also visualized the quantum critical wave functions with multi fractal scaling characteristics near an Anderson quantum phase transition (3). Recently, we have used STM to visualize the localized electronic states of a kagome flat band and found direct evidence for the non-trivial localization mechanism leading to so-called compact localized states (4). Tuning the kagome flat band occupation of CoSn with holes through chemical doping with Fe, we also demonstrated the emergence of nematic order and an orbital-selective Mott insulating state at partial flat band occupation (5).