Daniel Walkup

We examine graphene quantum dots in back-gated devices on hexagonal boron nitride (hBN) and visualize their merger using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. These dots are formed by the combination of nanoscale potential wells created by pulsing the voltage of an STM tip above charged defects in the hBN underlayer, and strong magnetic fields gapping the density of states, which add insulating rings in the potential wells. Control of the charge state is achieved via the back gate and sample bias voltages, and the position of the STM tip which serves as a mobile top gate. The individual quantum dots present a distinct phenomenology of single-electron charging due to multiple Landau levels crossing the Fermi energy concentrically. Here, we study side by side pairs of these quantum dots via STM, where we observe a tunable interdot coupling and mergeability. Specifically, with increasing charge filling, the quantum dots formed by electrons belonging to one Landau level merge into a single quantum dot, while the electrons in the next-higher Landau level remain spatially separated into two charge pockets. Using the probe tip as a multifunction tool, we visualize the evolution, growth, and merger of this unique double quantum dot system as a function of tip position and gate voltages.

For almost a century, magnetic oscillations have been a powerful "quantum ruler" for measuring Fermi surface topology. In this study, we used Landau-level spectroscopy to unravel the energy-resolved valley-contrasting orbital magnetism and large orbital magnetic susceptibility that contribute to the energies of Landau levels of twisted double-bilayer graphene. These orbital magnetism effects led to substantial deviations from the standard Onsager relation, which manifested as a breakdown in scaling of Landau-level orbits. These substantial magnetic responses emerged from the nontrivial quantum geometry of the electronic structure and the large length scale of the moire lattice potential. Going beyond traditional measurements, Landau-level spectroscopy performed with a scanning tunneling microscope offers a complete quantum ruler that resolves the full energy dependence of orbital magnetic properties in moire quantum matter.

Author(s): Daniel Walkup. When a gate-tunable two-dimensional electron system (2DES) lies atop a flake of hexagonal boron nitride (hBN), charged defects in the hBN can modify the gating locally, creating a potential landscape for the 2DES’s mobile electrons. By pulsing the voltage of a scanning tunneling microscope (STM) tip we create controlled, semi-permanent pockets of these

Research in new quantum materials requires multi-mode measurements spanning length scales, correlations of atomic-scale variables with a macroscopic function, and spectroscopic energy resolution obtainable only at millikelvin temperatures, typically in a dilution refrigerator. In this article, we describe a multi-mode instrument achieving a μeV tunneling resolution with in-operando measurement capabilities of scanning tunneling microscopy, atomic force microscopy, and magnetotransport inside a dilution refrigerator operating at 10 mK. We describe the system in detail including a new scanning probe microscope module design and sample and tip transport systems, along with wiring, radio-frequency filtering, and electronics. Extensive benchmarking measurements were performed using superconductor–insulator–superconductor tunnel junctions, with Josephson tunneling as a noise metering detector. After extensive testing and optimization, we have achieved less than 8 μeV instrument resolving capability for tunneling spectroscopy, which is 5–10 times better than previous instrument reports and comparable to the quantum and thermal limits set by the operating temperature at 10 mK.

An electronic wedding cake: In nanostructures such as quantum dots, spatial confinement forces electrons to assume discrete energy levels. Quantization can also occur in an external magnetic field, where electrons' energies group into so-called Landau levels (LLs). Gutiérrez et al. explored the interplay between these two mechanisms and electronic interactions in a circulator resonator made of graphene. As an external magnetic field was increased, the electron quantum states transformed from atomic-like states to LL-like states. Electronic interactions caused a characteristic wedding cake–like shape of electronic density at high fields. Science , this issue p. 789

Orbital degrees of freedom can have pronounced effects on the fundamental properties of electrons in solids. In addition to influencing bandwidths, gaps, correlation strength and dispersion, orbital effects have been implicated in generating novel electronic and structural phases. Here we show how the orbital nature of bands can result in non-trivial effects of strain on band structure. We use scanning–tunneling microscopy to study the effects of strain on the electronic structure of a heteroepitaxial thin film of a topological crystalline insulator, SnTe. By studying the effects of uniaxial strain on the band structure we find a surprising effect where strain applied in one direction has the most pronounced influence on the band structure along the perpendicular direction. Our theoretical calculations indicate that this effect arises from the orbital nature of the conduction and valence bands. Our results imply that a microscopic model capturing strain effects must include a consideration of the orbital nature of bands.

Flicking the Berry phase switch: When an electron completes a cycle around the Dirac point (a particular location in graphene's electronic structure), the phase of its wave function changes by π. This so-called Berry phase is tricky to observe directly in solid-state measurements. Ghahari et al. built a graphene nanostructure consisting of a central region doped with positive carriers surrounded by a negatively doped background. Scanning tunneling spectroscopy revealed sudden jumps in conductivity as the external magnetic field was increased past a threshold value. The jumps occurred when electron orbits started encompassing the Dirac point, reflecting the switch of the Berry phase from zero to π. The tunability of conductivity by such minute changes in magnetic field is promising for future applications. Science , this issue p. 845