Joseph A. Stroscio

Superconductivity in strontium titanate has remained enigmatic for over 50 years. The LaAlO ₃ /SrTiO ₃ (LAO/STO) heterointerface enables systematic dimensional confinement, from two-dimensional electron gas to quasi-one-dimensional nanostructures, providing unprecedented access to this quantum state. While transport measurements in patterned devices reveal puzzling phenomena, including width-independent critical currents and anomalous pairing, suggesting one-dimensional behavior, direct local probes for the patterned interface and its superconducting response have remained lacking. Here, we use ultra-low temperature non-contact atomic force microscopy with dissipation spectroscopy and Kelvin probe force microscopy to locally probe signatures of superconductivity in patterned LAO/STO devices. Spatially resolved energy dissipation measurements reveal signatures of superconductivity with some features confined to edge channels of order ≈200 nm in width. Dissipation spectra exhibit a characteristic nonlinear bias dependence that provides a local diagnostic of superconductivity consistent with the intermediate carrier density near the superconducting dome, which persists up to the critical field. These results demonstrate the ability of atomic force microscopy to probe superconductivity in patterned LAO/STO structures, potentially addressing fundamental, longstanding questions about quantum confinement and transport anomalies in these correlated nanostructures.

Thermopower, through its entropic sensitivity, has recently emerged as a powerful probe of correlated states. Here, we report the first thermopower measurements of fractional quantum Hall (FQH) states in monolayer graphene, a system that can host new correlated states due to its relativistic dispersion relation. Our thermopower measurements reveal FQH states appearing in the N=0 and higher Landau levels, including new even denominator states. We employ temperature-dependent magneto-thermopower measurements to extract the energy gaps associated with the more prominent FQH states.

Here we report on low temperature transport measurements of encapsulated bilayer graphene nano constrictions fabricated employing electrode-free AFM-based local anodic oxidation (LAO) nanolithography. This technique allows for the creation of constrictions as narrow as 20 nm much smaller than previous studies. In wider constrictions, we observe bulk transport characteristics. However, as the constriction’s width is reduced, a transport gap appears. Single quantum dot (QD) formation is observed within the narrowest constriction with addition energies exceeding 100 meV, which surpass previous experiments on patterned QDs. Our results suggest that transport through these narrow constrictions is governed by edge disorder combined with quantum confinement effects. Our findings introduce electrode-free AFM-LAO lithography as an easy and flexible method for creating nanostructures with tunable electronic properties without relying on patterning techniques such as e-beam lithography.

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.

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.

Interacting and tunable quantum dots (QDs) have been extensively exploited in condensed matter physics and quantum information science. Using a low-temperature scanning tunneling microscope (STM), we both create and directly image a new type of coupled QD system in graphene, a highly interacting quantum relativistic system with tunable density. Using detailed scanning tunneling spectroscopy (STS) measurements, we show that Landau quantization inside a potential well enables novel electron confinement via the incompressible strips between partially filled Landau levels (LLs), forming isolated and concentric LL QDs. By changing the charge density and the magnetic field we can tune continuously between single- and double-concentric LL QD systems within the same potential well. In the concentric QD regime, single-electron charging peaks of the two dots intersect, displaying a characteristic avoidance pattern. At moderate fields, we observe an unconventional avoidance pattern that differs significantly from that observed in capacitively coupled double-QD systems. We find that we can reproduce in detail this anomalous avoidance pattern within the framework of the electrostatic double-QD model by replacing the capacitive interdot coupling with a phenomenological charge-counting system in which charges in the inner concentric dot are counted in the total charge of both islands. The emergence of such strange forms of interdot coupling in a single potential well, together with the ease of producing such charge pockets in graphene and other two-dimensional (2D) materials, reveals an intriguing testbed for the confinement of 2D electrons in customizable potentials.

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