Utilizing specifically designed grooved quartz capillaries pulled into a sharp pipette we have fabricated the smallest SQUID-on-tip (SOT) devices with effective diameters down to 39 nm. We have developed integrated resistive shunt in close proximity to the pipette apex which significantly improves the SOT characteristics. As a result we have attained In and Sn SOTs with lowest flux noise and record low spin noise of 0.29 µBHz-1/2. In addition, the new SOTs function at sub-Kelvin temperatures and in high magnetic fields of over 2.5 T.
Nanoscale superconducting quantum interference devices on a tip are of high current interest for ultrasensitive scanning probe magnetic and thermal imaging. Their applicability, however, was curbed by volatility to ambient conditions and the limited range of operating fields and temperatures. We have developed a novel technique of collimated differential-pressure magnetron sputtering for versatile self-aligned fabrication of SOT nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point-source, deposition. The new technique provides access to a broad range of superconducting materials and alloys beyond the elemental superconductors employed in the existing thermal deposition methods, opening the route to greatly enhanced SOT characteristics and functionalities.
Dissipationless topologically protected states are of major fundamental interest as well as of practical importance in metrology and quantum information technology. Although topological protection can be robust theoretically, in realistic devices it is often fragile against various dissipative mechanisms, which are difficult to probe directly because of their microscopic origins. By utilizing scanning nanothermometry, we visualized and investigated the microscopic mechanisms undermining dissipationless transport in the quantum Hall state in graphene. Our simultaneous nanoscale thermal and scanning gate microscopy shows that the dissipation is governed by crosstalk between counterpropagating pairs of downstream and upstream channels that appear at graphene boundaries because of edge reconstruction. Our findings offer a crucial insight into the mechanisms concealing the true topological protection and suggest venues for engineering more robust quantum states for device applications.
Using a SOT, acting simultaneously as a tunable scanning electric charge and as ultrasensitive nanoscale magnetometer, we induced and directly imaged the microscopic currents generating the magnetic monopole response in a graphene quantum Hall electron system. We have found a rich and complex nonlinear behavior governed by coexistence of topological and nontopological equilibrium currents that is not captured by the monopole models. Furthermore, by utilizing a tuning fork that induces nanoscale vibrations of the SOT, we have directly imaged the equilibrium currents of individual quantum Hall edge states for the first time. We reveal that the edge states that are commonly assumed to carry only a chiral downstream current, in fact carry a pair of counterpropagating currents. The intricate patterns of the counterpropagating equilibrium-state orbital currents provide new insights into the microscopic origins of the topological and nontopological charge and energy flow in quantum Hall systems.
Utilizing our scanning SOT, we have attained tomographic imaging of the Landau levels in the quantum Hall state in magic angle graphene and mapped the local θ variations in hBN encapsulated devices with relative precision better than 0.002° and spatial resolution of a few moiré periods. We find that even state-of-the-art devices, exhibiting correlated states, Landau fans, and superconductivity, display significant θ variations of up to 0.1° with substantial gradients and a network of jumps, and may contain areas with no local MATBG behavior, highlighting the importance of percolation physics. The θ gradients generate large gate-tunable in-plane electric fields, unscreened even in the metallic regions, which drastically alter the quantum Hall state by forming edge channels in the bulk of the sample and may significantly affect the phase diagram of the correlated and superconducting states.
Vortices are the hallmarks of hydrodynamic flow. We have provided the first visualization of whirlpools in an electron fluid. By using the SOT, we imaged the current distribution in a circular chamber connected through a small aperture to a current-carrying strip in the high-purity type II Weyl semimetal WTe2. In this geometry, the Gurzhi momentum diffusion length and the size of the aperture determine the vortex stability phase diagram. We find that vortices are present for only small apertures, whereas the flow is laminar (non-vortical) for larger apertures. These findings suggest a new mechanism of hydrodynamic flow in thin pure crystals such that the spatial diffusion of electron momenta is enabled by small-angle scattering at the surfaces instead of the routinely invoked electron–electron scattering, which becomes extremely weak at low temperatures. This surface-induced para-hydrodynamics, which mimics many aspects of conventional hydrodynamics including vortices, opens new possibilities for exploring and using electron fluidics in high-mobility electron systems.
