Two dimensional electrons subjected to a strong magnetic field form highly degenerate Landau levels, whose flat dispersion makes them highly susceptible to interaction driven instabilities.  Among the most spectacular of such ground states are those leading to fractional quantum Hall effects–‘topologically ordered’ collective electron ground states where the elementary excitations of the system carry fractional charge, and follow fractional quantum statistics.

Driven by a string of technical advances in the field of two dimensional layered materials [1-2], including most recently in our group [3], graphene heterostructures are now among the best material platforms for studying these effects, combining pristine sample quality with highly controllable spin, valley, and orbital degeneracies. The Figure to the left shows data from our lab probing the density of states of a graphene bilayer, as the charge density and interlayer potential are varied.  Each thin vertical line is an incompressible fractional quantum Hall liquid, and the interlayer potential drives phase transitions between correlated states with differing layer occupation. (see [3] for details).

Ongoing projects include using these devices to experimentally detect nonabelian statistics using both interferometric and thermodynamic probes, and engineering nonabelian defect states in abelian fractional quantum Hall and ‘fractional Chern insulator’ phases.

[1] Dean et al., Nature Nanotechnology, 5:722-726 (2010).
[2] Wang et al., Science, 342:614-617 (2013).
[3] Zibrov et al., arXiv:1607.06461 (2016).

 

 
 

Understanding the nature of electronic states requires access to a variety of fundamental observables. Some can be extracted directly from a well designed sample–resistance and density of states, e.g.–but others require specialized tools.  We build and operate nanoSQUID-on-tip microscopes [1] that allow nanoscale mapping of magnetic and thermal properties. These microscopes are based on a superconducting quantum interference device fabricated at the tip of a quartz pipette, and enable world-leading combination of spatial resolution (down to ~40 nm), magnetic sensitivity (a few nT/rtHz), and thermal sensitivity in the the cryogenic temperature range. Results from the Young lab to date include direct imaging of orbital magnetism in graphene heterostructures [2] and unravelling the mechanism for current-induced magnetic switching by intrinsic spin Hall torque in transition metal dichalcogenide moire structures. Ongoing work includes direct imaging of electron flow in a electron fluids and revealing new mechanisms for spin-orbit coupling in strongly correlated electron systems. In addition, we are building new instrumentation that will bring nanoSQUID on tip microscopy to the 20mK regime, using a custom designed dilution refrigerator in which the entire scanning probe microscope is contained in superfluid helium cell.

[1] Vasyukov et al., Nature Nanotechnology 8:639-644 (2013).
[2] C. L. Tschirhart, M. Serlin, AFY et al., Science 372, 1323-1327 (2021).
[3] C. L. Tschirhart, E. Redekop, AFY et al., Nature Physics (in press, 2023).

 
 

The condition of equilibrium puts constraints on what types of matter can exist, and on their quantitative characteristics.  It has been proposed that strong electromagnetic drive can induce new phases not available as ground states, or quantitatively favor one or more competing ground states under a set of similar equilibrium conditions (temperature, magnetic field, etc.).  Accessing such phases with electrical measurements, however, is limited by the short timescales during which electromagnetic drive can be applied, itself limited both by the availability of pulsed laser sources and limits on power dissipation within the sample itself.  We have developed a versatile THz spectrometer-on-a-chip technique, allowing us to probe the excitation spectrum of deeply sub-diffraction limit sized samples—including van der Waals heterostructures—in the THz frequency range and on picosecond time scales [1]. Critically, our design features both cryogenic compatibility and fast sample exchange, allowing us to bring this technique to bear on highly tunable, complex heterostructures hosting a wide array of correlation driven electronic ground states.

[1] A. Potts, A. Nayak, AFY et. al., “On-chip time-domain terahertz spectroscopy of superconducting films below the diffraction limit.” arXiv:2302.05434