Fluctuation Probes of Quantum Materials
Transport and noise measurements can reveal different aspects of the same electronic system. Average current emphasizes net flow, while fluctuations expose correlations, nonequilibrium populations, and instability mechanisms. Agarwal's collaborations on graphene and high-mobility Corbino devices illustrate how theory can connect unusual signals to microscopic processes. The central research question is how to distinguish a proposed collective mechanism from more conventional explanations involving heating, scattering, contacts, or geometry.
Figure to read
Concentric contacts define a multiterminal Corbino device; the companion panel locates the samples by interaction parameter. Figure 1 in the source paper. Vijayakrishnan, Sujatha; Poitevin, F.; Yu, Oulin; Berkson-Korenberg, Z.; Petrescu, M.; Lilly, M. P; Szkopek, T.; Agarwal, Kartiek; West, K. W.; Pfeiffer, L. N.; Gervais, G.. Anomalous Electronic Transport in High Mobility Corbino Rings, Nature Communications 14, 3906 (2023). Paper. CC0 1.0, manuscript. Original manuscript graphic; no alterations.
Follow the contact geometry before interpreting resistance. The density comparison indicates distinct interaction scales for the two samples; the drawing itself does not establish hydrodynamic flow.
Graphene beyond a single effective temperature
Andersen, Agarwal, and collaborators combine global and local noise probes in current-driven graphene. Their observations are interpreted through an electron-phonon Cherenkov instability: carriers drifting faster than an appropriate sound velocity can amplify phonons rather than merely scatter from a passive thermal population. The physical comparison is between electron drift, phonon propagation, and the mechanisms that damp or remove the generated excitations. 1
The importance of the study is the joint interpretation of several measurements, including dependence on carrier density and temperature. Excess noise alone would not uniquely identify the mechanism. A driven electronic distribution can be nonthermal, and assigning it one noise temperature may hide the processes creating the fluctuations. The experiment therefore motivates a broader diagnostic strategy: compare signals with different spatial sensitivity and test their behavior as the proposed threshold is crossed.
What a local probe adds
The theory of magnetic noise above a two-dimensional conductor relates local spin relaxation to nonlocal current correlations. Changing sensor height changes the wavelengths that dominate the signal. This can help distinguish spatially structured nonequilibrium behavior from what a global terminal measurement reports. However, equilibrium fluctuation-dissipation relations cannot automatically be carried into a strongly driven state. The assumptions connecting noise and conductivity must be checked for the actual measurement regime. 2
The broader NV-magnetometry literature is relevant because sensor depth and spatial resolution are not ideal parameters. A local measurement still averages over an electromagnetic kernel, and backgrounds can vary with position. Agreement between a local and global diagnostic is most persuasive when each has an independently justified response model, rather than when both are reduced to the same phenomenological temperature by assumption. 3
Why use a Corbino geometry?
A Corbino device arranges contacts in an annular geometry instead of the familiar Hall-bar layout. Vijayakrishnan, Agarwal, and collaborators examine high-mobility GaAs/AlGaAs devices with multiple concentric contacts and report unusual temperature dependence. The geometry offers a complementary view of transport because current paths, contacts, and edge contributions differ from those in a Hall bar. The paper discusses ballistic and hydrodynamic scales and a possible Gurzhi interpretation. 4
Hydrodynamic transport requires a hierarchy in which momentum-conserving collisions establish local equilibrium before momentum-relaxing processes dominate. A nonmonotonic resistance curve can be consistent with such a hierarchy, but does not establish it uniquely. Contact effects, finite geometry, and other scattering contributions must be included. The cautious wording in the Corbino interpretation matters: the observation is a measured transport anomaly, while the mechanism is assessed through comparisons and physical modeling.
Symmetry-sensitive probes as a next step
Recent work on time-reversal-sensitive magnetic noise provides another route to separating mechanisms. Instead of relying only on a temperature trend or amplitude, opposite sensor polarizations can test the handedness of fluctuations. Such a comparison accesses response components distinct from the longitudinal transport usually emphasized in noise thermometry. Its application to a particular device would require a corresponding model of the material and geometry. 5
Together these examples show how Agarwal's theoretical work interacts with experiment: construct a response model, identify a discriminating observable, and compare against alternative mechanisms. The next useful measurements are those that change one physical assumption at a time—geometry, density, driving strength, or sensor polarization. Follow local magnetometry for the underlying kernels and driven systems for another warning against interpreting a transient or unusual response as a fully established transport phase. The research opportunity lies in making several incomplete observations constrain one consistent microscopic account.
References
- Trond I. Andersen; Bo L. Dwyer; Javier D. Sanchez-Yamagishi; Joaquin F. Rodriguez-Nieva; Kartiek Agarwal; Kenji Watanabe; Takashi Taniguchi; Eugene A. Demler; Philip Kim; Hongkun Park; Mikhail D. Lukin. Electron-phonon instability in graphene revealed by global and local noise probes. Science 364, 154–157 (2019).
- Agarwal, Kartiek; Schmidt, Richard; Halperin, Bertrand; Oganesyan, Vadim; Zaránd, Gergely; Lukin, Mikhail D.; Demler, Eugene. Magnetic noise spectroscopy as a probe of local electronic correlations in two-dimensional systems. Phys. Rev. B 95, 155107 (2017). Open manuscript.
- Casola, Francesco; van der Sar, Toeno; Yacoby, Amir. Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond. Nature Reviews Materials 3, 17088 (2018). Open manuscript.
- Vijayakrishnan, Sujatha; Poitevin, F.; Yu, Oulin; Berkson-Korenberg, Z.; Petrescu, M.; Lilly, M. P; Szkopek, T.; Agarwal, Kartiek; West, K. W.; Pfeiffer, L. N.; Gervais, G.. Anomalous Electronic Transport in High Mobility Corbino Rings. Nature Communications 14, 3906 (2023). Open manuscript.
- De, Suman Jyoti; Pereg-Barnea, Tami; Agarwal, Kartiek. Nanoscale Defects as Probes of Time-Reversal Symmetry Breaking. Physical Review X 16, 011001 (2026). Open manuscript.
Independent research synthesis. Literature checked 5 September 2026; preprints are identified in the references.