An eight-layer rhombohedral graphene device on a monolayer WSe2 substrate showed neighboring regions where resistance either fell below the measurement limit or settled at a finite value as it cooled. The study, published in Nature on September 23, maps the two behaviors in the same material platform.
The researchers call the finite-resistance state an “anomalous metal.” Like the superconducting region beside it, it suppresses resistance as temperature falls, but it does not reach zero. This unusual combination has been reported in thin superconductors for decades, yet its cause remains unsettled.
Top and bottom gates let the team tune the device’s carrier density and electric field. At a base temperature of about 11 millikelvin, the resulting map showed separate pockets of superconductivity and anomalous-metal behavior. In some gate settings, increasing current first shifted the device from zero resistance into the finite-resistance state, then into a normal metal at a higher threshold.
At low in-plane fields, both pockets widened across the gate-voltage map. At higher fields, the anomalous-metal region overtook the zero-resistance region and persisted to roughly four times the in-plane field at which the superconducting pocket disappeared. The boundary between them stayed abrupt as the gates were tuned.
A second device reproduced the main phase boundaries and several of these responses, although its transport was less uniform. The repeat makes a simple sample-specific disorder explanation less persuasive, the authors argue, but it does not identify the microscopic mechanism or rule out every effect of inhomogeneity. The result gives researchers a tunable setting for testing why some superconducting-like states retain resistance.
Illustration: Ququ / Wikimedia Commons, CC BY-SA 3.0. No changes made; this shows graphene’s lattice, not the experimental device. Image source.
