A quantum material’s surface charge approaches the expected quantized response
A laboratory experiment detected electric charge gathering on a quantum anomalous Hall material as the magnetic field changed. Reducing charge loss brought the measured response close to its predicted quantized value. The peer-reviewed study adds a direct surface-charge measurement to the electrical transport methods usually used to investigate this state.
Science··Midday
Surface charge adds a different view of the Hall state
Researchers detected how a changing magnetic field made electric charge gather on a quantum anomalous Hall material. In this state, the material’s own magnetization accompanies an electrical response fixed by fundamental constants. Conventional transport experiments measure conduction along the sample; the new observation follows charge building up on its surface. The result concerns two-dimensional surface physics in laboratory specimens of a magnetically doped topological insulator.[1]
An electrode above the sample senses the accumulating charge
The experiment used thin films containing chromium, bismuth, antimony and tellurium, formed into rings and disks. An electrode above the film acted as a capacitive sensor: changes in the sample induced an oppositely signed charge in that electrode, detectable as current. Some surface charge escaped through the material. The researchers examined that loss by changing measurement frequency and electrical conductance separately, testing a model of the resulting attenuation.[1]
Less charge loss reveals the expected response
Cooling a disk specimen to about 20 millikelvin reduced the dissipative conduction. At higher measurement frequencies, the charge response came within 5 percent of the expected quantized coefficient. The lower-loss disk measurements complemented ring experiments in which attenuation had obscured much of the signal. The detected charges had the same sign on opposing surfaces; the opposite-sign pattern required for the proposed topological magnetoelectric effect remains a different experimental target.[1]
