Smallest to date ΔR_K/R_K uncertainty with zero-field Quantum Anomalous Hall Effect in a V-doped (BiSb)₂Te₃ device

The quantum anomalous Hall effect (QAHE), the quantized version of the anomalous Hall effect, is realized in magnetically-doped topological insulator (TI) materials that present a ferromagnetic ground state. The Hall conductivity acquires then quantized values that are integer multiples of the (von Klitzing) conductance quantum e²/h. The integer, in turn, is equal to the Chern number arising out of topological properties of the material band structure, and the QAHE is, hence a topologically protected state. Since its first realization in V and Cr-doped (BiSb)₂Te₃, this phenomenon has attracted enormous interest in the condensed matter physics, magnetism, and metrology communities, for it provides a route to tune desired quantum functionality paving the way to a primary electrical resistance standard, without the complications of a magnetic field applied with He-cooled superconducting coils. To this end, we focus on a V-doped (BiSb)₂Te₃ Hall-bar device, measured using a precision resistance bridge based on a state-of-the-art 14-bit cryogenic current comparator (CCC). We combine higher-resolution electronics, improved Hall bar contacts, and advanced film growth techniques to accomplish the QAHE in zero applied magnetic field with the lowest uncertainties in the ΔR_K/R_K = 10nΩ/Ω range. These measurements were carried out in the temperature T = 30-700 mK range, with relatively high electrical currents of 40-300 nA, which make this device an excellent addition to the quantum metrology toolbox.