LOW-NOISE MEASUREMENTS OF THE QUANTIZED HALL RESISTANCE USING AN IMPROVED CRYOGENIC CURRENT COMPARATOR BRIDGE

Significant improvements have been made on the bridge based on a cryogenic current comparator (CCC) that is used at the Bureau International des Poids et Mesures (BIPM) for measurements of the quantized Hall resistance (QHR); a SQUID-based dc nanovoltmeter and a new CCC, with an increased number of turns, have been built and incorporated into the bridge. Using the SQUID-based nanovoltmeter, the noise in the two-terminal resistance of Hall samples in the quantum regime was measured for the temperature range 4.2 K to 1.3 K and was found to agree with the Nyquist formula. Measurements of the QHR with a low standard deviation (2 parts in 10(9), for an abservation time of 15 min and when 40-mu-A of current is passed through the Hall sample) were carried out using the new CCC and the SQUID nanovoltmeter. The agreement between QHR's corresponding to two different plateaus and measured on four GaAs-based heterostructures selected for their negligible residual longitudinal resistivity was found to be better than +/- 2 parts in 10(9). The agreement between the mean value of the QHR measured on a silicon MOSFET and that measured on GaAs-based heterostructures was found to be better than 1 part in 10(8).