Development of high frequency and wide bandwidth Johnson noise thermometry

We develop a high frequency, wide bandwidth radiometer operating at room temperature, which augments the traditional technique of Johnson noise thermometry for nanoscale thermal transport studies. Employing low noise amplifiers and an analog multiplier operating at 2~GHz, auto- and cross-correlated Johnson noise measurements are performed in the temperature range of 3 to 300~K, achieving a sensitivity of 5.5~mK (110 ppm) in 1 second of integration time. This setup allows us to measure the thermal conductance of a boron nitride encapsulated monolayer graphene device over a wide temperature range. Our data shows a high power law (T${}^{\sim 4}$) deviation from the Wiedemann-Franz law above T$\sim$100~K.