Ultra-fast calorimetric measurements of the electronic heat capacity of graphene

Abstract

Heat capacity is an invaluable quantity in condensed matter physics, yet it has been so far experimentally inaccessible in two-dimensional (2D) van der Waals (vdW) materials, owing to their ultra-fast thermal relaxation times and the lack of suitable nano-scale thermometers. Here, we demonstrate a novel thermal relaxation calorimetry scheme that allows the first measurements of the electronic heat capacity of graphene Ce. It is enabled by the grouping of a radio-frequency Johnson noise thermometer, which can measure the electronic temperature Te with a measurement sensitivity of δTe ~ 20 mK, and an ultra-fast photo-mixed optical heater, which can simultaneously modulate Te with a frequency of up to Ω=0.2 THz. This combination allows record sensitive and record fast measurements of the electronic heat capacity Ce < 10^(-19) J/K, with an electronic thermal relaxation time τe < 10^(-13), representing orders of magnitude improvements as compared to previous state-of-the-art calorimeters. These features embody a breakthrough in heat capacity metrology of nano-scale and low-dimensional systems, and provide a new avenue for the investigation of their thermodynamic quantities.