▎ 摘　　要

Heat capacity is an invaluable quantity in condensed matter physics and yet has been completely inaccessible in two-dimensional (2D) van der Waals (vdW) materials, owing to their ultrafast thermal relaxation times and the lack of suitable nanoscale thermometers. Her; we demonstrate a novel thermal relaxation calorimetry scheme that allows the first measurements of the electronic heat capacity of graphene. It is enabled by combining a radio frequency Johnson noise thermometer, which can measure the electronic temperature with a sensitivity of similar to 20 mK/Hz(1/2), and a photomixed optical heater that modulates T-e with a frequency of up to Omega = 0.2 THz. This allows record sensitive measurements of the electronic heat capacity C-e < 10(-19) J/K and the fastest measurement of electronic thermal relaxation time tau(e) < 10(-12) s yet achieved by a calorimeter. These features advance heat capacity metrology into the realm of nanoscale and low-dimensional systems and provide an avenue for the investigation of their thermodynamic quantities.