Conducting materials typically exhibit either diffusive or ballistic charge transport. When electron–electron interactions dominate, a hydrodynamic regime with viscous charge flow emerges^{1,2,3,4,5,6,7,8,9,10,11,12,13}. More stringent conditions eventually yield a quantum-critical Dirac-fluid regime, where electronic heat can flow more efficiently than charge^{14,15,16,17,18,19,20,21,22}. However, observing and controlling the flow of electronic heat in the hydrodynamic regime at room temperature has so far remained elusive. Here we observe heat transport in graphene in the diffusive and hydrodynamic regimes, and report a controllable transition to the Dirac-fluid regime at room temperature, using carrier temperature and carrier density as control knobs. We introduce the technique of spatiotemporal thermoelectric microscopy with femtosecond temporal and nanometre spatial resolution, which allows for tracking electronic heat spreading. In the diffusive regime, we find a thermal diffusivity of roughly 2,000 cm² s⁻¹, consistent with charge transport. Moreover, within the hydrodynamic time window before momentum relaxation, we observe heat spreading corresponding to a giant diffusivity up to 70,000 cm² s⁻¹, indicative of a Dirac fluid. Our results offer the possibility of further exploration of these interesting physical phenomena and their potential applications in nanoscale thermal management.

导电材料通常表现出扩散或弹道电荷传输。当电子 - 电子相互作用占主导地位时，会出现具有粘性电荷流的流体动力学状态^{1,2,3,4,5,6,7,8,9,10,11,12,13}。更严格的条件最终会产生量子临界狄拉克流体状态，其中电子热可以比电荷更有效地流动^{14、15、16、17、18、19、20、21、22}. 然而，迄今为止，在室温下观察和控制流体动力学状态下的电子热流仍然难以捉摸。在这里，我们观察了石墨烯在扩散和流体动力学状态下的热传输，并报告了在室温下向狄拉克流体状态的可控转变，使用载流子温度和载流子密度作为控制旋钮。我们介绍了具有飞秒时间和纳米空间分辨率的时空热电显微镜技术，该技术允许跟踪电子热扩散。在扩散状态下，我们发现热扩散率约为 2,000 cm ²  s ^-1，与电荷传输一致。此外，在动量弛豫之前的流体动力学时间窗口内，我们观察到热扩散对应于高达 70,000 cm ²  s ^-1的巨大扩散率，表明存在狄拉克流体。我们的结果为进一步探索这些有趣的物理现象及其在纳米级热管理中的潜在应用提供了可能性。