Extended Data Fig. 1: Calculated dielectric permittivities of materials used in this work.

a, Calculated dielectric permittivities of Au, SiO2, H2O, Fe3O4 and kerosene respectively. The inset is a zoom-in plot of the dielectric permittivities with the Matsubara energy in the range between 100 eV and 500 eV. b, Calculated dielectric permittivities of water-based ferrofluids with different volume fractions. The dielectric permittivities of Au and SiO2 are also shown for comparison.