Experiments with ⁴He heated from above, very near the lambda point

We report on new transport phenomena observed within a column of ⁴He heated from above and close to its transition to superfluidity. In this configuration the helium sample will self-organize to a state where the temperature gradient across the column remains equal to the pressure-induced gradient of T_λ (1.273 μK/cm) even as the heat flux is varied by a factor of about a thousand. On this self-organized state we have observed a new temperature-entropy wave that propagates opposite to the direction of a steady heat flux Q. This propagating mode is due to a non-linear temperature dependence of the thermoconductance of the helium near criticality. Such a mode had been predicted to exist in this state for Q less than about 100 nW/cm². We confirm that this mode exists in this regime, however we also observe that it propagates at higher values of Q, even when the helium is pushed away from the self-organized state into the normal state.