# A Brief Review on the Recent Experimental Advances in Thermal Rectification at the Nanoscale
## 2 Experimental Study
### Thermal Rectification of Electronic Heat Currents  
Recent theoretical studies have shown that the thermal rectification effect of the electronic heat current can be achieved in tunnel junctions of metal microcircuits at low temperatures [49; 50; 51]. [PERSON] et al. confirmed the feasibility of this thermal diode through experiments [52]. They used electron beam lithography, metal mask and in situ oxidation to design a \(\mathrm{N_{L}}\mathrm{INISIN_{R}}\) chain, where N represents common metal (composed of \(\mathrm{Al_{0.98}Mn_{0.02}}\)), I represent insulator, S represents superconductor (composed of Al with critical temperature at 1.5 K), and subscripts L and R refer to the left and right leads, respectively (Figure 11). The thermal symmetry is destroyed by inserting the S electrode into a common metal diode such that the normal-state resistance on the right side is much larger than the resistance on the left side. Here, the presence of band gap significantly suppresses the heat flow through the S electrode. Two common metal probes and four superconducting probes were prepared on both sides of the sample, which can be used as both a heater and detector. The thermal resistance could be measured in both directions by changing the role of the heater and detector. In the experiment, only the heat flow carried by the electrons was considered while the contribution of the phonons was ignored. This assumption is expected to hold because the lattice phonons in the entire sample structure were thermalized at the substrate temperature due to the very small Kapitza resistance at low temperatures [53; 54; 55]. Therefore, it is possible to increase the electronic temperature at one end and yield a temperature gradient across the thermal diode. The experimental results are shown in Figure 12 below. The temperature difference between positive and negative directions is drawn at different bias temperatures. The theoretical solution calculated by the thermal model is also included in the figure. It is seen that the experimental and theoretical results agree very well with each other. The thermal rectification ratio \(R\) is marked on the right axis. It can be seen that at low ambient temperatures, \(R\) is much larger than 1, confirming a very strong thermal rectification effect in the thermal diodes. Figure 11: False-color SEM image of the thermal rectification device. Reprinted with permission from Springer Nature Publishing Group. The presence of the superconducting electrode S is important for thermal rectification. Because the quasiparticles with energy less than the band gap cannot pass through the forbidden band, there is a large temperature difference at both ends of S. This results in the increasing temperature difference across the S-electrode in the forward temperature bias, while in the reverse, the S-electrode suppresses the temperature difference and eventually causes a much larger forward heat flow and strong thermal rectification effect. In Figure 12, as the temperature bias increases, \(1/R>2\) occurs, which implies another thermal rectification mechanism in the thermal diode. The energy transmitted from the N-electrode to the environment has been divided into two parts: one part transmitted to the bath temperature by the cold finger probe is correlated with the quadratic square of the temperature (\(\propto T^{2}\)), and the other part is transmitted by the lattice phonons, which is related to the sixth power of the temperature \(\left(\propto T^{6}\right)\). As Figure 13 shows, when the temperature bias in the system increases, the second part of the energy exceeds the first part, which becomes a dominant contribution in the heat conduction. This leads to an enhanced electron-phonon coupling and effectively suppresses the forward heat flow. The increase in ambient temperature will accelerate the latter's share of the former and further decrease the forward heat flow. The energy gap of the superconducting electrode is also affected by the bath temperature, causing an increase in the reverse current.