Two materials are usually necessary to manifest the Seebeck effect. However, the Seebeck effect occurs when we apply a temperature gradient to a material, and this material produces a potential difference. The usual reason to justify the need of two materials it is to break the symmetry.
Let's imagine a metal bar. If I apply a temperature gradient at the bar extremities, it will induce a flux of electrons, that is, an electric current, and a potential difference. After some time, the electrons will be in "balance", they will just vibrate in the extremities of the bar, but there will be no flux of these electrons. To break this symmetry, we join the extremities of a metal bar made of a different material to the extremities of the previous bar. Since the bars are different, we break the symmetry, and so we obtain a flux of electrons. But what happens if, instead of using a different material, we connect a resistance or a lamp (for example) to the first bar? Should it not break the symmetry? This way we just need one material to produce the Seebeck effect.
My doubt is, historically, the Seebeck effect it has been observed with two materials, but in reality, it can occur in a single material. I do not understand this concept. Why does sometimes it needs two materials, and other times just need one?
Honestly, the fact of using only one material makes much more sense when connecting any type of electronics. But if so, can all materials be considered thermoelectric? Since the application of a thermal gradient produces oscillation of electrons, and apart from the case of wood, which burns, there should be charge flow in any type of metal or semiconductor due to an increase in temperature. I am really confused, and if someone can help me, I would appreciate it!