Band Structure in Real Systems

There is a high electron density in the 3d band a few electron volts below the Fermi level. Cu preferentially absorbs blue light as these 3d electrons make transitions to 4s states giving Cu its characteristic reddish color. Allo)nng Cu with Zn shifts the absorption edge giving brass its yellowish color. Gold s yellow color is the result of similar interband transitions. 

Band structure of Cu. Transitions of the 3d electron states (flat lines just below the Fermi level to 4s states at the Fermi level absorb blue light giving Cu is reddish color. (From Segal, B., Phys. Rev., 125, 114, 1962. With permission.) 

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The band structure in real systems is more complex than the simple models we have developed in trying to understand the electronic properties of metals. This is especially true in the transition metals where the d-electrons contribute significantly to the band structure. We see that conductivity is a fimction of the Fermi surface and the density of states in the vicinity of the Fermi level. By examining the calculated or measured band structure we can obtain much more insight into interband transitions, which account for the color of metals, and why some metals are much better conductors than others. 

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