Hi, when calculating the effective mass at the band edge, why is k replaced by pi+delta k and not pi/2+delta k?
there is a division by 2 right? (a/2)
Yeah, I can clarify a bit on how I approached it. From the information above 2a the dispersion had the form (I don’t know how to use latex with the messages): E=+/- sqrt(4t^2+cos^2(ka/2)+epsilon^2). The band edge is the same as the bandgap, I presume, both at k= +/- pi/a. I wanted to replace k with pi/a + delta k in the cosine, but that is incorrect. So in short, how would you expand the cosine for the effective mass around a point? Hopefully this makes it a bit clearer