Almost every solid state physics textbook says crystal momentum is not really physical momentum. For example, phonons always carry crystal momentum but they do not cause a translation of the sample at all. However, I learned that in indirect-band-gap semiconductors, we need phonons to provide t...

I'm the new product manager for the DAG team. I’m excited to announce that the top bar design that has been on Stack Overflow for several months is coming soon to the Stack Exchange network. Details are below, but the short version is the changes will roll out on meta.se first this week. Assuming...

Kick questions off the Hot Network Questions list (or at least vote to do that.) Many Hot Network Questions are great, and they are great ways to funnel traffic to the better parts of a site and increase engagement. Other Hot Network Questions, however, are not great. The mechanism is strongly...

The Hot Network Questions sidebar is a bit of a contentious issue, because it tends to promote questions that are often perceived as 'cheap', of relatively low quality, or on the fringes of topicality for the site that hosts them. One reason for this is that they're subject to a feedback loop: t...

Recently I had to solve a simple problem in which I had a sphere of radius $R$ with a constant potential (but with different sign), on both of the hemispheres, and I was asked to get the electrostatic potential for every point in the space. And I had to do it with both hemispheres along the $z$-a...