Sunday, February 04, 2018

New readers: What is condensed matter physics? What is special about the nanoscale?

If you're a new reader, perhaps brought here by the mention of this blog in the Washington Post, welcome!   Great to have you here.  Just a couple of quick FAQs to get you oriented:

What is condensed matter physics?  Condensed matter (once known as "solid state) is a branch of physics that deals with the properties of matter consisting of large numbers of particles (usually atoms or (electrons+the rest of the atoms)) in "condensed" states like liquids and solids - basically the materials that make up an awful lot of the stuff you interact with all the time.  New properties can emerge when you bring lots of particles together.  See here for an example involving plastic balls, or here (pdf) for a famous essay about this general point.  Condensed matter physicists are often interested in identifying the different types of states or phases that can arise, and understanding transitions between those states (like how does water boil, or how does magnetism turn on in iron as its temperature is lowered from the melting point, or how does a ceramic copper oxide suddenly start letting electricity flow without resistance below some particular temperature).  Hard condensed matter typically deals with systems where quantum mechanics is directly important (electronic, magnetic, and optical properties of materials, for example), while soft condensed matter describes systems where the main actors (while quantum deep down like all matter) are not acting in a quantum way - examples include the jamming of grains of sand when you build a sand castle, or the spontaneous alignment of rod-like molecules in the liquid crystal display you're using to read this.

While particle physics tries to look at the tiniest bits of stuff, condensed matter hits on some of the same (literally the same concepts and math) deep ideas about symmetry, and often has direct implications for technologies that affect your daily life.   Understanding this stuff has given us things like the entire electronics industry, the telecommunications industry, and soon probably quantum computers.  

A powerful concept in physics in general and condensed matter in particular is universality.  For example, materials built out of all kinds of different ingredients can be mechanically rigid solids; there is something universal about mechanical rigidity that makes it emerge independent of the microscopic details.  Another example:  Lots of very different systems (metallic lead; waxy crystals of buckyball molecules with some alkaline metal atoms in between; ceramic copper oxides; hydrogen sulfide gas under enormous pressure) can conduct electricity without resistance at low temperatures - why and how is superconductivity an emergent property?

What is special about the nanoscale?  Because it's about collective properties, traditional condensed matter physics often uses a lot of nice approximations to describe systems, like assuming they're infinite in extent, or at least larger than lots of physically important scales.   When you get down to the nanoscale (recall that a typical atom is something like 0.3 nanometers in diameter), a lot of the typical approximations can fail.  As the size of the material or system becomes small compared to the length scales associated with various physical processes, new things can happen and the properties of materials can change dramatically.  Tightly confined liquids can act like solids.  Colorless materials can look brilliantly chromatic when structured on small scales.  Two electrical insulators brought together can produce a nanoscale-thick metallic layer.   We now have different techniques for structuring materials on the nanoscale and for seeing what we're doing down there, where the building blocks are often far smaller than the wavelengths of light.  Investigations at the nanoscale are tied to some of the most active topics in condensed matter, and verge into the interdisciplinary boundaries with chemistry, biology, materials science, electrical engineering, and chemical engineering.   That, and it's fun.

Please browse around through the archives, and I hope you find it interesting.


thm said...

Going to be a bit pedantic here: I've heard both phrases "soft condensed matter physics" and "soft matter physics" used almost interchangeably, often as part of a speaker introduction or general article. I'd say that "soft matter" is a small subset of "soft condensed matter" and is almost always the less accurate word choice, unless you're actually talking about foams or some such--or is most "soft condensed matter" also "soft matter"? Interestingly the google ngram for "soft matter" vs. "soft condensed matter" show "soft condensed matter" more common through the late 1990s, when I was an impressionable student, while "soft matter" has really taken off since, as I've become old enough to be a middle-aged curmudgeon.

Douglas Natelson said...

thm, you and I appear to be almost exact contemporaries, which explains our similar views on this vocabulary issue (as well as our common experience in scoring little league baseball games, as I read on your blog. Gamechanger is annoying - paper scorebooks are the way to go for me.).

thm said...

I think we're on the losing side of the vocabulary battle, though, because the APS's GSOFT is the "Topical Group on Soft Matter." Not that the name has kept me from joining, though.

The one thing about Gamechanger is that is lets my dad follow his grandson's games from 3000 miles away. So I'm glad there's another dad who Gamechangers the games. Although one of the ways my scorekeeping style evolved as it did was to write detailed post-game narratives for my dad. Expect more on my (mostly dormant) blog about scorekeeping: I make my own, and finally have settled on a design I'm willing to share. And I'll also blog more along the lines of my old March Meeting posts.