Tuesday, October 21, 2008

Fortuitous physics

Every now and then you stumble across a piece of physics, some detail about how the universe works, that is extremely lucky in some sense. For example, it's very convenient that Si is a great semiconductor, and at the same time SiO2 is an incredibly good insulator - in terms of the electric field that it can sustain before breakdown, SiO2 is about as good as it gets. Another example is GaAs. While it doesn't have a nice oxide, it does have some incredibly nice crystal growth properties. I've been told that through some fortunate happenstance of growth kinetics, you can do growth (e.g., in a molecular beam epitaxy system - a glorified evaporator) under nearly arbitrarily As-rich conditions and still end up with stoichiometric GaAs. Somehow the excess As just doesn't stick around. A third example is the phase diagram of 3He/4He mixtures. Mixtures of the two helium isotopes phase separate at low temperatures (T below 600 mK) >3He-rich phase that's almost pure, and a dilute phase with about 6% 3He in 4He. If you pump the 3He atoms out of the dilute phase, more 3He atoms are pulled from the concentrated phase to maintain the 6% concentration in the dilute phase. There is a latent heat associated with removing a 3He atom from the concentrated phase. The result is a form of evaporative cooling: the temperature of the concentrated phase decreases as the pumping continues, and unlike real evaporative cooling, the effective vapor pressure of the 3He in the dilute phase remains fixed even as T approaches zero. This happy piece of physics is the basis for the dilution refrigerator, which lets us cool materials down to within a few mK of absolute zero.

Any suggestions for other fortunate, useful pieces of physics?

9 comments:

Paul said...

Water strikes me as amazing sometimes. It's lighter density as a solid, strong hydrogen bonding, dipole moment and its amazing ability to solvate things.

Anonymous said...

Probably the most magical and fortuitous aspect of SiO2 is the amazingly benign interface that it forms with crystalline Si. Sticking with your theme, GaAs/AlGaAs is pretty darn amazing, too. Whether grown by LPE, MBE, or MOCVD, this lattice-matched interface can (under good circumstances) possess very low trap density, no dislocations, and allows both electrons and holes to be confined in the GaAs. Pretty sweet :)

Aaron said...
This comment has been removed by the author.
Aaron said...

Paul, don't forget about water's high specific heat. Our oceans do a great job of regulating the surface temperature of the planet.

How about the exponential dependence on gap size in tunneling current? Without this, scanning tunneling microscopy would be pretty tough.

Paul said...

Oh yeah, and it's ability to act well as both an acid and base, really strong capillary action and surface tension (though that's both due to hydrogen bonding).

Peter Armitage said...

I still marvel about Silicon. It is a miracle material. In addition to the remarkable Si/SiO2 thing Doug mentioned, It ...

- has chemical properties that allow it to be easily refined and made ultra pure.
-is incredible abundant on the planet (Sand!)
-has xtal structure that allows well excellent mechanical properties and surface plane specific etching
- has trivially passivated surfaces.

Chad Orzel said...

Manual TrackBack ping.

Patrick said...

One of my professors used to call SiO2 "God's gift to microelectronics."

And it's not just the material properties that are fortuitous, but also the serendipitous discoveries that are wonderful, such as the accidental washing off of germanium oxide that led to the invention of the transistor.

Incoherent Ponderer said...

I think the most lucky fact is that the kinetic energy is mv^2/2. Imagine all the injuries we would have to endure (as kids or even as adults) if it was mv^2, and the 1/2 factor was missing!