Phonons in High Temperature Superconductors

[phantom power]

BERKELEY – An elegant experiment conducted by University of California, Berkeley, and Lawrence Berkeley National Laboratory (LBNL) scientists, in collaboration with a group of scientists at Tokyo University, shows clearly that in high temperature superconductors, vibrations in the crystal lattice play a significant though unconventional role.

"The results we found provide the first direct evidence for a significant and unconventional role of phonons in the high temperature superconductivity, meaning that all the reasons that have been used so far to disregard the importance of phonons are not valid anymore," Lanzara said.

Lanzara, Gweon and theorist Dung-Hai Lee, UC Berkeley professor of physics, agree with many of their colleagues that in high temperature superconductors, the repulsive electron-electron interaction is very strong, and that the tendency for electrons of opposite spins to pair up into a "singlet'' has a lot to do with the antiferromagnetic interaction that's responsible for making the undoped ceramic materials antiferromagnets. However, they also believe that this tendency to form spin singlets enhances the interaction between the electrons and the phonons.

Such an electronically enhanced electron-phonon coupling is similar to what happens in the so-called spin-Peierls systems, where alternating electrons in a solid lattice adopt opposite spins and, as a result, pair up into an ordered system similar to Cooper pairing, though these pairs do not roam the solid but stay near their lattice electrons. The alternating spin arrangement that characterizes spin-Peiels behavior is identical to the antiferromagnetic situation in high temperature superconductors.
http://www.sciencedaily.com/releases/2004/08/040824014758.htm

[kestrel91316]

1. Uh, excuse me....................(raises hand for question)

WHAT IS A "PHONON"???????? I only know "photons? Typo????????????

[Dogmudgeon]

2. Looks like some kind of quantum of kinetic energy to me

Anyone here know for sure?

--bkl

[phantom power]

4. They are acoustic "quanta". I'm not sure if they are quanta, in

the same strict way that things like photons are, but physicists refer to them that way.

[GAspnes]

5. phonons

http://en.wikipedia.org/wiki/Phonon

A phonon is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The study of phonons is an important part of solid state physics, because they contribute to many of the physical properties of materials, such as thermal and electrical conductivity. For example, the propagation of phonons is responsible for the conduction of heat in insulators, and the properties of long-wavelength phonons gives rise to sound in solids (hence the name phonon).

[bemildred]

3. Phonons:

... The pairing is made possible by interactions between the electrons and the metal atoms vibrating in place in the crystal lattice. The lattice is the ordered three-dimensional arrangement of atoms in a solid, like the scaffolding of a crystal.

Most researchers studying high temperature superconductors have disregarded these lattice vibrations, called phonons, under the assumption that they play no role in the high temperature superconductors.

Not the best explanation, but some sort of lattice resonation, most
likely having a characteristic frequency. The name suggests a sort
of standing sound wave or vibration in the lattice.

---

On a different subject, last year some fellows found sound waves in
the intergalactic dust, a very low note caused by the Big Bang.

[midnight armadillo]

7. That's right

Not the best explanation, but some sort of lattice resonation, most
likely having a characteristic frequency. The name suggests a sort
of standing sound wave or vibration in the lattice.

Phonons are lattice vibrations that act like particles obeying Bose-Einstein statistics, like photons do. Essentially, they're quantized sound waves in the material.

[bemildred]

8. Almost right, they move, hence not "standing".

But a very interesting subject. One wonders if they obey
exclusion and uncertainty rules? I would guess they do because
of the quantization.

It makes sense that the lattice would have characteristic
frequencies, and quantization makes sense at the atomic scale.
Sound is energy, and all energy is quantized (AFAWK).

I expect there is some sort of resonance between the phonons and
the electron pairs that facilitates resistanceless movement at the
higher temperatures (speculating ...).

But I'm not up to the math any more ...

[midnight armadillo]

9. Bose-Einstein statistics

Integer spin particles (i.e. bosons) obey Bose-Einstein statistics, which means they can pile up on top of each other and so forth without interacting. Or, in other words, they can all occupy the same energy state. Phonons aren't real particles and don't have spin but do follow B-E, so they could perhaps be considered spin 0.

Electrons and other fermions have spin values of 1/2 (gluons are 3/2 if memory serves) and obey Fermi-Dirac statistics, the Pauli exclusion principle, and so on.

[bemildred]

10. Ah, "bosons" and "fermions".

I believe you are right about the gluons.
From what I have read, the distinction between "real"
particles and "virtual" ones is as clear as mud, but in
the case of phonons I can see the point.
Thanks for the explanation. :thumbsup:

[kestrel91316]

6. Sheesh, I've been away from physics way too long.......