Lattice Dynamics related to movement of atoms about their equilibrium positions determined by electronic structure Physical properties of solids Sound velocity Electronic properties the topic coming up after phonons and thermal effects (have flexible ending because the speed of this lecture varies a lot due to questions) Thermal properties: -specific heat -thermal expansion -thermal conductivity (for semiconductors) Hardness of perfect single crystals (without defects)

Uniform Solid Material There is energy associated with the vibrations of atoms. They are tied together with bonds, so they can't vibrate independently. The vibrations take the form of collective modes which propagate through the material. Last time we learned that there is energy associated with the vibrations of atoms. But they are tied together, so they don’t vibrate independently. Sound velocity, thermal expansion, thermal conductivity, specific heat need lattice dynamics to get correct (X-1) (X) (X+1)

Wave-Particle Duality They don’t like to be seen together  Phonons are another quasi-particle. What did it mean for something to be a quasi-particle? It acted like a free particle, but could be due to collective interactions including collisions. We are much more familiar with treating light as a particle, what does a phonon particle look like? Unlike static lattice model , which deals with average positions of atoms in a crystal, lattice dynamics extends the concept of crystal lattice to an array of atoms with finite masses that are capable of motion. Just as light is a wave motion that is considered as composed of particles called photons, we can think of the normal modes of vibration in a solid as being particle-like. Quantum of lattice vibration is called the phonon.

Phonon: A Lump of Vibrational Energy Propagating lattice vibrations can be considered to be sound waves, and their propagation speed is the speed of sound in the material. Roughly how big is ? Phonon: Sound Wavepackets So how big is this? (animation and picture from Wikipedia) If you wanted to probe this oscillation, what wavelength would you want your probe to have? We will come back to this.

Reminder to the physics of oscillations and waves (a few slides) Harmonic oscillator in classical mechanics Equation of motion: Example: vertical springs or Hooke’s law where Solution with where X=A cos ωt X Kx Do you remember why we treat waves like harmonic motion? Waves are generated by a oscillation. x

Traveling plane waves: Displacement as a function of time and k Y or X (Phonon wave vector also often given as q instead of k) Consider a particular state of oscillation Y=constant traveling along In a crystal, there are often traveling waves. Different waves might result in the atom movement, which we’ll come back to later. To illustrate some points, let’s consider a wave that causes all of the atoms in a region to move together. You sometimes see q used instead of k for electrons too, but I have seen it more with phonons. In order for Y to be constant, the argument inside of the cosine must be constant. If it’s constant, it’s derivative is equal to zero; let’s see what that implies. Also get the same solution if you solve for the wave equation. solves wave equation

Transverse wave Longitudinal wave Standing wave What if you subtracted the waves? Standing wave

Like diffraction, simple for large wavelengths Crystal can be viewed as a continuous medium: good for Speed of longitudinal wave: where Bs: bulk modulus (ignoring anisotropy of the crystal) Bs determines elastic deformation energy density (click for details in thermodynamic context) Relate bulk modulus to our conversation on volume stress and strain dilation E.g.: Steel Bs=160 109N/m2 ρ=7860kg/m3

> interatomic spacing continuum approach fails It is only by considering these quantized phonons that we can finally get an accurate calculation for specific heat (which we will discuss more after a few classes) In addition: vibrational modes quantized phonons

Vibrational Modes of a Monatomic Lattice Linear chain: Remember: two coupled harmonic oscillators Symmetric mode Anti-symmetric mode Superposition of normal modes Today 1D, Next time: 3D

? generalization Infinite linear chain How to derive the equation of motion in the harmonic approximation n n-2 n-1 n+1 n+2 a C un-2 un-1 un+1 un+2 un un un+1 un+2 un-1 un-2 fixed

? Total force driving atom n back to equilibrium n n equation of motion Old solution for continuous wave equation was . Use similar? ? approach for linear chain Let us try! , ,

Continuum limit of acoustic waves: k Note: here pictures of transversal waves although calculation for the longitudinal case Continuum limit of acoustic waves:

Technically, only have longitudinal modes in 1D (but transverse easier to see what’s happening) x = na un un Will need to trim out some of this but wait until see how much we get through in prior lecture (a) Chain of atoms in the absence of vibrations. (b) Coupled atomic vibrations generate a traveling longitudinal (L) wave along x. Atomic displacements (un) are parallel to x. (c) A transverse (T) wave traveling along x. Atomic displacements (un) are perpendicular to the x axis. (b) and (c) are snapshots at one instant.

Results in same motion of the atoms k Results in same motion of the atoms , here h=1 1-dim. reciprocal lattice vector Same as before! I improved this a bit. Did the class flow better in this section this time? Region is called first Brillouin zone Means we only have to consider a small range of k values range than all possible wavelengths!

Note different convention for a than Kittel. I have a good reason! Vibrational Spectrum for structures with 2 or more atoms/primitive basis Linear diatomic chain: 2n 2n-2 2n-1 2n+1 2n+2 Note different convention for a than Kittel. I have a good reason! C a 2a u2n u2n+1 u2n+2 u2n-1 u2n-2 Equation of motion for atoms on even positions: A and B are amplitudes of the motion of the two different atoms. The amplitudes need not be the same, but they could be. The book actually makes the distance between same atoms a instead of my choice of 2a. Both are common. You will see shortly why I choose the axis this way. By doing this, I’m able to directly compare my result to the monatomic lattice as I make the masses more similar (next slide) Equation of motion for atoms on odd positions: Solution with: and

, Click on the picture to start the animation M->m note wrong axis in the movie The books solves this with matrices, which is also fine. I’ll show you another way. Only got here in 75 minute class (slide 18), got to 24 nice time (have flexible ending) 2 2 ,

Transverse optical mode for diatomic chain Amplitudes of different atoms A/B=-m/M Transverse acoustic mode for diatomic chain Again, in 1D, these should all be longitudinal, but in 3D we have both. (longitudinal is harder to interpret/visualize in my opinion, so that’s why I keep showing these tranverse ones A/B=1

Longitudinal Eigenmodes in 1D What if the atoms were opposite charged? Optical Mode: These atoms, if oppositely charged, would form an oscillating dipole which would couple to optical fields with λ~a For example, an ionic crystal, where we know an electron transfers from one atom to the other. What would be different between these two?

Summary: What is a phonon? Consider the regular lattice of atoms in a uniform solid material. There should be energy associated with the vibrations of these atoms. But they are tied together with bonds, so they can't vibrate independently. The vibrations take the form of collective modes which propagate through the material. Such propagating lattice vibrations can be considered to be sound waves. And their propagation speed is the speed of sound in the material.

Amplitude of vibration is strongly exaggerated! Analogy with classical mechanical pendulums attached by spring Amplitude of vibration is strongly exaggerated!