Thermal Properties of Crystal Lattices Introduction to Solid State Physics http://www.physics.udel.edu/~bnikolic/teaching/phys624/phys624.html

Statistical mechanics of Phonons (=bosons) →Motion of harmonic crystal is described by a set of decoupled harmonic oscillators (phonons): →Average excitation level of normal mode at temperature can also be interpreted as the number of indistinguishable quanta, i.e., phonons, per oscillator: →Crystal in equilibrium with a heath bath at temperature → justified by dividing infinite system into a finite number of subsystems which interact weakly with the remaining system (acting as a heat bath for the give subsystem): PHYS 624: Thermal Properties of Crystal Lattices

Partition function for Phonons PHYS 624: Thermal Properties of Crystal Lattices

Evaluating : Virial theorem →For harmonic oscillator: PHYS 624: Thermal Properties of Crystal Lattices

Evaluating : Phonon density of states PHYS 624: Thermal Properties of Crystal Lattices

Example: Phonon DoS in 1D chain PHYS 624: Thermal Properties of Crystal Lattices

Average RMS displacement of ions from equilibrium positions →One atom per unit cell: →Use virial theorem and phonon DOS: PHYS 624: Thermal Properties of Crystal Lattices

Models of lattice dispersion: Debye →For thermodynamic properties optical modes are irrelevant: Retain only acoustic modes, while replacing them with a purely linear mode with the same initial dispersion. →Since the total DoS is finite, we have to introduce a cutoff at Debye frequency. PHYS 624: Thermal Properties of Crystal Lattices

Models of lattice dispersion: Einstein →Each atom oscillates independently of other atoms — model is dispersionless: →Helium absorbed on atomically perfect surface – each atom is attracted weakly to the surface by van der Walls forces and sits in the local minimum of the surface lattice potential: it oscillates with a frequency without interacting with its neighbors. PHYS 624: Thermal Properties of Crystal Lattices

→Example of the Mermin-Wagner theorem! Long-range order →Long-range order (which is an initial assumption for introducing phonons!) in the lattice at low temperature (at high temperature all lattices melt) exists if and only if : →Example of the Mermin-Wagner theorem! PHYS 624: Thermal Properties of Crystal Lattices

One-dimensional systems: no long-range order →Random fluctuations of atoms in 1D lattice can accumulate to produce a very large average RMS displacement of the atoms out of small interatomic displacements. In higher dimensional systems: the displacements in any directions are constrained by the neighbors in orthogonal directions. “Real” 2D systems (monolayer of gas deposited on atomically perfect surface) do have long-range order due to the surface potential (corrugation of surface). PHYS 624: Thermal Properties of Crystal Lattices

Specific heat of Phonon gas →Debye approximation: Elastic isotropic medium where cutoff frequency is the same for all three acoustic modes (this crude approximation better fits experiments than introducing separate cutoff for longitudinal and transverse branches!). PHYS 624: Thermal Properties of Crystal Lattices

Specific heat of Phonon gas: low vs. high temperature limits Characteristic signature of low-energy phonon excitations! → Debye temperature is a measure of the stiffness of the crystal: above all modes are getting excited, and below modes begin to be “frozen out”. PHYS 624: Thermal Properties of Crystal Lattices

Anharmonic effects: Thermal expansion →An unconstrained cubic system of linear dimension L will change its length with temperature — described by a coefficient of free (p=0) expansion: →Harmonic crystals do not expand when heated! →Introduce anharmonicity in the potential — quasi-harmonic approximation: PHYS 624: Thermal Properties of Crystal Lattices

Cubic terms generates thermal expansion →As the average energy (temperature) of particle trapped in a cubic potential increases, its mean positions shifts! →Cubic potential is not any more exactly solvable — use mean-field approximation: PHYS 624: Thermal Properties of Crystal Lattices

Grüneissen number →If free energy is expressed as a function of the volume, than the condition of zero stress for every temperature yields the relation between V and T: →Generalized to “real” solid: PHYS 624: Thermal Properties of Crystal Lattices

Anharmonicity: Three-phonon processes →Physically: Phonon can decay into two other phonons while conserving the crystal momentum. → Graphically: Three phonon processes arising from cubic terms in the inter-ion potential (six other three phonon-processes are also possible). PHYS 624: Thermal Properties of Crystal Lattices

Thermal conduction 273.2K 298.2K Carbon 26.2 23.2 Copper 4.03 4.01 →Thermal current density = heat conductivity times temperature gradient = energy density times the velocity: →Metals carry heat via free electrons, and are good conductors of both heat and electricity. →Insulators lack free electrons and, therefore, carry heat via lattice vibrations ↔ phonons. Although most insulators are not good thermal conductors, some very stiff insulating crystals have very high thermal conductivities (often highly temperature dependent): 273.2K 298.2K Carbon 26.2 23.2 Copper 4.03 4.01 PHYS 624: Thermal Properties of Crystal Lattices

Linear response theory of transport coefficients →If temperature gradient is small, will deviate only slightly from its equilibrium value : →Thus, to first order, heat current density is given by: →Since we already know , the calculation of and heat conductivity reduces to finding linear change in the phonon density due to the transport of energy. PHYS 624: Thermal Properties of Crystal Lattices

Changing phonon number: decay →Change of phonon density occurs either by phonon decay or by phonon diffusion in and out of the region. PHYS 624: Thermal Properties of Crystal Lattices

Changing phonon number: diffusion PHYS 624: Thermal Properties of Crystal Lattices

Properties of thermal conductivity →Phonons near the BZ boundary or optical modes with small velocity contribute very little to thermal conduction. →Stiff materials, with very fast velocity of the acoustic modes will have a large thermal conductivity. →Thermal conductivity is small for material with short mean free path — affected by defects, anharmonic umklapp processes, … PHYS 624: Thermal Properties of Crystal Lattices

Umklapp processes →At low temperature, only low-energy acoustic modes are excited for which and crystal momentum , so that one has to worry about anharmonic processes which do not involve a reciprocal lattice vector in requirement for momentum conservation. →At high temperatures, conservation of momentum in an anharmonic process may involve a reciprocal lattice vector if of an excited mode is large enough, and there exists sufficiently small reciprocal vector , so that momentum reversal occurs when: Umklapp process: Involves a very large change in the heat current (almost a reversal) — therefore and can become small at high temperatures. PHYS 624: Thermal Properties of Crystal Lattices