physics

Phason

Phason is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Phason rather than just read about it. In short: In physics, a phason is a form of collective excitation found in aperiodic crystal structures. Phasons are a type of quasiparticle: an emergent phenomenon of many-particle systems.

Key takeaways

Phason belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phason to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phason from memory before moving on to harder problems.

Reference excerpt

In physics, a phason is a form of collective excitation found in aperiodic crystal structures. Phasons are a type of quasiparticle: an emergent phenomenon of many-particle systems. The phason can also be seen as a degree of freedom unique to quasicrystals. Similar to phonons, phasons are quasiparticles associated with atomic motion. However, whereas phonons are related to the translation of atoms, phasons are associated with atomic rearrangement. As a result of this rearrangement, or modulation, the waves that describe the position of atoms in the crystal change phase—hence the term "phason". In the language of the superspace picture commonly employed in the description of aperiodic crystals in which the aperiodic function is obtained via projection from a higher dimensional periodic function, the 'phason' displacement can be seen as displacement of the (higher-dimensional) lattice points in the perpendicular space. Phasons can travel faster than the speed of sound within quasicrystalline materials, giving these materials a higher thermal conductivity than materials in which the transfer of heat is carried out only by phonons. Different phasonic modes can change the material properties of a quasicrystal. In the superspace representation, aperiodic crystals can be obtained from a periodic crystal of higher dimension by projection to a lower dimensional space– this is commonly referred to as the cut-and-project method. While phonons change the position of atoms relative to the crystal structure in space, phasons change the position of atoms relative to the quasicrystal structure and the cut-through superspace that defines it. Therefore, phonon modes are excitations of the "in-plane" real (also called parallel, direct, or external) space, whereas phasons are excitations of the perpendicular (also called internal or virtual) space. Phasons may be described in terms of hydrodynamic theory: when going from a homogenous fluid to a quasicrystal, hydrodynamic theory predicts six new modes arising from the translational symmetry breaking in the parallel and perpendicular spaces. Three of these modes (corresponding to the parallel space) are acoustic phonon modes, while the remaining three are diffusive phason modes. In incommensurately-modulated crystals, phasons may be constructed from a coherent superposition of phonons of the unmodulated parent structure, though this is not possible for quasicrystals. Hydrodynamic analysis of quasicrystals predicts that, while the strain relaxation of phonons is relatively rapid, relaxation of phason strain is diffusive and is much slower. Therefore, metastable quasicrystals grown by rapid quenching from the melt exhibit built-in phason strain associated with shifts and anisotropic broadenings of X-ray and electron diffraction peaks.

Worked examples

Example 1 — a first encounter with Phason

Start with the simplest possible case. Write down what Phason claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Phason before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Phason ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Phason

In research: Phason appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Phason in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Phason is common in secondary-school and first-year university syllabi. It links to neighbouring topics Condensed matter stubs, Crystallography, Quasiparticles, so understanding it makes those chapters shorter.
In everyday life: Look for Phason outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Phason in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phason means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phason out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phason in simple terms?

In physics, a phason is a form of collective excitation found in aperiodic crystal structures. Phasons are a type of quasiparticle: an emergent phenomenon of many-particle systems.

Why does Phason matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Phason?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Phason.