Monday, July 20, 2020

Materials physics versus condensed matter physics

How do you define a distinct scientific discipline? Should it be defined in terms of the subject of study, methods used, concepts, goals, history, and/or sociology? Who gets to decide the definition: the practitioners, a broader scientific community, or administrators? How clear do the boundaries between disciplines need to be? And, does it really matter?

Condensed matter physics has a close relationship between materials physics, both intellectually and organisationally. The flagship journal Physical Review B has the subtitle "covering condensed matter and materials physics". The largest physics meeting in the world is the American Physical Society March Meeting which is largely organised by two APS divisions, those of Condensed Matter and Materials Physics. According to the APS website
The Division of Condensed Matter Physics
Originally called the Division of Solid State Physics (DSSP), the unit was formed in 1947, the third society division. In 1978 the DSSP was renamed the Division of Condensed Matter Physics to recognize that disciplines covered in the division included liquids (quantum fluids) as well as solids. Today the DCMP is the largest of all APS divisions. Condensed Matter Physics concentrates on such topics as superconductivity, semiconductors, magnetism, complex fluids, and thin films. A broad range of physical problems, both applied and basic, are investigated.

The Division of Materials Physics was established in 1984. Materials Physics applies fundamental condensed matter concepts to complex and multiphase media, including materials of technological interest. 
I suggest that although there is much common ground, particularly in the materials and techniques that are involved, there is a significant difference in the goals, values, orientation, and questions that are the focus of the two fields, CMP and MP.

CMP is largely concerned with
-the big picture
-states of matter and the transitions between them
-ideal systems (e.g, where the amount of disorder is minimal or where it is homogeneous)
-universality, i.e. properties that are largely independent of the structural and chemical details of a material
-qualitative changes in the properties of a material
-fundamental questions
-understanding for its own sake

In contrast, MP is largely concerned with
-the details
-real imperfect  ``everyday'' materials, especially including impurities, defects, and inhomogeneities.
-incremental improvements in a specific property of a material
-practical questions
-understanding to enable technological applications

I want to stress that there is significant overlap and one cannot clearly separate the two fields. Furthermore, it is not that one is better than the other. They are just different. We need both and we need healthy interaction between the two fields. That is why I think understanding the differences between MP and CMP really does matter.

This post was stimulated by my son asking me the simple but profound question, ``What is the difference between a material and a state of matter?"

What do you think about the difference between MP and CMP? Does it matter?


  1. This reminds me of an exchange I once heard between Brian Coles and Jens-Als Nielsen. Nielsen had just given a beautiful talk showing precise measurements of critical exponents in a 2D-Ising compound. Brian Coles said he was impressed but also depressed by the idea of working on perfect model systems. He thought we should study "real" materials. Nielsen listened patiently and then replied, "That's your problem." I don't think Brian Coles, who I believe first coined the term "spin glass" would have considered himself a "materials physicist" but I have sympathies for both sides of this debate and have been torn between the two ever since.

  2. IMHO, CMP emphasizes the "science" aspects whereas MP emphasizes the "engineering" aspects. A circumstantial piece of evidence in support of this view is that several prominent North American material science and engineering departments are under school of engineering's umbrella.

  3. Great questions! The Deutsche Physikalische Gesellschaft seems to define CMP as the umbrella branch of Physics that includes Materials Physics, among others. To be precise, the Sektion Kondensierte Materie of the DPG Meetings encompass: Biological Physics, Chemical Physics, Thin Films, Solid-state crystals and microstructures, Semiconductor Physics, Metal and Materials Physics, Surface Physics, and interestingly, also Statistical Physics and the Physics of Socio-economic Systems.