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?

Monday, July 13, 2020

The most accurate equations in all of physics

A beautiful and profound article, Superconductivity and the Quantization of Energy, by D.G. McDonald was published in Science in 1990.
Ideas about quantized energy levels originated in atomic physics, but research in superconductivity has led to unparalleled precision in the measurement of energy levels. A comparison of levels produced by two Josephson junctions shows that they differ by no more than 3 parts in 10^19 at an energy of 0.0003 electron volt. The fact that the myriad of interactions of 10^12 particles in a macroscopic body, a Josephson junction, can produce sharply defined energy levels suggests a dynamical state effectively divorced from the complexities of its environment. The existence of this state, the macroscopic quantum state of superconductors, is well established, but its isolation from intrinsic perturbations has recently been shown to be extraordinary. These new results, with an improved precision of about ten orders of magnitude, are discussed in the context of highly accurate results from quantum electrodynamics, atomic spectroscopy, and the standards of metrology.  
McDonald suggests that the equations associated with the AC Josephson effect are the ``most accurate equations in all of physics". The central equation is the relationship between the voltage V across a junction and the frequency, nu, of the radiation shone on the junction.
The proportionality constant is the Josephson constant,
where e is the electronic charge and h is Planck's constant. Thus, the Josephson effect provides a means to make a macroscopic measurement of fundamental constants that are normally associated with measurements of atomic systems.

The relative value of the Voltage V in two different Josephson junctions can be measured with great precision using a SQUID. One obtains the same value for a range of systems of different chemical composition and Josephson junctions of different design.

On a philosophical level, these results are a beautiful illustration that superconductivity is an emergent state of matter and that the Josephson effect is an emergent effect. An important characteristic of emergent phenomena are that they are independent of most of the chemical and structure details of the materials involved.

These observations had important implications for metrology (the scientific study of measurement and the associated units and standards). In 1990 an international agreement was made that the voltage standard based on Weston cells (a particular type of electrical battery) would be replaced by the Josephson voltage standard. This change was not just one of precision, but also portability, reproducibility, and flexibility. The old voltage standard involved a specific material and device and required duplication in order to have an accurate standard. In contrast, the Josephson standard is independent of the materials used and the details of the device.

All of the above illustrates how much Josephson matters.

Friday, July 10, 2020

The discipline of scientific writing

Writing a paper is hard work. Writing a paper that is clear, engaging, and accurate is even harder. After you have written a draft or are reading a draft of a colleague or co-author I think the following discipline is important and worthwhile.

Go through every sentence and ask, is this true? Is it precise and accurate?

Let me illustrate with a concrete example. Consider the following different claims.

The experiments of Jones et al. proved that GaAs quantum wires are Luttinger liquids.

Jones et al. interpreted their experimental results on GaAs quantum wires in terms of the framework of Luttinger liquid theory.

Jones et al. fit their experimental data for the temperature dependence of the resistivity of a GaAs quantum wire to a power law, such as predicted by Luttinger liquid theory.

Jones et al. showed that their experimental data was inconsistent with Fermi liquid theory, but consistent with Luttinger theory.

Hopefully, the differences between these claims are clear.

This discipline becomes even more important when reporting one's own research. For example, just replace ``Jones et al." in the sentences above with "We".

Unfortunately, the rush to publish in luxury journals has increased the tendency of authors to not exercise the appropriate restraint and discipline required by scientific integrity.