Expt 4) Electrical noise is quantified in 1928

In 1928, J.B. Johnson measured a rather peculiar quantity. He wanted to quantify where electrical noise came from when performing measurements of resistance. The noise was a problem for vacuum tube amplifiers, which would necessarily amplify not only wanted signals, but also the unwanted noise. In one of the famous early manifestations of the relationship between equilibrium fluctuations and non-equilibrium dissipation, Johnson observed that the mean-squared voltage was linearly proportional to both the temperature and the resistance, i.e. .

Soon after, Harry Nyquist was able to explain the result. But it was only in 1951 when Callen and Welton produced the fluctuation-dissipation theorem, was the full importance of Johnson noise understood within a larger context. Whenever energy is dissipated into heat, there is always a reverse process where thermal fluctuations give rise to noise. In this case, the electric current in a resistive wire will rapidly go to zero in the absence of a sustained driving force (dissipation). The corresponding fluctuation is Johnson noise, where a small, rapidly fluctuating current is generically present due to thermal fluctuations. Below is the plot from Johnson’s famous paper in 1928 showing the relationship between the noise, resistance and temperature in a half megaOhm “Advance” wire, aqueous solutions of various salts, and in a carbon filament:

Mean-squared voltage vs. resistance (left) and vs. temperature (right).

Expt 3) Electron diffraction

Electron diffraction was first observed in 1927 by Clinton Davisson and Lester Germer from the surface of crystalline nickel. This experiment was profound for two reasons. First, it established the wave nature of the electron experimentally. Although Einstein and Planck had suggested that photons possessed particle-like properties in the early 20th century, it took almost 20 years for someone to suggest that particles may correspondingly possess wave-like properties. In 1924, de Broglie made such a hypothesis, and Schrodinger followed up on his suggestion by writing down his eponymous equation in 1926. The Davisson-Germer experiment showed that electrons can also, like X-rays, diffract from crystals.

Second, this experiment set the stage for electron diffraction and microscopy to be used as a tool in various fields of science. Because electrons are massive, it is much easier to obtain short wavelength electrons (where ) than short wavelength photons (where ) without radiation concerns. Electrons in the eV range of energies are enough to yield a wavelength on the order of the interatomic spacing, whereas photons in the keV range of energies are required to obtain similar wavelengths. Below is Davisson and Germer’s classic figure showing diffraction peaks at expected angles with an incident beam energy of 54 eV.

In this image, diffraction peaks characteristic of nickel are observed, demonstrating the wave nature of the electron. Image is taken from here.

Expt 2) X-ray diffraction from crystals

Diffraction from crystals was first observed in 1912 by Max von Laue, Paul Knipping and Walter Friedrich and soon after by William Lawrence Bragg (son) and William Henry Bragg (father).

Although we normally think of X-ray diffraction as being extremely strong evidence that crystals comprise a periodic array of atoms, this is not the reason that the group led by Laue carried out the first experiments of diffraction from crystals. That periodic arrays of atoms laid beneath the geometry of macroscopic crystals was already known from the fact that exact integers index the orientation of crystal facets.

Instead, Laue wanted to establish one of the key important facts concerning “Rontgen rays”, as X-rays were then known. At the time, it was debated whether X-rays consisted of “corpuscles” or waves. Laue, in conversation with Ewald, proposed that if X-rays were wave-like, they should show diffraction from crystals, as their wavelength would be on the order of the separation between atoms in the crystal lattice. When Laue was able to persuade Knipping and Friedrich to carry out the experiment, the answer came out decisively in favor of the wave interpretation (see publication in the original German here). The Braggs followed up this work shortly thereafter and showed that the angular distribution of bright spots could be explained by diffraction from planes obeying the formula, (see publication here).

Looking back, however, it seems like the most important aspect of their work was to establish that the structure of the periodic arrays could be backed out from diffraction patterns (modulo the phase problem). It is this very method that eventually led to one of the most important experimental discoveries in the history of science: the determination of the double-helix structure of DNA using X-ray diffraction by Rosalind Franklin, Maurice Wilkins, James Watson and Francis Crick.

X-ray diffraction from ZnS taken by Knipping, Friedrich and Laue in 1913. Image reprinted without permission from this paper.

Expt 1) Superconductivity in Hg in 1911

In this famous experiment, Heike Kamerlingh Onnes showed that mercury exhibits vanishing resistance below 4.2 K — he had discovered superconductivity. (More precisely, it was Kamerlingh Onnes’ lab assistants, Gilles Holst and Cornelius Dorsman, that performed the experiments while being overseen by Kamerlingh Onnes and his chief of technical staff, Gerrit Flim.) Below is the legendary plot from Kamerlingh Onnes’ lab notebook:

The famous plot from Kamerlingh Onnes’ notebook showing that the resistance of mercury becomes vanishingly small at roughly 4.2K.

Kamerlingh Onnes’ result holds a special place in the history of the field for two reasons. First, this experiment started a completely new field — low-temperature physics. At the time, Kamerlingh Onnes’ lab was the only place in the world that had liquified helium and his group could uniquely conduct experiments at such low temperatures. He won the initial stage of the low-temperature race that still continues today. Second, the implications of achieving such low temperatures was profound. Kamerlingh Onnes himself understood the importance of his discovery as he details in his Nobel lecture, given in 1913. He realized that cooling matter down to low temperatures would “contribute towards lifting the veil which thermal motion at normal temperature spreads over the inner world of atoms and electrons.” It was probably this sentiment, permitted by his own scientific breakthrough, that, in hindsight, was Kamerlingh Onnes’ most profound gift to science. It is often said that performing experiments under idealized conditions is required not to discover exotic effects, but to strip away complications so that nature can reveal her true self. Kamerlingh Onnes’ achievement is definitely in that spirit.

Landmark experiments in condensed matter physics

When we look at science at a whole, the distinguishing property it possesses compared to other branches of knowledge is its reliance on experiment and observation. Though Galileo wasn’t the first person to emphasize the relationship between our conceptions of the world and experimentation/observation, he was certainly the most influential.

Over the next few weeks and potentially even months, I will write short posts on classic experiments that have been conducted in the field of condensed matter physics (post-19th century). These standout experiments are either important because of the discoveries they made, the spectacular nature of the experiments, or because of the clever methods the experimenters used to tease out an answer to a question. The list I will present is subjective and therefore potentially incomplete. Hence, I urge readers to make suggestions in the comments section concerning experiments they think belong (or perhaps do not belong!) on the list.