# Tag Archives: Experiment

## Coupled and Synchronized Metronomes

A couple years ago, I saw P. Littlewood give a colloquium on exciton-polariton condensation. To introduce the idea, he performed a little experiment, a variation of an experiment first performed and published by Christiaan Huygens. Although he performed it with only two metronomes, below is a video of the same experiment performed with 32 metronomes.

A very important ingredient in getting this to work is the suspended foam underneath the metronomes. In effect, the foam is a field that couples the oscillators.

## Data Representation and Trust

Though popular media often portrays science as purely objective, there are many subjective sides to it as well. One of these is that there is a certain amount of trust we have in our peers that they are telling the truth.

For instance, in most experimental papers, one can only present an illustrative portion of all the data taken because of the sheer volume of data usually acquired. What is presented is supposed to be to a representative sample. However, as readers, we are never sure this is actually the case. We trust that our experimental colleagues have presented the data in a way that is honest, illustrative of all the data taken, and is reproducible under similar conditions. It is increasingly becoming a trend to publish the remaining data in the supplemental section — but the utter amount of data taken can easily overwhelm this section as well.

When writing a paper, an experimentalist also has to make certain choices about how to represent the data. Increasingly, the amount of data at the experimentalist’s disposal means that they often choose to show the data using some sort of color scheme in a contour or color density plot. Just take a flip through Nature Physics, for example, to see how popular this style of data representation has become. Almost every cover of Nature Physics is supplied by this kind of data.

However, there are some dangers that come with color schemes if the colors are not chosen appropriately. There is a great post at medvis.org talking about the ills of using, e.g. the rainbow color scheme, and how misleading it can be in certain circumstances. Make sure to also take a look at the articles cited therein to get a flavor of what these schemes can do. In particular, there is a paper called “Rainbow Map (Still) Considered Harmful”, which has several noteworthy comparisons of different color schemes including ones that are and are not perceptually linear. Take a look at the plots below and compare the different color schemes chosen to represent the same data set (taken from the “Rainbow Map (Still) Considered Harmful” paper):

The rainbow scheme appears to show more drastic gradients in comparison to the other color schemes. My point, though, is that by choosing certain color schemes, an experimentalist can artificially enhance an effect or obscure one he/she does not want the reader to notice.

In fact, the experimentalist makes many choices when publishing a paper — the size of an image, the bounds of the axes, the scale of the axes (e.g. linear vs. log), the outliers omitted, etc.– all of which can have profound effects on the message of the paper. This is why there is an underlying issue of trust that lurks in within the community. We trust that experimentalists choose to exhibit data in an attempt to be as honest as they can be. Of course, there are always subconscious biases lurking when these choices are made. But my hope is that experimentalists are mindful and introspective when representing data, doubting themselves to a healthy extent before publishing results.

To be a part of the scientific community means that, among other things, you are accepted for your honesty and that your work is (hopefully) trustworthy. A breach of this implicit contract is seen as a grave offence and is why cases of misconduct are taken so seriously.

## Drought

Since the discovery of superconductivity, the record transition temperature held by a material has been shattered many times. Here is an excellent plot (taken from here) that shows the critical temperature vs. year of discovery:

Superconducting Transition Temperature vs. Year of Discovery (click to enlarge)

This is a pretty remarkable plot for many reasons. One is the dramatic increase in transition temperature ushered in by the cuprates after approximately 70 years of steady and slow increases in transition temperatures. Another more worrying signature of the plot is that we are currently going through an unprecedented drought (not caused by climate change). The highest transition temperature has not been raised (at ambient pressures) for more than 23 years, the longest in history since the discovery of superconductivity.

It was always going to be difficult to increase the transition temperatures of superconductors once the materials in the  cuprate class were (seemingly) exhausted. It is interesting to see, however, that the mode of discovery has altered markedly compared with years prior. Nowadays, vertical lines are more common, with a subsequent leveling out. Hopefully the vertical line will reach room temperature sooner rather than later. I, personally, hope to still be around when room temperature superconductivity is achieved — it will be an interesting time to be alive.

## Schrodinger’s Cat and Macroscopic Quantum Mechanics

A persisting question that we inherited from the forefathers of the quantum formalism is why quantum mechanics, which works emphatically well on the micro-scale, seem at odds with our intuition at the macro-scale. Intended to demonstrate the absurdity of applying quantum mechanics on the macro-scale, the mirco/macro logical disconnect was famously captured by Schrodinger in his description of a cat being in a superposition of both alive and dead states. There have been many attempts in the theoretical literature to come to grips with this apparent contradiction, the most popular of which goes under the umbrella of decoherence, where interaction with the environment results in a loss of information.

Back in 1999, Arndt, Zellinger and co-workers observed a two-slit interference of C60 molecules (i.e. buckyballs), in what was the largest molecule to exhibit such interference phenomena at the time. The grating used had a period of about 100 nm in the experiment, while the approximate de Broglie wavelength of the C60 molecules was 2.5 picometers. This was a startling discovery for a couple reasons:

1. The beam of C60 molecules used here was far from being perfectly monochromatic. In fact, there was a pretty significant spread of initial velocities, with the full width at half maximum ($\Delta v/v$) getting to be as broad as 60%.
2. The C60 molecules were not in their ground state. The initial beam was prepared by sublimating the molecules in an oven which was heated to 900-1000K. It is estimated, therefore, that there were likely 3 to 4 photons exchanged with the background blackbody field during the beam’s passage through the instrument. Hence the C60 molecules can be said to have been strongly interacting with the environment.
3. The molecule consists of approximately 360 protons, 360 neutrons and 360 electrons (about 720 amu), which means that treating the C60 molecule as a purely classical object would be perfectly adequate for most purposes.

In the present, the record set by the C60 molecule has since been smashed by the larger molecules with mass up to 10,000 amu. This is now within one order of magnitude of a small virus. If I was a betting man, I wouldn’t put money against viruses exhibiting interference effects as well.

This of course raises the question as to how far these experiments can go and to what extent they can be applied to the human scale. Unfortunately, we will probably have to wait for a while to be able to definitively have an answer to that question. However, these experiments are a tour-de-force and make us face some of our deepest discomforts concerning the quantum formalism.

## A First-Rate Experiment: The Damon-Eshbach Mode

One of the things I have tried to do on this blog is highlight excellent experiments in condensed matter physics. You can click the following links for posts I’ve written on illuminating experiments concerning the symmetry of the order parameter in cuprate superconductors, Floquet states on the surface of topological insulators, quantized vortices in superfluid 4He, sonoluminescence in collapsing bubbles and LO-TO splitting in doped semiconductors, just to highlight a few. Some of these experiments required some outstanding technical ingenuity, and I feel it important to document them.

In a similar vein, there was an elegant experiment published in PRL back in 1977 by P. Grunberg and F. Metawe that shows a rather peculiar spectral signature observed with Brillouin scattering in thin film EuO. The data is presented below. For those who don’t know, Brillouin scattering is basically identical to Raman scattering, but the energy scale observed is much lower, typically a fraction of a $cm^{-1}$ ~ 5 $cm^{-1}$ (1 $cm^{-1} \approx$ 30GHz). Brillouin scattering is often used to observe acoustic phonons.

From the image above there is immediately something striking in the data: the peak labeled M2 only shows up on either the anti-Stokes side (the incident light absorbs a thermally excited mode) or the Stokes side (the incident light excites a mode) depending on the orientation of the magnetic field. In his Nobel lecture, Grunberg revealed that they discovered this effect by accident after they had hooked up some wires in the opposite orientation!

Anyway, in usual light scattering experiments, depending on the temperature, modes are observed on both sides (Stokes and anti-Stokes) with an intensity difference determined by Bose-Einstein statistics. In this case, two ingredients, the slab geometry of the thin film and the broken time-reversal symmetry give rise to the propagation of a surface spin wave that travels in only one direction, known as the Damon-Eshbach mode. The DE mode propagates on the surface of the sample in a direction perpendicular to the magnetization, B, of the thin film, obeying a right-hand rule.

When one thinks about this, it is truly bizarre, as the dispersion relation would for the DE mode on the surface would look something like the image below for the different magnetic field directions:

One-way propagation of Damon Eshbach Mode

The dispersion branch only exists for one propagation direction! Because of this fact (and the conservation of momentum and energy laws), the mode is either observed solely on the Stokes or anti-Stokes side. This can be understood in the following way. Suppose the experimental geometry is such that the momentum transferred to the sample, q, is positive. One would then be able to excite the DE mode with the incident photon, giving rise to a peak on the Stokes side. However, the incident photon in the experiment cannot absorb the DE mode of momentum -q, because it doesn’t exist! Similar reasoning applies for the magnetization in the other direction, where one would observe a peak in only the anti-Stokes channel.

There is one more regard in which this experiment relied on a serendipitous occurrence. The thin film was thick enough that the light, which penetrates about 100 Angstroms, did not reach the back side of the film. If the film had been thin enough, a peak would have shown up in both the Stokes and anti-Stokes channels, as the photon would have been able to interact with both surfaces.

So with a little fortune and a lot of ingenuity, this experiment set Peter Grunberg on the path to his Nobel prize winning work on magnetic multilayers. As far as simple spectroscopy experiments go, one is unlikely to find results that are as remarkable and dramatic.