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 ~ 5 (1 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.

Flash Boys: A Few Lessons

In the past couple weeks, I was able to get my hands on and read (finally!) Flash Boys by Michael Lewis. It tells the story of a few honest guys that try to stir up the way business is done on Wall Street, with the main protagonist being Brad Katsuyama, a former employee at the Royal Bank of Canada. There are some startling revelations in this book, some of which are relevant to physicists that go onto work on Wall Street, and some that apply more generally.

During my time in graduate school, I saw a fair share of theoretically-trained physicists (that tended to be quite computationally proficient) go onto work at high-frequency trading (HFT) and investment banking firms. I don’t see this as necessarily a negative trend (especially for those that are working in investment banks rather than HFT firms), but this largely depends on the roles the physicists are hired to fill. In speaking to the physicists who have gone onto work on Wall Street, many of them have been attracted by the interesting puzzles/problems they are given to solve.

One of the main themes of the book is that the physicists, mathematicians and other STEM PhDs that work on Wall Street are often prevented from understanding their own roles within their companies. What I mean by this is that upper management in many Wall Street companies actively try to impede people with a more technical leaning from gaining a broad overview of the firm’s intentions and its role in the economy as a whole. The PhDs are hired to solve puzzles, not to understand the meaning of the puzzles they are solving. Indeed, many STEM PhDs are not even interested in knowing the consequences of the problems they are solving. This is just one of the parts of the book that I found to be particularly disturbing.

For those STEM PhDs that are thinking of going to work on Wall Street, Flash Boys is one of the most insightful and accessible reads one is likely to find. In stark contrast to the management at many of these firms, the book seeks to provide one with an overview of what has occurred on Wall Street since 2007. In it, Lewis describes the reasons behind the rise of dark pools and other public stock exchanges (i.e. the fragmentation of trading sites), why optical fibers that connect, e.g. Chicago exchanges to New York exchanges, are of immense value to HFT firms, how HFT firms essentially provide an unwanted tax to investors in the American stock market, and how investment banks’ (e.g. Goldman Sachs’) incentives don’t always align with those of their clients.

Since the writing of the book, things have started to change somewhat on Wall Street. Brad Katsuyama and his team have opened up the IEX (Investors Exchange), which seeks to prevent high-frequency traders from teasing out information about investment strategies employed by mutual funds, hedge funds, and individuals who invest from home. (This information can be used by HFT firms to front-run.) Even as things change, the book is without a doubt still very relevant today and is highly recommended, especially for those seeking a job on Wall Street.

On a more general level, one of the lessons I took from the book was about the need for introspection. It is sometimes necessary to ask oneself questions such as:

1. What are the broader consequences of my work?
2. What are the possible unintended consequences?
3. What are the societal impacts?
4. Are these consequences long or short term?

Even though we choose to pursue the seemingly singular goal of scientific knowledge and understanding, we do have a role to play in the broader society as well.