Monthly Archives: April 2016


Recently, I was invited to sign up for SciPost, an online platform similar to the arXiv. However, the major difference is that SciPost is creating a suite of free and open-access peer-reviewed online journals. Moreover, copyrights will be held by the authors of the papers, and not by publishers.

Publications will be free for both authors and readers. The journal articles will be completely open to everyone.

To be honest, such a platform has probably been a long time coming for our community. The FAQ page on the website states that SciPost is launching because:

The publishing landscape is evolving rapidly, and it is not clear that the best interests of science and scientists are being represented. SciPost offers a grassroots solution to the problem of scientific publishing, designed and implemented by scientists in the best interests of science itself.

SciPost is open for submissions starting June 2016. I sincerely hope that those in charge of SciPost have it running smoothly by then and that it reaches the critical mass to be successful. Good luck to the team and particularly J.-S. Caux, the condensed matter theorist who started this endeavor.

Loudspeaker Crossover

The goal of an audio speaker is to reproduce the sound of the input as best as possible. This may sound like a simple statement, but it is a notoriously difficult one to engineer. Furthermore, sound to peoples’ ears is subjective, which only serves to complicate this task.

Today, with currently available technology, it is not possible to reproduce a flat frequency response across the entire spectrum of sound within the human-audible regime (i.e. ~20Hz to ~20kHz) with a single driving cone. This is why a decent set of speakers usually comprise two or more components, for example, a sub-woofer and a tweeter. The sub-woofer plays low-frequency sound, while the tweeter plays the high-frequency sound, which you would probably guess just from the onomatopoeic value of their names. Here is an example of a sound pressure level vs. frequency graph for this sub-woofer:


Black Line: Sound Pressure Level in dB vs. Frequency

You can see that the sound level dies off dramatically at higher frequencies, which is why we need the tweeter. For those that are curious, the plots like the one above are usually obtained by hanging a microphone in front of the speaker cone in an anechoic chamber and sweeping through the frequencies with a frequency generator. Due to the directional nature of sound from a speaker, the microphone is hung directly in front of the cone at a distance of 1 meter.

Because sound has to be routed through two separate cones, the speaker manufacturer has to make an electronics decision about how to do this. The electronics components that are used to do this are usually referred to as the “crossover circuit”. This is really just a fancy name for a high- and low-pass filter (or band-pass filter if one is also using a mid-range speaker). Crossover circuits tend to come in two varieties: active and passive. Active crossovers usually use operational amplifiers (or op-amps) to make the filters (which require the use of external power), while passive crossovers use inductors and capacitors (and do not need to be plugged into the wall).

For the sake of simplicity, let’s suppose that we have both 8 Ohm tweeters and sub-woofers for our speaker system. Suppose we want our cross-over frequency to be 500Hz. We can make the world’s simplest high-pass filter like so for the tweeter:


High-Pass Filter

So what would the capacitance need to be? Well, simply enough, it’s just the cut-off frequency of an RC circuit. In this case, for the 500 Hz crossover, we can calculate it as so:

f_c = \frac{1}{2 \pi R*C} = \frac{1}{2 \pi *8*C}

Solving for C with f_c = 500 Hz gives 39.79\mu F.

For the sub-woofer’s low-pass filter, we would replace the capacitor with the inductor and the equation would be f_c = \frac{L}{2 \pi R}. I should mention that these simple circuits also give one a 6dB/octave rolloff. This point is explained well on Wikipedia, so I don’t think I need to repeat it here.

The stunning thing about this simple passive crossover is that manufacturers can make a set of pretty high-end speakers with just these circuits. Of course, one would have to use some reasonable capacitors and inductors — but it can be that simple.

Manufacturers have many other design considerations besides the electronics to make as well (such as the enclosure!), but I hope this helps you understand the basics of what’s inside your speaker box.

Is it really as bad as they say?

It’s been a little while since I attended A.J. Leggett’s March Meeting talk (see my review of it here), and some part of that talk still irks me. It is the portion where he referred to “the scourge of bibliometrics”, and how it prevents one from thinking about long-term problems.

I am not old enough to know what science was like when he was a graduate student or a young lecturer, but it seems like something was fundamentally different back then. The only evidence that I can present is the word of other scientists who lived through the same time period and witnessed the transformation (there seems to be a dearth of historical work on this issue).


It was easy for me to find articles corroborating Leggett’s views, unsurprisingly I suppose. In addition to the article I linked last week by P. Nozieres, I found interviews with Sydney Brenner and Peter Higgs, and a damning article by P.W. Anderson in his book More and Different entitled Could Modern America Have Invented Wave Mechanics? In his opinion piece, Anderson also refers to an article by L. Kadanoff expressing a similar sentiment, which I was not able to find online (please let me know if you find it, and I’ll link it here!). The conditions described at Bell Labs in David Gertner’s book The Idea Factory also paint a rather stark contrast to the present status of condensed matter physics.

Since I wasn’t alive back then, I really cannot know with any great certainty whether the current state of affairs has impeded me from pursuing a longer-term project or thinking about more fundamental problems in physics. I can only speak for myself, and at present I can openly admit that I am incentivized to work on problems that I can solve in 2-3 years. I do have some concrete ideas for longer-term projects in mind, but I cannot pursue these at the present time because, as an experimentalist and postdoc, I do not have the resources nor the permanent setting in which to complete this work.

While the above anecdote is personal and it may corroborate the viewpoints of the aforementioned scientists, I don’t necessarily perceive all these items as purely negative. I think it is important to publish a paper based on one’s graduate work. It should be something, however small, that no one has done before. It is important to be able to communicate with the scientific community through a technical paper — writing is an important part of science. I also don’t mind spending a few years (not more than four, hopefully!) as a postdoc, where I will pick up a few more tools to add to my current arsenal. This is something that Sydney Brenner, in particular, decried in his interview. However, it is likely that most of what was said in these articles was aimed at junior faculty.

Ultimately, the opinions expressed by these authors is concerning. However, I am uncertain as to the extent to which what is said is exaggeration and the extent to which it is true. Reading these articles has made me ask how the scientific environment I was trained in (US universities) has shaped my attitude and scientific outlook.

One thing is undoubtedly true, though. If one chooses to resist the publish-or-perish trend by working on long-term problems and not publishing, the likelihood of landing an academic job is close to null. Perhaps this is the most damning consequence. Nevertheless, there is still some outstanding experimental and theoretical science done today, some of it very fundamental, so one should not lose all hope.

Again, I haven’t lived through this academic transformation, so if anyone has any insight concerning these issues, please feel free to comment.

What’s NDT Been Up To?

Readers of this blog will know that I’m a big fan of what Neil DeGrasse Tyson does for science in the public eye. Recently, he sent out a couple tweets, which I thought were hilarious that I thought I’d share here as well. I hope you enjoy these as much as I did!



Rather than being linear, the historical progression of topics in physics sometimes takes a tortuous route. There are two Annual Reviews of Condensed Matter Physics articles, one by P. Nozieres and one by M. Dresselhaus, that describe how widespread interest on certain subjects in the study of condensed matter were affected by timing.

In the article by Dresselhaus, she notes that HP Boehm and co-workers had actually isolated monolayer graphene back in 1962 (pdf!, and in German). On the theoretical front, P. Nozieres says in his article:

But neither I nor any of these famous people ever suspected what was hiding behind that linear dispersion. Fifty years later, graphene became a frontier of physics with far-reaching quantum effects.

Dresselhaus also mentions that carbon nanotubes were observed in 1952 in Russia followed by another reported discovery in the 1970s by M. Endo. These reports occurred well before its rediscovery in 1991 by Iijima that sparked a wealth of studies. The controversy over the discovery of nanotubes actually seems to date back even further, perhaps even to 1889 (pdf)!

In the field of topological insulators, again there seems to have been an oversight from the greater condensed matter physics community. As early as 1985, in the Soviet journal JETP, B.A. Volhov and O.A. Pankratov discussed the possibility of Dirac electrons at the surface between a normal band-gap semiconductor and an “inverted” band-gap semiconductor (pdf). Startlingly, the authors suggest CdHgTe and PbSnSe as materials in which to investigate the possibility. A HgTe/(Hg,Cd)Te quantum well hosted the first definitive observation of the quantum spin hall effect, while the Pb_{1-x}Sn_xSe system was later found to be a topological crystalline insulator.

One can probably find many more examples of historical inattention if one were to do a thorough study. One also wonders what other kinds of gems are hidden within the vastness of the scientific literature. P. Nozieres notes that perhaps the timing of these discoveries has something to do with why these initial discoveries went relatively unnoticed:

When a problem is not ripe you simply do not see it.

I don’t know how one quantifies “ripeness”, but he seems to be suggesting that the perceived importance of scientific works are correlated in some way to the scientific zeitgeist. In this vein, it is amusing to think about what would have happened had one discovered, say, topological insulators in Newton’s time. In all likelihood, no one would have paid the slightest attention.

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.

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.