Tuesday, November 13, 2018

Blog stats weirdness

This blog is hosted on blogger, google's free blogging platform.  There are a couple of ways to get statistics about the blog, like rates of visits and where they're from.  One approach is to start from the nanoscale views blogger homepage and click "stats", which can tell me an overview of hit rates, traffic sources, etc.  The other approach is to go to analytics.google.com and look at the more official information compiled by google's tracking code. 

The blogger stats data has always looked weird relative to the analytics information, with "stats" showing far more hits per day - probably tracking every search engine robot that crawls the web, not just real hits.  This is a new one, though:  On "stats" for referring traffic, number one is google, and number three is Peter Woit's blog.  Those both make sense, but in second place there is a site that I didn't recognize, and it appears to be associated with hardcore pornography (!).  That site doesn't show up at all on the analytics page, where number one is google, number two is direct linking, and number three is again Woit's blog.  Weird.  Very likely that this is the result of a script trying to put porn spam in comments on thousands of blogs.  Update:  As I pointed out on social media to some friends, it's not that this blog is porn - it's just that someone somewhere thinks readers of this blog probably like porn.  :-)


Monday, November 12, 2018

Book review: Solid State Insurrection

Apologies for the slow updates.  Between administrative responsibilities and trying to get out a couple of important papers, posting has been a bit slower than I would like, and this is probably going to continue for a few weeks.

If you've wondered how condensed matter physics got to where it is, more in terms of the sociology of physics rather than the particular scientific advances themselves, I strongly recommend Solid State Insurrection:  How the Science of Substance Made American Physics Matter, by Joseph D. Martin.  This book follows the development of condensed matter physics from its beginnings before WWII through to what the author views as the arrival of its modern era, the demise of the Superconducting Supercollider in the early 1990s, an event strongly associated by some with critiques by Phil Anderson.  

I got into condensed matter physics starting in the early 1990s, in the post-"More is Different" era, and CMP had strongly taken on its identity as a field dedicated to understanding the states of matter (and their associated structural, electronic, and magnetic orders) that emerge collectively from the interactions of many underlying degrees of freedom.  While on some level I'd known some of the history, Prof. Martin's book was eye-opening for me, describing how solid-state physics itself emerged from disparate, fluctuating subfields (metallurgy, in particular).   

Martin looks at the battles within the APS and the AIP into the 1940s about whether it's good or bad to have topical groups or divisions; whether it's a good or bad thing that the line between some of solid-state physics and electrical engineering can be blurry; how the societies' publication models could adapt.  Some of that reads a bit like the standard bickering that can happen within any professional society, but the undercurrent throughout is interesting, about the sway held in the postwar era by nuclear and later particle physicists.  

The story of the founding of the National Magnet Lab (originally at MIT, originally funded by the Air Force before switching to NSF) was new to me.  It's an interesting comparison between the struggles to get the NML funded (and how "pure" vs "applied" its mission should be) and the rate at which accelerator and synchrotron and nuclear science facilities were being built.  To what extent did the success of the Manhattan Project give the nuclear/particle community carte blanche from government funders to do "pure" science?  To what degree did the slant toward applications and away from reductionism reinforce the disdain which some held for solid-state (or should I say squalid state or schmutzphysik)?

Martin also presents the formalization of materials science as a discipline and its relationship to physics, the rise of the antireductionist/emergence view of condensed matter (a rebranding that began in the mid-60s and really took off after Anderson's 1972 paper and a coincident NRC report), and a recap of the fight over the SSC along the lines of condensed matter vs. high energy.   (My take:  there were many issues behind the SSC's fate.  The CM community certainly didn't help, but the nature of government contracting, the state of the economy at the time, and other factors were at least as contributory.)

In summary:  Solid State Insurrection is an informative, interesting take on the formation and evolution of condensed matter physics as a discipline.  It shows the very human, social aspects of how scientific communities grow, bicker, and change.



Saturday, November 03, 2018

Timekeeping, or why helium can (temporarily) kill your iphone/ipad

On the day when the US switches clocks back to standard time, here is a post about timekeeping and its impact.  

Conventional computers need a clock, some source of a periodic voltage that tells the microprocessor when to execute logic operations, shift bits in registers, store information in or retrieve information from memory.  

Historically, clocks in computer systems have been based on quartz oscillators or similar devices.  Quartz is an example of a piezoelectric, a material that generates a voltage when strained (or, conversely, deforms when subjected to a properly applied voltage).  Because quartz is a nice material with a well-defined composition, its elastic properties are highly reproducible.  That means that it's possible to carve it into a mechanical resonator (like a tuning fork), and as long as you can control the dimensions well, you will always get very close to the same mechanical resonance frequency.  Pattern electrodes on there, making the quartz into a capacitor, and it's possible to set up an electrical circuit that takes the voltage produced when the quartz is resonantly deforming, amplifies that signal, and feeds it back onto the material, so that the quartz crystal resonator will ring at its natural frequency (just like a microphone pointed at a speaker can lead to a ringing).  Because quartz's elastic and electrical properties depend only weakly on temperature, this can act as a very stable clock, either for a computer like your desktop machine or tablet or smartphone, or in an electric wristwatch.  

In recent years, though, it's become attractive for companies to start replacing quartz clocks with microelectromechanical resonators.  While silicon is not piezoelectric, and so can't be used directly as a substitute for quartz, it does have extremely reproducible elastic properties.  Unlike piezoelectric resonators, though, MEMS resonators typically have to be packaged so that the actual paddle or cantilever or tuning fork is in vacuum.  Gas molecules can damp the resonator, lowering its quality factor and therefore hurting its frequency stability (or possibly damping its motion enough that it just can't function as part of a stable self-resonating circuit).  

The issue that's come up recently (see this neat article) is that too much helium gas in the surrounding air can kill (at least temporarily) iphones and such devices that use these MEMS clocks.  In a helium-rich environment like when filling up superconducting magnets, helium molecules can diffuse through the packaging into the resonator environment.  Whoops.  Assuming the device isn't permanently damaged (I could imagine feedback circuits doing weird things if the damping is way out of whack), the helium has to diffuse out again to resolve the problem.  Neat physics, and something for helium-users to keep in mind. 

Thursday, November 01, 2018

Imposter syndrome

If you're reading this, you've probably heard of imposter syndrome before - that feeling that, deep down, you don't really deserve praise or recognition for your supposed achievements, because you feel like you're not as good at this stuff as your colleagues/competitors, who must really know what they're doing.  As one of my grad school roommates said as a bunch of us were struggling with homework:  "Here we are, students in one of the most prestigious graduate programs in the country.  I sure hope someone knows what they're doing."  

This feeling can be particularly prevalent in fields where there is great currency in the perception of intellectual standing (like academia, especially in science).  My impression is that a large majority of physicists at all levels (faculty, postdocs, grad students, undergrads) experience this to greater or lesser degrees and frequencies.  We're trained to think critically, and driven people tend to overthink things. If you're fighting with something (some homework set, or some experiment, or getting some paper out, or writing a proposal), and your perception is that others around you are succeeding while you feel like you're struggling, it's not surprising that self-doubt can creep in.  

I'm not posting because I've had a great insight into mitigating these feelings (though here are some tips).  I'm posting just to say to readers who feel like that sometimes: you're not alone.   

Wednesday, October 24, 2018

Scalable materials for quantum information

There is no question that the explosive spread of electronics and optoelectronic technology in the 20th century has its foundation in the growth and preparation of high quality materials - silicon with purity better than parts per billion, single crystals cut and polished to near-atomic flatness, with exquisite control of impurity concentrations; III-V compound semiconductors for high speed transistors, LEDs, and lasers; even ultrapure SiO2 for millions of km of ultralow loss optical fiber.

Any new electronics-based technology intended to supplant or supplement now-traditional electronic materials at scale is going to need a material platform that can credibly reach similar quality.  Many of the 2d materials have a long way to go in that regard.  However, there have been recent advances in a couple of specific systems targeted for particular forms of quantum information devices.  

arXiv:1810.09350 - Nelz et al., Towards wafer-scale diamond nano- and quantum technologies
It is possible to grow single-crystal diamond films on the 100 mm wafer scale, starting with Si substrates coated with iridium/yttria-stabilized zirconia.  There are dislocations and stacking faults, but it's getting there.  If the native defect density can be controlled and eliminated to a very fine level, and ion implantation can be used to create well-defined defects (NV centers and the like), that would be a big boost to hopes of wide-spread use and mass fabrication of quantum devices based on these systems.

arXiv:1810.06521 - Sabbagh et al., Wafer-scale silicon for quantum computing
Those who want to use electron spins in Si as quantum bits need to worry about whether nuclear spins from naturally abundant 29Si.  It has now been shown that it is possible to use isotopically enriched silane made from 28Si to grow epitaxial layers of material almost devoid of 29Si, and that MOS devices made from this stuff can be of high quality.  It's worth noting:  Isotope separation of different Si isotopic variants of silane by centrifuge is easier than trying the same thing with, e.g, uranium hexafluoride to enrich 235U, because the percentage mass difference is considerably higher in the Si case.



Sunday, October 14, 2018

Faculty position at Rice - theoretical biological physics

Faculty position in Theoretical Biological Physics at Rice University

As part of the Vision for the Second Century (V2C2), which is focused on investments in research excellence, Rice University seeks faculty members, preferably at the assistant professor level, starting as early as July 1, 2019, in all areas of Theoretical Biological Physics. Successful candidates will lead dynamic, innovative, and independent research programs supported by external funding, and will excel in teaching at the graduate and undergraduate levels, while embracing Rice’s culture of excellence and diversity.  This search will consider applicants from all science and engineering disciplines. Ideal candidates will pursue research with strong intellectual overlap with physics, chemistry, biosciences, bioengineering, chemical and biomolecular engineering, or other related disciplines. Applicants pursuing all styles of theory and computation integrating the physical and life sciences are encouraged to apply.

For full details and to apply, please visit https://jobs.rice.edu/postings/17099.  Applicants should please submit the following materials: (1) cover letter (2) curriculum vitae, (3) research statement, (4) statement of teaching philosophy, and the names and contact information for three references. Application review will commence no later than November 30, 2018 and continue until the positions are filled. Candidates must have a PhD or equivalent degree and outstanding potential in research and teaching. We particularly encourage applications from women and members of historically underrepresented groups who bring diverse cultural experiences and who are especially qualified to mentor and advise members of our diverse student population.

Rice University, located in Houston, Texas, is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability, or protected veteran status.

Friday, October 12, 2018

Short items

A few interesting things I've found this past week:

  • The connection between particle spin and quantum statistics (fermions = half-integer spin, bosons = integer spin) is subtle, as I've mentioned before.  This week I happened upon a neat set of slides (pdf) by Jonathan Bain on this topic.  He looks at how we should think about why a pretty restrictive result from non-interacting relativistic quantum field theories has such profound, general implications.  He has a book on this, too.  
  • There is a new book about the impact of condensed matter physics on the world and why it's the comparatively unsung branch of the discipline.   I have a copy on the way; once I read it I'll post a review.
  • It's also worth reading about why mathematics as a discipline is viewed the way it is culturally.
  • This is a really well-written article about turbulence, and why it's hard even though it's "just \(\mathbf{F} = m\mathbf{a}\)" for little blobs of fluid.
  • Humanoid robots are getting more and more impressive.  I would really like to know the power consumption of one of those, though, given that the old ones used to have either big external power cables or on-board diesel engines.  The robot apocalypse is less scary if they have to recharge every ten minutes of operating time.
  • I always wondered if fidget spinners were good for something.