## Saturday, April 04, 2020

### Brief items

A couple of interesting links:

• From City University of New York, a paper on a bit of the physics relevant to the pandemic - specifically the issue of aerosolized droplets and air circulation in rooms.  The conclusion is that, based on common convection patterns, the best approach to clearing airborne contaminants is a ceiling-mounted suction filter as in surgical operating rooms.  (I suspect that vertical flow ceiling HEPA fan filter units with many air changes per hour as in microfabrication cleanrooms would also work, but it's not like anyone is going to install elevated, gridded flooring everywhere.)  Some of the basic physics of particle suspension is simple enough to teach to high school students, without even getting into viscosity and drag anf real fluid mechanics.  The typical amount of kinetic energy that a would-be suspended particle picks up in collisions with its surroundings is on the order of $k_{\mathrm{B}}T$, or about 26 meV ($4.14 \times 10^{-21}$ J).  For a particle to stay readily suspended, that has to be comparable to the gravitational potential energy that it would cost to elevate the particle by its own typical size.  For a spherical droplet of the density of water, you'd be looking at something like $(4/3)\pi R^{3} \cdot \rho \cdot g \cdot 2R$, where $R$ is the droplet radius, $\rho$ is the density of water, 1000 kg/m3, and $g$ is the gravitational acceleration, 9.807 m/s2.  Setting those equal and solving gives $R \approx 470$ nm.
• The always excellent Natalie Wolchover has a new article in Quanta about how one limiting factor in gravitational interferometers is the quality of the glass used in the dielectric mirrors.  Specifically, the tunneling two-level systems (see here and here) in ordinary amorphous insulating dielectrics at low temperatures are a problem.  It's like I've said ever since my doctoral work:  tunneling two-level systems are everywhere, and they're evil.
• As pointed out by many, this paper has a novel approach to room temperature superconductivity.  This is a bit like my idea of converting my entire lab into ultra-high vacuum workspace.  Sure, personnel would all have to wear special spacesuits, but it would really help preserve samples.
• In these days of social distancing, this was also amusing.
Please stay safe.  I know it's hard to stay positive while all of this is going on, but remember that you're not alone.

## Monday, March 30, 2020

### Phil Anderson and the end of an era

Social media spread the word yesterday evening that Phil Anderson, intellectual giant of condensed matter physics, had passed away at the age of 96.

It is hard to overstate the impact that Anderson had on the field.  In terms of pure scientific results, there are others far more skilled than I who can describe his contributions, but I will mention a few that are well known:

• He developed what is now known as the Anderson model, a theoretical treatment originally intended to capture the essential physics in some transition metal-based magnets.  The model considers comparatively localized d orbitals and includes both hopping to neighboring sites in a lattice as well as the "on-site repulsion" U that makes it energetically expensive to have two electrons (in a spin singlet) on the same site.  This leads to "superexchange" processes, where energetically costly double-occupancy is a virtual intermediate state.  The Anderson model became the basis for many developments - allow coupling between the local sites and delocalized s or p bands, and you get the Kondo model.  Put in coupling to lattice vibrations and you get the Anderson-Holstein model.  Have a lattice and make the on-site repulsion really strong, and you get the Hubbard model famed in correlated electron circles and as the favored treatment of the copper oxide superconductors.
• Anderson also made defining contributions to the theory of localization.  Electrons in solids are wavelike, and in perfect crystal lattices the ones in the conduction and valence bands propagate right past the ions because the waves themselves account for the periodicity of the lattice.  Anderson showed that even in the absence of interactions (the electron-electron repulsion), disorder can scatter those waves, and interference effects can lead to situations where the final result is waves that are exponentially damped with distance.  This is called Anderson localization, and it applies to light and sound as well as electrons.  With strict conditions, this result implies that (ignoring interactions) infinitesimal amounts of disorder can make a 2D electronic system an insulator.
• Here is his Nobel Lecture, by the way, that really focuses on these two topics.
• In considering superconductivity, Anderson also discovered what is now known as the Higgs mechanism, showing that while the bare excitations of some quantum field theory could be massless, coupling those excitations to some scalar field whose particular value broke an underlying symmetry could lead to an effective mass term (in the sense of how momentum and energy relate to each other) for the originally massless degrees of freedom.  Since Anderson himself wrote about this within the last five years, I have nothing to add.
• Anderson also worked on superfluidity in 3He, advancing understanding of this first-discovered non-electronic paired superfluid and its funky properties due to p-wave pairing.
• With the discovery of the copper oxide superconductors, Anderson introduced the resonating valence bond (RVB) model that still shapes discussions of these and exotic spin-liquid systems.
Beyond these and other scientific achievements, Anderson famously articulated a key intellectual selling point of condensed matter physics:  emergent properties from collective actions of large numbers of interacting degrees of freedom can be profound, non-obvious, and contain foundational truths - that reductionism isn't always the path to understanding or "fundamental" insights.  More is different.  He also became a vocal critic about the Superconducting Supercollider.  (For what it's worth, while this certainly didn't help collegiality between high energy and condensed matter physics, there were many factors at play in the demise of the SSC.  Anderson didn't somehow single-handedly kill it.)

Anderson was unquestionably a brilliant person who in many ways defined the modern field of condensed matter physics.  He was intellectually active right up to the end, and he will be missed.  (For one of my own interactions with him, see here.)

## Friday, March 20, 2020

### (Experimentalist) grad students + postdocs in the time of covid-19

As I write this, a very large fraction of the research universities in the US (and much of the world) are either in a shutdown mode or getting there rapidly.  On-campus work is being limited to "essential" operations.  At my institution (and most of the ones I know about), "essential" means (i) research directly related to diagnosing/treating/understanding covid-19; (ii) minimal efforts necessary to keep experimental animals and cell lines going, as the alternative would be years or decades of lost work; (iii) maintenance of critical equipment that will be damaged otherwise; (iv) support for undergraduates unable to get home.

For people in some disciplines, this may not be that disruptive, but for experimentalists (or field researchers), this is an enormous, unplanned break in practice.  Graduate students face uncertainty (even more than usual), and postdocs doubly so (and I haven't seen anything online discussing their situation.  An eight week hitch in the course of a six year PhD is frustrating, but in a limited-duration postdoc opportunity, it's disproportionately worse.  The economics faced by universities and industry will also complicate the job market for a while.), and are often far from their families.

If we'd experienced something like this before, I could offer time-worn wisdom, but we've never had circumstances like this in the modern (post-WWII) research era.  This whole situation feels surreal to me.  Frankly, focusing and concentrating on science and the routine parts of the job have been a challenge, and I figure it has to be worse for people not as ancient established.  Here are a few thoughts, suggestions, and links as we move to get through this:

• While we may be physically socially distancing, please talk with your friends, family, and colleagues, by phone, skype, zoom, slack, wechat, whatever.  Try not to get sucked into the cycle of breaking news and the toxic parts of social media.  Please take advantage of your support structure, and if you need to talk to someone professional, please reach out.  We're in this together - you don't have to face everything by yourself.
• Trying to set up some kind of routine and sticking to it is good.  Faculty I know are trying to come up with ways to keep their folks intellectually engaged - regular group meetings + presentations by zoom; scheduled seminars and discussions via similar video methods across research groups and in some cases even across different universities.  For beginning students, this is a great time to read (really read) the literature and depending on your program, study for your candidacy/qualifier.  Again, you don't have to do this alone; you can team up with partners on this.  For students farther along, data analysis, paper writing, planning the next phase of your research, starting to work on the actual thesis writing, etc. are all possibilities.  For postdocs interested in academia, this is potentially a time to comb the literature and think about what you would like to do as a research program.  Some kind of schedule or plan is the way to divide this into manageable pieces instead of feeling like these are gigantic tasks.
• The Virtual March Meeting has continued to add talks.
• My friend Steve Simon's solid state course lectures are all available.  They go with his book.  They are also just one example of the variety of talks available from Oxford - here are the other physics ones.
• Here is a set of short pieces about topology in condensed matter from a few years ago.
• And here is a KITP workshop on this topic from this past fall.
• These are some very nice lecture notes about scientific computing using python.  Here is something more in-depth on github.  Could be a good time to learn this stuff....
• On the lighter side, here are both PhD Comics movies for free streaming.
Feel free to leave more suggestions and links in the comments.  I'm sure we could all use them.  Stay safe.

## Thursday, March 12, 2020

### Exponentials, extrapolation, and prudence

It's been a remarkable week.  There seems to be a consensus among US universities, based in part on CDC guidelines, and in part on the logistically and legally terrifying possibility of having to deal with dormitories full of quarantined undergraduates, that the rest of the 2019-2020 academic year will be conducted via online methods.  This will be rough, but could well be a watershed moment for distance education techniques.  The companies that make the major software platforms (e.g. zoom, canvas) and their web storage are facing a remarkable trial by fire when the nation's large universities all come back from break and hundreds of thousands of students all try to use these tools at once.

At the same time that all this is going on, many doctoral programs around the country (including ours) that had not already done their graduate recruiting visitations were canceling open houses and trying to put together virtual experiences to do the job.

There is a lot to unpack here, but it's worth asking:  Are people over-reacting?  I don't think so, and over-reacting would be better than the alternative, anyway.  Different estimates give a range of values, but it would appear that the age-averaged mortality rate of covid-19 is somewhere between 0.7% and 3%.  (The current number in the US is something like 2.9%, but that's probably an overestimate due to appallingly too little testing; in the non-Wuhan parts of China it's like 0.6%, but in Italy it's over 3%.)  The disease seems comparable in transmission to the annual influenza, which in the US is estimated to infect 35-40M people every year, and with a mortality rate of around 0.1% leads to something like 35-40K deaths per year.  Given this, it's not unreasonable to think that, unchecked, there could be between 250K and 1.2M deaths from this in the US alone.  A key issue in Italy stems from the hospitalization rate of around 10-15%.  If the cases come too rapidly in time, there just aren't enough hospital beds.  This is why flattening the curve is so important.

It annoys me to see some people whom I generally respect scientifically seem to throw their numerical literacy out the window on this.  We shouldn't freak out and panic, but we should understand the underlying math and assumptions and take this appropriately seriously.

Update:  Notes from a meeting at UCSF (web archive version of link) hosted by, among others, Joe DeRisi.  I first met Joe when we became Packard Fellows back in 2003.  He's a brilliant and very nice guy, who with colleagues created the viral phylogeny chip that identified SARS as a previously unknown coronavirus and pinpointed its closest relatives.

## Friday, March 06, 2020

### More about the APS meeting(s) and covid-19

Just to follow up:

• The APS is partnering with the Virtual March Meeting, as well as collecting talks and slides and linking them to the online meeting program.
• There is going to be a Virtual Science Forum this afternoon (Eastern Standard Time, Friday, March 6) using zoom as a meeting platform, featuring what would have been March Meeting invited talks by Florian Marquardt, Eun-Ah Kim, and Steve Girvin.
• The APS is working on refunds.  All told, the society is going to lose millions of dollars on this.
• I am very surprised that the webpage for the APS April Meeting does not, as of this writing, have anything on it at all about this issue.  I've already passed on my strong suggestion that they at least put up a notice that says "We are closely monitoring the situation and will make a firm decision about the status of the meeting by [date]."
• The ACS has a notice on their page about their national meeting scheduled for Philadelphia on March 22-26.  I'm rather surprised that they are still going ahead. Update:  ACS has now cancelled their spring meeting.
• The MRS seems to have nothing up yet regarding their April meeting.
People tend to have poor intuition about exponential functions.  I'm not an alarmist, but it's important to consider:  total US cases of covid-19 today are the level Wuhan was seven weeks ago. Hopefully measures people are taking (social distancing, hand washing, dropping non-critical travel) plus seasonality of illness plus lower population density plus fewer smokers will help keep things comparatively manageable. The US government realistically will not take some of the steps taken by the Chinese government (e.g., wholesale travel restrictions, military-enforced quarantines).

## Tuesday, March 03, 2020

### Virtual March Meeting

In the wake of the cancellation of the 2020 APS March Meeting due to concerns about COVID-19, an effort has sprung up, the Virtual March Meeting, with the idea of having would-be speakers record and upload their presentations.   (I believe that this was spearheaded by q-ctrl, but I'm not certain.  If someone knowledgeable about this would like to explain in the comments, that would be very helpful.)

In general, this is a great idea.  There were a number of talks, particularly some invited sessions, that I was very much hoping to see at the meeting, and if this is a way of providing access to at least some of that content, I'm all in favor.

There are some downsides.  No interactive Q&A.  Some people are willing to be speculative and show a couple of in-progress/not-yet-submitted slides in their talks, but they are unlikely to want their pre-publication ideas out there on the internet forever.  It seems unlikely that there will be large-scale participation, particularly by the generally busy folks who are giving the longer invited talks and prize talks.  Still, some effort to accommodate limited travel is better than nothing.

I've attempted to upload my own contributed talk, though it doesn't seem to have materialized yet on their siteHere it is.  If you really want to get the March Meeting experience, you should watch this from the back of a small, uncomfortably crowded room with dodgy air temperature and unreliable audio.  Also, you should pretend that the session chair stands up and starts glowering at me on slide 18.   (This is in the spirit of a comment made by a friend who once said that he couldn't make it to Princeton reunions, so instead he was going to simulate the experience by pouring beer and mud in his shoes and squishing around in the humidity.)

## Saturday, February 29, 2020

### APS March Meeting cancelled

Hello all - I have just heard from Dan Arovas, program chair of the APS March Meeting, that the APS has decided to cancel the meeting, which was scheduled to begin tomorrow: "Just finished a Zoom meeting with APS CEO Kate Kirby, APS presidential line, secretary treasurer, counselor. APS is preparing a statement for release to the press. Right now you can help by informing all your students, postdocs, and colleagues. The web site will be updated as soon as possible."

This is a response to COVID19. As I post this, the meeting website has not yet been updated.  I will post more when I learn more.

Update: The text of the APS email: "Due to rapidly escalating health concerns relating to the spread of the coronavirus disease (COVID-19), the 2020 APS March Meeting in Denver, CO, has been canceled. Please do not travel to Denver to attend the March Meeting. More information will follow shortly."

Update: APS website now confirms.

Update: Here is the text of the letter from the APS president and CEO about the decision.
To the Board, the Council and Unit Leaders of APS:
You have probably already heard that on Saturday, February 29, the APS Leadership decided to cancel the 2020 March Meeting in Denver. We are writing to give you some of the details that led to this difficult decision, which was made in consultation with the APS senior management and the March Meeting program chair.
APS leadership has been monitoring the spread of the coronavirus disease (COVID-19) in the days leading up to the meeting. As you know, a large number of March Meeting attendees come from outside the US. Many have already canceled their attendance, particularly those from China, where travel to the meeting is not currently possible. In addition, we had many planning to come from countries where the CDC has upgraded its warning to level 3 as recently as the day of our decision, yesterday February 29. Even more were coming from countries where the virus appears to be establishing itself in the general population, so that the warning level could rise during the course of our meeting, which might significantly delay their return travel or even lead to quarantines.
In this case the safety of the attendees has to be a primary concern. There is a reasonable expectation that in a meeting with many thousands of participants, some will fall ill. This always happens of course, but it presently takes some time to establish whether an illness is seasonal flu or COVID-19, and many attendees who have come into contact might need to be quarantined during the testing. In light of this danger, we realized that ordinary social events such as the evening receptions would have to be cancelled out of caution.
We appreciate the high cost of our decision, both for the APS and also the attendees. We don’t know the actual loss yet, but the APS portion alone is certain to be in the millions of dollars. We want to assure the APS Board, Council, and Unit Leaders, that we have considered this carefully. Our society is strong financially, and we can absorb this loss. The welfare of our community is certainly a greater concern.
We know you have many questions about the path forward following this decision. We will continue to communicate and confer with you regularly in the coming weeks, as we all come to terms with the need to find new ways to maintain strong international science contacts.
Phil Bucksbaum, APS President
Kate Kirby, APS CEO

## Monday, February 24, 2020

### BAHFest 2020 at Rice, Sunday March 8 UPDATE: postponed.

Update:  This event is going to be postponed until the fall semester.

For those in the Houston area:

Spread the word -

BAHFest is a celebration of well-argued and thoroughly researched but completely incorrect scientific theory. Our brave speakers present their bad theories in front of a live audience and a panel of judges with real science credentials, who together determine who takes home the coveted BAHFest trophy. And eternal glory, of course. If you'd like to learn more about the event, you can check out these articles from the Wall Street Journal and NPR's Science Friday

Here are some examples from past shows:

Our keynote for this year's event is the hilarious Phil Plait (AKA the Bad Astronomer)! Phil will be doing a book signing of his book "Death from the Skies" before and after the show.

The event is brought to you by BAHFest, and the graduate students in Rice University's Department of BioSciences. Click here for more information about the show, including how to purchase tickets. We hope to see you there!

[Full disclosure:  I am one of the judges at this year's event.]

## Saturday, February 22, 2020

### Brief items

As we head out of a very intense week here and toward the March APS meeting, a few brief items:

• Speaking of the March Meeting, I hear (unofficially) that travel restrictions due to the coronavirus have made a big dent - over 500 talks may be vacant, and the program committee is working hard to explore options for remote presentation.  (For the record, I fully endorse the suggestion that all vacant talks be delivered in the form of interpretive dance by Greg Boebinger.)
• There will be many talks about twisted bilayers of various 2D materials at the meeting, and on that note, this PRL (arxiv version here) shows evidence of "strange metallicity" in magic-angle bilayer graphene at temperatures above the correlated insulator state(s).
• Following indirectly on my post about condensed matter and Christmas lights, I want to point out another example of how condensed matter physics (in the form of semiconductor physics and the light emitting diode) has changed the world for the better in ways that could never have been anticipated.  This video shows and this article discusses the new film-making technique pioneered in the making of The Mandalorian.  Thanks to the development of organic LED displays, infrared LEDs for motion tracking, and lots of processing power, it is possible to create a floor-to-ceiling wraparound high definition electronic backdrop.  It's reconfigurable in real time, produces realistic lighting on the actors and props, and will make a lot of green screen compositing obsolete.  Condensed matter:  This is The Way.
• Superconducting gravimeters have been used to check to see if there are compact objects (e.g., hunks of dark matter, or perhaps microscopic black holes) orbiting inside the earth.  I remember reading about this issue while in college.  Wild creative idea of the day:  Maybe we should use helioseismology to try to infer whether there are any such objects orbiting inside the sun....

## Thursday, February 13, 2020

### Film boiling and the Leidenfrost point

While setting up my eddy current bounce demonstration, I was able to film some other fun physics.

Heat transfer and two-phase (liquid+gas) fluid flow is a complicated business that has occupied the time of many scientists and engineers for decades.  A liquid that is boiling at a given pressure is pinned to a particular temperature - that's the way the first-order liquid-vapor transition works.  Water at atmospheric pressure boils at 100 C; adding energy to the liquid water at 100 C via heat transfer converts water into vapor rather than increasing the temperature of the liquid.

Here we are using liquid nitrogen (LN2), which boils at 77 K = -196 C at atmospheric pressure, and are trying to cool a piece of copper plate that initially started out much warmer than that.  When the temperature difference between the copper and the LN2 is sufficiently high, there is a large heat flux that creates a layer of nitrogen vapor between the copper and the liquid.  This is called film boiling.   You've seen this in practice if you've ever put a droplet of water into a really hot skillet, or dumped some LN2 on the floor.  The droplet slides around with very low friction because it is supported by that vapor layer.

Once the temperature difference between the copper and the LN2 becomes small, the heat flux is no longer sufficient to support film boiling (the Leidenfrost point), and the vapor layer collapses - that brings more liquid into direct contact with the copper, leading to more vigorous boiling and agitation.  That happens at about 45 seconds into the video.  Then, once the copper is finally at the same temperature as the liquid, boiling ceases and everything gets calm.

For a more technical discussion of this, see here.  It's written up on a site about nuclear power because water-based heat exchangers are a key component of multiple power generation technologies.

## Tuesday, February 11, 2020

### Eddy currents - bouncing a magnet in mid-air

Changing a magnetic field that permeates a conductor like a metal will generate eddy currents.  This is called induction, and it was discovered by Michael Faraday nearly 200 years ago.   If you move a ferromagnet near a conductor, the changing field produces eddy currents and those eddy currents create their own magnetic fields, exerting forces back on the magnet.  Here is a rather dramatic demo of this phenomenon, shamelessly stolen by me from my thesis adviser.

In the video, you can watch in slow motion as I drop a strong NdFe14B2 magnet from about 15 cm above a 2 cm thick copper plate.  The plate is oxygen-free, high-purity copper, and it has been cooled to liquid nitrogen temperatures (77 K = -196 C).   That cooling suppresses lattice vibrations and increases the conductivity of the copper by around a factor of 20 compared with room temperature.  (If cooled to liquid helium temperatures, 4.2 K, the conductivity of this kind of copper goes up to something like 200 times its room temperature value, and is limited by residual scattering from crystalline grain boundaries and impurities.)

As the magnet falls, the magnetic flux $\Phi$ through the copper increases, generating a circumferential electromotive force and driving eddy currents.  Those eddy currents produce a magnetic field directed to repel the falling magnet.  The currents become large enough that the resulting upward force becomes strong enough to bring the magnet to a halt about 2 cm above the copper (!).  At that instant, $d\Phi/dt = 0$, so the inductive EMF is zero.  However, the existing currents keep going because of the inductance of the copper.  (Treating the metal like an inductor-resistor circuit, the timescale for the current to decay is $L/R$, and $R$ is quite small.)  Those continuing currents generate magnetic fields that keep pushing up on the magnet, making it continue to accelerate upward.  The magnet bounces "in mid air".  Of course, the copper isn't a perfect conductor, so much of the energy is "lost" to resistively heating the copper, and the magnet gradually settles onto the plate.  If you try this at room temperature, the magnet clunks into the copper, because the copper conductivity is worse and the eddy currents decay so rapidly that the repulsive force is insufficient to bounce the magnet before it hits the plate.

(Later I'll make a follow-up post about other neat physics that happens while setting up this demo.)

## Sunday, February 09, 2020

### Updated: Advice on choosing a grad school

I realized it's been several years since I've run a version of this, and it's the right season....

This is written on the assumption that you have already decided, after careful consideration, that you want to get an advanced degree (in physics, though much of this applies to any other science or engineering discipline).  This might mean that you are thinking about going into academia, or it might mean that you realize such a degree will help prepare you for a higher paying technical job outside academia.  Either way,  I'm not trying to argue the merits of a graduate degree.
• It's ok at the applicant stage not to know exactly what you want to do.  While some prospective grad students are completely sure of their interests, that's more the exception than the rule.  I do think it's good to have narrowed things down a bit, though.  If a school asks for your area of interest from among some palette of choices, try to pick one (rather than going with "undecided").  We all know that this represents a best estimate, not a rigid commitment.
• If you get the opportunity to visit a school, you should go.  A visit gives you a chance to see a place, get a subconscious sense of the environment (a "gut" reaction), and most importantly, an opportunity to talk to current graduate students.  Always talk to current graduate students if you get the chance - they're the ones who really know the score.  A professor should always be able to make their work sound interesting, but grad students can tell you what a place is really like.
• International students may have a very challenging time being able to visit schools in the US, between the expense (many schools can help defray costs a little but cannot afford to pay for airfare for trans-oceanic travel) and visa challenges.  Trying to arrange skype discussions with people at the school is a possibility, but that can also be challenging.  I understand that this constraint tends to push international students toward making decisions based heavily on reputation rather than up-close information.
• Always go someplace where there is more than one faculty member with whom you might want to work.  Even if you are 100% certain that you want to work with Prof. Smith, and that the feeling is mutual, you never know what could happen, in terms of money, circumstances, etc.  Moreover, in grad school you will learn a lot from your fellow students and other faculty.  An institution with many interesting things happening will be a more stimulating intellectual environment, and that's not a small issue.
• You should not go to grad school because you're not sure what else to do with yourself.  You should not go into research if you will only be satisfied by a Nobel Prize.  In both of those cases, you are likely to be unhappy during grad school.
• I know grad student stipends are low, believe me.  However, it's a bad idea to make a grad school decision based purely on a financial difference of a few hundred or a thousand dollars a year.  Different places have vastly different costs of living - look into this.  Stanford's stipends are profoundly affected by the cost of housing near Palo Alto and are not an expression of generosity.  Pick a place for the right reasons.
• Likewise, while everyone wants a pleasant environment, picking a grad school largely based on the weather is silly.
• Pursue external fellowships if given the opportunity.  It's always nice to have your own money and not be tied strongly to the funding constraints of the faculty, if possible.  (It's been brought to my attention that at some public institutions the kind of health insurance you get can be complicated by such fellowships.  In general, I still think fellowships are very good if you can get them.)
• Be mindful of how departments and programs are run.  Is the program well organized?  What is a reasonable timetable for progress?  How are advisors selected, and when does that happen?  Who sets the stipends?  What are TA duties and expectations like?  Are there qualifying exams?  Where have graduates of that department gone after the degree?  Know what you're getting into!  Very often, information like this is available now in downloadable graduate program handbooks linked from program webpages.
• It's fine to try to communicate with professors at all stages of the process.  We'd much rather have you ask questions than the alternative.  If you don't get a quick response to an email, it's almost certainly due to busy-ness, and not a deeply meaningful decision by the faculty member.  For a sense of perspective:  even before I was chair, I would get 50+ emails per day of various kinds not counting all the obvious spam that gets filtered.
There is no question that far more information is now available to would-be graduate students than at any time in the past.  Use it.  Look at departmental web pages, look at individual faculty member web pages.  Make an informed decision.  Good luck!

## Wednesday, January 29, 2020

### Charles Lieber

As one of the only surviving nano-related blogs, I feel somewhat obligated to write a post about this.  Charles Lieber, chair of the department of chemistry and chemical biology at Harvard, was arrested yesterday by the FBI on charges of fraud.  Lieber is one of the premier nano researchers in the world.  The relevant documents are here (pdf) and they make for quite a read.

In brief, Lieber is alleged to have signed on to China's Thousand Talents program with an affiliation at Wuhan University of Technology back in 2011.  This involved the setting up of a joint research lab in Wuhan and regular interactions, including WUT students to come to Harvard.  That in itself is not necessarily problematic.  Much more concerning is the claim that WUT would pay \$50K/month (plus living expenses) for his involvement, and the stipulation in the agreement that he would be working at least nine months/yr with them.  That alone would raise serious conflict-of-commitment and percentage-effort issues.  Worse is the allegation that this went on for years, none of this was disclosed appropriately, and in fact was denied to both DOD and (via Harvard internal folks) NIH.

These allegations are shocking, and the story is hard to fathom for multiple reasons.

Putting on my department chair hat, I can't help but think about how absolutely disruptive this will be for his students and postdocs, since he was placed on immediate leave.  It will be a nontrivial task for the department and the Faculty of Arts and Sciences at Harvard to come up with a way to transition the students to other advising and pay circumstances, and even more challenging for the postdocs.  What a mess.

## Wednesday, January 22, 2020

### Stretchy bioelectronics and ptychographic imaging - two fun talks

One of the great things about a good university is the variety of excellent talks that you can see.

Yesterday we had our annual Chapman Lecture on Nanotechnology, in honor of Rice alum Richard Chapman, who turned down a first-round draft selection to the Detroit Lions to pursue a physics PhD and a career in engineering.  This year's speaker was Zhenan Bao from Stanford, whom I know from back in my Bell Labs postdoc days.  She spoke about her group's remarkable work on artificial skin:  biocompatible, ultraflexible electronics including active matrices of touch sensors, transistors, etc.  Here are a few representative papers that give you some idea of the kind of work that goes into this: Engineering semiconducting polymers to have robust elastic properties while retaining high charge mobilities; a way of combining conducting polymers (PEDOT) with hydrogels so that you can pattern them and then hydrate to produce super-soft devices; a full-on demonstration of artificial skin for sensing applications.  Very impressive stuff.

Today, we had a colloquium by Gabe Aeppli of ETH and the Paul Scherrer Institute, talking about x-ray ptychographic imaging.  Ptychography is a simple enough idea.  Use a coherent source of radiation to illuminate some sample at some spot, and with a large-area detector, measure the diffraction pattern.  Now scan the spot over the sample (including perhaps rotating the sample) and record all those diffraction patterns as well.  With the right approach, you can combine all of those diffraction patterns and invert to get the spatial distribution of the scatterers (that is, the matter in the sample).  Sounds reasonable, but these folks have taken it to the next level (pdf here).   The video I'm embedding here is the old result from 2017.  The 2019 paper I linked here is even more impressive, able to image, nondestructively, in 3D, individual circuit elements within a commercial integrated circuit at nanoscale resolution.  It's clear that a long-term goal is to be able to image, non-destructively, the connectome of brains.

## Monday, January 20, 2020

### Brief items

Here are some items of interest:

• An attempt to lay out a vision for research in the US beyond Science: The Endless Frontier.  The evolving roles of the national academies are interesting, though I found the description of the future of research universities to be rather vague - I'm not sure growing universities to the size of Arizona State is the best way to provide high quality access to knowledge for a large population.  It still feels to me like an eventual successful endpoint for online education could be natural language individualized tutoring ("Alexa, teach me multivariable calculus."), but we are still a long way from there.
• Atomic-resolution movies of chemistry are still cool.
• Dan Ralph at Cornell has done a nice service to the community by making his lecture notes available on the arxiv.  The intent is for these to serve as a supplement to a solid state course such as one out of Ashcroft and Mermin, bringing students up to date about Berry curvature and topology at a similar level to that famous text.
• This preprint tries to understand an extremely early color photography process developed by Becquerel (the photovoltaic one, who was the father of the radioactivity Becquerel).  It turns out that there are systematic changes in reflectivity spectra of the exposed Ag/AgCl films depending on the incident wavelength.  Why the reflectivity changes that way remains a mystery to me after reading this.
• On a related note, this led me to this PNAS paper about the role of plasmons in the daguerreotype process.  Voila, nanophotonics in the 19th century.
• This preprint (now out in Nature Nano) demonstrates incredibly sensitive measurements of torques on very rapidly rotating dielectric nanoparticles.  This could be used to see vacuum rotational friction.
• The inventors of chemically amplified photoresists have been awarded the Charles Stark Draper prize.  Without that research, you probably would not have the computing device sitting in front of you....

## Tuesday, January 14, 2020

### The Wolf Prize and how condensed matter physics works

The Wolf Prize in Physics for 2020 was announced yesterday, and it's going to Pablo Jarillo-Herrero, Allan MacDonald, and Rafi Bistritzer, for twisted bilayer graphene.  This prize is both well-deserved and a great example of how condensed matter physics works.

MacDonald and Bistritzer did key theory work (for example) highlighting how the band structure of twisted bilayer graphene would become very interesting for certain twist angles - how the moire pattern from the two layers would produce a lateral periodicity, and that interactions between the layers would lead to very flat bands.  Did they predict every exotic thing that has been seen in this system?  No, but they had the insight to get key elements, and the knowledge that flat bands would likely lead to many competing energy scales, including electron-electron interactions, the weak kinetic energy of the flat bands, the interlayer coupling, effective magnetic interactions, etc.  Jarillo-Herrero was the first to implement this with sufficient control and sample quality to uncover a remarkable phase diagram involving superconductivity and correlated insulating states.  Figuring out what is really going on here and looking at all the possibilities in related layered materials will keep people busy for years.   (As an added example of how condensed matter works as a field, Bistritzer is in industry working for Applied Materials.)

All of this activity and excitement, thanks to feedback between well-motivated theory and experiment, is how the bulk of physics that isn't "high energy theory" actually works.

## Monday, January 13, 2020

### Popular treatment of condensed matter - topics

I'm looking more seriously at trying to do some popularly accessible writing about condensed matter.  I have a number of ideas about what should be included in such a work, but I'm always interested in other peoples' thoughts on this.   Suggestions?

## Sunday, January 05, 2020

### Brief items

Happy new year.  As we head into 2020, here are a few links I've been meaning to point out:

• This paper is a topical review of high-throughput (sometimes called combinatorial) approaches to searching for new superconductors.   The basic concept is simple enough:  co-deposit multiple different elements in a way that deliberately produces compositional gradients across the target substrate.  This can be done via geometry of deposition, or with stencils that move during the deposition process.  Then characterize the local properties in an efficient way across the various compositional gradients, looking for the target properties you want (e.g., maximum superconducting transition temperature).  Ideally, you combine this with high-throughput structural characterization and even annealing or other post-deposition treatment.  Doing all of this well in practice is a craft.
• Calling back to my post on this topic, Scientific American has an article about wealth distribution based on statistical mechanics-like models of economies.   It's hard for me to believe that some of these insights are really "new" - seems like many of these models could have been examined decades ago....
• This is impressive.  Jason Petta's group at Princeton has demonstrated controlled entanglement between single-electron spins in Si/SiGe gate-defined quantum dots separated by 4 mm.  That may not sound all that exciting; one could use photons to entangle atoms separated by km, as has been done with optical fiber.  However, doing this on-chip using engineered quantum dots (with gates for tunable control) in an arrangement that is in principle scalable via microfabrication techniques is a major achievement.
• Just in case you needed another demonstration that correlated materials like the copper oxide superconductors are complicated, here you go.  These investigators use an approach based on density functional theory (see here, here, and here), and end up worrying about energetic competition between 26 different electronic/magnetic phases.  Regardless of the robustness of their specific conclusions, just that tells you the inherent challenge of those systems:  Many possible ordered states all with very similar energy scales.