## Thursday, February 26, 2015

### Brief items + the March APS Meeting

This has been an absurdly busy period for the last few weeks; hence my lower rate of posting.  I hope that this will resolve itself relatively soon, but you never know.  I am going to the first three days of the March APS meeting, and will try to blog about what I see and learn there, as I have in past years.

In the meantime, a handful of items that have cropped up:
• If you go to the APS meeting, you can swing by the Cambridge University Press table, and pre-order my nano textbook for a mere \$64.  It's more than 600 pages with color figures - that's a pretty good deal.  They will have a couple of bound proof copies, so you can see what it looks like, to a good approximation.  If you teach a senior undergrad or first-year grad sequence on this stuff and think you might have an interest in trying this out as a text, please drop me an email and I can see about getting you a copy.  (My editor tells me that the best way to boost readership of the book is to get a decent number of [hopefully positive] reviews on Amazon....)
• On a related note, you should really swing by the Cambridge table to order yourself a copy of the 19-years-in-the-making third edition of Horowitz and Hill's Art of Electronics.  I haven't seen it yet, but I have every reason to think that it's going to be absolutely fantastic.  Seriously, from the experimental physics side, this is a huge deal.
• This is a fun video, showing a "motor" made from an alkaline battery, a couple of metal-coated rare-earth magnets, and a coil of uninsulated wire.  It's not that crazy to see broadly how it works (think inhomogeneous fields from a finite solenoid + large magnetic moment), but it's cool nonetheless.
• Here's an article (pdf) that's very much worth reading about the importance of government funding of basic research.  It was favorably referenced here by that (sarcasm mode = on) notorious socialist organization (/sarcasm), the American Enterprise Institute.

## Monday, February 16, 2015

### Centers, institutes, and all that

One hallmark of the modern research university is the proliferation of Centers and Institutes, groupings of investigators outside the hierarchy of traditional academic departments.  I'd like to explain a bit about what these entities are, what (in my view) makes an effective one, and some challenges that these organizations face.  There is a great deal of variance across universities with these terms; the version I'm going to describe is mostly what we have at my home institution.  Your mileage may vary, and I'd like to hear your thoughts on what makes a great center or institute.

An Institute is an organization that draws members from across different departments (indeed, often from across different Schools such as Natural Science and Engineering), with a strong, usually broad, thematic focus, and with an annual budget for staff and programs that comes largely from internal university funds.  Institutes support programs that benefit their membership.  Examples of programs include:  seminar series; topical workshops and conferences, including interaction with companies, political entities, or the media; visitor programs; educational forays such as interdisciplinary graduate programs, training grants, research experience for undergraduates or teachers, K12 outreach days; endowed postdoctoral fellowships; etc.  An Institute is meant to act as a a catalyst or enabling structure to bring together researchers with a common intellectual interest, to foment new and support existing collaborations, and to further research activity in that area.  At some universities, an Institute may have its own building or administer shared infrastructure.

A Center is usually a smaller, more focused group of researchers that is often expected to be financially self-supporting through multi-investigator external funding.  (Sometimes these are called Laboratories.)   Centers often exist within or are founded as the result of institutes.  A Center likely concentrates on a portfolio of specific research-related projects, rather than having broad programmatic efforts like an Institute.  The US NSF sponsors a number of center programs (MRSECs, ERCs, STCs, CCIs, and formerly NSECs), as does the US DOE (EFRCs).

An effective Center is almost self-defining:  It is able to accomplish focused research goals and to raise sufficient external resources to be self-supporting at least on the several year to decade timescale.  A good Center is able to identify and adapt to new research opportunities, while realizing which avenues are becoming played-out and should be set aside - basically, effective self-criticism while encouraging creativity through seed projects to generate new activity.   Longevity and research productivity metrics are two ways to assess the utility of a Center.

An effective Institute needs to serve its members by successfully supporting and carrying out its programs.  Because many Institutes have very broad programmatic goals, this requires serious "buy-in" - a decent fraction of the membership have to be willing to invest their time and energies to ensure that these programs are a success.  This only happens when the people involved really believe in the efforts and can see that they and the institution derive real benefit from the work.   This means that the Institute has to be responsive to the needs of its membership.   At the same time, the university has to assess (via research and funding metrics) the impact of the Institute and its programs, since the university has to decide whether the internal resources of the Institute could have been better spent elsewhere.

Both Institutes and Centers can be vulnerable to budgetary problems (internal and external, respectively) and to lack of engagement by membership.  At most places (the University of Chicago seems to be a big exception, since there Institutes have a lot of power) an Institute can be particularly exposed in tight times, since departments and much of their budgets are explicitly necessary for the reaching mission of the university, while Institutes are often viewed as elective or discretionary expenses.  In terms of engagement with members, like many organizations, Institutes and Centers succeed by succeeding and fail by failing.  You can't force people to collaborate, but once some do arise, productive collaborations lead to further productive collaborations.  Overall, Centers and Institutes appear to be key components of successful research universities.  It's not clear how these organizational structures (and their associated programs) will fare if we are in a long era of declining federal funding and internal cost cutting.

## Friday, February 13, 2015

### 2d metallicity at low temperatures - a nice new result

To quote this blog from about 8.5 years ago (!):
For years now, there has been a fairly heated debate about the nature of an apparent metal-insulator transition (as a function of carrier density) seen in various 2d electronic and hole systems. The basic observation, originally made in some Si MOSFETs of impressively high interface quality made in Russia, is that as the 2d carrier density is reduced, the temperature dependence of the sheet resistance changes qualitatively, from a metallic dependence (lower T = lower resistance) at high carrier concentration to an insulating dependence (lower T = higher resistance) at low concentration, with a separatrix in between with nearly T-independent resistance at some critical carrier density. A famous 1979 paper by the "Gang of Four" (Anderson, Abrahams, Licciardello, and Ramakrishnan) on the scaling theory of localization had previously argued that 2d systems of noninteracting carriers all become insulating at T=0 for arbitrarily weak disorder.
So, there has been a long-simmering controversy about why some 2d systems (electrons or holes) seem to show a really metallic temperature dependence of their conductance at low temperatures.  This dependence, where the conductivity apparently increases by, say, a factor of 2 from $T =$ 4.2K down to 0.1 K, takes place over a temperature range where the scattering of electrons by lattice vibrations (the mechanism responsible for the increase in conductivity of ordinary metals as they are cooled from room temperature down to cryogenic temperatures) is supposed to be all finished.   I mentioned this as an ongoing controversy in '06 and again in '12.  What is going on here?

There is a new preprint from Bruce Kane and colleagues at Maryland that clarifies things considerably, in my view.  Kane, probably best known for proposing a quantum computing scheme involving individual phosphorus donors in Si, is a very clever experimentalist.  He has developed a method of creating field-effect transistors,where the conducting channel is the hydrogen-terminated surface of a Si wafer, and the gate dielectric is vacuum (!).  Using these devices, his group has been able to look at the apparent metallicity in both electrons and holes in the same system.  They find that the improvement in conduction at low temperatures has to do with the screening of charged impurities by the conducting system (and for the experts:  in Si the electrons are able to do this better than the holes because there are 6 conduction band valleys, while there is no valley degeneracy for the holes).  This doesn't directly get to the "fundamental" question about whether the true, zero-temperature ground state is insulating in a real, interacting system, but it does go a long way toward demonstrating why the conductivity still has a metallic change with temperature even though phonons should be out of the picture.

## Friday, February 06, 2015

Over the last week I've run into a couple of readers of this blog who pointed out that many people never find older posts (unless they happen to use google with just the right search terms), and that it might be valuable to re-run updated versions of some of those, particularly the ones geared toward career advice.  This makes lots of sense, given how long this blog has been running and how readership has evolved.  So, here is the first of these updated re-runs (from 2011):  Advice on choosing a grad school.

This is written on the assumption that you have 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.
• 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.