Saturday, July 30, 2016

Ask me something.

I realized that I haven't had an open "ask me" post in almost two years.  Is there something in particular you'd like me to write about?  As we head into another academic year, are there matters of interest to (grad or undergrad) students?


Grumpy said...

How do you feel about the traditional lecture format for sophomore level physics? Do you have any tips for delivering the best learning environment with the least amount of prep?

Anonymous said...

Why don't my professors care about my personal and professional development? Investing in the human capital of young, promising researchers is literally their job. I regret graduate school. Keeping this anonymous because I don't want this coming back to me.

Anonymous said...

Ok, here's a real question: What's the best way to explain solid state physics phenomena like band gaps and excitations and conduction *without* relying on the single-particle fiction? I feel like we have to lie to students all the time (or at least speak very imprecisely) since we lack the language and understanding of multi-electron wavefunctions. Thoughts?

Anonymous said...

In a graduate course I learned about Green Functions as elementary building blocks for many particle physics in condensed matter. The lecture started with linear response theory (Kubo) and developed the Feynman concept for diagrams in order to address interacting Hamiltonians.

I clearly lack the understanding how a formalism like Kubo's can be used as basis for approximations given that Kubo itself is a drastic simplification (first order).

Moreover, how can we measure (!) single particle Green's functions and what information do they really provide? We spent tons of time to develop a formalism for them but nobody motivated the practical and experimental relevance in further detail. :/

Anonymous said...

What is your opinion on 2D materials and their applications? I've noticed several fantastic claims by both media and researchers.

Anonymous said...

1. Can you shed light on out-of-plane plasmonic resonances which have not been studied much ?

2. What future do you see for the plasmonic based research in the coming years ?

Douglas Natelson said...

I'll try to take these from the top, though it may take me a bit to get through them.

Grumpy, it depends a bit what you mean by "sophomore level". Are you referring to waves + optics, and some lead in to quantum? Or Lagrangians/Hamiltonians? The more technically detailed and matehmatical the material (and the farther removed from classroom demos), the more challenging it is to move toward nontraditional formats, it seems to me. In terms of "least amount of prep", I assume you mean from the faculty side, and further that there is an unwritten "yet still having the course be well run and pedagogically valuable". Some kind of flipped lecture scheme, with recorded video snippets watched with a rubric by students pre-class, and then class-as-discussion-and-examples would probably work long-term, though the initial investment of time and effort to set this up and make it good would be large. An example of the online component that worked well is here, by my colleague Jason Hafner.

Anon@6:01, I'm sorry to hear that things aren't going well. I can only speak for myself when I say that I definitely care about the professional development of my students, and I believe that this is true of the large majority of my colleagues. Altruism is part of this (it is our job, as you say), and even self-serving professors should care - successful students = good for the lab, and while not as obvious as things like publications and citation counts, the post-PhD careers of students is one way that some professors "keep score". That being said, professors are almost never trained in how to manage students, and I now have a much greater appreciation for how the mentoring relationship can go off the rails. Take advantage of support structure available to you - other members of your committee or department, an ombudsperson. Feel free to contact me by email if that would be helpful.

Anon@6:03, the thing is, very often the single-particle language isn't really (much of) a fiction. I mean, would you call it a lie to talk about the 6s electrons in gold, when technically speaking the true description of the electrons in an atom of Au requires a totally antisymmetric wavefunction involving 79 electrons, including relativistic corrections? I know what you mean, though. I ran into this when writing my book. People talk about "the wavefunction of an electron in a quantum dot" like it's a particle-in-a-box, for example, when what they really mean is the "envelope" function that is a good approximation to constructing the true electronic state out of Bloch functions that take into account the atoms in the lattice, and that true state is really closer to some totally antisymmetric linear combination of Slater determinants, etc. etc. I think the best we can do is make sure that students know "single-particle state" is an approximation that is good a lot of the time, and to remind them periodically that it's an approximation as well as linguistic shorthand for something more complicated. I'm not sure this helps, but it's the best I can do. Others may have more to say on this.

Grumpy said...

Hi Doug,

Thank you very much for the thorough answer!! I just watched the edx videos by your colleague. Wow they are great. I am really in over my head (first big lecture course as an assistant professor). Yes this is the waves/SR/baby quantum version.

Would you mind clarifying what sort of "rubric" the students should use when watching the videos?

I am a big fan of the flipping deal, but to be honest I never took a class that way. I rarely got much out of lectures as a student; I mostly learned by doing problem sets at cafés.

Unknown said...

What steps, if any, would you want to take to reform undergraduate physics education?

Anonymous said...

What is the meaning of life?

Anonymous said...

Read Erwin Schrodinger's masterpiece " What is life ? "
type in google , to get pdf file.

Douglas Natelson said...

Anon@11:36pm, I might not be the best person to answer this, and if a theorist reading this has a better answer, I invite their contribution. I think you already have said part of the answer - when trying to figure out response to some perturbation, a conventional approach is to start with linear response basically as an assumption (though the validity of the linear response treatment can be established with some rigor within its formalism, I believe). Then you seek some way of looking at additional terms in the perturbative expansion beyond first order. Feynman diagrams are one formalism for this. If you're lucky, you can find ways to sum some kinds of terms to all orders (basically trying to leverage 1/(1-x) expands to 1 + x + x^2 +.... for example). As for ways of measuring the single-particle Green's fn, that's tricky, as you've surmised. Photoemission is the canonical way to do something like this (you can see how many true single electrons are extracted to infinity as a function of energy and momentum transfer), though this gives something more like the spectral function of the retarded Greens fn - see here. Personally I think of the Greens fn in much the way I think of the wavefunction itself. Hard to measure the whole thing directly, but part of a formalism that lets you calculate other measurable quantities (correlation fns measured through scattering experiments, for example).

Jake, yes, there is an enormous amount of hype floating around regarding 2d materials. They really do have some remarkable properties, and thanks to advances in growth techniques, patterning methods, and the development of new characterization tools and theoretical methods, people are making much more progress with these systems than they did ~ 40 years ago, when transition metal dichalcogenides also had a mini-boom in popularity. It's important to remember that Si and III-V semiconductors are ubiquitous because of many decades of effort by thousands of people. Introducing a new materials system and having it be rapidly translated into technologies, especially in electronics or optoelectronics, is usually a slow process, so statements about immediate technological impact have to be taken with a big grain of salt. Also, the semiconductor industry in particular is incredibly rigid (for a variety of economic reasons) about adopting new materials. Still, the whole cycle of materials growth -> characterization -> theoretical modeling -> new growth is potentially much faster now, so there is reason for measured optimism.

Anonymous said...

I'm not an undergrad or grad student anymore, but I'm interested in condensed matter physics.
One important question regarding future technology is about batteries:
What are the current obstacles in building more efficient and higher capacity batteries, and is there a theoretical limit to how much energy a unit volume of battery may be able to hold? Is there a breakthrough around the corner?
I'm only interested in somewhat conventional and safe battery technologies, not exotic ones that rely on nuclear fission or fusion or other nonsense.

Anonymous said...

I know you've addressed the academic job market process before, but do you have any advice to postdocs of when to start going on the job market? I'm just drafting my first postdoc manuscript, which I'm hoping to have submitted right before this season of applications begin.

Is there a draw back of applying two rounds? Also, any advice for balancing asking your advisor's (and former advisor) about applying? I'm nervous about talking to them about it, but also so ready to have my own lab.

I guess a lot of this is stemming from after starting my postdoc, I really do not see the benefit of it. It's too short of time and extremely stressful to be in such temporary employment. The only benefit I see is to the PI's getting fast labor out of someone. The longer I'm in this postdoc, the more my confidence to do, and stay in, science declines.

Douglas Natelson said...

Anon@2:56am, I'm not really familiar with out-of-plane plasmon modes - I'd have to read up on them. Do you have a particular reference in mind? Regarding plasmons in general, there are many directions being pursued - energy harvesting (photovoltaics); plasmon-assisted photodetection; part of the palette of tools for metamaterials and metasurfaces; photothermal heat generation; flavors of quantum optics using plasmons; hot electron photochemistry and photocatalysis; the now-usual surface-enhanced spectroscopies. It's still a bit murky which of these will pan out into technologies, but a deep understanding the dynamics and the roles of disorder and symmetry breaking are important basic issues.

Grumpy, I'll pass along your kind words. By "rubric", I mean something like what UIUC does in their flipped freshman physics classes. Students watch brief video segments in a specific order and have to answer brief questions in between before getting to see the next segment. The students are expected to have watched everything and answered the questions prior to class. Then the instructor can see what the answers look like and can tailor conversation appropriately. (Full disclosure: I have not done this flipped video approach myself, so you shouldn't take what I say as anything like the gospel. I, too, got the most out of working on problem sets, though I did appreciate very well-prepared lectures like the ones I had in my undergrad quantum sequence.)

Paul, that's a big question. I have my personal tastes, but everyone has their own opinions. Answers depend strongly on the target audience: would-be doctoral candidates, or people interested more broadly in science and technical fields. I think a looming issue is making sure that undergrads get an appreciation that physics is a living field and that there's more to it than string theory stuff on the one hand and ancient received wisdom on the other.

Unknown said...

It will be interesting a post on the recent experiments which sent the field of topological insulators into a crisis. These are the observation of perfectly conducting edge states in 2D QSH insulators even in the presence of strong magnetic fields as high as 12T [PRL 114, 096802 (2015) and NATURE COMMUNICATIONS 6, 7252 (2015)], and the discovery that magnetic impurities can localize the entire surface spectrum of 3D TRS topological insulators (as opposed to opening a harmless tiny gap) [Nature Communications 7, 10559 (2016)]. These are both in stark contrast with the theoretical predictions.

Matt said...

If you were a beginning grad student now, what field would you go into and why?

If you happened to have a spare million to invest in a nanoscale-related field, where would you put it?

Thanks for the great blog!

Douglas Natelson said...

Anon@9:55, those are good questions. Battery materials (anodes and cathodes) should ideally be able to take in or lose large amounts of lithium (assuming we continue down that road and don't switch to sodium or something) rapidly, with minimal structural damage or other irreversibility, thousands of times. The electrolyte that transports the lithium ions has to be electrochemically stable while all this is going on (perhaps not counting the formation of the Mysterious Magic Goop - that is, the solid-electrolyte interphase - that appears on the two electrodes after a few cycles). The separator has to be boring, inert, and exceedingly stable in this environment. All of these constituents need to be reliably and inexpensively manufacturable. It's a pain. As far as I know, the biggest obstacle to greatly improved Li-ion battery capacities is the lack of a really good high capacity cathode material, though others more in the field may disagree.

As for the ultimate limits of energy density available in batteries, a rough estimate would be on the order of what you have in gasoline. That's a very dense packing of covalent bonds that get undone and rearranged when combustion takes place. It's hard to imagine that you could do better than that via electrochemical lithiation/delithiation, given the need to have some kind of structurally meaningful electrodes.

I don't know of any impending breakthrough, though materials discovery could result in an improved cathode at any time. More to the point, there are far more people with far more resources and economic motivation working on this problem than ever before. That has to improve the odds....

Anon@11:13, your questions are deserving of a longer response than I can really give right now. When you say two rounds, do you mean in subsequent years? The short version: You should go on the market when you are most likely to be competitive. A postdoc appointment is a chance to learn something new beyond your PhD and demonstrate that you can shift gears a bit. Producing nice result(s) as a postdoc helps support that your doctoral work's awesomeness is a reflection at least in large part of you, rather than the adviser. Competitiveness for jobs depends on the area. In many engineering disciplines faculty are hired with no postdoc experience. In physics, it would be very unusual for someone to be strongly competitive on the job market without that additional post-PhD demonstration of productivity these days.

David Brown said...

Does the discovery described in the following publication have economically important implications?
"Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity" by Bianconi & Jarlborg, 2015

Gautam Menon said...

Is there a case to be made for the importance of quantum mechanical effects in biological systems?

Anonymous said...

A damaging misconceptions ..... that research dominates and diminishes teaching.
starts the write up way back in 1996 in the following pdf and then concludes the para ' teaching makes for better research"

The general feeling by many are teaching and research are split wide open for evaluating the academics capability even for tenure.

The questions are

1) Is it due to heavy emphasis on metrics ?
Many students have reported that many academics have a Jack Nicholson ( As good as it gets) type compulsive obsessive disorder ( or is it an order) { COD or COO ?) with metrics.
2) Do you feel that attention span of undergrads has come down, due to technology overwhelming and academics find it difficult to reign the undergrads attention for sustained period of teaching.
3) The large class size ( which many universities have adopted) is it responsible for decline in academics teaching. online can be done for a large class , but the number of queries which come by email from a large class for each different question takes a lot of time to answer . In pre technology days , a good academic would one day in the morning, answer the volley of questions and sign off. email queries and answers discourages cohort learning because it is one is to one.
4) Some universities , only some univ give importance to good teaching for tenure? Why not all?

Roald Hoffman first para in the pdf has come true.

Douglas Natelson said...

Emil, nice to hear from you! Yes, I should probably write a longer post at some point about the issues in these systems. In the InAs/GaSb case, there is quite some controversy about the nature of edge state transport (e.g., can you get "trivial" edge state transport in these systems due to band bending that can look a lot like interesting edge state transport? What is the best way to determine unambiguously that you are in the topologically nontrivial regime? What limits ballistic transport in that regime?). I am actually working on a proposal involving that system, so I'm a little leery of saying too much in this format b/c I don't want to oversimplify or misstate anything.

Matt, great question. Assuming we're talking physics, I probably would still go into condensed matter (I really like the scale of the work, the physics involved, and the puzzle solving aspects of the experiments), though I've always had a love of astronomy/observational astrophysics. I still remember going to one of my undergrad advisers fall of senior year for advice about grad school, and this highly successful and recognized condensed matter guy spent twenty minutes telling me how we were entering the golden age of astrophysics :-)

As for where I'd invest a spare $1M in nano, apart from my own research program (hey, that should net something like $50K/yr, right?), I'd look at heteroepitaxial growth of 2d materials. If you were wondering about investing in nano-related companies or businesses, the safest investment would be a "tool maker" - there's an ever-expanding market for electron microscopes, and for increasingly high end rf generators and analyzers. [obviously no one should take investment advice here seriously!]

Douglas Natelson said...

David, regarding H3S superconductivity: I haven't read that paper in detail. Superconductivity at high temperatures in H3S under pressure is very neat - challenging experiments and an exciting result that shows that there really is no reason in principle why we can't have room temperture superconductivity. As far as I know, it seems to be consistent with established theoretical understanding, meaning BCS and related physics (as in this paper). Direct economic impact is unlikely, since the pressure conditions needed to do this in H3S require specialized apparatus (diamond anvil cells). Still, between this and other work on nonequilibrium superconductivity-like response, we seem to be getting closer....

Gautam, I believe (without digging up references) that there is strong evidence of quantum effects in particular processes (photosynthesis; the way light detection works in the eye). I am extremely skeptical that there are (electronically) quantum coherent effects involved in consciousness as some people try to argue. Obviously quantum mechanics is directly responsible for chemistry and all the noncovalent interactions in biological systems, too, but presumably that's not what you mean....

Anon@8:50, there is no question that the incentive structure at research universities strongly rewards success in research and at best only moderately considers pedagogy and classroom teaching. Part of this may have to do with it being easier to quantify research ($ brought in, papers published, citation counts, etc.), while serious assessment of teaching is difficult. I don't think there has been a big change in attention span of students, but there are certainly more distractions now (ubiquitous internet devices) and a shift such that mere facts are available virtually instantly. To get student buy-in for teaching, we have to convey that we're teaching how to think critically and analytical skills, not just transferring facts, which they can look up on their phones. Regarding class size, there is near-universal recognition that smaller classes are better, and many places are pushing toward smaller sections and fewer giant lectures. As for online instruction, I think we are all still finding out what approach works best under which circumstances. There is no substitute for student-instructor interaction, and you can't run an effective online course at a serious level (I don't mean some fun thing, but something with real assignments that is the equivalent of a for-credit class) without an appropriate amount of personnel support to have those interactions.

Unknown said...

Hi Doug, is very simple to decide that. QSH is a distinct phase, one should identify the phase boundary as one usually does when studying a (bulk!) phase transition. One should take note from IQHE where this has been pushed experimentally to the limits (so much that the experimental scaling analysis looks like textbook theory). Chern insulators are now at par with IQHE, i.e. the critical point between trivial and topological phases has been identified beyond any doubt. This was never achieved for QSH class, hence QSH cannot be declared a distinct condensed matter phase. It is amazing how the experimentalists missed this point and never looked into it seriously.

David Schroeder said...

Have you ever heard of Podkletnov's work with high temperature superconductors? In those experiments Podkletnov (back between 1996 and 2001) claimed to detect, at first small (.05 g) acceleration signals from spinning superconductors. Later (2001) he claimed totally fantastic (and unbelievable) reports of 1000 g impulses, persisting for 1/10,000th of a second, when his YBCO superconductor was subjected to 2 million volt discharges. It would be great if you could do a write-up on this.

I was totally fascinated by these claims, while keeping Carl Sagan's cautionary mantra: "Extraordinary claims require extraordinary evidence" in mind. Indeed, I was so intrigued that I ran some simple experiments with commercial one inch YBCO superconductors. On a couple of occasions I detected .0001 g signals. But I'm convinced that these resulted from an electromagnetic pulse as 1.2 megawatts (600 volts, 2000 amps) coursed through the superconductor, affecting my accelerometer (I've completely isolated the acoustic 'pop' from expanding cryofluid with electronic timing circuits). Nonetheless I'm going to try additional experiments with improved shielding of the ADXL203 accelerometer.

Luis Gregório Dias said...

Hello Doug. Here's my question: how do you manage your time?

For instance: do you set aside a particular time to write/read e-mails? Do you use "to-do-list" apps such as Wunderlist, Google Keep, etc.? Any tips for keeping a high productivity overall while juggling with all the demands of the academic life (doing good science, writing/reviewing papers, advising students, writing blog posts, teaching, etc.)? ;)

Douglas Natelson said...

David, you've already hit on the key aspects of Podkletnov and his more recent kin, those that are looking at "EM Drive" and "Mach Effect" devices: extraordinary claims require extraordinary evidence, and sensitive force measurements in the presence of large oscillating or pulsed currents are tricky.

Luis, the short answer is "not as well as I should". I keep lists - I got into that habit in grad school. I use google calendar. I do try to stick with some routines, and once I had kids I tried to figure out what kind of work I can do well at home vs. what I need to do at the office/lab. It's hard.

sam north said...

Hello Douglas
Could you give an opinion on this report.

Douglas Natelson said...

Sam, I flipped through it. The report does not really conform to what one would expect for a scientific paper, and the author takes it basically as a given that there are definitely low energy nuclear reactions happening, going all the way back to Pons and Fleischman in '89. My views on this stuff have not changed since I wrote this or this, the passion of LENR supporters notwithstanding. The TL/DR version: It would be incredible and awesome if it were true, but I have yet to see experimental work that actually meets the standards of reproducibility one would expect for a real effect.

sam north said...

Thanks for reply Douglas.If you do not know.
Bob Greenyer is working with Aarhus University.
Andrea Rossi Robert E Godes and others
keep plugin away at LENR.
I will post on your blog if anything important

sam north said...

sam north said...

Hi Douglas
Do you follow DR Randell Mills
and his Sun Cell technology.

Have a good New Year

Douglas Natelson said...

Sam, Randall Mills has been pushing hydrinos since before I was in grad school. I actually read through his manifesto/long paper on this back in 1994. It's just not correct, nor are hydrinos backed up by any experimental evidence taken in serious labs.

sam north said...

Hi Douglas
I do not know if Dr Mills hydrino theory is correct
but according to Brett Holverstott who
worked with Dr mills his Sun Cell energy device is almost ready for the market.

I hope you are having a good summer.