Remember, you should be scared. Very scared.
Stop trying to frighten me. To be trite, that's just what the terrorists want.
Update: Here's someone who agrees with me. Feel the irony.
Friday, February 29, 2008
Tuesday, February 26, 2008
This week in cond-mat
A brief look at three papers from the past week that I thought looked particularly interesting.
arxiv:0802.3236 - Bleszynski-Jayich et al., Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
For a number of years now the Westervelt group at Harvard has been at the forefront of using scanned probe microscopy to examine the electronic states in semiconductor nanostructures. The basic idea is simple: use a conducting AFM tip as a local gate, and measure the transport through the nanodevice as a function of the tip position. If the gate is located somewhere irrelevant to the current paths through the device, you see no effect. By mapping the device response to the gate, you can map out many interesting features in the electronic states that contribute to transport. This is another example of applying this basic technique, this time to one of the InAs-based structures that Lars Samuelson has been developing extensively in recent years. Very nice. The data are rather psychedelic.
arxiv:0802.2350 - Geraci et al., Improved constraints on non-Newtonian forces at 10 microns
These kinds of experiments are "small scale physics" at its best. The high energy theory community has been talking for a while about whether "large" extra dimensions (beyond the usual 3+1 of ordinary space-time) can show themselves through deviations in Newtonian gravity at the sub-mm scale. Measurements of G, the gravitational constant, at these distances are extremely challenging. Remember, electromagnetic forces can swamp gravity by 40 orders of magnitude, and there are all kinds of complications that can arise in such measurements. I always enjoy these experiments, where extreme skill and cleverness are used to go after big foundational questions without gigadollar particle accelerators.
arxiv:0802.3462 - Min et al., Room-temperature superfluidity in graphene bilayers?
There's an old saying that the answer to any rhetorical question in the title of a paper is always "no". Here, however, Allan MacDonald and company suggest the opposite. It would appear that the special properties of graphene's unusual band structure may lead to superfluidity of bilayer excitons (a hole in one layer bound electrostatically to an electron in the neighboring layer to form an effective composite boson that is overall charge-neutral) at room temperature. There's been evidence for a while of low-T excitonic superfluidity in 2d electron/hole bilayers. This would be very neat, and it's always nice to see theorists making provocative predictions. (It would not lead to room temperature superconductivity, though! Since the excitons are neutral, their superfluid state doesn't carry a net current.)
arxiv:0802.3236 - Bleszynski-Jayich et al., Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
For a number of years now the Westervelt group at Harvard has been at the forefront of using scanned probe microscopy to examine the electronic states in semiconductor nanostructures. The basic idea is simple: use a conducting AFM tip as a local gate, and measure the transport through the nanodevice as a function of the tip position. If the gate is located somewhere irrelevant to the current paths through the device, you see no effect. By mapping the device response to the gate, you can map out many interesting features in the electronic states that contribute to transport. This is another example of applying this basic technique, this time to one of the InAs-based structures that Lars Samuelson has been developing extensively in recent years. Very nice. The data are rather psychedelic.
arxiv:0802.2350 - Geraci et al., Improved constraints on non-Newtonian forces at 10 microns
These kinds of experiments are "small scale physics" at its best. The high energy theory community has been talking for a while about whether "large" extra dimensions (beyond the usual 3+1 of ordinary space-time) can show themselves through deviations in Newtonian gravity at the sub-mm scale. Measurements of G, the gravitational constant, at these distances are extremely challenging. Remember, electromagnetic forces can swamp gravity by 40 orders of magnitude, and there are all kinds of complications that can arise in such measurements. I always enjoy these experiments, where extreme skill and cleverness are used to go after big foundational questions without gigadollar particle accelerators.
arxiv:0802.3462 - Min et al., Room-temperature superfluidity in graphene bilayers?
There's an old saying that the answer to any rhetorical question in the title of a paper is always "no". Here, however, Allan MacDonald and company suggest the opposite. It would appear that the special properties of graphene's unusual band structure may lead to superfluidity of bilayer excitons (a hole in one layer bound electrostatically to an electron in the neighboring layer to form an effective composite boson that is overall charge-neutral) at room temperature. There's been evidence for a while of low-T excitonic superfluidity in 2d electron/hole bilayers. This would be very neat, and it's always nice to see theorists making provocative predictions. (It would not lead to room temperature superconductivity, though! Since the excitons are neutral, their superfluid state doesn't carry a net current.)
Sunday, February 17, 2008
This week in the arxiv
One particularly nice paper from this past week:
arxiv:0802.0930 - Dolev et al., Towards identification of a non-Abelian state: observation of a quarter of electron charge and \nu=5/2 quantum Hall state
I've written in the past a couple of times about how the low energy electronic excitations of some condensed matter systems can be particle-like (that is, they have a well-defined set of quantum numbers and interact relatively weakly with one another) but with properties quite different from those of free electrons. The fractional quantum Hall system is a perfect example of this. For cold electrons confined to a two-dimensional layer in the presence of a large magnetic field, the best way to think about the low energy excitations of the electronic system is not as free electrons. Rather, interactions between the electrons in the presence of the field lead to the formation of a new description (the so-called Laughlin liquid) when the ratio of electron density to magnetic flux quanta is certain rational fractions with odd denominators. The quasiparticles in those states have fractional charge (!) rather than the usual -e of an electron. One particularly exotic state happens in very very clean 2d electron systems when that ratio is 5/2. Even though this is an even-denominator state, and the usual expectation would be that the quasiparticles (called composite fermions) should be rather like free electrons, the quantum Hall state shows that something else is going on. The proposed explanation is that the composite fermions pair up to form a special condensate (not unlike in a superconductor), and the excitations of this paired state are predicted to have all sorts of weird properties. Swapping two such quasiparticles around each other is supposed to leave a topological imprint on the system, a bit like braiding the ends of ropes. There is a lot of interest in using such a system to do quantum computation. There's only one problem: so far no one has proven that the 5/2 state really has these exotic properties. This paper by the always-impressive group at the Weizmann goes part of the way there, demonstrating via shot noise that the excitations at nu=5/2 have charge e/4 (!), consistent with the theories of an exotic state. This is a major experimental achievement - historically the kind of surface processing required to do these shot-noise or more complex measurements usually degrades the charge mobility in the 2d layer enough to kill the 5/2 state altogether.
arxiv:0802.0930 - Dolev et al., Towards identification of a non-Abelian state: observation of a quarter of electron charge and \nu=5/2 quantum Hall state
I've written in the past a couple of times about how the low energy electronic excitations of some condensed matter systems can be particle-like (that is, they have a well-defined set of quantum numbers and interact relatively weakly with one another) but with properties quite different from those of free electrons. The fractional quantum Hall system is a perfect example of this. For cold electrons confined to a two-dimensional layer in the presence of a large magnetic field, the best way to think about the low energy excitations of the electronic system is not as free electrons. Rather, interactions between the electrons in the presence of the field lead to the formation of a new description (the so-called Laughlin liquid) when the ratio of electron density to magnetic flux quanta is certain rational fractions with odd denominators. The quasiparticles in those states have fractional charge (!) rather than the usual -e of an electron. One particularly exotic state happens in very very clean 2d electron systems when that ratio is 5/2. Even though this is an even-denominator state, and the usual expectation would be that the quasiparticles (called composite fermions) should be rather like free electrons, the quantum Hall state shows that something else is going on. The proposed explanation is that the composite fermions pair up to form a special condensate (not unlike in a superconductor), and the excitations of this paired state are predicted to have all sorts of weird properties. Swapping two such quasiparticles around each other is supposed to leave a topological imprint on the system, a bit like braiding the ends of ropes. There is a lot of interest in using such a system to do quantum computation. There's only one problem: so far no one has proven that the 5/2 state really has these exotic properties. This paper by the always-impressive group at the Weizmann goes part of the way there, demonstrating via shot noise that the excitations at nu=5/2 have charge e/4 (!), consistent with the theories of an exotic state. This is a major experimental achievement - historically the kind of surface processing required to do these shot-noise or more complex measurements usually degrades the charge mobility in the 2d layer enough to kill the 5/2 state altogether.
Allocation of resources
When probabilities of some events become very low, it can be hard to calibrate your thinking and planning about them. The classic large-scale example is that of asteroid defense. The odds of an asteroid hitting the earth within our lifetimes are very low. On the other hand, the likelihood isn't zero, the negative consequences would be severe for millions if not billions of people, and we actually have the technical capability to do something about the problem with enough advanced warning. So, how much money should we as a species spend on asteroid defense? A bit closer to home, there are funding opportunities out there sometimes that are game-changing amounts of money, but getting the grant is something like a 0.5% chance, and the criteria are quite opaque. It's tough to get a good handle on how much time one should invest in the (relatively short) proposal....
Saturday, February 09, 2008
Where to publish
I've had two different conversations in the last couple of days about how people choose where to submit papers, and it's a decent topic for a blog post. I can only speak for myself, but I think I'm pretty typical. To frame the discussion, consider why we publish journal articles in the first place. We want the scientific community to know what we've been doing, so that our work can be built upon - if we've answered a question that many people want answered, those people should know. If we've developed a new technique that will be useful, or if we've learned something that changes the way we think about some (ideally important) system, the rest of the community should know. Of course, publications and citations are also one metric of performance. It's a marketplace of ideas out there, and if no one cites your papers, then that says that you may not be having a major influence in moving the field forward.
The desire to disseminate knowledge and get recognition both provide a motive to try to publish in the highest impact journals that are appropriate. On the other hand, not every publication-worthy result is necessarily earth-shaking in significance. I know that there are some people who apparently send every halfway-decent paper to Science and Nature first, because "why not?" I tend to be more conservative and self-assessing. Not everything is of interest to a broad readership. Similarly, there are physicists who send every result to PRL. Again, let's be honest - not every physics result is PRL-worthy. Furthermore, in the nano arena, sometimes the chemistry or engineering literature may really be more appropriate than Phys Rev, and that's fine. I do try to aim for the highest "impact factor" journal that seems topical and reasonable - that's just common sense.
An additional factor is the time-to-publication. If you're working in a competitive area, you may want to get a result out in the peer-reviewed literature fast, and the best way to do that may be to publish in something other than PRL. The arxiv mitigates this a bit, but not all publishers like electronic preprints.
The desire to disseminate knowledge and get recognition both provide a motive to try to publish in the highest impact journals that are appropriate. On the other hand, not every publication-worthy result is necessarily earth-shaking in significance. I know that there are some people who apparently send every halfway-decent paper to Science and Nature first, because "why not?" I tend to be more conservative and self-assessing. Not everything is of interest to a broad readership. Similarly, there are physicists who send every result to PRL. Again, let's be honest - not every physics result is PRL-worthy. Furthermore, in the nano arena, sometimes the chemistry or engineering literature may really be more appropriate than Phys Rev, and that's fine. I do try to aim for the highest "impact factor" journal that seems topical and reasonable - that's just common sense.
An additional factor is the time-to-publication. If you're working in a competitive area, you may want to get a result out in the peer-reviewed literature fast, and the best way to do that may be to publish in something other than PRL. The arxiv mitigates this a bit, but not all publishers like electronic preprints.
Wednesday, February 06, 2008
Combined single-molecule electronics and optics
This'll be my last self-referential post for a while. Now that it's out online, I want to write a post about our latest result. As readers of this blog know, I am not a big fan of the hype that accompanies a lot of nano research. I cringe everytime someone claims that a minor development is a breakthrough, and it drives me crazy when people who know better feel compelled to imply that self-reproducing nanobots are going to build spaceships out of single-crystal diamond in five years. That being said, I really do think that this result is a major advance, both in molecular-scale electronics and in ultrasensitive chemical sensing.
The two-sentence summary: we can do simultaneous electronic and optical measurements on single molecules (!) by using our electrodes as optical antennas. This opens up lots of science to be done as well as some very intriguing technological possibilities.
Over the last decade, a number of techniques have been developed to measure electronic conduction through single molecules. There are lots of basic physics and physical chemistry questions that still need to be answered in such systems (e.g., how does dissipation work at these scales? What happens when electronic correlations are strong and the system is driven out of equilibrium?). One long-standing problem, though, has been the lack of any independent (non-transport) way to confirm that conduction is taking place through a particular molecule of interest. Except for scanning tunneling microscopy (great for science, but impractical for some measurements and definitely not scalable for devices), there are no good imaging techniques to see the object (molecule of interest? contaminant? accidental nanoparticle?) through which the current is passing. The resulting approach to these devices has been essentially statistical, requiring the fabrication of large numbers of devices with many control experiments, etc.
Over the same period, as I discussed here in reference to an earlier paper from our group, surface-enhanced Raman spectroscopy (SERS) has been studied extensively. Raman spectroscopy is a very common physical chemistry technique to probe the vibrational spectrum of materials. Light comes in at some frequency, dumps some energy into the vibrational modes of the material (this is called Stokes scattering), and leaves with less energy. By measuring the energy shift between incoming and outgoing light, it's possible to pick out a material's characteristic vibrational modes - a kind of chemical fingerprint. In SERS, nanostructured metal surfaces act like little optical antennas when illuminated, creating so-called hotspots where the local electromagnetic intensity can be as much as a million times greater than the incident intensity, leading to greatly enhanced Raman emission. People have reported SERS capable of measuring single molecules, but demonstrating that conclusively is extremely difficult.
In our new paper, we've been able to kill two birds with one stone. We have been able to perform simultaneous electronic transport and Raman spectroscopy on individual molecules. The same metal electrodes used to push current through the molecules also function as a plasmonic antenna, giving enormous SERS enhancements. Conduction between the electrodes is known to be by tunneling, and tunneling depends so steeply on distance that the total volume through which the current is passing can only contain at most one or two molecules. (This steep distance dependence is the reason STMs work!) At room temperature (and in air), we see that the conduction from one electrode to the other bops around a bit as a function of time. This is due to molecular motion and the changing molecular environment, and isn't surprising. However, we can simultaneously measure the Raman signal from the region between the electrodes. We find that the time variation in the Raman emission correlates extremely well with the time variation in the interelectrode conduction. Since the conduction occurs via tunneling and probes about one molecular volume, the Raman emission must be from the same single molecule in question.
This demonstrates that we can mass-fabricate single-molecule sensitive SERS hotspots in high yield in pre-defined locations. At the same time, this multimodal single-molecule sensing shows via the Raman signature that we are pushing current through the specific molecule of interest in a given device.
We've got lots of ideas on where to go with this - it's very exciting.
The two-sentence summary: we can do simultaneous electronic and optical measurements on single molecules (!) by using our electrodes as optical antennas. This opens up lots of science to be done as well as some very intriguing technological possibilities.
Over the last decade, a number of techniques have been developed to measure electronic conduction through single molecules. There are lots of basic physics and physical chemistry questions that still need to be answered in such systems (e.g., how does dissipation work at these scales? What happens when electronic correlations are strong and the system is driven out of equilibrium?). One long-standing problem, though, has been the lack of any independent (non-transport) way to confirm that conduction is taking place through a particular molecule of interest. Except for scanning tunneling microscopy (great for science, but impractical for some measurements and definitely not scalable for devices), there are no good imaging techniques to see the object (molecule of interest? contaminant? accidental nanoparticle?) through which the current is passing. The resulting approach to these devices has been essentially statistical, requiring the fabrication of large numbers of devices with many control experiments, etc.
Over the same period, as I discussed here in reference to an earlier paper from our group, surface-enhanced Raman spectroscopy (SERS) has been studied extensively. Raman spectroscopy is a very common physical chemistry technique to probe the vibrational spectrum of materials. Light comes in at some frequency, dumps some energy into the vibrational modes of the material (this is called Stokes scattering), and leaves with less energy. By measuring the energy shift between incoming and outgoing light, it's possible to pick out a material's characteristic vibrational modes - a kind of chemical fingerprint. In SERS, nanostructured metal surfaces act like little optical antennas when illuminated, creating so-called hotspots where the local electromagnetic intensity can be as much as a million times greater than the incident intensity, leading to greatly enhanced Raman emission. People have reported SERS capable of measuring single molecules, but demonstrating that conclusively is extremely difficult.
In our new paper, we've been able to kill two birds with one stone. We have been able to perform simultaneous electronic transport and Raman spectroscopy on individual molecules. The same metal electrodes used to push current through the molecules also function as a plasmonic antenna, giving enormous SERS enhancements. Conduction between the electrodes is known to be by tunneling, and tunneling depends so steeply on distance that the total volume through which the current is passing can only contain at most one or two molecules. (This steep distance dependence is the reason STMs work!) At room temperature (and in air), we see that the conduction from one electrode to the other bops around a bit as a function of time. This is due to molecular motion and the changing molecular environment, and isn't surprising. However, we can simultaneously measure the Raman signal from the region between the electrodes. We find that the time variation in the Raman emission correlates extremely well with the time variation in the interelectrode conduction. Since the conduction occurs via tunneling and probes about one molecular volume, the Raman emission must be from the same single molecule in question.
This demonstrates that we can mass-fabricate single-molecule sensitive SERS hotspots in high yield in pre-defined locations. At the same time, this multimodal single-molecule sensing shows via the Raman signature that we are pushing current through the specific molecule of interest in a given device.
We've got lots of ideas on where to go with this - it's very exciting.
Sunday, February 03, 2008
political advertising: good and bad
I don't want to start a political flamewar, but I find the contrast between these two political ads very striking:
Obama's Yes, we can - feel-good, inspirational video with lots of stars, excerpts of Obama's NH concession (!) speech.
Clinton's Freefall - be scared! Only we can save you from certain doom!
Obama's Yes, we can - feel-good, inspirational video with lots of stars, excerpts of Obama's NH concession (!) speech.
Clinton's Freefall - be scared! Only we can save you from certain doom!
One PRL/arxiv paper
I'll write more in the next day or two about what I think is a very exciting new result of ours. For now, I wanted to write a little about this paper:
arxiv:0801.4021, Frolov et al., Electrical generation of pure spin currents in a two-dimensional electron gas
For quite some time there has been a strong interest in using the spin degree of freedom of electrons for information processing. In some sense this is old news (see this past year's Nobel in physics), but the real trick is to see whether one can generate currents of only spin, rather than pushing whole, spin-polarized electrons through a circuit. In principle pure spin currents can be moved without dissipation, so if they can be generated and detected in a "nice" way, it may be possible to reduce the power required for certain computations. Of course, unlike charge, spin polarization is not conserved - spins generally prefer to relax back to an unpolarized state in the absence of big magnetic fields. This paper reports a way of generating spin currents that is quite clever - use quantum point contacts + spin-orbit scattering to generate an excess spin population in a region of 2d electron gas, and then the excess spin population diffuses away (without a net flow of charge). This paper also demonstrates that reducing the dimensionality of the system leads to an enhanced spin lifetime. It's a neat result and a very pretty experiment.
arxiv:0801.4021, Frolov et al., Electrical generation of pure spin currents in a two-dimensional electron gas
For quite some time there has been a strong interest in using the spin degree of freedom of electrons for information processing. In some sense this is old news (see this past year's Nobel in physics), but the real trick is to see whether one can generate currents of only spin, rather than pushing whole, spin-polarized electrons through a circuit. In principle pure spin currents can be moved without dissipation, so if they can be generated and detected in a "nice" way, it may be possible to reduce the power required for certain computations. Of course, unlike charge, spin polarization is not conserved - spins generally prefer to relax back to an unpolarized state in the absence of big magnetic fields. This paper reports a way of generating spin currents that is quite clever - use quantum point contacts + spin-orbit scattering to generate an excess spin population in a region of 2d electron gas, and then the excess spin population diffuses away (without a net flow of charge). This paper also demonstrates that reducing the dimensionality of the system leads to an enhanced spin lifetime. It's a neat result and a very pretty experiment.
Saturday, January 26, 2008
updated group webpage + grad recruiting
It took a little work, but I finally did a major overhaul of my group's webpage. It's hard to get a good sense of how important it is to have a good webpage. My impression is that prospective students put a surprisingly large amount of weight on this, and I'm pretty sure that many others (funding agency personnel, referees, news media) do a lot of web-based background chasing, too. Nothing says "disorganized" like having a webpage that clearly hasn't been updated in four years.
My department is in the process of overhauling its webpage as well. We are very interested in increasing our applicant pool, and this certainly can't hurt (provided that it's done well, of course). Rice's big challenge is one of overall visibility, in my opinion. I just want to make sure that undergrads at the top 25 places in the country are at least aware that we have a thriving graduate program. That's easier said than done.... When I was a senior Rice was not even on my radar screen - growing up and going to school in the northeast, I just didn't think about them. Of course, the best thing to do to boost graduate applications is to do great science and make sure that people know about it, but that takes time.
My department is in the process of overhauling its webpage as well. We are very interested in increasing our applicant pool, and this certainly can't hurt (provided that it's done well, of course). Rice's big challenge is one of overall visibility, in my opinion. I just want to make sure that undergrads at the top 25 places in the country are at least aware that we have a thriving graduate program. That's easier said than done.... When I was a senior Rice was not even on my radar screen - growing up and going to school in the northeast, I just didn't think about them. Of course, the best thing to do to boost graduate applications is to do great science and make sure that people know about it, but that takes time.
Monday, January 21, 2008
Two recent ACS papers
Real life is very time consuming. This is a quick post about a couple of cute papers that are up as asap on Nano Letters at the moment.
Coraux et al., Structural coherence of graphene on Ir(111) - This is the first of what I suspect will be a large number of papers coming from people trying to find nice surfaces on which to grow graphene. Sure, it's possible to get graphene over large areas of SiC wafers, as the Georgia Tech group has shown very prettily. Still, for graphene to really come into its own as a technology, it would sure be nice to grow it by some technique like CVD or MBE. These folks have shown that it's possible to grow good graphene over large areas using single-crystal Ir as a substrate. Unfortunately, that's not too useful in and of itself. Still, it's a start, and it lets them look at things like the growth mechanisms and the way graphene accommodates substrate roughness and point defects.
Yang et al., Experimental observation of an extremely dark material made by a low-density nanotube array - This immediately calls to mind the famous line of Nigel Tufnel in This is Spinal Tap, when discussing the cover of their "Black Album": "It's like, 'How much more black could it be?', and the answer is 'None - none more black.'" These folks have been able to use the intrinsic optical properties of nanotubes + the fact that they can be grown in "carpet" form to make the blackest material ever. Pretty cool.
Coraux et al., Structural coherence of graphene on Ir(111) - This is the first of what I suspect will be a large number of papers coming from people trying to find nice surfaces on which to grow graphene. Sure, it's possible to get graphene over large areas of SiC wafers, as the Georgia Tech group has shown very prettily. Still, for graphene to really come into its own as a technology, it would sure be nice to grow it by some technique like CVD or MBE. These folks have shown that it's possible to grow good graphene over large areas using single-crystal Ir as a substrate. Unfortunately, that's not too useful in and of itself. Still, it's a start, and it lets them look at things like the growth mechanisms and the way graphene accommodates substrate roughness and point defects.
Yang et al., Experimental observation of an extremely dark material made by a low-density nanotube array - This immediately calls to mind the famous line of Nigel Tufnel in This is Spinal Tap, when discussing the cover of their "Black Album": "It's like, 'How much more black could it be?', and the answer is 'None - none more black.'" These folks have been able to use the intrinsic optical properties of nanotubes + the fact that they can be grown in "carpet" form to make the blackest material ever. Pretty cool.
Sunday, January 13, 2008
the arxiv and publishing
Journals seem to have a love/hate relationship with the arxiv. For years Nature in particular used to campaign actively against electronic preprint servers, though they have given in and no longer consider posting preprints to compromise a journal submission. Science still forbids web posting of preprints before publication, presumably because at the end of the day they think that people won't buy the journal if they can get versions of the paper for free. This comes up most often when someone has a hot result and they want to get it out there to the community quickly (perhaps to establish priority), rather than wait weeks or months for the editorial process at a journal. Other publishers have attitudes that run the gamut from embracing preprint servers to ignoring their existence. Does anyone out there know if there is any actual evidence to support or refute the idea that preprint posting harms journal circulation?
Back to science soon....
I will start blogging about actual physics again soon. It's been a busy time lately, between the start of the new semester and having two proposals due - one this past Thursday for the IMR program, and one this coming Friday for the MRSEC program. Writing equipment proposals like the IMR these days feels like a complete throw of the dice. For those who don't know, after your startup period, it can be challenging to get the resources needed to buy significant pieces of equipment (say $200K-$400K). You can't just ask for that kind of equipment as part of a standard grant proposal, which is why they set up separate proposals just for gadgetry. I'm trying to get a piece of equipment that will be a major boost to all areas of my research program, and after the initial grant period I'm planning on adding it to Rice's shared instrumentation pool so that it's there to help the whole campus community. The big question is, with the current budget woes (described so completely by Gordon and Chad, among others, so there's no need for me to say much more), what's going to happen to this program? Is the proposal success rate going to be 10%? Lower? The equivalent program at DOE just got cancelled entirely. The reason that the recent remarks of the Harvard president hit a nerve is that there is a grain of truth to them....
Monday, January 07, 2008
EndNote
Like many physicists, given my druthers I'd write everything in LaTeX, but there are times when it's necessary to write large documents (like proposals) in Word. While LaTeX has the BibTeX bibliography management tool, in Word the main choice is EndNote, the offering from ISI (the folks who do Web of Knowledge). EndNote isn't too bad, but there are times when it's dreadfully painful, particularly if you're merging documents with contributions from multiple authors, each with their own EndNote library. This afternoon I had to upgrade to version X1 - the joys of planned obsolescence, since I couldn't read the libraries of my colleagues with more recent versions than mine. What a racket. I wonder if the new version is smart enough to read article numbers (as in PRL) as page numbers, instead of me having to type them in by hand.
Sunday, December 30, 2007
Nanotechnology - how to get into it, and where it's going
This post is in response to a comment here seeking some advice about nanotechnology, and is relatively brief.
What is nanotechnology? Nanotechnology is a vague, overly broad term. The most commonly accepted definition is something like "nanotechnology is any technology making use of the unique properties of matter structured on length scales smaller than 100 nm." By this definition the semiconductor industry has been doing nanotechnology for a long time now. The point is, in the last ten to twenty years, we've learned a lot about how to engineer materials and structure them in all three dimensions (under the right circumstances) on scales much smaller than 100 nm. This capability has a real chance of having a major impact on a large number of industries, from biomedical sensing and treatment to light strong structural composites to energy generation to waste remediation.
What should I study if I'm interested in nanotechnology? Nanoscale science and engineering is broad and interdisciplinary. The main avenues for getting into cutting edge work at these scales remain condensed matter physics, physical chemistry, and electrical engineering programs, though there are exceptionally good people working at the nanoscale in bio, bioengineering, chemical engineering, and mechanical engineering programs as well. The best approach, in my opinion, is to get a first-rate education in one of these traditional disciplines and focus on the nano, if you want to make scientific or engineering research contributions. Broad nano overview programs right now are better suited to people who want to be scientifically literate for decision-making (e.g. managers or patent lawyers) rather than those who want to do the science and engineering.
Is there really substance behind the hype? Is nanotechnology actually going somewhere? There is definitely substance behind some of the hype. As a very recent example, this new paper in Nature Nanotechnology reports a way of making lithium ion battery electrodes from silicon nanowires. Because it's in nanowire form, the Si can take up huge amounts of Li without the resulting strain pulverizing the Si. Between that and the huge specific surface area of the nanowires, real gains over conventional batteries should be possible. Best of all, industrial scaleup of Si nanowire growth looks achievable.
That's just one example from the past week. There is an awful lot of silliness out there, too, however. We're not going to have nanorobots swimming through our bodies repairing our capillaries. We're not going to have self-reproducing nanomachines assembling rocket engines one atom at a time out of single-crystal diamond. Getting a real science or engineering education gives you the critical skills and knowledge to tell the difference between credible and incredible claims.
Is going into nanotechnology a stable career path relative to alternatives? Another reason to get a solid education in a traditional science or engineering discipline is that you shouldn't be limited to just "nano" stuff. Frankly, I think this would be far more useful in just about any career path (including law or medicine) than an undergrad degree in business. Still, there are no guarantees - learn to be flexible, learn to think critically, and learn to solve problems.
What is nanotechnology? Nanotechnology is a vague, overly broad term. The most commonly accepted definition is something like "nanotechnology is any technology making use of the unique properties of matter structured on length scales smaller than 100 nm." By this definition the semiconductor industry has been doing nanotechnology for a long time now. The point is, in the last ten to twenty years, we've learned a lot about how to engineer materials and structure them in all three dimensions (under the right circumstances) on scales much smaller than 100 nm. This capability has a real chance of having a major impact on a large number of industries, from biomedical sensing and treatment to light strong structural composites to energy generation to waste remediation.
What should I study if I'm interested in nanotechnology? Nanoscale science and engineering is broad and interdisciplinary. The main avenues for getting into cutting edge work at these scales remain condensed matter physics, physical chemistry, and electrical engineering programs, though there are exceptionally good people working at the nanoscale in bio, bioengineering, chemical engineering, and mechanical engineering programs as well. The best approach, in my opinion, is to get a first-rate education in one of these traditional disciplines and focus on the nano, if you want to make scientific or engineering research contributions. Broad nano overview programs right now are better suited to people who want to be scientifically literate for decision-making (e.g. managers or patent lawyers) rather than those who want to do the science and engineering.
Is there really substance behind the hype? Is nanotechnology actually going somewhere? There is definitely substance behind some of the hype. As a very recent example, this new paper in Nature Nanotechnology reports a way of making lithium ion battery electrodes from silicon nanowires. Because it's in nanowire form, the Si can take up huge amounts of Li without the resulting strain pulverizing the Si. Between that and the huge specific surface area of the nanowires, real gains over conventional batteries should be possible. Best of all, industrial scaleup of Si nanowire growth looks achievable.
That's just one example from the past week. There is an awful lot of silliness out there, too, however. We're not going to have nanorobots swimming through our bodies repairing our capillaries. We're not going to have self-reproducing nanomachines assembling rocket engines one atom at a time out of single-crystal diamond. Getting a real science or engineering education gives you the critical skills and knowledge to tell the difference between credible and incredible claims.
Is going into nanotechnology a stable career path relative to alternatives? Another reason to get a solid education in a traditional science or engineering discipline is that you shouldn't be limited to just "nano" stuff. Frankly, I think this would be far more useful in just about any career path (including law or medicine) than an undergrad degree in business. Still, there are no guarantees - learn to be flexible, learn to think critically, and learn to solve problems.
Texas and "creation science"
Is it a coincidence that every state panel staffed with Rick Perry appointees does something to undermine science education and science literacy in this state? The latest ridiculousness comes from an advisory panel to the Texas Higher Education Coordination Board, who have recommended in favor of recognizing Masters of Science Education degrees granted by the Institute for Creation Science Research. Yes, that's right - this isn't even the subtle creationism of "Intelligent Design". This is full-on young Earth creationism, as explained on the ICR's own FAQ page:
To the arguments of the Houston Chronicle against granting the ICR request, let me add two more (both admittedly self-serving): this seriously hurts our ability to recruit high tech professionals to this state, and this puts Texas science and engineering faculty at a competitive disadvantage for funding. For example, ordinarily it would be a plus for a large center proposal to the NSF to be coupled to the state's education initiatives. In Texas, that's not clear.
All things in the universe were created and made by God in the six literal days of the creation week described in Genesis 1:1-2:3, and confirmed in Exodus 20:8-11. The creation record is factual, historical and perspicuous; thus all theories of origins or development which involve evolution in any form are false. All things which now exist are sustained and ordered by God's providential care. However, a part of the spiritual creation, Satan and his angels, rebelled against God after the creation and are attempting to thwart His divine purposes in creation.Remember, if you believe in evolution in any form, or that the universe is actually 13 billion years old, according to these folks who want to staff science faculty positions in Texas you have been corrupted by Satan and his agents. Great.
To the arguments of the Houston Chronicle against granting the ICR request, let me add two more (both admittedly self-serving): this seriously hurts our ability to recruit high tech professionals to this state, and this puts Texas science and engineering faculty at a competitive disadvantage for funding. For example, ordinarily it would be a plus for a large center proposal to the NSF to be coupled to the state's education initiatives. In Texas, that's not clear.
Saturday, December 22, 2007
Books on science this holiday season
From his new book, I am America (and so can you!), part of Stephen Colbert's view of science:
" 'Why?' -- The question scientists are always asking. You know who else is always asking 'why?' ? Five year olds! That's the kind of intellectual level we're dealing with here."
That's the best justification for my desire to be a scientist that I've seen since I read Tom Weller's book Science Made Stupid back when I was in high school:
"What is science? Put most simply, science is a way of dealing with the world around us. It is a way of baffling the uninitiated with incomprehensible jargon. It is a way of obtaining fat government grants. It is a way of achieving mastery over the physical world by threatening it with destruction."
I've also been reading Uncertainty, a very well-written book about the birth of quantum mechanics that focuses mostly on the personalities of the major players. It's a compelling story, though there are no major surprises: Heisenberg was ludicrously bright; Bohr was incapable of writing a short, declarative statement; Pauli was a sarcastic bastard who could get away with it because he was brilliant; Einstein was already the grand old man.
I can also recommend American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer. I'm not done with this one yet, but it's extremely interesting. If you thought that socially awkward, neurotic people going into science was a recent phenomenon, think again.
" 'Why?' -- The question scientists are always asking. You know who else is always asking 'why?' ? Five year olds! That's the kind of intellectual level we're dealing with here."
That's the best justification for my desire to be a scientist that I've seen since I read Tom Weller's book Science Made Stupid back when I was in high school:
"What is science? Put most simply, science is a way of dealing with the world around us. It is a way of baffling the uninitiated with incomprehensible jargon. It is a way of obtaining fat government grants. It is a way of achieving mastery over the physical world by threatening it with destruction."
I've also been reading Uncertainty, a very well-written book about the birth of quantum mechanics that focuses mostly on the personalities of the major players. It's a compelling story, though there are no major surprises: Heisenberg was ludicrously bright; Bohr was incapable of writing a short, declarative statement; Pauli was a sarcastic bastard who could get away with it because he was brilliant; Einstein was already the grand old man.
I can also recommend American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer. I'm not done with this one yet, but it's extremely interesting. If you thought that socially awkward, neurotic people going into science was a recent phenomenon, think again.
Tuesday, December 18, 2007
Research and corporations
This article from the NY Times is worth reading. Basically it points out something I've said repeatedly in the past: some corporations think that they can substitute university-funded research for what used to get done in big industrial research labs (like Bell Labs, IBM, Xerox, GM, Westinghouse, Ford Scientific, RCA, etc.). I think that there's definitely a place for corporate funding of university research, provided the company goes into this with their eyes open and conscious of the realities of academic work. However, I don't think university research projects will ever be able to approach the total intellectual effort that IBM or Bell could bring to bear on an important problem. If Bell decided that some piece of solid state physics was important, they could put 35 PhDs onto the problem. There just isn't that kind of concentration of expertise at a university - we're all working on different areas, for the most part.
Monday, December 17, 2007
Magnetite
Now that it's been published online, I can talk about our new paper about magnetite. Back in August I wrote a post about the different types of papers that I've been involved with. This one fits the third category that I'd mentioned, the (Well-Motivated) Surprise, and it's been a fun one.
Background: Magnetite is Fe3O4, also known as lodestone. This material is a ferrimagnet, meaning that it has two interpenetrating lattices of magnetic ions with oppositely directed polarizations of different magnitudes. Since one polarization wins, the material acts in many ways like a ferromagnet, which is how it was first used in technology: to make primitive compasses. The magnetic ordering temperature for magnetite is about 860 K. Anyway, at room temperature magnetite has a crystal structure called inverse spinel, with two kinds of lattice sites for iron atoms. The A sites are each in the center of a tetrahedron with oxygen atoms at the corners, and are occupied by Fe(3+). The B sites (there are twice as many as A sites) are each in the center of an oxygen octahedron, and are occupied by a 50/50 mix of Fe(3+) and Fe(2+), according to chemical formal charges.
It's been known for nearly 70 years that the simple single-electron band theory of solids (so good at describing Si, for example) does a lousy job at describing magnetite. Fe3O4 is a classic example of a strongly correlated material, meaning that electron-electron interactions aren't negligible. At room temperature it's moderately conducting, with a resistivity of a few milli-Ohm-cm. That's 1000 times worse than Cu, but still not too bad. When cooled, the resistivity goes weakly up with decreasing temperature (not a standard metal or semiconductor!), and at about 120 K the material goes through the Verwey transition, below which it becomes much more insulating. Verwey first noticed this in 1939, and suggested that conduction at high temperatures was through shifting valence of the B-site irons, while below the transition the B-site irons formed a charge ordered state. People have been arguing about this ever since, sometimes with amusing juxtapositions (hint: look at the titles and publication dates on those links).
Motivation: I'd been interested for a while about trying to do some nanoscale transport measurements in strongly correlated systems. The problem is, most relevant materials are very difficult to work with - not air stable, difficult to prepare, etc. Magnetite is at least a well-defined compound, and the Verwey transition acts as something of a gauge of material quality, at least in bulk. Screw up the oxygen content by a couple of percent, and the transition temperature falls through the floor.
What did we find: In two different kinds of magnetite nanostructures, we found that the I-V characteristics become dramatically hysteretic once the sample is cooled below the Verwey transition. This was completely unexpected! Basically it looks like you can take the system, which wants to be a decent insulator in equilibrium at low temperatures, and kick it back into a conducting state by applying a large enough electric field. Reduce the field back down, and the system remains in the conducting state until you pass a lower threshold, and then the magnetite snaps back into being an insulator. We worked very hard to check that this was not just some weird self-heating problem, and that's all described in the paper. I should point out that other strongly correlated insulators (vanadium oxides; some perovskite oxides) seem to be capable of qualitatively similar transitions. Hopefully we'll be able to use this transition as a way of getting a better handle on the nature of the Verwey transition itself - in particular, the role of structural degrees of freedom as well as electronic correlations.
Background: Magnetite is Fe3O4, also known as lodestone. This material is a ferrimagnet, meaning that it has two interpenetrating lattices of magnetic ions with oppositely directed polarizations of different magnitudes. Since one polarization wins, the material acts in many ways like a ferromagnet, which is how it was first used in technology: to make primitive compasses. The magnetic ordering temperature for magnetite is about 860 K. Anyway, at room temperature magnetite has a crystal structure called inverse spinel, with two kinds of lattice sites for iron atoms. The A sites are each in the center of a tetrahedron with oxygen atoms at the corners, and are occupied by Fe(3+). The B sites (there are twice as many as A sites) are each in the center of an oxygen octahedron, and are occupied by a 50/50 mix of Fe(3+) and Fe(2+), according to chemical formal charges.
It's been known for nearly 70 years that the simple single-electron band theory of solids (so good at describing Si, for example) does a lousy job at describing magnetite. Fe3O4 is a classic example of a strongly correlated material, meaning that electron-electron interactions aren't negligible. At room temperature it's moderately conducting, with a resistivity of a few milli-Ohm-cm. That's 1000 times worse than Cu, but still not too bad. When cooled, the resistivity goes weakly up with decreasing temperature (not a standard metal or semiconductor!), and at about 120 K the material goes through the Verwey transition, below which it becomes much more insulating. Verwey first noticed this in 1939, and suggested that conduction at high temperatures was through shifting valence of the B-site irons, while below the transition the B-site irons formed a charge ordered state. People have been arguing about this ever since, sometimes with amusing juxtapositions (hint: look at the titles and publication dates on those links).
Motivation: I'd been interested for a while about trying to do some nanoscale transport measurements in strongly correlated systems. The problem is, most relevant materials are very difficult to work with - not air stable, difficult to prepare, etc. Magnetite is at least a well-defined compound, and the Verwey transition acts as something of a gauge of material quality, at least in bulk. Screw up the oxygen content by a couple of percent, and the transition temperature falls through the floor.
What did we find: In two different kinds of magnetite nanostructures, we found that the I-V characteristics become dramatically hysteretic once the sample is cooled below the Verwey transition. This was completely unexpected! Basically it looks like you can take the system, which wants to be a decent insulator in equilibrium at low temperatures, and kick it back into a conducting state by applying a large enough electric field. Reduce the field back down, and the system remains in the conducting state until you pass a lower threshold, and then the magnetite snaps back into being an insulator. We worked very hard to check that this was not just some weird self-heating problem, and that's all described in the paper. I should point out that other strongly correlated insulators (vanadium oxides; some perovskite oxides) seem to be capable of qualitatively similar transitions. Hopefully we'll be able to use this transition as a way of getting a better handle on the nature of the Verwey transition itself - in particular, the role of structural degrees of freedom as well as electronic correlations.
Tuesday, December 11, 2007
Hahvahd and the burden of financial excess.
As pointed out by Julianne at Cosmic Variance, the president of Harvard had this to say about the combined issue of declining federal science research (in real dollars) and Harvard's soul-crushing dilemma of extreme wealth:
"One thing we all must worry about — I certainly do — is the federal support for scientific research. And are we all going to be chasing increasingly scarce dollars?" says Drew Gilpin Faust, Harvard's new president.Wow. So much for thinking that Larry Summers' arrogance was anomalous.Not that Faust seems worried about Harvard or other top-tier research schools. "They're going to be—we hope, we trust, we assume—the survivors in this race," she says. As for the many lesser universities likely to lose market share, she adds, they would be wise "to really emphasize social science or humanities and have science endeavors that are not as ambitious" as those of Harvard and its peers.
Thursday, December 06, 2007
Abstract fun
I spent my day at APS headquarters sorting abstracts for the March Meeting, the big condensed matter gathering that now approaches 7000 talks and posters. This is the second time I've done this, and it's always an interesting experience. When people submit abstracts they are supposed to choose a sorting category so that their talk ends up in an appropriate session - that way the audience will hopefully include people that actually are interested in the subject of the work. The contributed talks at the March Meeting are each 10 minutes, with 2 minutes for questions. Often these talks are the first chance a graduate student gets to present their work in a public forum before other scientists. Unfortunately 10 minutes is very short, so much so that often only near-experts in an area can get much out of such a brief explanation of results. There are also invited talks that are 30 minutes with 6 minutes for questions. These can be arranged in Invited Sessions, where all the talks are invited, and the session theme and potential speakers are nominated and voted upon by the program committee. Alternately, there are mixed Focus Topic sessions that typically have one or two invited talks mixed in with contributed ones.
The first big challenge in sorting the abstracts is that the sorting categories often overlap. For example, there were at least four different categories where people could have submitted abstracts about electronic properties of quantum dots. Surprisingly, about 80 people pushing around 7000 slips of barcoded paper is a reasonably efficient way of sorting. The second major issue in organizing the meeting is that space is very limited, and sessions are highly constrained - you don't want a contributed session to take place at the same time as an invited session on a closely related area, for example.
Helping to put together meetings like this is a bit like the scientific equivalent of jury duty. You want to make sure that it gets done well by people whose judgment you trust, but you don't want to have to do it yourself very often. It is a good way to get meet your fellow physicists, though.
The first big challenge in sorting the abstracts is that the sorting categories often overlap. For example, there were at least four different categories where people could have submitted abstracts about electronic properties of quantum dots. Surprisingly, about 80 people pushing around 7000 slips of barcoded paper is a reasonably efficient way of sorting. The second major issue in organizing the meeting is that space is very limited, and sessions are highly constrained - you don't want a contributed session to take place at the same time as an invited session on a closely related area, for example.
Helping to put together meetings like this is a bit like the scientific equivalent of jury duty. You want to make sure that it gets done well by people whose judgment you trust, but you don't want to have to do it yourself very often. It is a good way to get meet your fellow physicists, though.
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