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.
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