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.
Sunday, December 30, 2007
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.
Saturday, December 01, 2007
Texas, you're not making this any easier.
Well, looks like it's time for another of my once-every-few-months occasions to be severely disappointed in public agencies in Texas. This time the director of the state's public school science curriculum has been forced out, apparently because she prefers evolution to "intelligent design". This is just pathetic. While I appreciate Eric Berger's spirited defense of Texas (in short, we're not all antiscience zealots), the steady stream of this stuff from Austin is unquestionably depressing.
Tuesday, November 27, 2007
"Unparticles" and condensed matter
At the risk of contributing to what has recently been called the intellectual wasteland that is the physics blogosphere, I want to point out a nice review paper on the arxiv, and its connection to high energy physics. Subir Sachdev at Harvard has put up a relatively pedagogical review about quantum magnetism and criticality. Back when I was a grad student, I didn't appreciate that quantum magnetic systems were so interesting - I thought that they were a zoo or menagerie of semi-random compounds that happened to have effective model Hamiltonians of interest only to rather esoteric theorists. Now I understand the appeal - the relevant Hamiltonians can have some truly bizarre solutions that can be relevant (intellectually if not directly) to whole classes of systems. One class of such systems is the heavy fermion compounds that are non-Fermi liquids, and another comprises some exotic "spin liquids". The low energy excitations of these strongly correlated quantum systems are not readily described as particle-like or wave-like. They don't have simple quantum numbers and simple dispersion relations, and they result from complicated, correlated motion of electrons (or spins, or both). This has been known in condensed matter circles for some time, and is very neat. Much exciting theory work is being done to come up with good ways to treat such systems.
What I don't understand, and perhaps a reader can enlighten me, is how these ideas relate to "unparticles". Howard Georgi, also of Harvard, made a pretty big splash this past year by publishing a PRL (linked above in free form) about the possibility that there may be fundamental excitations of quantum fields (like the ones thought to be relevant in high energy physics) that are not simply described as particles. Since this paper came out, there are now 78 papers on the arxiv that deal with unparticles. So, is this a case of high energy physics reinventing an idea that's been known conceptually for some time in condensed matter? Or is there really a basic underlying difference here? I should point out that at present, while there is experimental evidence for non-particle-like excitations in condensed matter, there is not yet any evidence for such things in high energy experiments as far as I know.
What I don't understand, and perhaps a reader can enlighten me, is how these ideas relate to "unparticles". Howard Georgi, also of Harvard, made a pretty big splash this past year by publishing a PRL (linked above in free form) about the possibility that there may be fundamental excitations of quantum fields (like the ones thought to be relevant in high energy physics) that are not simply described as particles. Since this paper came out, there are now 78 papers on the arxiv that deal with unparticles. So, is this a case of high energy physics reinventing an idea that's been known conceptually for some time in condensed matter? Or is there really a basic underlying difference here? I should point out that at present, while there is experimental evidence for non-particle-like excitations in condensed matter, there is not yet any evidence for such things in high energy experiments as far as I know.
Friday, November 23, 2007
Really? Seriously?
Sometimes I read a science article online or in the newspaper that I think is poor. This one I just don't know how to interpret. Lawrence Krauss is a solid guy, a very strong public advocate for science, and a very good popularizer of physics. Still, the idea that our observations of dark energy have somehow collapsed the quantum state of the entire universe is, umm, nuts on the same level as saying that the moon doesn't exist if no one is looking at it. There's no question that there are subtleties in worrying about applying quantum mechanics to the universe as a whole. Still, this carries the "spooky action at a distance" idea a bit far.
Saturday, November 17, 2007
This week in cond-mat
Two papers this week. I'll write about our own at a later date. These two are both connected to on-going long-term controversies in condensed matter/mesoscopic/nanoscale physics.
arxiv:0711.1810 - Capron et al., Low temperature dephasing in irradiated metallic wires
In the orthodox picture of metals (thought to be valid for relatively weak disorder), the quantum coherence time of electrons is expected to diverge toward infinity as the temperature approaches zero. Think about a two-slit experiment for electrons. If the electrons are well isolated from their environment, they can diffract off the slits and land on the screen, producing an interference pattern. If the electrons are coupled to environmental degrees of freedom that can change their state when the electron goes by, the relative phase of the electron wavefunctions going through each of the slits gets scrambled by that interaction, washing out the interference. In the usual 2-slit experiment, the degrees of freedom are those of detectors at the slits. Within a disordered metal, those environmental degrees of freedom can be lattice vibrations, other electrons, or magnetic impurities. For a decade now there has been an ongoing controversy about whether the coherence time (as inferred from some quantum correction to the classical electronic conductance) really does diverge, or whether it saturates as T -> 0. Intrinsic saturation would be a big deal - it would imply that the quasiparticle picture of electrons (Fermi liquid theory) fails at the low T limit. In this paper, the authors perform a very careful control experiment, looking at whether structural damage to silver nanowires can, by itself, introduce extra degrees of freedom that cause decoherence. They get this damage by ion-implanting Ag ions into Ag nanowires. The results show no sign of extra decoherence due to this irradiation.
arxiv:0711.1464 - Baenninger et al., Low-temperature collapse of electron localisation (sic) in two dimensions
Another ongoing brouhaha has been about whether electrons confined to two dimensions have an insulating or metallic ground state in the presence of any disorder. Without interactions, the "Gang of Four" (Anderson, Abrahams, Ramakrishnan, and Licciardello) showed that even infinitesimal disorder leads to localization and an insulating ground state for an infinite 2d system. Of course, real electrons do interact with each other, and real systems are of finite size. One big complication in this whole discussion is in trying to tell the difference between a real, uniform, insulating state and the breakup of your system into inhomogeneous "puddles" of electrons due to the disorder potential. The Cambridge group has done some careful experiments in mesoscopic samples of rather clean 2d electron gas, and they've found that small regions with higher temperature resistances far exceeding the quantum of resistance (~ h/e^2 ~ 26 kOhms) can show a crossover at low temperatures to what looks like a metallic state. I haven't been following this controversy in detail, but these data look very interesting, and I will have to read this closely.
arxiv:0711.1810 - Capron et al., Low temperature dephasing in irradiated metallic wires
In the orthodox picture of metals (thought to be valid for relatively weak disorder), the quantum coherence time of electrons is expected to diverge toward infinity as the temperature approaches zero. Think about a two-slit experiment for electrons. If the electrons are well isolated from their environment, they can diffract off the slits and land on the screen, producing an interference pattern. If the electrons are coupled to environmental degrees of freedom that can change their state when the electron goes by, the relative phase of the electron wavefunctions going through each of the slits gets scrambled by that interaction, washing out the interference. In the usual 2-slit experiment, the degrees of freedom are those of detectors at the slits. Within a disordered metal, those environmental degrees of freedom can be lattice vibrations, other electrons, or magnetic impurities. For a decade now there has been an ongoing controversy about whether the coherence time (as inferred from some quantum correction to the classical electronic conductance) really does diverge, or whether it saturates as T -> 0. Intrinsic saturation would be a big deal - it would imply that the quasiparticle picture of electrons (Fermi liquid theory) fails at the low T limit. In this paper, the authors perform a very careful control experiment, looking at whether structural damage to silver nanowires can, by itself, introduce extra degrees of freedom that cause decoherence. They get this damage by ion-implanting Ag ions into Ag nanowires. The results show no sign of extra decoherence due to this irradiation.
arxiv:0711.1464 - Baenninger et al., Low-temperature collapse of electron localisation (sic) in two dimensions
Another ongoing brouhaha has been about whether electrons confined to two dimensions have an insulating or metallic ground state in the presence of any disorder. Without interactions, the "Gang of Four" (Anderson, Abrahams, Ramakrishnan, and Licciardello) showed that even infinitesimal disorder leads to localization and an insulating ground state for an infinite 2d system. Of course, real electrons do interact with each other, and real systems are of finite size. One big complication in this whole discussion is in trying to tell the difference between a real, uniform, insulating state and the breakup of your system into inhomogeneous "puddles" of electrons due to the disorder potential. The Cambridge group has done some careful experiments in mesoscopic samples of rather clean 2d electron gas, and they've found that small regions with higher temperature resistances far exceeding the quantum of resistance (~ h/e^2 ~ 26 kOhms) can show a crossover at low temperatures to what looks like a metallic state. I haven't been following this controversy in detail, but these data look very interesting, and I will have to read this closely.
Monday, November 12, 2007
Potpourri
A small selection of links....
This game is very addictive, educational, and as you play, you feed the hungry (albeit extremely slowly).
Now this is a nanotube radio! Rather than having the nanotube just be the nonlinear element responsible for demodulating the AM signal on the carrier wave, this one has the nanotube acting as the antenna and amplifier as well, effectively. I heard Alex Zettl get interviewed on NPR about it.
The FSP has an interesting post about ambition. Physics as a discipline has issues with this, with an historical attitude that anything less than a tenured job at Harvard is somehow inadequate - a notion that's wrongheaded and sociologically unhealthy.
Schlupp has a post about a little frustrating science journalism. It is a shame that sometimes the media can't tell the difference between good science or engineering and crackpottery. On a plane last week I had someone (who realized I was a physicist from my reading material) ask me about the guy who can get hydrogen from seawater by hitting it with microwaves. Kind of cool, yes. Source of energy? Of course not - it takes more microwave power to break the water into hydrogen and oxygen than you can get back by burning the resulting hydrogen. It's called thermodynamics.
This game is very addictive, educational, and as you play, you feed the hungry (albeit extremely slowly).
Now this is a nanotube radio! Rather than having the nanotube just be the nonlinear element responsible for demodulating the AM signal on the carrier wave, this one has the nanotube acting as the antenna and amplifier as well, effectively. I heard Alex Zettl get interviewed on NPR about it.
The FSP has an interesting post about ambition. Physics as a discipline has issues with this, with an historical attitude that anything less than a tenured job at Harvard is somehow inadequate - a notion that's wrongheaded and sociologically unhealthy.
Schlupp has a post about a little frustrating science journalism. It is a shame that sometimes the media can't tell the difference between good science or engineering and crackpottery. On a plane last week I had someone (who realized I was a physicist from my reading material) ask me about the guy who can get hydrogen from seawater by hitting it with microwaves. Kind of cool, yes. Source of energy? Of course not - it takes more microwave power to break the water into hydrogen and oxygen than you can get back by burning the resulting hydrogen. It's called thermodynamics.
Monday, November 05, 2007
This week in cond-mat
Several entries from the arxiv this week. My descriptions here are a bit brief b/c of continued real-world constraints.
arxiv:0711.0343 - Dietl, Origin and control of ferromagnetism in dilute magnetic semiconductors and oxides
arxiv:0711.0340 - Dietl, Origin of ferromagnetic response in diluted magnetic semiconductors and oxides
These are two review articles by Tomasz Dietl, one of the big names in the dilute magnetic semiconductor (DMS) game. DMS are semiconductor materials that exhibit ferromagnetic order usually because of doping with transition metal atoms that contain unpaired d electrons, such as manganese. The idea of integrating magnetic materials directly with semiconductor devices, and ideally controlling magnetism via electrical or optical means, is quite appealing. However, it is very challenging to achieve high magnetic ordering temperatures (e.g., room temperature) and decent electronic properties at the same time. In many systems the high doping levels required for the magnetism go hand in hand with lots of disorder, in part because crystal growth must be performed under nonequilibrium conditions to force enough transition metal atoms to sit on the appropriate lattice sites. Anyway, these articles (one coming out in J. Phys.: Cond. Matt.
and the other coming out in J. Appl. Phys.) should give you plenty of reading material if you're interested in this area.
arxiv:0711.0218 - Leek et al., Observation of Berry's phase in a solid state qubit
In basic quantum mechanics we learn that particles are described by a complex wavefunction that has a phase factor. Propagation of a particle in space racks up phase at a rate proportional to the particle's momentum. As Feynman would tell us, each possible trajectory of a particle from A to B then contributes some complex amplitude (with a phase). The total probability of finding the particle at B is the squared magnitude of the sum of all of those amplitudes, rather than the classical sum of the probabilities of each path. Phase differences between paths lead to interference terms, and are the sort of thing responsible for electro diffraction, for example. Besides propagating through space, there are other ways of accumulating phase. In the case of the Aharanov-Bohm effect, the vector potential leads to an additional phase factor that depends on trajectory. In the general case of Berry's Phase, the slow variation of some external parameters (such as electric fields) can lead to a similar geometrical phase factor. The intro to this paper gives a nice discussion of the classical analog of this in terms of moving a little vector on the surface of a sphere. Anyway, this team has used a solid-state superconducting qubit to demonstrate this geometric phase explicitly. Quite nice.
arxiv:0710.5515 - Castelnovo et al., Magnetic monopoles in spin ice
One of the things that I find so interesting about condensed matter physics is the idea of emergent degrees of freedom. For example, phonons (quantized sound waves) are quantum mechanical quasiparticles in solids that can have well-defined quantum numbers, and arise because of the collective motion of large numbers of atoms. In a more exotic example, Cooper pairs in ordinary superconductors are objects with spin 0, charge -2e, yet are "built" out of electrons plus phonons. In a very exotic example, the quasiparticles in the fractional quantum Hall effect can have fractional charges and obey exotic statistics. In an even more extreme case, these authors propose that there are quasiparticle excitations in a kind of magnetically ordered insulator that act like magnetic monopoles. It seems that magnetic monopoles do not exist as elementary particles. Indeed, they would require a modification of Maxwell's equations. (In this solid state system the argument is that they exist as monopole/antimonopole pairs, so that the net divergence of the magnetic field is still zero). "Forbidden" particles emerging from the collective action of many electrons - a very neat idea, and it would appear that there may even be some experimental evidence for this already.
arxiv:0711.0343 - Dietl, Origin and control of ferromagnetism in dilute magnetic semiconductors and oxides
arxiv:0711.0340 - Dietl, Origin of ferromagnetic response in diluted magnetic semiconductors and oxides
These are two review articles by Tomasz Dietl, one of the big names in the dilute magnetic semiconductor (DMS) game. DMS are semiconductor materials that exhibit ferromagnetic order usually because of doping with transition metal atoms that contain unpaired d electrons, such as manganese. The idea of integrating magnetic materials directly with semiconductor devices, and ideally controlling magnetism via electrical or optical means, is quite appealing. However, it is very challenging to achieve high magnetic ordering temperatures (e.g., room temperature) and decent electronic properties at the same time. In many systems the high doping levels required for the magnetism go hand in hand with lots of disorder, in part because crystal growth must be performed under nonequilibrium conditions to force enough transition metal atoms to sit on the appropriate lattice sites. Anyway, these articles (one coming out in J. Phys.: Cond. Matt.
and the other coming out in J. Appl. Phys.) should give you plenty of reading material if you're interested in this area.
arxiv:0711.0218 - Leek et al., Observation of Berry's phase in a solid state qubit
In basic quantum mechanics we learn that particles are described by a complex wavefunction that has a phase factor. Propagation of a particle in space racks up phase at a rate proportional to the particle's momentum. As Feynman would tell us, each possible trajectory of a particle from A to B then contributes some complex amplitude (with a phase). The total probability of finding the particle at B is the squared magnitude of the sum of all of those amplitudes, rather than the classical sum of the probabilities of each path. Phase differences between paths lead to interference terms, and are the sort of thing responsible for electro diffraction, for example. Besides propagating through space, there are other ways of accumulating phase. In the case of the Aharanov-Bohm effect, the vector potential leads to an additional phase factor that depends on trajectory. In the general case of Berry's Phase, the slow variation of some external parameters (such as electric fields) can lead to a similar geometrical phase factor. The intro to this paper gives a nice discussion of the classical analog of this in terms of moving a little vector on the surface of a sphere. Anyway, this team has used a solid-state superconducting qubit to demonstrate this geometric phase explicitly. Quite nice.
arxiv:0710.5515 - Castelnovo et al., Magnetic monopoles in spin ice
One of the things that I find so interesting about condensed matter physics is the idea of emergent degrees of freedom. For example, phonons (quantized sound waves) are quantum mechanical quasiparticles in solids that can have well-defined quantum numbers, and arise because of the collective motion of large numbers of atoms. In a more exotic example, Cooper pairs in ordinary superconductors are objects with spin 0, charge -2e, yet are "built" out of electrons plus phonons. In a very exotic example, the quasiparticles in the fractional quantum Hall effect can have fractional charges and obey exotic statistics. In an even more extreme case, these authors propose that there are quasiparticle excitations in a kind of magnetically ordered insulator that act like magnetic monopoles. It seems that magnetic monopoles do not exist as elementary particles. Indeed, they would require a modification of Maxwell's equations. (In this solid state system the argument is that they exist as monopole/antimonopole pairs, so that the net divergence of the magnetic field is still zero). "Forbidden" particles emerging from the collective action of many electrons - a very neat idea, and it would appear that there may even be some experimental evidence for this already.
Wednesday, October 31, 2007
In honor of Halloween....
Three of my favorite science-related quotes from the movies, all from Ghostbusters:
Dean Teager: Your theories are the worst kind of popular tripe; your methods are sloppy, and your conclusions are highly questionable. You are a poor scientist, Dr. Venkman.
---
Ray Stantz: Personally, I like the University. They gave us money and facilities, we didn't have to produce anything. You've never been out of college. You don't know what it's like out there. I've worked in the private sector. They expect results.
---
Peter Venkman: Back off, man! I'm a scientist!
Any other good ones to share? (Real science post coming in a day or two....)
Dean Teager: Your theories are the worst kind of popular tripe; your methods are sloppy, and your conclusions are highly questionable. You are a poor scientist, Dr. Venkman.
---
Ray Stantz: Personally, I like the University. They gave us money and facilities, we didn't have to produce anything. You've never been out of college. You don't know what it's like out there. I've worked in the private sector. They expect results.
---
Peter Venkman: Back off, man! I'm a scientist!
Any other good ones to share? (Real science post coming in a day or two....)
Friday, October 26, 2007
Jobs jobs jobs
I figure it's probably a good idea to take advantage of the staggeringly enormous readership of this blog to point out several searches going on at Rice right now.
First, three searches are going on here at Rice in the Physics and Astronomy department at the moment. These are:
Finally, the Chemistry department is doing a search for inorganic or physical chemists, broadly defined. The ad is on the departmental homepage.
Share and enjoy! If you want to discuss what Rice is like as a faculty member, please feel free to contact me and I'll be happy to talk.
First, three searches are going on here at Rice in the Physics and Astronomy department at the moment. These are:
- A tenure-track faculty position in atomic/molecular/optical theory, ultracold atoms in particular.
- A tenure-track faculty position in nuclear experiment, heavy ion collisions in particular.
- A prestigious named instructor position.
Finally, the Chemistry department is doing a search for inorganic or physical chemists, broadly defined. The ad is on the departmental homepage.
Share and enjoy! If you want to discuss what Rice is like as a faculty member, please feel free to contact me and I'll be happy to talk.
Friday, October 19, 2007
Three papers and a video.
Three interesting papers on ASAP at Nano Letters at the moment:
http://dx.doi.org/10.1021/nl0717715 and http://dx.doi.org/10.1021/nl072090c are both papers where people have taken graphite flakes, oxidized them to make graphite oxide, and then suspended the graphene oxide sheets in solvent. They then deposit the sheets onto substrates and made electronic devices out of them after trying to reduce the graphene oxide back to just graphene. There are a couple of people here at Rice trying similar things from the chemistry side. Interesting that a number of groups are all working on this at about the same time. That's one reason why it can be dangerous to try to jump into a rapidly evolving hot topic - it's easy to get scooped.
This one is a cute paper titled "Carbon nanotube radio". The science is nicely done, though not exactly surprising. AM radio works by taking an rf carrier signal and demodulating it to get back just the envelope of that carrier signal. Back in the early 20th century (or more recently, if you bought an old kit somewhere), people used to do the demodulating using a diode made semi-reliably by jamming a metal needle (a "cat's whisker") into a lead sulfide crystal - hence the term "crystal radio". It's simple trig math to see that a nonlinear IV curve (one with a nonzero d^2I/dV^2) can rectify an ac signal of amplitude V0 to give a dc signal of (1/4)(d^2I/dV^2)V0^2. Well, in this case the nonlinear element is a nanotube device. Cute, though I have to admit that I found the media hype a bit much. Wilson Ho did the same essential thing very nicely with an STM, but didn't talk about atomic-scale radio receivers....
Lastly, via Scott Aaronson, a link to a fantastic math presentation. Watch the whole thing - this really is a model of clarity and public outreach. On a bitter-sweet note, in the credits at the end I realized that one of the people responsible for this was an acquaintance from college who has since passed away. Small world.
http://dx.doi.org/10.1021/nl0717715 and http://dx.doi.org/10.1021/nl072090c are both papers where people have taken graphite flakes, oxidized them to make graphite oxide, and then suspended the graphene oxide sheets in solvent. They then deposit the sheets onto substrates and made electronic devices out of them after trying to reduce the graphene oxide back to just graphene. There are a couple of people here at Rice trying similar things from the chemistry side. Interesting that a number of groups are all working on this at about the same time. That's one reason why it can be dangerous to try to jump into a rapidly evolving hot topic - it's easy to get scooped.
This one is a cute paper titled "Carbon nanotube radio". The science is nicely done, though not exactly surprising. AM radio works by taking an rf carrier signal and demodulating it to get back just the envelope of that carrier signal. Back in the early 20th century (or more recently, if you bought an old kit somewhere), people used to do the demodulating using a diode made semi-reliably by jamming a metal needle (a "cat's whisker") into a lead sulfide crystal - hence the term "crystal radio". It's simple trig math to see that a nonlinear IV curve (one with a nonzero d^2I/dV^2) can rectify an ac signal of amplitude V0 to give a dc signal of (1/4)(d^2I/dV^2)V0^2. Well, in this case the nonlinear element is a nanotube device. Cute, though I have to admit that I found the media hype a bit much. Wilson Ho did the same essential thing very nicely with an STM, but didn't talk about atomic-scale radio receivers....
Lastly, via Scott Aaronson, a link to a fantastic math presentation. Watch the whole thing - this really is a model of clarity and public outreach. On a bitter-sweet note, in the credits at the end I realized that one of the people responsible for this was an acquaintance from college who has since passed away. Small world.
Tuesday, October 16, 2007
This week in cond-mat
Real life continues to be very busy this semester. Two interesting papers on the arxiv this week....
arxiv:0710.2845 - Fratini et al., Current saturation and Coulomb interactions in organic single-crystal transistors
The technology finally exists to do what He Who Must Not Be Named claimed to have done: use a field-effect geometry to gate significant charge densities (that is, a good fraction of a charge carrier per molecule) into the surface of a clean single crystal of an organic semiconductor. The Delft group has used Ta2O5 as a high-k gate dielectric, and are able to get 0.1 holes per rubrene atom in a single-crystal FET geometry. In typical organic FETs, increasing the charge density in the channel improves transport by filling trap states and by moving the chemical potential in the channel toward the mobility edge in the density of states. Surprisingly, Fratini et al. have found that the channel conductance actually saturates at very high charge densities instead of continuing to increase. The reason for this appears to be Coulomb interactions in the channel due to the high carrier density and the polaronic nature of the holes. The strong coupling between the carriers and the dielectric layer leads to a tendency toward self-trapping; add strong repulsion and poor screening into the mix, and you have a more insulating state induced by this combination of effects. Very interesting!
arxiv:0710.2323 - Degen et al., Controlling spin noise in nanoscale ensembles of nuclear spins
Dan Rugar at IBM has been working on magnetic resonance force microscopy for a long time, and they've got sensitivity to the point where they can detect hundreds of nuclear spins (!). (That may not seem impressive if you haven't been following this, but it's a tour de force experiment that's come very far from the initial work.) The basic idea of MRFM is to have a high-Q cantilever that is mechanically resonant at the spin resonance frequency and coupled via magnetic interactions to the sample - that way the polarized spins precess, they drive the cantilever resonance mode. When they look at such a small number of spins, the statistical fluctuations in the spin polarization are readily detected. This is a problem for imaging, actually - the timescale for the natural fluctuations is long enough that the signal bops around quite a bit during a line scan. Fortunately, Degen et al. have demonstrated in this paper that one can deliberately randomize the magnetization by bursts of rf pi/2 pulses, and thus suppress the fluctuation impact on imaging by making the effective fluctuations much more rapid. This is a nice mix of pretty physics and very clever experimental technique.
arxiv:0710.2845 - Fratini et al., Current saturation and Coulomb interactions in organic single-crystal transistors
The technology finally exists to do what He Who Must Not Be Named claimed to have done: use a field-effect geometry to gate significant charge densities (that is, a good fraction of a charge carrier per molecule) into the surface of a clean single crystal of an organic semiconductor. The Delft group has used Ta2O5 as a high-k gate dielectric, and are able to get 0.1 holes per rubrene atom in a single-crystal FET geometry. In typical organic FETs, increasing the charge density in the channel improves transport by filling trap states and by moving the chemical potential in the channel toward the mobility edge in the density of states. Surprisingly, Fratini et al. have found that the channel conductance actually saturates at very high charge densities instead of continuing to increase. The reason for this appears to be Coulomb interactions in the channel due to the high carrier density and the polaronic nature of the holes. The strong coupling between the carriers and the dielectric layer leads to a tendency toward self-trapping; add strong repulsion and poor screening into the mix, and you have a more insulating state induced by this combination of effects. Very interesting!
arxiv:0710.2323 - Degen et al., Controlling spin noise in nanoscale ensembles of nuclear spins
Dan Rugar at IBM has been working on magnetic resonance force microscopy for a long time, and they've got sensitivity to the point where they can detect hundreds of nuclear spins (!). (That may not seem impressive if you haven't been following this, but it's a tour de force experiment that's come very far from the initial work.) The basic idea of MRFM is to have a high-Q cantilever that is mechanically resonant at the spin resonance frequency and coupled via magnetic interactions to the sample - that way the polarized spins precess, they drive the cantilever resonance mode. When they look at such a small number of spins, the statistical fluctuations in the spin polarization are readily detected. This is a problem for imaging, actually - the timescale for the natural fluctuations is long enough that the signal bops around quite a bit during a line scan. Fortunately, Degen et al. have demonstrated in this paper that one can deliberately randomize the magnetization by bursts of rf pi/2 pulses, and thus suppress the fluctuation impact on imaging by making the effective fluctuations much more rapid. This is a nice mix of pretty physics and very clever experimental technique.
Wednesday, October 10, 2007
Giant magnetoresistance
I think it's great that the physics Nobel this year went for giant magnetoresistance (GMR). GMR is intrinsically a quantum mechanical effect, an example of a nanoscale technology that's made it out of the lab and into products, and one of the big reasons that you can buy a 500GB hard drive for $100. (Good job, Sujit, for the advanced pick!).
The story in brief: Back in the ancient past (that is, the 1980s), the read heads on hard drives operated based on the anisotropic magnetoresistance (AMR). For band structure reasons, the electrical resistivity of ferromagnetic metals depends a bit on the relative orientations of M, the magnetization, and J, the current density. In the common NiFe alloy permalloy, for example, the resistivity is about 2% larger when M is parallel to J than when M is perpendicular to J. To read out the bits on magnetic media, a strip of very coercible magnetic material was used, and the fringing fields from the disk media could alter the direction of that strip's M, leading to changes in the resistance that were translated into voltage changes that correspond to 1s and 0s.
In the late 1980s, Fert and Grunberg demonstrated that stacks of nanoscale layers of alternating magnetic and nonmagnetic metals had remarkable magnetoresistive properties. When the M of the FM layers are aligned, the mobile electrons can move smoothly between the layers, leading to relatively low resistance. However, when the M of the FM layers are anti-aligned, there is a mismatch between the densities of states for spin-up and spin-down electrons between anti-aligned layers. The result is enhanced scattering of spin-polarized electrons at the interfaces between the normal and FM layers. (Crudely, a spin-down electron that comes from being the majority spin in one FM layer goes through the normal metal and runs into the anti-aligned FM layer, where that spin orientation is now the minority spin - there are too few empty states available for that electron in the new FM layer, so it is likely to be reflected from the interface.) More scattering = higher resistance. The resulting GMR effect can be 10x larger than AMR, meaning that read heads based on GMR multilayers could read much smaller bits (with smaller fringing fields) for the same signal-to-noise ratio.
The story in brief: Back in the ancient past (that is, the 1980s), the read heads on hard drives operated based on the anisotropic magnetoresistance (AMR). For band structure reasons, the electrical resistivity of ferromagnetic metals depends a bit on the relative orientations of M, the magnetization, and J, the current density. In the common NiFe alloy permalloy, for example, the resistivity is about 2% larger when M is parallel to J than when M is perpendicular to J. To read out the bits on magnetic media, a strip of very coercible magnetic material was used, and the fringing fields from the disk media could alter the direction of that strip's M, leading to changes in the resistance that were translated into voltage changes that correspond to 1s and 0s.
In the late 1980s, Fert and Grunberg demonstrated that stacks of nanoscale layers of alternating magnetic and nonmagnetic metals had remarkable magnetoresistive properties. When the M of the FM layers are aligned, the mobile electrons can move smoothly between the layers, leading to relatively low resistance. However, when the M of the FM layers are anti-aligned, there is a mismatch between the densities of states for spin-up and spin-down electrons between anti-aligned layers. The result is enhanced scattering of spin-polarized electrons at the interfaces between the normal and FM layers. (Crudely, a spin-down electron that comes from being the majority spin in one FM layer goes through the normal metal and runs into the anti-aligned FM layer, where that spin orientation is now the minority spin - there are too few empty states available for that electron in the new FM layer, so it is likely to be reflected from the interface.) More scattering = higher resistance. The resulting GMR effect can be 10x larger than AMR, meaning that read heads based on GMR multilayers could read much smaller bits (with smaller fringing fields) for the same signal-to-noise ratio.
Thursday, October 04, 2007
Challenges in measurement
This post is only going to be relevant directly for those people working on the same kind of stuff that my group does. Still, it gives a flavor of the challenges that can pop up unexpectedly in doing experimental work.
Often we are interested in measuring the electronic conductance of some nanodevice. One approach to doing this is to apply a small AC voltage to one end of the device, and connect the other end to something called a current preamplifier (or a current-to-voltage converter, or a glorified ammeter) to measure the amount of current that flows. It's possible to build your own current preamp, but many nanodevice labs have a couple of general purpose ones lying around. A common one is the SR570, made by Stanford Research. This gadget is pretty nice - it has up to a 1 MHz bandwidth, it has built-in filter stages, it is remotely programmable, and it has various different gain settings depending on whether you want to measure microamps or picoamps of current.
Here's the problem, though. One of my students observed that his devices seemed to fail at a surprisingly high rate when using the SR570, while the failure rate was dramatically lower when using a different (though more expensive) preamp, the Keithley 428. After careful testing he found that when the SR570 changes gain ranges (there is an audible click of an internal relay when this happens, as the input stage of the amplifier is switched), spikes of > 1V (!) lasting tens of microseconds show up on the input of the amplifier (the part directly connected to the device), at least when hooked up to an oscilloscope. Our nanoscale junctions are very fragile, and these spikes irreversibly damage the devices. The Keithley, on the other hand, doesn't do this and is very quiet. Talking to SRS, this appears to be an unavoidable trait of the SR570. We're working to mitigate this problem, but it's probably good for people out there in the community using these things to know about this.
Often we are interested in measuring the electronic conductance of some nanodevice. One approach to doing this is to apply a small AC voltage to one end of the device, and connect the other end to something called a current preamplifier (or a current-to-voltage converter, or a glorified ammeter) to measure the amount of current that flows. It's possible to build your own current preamp, but many nanodevice labs have a couple of general purpose ones lying around. A common one is the SR570, made by Stanford Research. This gadget is pretty nice - it has up to a 1 MHz bandwidth, it has built-in filter stages, it is remotely programmable, and it has various different gain settings depending on whether you want to measure microamps or picoamps of current.
Here's the problem, though. One of my students observed that his devices seemed to fail at a surprisingly high rate when using the SR570, while the failure rate was dramatically lower when using a different (though more expensive) preamp, the Keithley 428. After careful testing he found that when the SR570 changes gain ranges (there is an audible click of an internal relay when this happens, as the input stage of the amplifier is switched), spikes of > 1V (!) lasting tens of microseconds show up on the input of the amplifier (the part directly connected to the device), at least when hooked up to an oscilloscope. Our nanoscale junctions are very fragile, and these spikes irreversibly damage the devices. The Keithley, on the other hand, doesn't do this and is very quiet. Talking to SRS, this appears to be an unavoidable trait of the SR570. We're working to mitigate this problem, but it's probably good for people out there in the community using these things to know about this.
Sunday, September 30, 2007
This week in cond-mat
Two recent papers in cond-mat this time, both rather thermodynamics-related. That's appropriate, since I'm teaching undergrad stat mech these days.
arxiv:0709.4181 - Kubala et al., Violation of Wiedemann-Franz law in a single-electron transistor
The Wiedemann-Franz law is one of those things taught in nearly every undergraduate solid-state physics class. It also happens to be extremely useful for doing cryogenic engineering, as I learned during my grad school days. The idea is simple: simple kinetic theory arguments (and dimensional analysis) imply that the conductivity of some parameter via some excitations is given by the product (carrying capacity of that parameter per excitation)*(speed of excitation carrying that parameter)*(mean free path of that excitation), with some geometric factor out in front (e.g., 1/3 for three dimensional diffusive motion of the excitation). For example, the electrical conductivity in a 3d, diffusive, ordinary metal is (1/3)(e)(v_F)(\ell), where e is the electronic charge, v_F is the Fermi velocity for conduction electrons, and \ell is the mean free path for those electrons (at low T, \ell is set by impurity scattering or boundary scattering). However, in a normal metal electrons can also carry thermal energy with some heat capacity c_v per electron that scales like T, while the speed and mean free path of the electrons are as above. This implies the Wiedemann-Franz law, that the ratio of the thermal conductivity to the (electrical conductivity*T) in an ordinary metal should be a constant (the Lorenz number, ~25 nanoOhms W/K^2). Deviations from the W-F law are indicators of interesting physics - basically that simple metal electrons either aren't the dominant carriers of the electrical current, or that the charge carriers don't carry thermal energy as normal. This paper is a theory piece by the Helsinki group showing that the W-F law fails badly for single-electron transistors. In particular, in the co-tunneling regime, when current is carried via quantum coherent processes, the Lorenz number is predicted to be renormalized upward by a factor of 9/5. This will be challenging to measure in experiments, but exquisite thermal conductivity measurements have been performed in similar systems in the past.
arxiv:0709.4125 - Allahverdyan et al., Work extremum principle: structure and function of quantum heat engines
Marlan Scully (also here) caused a bit of a flurry of excitement a few years ago by proposing a form of heat engine that uses quantum coherence and its destruction to do work, in addition to the conventional approach of using two thermal baths at different temperatures. This paper is a theoretical analysis of some such quantum heat engines. Carnot can sleep easy - in the end you can't violate the Carnot efficiency even with quantum heat engines, if you dot all the "i"s and cross all the "t"s. Neat to think about, though, and of some experimental relevance to the cold atom community, who can prepare highly coherent atomic gases at very low temperatures. This paper is long and detailed and I don't claim to have read it in depth, but it looks interesting.
arxiv:0709.4181 - Kubala et al., Violation of Wiedemann-Franz law in a single-electron transistor
The Wiedemann-Franz law is one of those things taught in nearly every undergraduate solid-state physics class. It also happens to be extremely useful for doing cryogenic engineering, as I learned during my grad school days. The idea is simple: simple kinetic theory arguments (and dimensional analysis) imply that the conductivity of some parameter via some excitations is given by the product (carrying capacity of that parameter per excitation)*(speed of excitation carrying that parameter)*(mean free path of that excitation), with some geometric factor out in front (e.g., 1/3 for three dimensional diffusive motion of the excitation). For example, the electrical conductivity in a 3d, diffusive, ordinary metal is (1/3)(e)(v_F)(\ell), where e is the electronic charge, v_F is the Fermi velocity for conduction electrons, and \ell is the mean free path for those electrons (at low T, \ell is set by impurity scattering or boundary scattering). However, in a normal metal electrons can also carry thermal energy with some heat capacity c_v per electron that scales like T, while the speed and mean free path of the electrons are as above. This implies the Wiedemann-Franz law, that the ratio of the thermal conductivity to the (electrical conductivity*T) in an ordinary metal should be a constant (the Lorenz number, ~25 nanoOhms W/K^2). Deviations from the W-F law are indicators of interesting physics - basically that simple metal electrons either aren't the dominant carriers of the electrical current, or that the charge carriers don't carry thermal energy as normal. This paper is a theory piece by the Helsinki group showing that the W-F law fails badly for single-electron transistors. In particular, in the co-tunneling regime, when current is carried via quantum coherent processes, the Lorenz number is predicted to be renormalized upward by a factor of 9/5. This will be challenging to measure in experiments, but exquisite thermal conductivity measurements have been performed in similar systems in the past.
arxiv:0709.4125 - Allahverdyan et al., Work extremum principle: structure and function of quantum heat engines
Marlan Scully (also here) caused a bit of a flurry of excitement a few years ago by proposing a form of heat engine that uses quantum coherence and its destruction to do work, in addition to the conventional approach of using two thermal baths at different temperatures. This paper is a theoretical analysis of some such quantum heat engines. Carnot can sleep easy - in the end you can't violate the Carnot efficiency even with quantum heat engines, if you dot all the "i"s and cross all the "t"s. Neat to think about, though, and of some experimental relevance to the cold atom community, who can prepare highly coherent atomic gases at very low temperatures. This paper is long and detailed and I don't claim to have read it in depth, but it looks interesting.
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