## Sunday, December 28, 2008

### More about insulators

I've been thinking more about explaining what we mean by "insulators", in light of some of the insightful comments. As I'd said, we can think about three major classes of insulators: band insulators (a large gap due to single-particle effects (more below) exists in the ladder of electronic states above the highest occupied state); Anderson insulators (the highest occupied electronic states are localized in space, rather than extending over large distances; localization happens because of disorder and quantum interference); and Mott insulators (hitherto neglected electron-electron interactions make the energetic cost of moving electrons prohibitively high).

The idea of an energy gap (a big interval in the ladder of states, with the states below the gap filled and the states above the gap empty) turns out to be a unifying concept that can tie all three of these categories together. In the band insulator case, the states are pretty much single-particle states (that is, the energy of each state is dominated by the kinetic energies of single electrons and their interactions with the ions that supply the electrons). In the Anderson insulator case, the gap is really the difference in energy between the highest occupied state and the nearest extended state (called the mobility edge). In the Mott case, the states in question are many-body states that have a major contribution due to electron-electron interactions. The electron-electron interaction cost associated with moving electrons around is again an energy gap (a Mott gap), in the ladder of many-body (rather than single-particle) states.

I could also turn this around and talk in terms of the local vs. extended character of the highest occupied states (as Peter points out). In the ideal (infinite periodic solid) band insulator case, all (single-particle) electronic states are extended, and it's the particular lattice arrangement and electronic population that determines whether the highest occupied state is far from the nearest unoccupied state. In the Anderson case, quantum interference + disorder leads to the highest occupied states looking like standing waves - localized in space. In the Mott case, it's tricky to try to think about many-body states in terms of projections onto single-particle states, but you can do so, and you again find that the highest relevant states are localized (due, it turns out, to interactions). Like Peter, I also have been meaning to spend more time thinking hard about insulators.

Coming soon: a discussion of "metals".

It's a small world. Just last week I finished reading this book, a very nice biography of Michael Faraday, possibly the greatest experimental physicist ever. Lo and behold, this week there are two long blog postings (here and here) also talking about Faraday. What an impressive scientist. Now I need to find a good bio of Maxwell....

## Friday, December 26, 2008

### What does it mean for a material to be an "insulator"?

I've been thinking for a while about trying to explain some physics concepts on, well, a slightly more popular level. This is a first pass at this, focusing on electrical insulators. Feedback is invited. I know that this won't be perfect for nonscientists at a first cut.

Very often we care about the electrical properties of materials. Conceptually, we imagine hooking the positive terminal of a battery up to one end of a material, hooking the negative terminal up to the other end, and checking to see if any current is flowing. We broadly lump solids into two groups, those that conduct electricity and those that don't. Materials in the latter category are known as insulators, and it turns out that there are at least three different kinds.
• Band insulators. One useful way of thinking about electrons in solids is to think about the electrons as filling up single-particle states (typically with two electrons per state). This is like what you learn in high school chemistry, where you're taught that there are certain orbitals within atoms that get filled up, two electrons per orbital. Helium has two electrons in the 1s orbital, for example. In solids, there are many, many states, each one with an associated energy cost for being occupied by an electron, and the states are grouped into bands separated by intervals of energy (band gaps) with no states. (Picture a ladder with groups of closely spaced rungs, and each rung has two little divots where marbles (the electrons) can sit.) Now, in clean materials, you can think of some states as corresponding to electrons moving to the left. Some states correspond to electrons moving to the right. In order to get a net flow of electrons when a battery is used to apply a voltage difference across a slab of material, there have to be transitions that, for example, take electrons out of left-moving states and put them into right-moving states, so that more electrons are going one way than the other. For this to happen, there have to be empty states available for the electrons to occupy, and the net energy cost of shifting the electrons around has to be low enough that it's supplied by the battery or by thermal energy. In a band insulator, all of the states in a particular band (usually called the valence band) are filled, and the energetically closest empty states are too far away energetically to be reached. (In the ladder analogy, the next empty rung is waaay far up the ladder.) This is the situation in materials like diamond, quartz, and sapphire.
• Anderson insulators. These are materials where disorder is responsible for insulating behavior. In the ladder analogy above, each rung of the ladder corresponded to what we would call an "extended" state. To get a picture of what this means, consider looking at a smooth, grooved surface, like a freshly plowed field, and filling it partially with water. Each furrow would be an extended state, since on a level field water would extend along the furrow from one end of the field to the other. Now, a disordered system in this analogy would look more like a field pockmarked with hills and holes. Water (representing the electrons) would pool in the low spots rather than forming a continuous line from one end of the field to the other. These local low spots are defects, and the puddles of water correspond to localized states. In the real quantum situation things are a bit more complicated. Because of the wavelike nature of electrons, even weak disorder (shallow dips rather than deep holes in the field) can lead to reflections and interference effects that can cause states to be localized on a big enough "field". Systems like this are insulating (at least at low temperatures) because it takes energy to hop electrons from one puddle to another puddle. For small applied voltages, nothing happens (though clearly if one imagines tilting the whole field enough, all the water will run down hill - this would correspond to applying a large electric field.). Examples of this kind of insulating behavior include doped polymer semiconductors.
• Mott insulators. Notice that nowhere in the discussion of band or Anderson insulators did I say anything at all about the fact that electrons repel each other. Electron-electron interactions were essentially irrelevant to those two ways of having an insulator. To understand Mott insulators, think about trying to pack ping-pong balls closely in a 2d array. The balls form a triangular lattice. Now the repulsion of the electrons is represented by the fact that you can't force two ping-pong balls to occupy the same site in the 2d lattice. Even though you "should" be able to put two balls (electrons) per site, the repulsion of the electrons prevents you from doing so without comparatively great energetic cost (associated with smashing a ping-pong ball). The result is, for exactly 1 ball (electron) per site ("half-filled band") in this situation dominated by ball-ball interactions ("on-site repulsion"), no balls are able to move in response to an applied push (electric field). To get motion (conduction) in this case, one approach is to remove some of the balls (electrons) to create vacancies in the lattice. This can be done via chemical doping. Examples of Mott insulators are some transition metal oxides like V2O3 and the parent compounds of the high temperature superconductors.
Often when speaking of "metals" vs. "insulators", we are interested in the ground state of the material, the state that would describe the material in equilibrium as T approaches 0. Materials with an electrical resistivity that tends toward infinity as T approaches 0 are insulators in this sense.

## Tuesday, December 23, 2008

### Quantum dots in graphene

The progress in graphene experiments continues. Unsurprisingly, many people are interested in using graphene, a natural 2d electronic system, to make what some would call quantum dots: localized puddles of electrons separated from "bulk" leads by tunnel barriers. Step one in making graphene quantum dots is to etch graphene into a narrow constriction. You can end up with localized electronic states due to disorder (from the edges and the underlying substrate). This Nano Letter shows examples of this situation. Alternately, you can make structures using gates that can define local regions of p-type (local chemical potential biased into the valence band) and n-type (conduction band) conduction, separated by tunnel junctions formed when p and n regions run into each other. That situation is described here. Neat stuff. I would imagine that low temperature transport measurements through such structures in the presence of applied magnetic fields should be very revealing, given the many predictions about exotic magnetic properties in edge states of graphene ribbons.

## Saturday, December 20, 2008

### The new science and technology team

The President-elect has named his science team. Apart from the fact that these folks are all highly qualified (the new administration will have two Nobel laureates advising it directly), I'm told by a senior colleague well-versed in policy that the real good news is the re-promotion of the science advisor position back to the level of authority that it had prior to 2001, and the reinvigoration of PCAST.

## Friday, December 19, 2008

### At the risk of giving offense....

I see that New Scientist has an article effectively making fun of the Department of Defense for asking their major scientific advisory panel, JASON, to look into a company's claim that it could use gravity waves as an imaging tool. JASON rightly determined that this was not something to worry about. Seems like a non-story to me. Thank goodness New Scientist has never actively promoted something manifestly scientifically wacky on their front cover, like a microwave cavity that violates conservation of momentum. Oh wait.

## Wednesday, December 17, 2008

### Outside shot

Since the President-Elect has not yet named his science advisor (though his transition team has named point people, and the nominee for Secretary of Energy has impeccable credentials), I thought I'd point out another crucial way that I would fit in. Sure, I'm under 5' 8" tall, but I can (sometimes) shoot; as some of my college friends can attest, I won a gift certificate in undergrad days by sinking a shot from the top of the key at a women's basketball game halftime promo. (For the humor-impaired: I'm not really in the running to be part of the Obama administration.)

### Let them fail.

Please explain to me why we should give AIG another penny.

### A couple of ACS papers

Two recent papers in the ASAP section of Nano Letters caught my eye.

The first is van der Molen et al., "Light-controlled conductance switching of ordered metal−molecule−metal devices". I've written a blurb about this for the ACS that will eventually show up here. The Schönenberger group has been working for a while on an approach for measuring molecular conductances that is based on networks of metal nanoparticles linked by molecules of interest. The idea is to take metal nanoparticles and form an ordered array of them with neighbors linked by molecules of interest covalently bound to the particle surfaces. The conductance of the array tells you something about the conductance of the particle-molecule-particle junctions. This is simple in concept and extremely challenging in execution, in part because when the metal nanoparticles are made by chemical means they are already coated with some kind of surfactant molecules to keep them suspended in solution. Performing the linking chemistry in a nice way and ending up with an ordered array of particles rather than a blob of goo requires skill and expertise. These folks have now made arrays incorporating molecules that can change reversibly change their structure upon exposure to light of the appropriate wavelength. The structural changes show up in photo-driven changes in the array conductance.

The second is Ryu et al., "CMOS-Analogous Wafer-Scale Nanotube-on-Insulator Approach for Submicrometer Devices and Integrated Circuits Using Aligned Nanotubes". Lots of people talk a good game about trying to make large-scale integrated circuits using nanotubes, but only a couple of groups have made serious progress. This paper by Chongwu Zhou's group shows that they can take arrays of tubes (grown by chemical vapor deposition on quartz or sapphire substrates), transfer them to Si wafers via a clever method involving gold, pattern the tubes, put down electrodes for devices, burn out the metallic tubes, and dope the semiconductor tubes chemically to do either p or n-type conduction. They are also working on fault-tolerant architectures to deal with the fact that each transistor (which in this case incorporates an ensemble of tubes) has slightly different characteristics.

## Tuesday, November 25, 2008

### A (serious) modest proposal

Hopefully someone in the vast (ahem.) readership of this blog will pass this along to someone with connections in the Obama transition team. I've already submitted this idea to change.gov, but who knows the rate at which that gets read.

As part of the forthcoming major economic stimulus package, I propose that the Obama administration fully fund the America Competes initiative immediately. If the goal of the package is to stimulate the economy while doing something for the long-term health of the country (e.g., creating jobs while fixing roads, bridges, etc.), then funding basic research via the various agencies is a great thing to do. Think about it: the US spends less as a percentage of GDP than most of the rest of the developed world on science research. Rectifying that to some degree would (a) help the long-term prospects for technological innovation in the US; (b) create jobs; (c) support the goal of developing energy-related technologies; (d) support our universities, many of which are getting hammered by falling state revenues and/or poor endowment returns. Best of all, you could do all of this and it would be a freakin' bargain! You could double the research funding in NSF, NIH, DOE, NASA, and NIST, and not even come close to the amount of money we've already given to AIG. I'm suggesting something far more modest and much less disruptive. Seriously, ask yourself what's better for the long-term health of the country. Cutting basic science to pay for propping up Goldman Sachs is perverse.

Update: If you think that this is a good idea, I encourage you to submit your suggestion here, here, and/or here.

## Monday, November 24, 2008

### Spin

Many particles possess an internal degree of freedom called "spin" that is an intrinsic amount of angular momentum associated with that particle. The name is meant to evoke a spinning top, which has some rotational angular momentum about its axis when, well, spinning. Electrons have "spin 1/2", meaning that if you pick a convenient axis of reference ("quantization axis") that we'll call z, the z-component of the electron's spin angular momentum is either +1/2 hbar or -1/2 hbar. All too often we treat spin in a rather cavalier way. When people talk about "spintronics", they are interested in using the spin degree of freedom of electrons to store and move information, rather than using the charge as in conventional electronics. One complication is that while charge is strictly conserved, spin is not. If you start off with a population of spin-aligned electrons and inject them into a typical solid, over time the spin orientation of those electrons will become randomized. Now, angular momentum is strictly conserved, so this relaxation of the electron spins must coincide with a transfer of angular momentum to the rest of the solid. Feynman pointed this out (somewhere in vol. III of his lectures on physics) - if you fire a stream of spin-polarized electrons into a brick hanging on the end of a thread, you are really applying a torque to the brick since you are supplying a flow of angular momentum into it, and the thread will twist to apply a balancing torque. Well, Zolfagharkhani et al. have actually gone and done this experiment. They use a ferromagnetic wire to supply a polarized spin current and an extremely sensitive nanomechanical torsional oscillator to measure the resulting torque. Very nice stuff.

## Thursday, November 20, 2008

### Nature Journal Club

My media onslaught continues. This past week I had a Journal Club contribution in Nature, which was fun and a nice opportunity for a wider audience. Here's a version of it before it was (by necessity) trimmed and tweaked, with added hyperlinks....

Tunable charge densities become very large, with super consequences

The electronic properties of materials depend dramatically on the density of mobile charge carriers. One way to tune that density is through doping, the controlled addition of impurity atoms or molecules that either donate or take up an electron from the rest of the material. Unfortunately, doping also leads to charged dopants that can act as scattering sites.

Fortunately, there is a way to change the carrier concentration without doping. In 1925 J. E. Lilienfeld first proposed what is now called the “field effect”, in which the sample material of interest is used as one electrode of a capacitor. When a voltage is applied to the other (“gate”) electrode, equal and opposite charge densities accumulate on the gate and sample surfaces, provided charge can move in the sample without getting trapped. While the density of charge that can be accumulated this way is rather limited by the properties of the insulating spacer between the gate and the sample, the field effect has been incredibly useful in transistors, serving as the basis for modern consumer electronics.

Recently it has become clear that another of Lilienfeld’s inventions, the electrolytic capacitor, holds the key to achieving much higher field effect charge densities. The dramatic consequences of this were made clear by researchers at Tohoku University in Sendai, Japan (K. Ueno et al., Nature Mater. 7, 856-858 (2008)), who used a polymer electrolyte to achieve gated charge densities at a SrTiO3 surface sufficiently large to produce superconductivity. While superconductivity had been observed previously in highly doped SrTiO3, this new approach allows the exploration of the 2d superconducting transition without the disorder inherent in doping.

The most exciting aspect of this work is that this approach, using mobile ions in an electrolyte for gating, can reach charge densities approaching those in chemically doped, strongly correlated materials such as the high temperature superconductors. As an added bonus, this approach should also be very flexible, not needing special substrates. Tuning the electronic density in strongly correlated materials without the associated pain of chemical doping would, indeed, be super.

## Tuesday, November 18, 2008

### This week in cond-mat

One paper today in the arxiv:
arxiv:0811.2914 - Zwanenburg et al., Spin states of the first four holes in a silicon nanowire quantum dot
This is another typically exquisite paper by the Kouwenhoven group at Delft, in collaboration with Charlie Lieber at Harvard. The Harvard folks have grown a Si wire segment in the middle of a long NiSi wire. The NiSi ends act as source and drain electrodes for conduction measurements, and the Si segment acts as a quantum dot, with the underlying substrate acting as a gate electrode. As usual, the small size of the Si segment leads to a discrete level spectrum, and the weak electronic coupling of the Si segment to the NiSi combined with the small size of the Si segment results in strong charging effects (Coulomb blockade, which I'll explain at length for nonexperts sometime soon). By measuring at low temperatures very carefully, the Delft team can see, in the conductance data as a function of source-drain voltage and gate voltage, the energy level spectrum of the dot. By looking at the spectrum as a function of magnetic field, they can deduce the spin states of the ground and excited levels of the dot for each value of dot charge. That's cute, but the part that I found most interesting was the careful measurement of excited states of the empty dot. The inelastic excitations that they see are not electronic in nature - they're phonons. They have been able to see evidence for the launching (via inelastic tunneling) of quantized acoustic vibrations. Figure 5 is particularly nice.

## Sunday, November 16, 2008

### Workshop on new iron arsenide superconductors

This weekend is a big workshop at the University of Maryland on the new iron arsenide high temperature superconductors. Since it's not really my area, I didn't go. Anyone want to give a little update? Any cool news?

## Tuesday, November 11, 2008

### Poor Doug's Almanack

Welcome, readers of Discover Magazine! Thanks for coming by, and I hope that you find the discussion here interesting. The historical target audience of this blog has been undergrads, grad students, and faculty interested in condensed matter (solid state) physics and nanoscience. The readership also includes some science journalists and other scientific/engineering professionals. I would like very much to reach a more general lay-audience as well, since I think we condensed matter types historically have been pretty lousy at explaining the usefulness and intellectually richness of our discipline. Anyway, thanks again.

(By the way, I don't compare in any serious way with Ben Franklin - that was a bit of hyperbole from Discover that I didn't know was coming. Fun science fact: Franklin's to blame that the electron charge is defined to be negative, leading to the unfortunate annoyance that current flow and electron flow point in opposite directions. He had a 50/50 chance, and in hindsight his choice of definition could've been better.)

## Sunday, November 09, 2008

### This week in cond-mat

This week the subject is boundary conditions. When we teach about statistical physics (as I am this semester), we often need to count allowed states of quantum particles or waves. The standard approach is to show how boundary conditions (for example, the idea that the tangential electric field has to go to zero at the walls of a conducting cavity) lead to restrictions on the wavelengths allowed. Boundary conditions = discrete list of allowed wavelengths. We then count up those allowed modes, converting the sum to an integral if we have to count many. The integrand is the density of states. One remarkable feature crops up when doing this for confined quantum particles: the resulting density of states is insensitive to the exact choice of boundary conditions. Hard wall boundary conditions (all particles bounce off the walls - no probability for finding the particle at or beyond the walls) and periodic boundary conditions (particles that leave one side of the system reappear on the other side, as in Asteroids) give the same density of states. The statistical physics in a big system is then usually relatively insensitive to the boundaries.

There are a couple of physical systems where we can really test the differences between the two types of boundary conditions.
arxiv:0811.1124 - Pfeffer and Zawadzki, "Electrons in superlattices: birth of the crystal momentum"
This paper considers semiconductor superlattices of various sizes. These structures are multilayers of nanoscale thickness semiconductor films that can be engineered with exquisite precision. The authors consider how the finite superlattice result (nonperiodic potential; effective hardwall boundaries) evolves toward the infinite superlattice result (immunity to details of boundary conditions). Very pedagogical.

arxiv:0811.0565, 0811.0676, 0811.0694 all concern themselves with graphene that has been etched laterally into finite strips. Now, we already have a laboratory example of graphene with periodic boundary conditions: the carbon nanotube, which is basically a graphene sheet rolled up into a cylinder. Depending on how the rolling is done, the nanotube can be metallic or semiconducting. In general, the larger the diameter of a semiconducting nanotube, the smaller the bandgap. This makes sense, since the infinite diameter limit would just be infinite 2d graphene again, which has no band gap. So, the question naturally arises, if we could cut graphene into narrow strips (hardwall boundary conditions transverse to the strip direction), would these strips have an electronic structure resembling that of nanotubes (periodic boundary conditions transverse to the tube direction), including a bandgap? The experimental answer is, yes, etched graphene strips to act like they have a bandgap, though it's clear that disorder from the etching process (and from having the strips supported by an underlying substrate) can dominate the electronic properties.

## Thursday, November 06, 2008

### Two new papers in Nano Letters

Two recent papers in Nano Letters caught my eye.

Kuemmeth et al., "Measurement of Discrete Energy-Level Spectra in Individual Chemically Synthesized Gold Nanoparticles"
One of the first things that I try to teach student in my nano courses is the influence of nanoscale confinement on the electronic properties of metals. We learn in high school chemistry about the discrete orbitals in atoms and small molecules, and how we can think about filling up those orbitals. The same basic idea works reasonably well in larger systems, but the energy difference between subsequent levels becomes much smaller as system size increases. In bulk metals the single-particle levels are so close together as to be almost continuous. In nanoparticles at low temperatures, however, the spacing is reasonably large compared to the available thermal energy that one can do experiments which probe this discrete spectrum. Now, in principle the detailed spectrum depends on the exact arrangement of metal atoms, but in practice one can look at the statistical distribution of levels and compare that distribution with a theory (in this case, "random matrix theory") that averages in some way over possible configurations. This paper is a beautiful example of fabrication skill and measurement technique. There are no big physics surprises here, but the data are extremely pretty.

Xiao et al., "Flexible, stretchable, transparent carbon nanotube thin film loudspeakers"
This is just damned cool. The authors take very thin films of carbon nanotubes and are able to use them as speakers even without making the films vibrate directly. The idea is very simple: convert the acoustic signal into current (just as you would to send it through an ordinary speaker) and run that current through the film. Because of the electrical resistance of the film (low, but nonzero), the film gets hot when the current is at a maximum. Because the film is so impressively low-mass, it has a tiny heat capacity, meaning that small energy inputs result in whopping big temperature changes. The film locally heats the air adjacent to the film surface, launching acoustic waves. Voila. A speaker with no moving parts. This is so simple it may well have real practical implementation. Very clever.

## Wednesday, November 05, 2008

### To quell speculation....

Yes, if asked, I would serve as President Obama's science advisor. (Come on - you would, too, right? Of course, it's easy for me to joke about this since it's about as probable as me being asked to serve as head of the National Science Board.)

## Monday, November 03, 2008

### This one's easy.

Has Bush been good for science? I agree with ZapperZ: No. How Marburger can argue that research funding has kept pace with inflation is beyond me, given the last three years of continuing resolutions, unless one (a) fudges the definition of research to include a lot of military development, and (b) fudges the definition of inflation to ignore things like food, fuel, and health care costs.

Could Bush have been even worse? Yes.

### Statistical physics

This fall I'm teaching Statistical and Thermal Physics, a senior (in the most common Rice physics curriculum, anyway) undergraduate course, and once again I'm struck by the power and profundity of the material. Rather like quantum, stat mech can be a difficult course to teach and to take; from the student perspective, you're learning a new vocabulary, a new physical intuition, and some new mathematical tools. Some of the concepts are rather slippery and may be difficult to absorb at a first hearing. Still, the subject matter is some of the best intellectual content in physics: you learn about some of the reasons for the "demise" of classical physics (the Ultraviolet Catastrophe; the heat capacity problem), major foundational issues (macroscopic irreversibility and the arrow of time; the precise issue where quantum mechanics and general relativity are at odds (or, as I like to call it, "Ultraviolet Catastrophe II: Electric Boogaloo")), and the meat of some of the hottest topics in current physics (Fermi gases and their properties; Bose Einstein condensation). Beyond all that you also get practical, useful topics like thermodynamic cycles, how engines and refrigerators work, chemical equilibria, and an intro to phase transitions. Someone should write a popular book about some of this, along the lines of Feynman's QED. If only there were enough hours in the day (and my nano book was further along). Anyway, I bring this up because over time I'm thinking about doing a series of blog posts at a popular level about some of these topics. We'll see how it goes.

## Sunday, November 02, 2008

### Ahh, Texas, again.

Stories like this one depress me. Is it really any wonder that our state has a difficult time attracting large high-tech companies from, e.g., California and Illinois, even though corporate taxation policies are very friendly here?

## Thursday, October 30, 2008

### Local news

You can learn all sorts of things reading your local paper. For example, I read yesterday that Rice and Baylor College of Medicine are talking about a possible merger. Unsurprisingly, everyone on campus already knew about that, but it's interesting to see the reporter's take on things, including various quotes from unnamed professors.

## Wednesday, September 17, 2008

### Because I'm a big musical nerd...

... I couldn't pass this up. Very well done, though someone should point out to the Obama supporters behind this that things didn't work out too well for most of the characters singing this in Les Miserables.

I will return to actual physics blogging soon, once the immediate disarray settles out.

## Sunday, September 14, 2008

### Ike follow-up

Well, that was interesting. Thankfully we're all fine and our house is undamaged. The prospect of being without power for an extended period continues to suck, to put it bluntly. 90 degree weather, near 100% humidity, and no air conditioning or refrigeration. On the plus side, my university still has power and AC. On the downside, they've disabled access (card keys) to most buildings and water service (i.e. sanitary plumbing) is spotty on campus.

## Friday, September 12, 2008

### Hurricane Ike

Hello - for those readers who don't know, I live in Houston, which is about to get hit by Hurricane Ike. I'm hopeful that this won't be a big deal, but there's always the chance that I'll be without electricity for a few days. So, blogging may be slow. In the mean time, check out this cool site for following tropical storm systems, and this explanation of how hurricanes are heat engines.

## Thursday, September 11, 2008

### Ahh, the Gulf coast.

You know, I lived the first twenty-nine years of my life without having to pay close attention to stuff like this.

## Wednesday, September 10, 2008

### Important online resource

The internet is definitely the best way to keep up with current events. Check here often (look at the link text). (Thanks, Dan.)

## Tuesday, September 09, 2008

### Final Packard highlights + amusing article

One of my former professors, Michael Peskin, has a nice article about why the LHC will not destroy the earth. He taught me graduate-level mechanics, and my brain still hurts from his take-home final.

A last few things I learned at the Packard meeting:
• The stickleback is a very useful fish for addressing the question, if natural selection removes variation in phenotypes, then why do we still see so much variation?
• There are structures on the membranes of many cells (the primary cilium; the protein known as rhomboid) that seem to have really profound effects on many cellular processes. Understanding how and why they do what they do demonstrates why systems biology is hard.
• It may be possible to do some kind of "safe" cryptographic key exchange based on functions that are not algebraic (as opposed to usual RSA-type encryption which is based on the asymmetry in difficulty between multiplication and factorization).
• There are deep connections between random permutations and the distribution of the number of prime factors.
• It's possible to run live small animals (zebrafish, c. elegans) through microfluidic assay systems in massively parallel fashion.
• Stem cell differentiation can apparently be influenced by the mechanical properties (e.g., squishy vs. hard) of the substrate. Weird.
• Artificial sieve structures can be very useful for electrophoresis of long segments of DNA.
• There may be clever ways to solve strongly correlated electronic structure problems using tensor networks.
• Natural synthesis of useful small molecules (e.g., penicillin, resveratrol) is pretty amazing. Makes me want to learn more about bacteria, actomycetes, and fungi.
• By clever trading of time and statistics for intensity, 3d superresolution imaging is possible under some circumstances.
• DNA can be used as a catalyst.
• Some bacteria in biofilms secrete molecules that look like antibiotic byproducts, but may actually serve as a way of carrying electrons long distances so that the little buggers far from the food source can still respirate.
• Virus chips are awesome.
• Don't ever get botfly larvae growing in your scalp. Ever.
• Tensegrity structures can be very useful for biomimetic machines.
• Sub-mm arrays are going to be a boon for astronomy.
• It looks like much of the Se and Br in the universe was actually produced by the same compact object mergers that give short gamma ray bursts.
• Dark energy remains a major enigma in physics and astrophysics. It's a big one.