Cool Papers 1: General

I’ve come across a number of pretty cool papers in the past few months. Some of them deal with particular phenomena (stay tuned for possible upcoming posts on molecules at surfaces, biomimetics, phononics, crystallization, nanoparticles, wetting phenomena, computational physics, etc. etc. - at some point), and so are probably better off getting their own blog posts. Here are a few papers that didn’t fall into specific categories…

1. Frictional Anisotropy on a Quasicrystal Surface
Along with ~10 other things, a subject that I’ve recently become interested in is nanoscale mechanics, broadly defined. A key experimental tool in this field is the use of local probes to push or pull on things controllably. Miquel Salmeron’s STM group at Berkeley does work on this and related subjects, and I finally got around to reading this paper of theirs from a few years back.

The idea is conceptually very simple: while friction unsurprisingly depends on commensurability (that is, if two surfaces in contact are structurally ‘complementary’, they will ‘lock in’ to each other and hence have high friction between them - an idea that apparently dates back to da Vinci), trying to think about friction using just this notion is unrealistic. For starters, most contacting surfaces are probably incommensurate, and other factors - such as periodicity(?) - contribute, as well.

This paper nicely singles out the role of periodicity by looking at different directions along Al-Ni-Co quasicrystal surfaces using STM (to image the surface and hence distinguish the periodic and aperiodic directions of atom ordering) and AFM (to measure the probe tip-surface friction along these directions) in ultra-high vacuum. The AFM friction data can be modeled using a classical model relevant to the experimental situation (the Derjaguin-Muller-Toporov or DMT model, which I need to learn more about), enabling key parameters to be derived from the measurements.

In particular, the authors find a larger friction force (8x) along the periodic direction than along the aperiodic direction. Unsurprisingly, they ascribe this to differences in energy dissipation via electron or phonon excitation+propagation along the different directions, although it is unclear to what extent each kind of excitation plays a role. Perhaps similar local-probe measurements of a different kind (e.g. ones sensitive to electrical versus mechanical properties) might be useful… At the end of the day, I like this paper because it is an elegant example of using a unique microstructure, in which just one variable (here periodicity) changes in ways that are well understood, to study something interesting as a function of just that variable.

2. Liquid Crystals and the Origins of Life
Noel Clark gave a great talk about this work here at Penn not too long ago. I won’t write too much about this since Randy has a nice description of it over at the condmat journal club.

Here’s the executive summary: according to extensions of Onsager’s rigid-rod model for the formation of liquid crystal phases, individual molecules must be sufficiently anisotropic (i.e. the aspect ratio has to be above a certain minimum) to form a liquid crystal (LC). Surprisingly, the authors of this paper observed LC phases consisting of single-stranded (ss) DNA molecules too short to satisfy this criterion. Optical and x-ray measurements indicate that this results from end-to-end stacking of duplexes of complementary short ss-DNA molecules (known as ‘living polymerization’) into larger rods that satisfy the Onsager criterion, even at low temperatures (in concentrated phases of duplexes separated from the isotropic phase of unpaired ss-DNA molecules).

This autocatalytic behavior is like positive feedback, in a sense, and is why this work is so interesting from a biological point of view: it provides a mechanism by which the right molecules can be ’selected’ out from a ’soup’, and ‘evolve’ into larger ones as part of an RNA world. It’s an interesting idea - definitely one that’s gotten a lot of press, it seems - and while this work doesn’t provide much hard evidence for it, I’ll be interested to see what it stimulates.

3. Suprafroth!
This is a very interesting paper out recently on the arxiv, I think to be published in Nature Physics. While I don’t understand all the details, I like this particularly because it’s a nice combination of ideas from soft- and hard-condensed matter physics, like electronic liquid crystals.

The authors used magneto-optical imaging, which I need to learn more about, to image the flux pattern of superconducting lead (a type-I superconductor). Turns out that the magnetic field on the edge of a disc-shaped sample of lead is larger than the actual applied field, and for large enough magnetic field some flux can penetrate the sample. This leads to a phase intermediate between the normal and superconducting phases, possessing a froth-like magnetic structure - specifically, the froth cell boundaries are superconducting, while the interiors are normal metal. This shows up very clearly in the magneto-optical images (see figures in the paper).

The nice thing is that, unlike ‘conventional’ froths, mass-transport processes like drying or drainage are not present here (as the authors point out, “this superconducting froth involves only electrons”). This means that the froth structure can be tuned reversibly using the applied magnetic field or temperature, and the nice magneto-optical images allow for quantitative analysis of the froth structure as a function of just these parameters.

This is philosophically similar (loosely speaking) to paper #1 - the friction measurements of quasicrystals: again, it is a very nice example of using a unique microstructure (here, a froth structure that doesn’t suffer from irreversible processes, and can be controlled by magnetic field or temperature) to study something interesting (here, the structure and dynamics of froths) as a function of just the variables that you can control.

4. Universality in Conference Registration
This is a cute correspondence recently sent to Nature Physics describing an intriguing social application of statistical mechanics.

The authors used registration data from two physics conferences (# of registrants as a function of time to the deadline), saw that they matched up remarkably well (after rescaling), and came up with a simple model to capture the observed phenomenon in which the ‘pressure’ felt by potential attendees to register varies inversely with respect to the time to the deadline. Also, incorporating a Boltzmann-like factor (instead of uniform probability to register over the period of time) leads to a prediction that agrees well with # of payments as a function of time to the deadline data.

Of course, there are a number of assumptions and fitting parameters floating around here, and I’m not entirely sure this work will change the world of physics, but I always find things like this fun.

‘Hard’ measurements, ’soft’ materials

So it’s been what, a little less than two months since I last posted? I tend to work on many projects at once - some are ones I’ve been plugging away at for a while, while others are “let’s see what happens” experiments that I work on when I get the time, motivated by some half-brained idea. In particular, I’ve made significant progress on a project of the latter category, and the month-and-a-half has been spent making samples, furiously taking and analyzing data, trying to figure out what it means/delving through the literature, &c. - and of course, effectively disrupting any prospects of sleep or studying for pesky standardized tests. And making headway on my other projects, too. The good news is that I, for one, find the data pretty exciting.

(Oh, and moving to my sweet new apartment, which apparently scores a very respectable 98/100 on the walkability scale. Not too bad, especially given the relatively low rent.)

Anyway, when I haven’t been concentrating on my research, I’ve been reading up on things like organic semiconductors and STM modification of molecules (I suppose what one could call ‘hard’ condensed matter measurements of ’soft’ materials, although admittedly some of my own research falls into this genre). I find people like Paul Chaikin, Heinrich Jaeger and George Gruner particularly fascinating since they seem to be actively doing this kind of research in addition to hard condensed matter physics of the more ‘traditional’ kind (superconductivity/correlated electron systems…). I wonder how many other PIs do this kind of thing?

And of course, two new additions to the reading list: “charge transfer on the nanoscale: current status“, and “electrostatic modification of novel materials” - both hefty reviews of topics relevant to this post.

Also: Heinzel’s book on mesoscopic physics is a new addition to my list of the greatest books of all time - in particular, its clarity is unmatched by many other books I’ve come across on the subject.

Talks Part 3: Biomaterials

Another talk that was particularly interesting was Angela Belcher’s Grace Hopper lecture on “Genetic Control of the Synthesis and Assembly of Materials for Electronics and Energy”. I’m not going to post much on it save for a number of references, because I’ve been aware of a lot of her group’s work for a good deal of time now. In general, what they do is try to combine man-made fabrication tools and the specificity inherent in living systems (via millions of years of evolution) to figure out easy, controllable, environmentally-friendly ways to make new materials for a variety of purposes. I was particularly struck by her emphasis on the simplicity of everything they do - if it can’t be transferred to industry or undergrad labs within several years, they won’t do it, which is an interesting philosophy. Anyway, one of the particularly cool things Prof. Belcher’s group has come up with recently is the use of viruses to direct the formation of nanowires, and they’ve been working to use them to make things like self-assembling, cheap and efficient Li-ion batteries. This kind of work definitely appeals to the part of me that likes science because of all the neat things that it enables us to make. Anyway, here are some of her publications that I’ve found most useful:

- B. D. Reiss et al., “Biological Routes to Metal Alloy Ferromagnetic Nanostructures“, Nano Lett. 4 1127 (2004).
- S. Jaffar et al., “Layer-by-Layer Surface Modification and Patterned Electrostatic Deposition of Quantum Dots“, Nano Lett. 4 1421 (2004).
- P. J. Yoo et al., “Spontaneous assembly of viruses on multilayered polymer surfaces“, Nature Materials 5 234 (2006).
- K. T. Nam et al., “Virus-Enabled Synthesis and Assembly of Nanowires for Lithium Ion Battery Electrodes“, Science 312 885 (2006).
- Y. Huang et al., “Programmable Assembly of Nanoarchitectures Using Genetically Engineered Viruses“, Nano Lett. 5 1429 (2005).
- C. Mao et al., “Viral assembly of oriented quantum dot nanowires“, PNAS 100 6946 (2003).

Talks Part 2: Imaging Spins

Imaging electrical spin injection/transport in spintronics devices: Scott Crooker (Los Alamos)
This was another cool (and very understandable) talk based on recent work on trying to understand the physical processes involved in spin injection and transport in lateral feromagnet/semiconductor structures. In a seminal paper in 1990, Datta (no relation to me) and Das proposed one of the earliest versions of a spin-FET: that is, a field-effect transistor made from doped silicon (versus the carbon nanotube FETs we make in our lab on a regular basis) with ferromagnetic contacts. The point, of course, is that the functionality of the device is to come not from coupling to the charge of the electron, but to its spin degree of freedom. It’s just a very cool idea, and people have taken it pretty far since then (although I’m not sure that industry will be ’switching’ to spin-based transistors anytime soon. Get it - switching to transistors? Hilarious.) Although many, many proposals currently exist, they all need certain things: a way to electrically inject spin-polarized electrons into the semiconducting channel, a way for these spins to be transported, a way to controllably manipulate the spins (i.e. with an external field), and a way to electrically detect this spin-polarized current. In particular, one of the key ways of confirming this electrical detection is using the ‘Hanle effect’ due to precession and dephasing of the spins in a transverse field.

Although this had been observed in all-metal devices (including the channel), a number of subtleties prevented a similar observation in semiconductor devices, until Crooker et al.’s work. What they did was use scanning Kerr rotation microscopy (using a continuous-wave (cw) probe laser and a sample resting on the cold finger of an optical cryostat, with an applied transverse field) to measure the out-of-plane component of the spin, and sure enough, they were able to obtain a Hanle signal. They extended (and continue to extend) this in a number of ways, comparing their data to a drift-diffusion model, injecting spins optically and seeing how the conductance changes, and even studying the effect of an applied strain (which interestingly leads to a term in the Hamiltonian that looks like a Rashba spin-orbit interaction with E replaced by the strain). A number of questions remain to be answered, but this work represents an interesting step forward.

Further reading…
- S. A. Crooker et al., “Imaging Spin Transport in Lateral Ferromagnet/Semiconductor Structures“, Science 309 2191 (2005), X. Lou et al., “Electrical Detection of Spin Accumulation at a Ferromagnet-Semiconductor Interface“, PRL 96, 176603 (2006), and X. Lou et al., “Electrical Detection of Spin Transport in Lateral Ferromagnet-Semiconductor Devices“, Nature Physics 3, 197 (2007).
- Some of the first work on strain-induced effects: G. L. Bir and G. E. Pikus, “Symmetry and strain-induced effects in semiconductors” (Wiley, 1974) - can be found on BorrowDirect.
- First spin-FET: S. Datta and B. Das, “Electronic analog of the electro-optic modulator“, APL 56, 665 (1990).
- Somewhat related: chapter 2 of D. D. Awschalom, D. Loss and N. Samarth eds., “Semiconductor Spintronics and Quantum Computation” (Springer 2002).

Funny Journal Content

1. A candidate for the funniest journal title/paper graphic…
Here’s a cute paper: rolling a single molecular at the atomic scale. The authors look at C₄₄H₂₄, a molecule possessing two triptyene ‘wheels’ (with three ‘paddles’, each) and thus two intramolecular degrees of freedom when adsorbed on a metal surface (the independent rotation of each wheel), and push it along with an STM tip. Interestingly, the STM current is a good indicater of what kind of motion the molecule is undergoing (’rolling’ versus ‘hopping’). What I find most amusing is that the molecule was previously used to construct a ‘molecular wheelbarrow’, a result which was published in Tetrahedron Letters - probably the funniest journal title I’ve come across - and includes the following priceless graphic:

2. Can a biologist fix a radio? Or, what one scientist learned while studying apoptosis
Speaking of funny papers, this paper by Yuri Lazebnick (via Structure+Strangeness) is great. Here’s an excerpt, dealing with the question of how would a biologist fix a radio, knowing only that it is a box meant to play music?

How would we begin? First, we would secure funds to obtain a large supply of identical functioning radios in order to dissect and compare them to the one that is broken. We would eventually find how to open the radios and will find objects of various shape, color, and size. We would describe and classify them into families according to their appearance. We would describe a family of square metal objects, a family of round brightly colored objects with two legs, round-shaped objects with three legs and so on. Because the objects would vary in color, we will investigate whether changing the colors affects the radio’s performance. Although changing the colors would have only attenuating effects (the music is still playing but a trained ear of some people can discern some distortion), this approach will produce many publications and result in a lively debate.

3. Formation of a nematic fluid at high fields in Sr₃Ru₂O₇:
I had quite a lengthy post on electronic liquid crystals in 2-dimensional electron gases (e.g. GaAs/AlGaAs heterostructures) a while back, and briefly noted that:

Scientists in Europe have measured a large magnetoresistive anisotropy in the correlated electron oxide strontium ruthenate (Sr₃Ru₂O₇) near the ‘metamagnetic quantum critical point’, indicating the formation of a new quantum nematic phase. This is strikingly similar to the tranport anisotropy in 2DEGs I’ve been talking about… in particular, both show strong sensitivity to disorder - and the authors claim that the formation of this phase is tuned by the divergence in the quasiparticle effective mass near this critical point. One can only wonder what other kinds of systems could yield such behavior as well.

This European work is now one of the feature papers for the online Journal Club for Condensed Matter Physics, with a far more in-depth (yet very readable) commentary by Catherine Kallin of McMaster University in Canada.