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.

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).

This Week’s Science Roundup

This week, there have been some interesting papers dealing with new magnetic materials; using thin-films in new and interesting ways (such as in transistor memory devices and gate dielectrics in carbon nanotube transistors); nanoscale photonics using nanowires and nanotubes; exploring the possibility of creating quantum dots in graphene using electrostatic potential barriers; using scanning tunneling microscopy to look at the Kondo effect in molecules and carrier dynamics in p-n junctions while they’re being operated; figuring out what part of the brain is responsible for our wandering minds; and two interesting applications of quantum mechanics in biology - theoretically considering phonon-assisted tunneling of electrons in elucidating how we smell, and using computational quantum mechanical calculations to study protein splicing. Whew.

Nanoscale/Condensed Matter-Related:
- Hybrid metal-organic materials that are magnetic at room temperature
- Thin-film ferromagnetic devices whose magnetization is modulated via an applied electric field
- A new organic (pentacene) thin-film field-effect transistor (FET) as a possible non-volatile memory device
- Using self-assembled monolayers (SAMs) as the gate dielectric in carbon nanotube FETs
- Nanoscale photonics: nanowire LEDs and nanotube coaxial cables
- Creating quantum dots electrostatically in graphene
- Manipulating the Kondo effect in molecular systems using STM
- Using STM to study carrier dynamics in a p-n junction

Bio-related:
- Why your brain wanders when you’re bored
- Could Humans Recognize Odor by Phonon Assisted Tunneling?
- Studying Protein Cleavage Using Quantum Mechanical Calculations