Laser cooling (also known as optical cooling) is a well-established technique…but mostly for gases. The basic idea is to disperse the thermal energy of the atoms through shining light on them: the frequency of the laser is set to be slightly lower than the energy of transition between two configurations in the atoms, so that atoms’ motion provides that extra bit to lead to absorption. That converts energy of motion (kinetic energy) into light, slowing the atoms down. However, the trick doesn’t work for solids, because the kinetic energy takes the form of phonons, quasiparticles of sound. However, researchers figured out a way to annihilate the phonons using a laser, in a particular type of semiconductor. Using that trick, they cooled the material down by 40 degrees, opening the way to rapid refrigeration of some solids.

The authors of the new study used cadmium sulphide (CdS), a material known as a group-II-VI semiconductor. Commonly used in digital electronics, semiconductors are insulators under normal conditions, but can be induced to conduct electricity when impurity atoms are added. Group-II-VI semiconductors host both strong phonons, and an additional type of particle-like excitation known as an exciton. Excitons are created through interactions between electrons and “holes” that the electrons left behind. [Read more...]

Rapid cooling of semiconductors using lasers (but no sharks)

Just as a ratchet allows rotation in one direction but not the other, quantum ratchets break the symmetry of a microscopic system to facilitate preferential motion in one direction or another. Graphene is a two-dimensional hexagonal lattice of carbon atoms. As such, it’s highly symmetrical, but beneath that lurks a potentially exploitable hidden asymmetry. If you add hydrogen atoms (for example) to the top of graphene, an applied alternating current (in the form of a microwave-frequency light wave) induces electrons to flow preferentially one direction: a quantum ratchet.

The reason for this striking change in behavior is due to what’s called a structure inversion asymmetry in graphene. In the presence of an external influence—in this case, the introduction of hydrogen atoms and a strong magnetic field—the shape of the electron orbits in the carbon atoms gets distorted in one direction. When exposed to the oscillating electric field, the electrons felt a strong resistive force in one direction (which the authors liken to friction), but increased mobility in the opposite direction. [Read more…]

Turning graphene into an AC/DC quantum ratchet

Most solids compress when squeezed, though the effect isn’t very large for most technologically important materials (metals, ceramics, and so forth). A few rare materials exhibit negative compressibility: they expand in the direction the force is exerted, though again the effect is small. However, researchers figured out a way to produce extraordinarily large negative compressibility, by fabricating a material with a folding wine-rack structure. (This was a fun story to write, and I got to refer to caecilians. Which are breaking my heart. Which are shaking my confidence daily.)

No material can exhibit negative compressibility in all dimensions at once: if it expands in one direction, then it will contract in the two directions perpendicular to the expansion. The researchers’ insight was to realize that they could maximize negative compressibility in one dimension by simultaneously maximizing positive compressibility in the other two. This kind of “wine rack” configuration (their words) is similar to an example found in nature: tendon structures in the legless amphibians known as caecilians. [Read more…]

In Soviet Russia, material compresses *you*

There’s just one word for the ‘teens: plastics photovoltaics. A new experiment may have solved a problem in nanostructured silicon solar cells: a type of photovoltaic cell that uses pores to increase the effective surface area for collection of light.

By separating the contribution by surface and interior recombination effects, the NREL study found that Auger recombination—recombination by these interior charges—was actually more damaging to photocell efficiency. In other words, the very pores that offered advantages also led to problems, which was why nanostructured photovoltaics haven’t lived up to their promises thus far.

The researchers found that etching the silicon material with tetramethylammonium hydroxide (TMAH) greatly increased the efficiency of the photocell. The result was shallower, slightly wider, and more irregularly shaped pores. [Read more….]

Pore, pore, pitiful solar cells