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A laser that likes a good shake

13 Jan

In general, lasers are not like old television sets: a good whack does not make a laser miraculously work better. Lasers (at least big lasers) take a lot of careful alignment to get working, so whacking, shaking, and leaning on are all out of the question. If you are being trained in a new lab, you will probably hear a lot of “no-touchy” around the laser mountings.

Some scientists in Russia, however, have created a laser that likes a good shake. The laser works by using mirrors to direct light at quantum dots, causing them to emit. The problem is that the quantum dots aren’t always in the state you need them to be in to emit light of the correct wavelength. This is where the shaking comes in. Scientists found that if they exposed quantum dots to sound waves, the properties of the dots shifted in a way that caused them to spend more time in the state that emitted at the correct wavelength. Quantum dots are perfect for this, as their electrons are willing to hang out in an excited state long enough for the sound waves to affect their properties before emitting a photon. The laser with the sound waves had a power 200 times better than without, which is pretty huge. Cool.

Article here.

Still curious? Here’s a bit more info about quantum dots: Quantum dots are tiny bits of matter with special properties. First you need to know about excitons. Excitons are basically an electron and the hole where that electron wants to go, together in a bound state. They are electrically neutral quasi-particles that exist in insulators and semiconductors. Quantum dots’ excitons are confined in all spatial dimensions. The result is a tiny bit of matter that acts (electrically) a bit like a bulk semiconductor and a bit like a single particle. Quantum dots are being researched all over the scientific world nowadays, from lasers and solar cells to quantum computing.

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Applied magnetic field enhances superconductivity

22 Sep

Metals conduct electricity better at low temperatures. Superconductivity is a phenomenon exhibited by some materials where the resistance of that material drops to zero, meaning that it conducts electricity very well, below a certain critical temperature. And I mean VERY well…like electrical current can run around forever in there. (Picture shows levitation of a magnet due to an induced magnetic field by the superconductor beneath).

Superconductivity is a very useful feature, but the problem is that the critical temperature is usually extremely low. It is a bit impractical to work with something that has to be kept at such a low temperature. There are high-temperature superconductors, but we are talking 90K (-300 degrees F),which is still pretty cold. Scientists have been looking for ways to increase the critical temperature of materials to make superconductivity more applicable.

For a minute, let’s look at the theory. There are a couple different ways of explaining the phenomenon of superconductivity. BCS theory describes it as a “condensation” of elections into bound, boson-like, states due to electron-phonon (quantum mechanical characterization of vibration) interaction. Which pretty much means that the electrons pair up because of the attractions and repulsions caused by lattice vibrations. You can kind of think of it classically in that the electron will pull the positive parts of the lattice towards itself, causing a net positive density around the first electron, which then attracts another electron. These are known as Cooper pairs–Cooper is the “C” in BCS.

According to BCS theory, applied magnetic fields should lower the superconductivity by disrupting these pairs of electrons. It would break up the symmetry. There have been some cases where an applied magnetic field slightly enhances the superconductivity, but never actually raises the critical temperature. It was thought that the magnetic field suppressed the negative effects of paramagnetic impurities in the material, causing the slight increase in superconductivity. This paper, however, shows that applied magnetic field can actually raise the critical temperature of the material, while the presence of impurities still decreases it, which completely flies in the face of accepted theory.