Tuesday, March 06, 2007

Application of BECs

In my previous post, I discussed what Bose-Einstein condensates are and how they are created. It is great to have a new state of matter, but it is not that breathtaking if there is no application. This post is going to deal with the application of Bose-Einstein condensates and is mostly based on Wolfgang Ketterle’s second McGill lecture, given on March 2nd. I incorporated some of my own understand and research into this, especially to simplify concepts. The two main benefits of BECs I will deal with is the creation of an atomic toolbox and increased headway into room temperature superconductors.

Atomic Physics Toolbox

There have always been differences between theoretical and applied physicists. Theorists love using perfect models to make prediction and calculations. Applied physicists have to use real materials with imperfections and need constant estimates. The imprecision of applied physics has slowed the field down a little, staying several steps behind theoretical physics. MIT hopes their BECs and cooling technology can serve as an atomic toolbox for many-body scientists to develop new materials.

The nanokelvin toolbox makes new physics possible. With the temperature approaching 0.5 nanokelvins and less, several rare physical phenomena can be observed. As explained in the previous post, the BEC phase transitions can be studied at this temperature. Also the atoms start to display quantum reflection and coherent chemistry. More importantly, at such low temperatures, the intermolecular forces overshadow kinetic energy and strong molecular interactions can be observed. These phenomena enhance knowledge of quantum mechanics and in many cases many-body physics as well.

MIT’s cold samples also simplify manipulation. When particles in a sample have lower kinetic energies they become easier to trap and move around. The magnetic field needed to confine the sample is significantly lower than its microkelvin equivalent. Individual atoms can be easily trapped via lasers. A laser as weak as a standard pointer starts to have enough power to trap and transport atoms. The ease of manipulation makes making optical traps, lattices and atom chips simpler. Less energy can be invested after cooling to achieve better results.

Many-body physicists can use the nanokelvin samples to solve systems with strong interactions and correlations. Normally, a gas has very weak interaction due to individual particles’ high kinetic energies. When super cooled the gas starts to display solid-like interaction and cohesion, but on a much easier to study scale. The ultra low density and ultra low pressure (reviling mediocre vacuum chambers in pressure) make the sample constituents far apart and organized in perfect structures. The dominant intermolecular forces make the system a good modeling tool for more complex many-body systems. The toolbox serves as a long awaited balance between theoretical and real systems.

Superconductors

First, it is vital to understand what makes superconductors important. Currently, semiconductors waste energy during transmission due to internal resistance to electron flow. To avoid this, the resistance on individual electrons inside a conductor has to be eliminated, so that the electrons can travel freely. The creation of effective superconductors has applications ranging from fusion, to medicine, to computer science.

Although, seemingly impossible, superconductivity has already been achieved. The current superconductors are hard to construct and operate at temperatures of 20 Kelvin or less. In 1986, “high temperature” superconductors were found, that operated at around 90 Kelvin. Most semiconductors’ resistance decreases with temperature, but even at 0 Kelvin, standard conductive matters like silver and copper still display a nonzero resistance. Superconductors work because some materials’ resistance abruptly drops to zero at a certain “critical temperature”. The reason that superconductors behave the way they do is explained by the Ginzburg-Landau theory and BCS theory. The first examines the macroscopic properties by mathematics and the second explains the quantum mechanics.

Under the BCS theory electrons pair up in Cooper pairs to facilitate resistance free travel. Normally electrons repel each other due to Coulomb’s law (opposites attract, alike repel). However, under low enough temperatures and proper magnetic conditions a positive charge develops between a pair of electrons, letting them overcome their natural repulsion and pair up.

Before pairing up a single electron is made up of one constituent and thus is a fermion. Fermions differ from Bosons (the ones that make BEC) in the number of constituents, or more fundamentally in their spin. Fermions have half-integer spin and Bosons have integer spin. Due to the Pauli Exclusion Principle (which explains why matter occupies space) the fermions are unable to gather at the low state like bosons. This exclusion leads to fermions having high kinetic energy even at 0 Kelvin and not being able to form Bose-Einstein Condensates. However, when two half-integers are added, an integer value is achieved and the possibilities expand.

A Cooper pair is considered a boson. The whole pair when created has an integer spin, allowing several pairs to accumulate on the lowest energy level. By accumulating several pairs at low energy state, a Bose-Einstein condensate can be formed. The only big difference between a standard boson and a Cooper pair remains in the spacing of the constituents. In a normal boson they are close together, but in a Cooper-pair they are farther apart and the orbit might even cross over other Cooper pairs. This “far apart” state of Cooper pairs as bosons is known as the BCS state.

Sadly, BCS’s unlike BECs do not display superfluid properties. Superfluids differ from normal liquids in that they experience no friction. Since a superfluid is a matter-wave (much like a BEC) it also displays various other strange properties. When rotated, a superfluid can only take on integer values for speed. This leads to a mosaic pattern of rotating mini-tornados created in the rotating sample (as opposed to the single tornado effect of a normal liquid). More importantly if superfluidity could be achieved for electrons then new superconductors will be possible.

Achieving superfluidity for electrons is precisely what Ketterle’s team at MIT is doing. Currently they can not work with electron samples, so they pair up Lithium ions (which are fermions) into boson pairs. Then, through various manipulations and cooling they achieve a transition, or hybrid state between a BEC and a BCS. In this state the sample displays superfluidity. The temperatures are still in the nanokelvin range, but temperatures can not be compared directly between different materials. Values have to be scaled by density to get the proper results. When the Lithium sample is scaled for density it turns out that it is actually 200 times warmer than a helium superfluid and 20 times warmer than the hottest superconductor. That means once such a technology can be applied to electrons and scaled for density, superconductors will be possible significantly above room temperature.


Conclusion

Should we expect Cooper pair super conductors to be part of our gadgets in ten years? I doubt it. However, the field is moving quickly and in the coming years’ electron BEC/BCS hybrid forms should appear and physics should begin with them. Also, the atomic physics toolbox should help physicists and engineers develop better materials for everything from clothes to space shuttles. There should defiantly be a lot of cool (and maybe even room-temperature) science over the coming decade.

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