Maintaining Excitations For More Time

When materials receive energy either by incident light or heat some of this energy is given to the electrons and can cause them to be promoted into higher energy levels. These excited electrons are destined to de-excite and return to their original energy level in time. The problem is that in hot metals that have been excited by electromagnetic radiation, the electrons only remain excited for a couple of femtoseconds before relaxing back into a de-excited state. In semiconductors where the bands are much more separated it can be a hundred milliseconds before the electrons revert to their original energy. Ideally we want to be able to control the relaxing mechanism so that the metals are forced to remain excited for a similar length of time as semiconductors. If this could be achieved it would improve our ability to capture solar energy.

It was found that the electron and hole recombination event could be controlled through strong electron lattice correlations. This is where the material can no longer be modelled as if each electron was acting independently and so the idea that there is a flowing “sea” of electrons leads to incorrect predictions. Instead each electron has a complex effect on its local surroundings. By encouraging the formation of polarons (where the positive lattice is attracted and forms a phonon around the electron) in these conditions it is suggested that the lifetime of excitation would increase. By experimenting on a mix of praseodymium (Pr), calcium (Ca) and manganate (MnO3) collectively called PCMO it was found that at room temperature these polarons are indeed responsible for a slow de-excitation of about two nanoseconds. This polaron was observed to contain energy much greater than the equilibrium energy for the metal. Normally this would decay quickly back to the ground state but with assistance from the environment electrons it can remain in this unstable form for a (relatively) long time. The exact optical absorption lengths for these materials makes them perfect for absorbing infra red and visible light if their excitations can be maintained and so may be used in photovoltaic applications.

Paper links: Evolution of Hot Polaron States with a Nanosecond Lifetime in a Manganite Perovskite


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