Cooper pairs of an electron and a hole
Scientists from FOM and Utrecht University have discovered that Cooper pairs of electrons and holes can form in a semiconductor at a very low temperature. These electron-hole Cooper pairs are analogous to the electron-electron Cooper pairs that give rise to superconductivity. The results are therefore important for a better understanding of superconductors at high temperatures. The researchers published the results on 15 May in the journal Physical Review B.
Electrons and holes in a semiconductor are sometimes just like people. Under normal conditions they form reasonably stable pairs. However, if they continually come into contact with attractive members of the opposite sex they become unfaithful. It is the same with electrons and holes: if they come too close together then no stable pairs remain.
There is, however, an interesting exception to this. Back to the comparison with people: imagine a country has both well-heated and poorly heated homes. As long as it stays warm, it makes little difference where you live. However, if it becomes cold then all of the men and women will go and live in the best-heated houses. For a couple in a well-heated house the threshold to divorce then becomes high: all other well-heated houses are occupied and nobody wants to move to a cold and draughty house. So couples will continue to stay together. The same applies to electron-hole Cooper pairs: electrons and holes remain together at a high density and low-temperature as all other favourable positions are already occupied.
Emission of light
The FOM researchers Marijn Versteegh, Arjon van Lange, Henk Stoof and Jaap Dijkhuis from Utrecht University calculated just how far they would have to cool down a zinc oxide crystal so that electron-hole Cooper pairs would arise at a realisable density. When the calculations revealed that the low temperatures could be achieved experimentally, they decided to put the theory to the test. They created a high density of electrons and holes by firing ultrashort, highly intense laser pulses on a zinc oxide crystal (Fig. 1) and they achieved this at increasingly lower temperatures. When the crystal was cooled down to -269°C a very strong emission of light occurred (Fig. 2). The wavelength and intensity of the light accurately agreed with the quantum theoretical calculations the researchers had performed. Light was found to originate from electron-hole Cooper pairs in a non-condensed state.
There are indications that high-temperature superconductivity is also related to uncondensed electron-electron Cooper pairs. This discovery of condensed electron-hole Cooper pairs could help to provide an explanation for superconductivity at high temperatures.
The researchers would now like to produce a condensate of electron-hole Cooper pairs. Realising this will require an even lower temperature still. If this effort proves successful then the Cooper pair laser will become a reality.
Publication
M.A.M. Versteegh, A.J. van Lange, H.T.C. Stoof en J.I. Dijkhuis, ‘Observation of preformed electron-hole Cooper pairs in highly excited ZnO’, Physical Review B (2012).
Contact
Prof. dr. J.I. Dijkhuis
Debye Institute for Nanomaterials Science, Universiteit Utrecht
+31 (0)30 253 23 19
Dr. M.A.M. Versteegh
Kavli Institute of Nanoscience, Technische Universiteit Delft
+31 (0)15 278 35 52
Prof. dr. ir. H.T.C. Stoof
Institute for Theoretical Physics, Universiteit Utrecht
+31 (0)30 253 18 71