Cooper pairs of an electron and a hole

LL13172B - Superconductors conduct current without resistance, because many-body effects create pairs of two electrons, called “Cooper pairs.” Now, a paper appearing in Physical Review B reports the observation of similar pairs in a bulk semiconductor, but now of an electron and a hole. Rather than causing superconductivity, these electron-hole Cooper pairs give rise to strong light emission.

Physicists used ultrashort laser pulses to highly excite a bulk zinc oxide crystal. Upon cooling down the crystal to 4 kelvin, they discovered a strong new peak in the light emission spectrum. Both wavelength and intensity of the light excellently agreed with predictions from theory. Their conclusion is that the observed light emanates from electron-hole Cooper pairs in an uncondensed state. Since there are indications that high-temperature superconductivity might also be related to uncondensed Cooper pairs, these so-called “preformed electron-hole Cooper pairs” can be important in the quest to understand high-temperature superconductivity.

Quantum mechanics on a Mobius ring: Energy levels, symmetry, optical transitions, and level splitting in a magnetic field

BQ12069 - In this paper, the authors explore the quantum mechanical properties of an electron constrained to move on the one-sided surface of a nanoscale Möbius ring. The results are of more than theoretical interest, as recent advances in the production of graphene sheets suggest that it may be only a matter of time before it will be possible to create twisted graphene ribbons. By solving the Schrödinger equation on the Möbius surface, the authors show that the quantum numbers for the energy spectra correspond to those for cylindrical rings of the same dimension while their degeneracies are lifted. Rings with odd numbers of twists have quantum numbers with both whole and half integer values. Because the twisted rings lack the rotational symmetry of the cylindrical rings, the values for the orbital angular momentum component vary slightly from the integer and half-integer values seen in the cylindrical ring. Also, the non-zero variance in angular momentum permits the transition of photons from half-integral to integral angular momentum states, something normally prohibited by the requirement for the conservation of angular momentum. The Zeeman splitting in an external magnetic field shows level anti-crossing in the lowest two levels. Using high-accuracy finite element methods, the authors investigate rings with 1, 2, ...5 twists to identify a pattern in the level splitting that they explain using group representation theory. Beautiful wavefunctions with 2p and also 4p periodicity are also shown.

Magnetic control of Leidenfrost drops

LQ12932E - Oxygen is a common gas but its liquid state has interesting properties. Since its boiling point is at –183°C, a drop of liquid oxygen placed on a solid at room temperature levitates on a cushion of its own vapour (the so-called Leidenfrost effect, also observed with water on a hot pan at 300°C), which makes it ultra-mobile since it does not contact its substrate. In addition, oxygen is attracted by the poles of a magnet so that, as shown in our paper, magnets properly placed can be used to deviate, capture or accelerate these ultra-mobile and elusive drops. Figure 1 shows top views of trajectories observed when a drop of liquid oxygen travels across a horizontal glass plate below which a magnet is placed (grey circle). Beyond the understanding of these trajectories, this simple system allows to probe the dynamics of a liquid drop without touching it and it provides new possibilities for controlled experiments with levitating liquids.