Molecules can superflow
Some liquids such as helium exhibit peculiar properties: at very low temperatures, the liquid seems to lose all friction as it flows; spontaneous fountains can form; or the liquid could creep up the wall of its container. These behaviors are manifestations of a property called superfluidity. Scientists have long searched for such phenomena in other condensed substances to see if this behavior extends to other substances. We present the first direct experimental evidence of superfluidity in a molecular system, namely nano-clusters of hydrogen molecules. The laboratory observation is supported by theoretical analysis from computer simulations that predicted the presence of superfluidity. We observed the nearly free rotation of a carbon dioxide molecule embedded in the hydrogen nano-clusters. The carbon dioxide acts as a "paddle" to test wether the cluster presents any resistance to motion. The lack of resistance, or viscosity, due to the motion of the "paddle" indicates that superfluidity in the nano-cluster has been established. We have attributed the superfluidity to the delocalized nature of the hydrogen molecules, which means that at very low temperatures, the superfluid nano-cluster behaves as a new single entity and no longer as a liquid with independent molecules.
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LN12202BR
Geminate recombination absent in efficient organic solar cells
Experiments show that an expected loss mechanism is absent in the most efficiency organic solar cells. Light absorbed in an organic semiconductor creates a tightly bound electron and hole pair – an exciton – that cannot separate except at very high electric fields. The bulk heterojunction solar cell solves the problem with a nanoscale mixture of two semiconductors, which allows the exciton to split at internal interfaces and enables solar cells of steadily improving efficiency. However the split electron and hole are still close together and bound by their Coulomb energy. Many scientists expected that their immediate recombination – known as geminate recombination – limits the solar cell performance. We were able to show that transient photoconductivity measurements distinguish geminate from non-geminate recombination. Carriers undergoing geminate recombination do not contribute to the photoconductivity while carriers that separate before they recombine do contribute. We tested two commonly studied high efficiency organic solar cell and found no detectable geminate recombination in either. This result is good for solar cells as it removes one fundamental limitation on the efficiency.
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LS12743
Controlling the temperature of quantum objects by measurements
The act of measuring is one of the most enigmatic phenomena in quantum physics since, as opposed to classical observations, it can in and of itself change the properties of the observed quantum object. A new twist to this paradox, due to appear in Phys. Rev. Lett. [1], shows that the state purity or polarization in a quantum object may change as a result of how frequent it is observed. This would be akin to taking the temperature of an object and finding that this depends on the rate at which it is taken: when the rate is very high the temperature goes up, and when it is taken more slowly it goes down. Counter intuitive effects of this kind have now been experimentally observed in experiments designed at controlling the polarization of nuclear spins in an ensemble of carbon atoms placed in contact with a "bath" consisting of proton spins. This study confirms theoretical predictions first described in an article that appeared in Nature in 2008. These effects defy the standard rules of macroscopic thermodynamics, whereby the interaction between a large heat source (“heat bath”) and a much smaller system must bring them to thermal equilibrium; that is towards a common, fixed temperature, unaffected by measurements. On the fundamental side, the present results help us bridge the long-standing gap between the quantum dynamics typically characterizing small (“nano”) systems, and the thermodynamics that generally governs the behavior of macrosystems. On the applied side, these results indicate the possibility of principally novel heating and cooling schemes, that may be faster and more robust than the existing ones.
