单分子设备可作为纳米科学实验的有力工具

Cornell University researchers recently stretched（伸直，舒展） individual molecules¹ and watched electrons flow through them, proving that single-molecule² devices can be used as powerful new tools for nanoscale science experiments. The finding, reported in the June 11 issue of the journal Science, probes the effects of strong electron interactions that can be important when shrinking electronics to their ultimate small size limit--single-molecule devices. The work resulted in the first precision tests of a phenomenon known as the underscreened Kondo effect.
A team of Spanish researchers has made a high-speed recording³ of elves and sprites in storms, fleeting⁴ and luminous⁵ electric phenomena⁶ produced in the upper layers of the atmosphere. Their analysis of these observations has been published in the Journal of Geophysical Research. "This is the first time in Europe that we have been able to use high-speed video to detect transitory luminous phenomena taking place in the upper atmosphere – so-called sprites (in the form of a carrot or column) and elves (which are ring shaped)", Joan Montanyà, co-author of the study and a researcher at the Department of Electric Energy at the Polytechnic⁷ University of Catalonia (UPC), tells SINC.

The results have been published in the Journal of Geophysical Research and show there are many fewer elves in storms that form over land than those at sea, where electric currents apparently⁸ have greater energy, especially in winter. Some of the recordings⁹ show elves and sprites at the same time, evidence of the strength of lightning over the sea during winter storms.

The scientists also observed the interaction between two sprites. A branch of one of them hit and bounced off the second, giving clues about their dynamics¹⁰ and electric structure. Sprites normally appear for around 40 milliseconds and 20 or 30 kilometres away from the site of the lightning.

"All these phenomena are related with storms, particularly winter storms, but they only appear in mesoscale convective systems (usually in large fronts), which produce lightning with high levels of energy or extreme electric currents", explains Montanyà.

As it is difficult to record these phenomena in situ during storms, the researchers placed a high-speed video camera on land with an image intensifier. This was used to remotely record a winter storm in the Western Mediterranean¹¹ (at a distance of between 400 and 1,000 kilometres) between the coasts of Italy and Spain.

The physics of electric discharges

"The observations made it possible not only to capture images of these short duration events, but also mean we can study the structure and dynamics of these highly unique electric discharges", explains Montanyà.

"Understanding the physics behind lightning and events associated with it will help us to protect ourselves better", the scientist points out, stressing the importance of research into sprites and elves to better understand other phenomena, such as gamma rays from terrestrial sources (TGF, Terrestrial Gamma-ray Flash), which also develop above electric storms.

In fact, the European Space Agency (ESA)'s future Atmosphere-Space Interactions Monitor mission (ASIM) aims to monitor these phenomena by placing an instrument on the outside of the International Space Station, due to be launched in 2013.
"The main advance in our work is that we show single-molecule devices can be very useful as scientific tools to study an interesting phenomenon that has never before been experimentally accessible," said Dan Ralph, the Cornell physics professor who led the study.

The research was funded in part by the Cornell Center for Materials Research, which is supported by the National Science Foundation's (NSF) Division of Materials Research. NSF's Division of Chemistry also contributed to the project.

"Single-molecule devices can indeed be used as model systems for making detailed¹² quantitative¹³ studies（定量分析） of fundamental physics inaccessible¹⁴ by any other technique," said first author Joshua Parks, a postdoctoral associate in Cornell's Department of Chemistry and Chemical Biology.

Using a cobalt-based complex cooled to extremely low temperatures, Ralph, Parks and an international team of researchers watched electrons move through single molecules and accomplished¹⁵ a feat¹⁶ that until now escaped chemists and physicists¹⁷. They were able to study the resistance of the flow of electricity within a system's electric field as the temperature approaches absolute zero.

This is known as the Kondo effect.

In physics, the Kondo effect is perhaps the most important model for understanding how electrons interact within a system such as a molecule. Because of the Kondo effect, when a spinning（纺织的） molecule is attached to electrodes, interactions between the molecule and electrons lead to coordinated¹⁸ motion of the electrons, resulting in a localized cloud of electrons that cancels out the molecule's spin and permits the electrons to flow with decreasing resistance as the temperature approaches zero degrees Kelvin, -273 degrees Fahrenheit¹⁹.

However, theories since 1980 have also predicted for certain types of high-spin molecules the possibility of an underscreened Kondo effect, in which the spin of the molecule is not completely cancelled and the resulting correlations²⁰（相互关系） between the flowing electrons are not as complete.

The researchers tested the Kondo effect by placing the cobalt-based complex between two electrodes（电极） and slowly stretching individual spin-containing molecules. They were able to manipulate the molecule's magnetic properties and make precise tests of how electrical resistance changes with variations in temperature. The results were found to be in good agreement with predictions for the underscreened Kondo effect.

"The research shows mechanical control can be a realistic（现实的，逼真的） strategy for manipulating molecular²¹ spin states, to supplement or replace the use of magnetic fields in proposed applications such as quantum computing²² or information storage," said Parks.