news center

Science: Electron 'trains' signal new state of matter

Science: Electron 'trains' signal new state of matter

作者:公良箸锇  时间:2019-03-01 07:14:00  人气:

By ELISABETH GEAKE A new state of matter called a Luttinger liquid has been found by three teams of physicists in the US. In this state, electrons become linked, moving together like carriages in a train rather than independently as they normally do. The idea of such a state of matter was thought up 30 years ago by Joaquin Luttinger of Columbia University, New York. He regarded it merely as a mathematical tool to solve a physics problem: ‘I never in my wildest dreams expected it to be found experimentally in any real system,’ says Luttinger, who is now retired. Last year, Charles Kane at the University of Pennsylvania in Philadelphia and Matthew Fisher of the University of California in Santa Barbara suggested an experiment to verify that a Luttinger liquid could really exist. The experiment has now been performed and Kane says he is ’70 to 80 per cent’ certain that it confirms the state’s existence. At normal temperatures, electrons in a conducting material are in constant frenzied motion, and the movement of any one electron does not directly affect the others. Luttinger suggested that if electrons were confined to a very thin, perfectly clean wire, they might behave in an unusual way. In particular, if the wire were cooled to very close to absolute zero, -273 °C, electrons would pack together so closely that if one electron moved it would reduce the density of electrons locally. To restore the density, the other electrons would rearrange themselves and the net result would be a Luttinger liquid in which the electrons would appear to be connected. Because electrons in a Luttinger liquid can only move in unison, it requires a lot of energy to make them move at all. Close to absolute zero, this energy is not available, so the electrons are prevented from flowing through the wire. As a result, the wire’s resistance should rise towards infinity. In a normal wire, the electrical resistance never becomes infinite because a few electrons can always flow through. But setting up the conditions to create a Luttinger liquid is difficult. The electrons must be confined to a thin wire. This allows the electron ‘train’ to pass through lengthways rather than sideways, making the coupling between electrons more obvious. However, a single atom of an impurity in such a very thin wire can prevent it conducting properly. ‘Impurities, which are very difficult to get rid of, tend to destroy the Luttinger liquid state,’ says Kane. But four years ago, Xiao-Gang Wen of the Massachusetts Institute of Technology realised that if a magnetic field was applied across the material, the disruptive effect of impurities could be eliminated. Richard Webb, Frank Milliken and Corwin Umbach of IBM’s Yorktown Heights research laboratory in New York state set out to test Wen’s theory. They made a device from two semiconductors – gallium arsenide and gallium aluminium arsenide. They then measured its behaviour as they varied the voltage and the temperature down to 38 millikelvin. ‘We obtained data consistent with a Luttinger liquid,’ says Webb, now at the University of Maryland at College Park. Before the experiments began, the devices had to be refrigerated for three weeks in order to stabilise impurities. Also, they could be tested reliably only for two to three weeks before the impurities began to migrate through them again. Webb says he is ‘really excited’ about the results. ‘This is an example of a situation where the standard picture of electrons is no longer valid,’ he says. He believes their collective behaviour could give clues to the ways more complicated groups of electrons behave,