TU1 B- I . HIGH FREQUENCY ONE DIMENSIONAL SINGLE ELECTRON TRANSPORT IN GaAs

HETEROSTRUCTURES

M.Pepper*, J.M.Shilton, V.I.Talyanski, J.E.Cunningham,C.J.B.Ford, C.G.Smith, D. A.Ritchie, G.A.C. Jones

Cavendish Laboratory Cambridge

CB30HE UK

Work on surface acoustic wave (SAW) induced single electron flow is described. A current given by ef is obtained where e is the electron charge and f is the frequency, near 2.8 GHz. Present accuracy for measurement of e is better than 0.1 %, prospects improvement will be discussed.

By forming a patterned gate on top of a two dimensional (2D) electron gas in the GaAs heterojunction, and applying a negative voltage it is possible to squeeze the electron gas into a desired shape (1). The simplest structure which can be configured in this way is the 1D electron gas arising from the application of a voltage to split Schottky gates over a GaAs heterojunction, here the progressive formation of 1D sub-bands is observed (2) until eventually the device is pinched off. If the channel is sufficiently short that it is less than the mean free path for elastic scattering then electron transport is ballistic and the conductance takes

quantised values - where n is the number of one

dimensional sub-bands occupied and the factor of 2 arises because of spin degeneracy, (3). When the negative voltage becomes sufficiently great, the ground one dimensional sub-band is forced above the Fermi energy and the channel is pinched off. Thus the conductance shows a series of plateaux separated by 2 2 - as the gate voltage is varied.

2e2n A

A

In our experimental configuration the SAW is induced by applying voltage to a transducer fabricated on the chip, the wavelength being twice the separation of the transducer fingers, w, ( about 0.5 micron) and related to the frequency byfw=s where s is the sound velocity, giving a resonant frequency near 2.8GHz. The SAWS generate a current which is then forced to flow through the channel defined by the split gates, the current comprising electrons trapped in the potential minima which are separated by a wavelength.

By gradually increasing the negative gate voltage the number of electrons in the minima is reduced until in single figures, and then to .... 5,4,3,2,1,0 with an accompanying current given by I=nef where n is the number of electrons in the minima, e is the electron charge and f is the frequency. As the gate voltage is altered, over a short range the number of energy levels

133

in a minima is constant so giving a plateau of constant current.

In the talk a discussion will be presented on the dependence of the current quantisation on sample geometry showing that (at the time of writing) samples have been measured which show the existence of up to 5 plateaux of quantised current with the first two particularly pronounced. The system corresponds to a chain of moving quantum dots, or sliding charge density wave, and when only one electron is in each minimum this is essentially a sliding Mott insulator in which the energy separation of the one and two electron states is the Mott-Hubbard gap of the system. Increasing the SAW power deepens the potential well, widening the plateau and requiring a more negative gate voltage to reach the single electron state, (4). Results will be presented on factors affecting the accuracy and flatness of the plateaux which is at present better than one part in 10 , the temperature dependence of the effect, which is clear between 1K and 0.3K and observable at 4.2K and the prospects for increasing the accuracy of the quantisation to the level required for metrological interest.

*Also at Toshiba Cambridge Research Centre, 260 Cambridge Science Park, Cambridge, CB4 4WE.

References

T.J. Thornton, M. Pepper, H. Ahmed, D. Andrews, G.J. Davies, Phys. Rev. Lett. 56, pp. 1198, 1986.

K-F Berggren, T.J. Thornton, D.J. Newson, M. Pepper, Phvs. Rev. Lett. 57, pp. 1769, 1986.

D.A. Wharam, T.J. Thornton, R. Newbury, M. Pepper, D.A. Ritchie, G.A.C. Jones, J. Phys. C 21, pp. L209, 1988; B.J. van Wees, H. van Houten, C.W.J. Beenakker, J.G. Williamson, L.P. Kouwenhoven, D. van der Marel, C.T. Foxon, Phvs. Rev. Lett. 60, pp. 848, 1988.

J.M. Shilton, V.I. Talyanski, M. Pepper, D.A. Ritchie, J.E.F. Frost, C.J.B. Ford, G.A.C .Jones,J. Phys.: Condens. Matt. 8, pp. L531, 1996; V.I. Talyanski, J.M. Shilton, M. Pepper, C.G. Smith, C.J.B. Ford, E.H. Linfield, D.A. Ritchie, G.A.C. Jones, Phys. Rev. B 56, pp. 15180, 1997.