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Electron Emission under Microwaves

B. Ivlev
University of South Carolina, US

Keywords: electron emission, microwaves, quantum devices

Quantum tunneling of electrons through nonstationary potential barriers is very unusual. The problem was addressed in B. Ivlev and V. Melnikov, Phys.Rev.Lett. 55, 1614 (1985), T. Martin and G. Berman, Phys.Lett. A 196, 65 (1994), S. Keshavamurty and W. Miller, Chem.Phys.Lett. 218, 189 (1994), A. Defendi and M. Roncadelly, J.Phys. A 28, L516 (1995), N. Maitra and E. Heller, Phys.Rev.Lett. 78, 3035 (1997), J. Ankerhold and H. Grabert, Europh.Lett. 47, 285 (1999), B. Ivlev, Phys.Rev. A 66, 012102 (2002). Emission of electrons from surfaces of metals, semiconductors, etc. to vacuum requires applying a big d.c. electric field, of the order of 10 MV/cm, to increase the tunneling current through the potential barrier created by the applied field. In this case, according to rules of quantum mechanics, the potential barrier is sufficiently thin and therefore sufficiently transparent for electron tunneling. At the small d.c. field of 100 V/cm the potential barrier is thick and almost classical which prevents quantum effects of tunneling. If to apply only a weak a.c. field of the frequency of a few GHz, electrons can escape to vacuum solely via multiquanta absorption to reach the top of the potential barrier. This process goes with no tunneling; nevertheless, it has also an extremely small probability since an electron has to absorb a very big number of quanta. The new accomplishment is that a cooperative action of the above small d.c. and small a.c. fields may result in the unexpected phenomenon: a current of emission electrons becomes not small. The phenomenon is very sensitive to the initial electron energy. The increase of barrier penetration occurs only for electron energies close to the certain energy which can be called the resonance energy. The phenomenon is called Euclidean resonance. The key point for understanding of this phenomenon is that the small and relatively slow varying a.c. field results in the fast under-barrier instability. The fast energy exchange between an electron and a.c. field leads, according to uncertainty principle, leads to the instant broad distribution in electron energy. The certain energy in the broad distribution plays the main role in the barrier penetration. Under definite conditions, this main energy can exceed the barrier height resulting in the easy over-barrier transition with no tunneling. The phenomenon of Euclidean resonance can be used for practical purposes in nanotechnology to control electrical current in nanodevices. The conventional method of tunneling spectroscopy enables to measure the density of electron states in metals and semiconductors. When the energy scale of electron spectrum is less than room temperature one should use a low temperature technique. The new method of nanoscale tunneling spectroscopy, modified by microwave irradiation, enables to resolve a fine structure of electron spectrum even at room temperature due to very sharp peak in the energy dependence of emission electrons.

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