Photon-assisted tunneling

In this section we report the first study of the micro-SQUID response of a low-spin molecular system, V15, to electromagnetic radiation. The advantages of our micro-SQUID technique in respect to pulsed electron paramagnetic resonance (EPR) techniques consist in the possibility to perform time-resolved experiments (below 1 ns) [59] on submicrometer sizes samples (about 1000 spins) [22] at low temperature (below 100 mK). Our first results on Vi5 open the way for time-resolved observations of quantum superposition of spin-up and spin-down states in SMMs. Other results obtained in similar systems but with large spins concern for example EPR measurements [10], resonant photon-assisted tunneling in a Feg SMM [60]. 

Karl Unterrainer, Photon-Assisted Tunneling in Semiconductor Quantum Structures P. Haring Bolivar, T. Dekorsy, and H. Kurz, Optically Excited Bloch Oscillations-Fundamentals and Application Perspectives... 

Meyer C, Elzerman J, Kouwenhoven L (2007) Photon-assisted tunneling in a carbon nanotube quantum dot. Nano Lett 7 295-299... 

We note that going to the length gauge provides a coupling of the form So x cos(uit). This is to be contrasted with the form Vo cos(ut) which is often assumed to investigate photon assisted tunneling [13, 14,15]. However this is not of major consequence on the characterization of the different regimes which can be delineated in the phenomenon [9],... 

Thon A, Merschdorf M, Pfeiffer W, Klamroth T, Saalfrank P, Diesing D (2004) Photon-assisted tunneling versus tunneling of excited electrons in metal-insulator-metal junctions. 

Since in this case, electrons could only be excited in a single well the photocurrent was small. On the other hand, the quantum yield, that is, the number of transferred electrons per absorbed photons, reached values of up to = 0.63 [80]. This might appear surprisingly high for a relatively thick outer barrier layer. However, calculations and measurements of the temperature dependence of the photocurrent showed that at room temperature the mechanism of electron transfer out of the well was thermionic emission over the barrier [80]. The rate of thermionic emission at lattice temperatures in the range of 200—300 K was sufficient to keep up with the measured rate of interfacial electron transfer. Studies with very thin outer barriers (20 A) have shown that the mechanism of charge transfer was field-assisted tunneling, and the photocurrent was then independent of temperature. 

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