instituto de matemáticas universidad de sevilla
Antonio de Castro Brzezicki
Solectrons in polydiacetylene: the ideal one dimensional electronic chain
A ballistic solectron, comprising a lattice soliton with an associated electron, was discovered, (accidentally), by Donovan and Wilson in 1981 [1]. They found, by experiment, a photocarrier to travel at a velocity close to the sound velocity, when pulled by an electric field. However, the velocity was independent of field, even over 4 decades of field. Moreover, at the lowest field, the velocity was greater than what would be found in any semiconductor at the same field. ( In conventional semiconductor language, the low field mobility was ultra-high, > 20 m2 s-1V-1 ).The carrier had thus a low dissipation of energy to the lattice. The carrier was ballistic.

This solectron was found on the backbone of a polydiacetylene (PDA) crystal. The backbone is made of π conjugated carbon chains. The chains are long, straight and parallel for macroscopic distances. The lowest electronic excitations belong to the backbone chains. The chains are separated by a (electronically) large distance (0.7 nm). Thus they form ideal one dimensional semiconductors. Optical absorption above the band gap (2.4 eV) created electron and hole photocarriers.

It is the ideal one dimensionality which leads to the extraordinary transport properties found. A model of the solectron was presented [2] which described the motion as that of an electron trapped within the lattice distortion that it has created, the whole moving together. An estimate of the energy loss to the lattice of a moving solectron was found to be ultra-small; insufficient to prevent the solectron increasing its energy (but not its velocity) at the smallest fields used (102 V/m). It was concluded this was the beginnings of an explanation of the experimental results. A further model of the solectron has been presented in 2014 by the group of Velarde (Velarde, Chetverikov, Ebeling, Wilson, Donovan [3]).

It is important that a solectron of thermal energies kT (25 meV) already has a velocity approaching that of sound [2]. It is natural to consider if a device can be created which uses these solectron transport properties. A Solectron Field Effect (SFET) has been proposed by Velarde and Wilson [4]. Consider source (S) and drain (D) conducting electrodes evaporated onto the surface of a PDA crystal. Consider, further, that the work functions of the S and D allow either electron or hole injection into the PDA in thermal equilibrium. Then it is anticipated that thermal solectrons, of either electron or hole character, are launched, with velocity approaching the sound velocity, along the chains from beneath the electrodes, in thermal equilibrium, in the absence of any applied fields. (Space charge then returns the carriers to their origin).

The SFET is created when an insulator, thickness d, and a conducting gate electrode (G) are evaporated. The gate voltage VG then controls the charge, and hence the S to D current, ISD in the channel between S and D. The potential advantage over the conventional silicon FET is twofold. First, the SFET can work at very low power supply voltages V0 , V0 = kT/e = 25 mV. Nevertheless, the response time can be fast. This reflects the ballistic properties of the solectron. Secondly, the gate insulator thickness, d, can by large. This reduces the gate to channel capacitance C, which is proportional to (1/d). In contrast, in the silicon FET the gate insulator thickness has to be small in order to control the carrier density in the channel. Both these factors are important when considering the energy cost of switching bits in the inverters of computer chips, for the energy cost per bit is C × (V0 )2. Possible energy consumption reductions of several orders of magnitude in a proposed SFET chip compared to present silicon microprocessor chips, while retaining the operating speed, will be described.

[1] K J Donovan and E G Wilson, "Demonstration of an ultra high mobility organic polymer", Phil. Mag. B 44, 9 (1981).
[2] E G Wilson "A New Theory of Acoustic Solitary Wave Polaron Motion", J. of Physics C, 16, 6739 (1983)
[3] M.G. Velarde, A.P. Chetverikov, W. Ebeling, E.G. Wilson, K J Donovan, "On the electron transport in polydiacetylene crystals and derivatives", Eur. Phys. Lett. EPL 106, (2014) 27004.
[4] "Solectron field effect transistor and inverter", M.G. Velarde, E.G. Wilson,
UK Patent Application Publication Number, GB 2533105, 15 June 2016,
Application Number 1421866.3, Filed 9 December 2014.

Organized by the Nonlinear Physics Group (GFNL) of the University of Sevilla