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Abstract
When dipolar gases are condensed at sufficiently low temperature onto a solid surface, they form films that may spontaneously exhibit electric fields in excess of 108 V/m. This effect, called the ‘spontelectric effect’, was recently revealed using an instrument designed to measure scattering and capture of low energy electrons by molecular films. In this review it is described how this discovery was made and the properties of materials that display the spontelectric effect, so-called ‘spontelectrics’, are set out. A discussion is included of properties that differentiate spontelectrics from ferroelectrics and other species in which spontaneous polarisation may be found.
Spontelectric films may be composed of a number of quite mundane dipolar molecules that involve such diverse dipolar species as propane, nitrous oxide or methyl formate. Experimental results are presented for spontelectrics illustrating that the spontelectric field generally decreases monotonically with increasing deposition temperature, with the exception of methyl formate that shows an increase beyond a critical range of deposition temperature. Films of spontelectric material show a Curie temperature above which the spontelectric effect disappears. Heterolayers may also be laid down creating potential wells on the nanoscale.
A model is put forward based upon competition between dipole alignment and thermal disorder, which is successful in reproducing the variation of the degree of dipole alignment and the spontelectric field with deposition temperature, including the behaviour of methyl formate. This model and associated data lead to the conclusion that the spontelectric effect is new in solid-state physics and that spontelectrics represent a new class of materials.
Acknowledgements
With reference to our own work reported here, we gratefully acknowledge the help of H.C. Fogedby and A. Svane (Aarhus University) in aiding us to formulate the theory presented in Section 4. We also gratefully acknowledge support of the staff of the Aarhus Synchrotron Radiation Laboratory (ISA) without whom this work would not have been possible, the Danish Research Council, the LASSIE FP7 ITN network, grant number 238258 a Marie Curie Intra-European Fellowship 009786 (RB) and the Lundbeck Foundation (RB).
Notes
Dedicated to the memory of Jean-Pierre Ziesel, highly valued both as a wonderful scientist and a great friend.