Electrostatic doping in field-effect transistor (FET) configuration is a powerful tool for investigating electronic transport properties of complex materials such as correlated oxides or organic semiconductors.[1] Key issues in electrostatically doped organic semiconductors are carrier mobility, contact resistance, structural defects, and interfacial phenomena.To disentangle intrinsic transport features from other contributions, scanning probe microscopy potentiometric techniques can be used for investigations of the local electric potential under bias, but were so far restricted to sub-100 nm thick evaporated and solution-processed polycrystalline [2,3] and semi-crystalline [4][5][6] organic thin film FETs. Bulk (i.e., lm to mm thick) single-crystal organic field-effect transistors [7][8][9] (SCOFETs) offer alternative and attractive paths for probing ultimate performances of defect-free molecular semiconductors, with charge carrier mobility values as large as 10-40 cm 2 V -1 s -1 for rubrene [10][11][12][13] and pentacene. [14] While the nanostructure and surface electronic states of organic single crystals have been successfully probed by scanning tunnelling microscopy (STM) and its spectroscopic modes, [15] to our knowledge, there are no reports of local investigations under electrostatic doping on operating SCOFETs. A recent report of high mobility values in "ultra-thin" (thicknesses in the 100 nm-1 lm range) SCOFETs [16] prompted us to check the possibility of investigating the local electronic transport under gate and source-drain bias on sub-lm thick single-crystals. We here demonstrate that scanning Kelvin probe microscopy [17] (SKPM), an atomic force microscopy (AFM)-based potentiometric technique, [18] allows for an in-depth exploration of the local electrical potential at the organic/dielectric interface in sub-lm thick organic single-crystals (OSCs), [16] opening novel perspectives for a deeper understanding of intrinsic charge transport and interfacial effects in semi-conducting singlecrystals. [7,19,20] Electrical ( Fig. 1) and SKPM (Figs. 2 and 3) investigations were carried out at ambient conditions on electrostatically bonded sub-lm thick rubrene single-crystals on Si back-gated FET structures with SiO 2 dielectric and metal source-drain contacts recessed in the oxide (see the Experimental section).[21] A member of the oligoacene family, [20] e.g., rubrene, was chosen as a model organic semiconductor, both for its high charge carrier mobility values and for its low sensitivity to contamination by oxygen in ambient conditions. The quality of rubrene single-crystals [22,23] was confirmed by polarized optical microscopy (POM) experiments [10,16] for which large birefringence effects were observed under cross-polarised light and by AFM characterisations of the surface morphology which displays flat terraces with single mono-layers steps (Fig. 3b). However, for some SCOFET devices (Fig. 2c), the surface morphology probed by AFM over the electrostatically bonded single-crystals presents irregularities...
It is now well admitted that the orbital degrees of freedom play an important role in the occurrence of non-conventional properties in many low-dimensional highly-correlated electron systems. Thus, in quasi-2D high-Tc cuprates the existence of staggered orbital currents has been invoked to explain the peculiarities of the pseudo-gap phase. In these materials, the quantitative understanding requires to take into account the multi-spin exchange interactions, as well. Among other effects, electron circulations or exchanges should give rise to anomalous magnetic correlation functions, which can be accurately investigated by means of polarized neutron scattering. This will be illustrated by recent studies carried out on the spin-ladder Sr14Cu24O41 and on the quasi-2D helimagnet BaCo2As2O8. * )
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