ABSTRACT
Kondo effects, negative differential resistance, transistor-or diode-like behavior, as well as switching and hysteresis. (Gaudioso et al., 2000; Liang et al., 2002; Park et al., 2002; Elbing et al., 2005; Blum et al., 2005; Lörtscher et al., 2006; Choi et al., 2006; Osorio et al., 2010; van der Molen and Liljeroth, 2010).An important aspect that distinguishes nanoscale molecular conductors from mesoscopic semiconductor devices is the influence of the nuclear degrees of freedom of the molecular bridge on transport properties (Galperin et al., 2007b; Härtle and Thoss, 2011a; Härtle et al., 2013c). Due to the small size and mass of molecules, the charging of the molecular bridge is often accompanied by significant changes of the nuclear geometry, indicating strong electronic-vibrational (vibronic) coupling. This coupling manifests itself in vibronic structures in the transport characteristics and may result in a multitude of non-equilibrium phenomena such as current-induced local heating and cooling, multistability, switching and hysteresis, as well as decoherence (Galperin et al., 2005, 2007b; de Leon et al., 2008; Ioffe et al., 2008; Ward et al., 2008; Härtle et al., 2009; Hüttel et al., 2009; Repp et al., 2010; Hihath et al., 2010; Ballmann et al., 2010; Osorio et al., 2010; Arroyo et al., 2010; Secker et al., 2011; Härtle and Thoss, 2011a,b; Kim et al., 2011; Ward et al., 2011; Ballmann et al., 2012; Albrecht et al., 2012; Wilner et al., 2013; Ballmann et al., 2013; Härtle et al., 2013a,c). Vibrational signatures indicating strong vibronic coupling as well as strong excitation of vibrational modes were observed in experiments for a number of molecular junctions (Wu et al., 2004; LeRoy et al., 2004; Yu et al., 2004; Pasupathy et al., 2005; Sapmaz et al., 2006; Thijssen et al., 2006; Parks et al., 2007; Böhler et al., 2007; de Leon et al., 2008; Hüttel et al., 2009; Hihath et al., 2010; Ballmann et al., 2010; Jewell et al., 2010; Osorio et al., 2010). Novel experimental techniques based on measuring the force needed to break a junction (Huang et al., 2006) or employing Raman spectroscopy (Ward et al., 2008; Ioffe et al., 2008) allow, furthermore, a characterization of the current-induced vibrational non-equilibrium state of a single-molecule junction and, thus, complement the information carried by the respective current-voltage characteristics.Various theoretical approaches have been employed to describe vibrationally coupled electron transport through single molecules (for an earlier overview see Galperin et al. (2007b)).
