Quantum transport through single-molecule devices: spin and vibration

Abstract

Over the past few years the emerging field of molecular electronics has stimulated the interest in understanding the physics of single-molecule transistors. Not only is this a result of promising technological applications, it has also been shown that the presence of specific internal molecular degrees of freedom leads to numerous novel quantum transport phenomena that go beyond the physics observed in larger nanostructural objects such as quantum dots or carbon nanotubes. In the present work we investigate the coupling of electronic degrees of freedom to (i) vibrations, (ii) spins, and (iii) chemical conformations in transport through molecular junctions. Transport through magnetic molecules is discussed in relation to molecular spintronics, i.e. the idea of integrating the concepts of molecular electronics and spintronics. An essential requirement for spintronics devices is the ability to control and detect the spin. In this context, we find that magnetic anisotropy is crucial for slow spin relaxation. The spin moment transmitted from one lead of the molecular junction to the other depends strongly on the orientation of the molecular spin and can be much larger than the initial molecular spin itself. This effect of giant spin amplification allows one to effectively read out the spin information. Importantly, the leads need not be polarized. On the other hand, spin writing requires a molecular junction that consists of one ferromagnetic and one nonmagnetic lead. Interestingly, current-induced switching of the spin to a predetermined state only requires a finite bias voltage and is also possible in the absence of a magnetic field. Furthermore, the proposed setup leads to interesting physics beyond the effect of spin writing, including the occurrence of large negative differential conductance (NDC) at high temperatures. This effect is the consequence of a new spin blockade mechanism. By this we mean the suppression of the single-electron tunneling rates for electrons of one spin species due to density-of-states effects. The interplay of magnetic and vibrational degrees of freedom is investigated in transport through vibrating single-molecule transistors in the Kondo regime. We find that the dependence of the Kondo temperature on the gate voltage is much weaker than in conventional nanostructures in the regime of strong electron-phonon coupling. Moreover, the Coulomb blockade is strongly asymmetric about the charge degeneracy points (marking the transition from the non-Kondo to the Kondo valley), i.e. the peaks in the differential conductance are well pronounced on one side of the degeneracy points, whereas they almost vanish on the other side. Experimental evidence for these two unusual features has been obtained in recent transport experiments on organic complexes. The main requirement for an electric circuit in nanoscale dimensions is a molecular device that can be switched between two distinct conductive states. Because of intrinsic bistabilities many single-molecule junctions reveal current-induced switching behavior, e.g. involving cis and trans isomers of a molecule. We study this process for molecules which exhibit two (meta)stable conformations in the neutral state, but only a single stable conformation in the ionic state. While other recent works in this field consider switching processes which are stimulated by thermal activation or vibrational-assisted tunneling from one minimum of the double well to the other, we show that the switching may also be induced by the current involving two subsequent sequential tunneling processes. Here, our main focus of interest is the regime of strongly asymmetric couplings to the leads, corresponding to the experimental setup of a scanning tunneling microscope (STM) conductance measurement. We show that the transport dynamics can be described by a set of Fokker-Planck equations for the Wigner distribution function of the molecule. Our main result is that the average number of switching events per time becomes extremely small compared to the average electronic tunneling rate determined by the current. In other words, the time that the molecule is in one of the two conformations is long compared to the average time between subsequent tunneling events. Such remarkable behavior has been observed in recent STM experiments.