Molecular electronics is considered a promising approach for future nanoelectronic devices. In order that molecular junctions can be used as electrical switches or even memory devices, they need to be actuated between two distinct conductance states in a controlled and reproducible manner by external stimuli. Here we present a tripodal platform with a cantilever arm and a nitrile group at its end that is lifted from the surface. The formation of a coordinative bond between the nitrile nitrogen and the gold tip of a scanning tunnelling microscope can be controlled by both electrical and mechanical means, and leads to a hysteretic switching of the conductance of the junction by more than two orders of magnitude. This toggle switch can be actuated with high reproducibility so that the forces involved in the mechanical deformation of the molecular cantilever can be determined precisely with scanning tunnelling microscopy.

We report measurements of the shot noise on single-molecule Au-1,4-benzenedithiol-Au junctions, fabricated with the mechanically controllable break junction (MCBJ) technique at 4.2 K in a wide range of conductance values from 10(-2) to 0.24 conductance quanta. We introduce a simple measurement scheme using a current amplifier and a spectrum analyzer and that does not imply special requirements regarding the electrical leads. The experimental findings provide evidence that the current is carried by a single conduction channel throughout the whole conductance range. This observation suggests that the number of channels is limited by the Au-thiol bonds and that contributions due to direct tunneling from the Au to the π-system of the aromatic ring are negligible also for high conductance. The results are supported by quantum transport calculations using density functional theory.

We have calculated the temperature, magnetic field, and ion doping dependence of the magnetic and electric properties in Fe-doped BaTiO 3 using a microscopic model and the Green's function technique. It is shown that the ferromagnetic and multiferroic properties observed at room temperature in Fe doped BaTiO 3 could be due to the super exchange interactions between Fe 3þ ions in different occupational sites associated with oxygen vacancies and to the exchange coupling of Fe ions with mixed valence, Fe 3þ and Fe 4þ. There is a multiferroic region which depends strongly on the Fe-doping concentration. V

We present the chemical synthesis as well as charge transport measurements and calculations for a new tripodal platform based on a rigid 9,9'-spirobifluorene equipped with a phenylene-ethynylene wire. The transport experiments are performed with the help of the low-temperature mechanically controlled break junction technique with gold electrodes. By combining experimental and theoretical investigations of elastic and inelastic charge transport, we show that the current proceeds through the designated molecular wire and identify a binding geometry that is compatible with the experimental observations. The conductive molecular wire on the platform features a well-defined and relatively high conductance of the order of 10(-3)G0 despite the length of the current path of more than 1.7 nm, demonstrating that this platform is suitable to incorporate functional units like molecular switches or sensors.

Recently, there has been a great effort in studying magnetism in nonmagnetic semiconductors diluted with magnetic impurities due to possible applications in spinbased electronic systems. Moreover, nanoparticles (NP) of inorganic materials including otherwise nonmagnetic oxides such as CeO 2 , Al 2 O 3 , MgO, ZnO, In 2 O 3 and SnO 2 , nitrides, chalcogenides and other functional materials like superconductors and ferroelectrics were shown to be ferromagnetic at room temperature. The magnetism in these NP has been suggested to be intrinsic and originates from cation or anion vacancies at the surfaces of the NP. Very recently, it was reported from both experiments and firstprinciples calculations that typical ferroelectric materials such as BaTiO 3 (BTO) and PbTiO 3 (PTO) become multiferroic when they are prepared at the nanoscale [1 -8]. So, nanocrystalline BTO offers a room-temperature magnetic hysteresis as well as temperature-dependent dielectric constant and a polarization. Multiferroics that exhibit magnetoelectric coupling are widely discussed from quite different context, see [9 -12]. However, apart from a density functional calculation as vacancy-induced magnetism in BTO(001) thin films [13] a well accepted theoretical description of the ferroic properties of nanocrystalline BTO is still missing. In a previous paper [14] the static and dynamic properties of KH 2 PO 4 (KDP)-type and BTO-type ferroelectric NP have been reviewed. Based on that approach we propose a statistical model which reflects the multiferroic properties of BTO-NP. While the spontaneous polarization in a classic ferroelectric material like BTO is expected to diminish when the particle size is reduced [14-16], ferromagnetism cannot occur in bulk material [17]. It is observed experimentally that the multiferroic nature emerges in an intermediate size range of nanocrystalline BTO. Whereas the ferromagnetism is arising from the oxygen vacancies or point defects at the surface of the NP the ferroelectricity is originated from the core of the material [1][2][3].As demonstrated in [14] the Ising model in a transverse field is also appropriate to describe the properties of ferroelectric NP. The Hamiltonian reads e 1 2Since the ferroelectricity in BTO is originated from the off-centering of the Ti ions with respect to the cubic perovskite crystal we assume as the simplest model that there are two positions of the Ti atoms in a double-well potential. These two states are mapped on the S z -component of a pseudo spin-1/2 operator whereas the S x -component Based on a microscopic approach we demonstrate that the unexpected ferromagnetic properties of BaTiO 3 (BTO) or PTO observed recently at room temperature are due to oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ti 3+ or Ti 2+ ions with nonzero net spin. The resulting different valence states composed of Ti 3+ or Ti 2+ offer a nonzero magnetization which decreases with increasing particle size. The system shows a multiferroic behavior belo...

Using spin Hamiltonian models and Green’s function techniques, we study the ferroic order parameters of ferroelectric nanoparticles, and show how multiferroic behavior can be achieved in such systems. We present a theoretical study suggesting that unexpected ferromagnetic properties of perovskite ferroelectric ABO3 nanoparticles (A = K, Li; B = Ta, Nb or A = Ba, Sr, Pb; B = Ti) observed recently at room temperatures can be explained by considering oxygen vacancies at the surface of the nanocrystalline materials. Such vacancies lead to the appearance of Ta4+ and/or Ta3+ (Ti3+ and/or Ti2+) ions at the surface with nonzero net spin. The resulting different valence compared to the Ta5+ (or Ti4+) with S = 0 in the bulk offers a nonzero magnetization which increases with decreasing particle size. The system shows a multiferroic behavior below a critical size of the nanoparticles and the related polarization tends to a saturation value when the particle size is enhanced.

Using a microscopic model and a Green's function technique we have studied the critical behavior of some multiferroics such as hexagonal RMnO3. The temperature dependence and the external field dependence of the magnetization and susceptibility are determined. The critical exponents β and γ are calculated. Applying the scaling laws α, δ, and ν are also obtained. The critical exponents are in very good agreement with the existing experimental data.

We have proposed microscopic models for describing the multiferroic properties of BiFeO 3 and GaFeO 3. It is shown that the mechanisms of the multiferroism are different. In BiFeO 3 , the magnetoelectric coupling is biquadratic, whereas in GaFeO 3 it is linear. The site disorder between Ga and Fe is a primary source of the net magnetic moment in GaFeO 3. The temperature and magnetic field dependence of the polarization is calculated in order to show that the proposed models for these two multiferroics are correct. Near the magnetic phase transition temperature T N we obtain a kink in the electric properties. V