We consider circular currents driven by voltage bias in molecular wires with loop substructures studied within simple tight-binding models. Previous studies of this issue have focused on specific molecular structures. Here we address several general issues. First we consider the quantitative definition of a circular current and adopt a definition that identifies the circular component of a loop current as the sole source of the magnetic field induced in the loop. The latter may be associated with the field at the loop center, with the magnetic moment associated with this field or with the total magnetic flux threading the loop. We show that all three measures yield an identical definition of the loop current. Second, we study dephasing effects on the loop current and the associated induced magnetic field. Finally, we consider circular currents in several molecular structures—benzene, azulene, naphthalene, and anthracene—and show that circular currents occur generically in such structures; can be, in certa...

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Circular Currents in Molecular Wires

We consider circular currents in molecular wires with loop substructures studied within simple tight-binding models. Previous studies of this issue have focused on specific molecular structures. Here we address several general issues. First we consider the quantitative definition of a circular current and adopt a definition that identified the circular component of a loop current as the sole source of the magnetic field induced in the loop. The latter may be associated with the field at the loop center, with the magnetic moment associated with this field or with the total magnetic flux threading the loop. We show that all three definitions yield an identical measure of the loop current. Secondly, we study dephasing effects on the loop current and the associated induced magnetic field. Finally, we consider circular currents in several molecular structures: benzene, azulene, naphthalene and anthracene and show that circular currents occur generically in such structures and can be, in certain voltage ranges, considerably larger than the net current through the molecule, and are furthermore quite persistent to normal thermal dephasing.

While mesoscopic conducting loops are sensitive to external magnetic fields, as is pronouncedly exemplified by observations of the Aharonov-Bohm (AB) effect in such structures, the small radius of molecular rings implies that the field needed to observe the AB periodicity is unrealistically large. In this paper, we study the effect of magnetic field on electronic transport in molecular conduction junctions involving ring molecules, aiming to identify conditions where magnetic field dependence can be realistically observed. We consider electronic conduction of molecular ring structures modeled both within the tight-binding (H\"uckel) model and as continuous rings. We also show that much of the qualitative behavior of conduction in these models can be rationalized in terms of a much simpler junction model based on a two-state molecular bridge. Dephasing in these models is affected by two common tools: the B\"uttiker probe method and coherence damping within a density matrix formulation. We show that current through a benzene ring can be controlled by moderate fields provided that several conditions are satisfied: (a) conduction must be dominated by degenerate (in the free molecule) molecular electronic resonances, associated with multiple pathways as is often the case with ring molecules, (b) molecular-leads electronic coupling must be weak so as to affect relatively distinct conduction resonances, (c) molecular binding to the leads must be asymmetric (e.g., for benzene, connection in the meta or ortho, but not para, configurations), and (d) dephasing has to be small. When these conditions are satisfied, considerable sensitivity to an imposed magnetic field normal to the molecular ring plane is found in benzene and other aromatic molecules. Interestingly, in symmetric junctions (e.g., para-connected benzene), the transmission coefficient can show sensitivity to magnetic field that is not reflected in the current-voltage characteristic. The analog of this behavior is also found in the continuous ring and the two-level models. Although sensitivity to magnetic field is suppressed by dephasing, quantitative estimates indicate that magnetic field control can be observed in suitable molecular conduction junctions.

Magnetic fields effects on the electronic conduction properties of molecular ring structures

A new proposal is given to control local magnetic field in a molecular junction. In presence of finite bias a net circular current is established in the molecular ring which induces a magnetic field at its centre. Allowing a direct coupling between two electrodes, due to their close proximity, and changing its strength we can regulate circular current as well as magnetic field for a wide range, without disturbing any other physical parameters. We strongly believe that our proposal is quite robust compared to existing approaches of controlling local magnetic field and can be verified experimentally.

Modulation of circular current and associated magnetic field in a molecular junction: A new approach.

The properties of organic molecules can be influenced by magnetic fields, and these magnetic field effects are diverse. They range from inducing nuclear Zeeman splitting for structural determination in NMR spectroscopy to polaron Zeeman splitting organic spintronics and organic magnetoresistance. A pervasive magnetic field effect on an aromatic molecule is the aromatic ring current, which can be thought of as an induction of a circular current of π-electrons upon the application of a magnetic field perpendicular to the π-system of the molecule. While in NMR spectroscopy the effects of ring currents on the chemical shifts of nearby protons are relatively well understood, and even predictable, the consequences of these modified electronic states on the spectroscopy of molecules has remained unknown. In this work, we find that photophysical properties of model phthalocyanine compounds and their aggregates display clear magnetic field dependences up to 25 T, with the aggregates showing more drastic magnetic field sensitivities depending on the intermolecular interactions with the amplification of ring currents in stacked aggregates. These observations are consistent with ring currents measured in NMR spectroscopy and simulated in time-dependent density functional theory calculations of magnetic field-dependent phthalocyanine monomer and dimer absorption spectra. We propose that ring currents in organic semiconductors, which commonly comprise aromatic moieties, may present new opportunities for the understanding and exploitation of combined optical, electronic, and magnetic properties.

Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T

The nonequilibrium Green's function method is used to study the ballistic transport in metallic carbon nanotubes when a current is injected from the electrodes with finite bias voltages. We reveal, both analytically and numerically, that large loop currents circulating around the tube are induced, which come from a quantum-mechanical interference and are much larger than the current along the tube axis when the injected electron is resonant with a time-reversed pair of degenerate states, which are, in fact, inherent in the zigzag and chiral nanotubes. The loop current produces large orbital magnetic moments, making the nanotube a molecular solenoid.

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Large orbital magnetic moments in carbon nanotubes generated by resonant transport

Inspired by a recent experiment which has detected a large orbital magnetic moment when electrodes are attached to a carbon nanotube, we study the ballistic transport in metallic carbon nanotubes when a current is injected from electrodes with finite bias voltages with nonequilibrium Green's function method. We reveal, both analytically and numerically, that large loop currents circulating around the tube are induced, which come from a quantum mechanical interference, when the injected electron is resonant with a time-reversed pair of degenerate states inherent in the zigzag and chiral nanotubes. This results in large orbital magnetic moments, making the nanotube a molecular solenoid. The current distribution for various types of tube and the magnetic moment as a function of the bias are displayed.

Large magnetic moments generated from loop currents in carbon nanotube attached to electrodes — a theoretical picture

A fundamental study on the ductility of high strength steels under impact deformation is carried out to investigate the effect of the local ductility of various materials on crash performance. In this study, newly developed 980 and 1180 MPa grade steels are investigated to clarify their advantages in term of crash performance compared to conventional DP (Dual Phase) steels. The features of the developed steel, named as jetQ are higher yield strength and higher local ductility due to an optimized microstructure by the quenching and partitioning process (QP) [1, 2]. The bending test according to VDA 238-100 is performed while observing the fracture propagation during the bending test. Fracture strain in the tensile tests is evaluated by a three-dimensional shape measurement technique for the fracture surface.Both three-point bending tests and axial impact tests are performed to evaluate the crashworthiness of different types of steel. The three-point bending test simulates the “structural bending and intrusive deformation” that occur in the automotive side impact test. In the axial impact test, the “folding deformation” and energy absorption during the automotive front crash test are simulated in a laboratory testing machine. In this study, the fracture behavior and energy absorption performance of the high strength steels are investigated under both deformation modes.Based on the experimental results, the effect of the local ductility of the high strength steel on the risk of material fracture is discussed. The experimental results show that high strength steels with higher ductility of thickness strain can improve crash deformation with lower risk of crash fracture. The jetQ steels of grades over 980 MPa have excellent local ductility compared to conventional DP sheets. The jetQ steel also shows higher crash performance due to its higher yield strength. Considering these advantages, the applicability of the jetQ steel to automobile body structures is discussed from the viewpoint of light weight body structures.

Shigehiro Takajo

Papers

Large orbital magnetic moments in carbon nanotubes generated by resonant transport

Large magnetic moments generated from loop currents in carbon nanotube attached to electrodes — a theoretical picture

Effect of Local Ductility of Advanced High Strength Steels in 980MPa and 1180MPa Grades on Crash Performance of Automotive Structures