Extension of the source-sink potential (SSP) approach to multichannel quantum transport.

We present an extension of the single channel source-sink potential approach [F. Goyer, M. Ernzerhof, and M. Zhuang, J. Chem. Phys. 126, 144104 (2007)] for molecular electronic devices (MEDs) to multiple channels. The proposed multichannel source-sink potential method relies on an eigenchannel description of conducting states of the MED which are obtained by a self-consistent algorithm. We use the newly developed model to examine the transport of the 1-phenyl-1,3-butadiene molecule connected to two coupled rows of atoms that act as contacts on the left and right sides. With an eigenchannel description of the wave function in the contacts, we determined that one of the eigenchannels is effectively closed by the interference effects of the side chain. Furthermore, we provide an example where we observe a complete inversion (from bonding to antibonding and vice versa) of the transverse character of the wave function upon passage through the molecule.

Molecular conductance obtained in terms of orbital densities and response functions.

Using the source-sink potential (SSP) approach recently developed in our group, we study electron transmission through molecular electronic devices (MEDs). Instead of considering the source-sink potentials exactly, we use a perturbative approach to find an expression for the transmission probability T(E) = 1 - absolute value(r(E))(2) that depends on the properties of the bare molecule. As a consequence, our approach is limited to weak molecule-contact coupling. Provided that the orbitals of the isolated molecule are not degenerate, we show that it is the orbital density, on the atoms that connect the molecule to the contacts, that largely determines the transmission through the device. Corrections to this leading-order contribution involve the second- and higher-order molecular response functions. An explicit expression for T(E) is obtained that is correct up to first order in the molecular response function. Illustrating our approach, a qualitative explanation is provided for why orders of magnitude difference in the transmission probability are obtained [M. Mayor et al., Angew. Chem. Int. Ed. 42, 5834 (2003)] upon modification of the contact position in the molecule. An extension of the formalism to interacting systems is outlined as well.

Loss of GIMAP5 (GTPase of immunity-associated nucleotide binding protein 5) impairs calcium signaling in rat T lymphocytes.

The recessive lyp allele, which harbors a defective gimap5 (GTPase of immunity-associated nucleotide binding protein 5) gene, causes spontaneous apoptosis of T lymphocytes in the biobreeding diabetes-prone strain of rats. Mechanisms underlying the pro-survival function of GIMAP5 remain unclear. In this study, we show that gimap5(lyp/lyp) T cells display diminished calcium flux in response to thapsigargin or signaling via the T cell antigen receptor. This defect is manifested in mature single positive thymocytes, where the survival defect first occurs. We also show that GIMAP5 deficiency does not affect the thapsigargin-induced calcium release from the intracellular stores but impairs subsequent calcium entry across the plasma membrane. Our findings suggest that GIMAP5 is an important regulator of calcium response in T lymphocytes and impaired calcium signaling might underlie spontaneous apoptosis of gimap5(lyp/lyp) T cells.

Electron Transmission through Aromatic Molecules.

A prominent feature of aromatic compounds is the ring current that can be observed indirectly in nuclear magnetic resonance experiments. This current is generated by an external magnetic field. In molecular electronics, molecules serve as conductors, and they are connected to metallic contacts that act as electron sources and electron sinks. We show that ring currents can also be found in molecular electronic devices containing cyclic π-electron systems. The circular currents are related to interference phenomena that can render the molecule impenetrable to electrons. While only small currents pass through the molecule, large internal circular currents are stimulated. We conjecture that the internal currents should result in experimentally observable magnetic moments.

Approximate density functionals applied to molecular quantum dots.

Recently, molecular quantum dots (MQDs) have been investigated experimentally and found to exhibit the Kondo effect. The Kondo effect leads to an enhancement of the zero-voltage conductance. Here, we study a finite cluster model of a MQD by means of Kohn-Sham density functional theory. Furthermore, employing an implementation of Landauer's formula, we calculate the conductance of the dot. We find that the electronic structure and the molecular conductance depend strongly on the exchange-correlation functional employed. While the local spin density approximation and the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation qualitatively reproduce certain features of the Kondo effect, PBE hybrid does not. Based on the MQD, we discuss the limitations of using density functional theory to model molecular electronic devices.

Side-chain effects in molecular electronic devices.

We discuss the effect of an abundant structural element of molecules on the transmission probabilities of molecular electronic devices. We show that an attachment of side chains to a molecular conductor may lead to zero transmission probabilities. The gaps in the transmission-probability appear approximately at the eigenvalues of the isolated side chains, provided that the corresponding eigenstates are not localized away from the molecular conductor. Simple Hückel-type calculations serve to illustrate the described effect. Furthermore, we show that complex transmission-probability curves, obtained with Kohn-Sham density-functional theory, also exhibit the described side-chain effect.