Session X22: Organic Magnetics and Bio-Electronics

\textit{Ab initio} simulations of the transport properties of Mn$_{12}$ based spin-devices

Single-molecule magnets (SMMs) represent a unique playground for fundamental quantum physics and exhibit exotic phenomena such as magnetic hysteresis as well as magnetization reversal through quantum tunneling. Recently, transport measurements on Mn$_{12}$ based molecular magnets in single-molecule-transitor devices have been realized. In this work we present \textit{ab initio} transport[1] calculations of Mn$_{12}$ molecules functionalized by thioether groups and sandwiched between gold contacts. We find the transport properties of these SMMs to be dominated by tunneling type behaviour across the organic functional groups and asymmetric coupling to the leads. We observe asymmetric I-V curves under positive and negative bias. In addition we demonstrate that the I-V characteristic changes upon changing the magnetic state of the molecule, suggesting that electrical single-spin detection can be indeed obtained from a detailed knowledge of the I-V. \newline [1] Rocha \textit{et. al}, Spin and Molecular Electronics in an Atomically Generated Orbital Landscape; URL: http://www.smeagol.tcd.ie   

Modeling the organic magnet Fe[TCNE]$_2$

Recent experiments have revealed the crystal structure of Fe[TCNE]$_2$, (TCNE = tetracyanoethylene), an organic-based magnet with a transition temperature around 100 K and a saturation magnetization corresponding to an effective spin of 3/2 per formula unit. Its structure consists of undulating layers of TCNE anion-radicals bound to four Fe(II) ions, where Fe ions between adjacent layers coupled via diamagnetic $\sigma$-dimerized TCNE.$^1$ Since the angular momentum of Fe(II) is almost quenched due to the asymmetric crystal field, we model the system using a Heisenberg Hamiltonian with antiferromagnetic in-plane coupling between the Fe(II) S=2 spins and the near-neighbor (TCNE) S = 1/2 spins and also antiferromagnetic superexchange coupling between the Fe(II) spins at adjacent planes. By comparing our results with magnetization measurements as function of temperature and field, we extract the values of the inter- and intra-plane antiferromagnetic couplings. We discuss how to extend our approach to other TCNE-based magnets, such as the amorphous semiconducting V[TCNE]$_x$, a room temperature ferrimagnet and promising candidate for multifunctional spintronic applications.\\ $^1$ J.-H. Her, P. W. Stephens, K. I. Pokhodnya, M. Bonner and J. S. Miller, An gew. Chem. Int. Ed. 2007, 46, 1521   

Reversible Photoinduced Magnetism in V-Cr Prussian blue analogues

The cyano-bridged bi-metallic compounds, so called ``Prussian blue magnets,'' display a broad range of interesting photoinduced magnetic phenomena. A notable example is Fe-Co Prussian blue magnet, which exhibits light-induced changes in between magnetic states together with glassy behavior [1,2]. Here, we report reversible photoinduced magnetic phenomena in V-Cr Prussian blue analogue (\textbf{K}$_{1.54}$\textbf{V}$_{0.85}$\textbf{[Cr(CN)}$_{6}$\textbf{](SO}$_{4}$\textbf{)}$_{0.16}$\textbf{3.1H}$_{2}$\textbf{O}), one of the few room temperature molecule-based magnets. Illumination with UV light suppresses magnetization, whereas subsequent illumination with green light increases magnetization. This recovery effect of green light is observed only when the sample is previously UV-irradiated. This suggests a hidden metastable magnetic state with a long lifetime at low $T $ ($<$ 100 K). Results of detailed magnetic studies and the likely microscopic mechanisms will be discussed. [1] Hashimoto et al. science \textbf{272}, 5262 (1996); [2] Pejakovic, et al. PRL \textbf{85} 1994 (2000)   

Magnetic properties of organic-based Ni[TCNE](MeCN)$_{2}$][BF$_{4}$] magnet.

A new organic-based magnet of Ni[TCNE][BF$_{4}$]( MeCN)$_{2-\delta}$ (\textbf{1}) composition ($\delta $ = 0.15; TCNE = tetracyanoethylene) was synthesized via reaction of NBu$_{4}$(TCNE) and Ni(NCMe)$_{6}$(BF$_{4})_{2}$ in CH$_{2}$Cl$_{2}$. Zero field cooled and field cooled magnetizations, $M(T)_{ZFC}$ and $M(T)_{FC}$, at 0.5 mT rise sharply below 70 K indicative of an onset of a magnetic transition. $M(T)_{ZFC}$ reaches maximum at 25 K followed by a rapid decrease suggesting antiferromagnetic (AF) interaction. In contrast, $M(T)_{FC}$ rises upon further cooling signifying a strong irreversibility in accord with sharp increase of a remanant magnetization below 30 K and hysteretic behavior of $M(H)$. The $M(H)$ at 2 K increases rapidly with field and approaches saturation above $\sim $ 0.5 T. At 9 T $M(H)$ reaches 2.24 $\mu _{B}$ that is significantly higher than 1.30 $\mu _{B}$ expected for AF coupled Ni(II) S = 1 and [TCNE]$^{-}$ (S = 1/2) suggesting a ferromagnetic (FM) interaction. The unpaired Ni$^{II}$ spins and those on the [TCNE]$^{-}$ reside in orthogonal orbitals resulting in FM coupling. Assuming that similarly to Fe[TCNE][FeCl$_{4}$](MeCN)$_{2}$ \textbf{1} consists of Ni$^{II}-\mu _{4}$-[TCNE]$^{-}$ layers we believe that the decrease of $M(T)_{ZFC}$ below 25 K is due to AF coupling \textit{between the layers} while the interaction \textit{within the layer} is FM in contrast to the AF one reported for Fe, V, and Mn analogues.   

Magnetoresistance in bulk heterojunction solar cells

The magnetoresistance (MR) response of the poly(3-hexyl thiophene) and poly(3-hexyl thiophene):1-(3-methoxycarbonyl) propyl-1-phenyl-[6,6]-methanofullerene (PHT:PCBM) based bulk heterojunction solar cells have been studied. Positive MR was always observed at room temperature in both the devices. In both cases the magnitude of the MR signal decreases at lower temperature and shows positive to negative sign inversion at 100K for the solar cells and at 200K for P3HT. The detailed voltage and temperature dependence of MR will be presented which will give important insight of the magnetic field effect on the bulk carrier mobility in the organic solar cells. We have observed tendency of retaining magnetic history in both the devices and it has been studied.   

Extending transfer-matrix studies of charge transport in dsDNA: diagonal ladder model

The $\pi$-stacking of aromatic bases along the axis of the DNA double helix suggests that DNA should be capable of supporting electron transport. This possibility has been investigated by a variety of experimental methods, including charge-transfer between intercalated dye molecules and direct measurement of conductivity in DNA molecules bridging two electrodes. In order to explore either the biological or nanotechnological significance of charge transport in DNA, we need theoretical models capable of predicting the influence of DNA sequence and structure on its charge transport properties. Transfer matrix methods have been used in conjunction with a ladder model of dsDNA (incorporating charge transfer between adjacent bases along a strand, and between hydrogen-bonded base pairs) to predict different transport properties for random, repetitive, or coding DNA sequences. It has been suggested that DNA charge transport may be involved in cellular mechanisms to detect and repair damage to DNA strands. We present extensions to the ladder model to allow for, firstly, charge transfer ``diagonally'' (from a base on a 5' strand to an adjacent base on a 3' strand, for example), and secondly, variations in hopping amplitudes due to bending of the helix (for example, in wrapping round a histone complex). Hence we take into account the extent of the electronic states and the geometry of the DNA strand in our modeling.   

Sequence Dependent Charge Transport on Double Stranded DNA

The transport properties of different double-stranded DNA sequences are studied by transfer and scattering matrix methods. The DNA is described by a tight-binding model with realistic sequence-specific hopping integrals. Our results show that, in qualitative agreement with experimental results [1], even a single basis mismatch on the sequence can dramatically change the conductance of short DNA sequences. The change in conductance is larger if the mismatch is on the energetically favorable path of transmission: the path with the most bases with energy close to the Fermi energy of the contacts. This trend is independent on which strand is being connected to the electrodes, although similar sequences have drastically different conductance values. We also study the effect of structural ``nicks'' on the DNA conductance. In accordance with experimental results [2], the conductance is changed by several orders of magnitude in the presence of the nicks, depending on the position of the defect on the strand. As the conductance of a strand is found to be dependent on the sequence of bases, this suggests an electronic approach to sequencing [1]. [1] J. Hihath \emph{et al.}, Proc. Natl. Acad. Sci.U. S. A \textbf{102}, 16979 (2005). [2] B. Hartzel \emph{et al.}, Appl. Phys. Lett. \textbf{82}, 4800 (2003).   

Theory of electron conductance across a DNA basepair

In recent years, research on electron tunneling through DNA basepairs has become more important due to its potential application in DNA sequencing technology. The goal is to recognize and identify a specific DNA base by measuring the hydrogen bond mediated tunneling current across a DNA basepair junction. In this talk, we discuss the results of density functional theory on the intrinsic conduction through DNA basepairs (Watson-Crick basepairs, Wobble basepairs, etc), and in particular the role of the hydrogen bond on the tunneling current.   

Charge transport in guanine crystals

Charge-transport processes in organic molecular crystals exhibit similarities and differences to those in $\pi$-conjugated polymers. For both types of condensed matter the polaronic effects are of high importance. These effects can cause a transition from bandlike transport to themally activated hopping. While the hopping regime is prevalent for DNA polymers, it is not clear if the same holds also for crystalline guanine or if band transport dominates. Also the influence of the temperature is rarely discussed in literature. In our approach to the problem of charge-carrier transport in these systems [1], we discuss the temperature dependence of the polaron bandwidth and the mobility in guanine crystals [2]. \newline [1] K. Hannewald {\it et al.}, Phys. Rev. B {\bf 69}, 075211 (2004); 075212 (2004). \newline [2] F. Ortmann {\it et al.}, J. Phys. Chem. B (to be published).