Spin-birefringence in molecular currents: Tellurium and gold complexes Amlan K. Roy a , Joseph L. Speyer b , Lizette Bartell b , Daniel Neuhauser b, * a IISER, Division of Chemical Sciences, Mohanpur Campus, Nadia, WB 741252, India b Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569, United States article info Article history: Received 5 September 2009 In final form 1 December 2009 Available online 4 December 2009 abstract We simulate the spin-flip current and transmission function through rings containing elements with a spin–orbit interaction. In a previous study (J. Chem. Phys. 123 (2005) 204714) we predicted that such a system can show spin-birefringence, i.e., a spin current polarized parallel to the molecular axis can flip its direction due to a phase lag due to the spin–orbit interaction. Here we demonstrate the effect in a semi-empirical extended Hückel theory (EHT) molecular simulation. The ring systems studied are naphthalene–bitellurium, gold–porphyrin, and cyclometallated chlorogold, connected to polyacetylene. Ó 2009 Published by Elsevier B.V. 1. Introduction A rapidly emerging field in modern microelectronics is spin- tronics [1,2], the use of electron spin degrees of freedom to process, store and transmit information, in contrast to semiconductor elec- tronics where this role is played by the charge. Spintronics discov- eries include giant magnetoresistance and the spin-valve effect in metallic multilayers, and spintronics may eventually be crucial for quantum computation. It is natural to ask whether spin-dependent transport can be accomplished in molecular electronics. Although such a combina- tion has been mostly studied in the context of inorganic semicon- ductors [3,4], the possible use of other materials in spintronics, e.g., p conjugated semiconductor, organometallic, molecular wire, and atomic carbon wire, DNA molecular monolayer and carbon nano- tubes [5–10], has also been explored considerably in the past few years, leading to molecular spintronics. Recently, the use of spin- polarized graphene has also attracted much attention [11]. The manipulation of the spin-polarized current can be done by magnetic fields (responsible for the Zeeman and Aharonov–Bohm effects); this however is potentially difficult since the direct effect of the magnetic fields on the electrons is proportional to the area on which they act, making their effect small for nano and sub-nano systems unless very large fields are used. Recently it was proposed [12] that spin–orbit coupling could also be exploited to influence spintronics in ring-type devices which contain one or a few atoms with strong spin–orbit interac- tion. By coupling the ring to a lead at an angle the l z !l z symme- try of the loop can be broken, i.e., the coupling of the ring states to incoming and outgoing states is asymmetrical. The asymmetric coupling leads to an interesting birefringence phenomena in which there is a phase lag between the different conserved ‘z’ polarizations of a planar device. Birefringence implies that if the spin is initially polarized in, e.g., the ‘x’ direction, i.e., is the coherent sum of the two ‘z’ polarizations, then as a function of initial energy it can come out polarized in the ‘x’ direction or be polarized in the opposite direction, ‘x’, depending on the phase lag between the two ‘z’ polarizations. In short, a molecule can flip the spin. Here we move beyond the schematic model systems in Ref. [12] to a more explicit molecular simulation with atoms that have strong spin–orbit coupling. The first system is a tellurium dimer connected to a ring-like structure, in our case naphthalene. This creates an effective triangle with the spin–orbit atom in one vertex. The other two vertices are then coupled at an angle to two poly- acetylene wires. The second type of compounds contains gold. We examine first the connection of gold to a porphyrin group. In addition, we study chlorogold, a gold atom complexed to a chlorine atom and con- nected to a pyridine ring and two benzene rings, forming a triangu- lar structure again. In this case the compounds are again coupled at an angle to two polyacetylene wires. The presence of the non-lin- ear angle is important to produce an effect mimicking that of the original model system, where the shift away from the linear angle broke the l z !l z symmetry. The atomic model is studied here with a fairly quantitative approach, extended Hückel theory (EHT) [13], using the Landauer– Büttiker formalism with a non-equilibrium Green’s function methods (see e.g., [5,8,14,15]), based here on the use of absorbing potentials. EHT can handle large systems and describes the relevant excitation energies well. Section 2 gives details of the wire-loop geometry. Section 3 describes the EHT method used to construct the Hamiltonian corresponding to the extended system; spin–orbit interaction is 0009-2614/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.cplett.2009.12.001 * Corresponding author. E-mail address: dxn@chem.ucla.edu (D. Neuhauser). Chemical Physics Letters 484 (2010) 104–109 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett