Executive Summary: The discovery of carbon nanotubes, graphene and other allotropes of carbon have given rise to a new era of condensed matter physics of real life low dimensional systems that were once thought to be only in the imagination. Many old theoretical treatments and ideas on these exotic materials have come to life in experiments[1-5] and have spurred many more theoretical efforts that have resulted in a wealth of new predictions and discoveries such as Dirac fermions, tunable metal/semiconductor/Dirac insulators[6], topological insulators[7], edge state magnetism[8] among many others[9].
Due to the low dimensionality of these materials they lend themselves to relatively simple theoretical models[2,7] with surprising results. These can be derived by symmetry considerations as in particle physics and field theory or more interestingly through accurate analytical tight-binding models[7]. Learning from analytical models, in the context of graphene and nanotubes, we have developed two complementary routes to derive effective Hamiltonians for low dimensional systems, accounting for the salient orbital features that yield interactions controlled by external or internal fields. We recently applied them to spin transport in DNA[10] with very exciting results showing all the qualitative features of experiments. The focus of the treatment was to understand extraordinary electron spin polarizing properties of chiral molecules[11-13]. As these models naturally incorporate the structural aspects of molecular systems and in turn, the nature and size of the interactions depend on these features, we can attempt to manipulate such couplings or even generate new ones by inducing system deformations. This approach has been very successful for graphene, both from the fundamental point of view (new developments of Abelian Gauge theories[14]) and practical applications (very high effective magnetic fields[15] without external sources!). An interesting aspect of the manipulation of these new materials is the inclusion of periodically varying interactions that allow the exploration of new topological phases not available within the static regime. We have already proposed some models for laser irradiation control of graphene-like materials but have not yet combined these non equilibrium laser induced features with the curvature effects that might arise due to presence of the break junction. Indeed, some recent proposals have considered the role of curvature effects in the electronic transport properties of conventional two dimensional gases as well as for Dirac fermion systems. Thus, we consider that this exploration of the interplay of curvature and radiation effects is a natural extension of these ideas and we expect to explore new physical scenarios not presented so far in the relevant literature. The Floquet tool to approach the previous problem can be regarded as a first step towards non-equilibrium properties of the system. This latter aspect should be more fully addressed in future work. The aim of this short project is to explore the potential to harness spin active materials (chiral molecules of great abundance in biology) by manipulating through mechanical means or performing “stretch engineering”. We will address this problem for a set of chiral molecules of interest, including DNA, aminoacids, simple proteins, oligopeptides and sequences of ring based structures (Benzene, Naphtalene and continuing sequence benzologues) that can be chemically fashioned into chains and helices. The approach will be based on analytical tight-binding modeling and verification and further exploration, by way of small scale Semi-Empirical models and transport calculations on the computer. For the most successful models we will apply an additional layer of verification by performing more sophisticated Density Functional Theory (DFT) based models. The objectives of the theoretical part of the project will then be: a) Establish through the Hamiltonian formulation of discrete orbital based model, the universal features that determine spin activity in these compounds. b) Determine the mechanism of the spin-orbit interaction in these materials and its magnitude to consolidate a full theoretical description. c) To find venues for practical applications of the spin-polarizing capacity of these molecular systems in the context of information devices coded spin and charge. The second major point of the proposal is to build a Mechanically Controllable Break Junction setup for a molecular spintronics and transport laboratory in Ecuador. The well known Break junction[17] and Conductance probe[18] setups have increasingly become the exploration tool for single molecule electronics and will serve as the ideal experimental counterpart to test and challenge the theory. The counterplay between theory and experiment will result in better and higher impact publications in this area of research.
Objectives General: It must be achieved during the development of the project. It identifies the purpose towards which resources and efforts should be directed. It is the set of results that the project intends to achieve through the actions.
- Analytical modeling of chiral molecular structures (point and global) in order to derive Hamiltonians with the full geometrical parameterization to ascertain response to externally applied deformations accessible to experiments.
- Building a basic Mechanically Controllable Break Junction (MCBJ).
Specific objectives:
- Building analytical Hamiltonians for nonchiral with simple orbital-transport structure (eg benzene, naphtalene, antrancene series) with p orbitals, real and reciprocal space, plus wavefunctions with detailed geometrical parameterization.
- Building analytical Hamiltonians for chiral molecules, (eg Lysine, DNA, helicene, oligo peptides), real and reciprocal space, plus wave functions, with detailed geometrical parameterization. Assessment of spin mechanisms.
- Candidate Hamiltonians (chiral and nonchiral) coupled to external radiation. Coupling to mechanical modes.
- Setting up conductance calculations with Landauer-Buttiker formalism as a function of realistic stretched/compressed configurations. Analytical and numerical calculations.
- Building of MCBJ material selection and assembly.
- Testing MCBJ with 1,8-octanedi-thiol (HS(CH2)8SH).
- Data selection to identify the curves corresponding to a MMM junction.
- Comparison of results with reported data.
Participating Institutions:
YACHAY TECH, ESPOL, EPN.
Participants:
Project manager Ernesto Antonio Medina Dagger.
- Ernesto Antonio Medina Dagger
- Solmar Alexandra Varela Salazar
- Werner Bramer Escamilla
- alexander lopez
- Leonardo Alberto Basile Carrasco
- Henry Marcelo Osorio Calvopiña
Awarded budget: $42260
Project status: Signing of agreements.
