Croatian Science Fundation Research Project
Collective effects, tunnelling and topological transport in novel nanojunctions
Duration of the project:
March 15th, 2017 to March 14th, 2021
- Department of Physics, Faculty of Science, University of Zagreb, Croatia
- Goucher College, Baltimore, Maryland, USA
- Ruhr Universität, Bochum
One goal of this project is to formulate a theoretical approach for novel features under influence of magnetic field and other gauge fields, introduced by e.g. deformation of the lattice, on collective, structural, magneto-transport and electromechanical properties of materials with reduced dimension, namely of 2D and quasi-2D systems like graphene, graphene-based compounds and some high-Tc cuprates. Understanding such effects is important not only from the point of view of fundamental physics, but also from technological point of view due to the opening of various ways of potential applications in the mentioned materials. Our research is expected to give a new insight into understanding of recent experiments, and also to explore the parameter space of specific systems in checking validity of our explanations of existing experimental finding, as well as in the theoretical results and effects to be proposed for experimental search. Another goal is an experimental realization of nanojunctions with pronounced tunable order parameters, topological properties or spin texture affecting the transport properties.
(I) In contrast to the quasi-1D systems (e.g. organic conductors like Bechgaard salts) in which observed collective phenomena like spin/charge density wave (SDW/CDW) are usually driven by direct electron-electron/phonon interaction, augmented by “nesting” of the Fermi surface and by the role of external magnetic field in improving it, in 2D graphene-like systems the nesting concept is mainly absent thus requiring entirely different approach. The idea is to develop and implement the mechanism of magnetic breakdown – the quantum tunnelling between overlapping electron orbits encircling the Fermi surfaces in doped graphene-like structure due to the magnetic or some other synthetic field generated by lattice deformation. Electron orbits are then related to overlap due to the influence of periodic potential created either by external engineering or spontaneously by the formation of density wave. Magnetic breakdown can lower the total energy of electron condensate, opening the possibility of inducing the new type of the instability in the system, the so called magnetic breakdown-induced density wave (MBIDW), predicted by us earlier for quasi-1D systems. Also, as already mentioned above, we intend to put here into focus a very special feature, known in graphene-based system with Dirac-like electron dispersions: the appearance of strong pseudomagnetic fields due to mechanical deformations of crystal lattice. The influence of such pseudofields may be crucial for the understanding of recently observed CDW in the surface layer of intercalated graphite compound CaC6 and similar materials. Our preliminary estimations indicate a possibility of CDW instability due to the spontaneous deformation of crystal lattice, generating the synthetic pseudomagnetic field which induces magnetic breakdown along the array of slightly overlapping Fermi surfaces in the reciprocal space with the new Brillouin zones. Another feature fundamentally influenced by magnetic breakdown are quantum oscillations in magnetotransport observed in graphene-based systems and in high-Tc cuprates. The special attention is dedicated to the influence of new degrees of freedom, introduced by magnetic breakdown, to the geometric properties of quantum electron states thus giving rise to possible novel topological phenomena like non-integer Chern number and non-abelian anyons in Q1D and 2D systems.
(II) Small mass and large Young modulus constitute graphene-based systems (graphene, carbon nanotubes) the ideal candidates for electromechanical systems at nanoscales (NEMS). Within this project we focus on the role of magnetomotive coupling, i.e. the coupling of electronic and mechanical subsystems in NEMS due to applied magnetic field. Here we want to study the dc-driven self-oscillating systems at milikelvin temperatures applying our spin-gate based mechanism to the oscillators based on graphene nanoribbon suspended on magnetic (spin-polarized) leads. Namely, it is an aspect of electron spin-controlled nanomechanics through synchronization of mechanical subsystem velocity with Coulomb blockade-regulated occupation of quantized Zeeman-split electron states in NEMS through which the tunnelling current flows. We want to explore the onset of the negative differential conductance which is the textbook prerequisite for self-excited oscillations in quasi-adiabatic and nonmarkovian regime. Our special interest lies in exploring the influence of synthetic pseudomagnetic fields on the NEMS performance.
(III) In the experimental part of this project, we will design and fabricate nanoscale devices that utilize and investigate the magnetic field-driven spin-dependent phenomena. We will also explore creating novel collective states by combining materials with different order parameters or topologically protected states.