Collective effects, tunnelling and topological transport in novel nanojunctions

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, magnetotransport and electromechanical properties of materials with reduced dimension, namely of 2D and quasi-2D systems like graphene, graphene-based compounds and of some high-T_c cuprates. Understanding such effects is important not only from the point of view of fundamental physics, but also from technological one since it opens various ways of potential applications in the mentioned materials. Our research is expected to give a new insight into recent experiments, and also to explore the parameter space of specific systems in checking validity of our explanations, as well as in the theoretical results and effects to be proposed for future 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) The Fermi surface nesting concept that has been decisive for collective phenomena like spin/charge density waves (SDW/CDWs) in quasi-1D systems (e.g. organic conductors like Bechgaard salts) is not relevant for the 2D graphene-like systems for which one needs an entirely different approach. Our intention 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 structures due to the magnetic or some other synthetic field generated by lattice deformation. Overlap between electron orbits appears due to the periodic potential created either by external engineering or spontaneously by the formation of density wave. Magnetic breakdown then lowers the total energy of electron condensate, opening the possibility of generating the so called magnetic breakdown-induced density wave (MBIDW), a new type of instability predicted by us earlier for quasi-1D systems. We also 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. Magnetic breakdown may also be the origin of quantum oscillations in magnetotransport observed in graphene-based systems, and in high-Tc cuprates as well. The special attention will be devoted 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 the 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. In particular, we plan to develop our new technique of designing patterned superconductivity by using the electron beam lithography. To this end our aim is to improve the general understanding of locally protected superconductivity and of corresponding graphene interfaces at nanoscales.