Control of spin-polarised currents in graphene nanorings

We study electronic transport in systems comprising square graphene nanorings with a ferromagnetic insulator layer on top of them. The rings are connected symmetrically or asymmetrically to contacts. The proximity exchange interaction of electrons with agnetic ions results in spin-dependent transport properties. When a nanoring is connected asymmetrically, the occurrence of Fano-like antiresonances in the transmission coefficient can induce abrupt changes in the spin polarisation under minute variations of the Fermi energy. We also demonstrate that the spin polarization can be efficiently controlled by a side-gate voltage. This opens a possibility to use these effects for fabricating tunable sources of polarised electrons

Twisted graphene nanoribbons as nonlinear nanoelectronic devices

We argue that twisted graphene nanoribbons subjected to a transverse electric field can operate as a variety of nonlinear nanoelectronic devices with tunable current-voltage characteristics controlled by the transverse field. using the density-functional tight-binding method to address the effects of mechanical strain induced by the twisting, we show that the electronic transport properties remain almost unaffected by the strain in relevant cases and propose an efficient simplified tight-binding model which gives reliable results. the transverse electric field creates a periodic electrostatic potential along the nanoribbon, resulting in a formation of a superlattice-like energy band structure and giving rise to different remarkable electronic properties. we demonstrate that if the nanoribbon geometry and operating point are selected appropriately, the system can function as a field-effect transistor or a device with nonlinear current-voltage characteristic manifesting one or several regions of negative differential resistance. the latter opens possibilities for applications such as an active element of amplifiers, generators, and new class of nanoscale devices with multiple logic states

Quantum nanoconstrictions fabricated by cryo-etching in encapsulated graphene

More than a decade after the discovery of graphene, ballistic transport in nanostructures based on this intriguing material still represents a challenging field of research in two-dimensional electronics. The presence of rough edges in nanostructures based on this material prevents the appearance of truly ballistic electron transport as theo\-re\-tically predicted and, therefore, not well-developed plateaus of conductance have been revealed to date. In this work we report on a novel implementation of the cryo-etching method, which enabled us to fabricate graphene nanoconstrictions encapsulated between hexagonal boron nitride thin films with unprecedented control of the structure edges. High quality smooth nanometer-rough edges are characterized by atomic force microscopy and a clear correlation between low roughness and the existence of well-developed quantized conductance steps with the concomitant occurrence of ballistic transport is found at low temperature. In par\-ti\-cu\-lar, we come upon exact 2$e^{2}/h$ quantization steps of conductance at zero magnetic field due to size quantization, as it has been theoretically predicted for truly ballistic electron transport through graphene nanoconstrictions

Impact of device geometry on electron and phonon transport in graphene nanorings

Recent progress in nanostructuring of materials opens up possibilities to achieve more efficient thermoelectric devices. Nanofilms, nanowires, and nanorings may show increased phonon scattering while keeping good electron transport, two of the basic ingredients for designing more efficient thermoelectric systems. Here we argue that graphene nanorings attached to two leads meet these two requirements. Using a density-functional parametrized tight-binding method combined with Green's function technique, we show that the lattice thermal conductance is largely reduced as compared to that of graphene nanoribbons. At the same time, numerical calculations based on the quantum transmission boundary method, combined with an effective transfer matrix method, predict that the electric properties are not considerably deteriorated, leading to an overall remarkable thermoelectric efficiency. We conclude that graphene nanorings can be regarded as promising candidates for nanoscale thermoelectric devices

Propiedades electrónicas y térmicas de nanoestructuras de grafeno

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 26-06-2019Nanoscience is one of the most exciting research fields of modern science due to the novel and unexpected properties that materials exhibit at the scale of individual atoms. In order to exploit these properties in future applications, a deep understanding of the behavior of matter at the nanoscale is required.This thesis aims to present new findings related to the modeling of electronic and thermal transport in several graphene nanostructures, including graphene nanoconstrictions, twisted graphene ribbons and graphene rings...La Nanociencia es una de las disciplinas de mayor crecimiento y relevancia en la actualidad debido a las nuevas e inesperadas propiedades que exhiben los materiales al reducir sus dimensiones. Sin embargo, para poder explotar estas propiedades de forma óptima en posibles aplicaciones, aún se necesita una mayor comprensión del comportamiento de la materia en la nanoescala. El objetivo principal de esta tesis es estudiar el transporte electrónico y térmico en distintas nanoestructuras de grafeno, entre las que se incluyen nanoconstricciones, nanocintas de grafeno helicoidales y anillos de grafeno..Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEunpu