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Our research efforts focus on the following scientific areas that span across the wider fields of condensed matter physics and nanoelectronics. We are always looking for new research collaborations, so please get in touch if you are interested in working with us.

Quantum Transport in Topological Insulators for Nanoelectronic Integrated Circuit (nano-chip) Applications

Topological insulator materials have burst onto the scene to become a very important topic in condensed matter physics in the last decade. Topological insulators are insulators in the bulk but have topologically protected edge current carrying states at the surface. Quantum Transport properties of topologically protected states are also of strong contemporary electrical engineering interest as they act as perfect conducting channels, in potential nanoelectronic interconnects and quantum functional devices. We apply the Kwant Quantum Transport Simulator to investigate the effect of non-magnetic impurities, defects and vacancies, initially in 2D for Quantum Spin Hall (QSH) systems, described by the Bernevig-Hughes-Zhang (BHZ) Hamiltonian. Non-magnetic impurities have spin conserving scattering which don’t destroy the topologically protected states. However, the presence of these impurities will alter the bulk conduction and will introduce quantum interference effects. Lattice vacancies can cause the formation of current vortices. Kwant as well as calculating the conductance of a structure via the Landauer-Buttiker formula, can also be used to examine the local density of states (DOS) and local current. Looking at the local DOS and currents provides detailed information on the effects of impurities and vacancies on Quantum Transport. Our work is also extended by addressing Quantum Transport in impure 3D topological insulators, from the Bi2Se3 family.

Please contact Dr Gerard Edwards for further information.

Graph

A plot of the probability density P as a function of (x,y) i.e. P=P(x,y) for a 2D quantum wire region with length L=2000 a and width W=1000 a for the BHZ Hamiltonian with confinement along y, and an energy at the energy gap zero. 

Modelling the Switching of Molecular Quantum Dot Cellular Automata for Mixed Valence Molecules

The Quantum Dot Cellular Automata (QCA) paradigm for nano-computing, put forward by researchers at Notre Dame, USA, manages to achieve the minimal heat dissipation and the ultimate in coding one bit, in the charge configuration state of a mixed valence molecule. We model the switching response for a pair of cells composed of a mixed valence Diferrocenylacetylene (DFA) molecules. The physical system consists of the external driver molecular and the test molecule (electronic system) being driven. However the test molecule can vibrate (vibronic modes) so this aspect must be included. The vibration system can exchange energy with the thermal environment. The density matrix for this system, whose time evolution is given by a Lindblad equation, employing the Markovian approximation, is solved, giving a treatment of the molecular switching including dissipation. The MATLAB Quantum Optics Toolbox/Quantum Toolbox in Python are used to solve for the density matrix as they have advanced features (representation of tensor operators and superoperators) designed for this purpose.

Please contact Dr Gerard Edwards for further information.

Graph

The adiabatic potential energy curves for a symmetric mixed-valence DFA test molecule subject to an external static driver, as a function of the classical vibrational coordinate, Q for the ground and first excited states.

A Simulator for the Clocking of Molecular Quantum-Dot Cellular Automata

Molecular Quantum-Dot Cellular Automata (QCA) is a revolutionary nano-computing paradigm which is scalable to the molecular level, where a single QCA molecular cell encodes a single bit. Current Complementary Metal Oxide Semiconductor (CMOS) integrated circuit technology is coming to the end of its life, in terms of further transistor integration, with severe problems due to heat dissipation that can melt the chip and challenges to define the transistor off state, due to quantum mechanical tunnelling leakage. Clocked control of the QCA is essential to avoid the trapping of the system into metastable states, the QCA working with ground state computing. A four phase clock with voltages supplied to the clocking wires placed beneath the QCA molecular layer will induce the flow of data down a QCA line of cells. This system acts as a molecular shift register.

We develop a MATLAB based simulator for the clocking of Molecular QCA, containing a Graphical User Interface (GUI), to facilitate the setting of input parameters, to display the E field produced by the clocking wires, fed by the four phase clock, investigate the feasibility of the QCA molecular shift register and general clocking control. The Object Oriented Programming method was employed in the software development.

Please contact Dr Gerard Edwards for further information.

Screenshot of GUI

A screenshot of the GUI for the final product above shows how easily the input simulation parameters can be set and the display of the insightful calculated output information, for the spatial variation of the E field, for a specified four phase clock signal. There are panels to input geometric parameters and control the Simulation (Wire and Conductor Settings; Clocking Wire Signal Parameters; Time Settings; QCA Settings; Additional Setting Controlling the Graphical Display). There are Graphical Outputs for the geometry of molecular array/clocking with the E field distribution, the E field at the QCA surface as a function of both time and position, the E field at the QCA surface as a function of position, the Four Phase Clock Waveforms with voltage against time applied to adjacent wires, in groups of four.

Spin-polarised electron gas

The homogeneous electron gas (HEG), comprised of a sea of electrons together with a uniform positive background charge density to ensure overall electrical neutrality, is one of the most fundamental model systems in condensed matter physics. The HEG allows the study of electron interactions in isolation from the effects of any nuclei. It also forms the basis of the exchange-correlation functionals used in almost all density functional theory simulations of real materials, enabling, for the first time, first-principles modelling and prediction of material properties, chemical reactions, catalyst performance and many other phenomena across chemistry, engineering and biology. Via quantum Monte Carlo simulations, we are able to provide more accurate energy and electron correlation data than were previously available, especially for partially-spin-polarised electron gases.

Please contact Dr Graham Spink for further information.

Spin-resolved pair correlation functions (PCFs) for unpolarized HEGs calculated at the densities shown

Spin-resolved pair correlation functions (PCFs) for unpolarized HEGs calculated at the densities shown. The PCF is proportional to the distribution of electrons as a function of distance r  from any given electron; rs  is the typical distance between neighbouring electrons. The antiparallel-spin PCFs are translated upwards by 0.2 units. The data were obtained using twist-averaged diffusion Monte Carlo simulations. Reproduced from Ref. [1] (© 2013 American Physical Society).

1. G. G Spink, R. J. Needs, and N .D. Drummond, Phys. Rev. B 88, 085121 (2013).

Trion formation in hole-doped electron gas

The Coulomb attraction between electrons and holes in layered semiconductor systems leads to many useful and fascinating behaviours. At low densities, isolated few-body species emerge, such as neutral excitons, consisting of bound electron-hole pairs, and charged trions, which are bound states of two electrons and one hole, or two holes and one electron. These species, which can be thought of as artificial analogues of the hydrogen atom and H- ion, have technological applications in, for example, optoelectronic storage devices [1] and solar cells [2]. As the density of charge carriers increases, the properties of the excitonic species are modified and a gradual crossover is observed to collective, many-body behaviour. Highly accurate quantum Monte Carlo simulations are carried out to study this crossover for an electron gas containing a single hole, looking in particular at electron-hole correlation as a function of carrier density and electron-hole mass ratio [3].

Graph

Crossover of hole-in-HEG system from the high-density collective excitonic state to a localized trion immersed in a low density HEG as a function of electron-hole mass ratio (mh/me) and density parameter rs (shown in units of a modified [3] exciton Bohr radius, a0*). The simulations are in agreement with the experimental data shown [4-6], which is based on the evolution of absorption and photoluminescence spectra from the many-body Fermi-edge singularity (FES) to discrete trion and exciton peaks. Reproduced from Ref. [3] (© 2016 American Physical Society).

Please contact Dr Graham Spink for further information.

1. T. Lundstrom, W. Schoenfeld, H. Lee, and P.M. Petroff, Science 286, 2312 (1999).
2. G. D. Scholes and G. Rumbles, Nat. Mat. 5, 683 (2006).
3. G. G. Spink, P. López Ríos, N. D. Drummond, and R. J. Needs, Phys. Rev. B 94, 041410(R) (2016).
4. V. Huard, R.T. Cox, K. Saminadayar, A. Arnoult, and S. Tatarenko, Phys. Rev. Lett. 84, 187 (2000).
5. G. Yusa, H. Shtrikman, and I. Bar-Joseph, Phys. Rev. B 62, 15390 (2000).
6. M. Yamaguchi, S. Nomura, H. Tamura, and T. Akazaki, Phys. Rev. B 87, 081310(R) (2013).

Charge and exciton transport in organic thin films

We have developed a computational platform to model temperature-dependent charge and exciton transport in organic thin films. Our work is related to applications such as Organic Photovoltaics (OPVs), Organic Light Emitting Diodes (OLEDs), and Organic Field-Effect Transistors (OFETs). The theoretical methodology we use is based on the combination of Molecular Dynamics to predict the morphology of organic thin films and Kinetic Monte Carlo modelling, parameterised with ab initio calculations, to simulate electron, hole, or exciton transport. We carry out a systematic comparison with available experimental data, and analyse the role that the dynamic nature of positional and energetic disorder play on the temperature and electric field dependence of charge mobility [1], or exciton diffusion length [2]. Our work establishes a clear link between microscopic and macroscopic transport quantities and utilises nanoscale theoretical schemes as a tool for predictive material screening towards efficient organic optoelectronic applications.

Please contact Dr Theodoros Papadopoulos for further information.

Graph

Left: Molecular Dynamics simulation of an organic thin film consisting of indinofluorene trimers at 300 K and 500 K. Right: Temperature dependent exciton diffusion length Ld  and its corresponding components along x, y, and z  transport axis. Here, the fundamental problem of energy transfer in soft organic materials is addressed, from a molecular standpoint, taking into account the effect of temperature and disorder on the exciton diffusion length.

1. S. M. Gali, G. D’Avino, P. Aurel, G. Han, Y. Yi, T. A. Papadopoulos, V. Coropceanu, J.-L. Brédas, G. Hadziioannou, C. Zannoni, and L. Muccioli, J. Chem. Phys. 147, 134904 (2017).
2. T. A. Papadopoulos, L. Muccioli, S. Athanasopoulos, A. B. Walker, C. Zannoni, and D. Beljonne, Chem. Science 2, 1025 (2011).

Charge transfer at materials interfaces and electronic structure of surfaces in the presence of defects and impurities.

We use a combination of density functional theory and Poisson equation to explore charge transfer effects at the interface between organic and inorganic layers. Such layers, that are either conducting or semiconducting in nature, constitute the building blocks of organic optoelectronic devices, such as Organic Photovoltaics (OPVs), Organic Light Emitting Diodes (OLEDs), and Organic Field-Effect Transistors (OFETs). Upon the creation of the interface, we investigate i) the appearance of gap states on the n- or p-type organic semiconductor, ii) the occurrence of Fermi-level pinning, iii) the work function modification of the substrate due to the adsorption of the semiconductor, and iv) the consequent effect on energy level alignment at the very interface. Agreement is sought between theory and experiment via UPS and XPS measurements. Our computational methodology provides insight into how the work function of the anode and cathode could be tuned to provide efficient electron injection/collection and finally be transformed into high‐performance optoelectronic device electrodes [1, 2].

Please contact Dr Theodoros Papadopoulos for further information.

Charge transfer at materials interfaces

Left: Interface between MoO3 and 4,4′-N,N′-dicarbazole-biphenyl (CBP) optimised computationally using density functional theory. Top-right: Density of states (DOS) projected partial charge on a terminal oxygen vacancy site of the MoOx surface. Bottom-right: DOS of the interface between MoOx and CBP.

1. T. A. Papadopoulos, J. Meyer, H. Li, Z. Guan, A. Kahn and J.-L. Brédas, Adv. Funct. Mater. 23, 6091 (2013).
2. E. Polydorou, A. Zeniou, D. Tsikritzis, A. Soultati, I. Sakellis, S. Gardelis, T. A. Papadopoulos, J. Briscoe, L. C. Palilis, S. Kennou, E. Gogolides, P. Argitis, D. Davazoglou, and M. Vasilopoulou, J. Mater. Chem. A 4, 11844 (2016).

Electron transport in single-molecule junctions

Using a first principles approach that combine density functional theory, scattering theory and Green functions within the Landauer-Buttiker formalism, we study the electron transport properties of molecular wires, which constitute the building blocks of future molecular transistors. Conductance and IV curves are extracted to characterise a wide range of molecular nanodevices. Via our methodology, the occurrence of Breit-Wigner and Fano resonances are explored to identify whether electron transport can be controlled via imposing conformational modifications on the molecular wire itself, acting hence as a molecular sensor [1].

Please contact Dr Theodoros Papadopoulos for further information.

Graph

Top-left: Gold electrodes attached to a molecular wire forming a junction. Bottom-left: DOS projected partial charge, which indicates the delocalised nature of the electron wavefunction when the transmission coefficient is equal to unity. Right: Total transmission coefficient against energy when the side group of the molecular wire is rotated up to 90°. Fano resonances due to bound states on the side group become apparent.

1. T. A. Papadopoulos, I. M. Grace, and C. J. Lambert, Phys. Rev. B 74, 193306 (2006).