arXiv subjects

David R. Bowler

Publications and source records attributed to David R. Bowler.

At least 37 records · Page 2Linked to original sources

arXiv2019

Gate controlled quantum interference: direct observation of anti-resonances in single molecule charge transport

Quantum interference can profoundly affect charge transport in single molecules, but experiments can usually measure only the conductance at the Fermi energy. Because in general the most pronounced features of the quantum interference are not located at the Fermi energy, it is highly desirable to probe charge transport in a broader energy range. Here by the method of electrochemical gating, we measure the conductance and map the transmission functions of single molecules at and around the Fermi energy, and study signatures associated with constructive and destructive interference. With the electrochemical gate control, we tune the quantum interference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and directly observe anti-resonance, a distinct feature of destructive interference. By tuning the molecule in and out of anti-resonance, we achieve continuous control of the conductance over 2 orders of magnitude with a subthreshold swing of ~17 mV/dec, features relevant to high-speed and low-power electronics.

cond-mat.mes-hall↗

arXiv2018

Structural properties of Silicon-Germanium and Germanium-Silicon Core-Shell Nanowires

Core-shell nanowires made of Si and Ge can be grown experimentally with excellent control for different sizes of both core and shell. We have studied the structural properties of Si/Ge and Ge/Si core-shell nanowires aligned along the $[110]$ direction, with diameters up to 10.2~nm and varying core to shell ratios, using linear scaling Density Functional Theory (DFT). We show that Vegard's law, which is often used to predict the axial lattice constant, can lead to an error of up to 1\%, underlining the need for a detailed \emph{ab initio} atomistic treatment of the nanowire structure. We analyse the character of the intrinsic strain distribution and show that, regardless of the composition or bond direction, the Si core or shell always expands. In contrast, the strain patterns in the Ge shell or core are highly sensitive to the location, composition and bond direction. The highest strains are found at heterojunction interfaces and the surfaces of the nanowires. This detailed understanding of the atomistic structure and strain paves the way for studies of the electronic properties of core-shell nanowires and investigations of doping and structure defects.

cond-mat.mtrl-sci↗

arXiv2018

High-accuracy large-scale DFT calculations using localized orbitals in complex electronic systems: The case of graphene-metal interfaces

Over many years, computational simulations based on Density Functional Theory (DFT) have been used extensively to study many different materials at the atomic scale. However, its application is restricted by system size, leaving a number of interesting systems without a high-accuracy quantum description. In this work, we calculate the electronic and structural properties of a graphene-metal system significantly larger than in previous plane-wave calculations with the same accuracy. For this task we use a localised basis set with the \textsc{Conquest} code, both in their primitive, pseudo-atomic orbital form, and using a recent multi-site approach. This multi-site scheme allows us to maintain accuracy while saving computational time and memory requirements, even in our exemplar complex system of graphene grown on Rh(111) with and without intercalated atomic oxygen. This system offers a rich scenario that will serve as a benchmark, demonstrating that highly accurate simulations in cells with over 3000 atoms are feasible with modest computational resources.

cond-mat.mes-hall↗

arXiv2018

Effects of the Hubbard U on density functional-based predictions of BiFeO$_3$ properties

First principles studies of multiferroic materials, such as bismuth ferrite (BFO), require methods that extend beyond standard density functional theory (DFT). The DFT+U method is one such extension that is widely used in the study of BFO. We present a systematic study of the effects of the U parameter on the structural, ferroelectric and electronic properties of BFO. We find that the structural and ferroelectric properties change negligibly in the range of U typically considered for BFO (3-5 eV). In contrast, the electronic structure varies significantly with U. In particular, we see large changes to the character and curvature of the valence band maximum and conduction band minimum, in addition to the expected increase in band gap, as U increases. Most significantly, we find that the $t_{2g}$/$e_{g}$ ordering, expected from crystal field theory, is inverted for U values larger than 4 eV. We therefore recommend a U value of at most 4 eV to be applied to the Fe $d$ orbitals in BFO. More generally, this study emphasises the need for systematic investigations of the effects of the U parameter not merely on band gaps but on the electronic structure as a whole, especially for strongly correlated materials.

cond-mat.mtrl-sci↗

arXiv2017

Atomic layer doping of Mn magnetic impurities from surface chains at a Ge/Si hetero-interface

We realize Mn $δ$-doping into Si and Si/Ge interfaces using Mn atomic chains on Si(001). Highly sensitive X-ray absorption fine structure techniques reveal that encapsulation at room temperature prevents the formation of silicides / germanides whilst maintaining one dimensional anisotropic structures. This is revealed by studying both the incident X-ray polarization dependence and post-annealing effects. Density functional theory calculations suggest that Mn atoms are located at substitutional sites, and show good agreement with experiment. A comprehensive magnetotransport study reveals magnetic ordering within the Mn $δ$-doped layer, which is present at around 120\,K. We demonstrate that doping methods based on the burial of surface nanostructures allows for the realization of systems for which conventional doping methods fail.

cond-mat.mtrl-sci↗

arXiv2017

Reaction paths of alane dissociation on the Si(001) surface

Building on our earlier study, we examine the kinetic barriers to decomposition of alane, AlH$_3$, on the Si(001) surface, using the nudged elastic band (NEB) approach within DFT. We find that the initial decomposition to AlH with two H atoms on the surface proceeds without a significant barrier. There are several pathways available to lose the final hydrogen, though these present barriers of up to 1 eV. Incorporation is more challenging, with the initial structures less stable in several cases than the starting structures, just as was found for phosphorus. We identify a stable route for Al incorporation following selective surface hydrogen desorption (e.g. by STM tip). The overall process parallels PH$_3$, and indicates that atomically precise acceptor doping should be possible.

cond-mat.mtrl-sci↗

arXiv2017

Canonical-ensemble extended Lagrangian Born-Oppenheimer molecular dynamics for the linear scaling density functional theory

We discuss the development and implementation of a constant temperature (NVT) molecular dynamics scheme that combines the Nosé-Hoover chain thermostat with the extended Lagrangian Born-Oppenheimer molecular dynamics (BOMD) scheme, using a linear scaling density functional theory (DFT) approach. An integration scheme for this canonical-ensemble extended Lagrangian BOMD is developed and discussed in the context of the Liouville operator formulation. Linear scaling DFT canonical-ensemble extended Lagrangian BOMD simulations are tested on bulk silicon and silicon carbide systems to evaluate our integration scheme. The results show that the conserved quantity remains stable with no systematic drift even in the presence of the thermostat.

cond-mat.mtrl-sci↗

arXiv2017

Alane adsorption and dissociation on the Si(001) surface

We used DFT to study the energetics of the decomposition of alane, AlH3, on the Si(001) surface, as the acceptor complement to PH3. Alane forms a dative bond with the raised atoms of silicon surface dimers, via the Si atom lone pair. We calculated the energies of various structures along the pathway of successive dehydrogenation events following adsorption: AlH2, AlH and Al, finding a gradual, significant decrease in energy. For each stage, we analyse the structure and bonding, and present simulated STM images of the lowest energy structures. Finally, we find that the energy of Al atoms incorporated into the surface, ejecting a Si atom, is comparable to Al adatoms. These findings show that Al incorporation is likely to be as precisely controlled as P incorporation, if slightly less easy to achieve.

cond-mat.mtrl-sci↗

arXiv2017

Efficient calculation of electronic structure using O(N) density functional theory

We propose an efficient way to calculate the electronic structure of large systems by combining a large-scale first-principles density functional theory code, Conquest, and an efficient interior eigenproblem solver, the Sakurai-Sugiura method. The electronic Hamiltonian and charge density of large systems are obtained by \conquest and the eigenstates of the Hamiltonians are then obtained by the Sakurai-Sugiura method. Applications to a hydrated DNA system, and adsorbed P2 molecules and Ge hut clusters on large Si substrates demonstrate the applicability of this combination on systems with 10,000+ atoms with high accuracy and efficiency.

cond-mat.mtrl-sci↗

arXiv2017

Exact location of dopants below the Si(001):H surface from scanning tunnelling microscopy and density functional theory

Control of dopants in silicon remains the most important approach to tailoring the properties of electronic materials for integrated circuits, with Group V impurities the most important n-type dopants. At the same time, silicon is finding new applications in coherent quantum devices, thanks to the magnetically quiet environment it provides for the impurity orbitals. The ionization energies and the shape of the dopant orbitals depend on the surfaces and interfaces with which they interact. The location of the dopant and local environment effects will therefore determine the functionality of both future quantum information processors and next-generation semiconductor devices. Here we match observed dopant wavefunctions from low-temperature scanning tunnelling microscopy (STM) to images simulated from first-principles density functional theory (DFT) calculations. By this combination of experiment and theory we precisely determine the substitutional sites of neutral As dopants between 5 and 15A below the Si(001):H surface. In the process we gain a full understanding of the interaction of the donor-electron state with the surface, and hence of the transition between the bulk dopant (with its delocalised hydrogenic orbital) and the previously studied dopants in the surface layer.

cond-mat.mtrl-sci↗

arXiv2016

Raoult's law revisited: accurately predicting equilibrium relative humidity points for humidity control experiments

The equilibrium relative humidity values for a number of the most commonly used precipitants in biological macromolecule crystallisation have been measured using a new humidity control device. A simple argument in statistical mechanics demonstrates that the saturated vapour pressure of a solvent is proportional to its mole fraction in an ideal solution (Raoult's Law). The same argument can be extended to the case where solvent and solute molecules are of different size.

cond-mat.soft↗

arXiv2016

Generalized canonical purification for density matrix minimization

A Lagrangian formulation for the constrained search for the $N$-representable one-particle density matrix based on the McWeeny idempotency error minimization is proposed, which converges systematically to the ground state. A closed form of the canonical purification is derived for which no a posteriori adjustement on the trace of the density matrix is needed. The relationship with comparable methods are discussed, showing their possible generalization through the hole-particle duality. The appealing simplicity of this self-consistent recursion relation along with its low computational complexity could prove useful as an alternative to diagonalization in solving dense and sparse matrix eigenvalue problems.

math-ph↗

arXiv2015

A linear scaling DFT study of the growth of a new \{105\} facet layer on a Ge hut cluster

Ge hut clusters, which appear during heteroepitaxy of Ge on Si(001), are prototypical examples of islands formed through strain. Experimentally, complete facets are observed to form rapidly, though the mechanism is unknown. We model the growth of new faces on Ge hut clusters, using linear scaling DFT. We build realistic small huts on substrates, and show that the growth of \{105\} facets proceeds from top-to-bottom, even for these small huts. The growth of the facet is driven by the reconstruction on the \{105\} facet, and nucleates at the boundaries of the facet. We thus resolve any ambiguities in kinetic models of hut cluster growth.

cond-mat.mtrl-sci↗

arXiv2015

Optimized multi-site local orbitals in the large-scale DFT program CONQUEST

We introduce numerical optimization of multi-site support functions in the linear-scaling DFT code CONQUEST. Multi-site support functions, which are linear combinations of pseudo-atomic orbitals on a target atom and those neighbours within a cutoff, have been recently proposed to reduce the number of support functions to the minimal basis while keeping the accuracy of a large basis [J. Chem. Theory Comput., 2014, 10, 4813]. The coefficients were determined by using the local filter diagonalization (LFD) method [Phys. Rev. B, 2009, 80, 205104]. We analyse the effect of numerical optimization of the coefficients produced by the LFD method. Tests on crystalline silicon, a benzene molecule and hydrated DNA systems show that the optimization improves the accuracy of the multi-site support functions with small cutoffs. It is also confirmed that the optimization guarantees the variational energy minimizations with multi-site support functions.

cond-mat.mtrl-sci↗

arXiv2015

Linear Scaling Density Matrix Real Time TDDFT: Propagator Unitarity \& Matrix Truncation

Real time, density matrix based, time dependent density functional theory proceeds through the propagation of the density matrix, as opposed to the Kohn-Sham orbitals. It is possible to reduce the computational workload by imposing spatial cut-off radii on sparse matrices, and the propagation of the density matrix in this manner provides direct access to the optical response of very large systems, which would be otherwise impractical to obtain using the standard formulations of TDDFT. Following a brief summary of our implementation, along with several benchmark tests illustrating the validity of the method, we present an exploration of the factors affecting the accuracy of the approach. In particular we investigate the effect of basis set size and matrix truncation, the key approximation used in achieving linear scaling, on the propagator unitarity and optical spectra. Finally we illustrate that, with an appropriate density matrix truncation range applied, the computational load scales linearly with the system size and discuss the limitations of the approach.

physics.comp-ph↗

arXiv2014

Stable and Efficient Linear Scaling First-Principles Molecular Dynamics for 10,000+ atoms

The recent progress of linear-scaling or O(N) methods in the density functional theory (DFT) is remarkable. We expect that first-principles molecular dynamics (FPMD) simulations based on DFT can now treat more realistic and complex systems using the O(N) technique. However, very few examples of O(N) FPMD simulations exist so far and the information for the accuracy or reliability of the simulations is very limited. In this paper, we show that efficient and robust O(N) FPMD simulations are now possible by the combination of the extended Lagrangian Born-Oppenheimer molecular dynamics method, which was recently proposed by Niklasson et al (Phys. Rev. Lett. 100, 123004 (2008)), and the density matrix method as an O(N) technique. Using our linear-scaling DFT code Conquest, we investigate the reliable calculation conditions for the accurate O(N) FPMD and demonstrate that we are now able to do actual and reliable self-consistent FPMD simulation of a very large system containing 32,768 atoms.

cond-mat.mtrl-sci↗

arXiv2014

Efficient Calculations with Multisite Local Orbitals in the large-scale DFT Code CONQUEST

Multisite local orbitals, which are formed from linear combinations of pseudo-atomic orbitals from a target atom and its neighbor atoms, have been introduced in the large-scale density functional theory calculation code CONQUEST. Multisite local orbitals correspond to local molecular orbitals so that the number of required local orbitals can be minimal. The multisite support functions are determined by using the localized filter diagonalization (LFD) method [Phys. Rev. B 2009, 80, 205104]. Two new methods, the double cutoff method and the smoothing method, are introduced to the LFD method to improve efficiency and stability. The Hamiltonian and overlap matrices with multisite local orbitals are constructed by efficient sparse-matrix multiplications in CONQUEST. The investigation of the calculated energetic and geometrical properties and band structures of bulk Si, Al and DNA systems demonstrate the accuracy and the computational efficiency of the present method. The representability of both occupied and unoccupied band structures with the present method has been also confirmed.

cond-mat.mtrl-sci↗

arXiv2014

DFT Studies of Adsorption of Benzoic Acid on the Rutile (110) Surface: Modes and Patterns

Adsorption of benzoic acid on the (110) surface of rutile, both unreconstructed and (1x2)- reconstructed ones, at saturation coverage of one benzoate per two adjacent five-coordinated Ti5c sites has been studied using DFT simulations, as implemented through the Vienna Ab initio Simulations Package (VASP). In order to study the effects of hydrogen bonding and Van der Waals forces in influencing the relative stabilities of different adsorbate overlayer superstructures, these studies were performed through Local Density Approximation (LDA), Generalized Gradient Approximation(GGA) and DFT-D2. Through the calculations, it was found out that although the optimized structures corresponded with the proposed models for the experimental results, the relative energetic stabilities of different overlayer structures have shown some differences with the experimental results. In the GGA calculations, the overlayer structures involving the benzene rings aligned along the (001)-direction were shown to be more stable than those aligned perpendicular to it, regardless of whether the benzoates were arranged in (1x1) or (1x2) symmetries on the (1x1)-unreconstructed surface, or on the (1x2)-reconstructed surface. These are in contrast to the experimental studies using STM, whose observations revealed that configurations with the benzene rings aligned along the (110)-direction when adsorbed on the (1x2)-reconstructed surface.

cond-mat.mtrl-sci↗