arXiv subjects

A. J. Fisher

Publications and source records attributed to A. J. Fisher.

At least 19 recordsLinked to original sources

arXiv2025

Materials for Quantum Technologies: a Roadmap for Spin and Topology

In this Perspective article, we explore some of the promising spin and topology material platforms (e.g. spins in semi- and superconductors, skyrmionic, topological and 2D materials) being developed for such quantum components as qubits, superconducting memories, sensing, and metrological standards and discuss their figures of merit. Spin- and topology-related quantum phenomena have several advantages, including high coherence time, topological protection and stability, low error rate, relative ease of engineering and control, simple initiation and read-out. However, the relevant technologies are at different stages of research and development, and here we discuss their state-of-the-art, potential applications, challenges and solutions.

cond-mat.mes-hall↗

arXiv2020

Optical and magnetic signatures of the topological edge states in a dimerised donor chain

We have studied the excited states of a one-dimensional donor dimer array by using time-dependent Hartree-Fock and density-functional theories. We find that tuning the inter-donor distances can induce a topological phase transition from a topologically trivial anti-ferromagnetic ground state to a topological phase supporting spin-polarised edge states. Significant changes in the optical spectra accompany this transition, providing signatures for the phase transition and the existence of the edge state; the edge states lead to a robust $1s\rightarrow 2p$ atomic transition. By contrast, the bulk of the chain becomes spinless, inducing a reduction in the HOMO-LUMO gap and a decrease of the excitation energies in the optical spectra. The inclusion of physics beyond the Su-Schrieffer-Heeger-Hubbard model (long-range electron correlations and basis states beyond the 1s manifold) has a critical effect on the results and leads to the novel prediction of a change in optical response related to a topological phase transition. Our results provide a theoretical foundation for using artificial donor linear arrays in semiconductors to form topological edge states by design.

cond-mat.mes-hall↗

arXiv2020

Excited states of a phosphorus pair in silicon: Effects of valley-orbital interaction and electron-electron interactions

Excitations of impurity complexes in semiconductors can not only provide a route to fill the terahertz gap in optical technologies, but can also connect local quantum bits to scale up solid-state quantum-computing devices. However, taking into account both the interactions among electrons/holes, and the host band structures, is challenging. Here we combine first-principles band-structure calculations with quantum-chemistry methodology to evaluate the ground and excited states of a pair of phosphorous donors in silicon within s single framework. We use a broken-symmetry Hartree-Fock approach, followed by a time-dependent Hartree-Fock method to compute the excited states. Our Hamiltonian for each valley includes an anisotropic kinetic energy term, which splits the 2p_0 and 2p_+- transitions of isolated donors by ~4 meV, in good agreement with experiments. Our single-valley calculations show the optical response is a strong function of the optical polarisation, and suggest the use of valley polarisation to control optics and reduce oscillations in exchange interactions. When taking into account all valleys, including valley-orbital interactions, we find a gap opens between the 1s to 2p transition and low-energy charge-transfer states within 1s manifolds (which become optically allowed because of inter-donor interactions). In contrast to the single-valley case, we find charge-transfer excited states also in the triplet sector, thanks to the valley degrees of freedom. These states have a qualitatively correct energy as compared with the previous experiments; additionally, we predict new excitations below 20 meV that have not been analysed previously. A statistical average of nearest-neighbour pairs at different separations suggests that THz radiation could be used to excite pairs spin-selectively. Our approach can readily be extended to other donors and to other semiconducting hosts.

cond-mat.mes-hall↗

arXiv2020

A linear combination of atomic orbitals (LCAO) model for deterministically placed acceptor arrays in silicon

We develop a tight-binding model based on linear combination of atomic orbitals (LCAO) methods to describe the electronic structure of arrays of acceptors, where the underlying basis states are derived from an effective-mass-theory solution for a single acceptor in either the spherical approximation or the cubic model. Our model allows for arbitrarily strong spin-orbit coupling in the valence band of the semiconductor. We have studied pairs and dimerised linear chains of acceptors in silicon in the `independent-hole' approximation, and investigated the conditions for the existence of topological edge states in the chains. For the finite chain we find a complex interplay between electrostatic effects and the dimerisation, with the long-range Coulomb attraction of the hole to the acceptors splitting off states localised at the end acceptors from the rest of the chain. A further pair of states then splits off from each band, to form a pair localised on the next-to-end acceptors, for one sense of the bond alternation and merges into the bulk bands for the other sense of the alternation. We confirm the topologically non-trivial nature of these next-to-end localised states by calculating the Zak phase. We argue that for the more physically accessible case of one hole per acceptor these long-range electrostatic effects will be screened out; we show this by treating a simple phenomenologically screened model in which electrostatic contributions from beyond the nearest neighbours of acceptor each pair are removed. Topological states are now found on the end acceptors of the chains. In some cases the termination of the chain required to produce topological states is not the one expected on the basis of simple geometry (short versus long bonds); we argue this is because of a non-monotonic relationship between the bond length and the effective Hamiltonian matrix elements between the acceptors.

cond-mat.mes-hall↗

arXiv2017

Excited states of defect lines in silicon: A first-principles study based on hydrogen cluster analogues

Excited states of a single donor in bulk silicon have previously been studied extensively based on effective mass theory. However, a proper theoretical description of the excited states of a donor cluster is still scarce. Here we study the excitations of lines of defects within a single-valley spherical band approximation, thus mapping the problem to a scaled hydrogen atom array. A series of detailed full configuration-interaction and time-dependent hybrid density-functional theory calculations have been performed to understand linear clusters of up to 10 donors. Our studies illustrate the generic features of their excited states, addressing the competition between formation of inter-donor ionic states and intra-donor atomic excited states. At short inter-donor distances, excited states of donor molecules are dominant, at intermediate distances ionic states play an important role, and at long distances the intra-donor excitations are predominant as expected. The calculations presented here emphasise the importance of correlations between donor electrons, and are thus complementary to other recent approaches that include effective mass anisotropy and multi-valley effects. The exchange splittings between relevant excited states have also been estimated for a donor pair and for a three-donor arrays; the splittings are much larger than those in the ground state in the range of donor separations between 10 and 20 nm. This establishes a solid theoretical basis for the use of excited-state exchange interactions for controllable quantum gate operations in silicon.

cond-mat.mes-hall↗

arXiv2013

Switching and propagation of magneto-plasmon-polaritons in magnetic slot waveguides and cavities

The dispersion relations for surface plasmon-polaritons propagating in the Voigt geometry in a metal-insulator-metal waveguide with a magneto-optically active dielectric medium are derived. The symmetry between the upper and lower interfaces is broken by the introduction of the magnetic field; the balance between the field distributions on the two interfaces can be controlled by the applied field. This control is illustrated by finite-element method numerical simulations of the field distributions around a point dipole placed in the centre of the short waveguide; it is shown that both the total emission of radiation from the cavity and the distribution of the far-field radiation can be strongly modified by tuning the magnetisation of the waveguide. This raises the novel possibility of using magnetic fields to control light propagation in nanostructures.

physics.optics↗

arXiv2013

Half-filled orbital and unconventional geometry of a common dopant in Si(001)

The determining factor of the bulk properties of doped Si is the column rather than the row in the periodic table from which the dopants are drawn. It is unknown whether the basic properties of dopants at surfaces and interfaces, steadily growing in importance as microelectronic devices shrink, are also solely governed by their column of origin. The common light impurity P replaces individual Si atoms and maintains the integrity of the dimer superstructure of the Si(001) surface, but loses its valence electrons to surface states. Here we report that isolated heavy dopants are entirely different: Bi atoms form pairs with Si vacancies, retain their electrons and have highly localized, half-filled orbitals.

cond-mat.mtrl-sci↗

arXiv2010

Contribution of spin pairs to the magnetic response in a dilute dipolar ferromagnet

We simulate the dc magnetic response of the diluted dipolar-coupled Ising magnet LiHo$_{0.045}$Y$_{0.955}$F$_4$ in a transverse field, using exact diagonalization of a two-spin Hamiltonian averaged over nearest-neighbour configurations. The pairwise model, incorporating hyperfine interactions, accounts for the observed drop-off in the longitudinal (c-axis) susceptibility with increasing transverse field; with the inclusion of a small tilt in the transverse field, it also accounts for the behavior of the off-diagonal magnetic susceptibility. The hyperfine interactions do not appear to lead to qualitative changes in the pair susecptibilities, although they do renormalize the crossover fields between different regimes. Comparison with experiment indicates that antiferromagnetic correlations are more important than anticipated based on simple pair statistics and our first-principles calculations of the pair response. This means that larger clusters will be needed for a full description of the reduction in the diagonal response at small transverse fields.

cond-mat.stat-mech↗

arXiv2009

Entanglement in general two-mode continuous-variable states: local approach and mapping to a two-qubit system

We present a new approach to the analysis of entanglement in smooth bipartite continuous-variable states. One or both parties perform projective filterings via preliminary measurements to determine whether the system is located in some region of space; we study the entanglement remaining after filtering. For small regions, a two-mode system can be approximated by a pair of qubits and its entanglement fully characterized, even for mixed states. Our approach may be extended to any smooth bipartite pure state or two-mode mixed state, leading to natural definitions of concurrence and negativity densities. For Gaussian states both these quantities are constant throughout configuration space.

quant-ph↗

arXiv2008

Entangling neutrons via successive scattering from a substrate

This letter details a simple scheme to entangle two neutrons by successive scattering from a macroscopic sample. In zero magnetic field the entanglement falls as the sample size increases. However, by applying a field and tuning the momentum of the neutrons, one can achieve a substantial degree of entanglement irrespective of the size of the sample.

quant-ph↗

arXiv2008

Molecular Thin Films: a New Type of Magnetic Switch

The design and fabrication of materials that exhibit both semiconducting and magnetic properties for spintronics and quantum computing has proven difficult. Important starting points are high-purity thin films as well as fundamental theoretical understanding of the magnetism. Here we show that small molecules have great potential in this area, due to ease of insertion of localised spins in organic frameworks and both chemical and structural purity. In particular, we demonstrate that archetypal molecular semiconductors, namely the metal phthalocyanines (Pc), can be readily fabricated as thin film quantum antiferromagnets, important precursors to a solid state quantum computer. Their magnetic state can be switched via fabrication steps which modify the film structure, offering practical routes into information processing. Theoretical calculations show that a new mechanism, which is the molecular analogue of the interactions between magnetic ions in metals, is responsible for the magnetic states. Our combination of theory and experiments opens the field of organic thin film magnetic engineering.

cond-mat.mtrl-sci↗

arXiv2008

Structure-dependent exchange in the organic magnets Cu(II)Pc and Mn(II)Pc

We study exchange couplings in the organic magnets copper(II) phthalocyanine (Cu(II)Pc) and manganese(II) phthalocyanine (Mn(II)Pc) by a combination of Green's function perturbation theory and \textsl{ab initio} density-functional theory (DFT). Based on the indirect exchange model our perturbation-theory calculation of Cu(II)Pc qualitatively agrees with the experimental observations. DFT calculations performed on Cu(II)Pc dimer show a very good quantitative agreement with exchange couplings that we extract by using a global fitting for the magnetization measurements to a spin-1/2 Bonner-Fisher model. These two methods give us remarkably consistent trends for the exchange couplings in Cu(II)Pc when changing the stacking angles. The situation is more complex for Mn(II)Pc owing to the competition between super-exchange and indirect exchange.

cond-mat.mtrl-sci↗

arXiv2008

Hydroxyl vacancies in single-walled aluminosilicate and aluminogermanate nanotubes

We report the first theoretical study of hydroxyl vacancies in aluminosilicate and aluminogermanate single-walled metal-oxide nanotubes. The defects are modeled on both sides of the tube walls and lead to occupied and empty states in the band gap which are highly localized both in energy and in real space. We find different magnetization states depending on both the chemical composition and the specific side with respect to the tube cavity. The defect-induced perturbations to the pristine electronic structure are related to the electrostatic polarization across the tube walls and the ensuing change in Brønsted acid-base reactivity. Finally, the capacity to counterbalance local charge accumulations, a characteristic feature of these systems, is discussed in view of their potential application as insulating coatings for one-dimensional conducting nanodevices.

cond-mat.mtrl-sci↗

arXiv2007

Quantum Communication in Spin Systems With Long-Range Interactions

We calculate the fidelity of transmission of a single qubit between distant sites on semi-infinite and finite chains of spins coupled via the magnetic dipole interaction. We show that such systems often perform better than their Heisenberg nearest-neighbour coupled counterparts, and that fidelities closely approaching unity can be attained between the ends of finite chains without any special engineering of the system, although state transfer becomes slow in long chains. We discuss possible optimization methods, and find that, for any length, the best compromise between the quality and the speed of the communication is obtained in a nearly uniform chain of 4 spins.

quant-ph↗

arXiv2007

Local entanglement of multidimensional continuous-variable systems

We study the `local entanglement' remaining after filtering operations corresponding to imperfect measurements performed by one or both parties, such that the parties can only determine whether or not the system is located in some region of space. The local entanglement in pure states of general bipartite multidimensional continuous-variable systems can be completely determined through simple expressions. We apply our approach to semiclassical WKB systems, multi-dimensional harmonic oscillators, and a hydrogen atom as three examples.

quant-ph↗

arXiv2007

Exchange in multi-defect semiconductor clusters: assessment of `control-qubit' architectures

We present a variational method to calculate the exchange interactions among donor clusters in a semiconductor. Such clusters are candidates for a so-called control-qubit architecture for quantum information, where the effective exchange coupling between two atoms is controlled by the electronic state of a third. We use a combination of the effective-mass approximation and the quantum defect method; our variational ansatz is particularly suited to cases where an excited state of one of the donors (control) is partially delocalised over several different centres, forming an analogue of an extended molecular orbital. Our method allows calculations of the "on/off" ratios of exchange interactions in such cases. We compare exchange interactions when the control is in the "on" and "off" states, and find that both the magnitude and sign of the exchange interactions may be changed. To rationalize the sign-change, we carry out a simple Green's function perturbation-theory calculation. This simple model qualitatively explains the sign change and illustrates its origins both in ring-exchange processes and in the delocalization of the control electron. We also compute probability distributions for the coupling strengths over the ensemble of clusters, and show that excitation of the control causes narrowing of the distributions along with shifts to larger magnitudes and from anti-ferromagnetic to ferromagnetic coupling.

cond-mat.mtrl-sci↗

arXiv2007

Robust non-adiabatic molecular dynamics for metals and insulators

We present a new formulation of the correlated electron-ion dynamics (CEID) scheme, which systematically improves Ehrenfest dynamics by including quantum fluctuations around the mean-field atomic trajectories. We show that the method can simulate models of non-adiabatic electronic transitions, and test it against exact integration of the time-dependent Schroedinger equation. Unlike previous formulations of CEID, the accuracy of this scheme depends on a single tunable parameter which sets the level of atomic fluctuations included. The convergence to the exact dynamics by increasing the tunable parameter is demonstrated for a model two level system. This algorithm provides a smooth description of the non-adiabatic electronic transitions which satisfies the kinematic constraints (energy and momentum conservation) and preserves quantum coherence. The applicability of this algorithm to more complex atomic systems is discussed.

cond-mat.mtrl-sci↗

arXiv2007

Exchange between deep donors in semiconductors: a quantum defect approach

Exchange interactions among defects in semiconductors are commonly treated within effective-mass theory using a scaled hydrogenic wave-function. However such a wave-function is only applicable to shallow impurities; here we present a simple but robust generalization to treat deep donors, in which we treat the long-range part of the wavefunction using the well established quantum defect theory, and include a model central-cell correction to fix the bound-state eigenvalue at the experimentally observed value. This allows us to compute the effect of binding energy on exchange interactions as a function of donor distance; this is a significant quantity given recent proposals to carry out quantum information processing using deep donors. As expected, exchange interactions are suppressed (or increased), compared to the hydrogenic case, by the greater localization (or delocalization) of the wavefunctions of deep donors (or `super-shallow' donors with binding energy less then the hydrogenic value). The calculated results are compared with a simple scaling of the Heitler-London hydrogenic exchange; the scaled hydrogenic results give the correct order of magnitude but fail to reproduce quantitatively our calculations. We calculate the donor exchange in silicon including inter-valley interference terms for donor pairs along the $\{100\}$ direction, and also show the influence of the donor type on the distribution of nearest-neighbour exchange constants at different concentrations. Our methods can be used to compute the exchange interactions between two donor electrons with arbitrary binding energy.

cond-mat.mtrl-sci↗