arXiv subjects

J. H. Cole

Publications and source records attributed to J. H. Cole.

At least 19 recordsLinked to original sources

arXiv2024

The influence of pinholes and weak-points in aluminium-oxide Josephson junctions

Josephson junctions are the key components used in superconducting qubits for quantum computing. The advancement of quantum computing is limited by a lack of stability and reproducibility of qubits which ultimately originates in the amorphous tunnel barrier of the Josephson junctions and other material imperfections. Pinholes in the junction have been suggested as one of the possible contributors to these instabilities, but evidence of their existence and the effect they might have on transport is unclear. We use molecular dynamics to create three-dimensional atomistic models to describe Al-AlOx-Al tunnel junctions, showing that pinholes form when oxidation of the barrier is incomplete. Following this we use the atomistic model and simulate the electronic transport properties for tunnel junctions with different barrier thicknesses using the non-equilibrium Green's function formalism. We observe that pinholes may contribute to excess quasiparticle current flow in Al-AlOx-Al tunnel junctions with thinner barriers, and in thicker barriers we observe weak-points which facilitate leakage currents even when the oxide is continuous. We find that the disordered nature of the amorphous barrier results in significant variations in the transport properties. Additionally, we determine the current-phase relationship for our atomistic structures, confirming that devices with pinholes and weak-points cause a deviation from the ideal sinusoidal Josephson relationship.

cond-mat.mes-hall↗

arXiv2023

Excitons and singlet fission at hybrid inorganic-organic semiconductor interfaces

Excitons in organic crystalline semiconductors play a crucial role in the operation of optoelectronic devices such as organic solar cells, light-emitting diodes, and photodetectors. The excitonic properties of materials are dramatically affected by the presence of surfaces and interfaces. In this work, we investigate the influence of a neutral hydrogen-passivated 1x2 reconstructed (100) silicon substrate on excitons within the crystalline tetracene layer deposited on the top of it. Our findings reveal that singlet excitons in the contact tetracene layer are situated within the continuum of unbound Wannier-Mott excitonic states in silicon, with noteworthy hybridization between these states. Consequently, in the contact tetracene layer, all singlet excitons exhibit a pronounced interlayer charge transfer character, while the triplet exciton remains confined to the tetracene layer. This makes the singlet fission effect highly improbable for the contact tetracene layer. Additionally, the presence of the silicon substrate results in a modification of the singlet-triplet gap by 144 meV. This change is solely attributed to the hybridization with excitons in silicon, which influences the exchange energy. Our results show that the dynamic dielectric screening caused by the substrate does not impact the singlet-triplet gap but alters the exciton binding energies.

cond-mat.mtrl-sci↗

arXiv2022

Phase transformation-induced superconducting aluminium-silicon alloy rings

The development of a materials platform that exhibits both superconducting and semiconducting properties is an important endeavour for a range of emerging quantum technologies. We investigate the formation of superconductivity in nanowires fabricated with silicon-on-insulator (SOI). Aluminium from deposited contact electrodes is found to interdiffuses with the Si nanowire structures to form an Al-Si alloy along the entire length of the predefined nanowire device over micron length scales at temperatures well below that of the Al-Si eutectic. The resultant transformed nanowire structures are layered in geometry with a continuous Al-Si alloy wire sitting on the buried oxide of the SOI and a residual Si cap sitting on top of the wire. The phase transformed material is conformal with any predefined device patterns and the resultant structures are exceptionally smooth-walled compared to similar nanowire devices formed by silicidation processes. The superconducting properties of a mesoscopic AlSi ring formed on a SOI platform are investigated. Low temperature magnetoresistance oscillations, quantized in units of the fluxoid, h/2e, are observed.

cond-mat.mtrl-sci↗

arXiv2022

A many-body perturbation theory approach to energy band alignment at the crystalline tetracene-silicon interface

Hybrid inorganic-organic semiconductor interfaces are of interest for new photovoltaic devices operating above the Shockley-Queisser limit. Predicting energy band alignment at the interfaces is crucial for their design, but represents a challenging problem due to the large scales of the system, the energy precision required and a wide range of physical phenomena that occur at the interface. To tackle this problem, we use many-body perturbation theory in the non-self-consistent GW approximation, orbital relaxation corrections for organic semiconductors, and line-up potential method for inorganic semiconductors which allows for tractable and accurate computing of energy band alignment in crystalline van-der-Waals hybrid inorganic-organic semiconductor interfaces. In this work, we study crystalline tetracene physisorbed on the clean hydrogen-passivated 1x2 reconstructed (100) silicon surface. Using this computational approach, we find that the energy band alignment is determined by an interplay of the mutual dynamic dielectric screening of two materials and the formation of a dipole layer due to a weak hybridization of atomic/molecular orbitals at the interface. We also emphasize the significant role of the exchange-correlation effects in predicting band offsets for the hybrid inorganic-organic semiconductor interfaces.

cond-mat.mtrl-sci↗

arXiv2021

Microscopic quantum point contact formation as the electromigration mechanism in granular superconductor nanowires

Granular aluminium is a high kinetic inductance thin film superconductor which, when formed into nanowires can undergo an intrinsic electromigration process. We use a combination of experimental and computational approaches to investigate the role of grain morphology and distribution in granular aluminium thin films, when formed into nanowire constrictions. Treating the granular aluminium film as a network of randomly distributed resistors with parameters motivated by the film microstructure allows us to model the electrical characteristics of the nanowires. This model provides estimates of the dependence of sheet resistance on grain size and distribution, and the resulting device to device variation for superconducting nanowires. By fabricating a series of different length nanowires, we study the electromigration process as a function of applied current, and then compare directly to the results of our computational model. In doing so we show that the electromigration is driven by the formation of quantum point contacts between metallic aluminium grains.

cond-mat.supr-con↗

arXiv2021

Aharonov-Bohm interference as a probe of Majorana fermions

Majorana fermions act as their own antiparticle, and they have long been thought to be confined to the realm of pure theory. However, interest in them has recently resurfaced, as it was realized through the work of Kitaev that some experimentally accessible condensed matter systems can host these exotic excitations as bound states on the boundaries of 1D chains, and that their topological and non-abelian nature holds promise for quantum computation. Unambiguously detecting the experimental signatures of Majorana bound states has turned out to be challenging, as many other phenomena lead to similar experimental behaviour. Here, we computationally study a ring comprised of two Kitaev model chains with tunnel coupling between them, where an applied magnetic field allows for Aharonov-Bohm interference in transport through the resulting ring structure. We use a non-equilibrium Green's function technique to analyse the transport properties of the ring in both the presence and absence of Majorana zero modes. Further, we show that these results are robust against weak disorder in the presence of an applied magnetic field. This computational model suggests another signature for the presence of these topologically protected bound states can be found in the magnetic field dependence of devices with loop geometries.

cond-mat.mes-hall↗

arXiv2021

Emission of twisted light from quantum dot arrays with a discrete rotational symmetry

We theoretically explore the optical properties of a circular array of quantum dots. This structure, that we call a circular emitter array (CEA), can exchange optical angular momentum via the emission and absorption of circularly polarised light as well as light carrying orbital angular momentum. Analytical expressions are derived for both interband and intraband transition rates and selection rules are determined. Only the key properties of the quantum dots are considering when modelling the CEA. This extends the applicability of our model to CEAs composed of a variety of quantum dots. Finally design principles for the tuning of the specific optical properties of the CEA are determined. This opens up the prospect of the designing a metamaterial, consisting of a surface of CEAs, that upconverts optical angular momentum.

quant-ph↗

arXiv2020

NanoNET: an extendable Python framework for semi-empirical tight-binding models

We present a novel open-source Python framework called NanoNET (Nanoscale Non-equilibrium Electron Transport) for modelling electronic structure and transport. Our method is based on the tight-binding method and non-equilibrium Green's function theory. The core functionality of the framework is providing facilities for efficient construction of tight-binding Hamiltonian matrices from a list of atomic coordinates and a lookup table of the two-center integrals in dense, sparse, or block-tridiagonal forms. The framework implements a method based on $kd$-tree nearest-neighbour search and is applicable to isolated atomic clusters and periodic structures. A set of subroutines for detecting the block-tridiagonal structure of a Hamiltonian matrix and splitting it into series of diagonal and off-diagonal blocks is based on a new greedy algorithm with recursion. Additionally the developed software is equipped with a set of programs for computing complex band structure, self-energies of elastic scattering processes, and Green's functions. Examples of usage and capabilities of the computational framework are illustrated by computing the band structure and transport properties of a silicon nanowire as well as the band structure of bulk bismuth.

physics.comp-ph↗

arXiv2020

Simulating the fabrication of aluminium oxide tunnel junctions

Aluminium oxide (AlO$_\mathrm{x}$) tunnel junctions are important components in a range of nanoelectric devices including superconducting qubits where they can be used as Josephson junctions. While many improvements in the reproducibility and reliability of qubits have been made possible through new circuit designs, there are still knowledge gaps in the relevant materials science. A better understanding of how fabrication conditions affect the density, uniformity, and elemental composition of the oxide barrier may lead to the development of lower noise and more reliable nanoelectronics and quantum computers. In this paper we use molecular dynamics to develop models of Al-AlO$_\mathrm{x}$-Al junctions by iteratively growing the structures with sequential calculations. With this approach we can see how the surface oxide grows and changes during the oxidation simulation. Dynamic processes such as the evolution of a charge gradient across the oxide, the formation of holes in the oxide layer, and changes between amorphous and semi-crystalline phases are observed. Our results are widely in agreement with previous work including reported oxide densities, self-limiting of the oxidation, and increased crystallinity as the simulation temperature is raised. The encapsulation of the oxide with metal evaporation is also studied atom by atom. Low density regions at the metal-oxide interfaces are a common feature in the final junction structures which persists for different oxidation parameters, empirical potentials, and crystal orientations of the aluminium substrate.

cond-mat.mes-hall↗

arXiv2019

Does the boson peak survive in an ultrathin oxide glass?

Bulk glasses exhibit extra vibrational modes at low energies, known as the boson peak. The microscopic dynamics in nanoscale alumina impact the performance of qubits and other superconducting devices, however the existence of the boson peak in these glasses has not been previously measured. Here we report neutron spectroscopy on Al/Al$_2$O$_{3-x}$ nanoparticles consisting of spherical metallic cores from 20 to 1000 nm surrounded by a 3.5 nm thick alumina glass. An intense low-energy peak is observed at $ω_{BP}$ = 2.8 $\pm$ 0.6 meV for highly oxidised particles, concurrent with an excess in the density of states. The intensity of the peak scales inversely with particle size and oxide fraction indicating a surface origin, and is red-shifted by 3 meV with respect to the van-Hove singularity of $γ$-phase Al$_2$O$_{3-x}$ nanocrystals. Molecular dynamics simulations of $α$-Al$_2$O$_{3-x}$, $γ$-Al$_2$O$_{3-x}$ and a-Al$_2$O$_{3-x}$ show that the observed boson peak is a signature of the ultrathin glass surface, and the frequency is softened compared to that of the hypothetical bulk glass.

cond-mat.mtrl-sci↗

arXiv2019

The effect of atomic structure on the electrical response of aluminium oxide tunnel junctions

Many nanoelectronic devices rely on thin dielectric barriers through which electrons tunnel. For instance, aluminium oxide barriers are used as Josephson junctions in superconducting electronics. The reproducibility and drift of circuit parameters in these junctions are affected by the uniformity, morphology, and composition of the oxide barriers. To improve these circuits the effect of the atomic structure on the electrical response of aluminium oxide barriers must be understood. We create three-dimensional atomistic models of aluminium oxide tunnel junctions and simulate their electronic transport properties with the non-equilibrium Green's function formalism. Increasing the oxide density is found to produce an exponential increase in the junction resistance. In highly oxygen-deficient junctions we observe metallic channels which decrease the resistance significantly. Computing the charge and current density within the junction shows how variation in the local potential landscape can create channels which dominate conduction. An atomistic approach provides a better understanding of these transport processes and guides the design of junctions for nanoelectronics applications.

cond-mat.mes-hall↗

arXiv2019

Probing charge carrier movement in organic semiconductor thin films via nanowire conductance spectroscopy

Understanding the movement of charge within organic semiconducting films is crucial for applications in photo-voltaics and flexible electronics. We study the sensitivity of the electrical conductance of a silicon nanowire to changes of charge states within an organic semiconductor physisorbed on the surface of the nanowire. Elastic scattering caused by motion of charge carriers near the nanowire modifies the mean-free path for backscattering of electrons propagating within it, which we have mathematically expressed in terms of the causal Green's functions. The scattering potential has been computed using a combination of the polarizable continuum model and density functional theory with the range-separated exchange-correlation functional for organic molecules and the semi-empirical tight-binding model for silicon. As an example, the sensitivity to charge state changes in tetracene is computed as a function of operating temperature and geometrical parameters of a nanowire. For a single molecule, ultra-thin silicon nanowires with characteristic sizes of the cross-section below 2 nm produce a detectable conductance change at room temperature. For larger nanowires the sensitivity is reduced, however the conductance change grows with the number of charged molecules: with sub-4 nm nanowires being sensitive enough to detect several tens of charge carriers. We propose using noise spectroscopy to access the temporal evolution of the charge states. Information regarding the spatial distribution of charge carries in organic thin films can be obtained using a grid of nanowire resistors and electric impedance tomography.

cond-mat.mes-hall↗

arXiv2018

Optical vector network analysis of ultra-narrow transitions in $^{166}$Er$^{3+}$:$^7$LiYF$_4$

We present optical vector network analysis (OVNA) of an isotopically purified $^{166}$Er$^{3+}$:$^7$LiYF$_4$ crystal. The OVNA method is based on generation and detection of modulated optical sideband by using a radio-frequency vector network analyzer. This technique is widely used in the field of microwave photonics for the characterization of optical responses of optical devices such as filters and high-Q resonators. However, dense solid-state atomic ensembles induce a large phase shift on one of the optical sidebands which results in the appearance of extra features on the measured transmission response. We present a simple theoretical model which accurately describes the observed spectra and helps to reconstruct the absorption profile of a solid-state atomic ensemble as well as corresponding change of the refractive index in the vicinity of atomic resonances.

physics.ins-det↗

arXiv2016

Ab initio calculation of energy levels for phosphorus donors in silicon

The s manifold energy levels for phosphorus donors in silicon are important input parameters for the design and modelling of electronic devices on the nanoscale. In this paper we calculate these energy levels from first principles using density functional theory. The wavefunction of the donor electron's ground state is found to have a form that is similar to an atomic s orbital, with an effective Bohr radius of 1.8 nm. The corresponding binding energy of this state is found to be 41 meV, which is in good agreement with the currently accepted value of 45.59 meV. We also calculate the energies of the excited 1s(T) and 1s(E) states, finding them to be 32 and 31 meV respectively. These results constitute the first ab initio confirmation of the s manifold energy levels for phosphorus donors in silicon.

cond-mat.mes-hall↗

arXiv2015

Electronic transport in Si:P delta-doped wires

Despite the importance of Si:P delta-doped wires for modern nanoelectronics, there are currently no computational models of electron transport in these devices. In this paper we present a nonequilibrium Green's function model for electronic transport in a delta-doped wire, which is described by a tight-binding Hamiltonian matrix within a single-band effective-mass approximation. We use this transport model to calculate the current-voltage characteristics of a number of delta-doped wires, achieving good agreement with experiment. To motivate our transport model we have performed density-functional calculations for a variety of delta-doped wires, each with different donor configurations. These calculations also allow us to accurately define the electronic extent of a delta-doped wire, which we find to be at least 4.6 nm.

cond-mat.mes-hall↗

arXiv2015

Parity effect and single-electron injection for Josephson-junction chains deep in the insulating state

We have made a systematic investigation of charge transport in 1D chains of Josephson junctions where the characteristic Josephson energy is much less than the single-island Cooper-pair charging energy, $E_\mathrm{J}\ll E_{CP}$. Such chains are deep in the insulating state, where superconducting phase coherence across the chain is absent, and a voltage threshold for conduction is observed at the lowest temperatures. We find that Cooper-pair tunneling in such chains is completely suppressed. Instead, charge transport is dominated by tunneling of single electrons, which is very sensitive to the presence of BCS quasiparticles on the superconducting islands of the chain. Consequently we observe a strong parity effect, where the threshold voltage vanishes sharply at a characteristic parity temperature $T^*$, which is significantly lower than the the critical temperature, $T_c$. A measurable and thermally-activated zero-bias conductance appears above $T^*$, with an activation energy equal to the superconducting gap, confirming the role of thermally-excited quasiparticles. Conduction below $T^*$ and above the voltage threshold occurs via injection of single electrons/holes into the Cooper-pair insulator, forming a non-equilibrium steady state with a significantly enhanced effective temperature. Our results explicitly show that single-electron transport dominates deep in the insulating state of Josephson-junction arrays. This conduction process has mostly been ignored in previous studies of both superconducting junction arrays and granular superconducting films below the superconductor-insulator quantum phase transition.

cond-mat.supr-con↗

arXiv2012

Ambient Nanoscale Sensing with Single Spins Using Quantum Decoherence

Magnetic resonance detection is one of the most important tools used in life-sciences today. However, as the technique detects the magnetization of large ensembles of spins it is fundamentally limited in spatial resolution to mesoscopic scales. Here we detect the natural fluctuations of nanoscale spin ensembles at ambient temperatures by measuring the decoherence rate of a single quantum spin in response to introduced extrinsic target spins. In our experiments 45 nm nanodiamonds with single nitrogen-vacancy (NV) spins were immersed in solution containing spin 5/2 Mn^2+ ions and the NV decoherence rate measured though optically detected magnetic resonance. The presence of both freely moving and accreted Mn spins in solution were detected via significant changes in measured NV decoherence rates. Analysis of the data using a quantum cluster expansion treatment of the NV-target system found the measurements to be consistent with the detection of ~2,500 motionally diffusing Mn spins over an effective volume of (16 nm)^3 in 4.2 s, representing a reduction in target ensemble size and acquisition time of several orders of magnitude over state-of-the-art electron spin resonance detection. These measurements provide the basis for the detection of nanoscale magnetic field fluctuations with unprecedented sensitivity and resolution in ambient conditions.

cond-mat.mes-hall↗

arXiv2012

Interferometry using Adiabatic Passage in Dilute Gas Bose-Einstein Condensates

We theoretically examine three-well interferometry in Bose-Einstein condensates using adiabatic passage. Specifically, we demonstrate that a fractional coherent transport adiabatic passage protocol enables stable spatial splitting in the presence of nonlinear interactions. A reversal of this protocol produces a coherent recombination of the BEC with a phase-dependent population of the three wells. The effect of nonlinear interactions on the interferometric measurement is quantified and found to lead to an enhancement in sensitivity for moderate interaction strengths.

cond-mat.quant-gas↗