arXiv subjects

Jared H. Cole

Publications and source records attributed to Jared H. Cole.

At least 37 records · Page 2Linked to original sources

arXiv2022

Quintet formation and exchange fluctuations: The role of stochastic resonance in singlet fission

Singlet fission describes the spin-conserving production of two triplet excitons from one singlet exciton. The existence of a spin-2 (quintet) triplet-pair state as a product of singlet fission is well established in the literature, and control of quintet formation is an important step towards applying singlet fission in photovoltaics and quantum information. However, a definitive mechanism for quintet formation is yet to be established, which makes it difficult to design materials for optimal quintet formation. Here we outline a mechanism in which inter-triplet exchange coupling fluctuations drive fast and efficient quintet formation. In contrast with conventional wisdom, we show that quintet population can arise despite strong exchange coupling. We evaluate the performance of this quintet formation mechanism in two regimes of conformational freedom, and relate quintet dynamics to material properties of singlet fission molecules.

cond-mat.mes-hall↗

arXiv2022

Influence of device geometry and imperfections on the interpretation of transverse magnetic focusing experiments

Spatially separating electrons of different spins and efficiently generating spin currents are crucial steps towards building practical spintronics devices. Transverse magnetic focusing is a potential technique to accomplish both those tasks. In a material where there is significant Rashba spin--orbit interaction, electrons of different spins will traverse different paths in the presence of an external magnetic field. Experiments have demonstrated the viability of this technique by measuring conductance spectra that indicate the separation of spin-up and spin-down electrons. However the effect that the geometry of the leads has on these measurements is not well understood. We show that the resolution of features in the conductance spectra is affected by the shape, separation and width of the leads. Furthermore, the number of subbands occupied by the electrons in the leads affects the ratio between the amplitudes of the spin-split peaks in the spectra. We simulated devices with random onsite potentials and observed that transverse magnetic focusing devices are sensitive to disorder. Ultimately we show that careful choice and characterisation of device geometry is crucial for correctly interpreting the results of transverse magnetic focusing experiments.

cond-mat.mes-hall↗

arXiv2022

Optical interaction of the NV- centre in diamond with a plasmonic metal nanoparticle

We present a rigorous theoretical model for the optical interaction between a nitrogen-vacancy (NV) centre in diamond and a plasmonic metal nanoparticle (MNP), accompanied by a computationally efficient procedure to solve the evolution. The proposed model enables us to successfully explain existing optical emission measurements of NV centres both in the presence and absence of a metal nanoparticle. We show that the NV-plasmon interaction provides a versatile new avenue to enhance and control the optical emission of an NV centre. Changes to the MNP type and size, NV-MNP centre separation, submerging medium permittivity, and NV orientation with respect to the MNP surface can be exploited to improve a plethora of NV centre based nanodevices.

cond-mat.mes-hall↗

arXiv2021

Acoustic spectral hole-burning in a two-level system ensemble

Microscopic two-level system (TLS) defects at dielectric surfaces and interfaces are among the dominant sources of loss in superconducting quantum circuits, and their properties have been extensively probed using superconducting resonators and qubits. We report on spectroscopy of TLSs coupling to the strain field in a surface acoustic wave (SAW) resonator. The narrow free spectral range of the resonator allows for two-tone spectroscopy where a strong pump is applied at one resonance while a weak signal is used to probe a different mode. We map the spectral hole burnt by the pump tone as a function of frequency and extract parameters of the TLS ensemble. Our results suggest that detuned acoustic pumping can be used to enhance the coherence of superconducting devices by saturating TLSs.

quant-ph↗

arXiv2021

The effect of magnetic impurity scattering on transport in topological insulators

Charge transport in topological insulators is primarily characterised by so-called topologically projected helical edge states, where charge carriers are correlated in spin and momentum. In principle, dissipation-less current can be carried by these edge states as backscattering from impurities and defects is suppressed as long as time-reversal symmetry is not broken. However, applied magnetic fields or underlying nuclear spin-defects in the substrate can break this time reversal symmetry. In particular, magnetic impurities lead to back-scattering by spin-flip processes. We have investigated the effects of point-wise magnetic impurities on the transport properties of helical edge states in the BHZ model using the Non-Equilibrium Green's Function formalism and compared the results to a semi-analytic approach. Using these techniques we study the influence of impurity strength and spin impurity polarization. We observe a secondary effect of defect-defect interaction that depends on the underlying material parameters which introduces a non-monotonic response of the conductance to defect density. This in turn suggests a qualitative difference in magneto-transport signatures in the dilute and high density spin impurity limits.

cond-mat.mes-hall↗

arXiv2021

Singlet exciton dynamics of perylene diimide and tetracene based hetero/homogeneous substrates via an \textit{ab initio} kinetic Monte Carlo model

Luminescent solar concentrators (LSCs) are devices that trap a portion of the solar spectrum and funnel it towards photon harvesting devices. The modelling of LSCs at a quantum chemical level however, remains a challenge due to the complexity of exciton and photon dynamic modelling. This study examines singlet exciton dynamics occurring within a typical LSC device. To do this, we use a rejection-free kinetic Monte Carlo method to predict diffusion lengths, diffusion coefficients, substrate anisotropy, and average exciton lifetimes of perylene diimide (PDI) and tetracene based substrates in the low concentration scheme. \textit{Ab initio} rate constants are computed using time-dependant density functional theory based methods. PDI type substrates are observed to display enhanced singlet exciton transport properties when compared to tetracene. Simulations show that substrates with dipole-aligned chromophores are characterised by anisotropic exciton diffusion, with slightly improved transport properties. Finally, a PDI-tetracene substrate is simulated for both disordered and dipole-aligned chromophore configurations. In this multi-dopant substrate transport is predominantly mediated by PDI due to the asymmetry in the transport rates between the two dyes considered. We conclude discussing the properties of multi-dopant substrates and how they can impact the design of next generation LSCs.

cond-mat.mtrl-sci↗

arXiv2021

Enhanced Control of Quantum Dot Photoluminescence in Hybrid Assemblies

The distance-dependent interaction of an emitter with a plasmonic nanoparticle or surface forms the basis of the field of plexitonics. Semiconductor quantum dots (QDs) are robust emitters due to their photostability, and offer the possibility of understanding the fundamental photophysics between one emitter and one metal nanoparticle. A key enabling challenge is the formation of systems containing both QDs and plasmonic nanoparticles in high purity. We present the translation of DNA-based self-assembly techniques to assemble metal and semiconductor nanocrystals into discrete hybrid structures, including dimers, of high purity. This method gives control over the interparticle separation, geometry, and ratio of QD:metal nanoparticle, as well as the spectral properties of the metal/QD components in the assembly to allow investigation of plasmon-exciton interaction. The hybrid assemblies show the expected enhancement in steady-state photoluminescence accompanied by an increase in the QD emission rate for assemblies with a strong overlap between the QD emission and localised surface plasmon resonance. In contrast, lengthening of the QD emission lifetime (a reduction of the emission rate) of up to 1.7-fold, along with an enhancement in steady-state PL of 15-75% is observed upon detuning of the QD and metal nanoparticle spectral properties. This understood in terms of the Purcell effect, where the gold nanoparticle acts as a damped, nanoscale cavity. Considering the metal nanoparticle using generalised nonlocal optical response theory (GNOR) and the QD as an open quantum system, the response is driven by the interference experienced by the emitter for parallel and perpendicular field orientations. This provides a mechanism for control of the emission rate of a QD by a metal nanoparticle across a much wider range of lifetimes than previously understood.

cond-mat.mes-hall↗

arXiv2021

Accurate calculation of excitonic signatures in the absorption spectrum of BiSBr using semiconductor Bloch equations

In order to realize the significant potential of optical materials such as metal halides, computational techniques which give accurate optical properties are needed, which can work hand-in-hand with experiments to generate high efficiency devices. In this work a computationally efficient technique based on semiconductor Bloch equations (SBEs) is developed and applied to the material BiSBr. This approach gives excellent agreement with the experimental optical gap, and also agrees closely with the excitonic stabilisation energy and the absorption spectrum computed using the far more computationally demanding \textit{ab initio} Bethe-Salpeter approach. The SBE method is a good candidate for theoretical spectroscopy on large- or low dimensional systems which are too computationally expensive for an \textit{ab initio} treatment.

cond-mat.mtrl-sci↗

arXiv2021

Localized Wannier function based tight-binding models for two-dimensional allotropes of bismuth

With its monoelemental composition, various crystalline forms and an inherently strong spin-orbit coupling, bismuth has been regarded as an ideal prototype material to expand our understanding of topological electronic structures. In particular, two-dimensional bismuth thin films have attracted a growing interest due to potential applications in topological transistors and spintronics. This calls for an effective physical model to give an accurate interpretation of the novel topological phenomena shown by two-dimensional bismuth. However, the conventional semi-empirical approach of adapting bulk bismuth hoppings fails to capture the topological features of two-dimensional bismuth allotropes because the electronic band topology is heavily influenced by crystalline symmetries as well as atom spacings. Here we provide a new parameterization using localized Wannier functions derived from the Bloch states in first-principles calculations. We construct new tight-binding models for three types of two-dimensional bismuth allotropes: a Bi (111) bilayer, bismuthene and a Bi(110) bilayer. We demonstrate that our tight-binding models can successfully reproduce the band structures, symmetries and topological features of these two-dimensional allotropes. We anticipate that these models can be extended to other similar two-dimensional topological structures such as antimonene and arsenene. Moreover, these models can serve as a starting point for investigating the electron/spin transport and electromagnetic response in low-dimensional topological devices.

cond-mat.mtrl-sci↗

arXiv2021

Exciton transport in amorphous polymers and the role of morphology and thermalisation

Understanding the transport mechanism of electronic excitations in conjugated polymers is key to advancing organic optoelectronic applications, such as solar cells, OLEDs and flexible electronics. While crystalline polymers can be studied using solid-state techniques based on lattice periodicity, the characterisation of amorphous polymers is hindered by an intermediate regime of disorder and the associated lack of symmetries. To overcome these hurdles we use a reduced state quantum master equation approach based on the Merrifield exciton formalism. Using this model we study exciton transport in conjugated polymers and its dependence on morphology and temperature. Exciton dynamics consists of a thermalisation process, whose features depend on the relative strength of thermal energy, electronic couplings and disorder, resulting in remarkably different transport regimes. By applying this method to representative systems based on poly(p-phenylene vinylene) (PPV) we obtain insight into the role of temperature and disorder on localisation, charge separation, non-equilibrium dynamics, and experimental accessibility of thermal equilibrium states of excitons in amorphous polymers.

cond-mat.mtrl-sci↗

arXiv2020

Two-dimensional spectroscopy beyond the perturbative limit: the influence of finite pulses and detection modes

Ultra-fast and multi-dimensional spectroscopy gives a powerful looking glass into the dynamics of molecular systems. In particular two-dimensional electronic spectroscopy (2DES) provides a probe of coherence and the flow of energy within quantum systems which is not possible with more conventional techniques. While heterodyne-detected (HD) 2DES is increasingly common, more recently fluorescence-detected (FD) 2DES offers new opportunities, including single-molecule experiments. However in both techniques it can be difficult to unambiguously identify the pathways which dominate the signal. Therefore the use of numerically modelling of 2DES is vitally important, which in turn requires approximating the pulsing scheme to some degree. Here we employ non-pertubative time evolution to investigate the effects of finite pulse width and amplitude on 2DES signals. In doing so we identify key differences in the response of HD and FD detection schemes, as well as the regions of parameter space where the signal is obscured by unwanted artefacts in either technique. Mapping out parameter space in this way provides a guide to choosing experimental conditions and also shows in which limits the usual theoretical approximations work well and which limits more sophisticated approaches are required.

quant-ph↗

arXiv2020

Stability of superconducting resonators: motional narrowing and the role of Landau-Zener driving of two-level defects

Frequency instability of superconducting resonators and qubits leads to dephasing and time-varying energy-loss and hinders quantum-processor tune-up. Its main source is dielectric noise originating in surface oxides. Thorough noise studies are needed in order to develop a comprehensive understanding and mitigation strategy of these fluctuations. Here we use a frequency-locked loop to track the resonant-frequency jitter of three different resonator types---one niobium-nitride superinductor, one aluminium coplanar waveguide, and one aluminium cavity---and we observe strikingly similar random-telegraph-signal fluctuations. At low microwave drive power, the resonators exhibit multiple, unstable frequency positions, which for increasing power coalesce into one frequency due to motional narrowing caused by sympathetic driving of individual two-level-system defects by the resonator. In all three devices we probe a dominant fluctuator, finding that its amplitude saturates with increasing drive power, but its characteristic switching rate follows the power-law dependence of quasiclassical Landau-Zener transitions.

cond-mat.supr-con↗

arXiv2020

Insulating, metallic and superconducting behavior in a single nanowire

In systems with reduced dimensions quantum fluctuations have a strong influence on the electronic conduction, even at very low temperature. In superconductors this is especially interesting, since the coherent state of the superconducting electrons is strongly interacting with these fluctuations and therefore is a sensitive tool to study them. In this paper, we report on comprehensive measurements of superconducting nanowires in the quantum phase slip regime. Using an intrinsic electromigration process, we have developed a method to lower the resistance of lithographically fabricated highly resistive nanowires in situ and in small consecutive steps. At low temperature we observe critical (Coulomb) blockade voltages and superconducting critical currents, depending on the nanowire's normal-state resistance, in good agreement with theoretical models. Between these two regimes, we find a continuous transition displaying a nonlinear metallic-like behavior. The reported intrinsic electromigration technique is not limited to low temperatures as we find a similar change in resistance that spans over three orders of magnitude also at room temperature. Aside from superconducting quantum circuits, such a technique to reduce the resistance may also have applications in modern electronic circuits.

cond-mat.supr-con↗

arXiv2020

The influence of a planar metal nanoparticle assembly on the optical response of a quantum emitter

We develop an analytical framework to study the influence of a weakly intercoupled inplane spherical metal nanoparticle (MNP) assembly on a coherently illuminated quantum emitter (QE). We reduce the analytical expressions derived for the aforementioned generic planar setup into simple and concise expressions representing a QE mediated by a symmetric MNP constellation, by exploiting the symmetry. We use the recently introduced generalized nonlocal optical response (GNOR) theory that has successfully explained plasmonic experiments to model the MNPs in our system. Due to the use of GNOR theory, and our analytical approach, the procedure we suggest is extremely computationally efficient. Using the derived model, we analyse the absorption rate, resultant Rabi frequency, effective excitonic energy shift and dephasing rate shift spectra of an exciton bearing QE at the centre of a symmetric MNP setup. We observe that the QE experiences plasmon induced absorption rate spectral linewidth variations that increase in magnitude with decreasing MNP-QE centre separation and increasing number of MNPs. Our results also suggest that, parameter regions where the QE exhibits trends of decreasing linewidth against decreasing MNP-QE centre separation are likely to be associated with plasmon induced excitonic energy redshifts. Similarly, regions where the QE absorption rate linewidth tends to increase against decreasing MNP QE centre separation are likely to be accompanied by plasmon induced excitonic energy blueshifts. In both these cases, the magnitude of the observed red/blueshift was seen to increase with the number of MNPs in the constellation, due to enhancement of the plasmonic influence.

cond-mat.mes-hall↗

arXiv2020

Rabi oscillations in a superconducting nanowire circuit

We investigate the circuit quantum electrodynamics of anharmonic superconducting nanowire oscillators. The sample circuit consists of a capacitively shunted nanowire with a width of about 20 nm and a varying length up to 350 nm, capacitively coupled to an on-chip resonator. By applying microwave pulses we observe Rabi oscillations, measure coherence times and the anharmonicity of the circuit. Despite the very compact design, simple top-down fabrication and high degree of disorder in the oxidized (granular) aluminum material used, we observe lifetimes in the microsecond range.

cond-mat.supr-con↗

arXiv2020

Building a bigger Hilbert space for superconducting devices, one Bloch state at a time

Superconducting circuits for quantum information processing are often described theoretically in terms of a discrete charge, or equivalently, a compact phase/flux, at each node in the circuit. Here we revisit the consequences of lifting this assumption for transmon and Cooper-pair-box circuits, which are constituted from a Josephson junction and a capacitor, treating both the superconducting phase and charge as noncompact variables. The periodic Josephson potential gives rise to a Bloch band structure, characterised by the Bloch quasicharge. We analyse the possibility of creating superpositions of different quasicharge states by transiently shunting inductive elements across the circuit, and suggest a choice of eigenstates in the lowest Bloch band of the spectrum that may support an inherently robust qubit encoding.

quant-ph↗

arXiv2019

First-Principles Calculation of Triplet Exciton Diffusion in Crystalline Poly($p$-phenylene vinylene)

Understanding and controlling exciton transport is a strategic way to enhance the optoelectronic properties of high-performance organic devices. In this article we study triplet exciton migration in crystalline poly($p$-phenylene vinylene) polymer (PPV) using comprehensive electronic structure and quantum dynamical methods. We solve the coupled electron-nuclear dynamics for the triplet energy migrating between two neighboring Frenkel sites in J- and H-aggregate arrangements. From the two-site model we extract key parameters for use with a master-equation approach that allows us to treat nanosize systems where time-dependent Schrödinger equation becomes intractable. We calculate the transient exciton density evolution and determine the diffusion constants along the principal crystal axes of the PPV. The triplet diffusion is characterized by two distinctive components: fast intrachain, and slow interchain. At room temperature the interchain diffusion coefficients are found to be $D_a=0.89\cdot10^{-2}$ cm$^2$s$^{-1}$ and $D_b=1.49\cdot10^{-2}$ cm$^2$s$^{-1}$ along the respective $\bar{a}$- and $\bar{b}$-axes, and the intrachain is $D_c=3.03$ cm$^2$s$^{-1}$ along the fast $\bar{c}$-axis. The exceptionally high exciton mobility along the $π$-conjugated backbone facilitates rapid triplet migration over long distances. Our results can be utilized in the design of efficient energy conversion and light-emitting devices with desired solid-state properties.

cond-mat.mtrl-sci↗

arXiv2019

Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits

Amorphous solids show surprisingly universal behaviour at low temperatures. The prevailing wisdom is that this can be explained by the existence of two-state defects within the material. The so-called standard tunneling model has become the established framework to explain these results, yet it still leaves the central question essentially unanswered -- what are these two-level defects? This question has recently taken on a new urgency with the rise of superconducting circuits in quantum computing, circuit quantum electrodynamics, magnetometry, electrometry and metrology. Superconducting circuits made from aluminium or niobium are fundamentally limited by losses due to two-level defects within the amorphous oxide layers encasing them. On the other hand, these circuits also provide a novel and effective method for studying the very defects which limit their operation. We can now go beyond ensemble measurements and probe individual defects -- observing the quantum nature of their dynamics and studying their formation, their behaviour as a function of applied field, strain, temperature and other properties. This article reviews the plethora of recent experimental results in this area and discusses the various theoretical models which have been used to describe the observations. In doing so, it summarises the current approaches to solving this fundamentally important problem in solid-state physics.

cond-mat.mes-hall↗