Stray Charges Research Articles

Method for Estimating Locations of Laser-Induced Stray Charges on Surface-Electrode Ion Traps Using Secular Frequency Shift at Multiple Ion Positions

Method for Estimating Locations of Laser-Induced Stray Charges on Surface-Electrode Ion Traps Using Secular Frequency Shift at Multiple Ion Positions

A microfabricated ion trap chip with a sloped loading slot to minimize exposing trapped ions to stray charges

A novel microfabricated ion trap chip that can geometrically minimize exposing trapped ions to stray charges on dielectric surfaces is developed. The new design utilizes a sloped loading slot to make the dielectric layers of the loading slot sidewalls invisible to the trapped ions. The designed loading slot is realized by applying silicon anisotropic etching processes, and the ion trap chip that contains the sloped loading slot is fabricated using microelectromechanical system processes. Stray charges induced by injecting an ultraviolet laser on the surface of the fabricated ion trap chip are experimentally characterized by measuring stray electric fields at the location of the trapped ions. The measured results are compared to those of a previous ion trap chip with a vertical loading slot. The magnitude of the measured stray electric field in the axial direction is within 1.5 V m−1 for the proposed ion trap chip, whereas it is approximately 10 V m−1 for the previously reported ion trap chip.

Lifting of spin blockade by charged impurities in Si-MOS double quantum dot devices

One obstacle that has slowed the development of electrically gated metal-oxide-semiconductor (MOS) singlet-triplet qubits is the frequent lack of observed spin blockade, even in samples with large singlet-triplet energy splittings. We present theoretical and experimental evidence that the cause of this problem in MOS double quantum dots is the stray positive charges in the oxide inducing accidental levels near the device's active region that allow spin blockade lifting. We also present evidence that these effects can be mitigated by device design modifications, such as overlapping gates.

A pyramid MOT with integrated optical cavities as a cold atom platform for an optical lattice clock

We realize a two-stage, hexagonal pyramid magneto-optical trap (MOT) with strontium, and demonstrate loading of cold atoms into cavity-enhanced 1D and 2D optical lattice traps, all within a single compact assembly of in-vacuum optics. We show that the device is suitable for high-performance quantum technologies, focusing especially on its intended application as a strontium optical lattice clock. We prepare 2 × 104 spin-polarized atoms of 87Sr in the optical lattice within 500 ms; we observe a vacuum-limited lifetime of atoms in the lattice of 27 s; and we measure a background DC electric field of 12 V m−1 from stray charges, corresponding to a fractional frequency shift of (−1.2 ± 0.8) × 10−18 to the strontium clock transition. When used in combination with careful management of the blackbody radiation environment, the device shows potential as a platform for realizing a compact, robust, transportable optical lattice clock with systematic uncertainty at the 10−18 level.

A New Microfabrication Method for Ion-Trap Chips That Reduces Exposure of Dielectric Surfaces to Trapped Ions

Accumulated electrostatic charges on the dielectric surfaces of ion traps are known to induce stray fields, leading to ion micromotions. In typical microfabricated ion-trap chips, metal electrodes are electrically isolated using thick dielectric pillars, which can accumulate stray charges on their sidewalls. This letter presents a new microfabrication method for ion-trap chips that reduces the exposure of dielectric surfaces to trapped ions. The dielectric pillars are fabricated with large T-shaped overhangs, and the sidewalls and top surfaces are coated with AlCu (1%) films. The bottom sides of the overhang parts provide electrical isolation. To prevent oxidation of the AlCu (1%) films, the electrode surfaces are coated with an additional Au film. The fabricated chips were implemented to trap 174Yb+ ions, and the laser-induced stray fields were measured. The results indicated that the trap chip fabricated by the newly developed method generates significantly smaller stray fields as compared with previous chips. [2017-0233]

Cotunneling and polaronic effect in granular systems

We theoretically study the conductivity in arrays of metallic grains due to the variable-range multiple cotunneling of electrons with short-range (screened) Coulomb interaction. The system is supposed to be coupled to random stray charges in the dielectric matrix that are only loosely bounded to their spatial positions by elastic forces. The flexibility of the stray charges gives rise to a polaronic effect, which leads to the onset of Arrhenius-like conductivity behavior at low temperatures, replacing conventional Mott variable-range hopping. The effective activation energy logarithmically depends on temperature due to fluctuations of the polaron barrier heights. We present the unified theory that covers both weak and strong polaron effect regimes of hopping in granular metals and describes the crossover from elastic to inelastic cotunneling.

A discrete element and ray framework for rapid simulation of acoustical dispersion of microscale particulate agglomerations

In industry, particle-laden fluids, such as particle-functionalized inks, are constructed by adding fine-scale particles to a liquid solution, in order to achieve desired overall properties in both liquid and (cured) solid states. However, oftentimes undesirable particulate agglomerations arise due to some form of mutual-attraction stemming from near-field forces, stray electrostatic charges, process ionization and mechanical adhesion. For proper operation of industrial processes involving particle-laden fluids, it is important to carefully breakup and disperse these agglomerations. One approach is to target high-frequency acoustical pressure-pulses to breakup such agglomerations. The objective of this paper is to develop a computational model and corresponding solution algorithm to enable rapid simulation of the effect of acoustical pulses on an agglomeration composed of a collection of discrete particles. Because of the complex agglomeration microstructure, containing gaps and interfaces, this type of system is extremely difficult to mesh and simulate using continuum-based methods, such as the finite difference time domain or the finite element method. Accordingly, a computationally-amenable discrete element/discrete ray model is developed which captures the primary physical events in this process, such as the reflection and absorption of acoustical energy, and the induced forces on the particulate microstructure. The approach utilizes a staggered, iterative solution scheme to calculate the power transfer from the acoustical pulse to the particles and the subsequent changes (breakup) of the pulse due to the particles. Three-dimensional examples are provided to illustrate the approach.

Stray Charge in Quantum-dot Cellular Automata: A Validation of the Intercellular Hartree Approximation

The authors analyze the effect of stray charges near a line of quantum-dot cellular automata (QCA) cells. Considering both the ground-state polarization and the excitation energy of the system, it is determined that there is a 129-nm-wide region surrounding a QCA wire where a stray charge will cause the wire to fail. This calculation is the result of a full-basis-set simulation of a four-cell line. A comparison is made between cells with parallel-spin electrons and those with antiparallel spin electrons, showing that they yield essentially identical results. Therefore, the added complexity of accounting for antiparallel spins does not yield superior simulation results. Finally, a comparison is made between the full-basis calculations and the results of the same calculation using the intercellular Hartree approximation (ICHA). The similarity of these two results demonstrates that the ICHA method is a valid tool for studying the effect of stray charges in larger systems.

Design and simulation of molecular nonvolatile single-electron resistive switches

We have carried out a preliminary design and simulation of a single-electron resistive switch based on a system of two linear, parallel, electrostatically coupled molecules: one implementing a single-electron transistor and another serving as a single-electron trap. To verify our design, we have performed a theoretical analysis of this “memristive” device, based on a combination of ab initio calculations of the electronic structures of the molecules and the general theory of single-electron tunneling in systems with discrete energy spectra. Our results show that such molecular assemblies, with a length below 10 nm and a footprint area of about 5 nm2, may combine sub-second switching times with multi-year retention times and high (>103) ON/OFF current ratios, at room temperature. Moreover, Monte Carlo simulations of self-assembled monolayers, based on such molecular assemblies, have shown that such monolayers may also be used as resistive switches, with comparable characteristics and, in addition, be highly tolerant to defects and stray offset charges.

Electric field compensation and sensing with a single ion in a planar trap

We use a single ion as a movable electric field sensor with accuracies on the order of a few V/m. For this, we compensate undesired static electric fields in a planar radio frequency trap and characterize the static field and its curvature over an extended region along the trap axis. We observe a strong buildup of stray charges around the loading region on the trap resulting in an electric field of up to 1.3 kV/m at the ion position. We also find that the profile of the stray field remains constant over a time span of a few months.

Spatial and Intensity Modulation of Nanowire Emission Induced by Mobile Charges

Single-molecule optical experiments carried out in conjunction with externally applied electric fields show deliberate spatial and intensity control over CdSe nanowire (NW) emission. In particular, by applying external fields to electrically isolated (single) NWs, their emission can be localized in areas of the wire closest to the positive electrode. In a few cases, the resulting emission intensity increases over the corresponding zero-field value by nearly an order of magnitude. More often than not, factors of 2-3 are seen. Reversing the field polarity causes the emission to localize in opposite regions of the wire. Emission from individual NWs can therefore be modulated. Complementary ac electric field measurements show that the effect persists up to 500 kHz. To explain the phenomenon, the effective passivation of surface trap states by mobile carriers is speculated. This, in turn, causes local changes in the NW emission quantum yield (QY). To verify the presence of such mobile charges, both ensemble and single NW bundle electrophoresis experiments are conducted. By investigating subsequent NW rotational and translational dynamics, an estimate for the number of mobile carriers is determined. A lower limit (best case) linear charge density of approximately 0.45-1.2 mobile electrons per micrometer of the wire is obtained. Apart from self-consistently explaining the field-induced NW emission modulation, the resulting data and subsequent analysis also suggests that the same mobile carriers may be the root cause of NW emission intermittency. Furthermore, given the ubiquity of stray charges, the resulting hypothesis may have additional applicability toward explaining blinking in other systems, a problem of current interest especially within the context of colloidal QDs.

Coherent transport in Josephson-junction rhombi chain with quenched disorder

We consider a chain of Josephson-junction rhombi (proposed originally by Dou\ifmmode \mbox{\c{c}}\else \c{c}\fi{}ot and Vidal) in the quantum regime. In the maximally frustrated case when magnetic flux through each rhombi ${\ensuremath{\Phi}}_{r}$ is equal to one-half of the superconductive flux quantum ${\ensuremath{\Phi}}_{0}$, the Josephson current is due to the correlated transport of pairs of Cooper pairs; i.e., charge is quantized in units of $4e$. A sufficiently strong deviation $\ensuremath{\delta}\ensuremath{\Phi}\ensuremath{\equiv}\ensuremath{\mid}{\ensuremath{\Phi}}_{r}\ensuremath{-}{\ensuremath{\Phi}}_{0}∕2\ensuremath{\mid}>\ensuremath{\delta}{\ensuremath{\Phi}}^{c}$ from the maximally frustrated point brings the system back to the usual $2e$-quantized supercurrent. For a regular chain $\ensuremath{\delta}{\ensuremath{\Phi}}^{c}$ was calculated by us in Phys. Rev. B 70, 184519 (2004). Here we present a detailed analysis of the effect of quenched disorder (random stray charges and random fluxes piercing rhombi) on the pairing effect. For the case of a chain with random stray charges in the quantum regime we derive the probability to find the system in the $4e$-dominated regime and estimate the characteristic value of the critical deflection $\ensuremath{\delta}{\ensuremath{\Phi}}^{c}$.

OHMIC PROPERTY OF A SINGLE-ELECTRON TUNNEL-JUNCTION SYSTEM AND ITS APPLICATION TO NEURAL NETWORK SYNAPSES

We have studied the ohmic property of parallel coupled single-electron tunnel-junction arrays. When a current flows in one array ('driving' array), the Coulomb blockade state of the other array ('passive' array) is lifted, leading to an ohmic relationship between the tunneling current and the bias voltage. The driving current basically plays the role of a switch for the current in the passive array, as it can vary the ohmic conductance as well. Since the ohmic characteristic and the switching function are quite robust to the background or stray charges, the parallel coupled tunnel-junction system can be used as a variable resistor in logic circuits. As its specific application, we have proposed a Hopfield neural network with single-electron synapses.

Interference effects in isolated Josephson junction arrays with geometric symmetries

Reducing the size of the junctions in a Josephson junction array takes the system from the classical to the quantum regime and turns the condensate phases on the islands into periodic quantum variables. Under suitable conditions such quantum arrays exhibit Aharonov-Bohm-Casher-type interference effects which change the spectrum of this macroscopic object by the addition of one Cooper pair to the array. We discuss the case of an isolated symmetric loop of N identical junctions encircling one half superconducting quantum of magnetic flux. We argue that the ground state is nondegenerate if the total number of Cooper pairs on the array is divisible by N, and doubly degenerate otherwise (after the stray charges are compensated by the gate voltages). This result is first established in the two opposite limits of dominating Josephson and charging energies, and is further explained using symmetry arguments. As long as the ground-state symmetry is preserved, its degeneracy remains unchanged in the entire range of charging-to-Josephson energy ratio. Such interference effects may be relevant in applications of superconducting junction arrays for quantum computing.

Charge effects on gravitational wave detectors

We show that the mean-square displacement of a charged oscillator due to the zero point oscillations of the radiation field is unique in the sense that it is very sensitive to the value of the bare mass of the charge. Thus, a controlled experiment using gravitational wave detectors could lead to a determination of the electron bare mass and shed some light on quantum electrodynamic theory. We also speculate that the irregular signals of non-gravitational origin often observed in gravitational wave bar detectors could be caused by stray charges and that such charges could also adversely affect LIGO and other such detectors.

Influence of imperfections on the dynamical response in model quantum cellular automata

The time-dependent behavior of quantum cellular automata (QCA) with imperfections is examined. We have studied a chain of quantum cells with the imperfections introduced by (i) variations of intercellular distances, (ii) variations of intercell tunneling strengths, and (iii) stray charges. We find that imperfections like these may easily impair the switching properties of semiconductor based QCAs.

Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires

Nanometre-scale electronic structures are of both fundamental and technological interest: they provide a link between molecular and solid state physics, and have the potential to reach far higher device densities than is possible with conventional semiconductor technology1,2. Examples of such structures include quantum dots,which can function as single-electron transistors3,4 (although theirsensitivity to individual stray charges might make them unsuitable for large-scale devices) and semiconducting carbon nanotubes several hundred nanometres in length, which have been used to create a field-effect transistor5. Much smaller devices could be made by joining two nanotubes or nanowires to create, for example, metal–semiconductor junctions, in which the junction area would be about 1 nm2 for single-walled carbon nanotubes. Electrical measurements of nanotube ‘mats’ have shown the behaviour expected for a metal–semiconductor junction6. However, proposed nanotube junction structures7 have not been explicitly observed, nor have methods been developed to prepare them. Here we report controlled, catalytic growth of metal–semiconductor junctions between carbon nanotubes and silicon nanowires, and show that these junctions exhibit reproducible rectifying behaviour.

Pair-Production Avalanches Revisited

As noted previously in this meeting, any millisecond pulsar that is not perfectly aligned must obviously be strongly charged electrostatically simply because the wave zone is so close to the surface that any trapped neutralizing electrosphere over the polar caps will be driven away beyond 6 or 7 neutron-star radii. Acceleration of stray charges (e.g., from the ISM or a nearby orbiting disk) in such a highly charged system automatically produces coherently radiating bunches of downward moving particles (Michel 1990), this coherent radiation simply reflecting from the surface (or missing the star entirely, as an alternative possibility). This mechanism, suggestive of how pulsars might operate, we will call a pair-production avalanche.

Energy dependence of the fundamental parameters of theK0−K¯0system. II. Theoretical formalism

We present a detailed analysis of the previously reported anomalous energy dependence of the fundamental ${K}^{0}\ensuremath{-}{\overline{K}}^{0}$ parameters $\ensuremath{\Delta}m={m}_{L}\ensuremath{-}{m}_{S}$, ${\ensuremath{\tau}}_{S}$, $|{\ensuremath{\eta}}_{+\ensuremath{-}}|$, and ${tan\ensuremath{\varphi}}_{+\ensuremath{-}}$. Such variations with energy can arise from the interaction of the kaons with an external field or medium. A phenomenological formalism is introduced to describe such energy-dependent influences on the ${K}^{0}\ensuremath{-}{\overline{K}}^{0}$ system. Using this formalism we demonstrate that effects of the type suggested by the data cannot be ascribed to an interaction of the kaons with an electromagnetic, hypercharge, or gravitational field, or to the scattering of the kaons from stray charges or cosmological neutrinos. The data are, however, compatible with an interaction which is even under charge conjugation, and models of such an interaction are discussed. We also consider the possibility that such effects may arise from a fundamental violation of Lorentz invariance. All of the mechanisms which appear capable of describing the data also suggest that similar effects could arise in neutrino oscillations, and some of the consequences of such a possibility are outlined.

A solution to the problem of spontaneous deflection of the beam in TWT's with low helix voltage

The spontaneous deflection of the beam in low voltage, typically low noise, traveling-wave tubes has been studied and attributed to the accumulation of stray charges on the glazed helix rods. A means of measuring the tendency of the helix to show these charging effects will be described. A method of processing of the complete helix which prevents the accumulation of charges but does not seriously affect its cold loss or other microwave properties will be reviewed. Traveling-wave tubes that have been made with helices processed by this technique cannot be made to show charging effects or beam deflections. Six tubes have accumulated 22,000 tube hours with no failures.