Asymmetrical Dips Research Articles

Numerical simulations of exocomet transits: Insights from β Pic and KIC 3542116

In recent years, the topic of existence and exploration of exocomets has been gaining increasing attention. The asymmetrical decrease in the star’s brightness due to the passage of a comet-like object in front of the star was successfully predicted. It was subsequently confirmed on the basis of the light curves of stars observed by Kepler and TESS orbital telescopes. Since then, there have been successful attempts to fit the asymmetrical dips observed in the stars’ light curves utilizing a simple 1D model of an exponentially decaying optically thin dust tail. In this work, we propose fitting the photometric profiles of some known exocomet transits based on a Monte Carlo approach to build up the distribution of dust particles in a cometary tail. As the exocomet prototypes, we used the physical properties of certain Solar System comets belonging to the different dynamical groups and moving at heliocentric distances of 0.6 au, 1.0 au, 5.0 au, and 5.5 au. We obtained a good agreement between the observed and modeled transit light curves. We also show that the physical characteristics of dust particles, such as the particle size range, the power index of dust size distribution, the particle terminal velocity, and distance to the host star affect the shape of the transit light curve, while the dust productivity of the comet nucleus and the impact parameter influence its depth and duration. The estimated dust production rates of the transiting exocomets are at the level of the most active Solar System comets.

Experimental study of modified Tavis-Cummings model with directly-coupled superconducting artificial atoms.

The Tavis-Cummings model is intensively investigated in quantum optics and has important applications in generation of multi-atom entanglement. Here, we employ a superconducting circuit quantum electrodynamic system to study a modified Tavis-Cummings model with directly-coupled atoms. In our device, three superconducting artificial atoms are arranged in a chain with direct coupling through fixed capacitors and strongly coupled to a transmission line resonator. By performing transmission spectrum measurements, we observe different anticrossing structures when one or two qubits are resonantly coupled to the resonator. In the case of the two-qubit Tavis-Cummings model without qubit-qubit interaction, we observe two dips at the resonance point of the anticrossing. The splitting of these dips is determined by Δ λ=2g12+g32, where g1 and g3 are the coupling strengths between Qubit 1 and the resonator, and Qubit 3 and the resonator, respectively. The direct coupling J12 between the two qubits results in three dressed states in the two-qubit Tavis-Cummings model at the frequency resonance point, leading to three dips in the transmission spectrum. In this case, the distance between the two farthest and asymmetrical dips, arising from the energy level splitting, is larger than in the previous case. The frequency interval between these two dips is determined by the difference in eigenvalues (Δ λ=ε 1+-ε 1-), obtained through numerical calculations. What we believe as novel and intriguing experimental results may potentially advance quantum optics experiments, providing valuable insights for future research.

Comparative study of adsorptions, reactions and electronic properties of U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces

The mysterious properties of individual U atoms on transition metal surfaces play indispensable parts in supplementing our understanding of uranium-transition metal systems, which are important subjects for both nuclear energy applications and fundamental scientific studies. By using scanning tunneling microscopy and density functional theory calculations, the adsorptions, reactions and electronic properties of individual U atoms on Cu(111), Ag(111), Au(111) and Ru(0001) surfaces were comparatively studied for the first time in this work. Upon the deposition of a small amount of U onto Cu(111) or Ag(111) at 8 K, individual U atoms show relatively high activity and can either be adsorbed on intact substrate surfaces or induce various surface vacancies surrounded by clusters of substrate atoms. By contrast, the majority of U atoms tend to dispersedly adsorb on intact surfaces of Au(111) and Ru(0001) rather than producing surface vacancies at the same temperature. In all cases, Kondo resonance manifested as asymmetric dip feature around Fermi energy is only observed in the differential tunneling conductance spectra of single U adatoms on Ag(111).

Guided-mode resonance based humidity sensing using a multilayer dielectric structure.

We report on a highly sensitive measurement of the relative humidity of air, which utilizes a guided-mode resonance (GMR) of a multilayer dielectric structure (MDS) and the spectral interference of s- and p-polarized waves reflected from the MDS. We employ the MDS represented by four bilayers of TiO2/SiO2 with a termination layer of TiO2 and demonstrate that the GMR shows up as a shallow and asymmetric dip. The GMR enables us to measure the relative humidity (RH) of air with sensitivities of 0.031-0.114 nm/%RH. In addition, by employing a birefringent crystal of mica, which modifies the phase difference between the polarized waves, the GMR is transformed into the resonance with a sharp dip, and the measured sensitivity is enhanced to 0.120 nm/%RH at 81 %RH. We also determined the sensitivity to the refractive index and the figure of merit as high as 8000 nm/refractive index unit (RIU) and 702 RIU-1, respectively. The results demonstrate that the GMR based sensor employing the MDS and the spectral interference of polarized waves with their phase difference appropriately adjusted enables a highly sensitive, hysteresis-free humidity measurement, characterized by a high FOM. Humidity sensors employing dielectric multilayers thus represent an effective alternative to available sensors, with advantages such as better mechanical and chemical stability.

Asymmetries in the Field and Angle Dependences of the Critical Current in HTS Tapes

A sharp dip in critical current has been observed in the field dependence of the critical current Ic of commercial American Superconductor Corporation (AMSC) second-generation high-temperature superconducting tapes on Ni-5%W substrates, when the field is applied parallel to the plane of the tape. The dip occurs centered at a nonzero field for one field polarity only—the polarity in which the Lorentz force in the superconductor is toward the substrate. The polarity for which the dip occurs changes with the sample orientation and current direction, remaining consistent with the direction of the Lorentz force relative to the substrate. Angle dependences of Ic in these tapes are consistent with this effect, lacking the usual 180° periodicity when measured under magnetic field strengths matching that of the dip in the field dependence. These effects are not observed on otherwise identical tapes produced with nonmagnetic Ni-9%W substrates, indicating that the ferromagnetism of the substrate plays a determining role. A finite-element model shows that this asymmetrical dip occurs naturally as a result of field concentration in the magnetic substrate, leading to a significant perpendicular component of field in the superconductor at a particular nonzero field strength.

Main and auxiliary control strategies of DFIG under asymmetrical grid voltage dips

When the Doubly Fed Induction Generator (DFIG) asymmetrical dips in the grid, the stator current will not only generate transient DC components, but also generate negative sequence components, especially when the motor impedance is small. The negative sequence component is larger, which will further deteriorate the grid voltage. In this paper, the DFIG is taken as the research object. When the grid is unbalanced, the main and auxiliary control strategies are adopted for the rotor-side converter, which can control the rotor current quickly and effectively, and improve the uninterrupted operation of the entire wind turbine. The MATLAB simulation software was used to simulate the 1.5MW wind turbine model, and the test was carried out on the 1.5MW wind turbine test bench. The simulation results and test results verify the feasibility and effectiveness of the control strategy.

An effective control solution for doubly-fed induction generator under harsh balanced and unbalanced voltage sags

This paper presents a novel control algorithm for the rotor-side converter of Doubly-Fed Induction Generators. The main goal is to endow the system with effective Low Voltage Ride Through capability, under harsh balanced and unbalanced grid voltage sags, without relying on dedicated auxiliary hardware, which is commonly adopted to sustain severe line faults. In this respect, nonlinear control theory arguments are applied to design a controller capable of mitigating oscillations (particularly on rotor currents and voltages) arising during line faults, therefore preventing the system from disconnecting for protection. The proposed solution adopts both feedforward and feedback terms. The former stems from a thoughtful analysis of the system internal dynamics, taking into account the effects of line voltage perturbations, which is exploited to design feasible state trajectories for the generator electromagnetic variables. Specifically, such references do not contain poorly-damped oscillatory modes of the machine natural dynamics, expressed in synchronously rotating frames (such components turn into slowly varying DC ones in a stationary frames). Then, a state feedback unit is designed according to modern saturated control techniques, accounting for constraints on rotor voltage, and steering real variables toward references, where priority given to rotor currents, to avoid rotor-side converter tripping due to overcurrent. In addition, a non standard line voltage reconstruction and dip detection scheme, based on adaptive state observers, is designed, to reliably cope with challenging faulty conditions. Detailed numerical simulations validate the proposed method benefits under severe symmetric and asymmetric dip scenarios.

S4. ASYMMETRIC DRUG-INDUCED PARKINSONISM IS RELATED TO PSYCHOPATHOLOGY

BackgroundDrug-Induced Parkinsonism (DIP) is the most common movement disorder induced by antipsychotics. The prevalence of DIP in chronic psychiatric populations ranges between 17 and 72% (1–3). Although, DIP is mostly symmetric, asymmetric DIP is reported in 18 to 54% of the patients. (4). There are no studies to the clinical relevance of asymmetric DIP. We investigated the prevalence of motor asymmetry in DIP and its relationship to the severity of psychopathology in a prospective study.MethodsIn a cohort study of 207 long-stay psychiatric inpatients the prevalence of DIP was assessed at least two times (mean follow-up 1.1 year) in each patient (5). DIP was assessed with the Unified Parkinson Disease Rating Scale (UPDRS) and the prevalence of persistent DIP was 56.2%. Patients with at least one time parkinsonism in the upper/lower limb(s) were included for analyses. Asymmetry of parkinsonism was calculated with the symmetry index (Figure 1). A cut-off value of ≥ 0,20 was used for the definition of asymmetric DIP. Multilevel mixed models were built to explore the relationship between asymmetry in DIP and the severity of psychopathology, measured on the Clinical Global Impression-Schizophrenia scale severity index (CGI-SCH SI).ResultsIn a cohort study of 207 long-stay psychiatric inpatients the prevalence of DIP was assessed at least two times (mean follow-up 1.1 year) in each patient (5). DIP was assessed with the Unified Parkinson Disease Rating Scale (UPDRS) and the prevalence of persistent DIP was 56.2%. Patients with at least one time parkinsonism in the upper/lower limb(s) were included for analyses. Asymmetry of parkinsonism was calculated with the symmetry index (Figure 1). A cut-off value of ≥ 0,20 was used for the definition of asymmetric DIP. Multilevel mixed models were built to explore the relationship between asymmetry in DIP and the severity of psychopathology, measured on the Clinical Global Impression-Schizophrenia scale severity index (CGI-SCH SI).DiscussionDIP is asymmetric in 1 of 5 patients. Therefore, the clinical rule that Parkinson’s disease always starts asymmetrically and such may be helpful to differentiate between Parkinson’s disease and DIP is not valid. Asymmetric presentation of DIP is of clinical relevance as it is related to the severity of psychopathology. Asymmetric DIP may alert the clinician of more severe psychopathology. Replication is indicated to examine the robustness of the relationship.

Fano Resonance in an Electrically Driven Plasmonic Device.

We present an electrically driven plasmonic device consisting of a gold nanoparticle trapped in a gap between two electrodes. The tunneling current in the device generates plasmons, which decay radiatively. The emitted spectrum extends up to an energy that depends on the applied voltage. Characterization of the electrical conductance at low temperatures allows us to extract the voltage drop on each tunnel barrier and the corresponding emitted spectrum. In several devices we find a pronounced sharp asymmetrical dip in the spectrum, which we identify as a Fano resonance. Finite-difference time-domain calculations reveal that this resonance is due to interference between the nanoparticle and electrodes dipolar fields and can be conveniently controlled by the structural parameters.

Tunable dynamic Fano resonances in coupled-resonator optical waveguides

A route toward line-shape engineering of Fano resonances in photonic structures is theoretically proposed, which uses dynamic modulation of the refractive index of a microcavity. The method is exemplified by considering coupled-resonator optical waveguide systems. An exact Floquet analysis, based on coupled-mode theory, is presented. Two distinct kinds of resonances can be dynamically created, depending on whether or not the static structure sustains a localized mode. In the former case, a single Fano resonance arises, which can be tuned in both frequency and linewidth by varying the refractive index modulation amplitude and frequency. In the latter case, two resonances, in the form of narrow asymmetric dips in the transmittance, are found, which can overlap resulting in an electromagnetically induced transparency effect.

Biexciton cascade in telecommunication wavelength quantum dots

We report on polarisation correlation from the cascaded recombination of biexcitons in a quantum dot emitting at a telecommunication wavelength. The fine structure splitting of the exciton state in this InAs/GaAs quantum dot is of the order of 100 μeV and polarisation correlation is expected. Strong polarisation correlation between the biexciton and exciton emission lines is observed under both continuous wave (CW) and pulsed laser excitation so telecom wavelength quantum dots with lower energy splittings could be suitable for entangled photon pair generation. Measurements were performed using nanowire superconducting single photon detectors (SSPDs). SSPDs offer low time-jitter and improve the resolution of features in the correlation spectra, including the asymmetric dip and peak resulting from the cascaded emission with the peak extending more than an order of magnitude above the Poissonian level.

Influence of magnetic field offsets on the resistance of magnetic barriers in two-dimensional electron gases: Experiment and simulations

Strongly localized, magnetic field structures oriented perpendicular to two-dimensional electron gases, also known as magnetic barriers, are shifted in $B$ space by homogeneous, perpendicular magnetic fields. The magnetoresistance across the barrier shows a characteristic asymmetric dip in the regime where the polarity of the homogeneous magnetic field is opposite to that of the magnetic barrier. The measurements are in quantitative agreement with semiclassical simulations, which reveal that the magnetoresistance originates from the interplay of snake orbits with $E\ifmmode\times\else\texttimes\fi{}B$ drift at the edges of the Hall bar and with elastic scattering.

Electron transport in crossed nanotubes with a point contact

The electron transport in a four-terminal nanodevice consisting of two crossed nanotubes is investigated in the framework of the Landauer-B\"uttiker formalism. The evident formula for the ballistic conductance of the device is found using a model of crossed conductive cylinders with a point contact between them. Sharp conductance dips stipulated by resonance scattering on the contact are shown to appear in the conductance of the first cylinder. The conductance between the cylinders has resonant behavior. The form and the position of resonant peaks are studied. Our results indicate that the form of asymmetric dips and peaks in the conductance differs from the well-known Fano line shape [Phys. Rev. 124, 1866 (1961)]. We have shown that the maximal value of the conductance between cylinders does not exceed a unit of the conductance quantum.

Impact tectonics in the core of the Vredefort dome, South Africa: Implications for central uplift formation in very large impact structures

Abstract— The 80 km wide Vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure. Exposure of progressively older strata in the collar of the dome and of progressively higher‐grade metamorphic rocks toward its center is consistent with differential uplift; however, the deepest levels exposed correspond to pre‐impact midcrust, rather than lower crust, as has been suggested previously. Pre‐impact Archean gneissic fabrics in the core of the dome are differentially rotated, with the angle of rotation increasing sharply at a distance of ˜16–19 km from the center. The present asymmetric dips of the collar strata, with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector, and the eccentric distribution of the pre‐impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely NW‐dipping target sequence during central uplift formation. The rocks in the core of the dome lack distinctive megablocks or large‐slip‐magnitude faults such as have been described in other central uplifts. We suggest that the large‐scale coherent response of these rocks to the central uplift formation could have been accommodated by small‐scale shear and/or rotation along pervasive pseudotachylitic breccia vein‐fractures.

Anticorrelation effect in femtosecond-pulse pumped type-II spontaneous parametric down-conversion

We report an experimental demonstration of an anticorrelation effect in femtosecond-pulse pumped type-II spontaneous parametric down-conversion. Our experimental data, which is different from that of Atat\ure et al. [Phys. Rev. Lett. 83, 1323 (2000)], confirmed the shallow symmetric ``dip'' that is predicted theoretically by Keller and Rubin [Phys. Rev. A 56, 1534 (1997)] and Grice and Walmsley [Phys. Rev. A 56, 1627 (1997)]. We show in this paper that the asymmetric dip observed in the literature is an artifact, which is caused by the asymmetric optical elements introduced into the beam path after the down-conversion process. The ``partial distinguishability'' theory suggested by Atat\ure et al. is therefore incorrect.

Multiwave mixing in semiconductor laser media.

This paper analyzes the interaction of a strong wave and one or two probe waves with an intrinsic semiconductor medium which is partially inverted by an injection current. The many-body interactions of the electron-hole excitations are described using recently developed generalized Bloch equations for semiconductors. Coupled-mode equations for the different light fields are derived and solved, and predict that the gain and the coupling coefficients experience asymmetric dips. These dips are generated by scattering of the strong field off carrier-density pulsations induced by the pump-probe interference.

Nonlinear-optical properties of biexcitons: Single-beam propagation

A nonperturbational method is developed for the calculation of the nonlinear-optical susceptibility of a system composed of two types of bosons: excitons and biexcitons. When applied to CuCl, this calculation accounts for the renormalization of the polariton dispersion relation due to the presence of the biexciton transition. It also gives a two-photon absorption towards the biexciton, with a resonance frequency which shifts with intensity. The calculated transmission spectrum for a light beam propagating through CuCl reproduces the features of the experimental spectrum. It contains an asymmetric dip at half the biexciton frequency, with a width dependent on the incident intensity. The nonperturbational treatment suggests that this width is due essentially to the shift of the absorption frequency during propagation, rather than to the collisional mechanism postulated within the perturbational framework of nonlinear optics.