Single Resonant State Research Articles

Multi-mode coupling in a H-shaped metamaterial structure in terahertz frequency

Mode coupling can not only effectively control the frequency, amplitude and linewidth of the transmission spectrum, but also improve the Q-factor of the spectrum. Herein, we propose a H-shaped metamaterial, in which the dipole mode, LC mode and lattice mode can be excited selectively, and each mode frequency can be independently tuned by changing the polarization of incident THz wave as well as the lattice constant of the metamaterial structure, thus allowing greater degrees of freedom to customize the polarization components of different properties in the system. Under different polarization directions, the strong coupling between the lattice mode and the inductance-capacitance (LC) mode as well as the lattice mode and the dipole mode is realized, which makes the transmission resonance Q-factor of the hybrid state increase to 8 times that of the single resonance state, and the obvious anti-crossing phenomenon is observed. In addition, the LC mode and the dipole mode can be excited simultaneously, and the coupling between these two modes successfully excites a bound states in the continuum with an infinite Q-factor.

Magnetic Dipole Transition in ^{48}Ca.

The magnetic dipole transition strength B(M1) of ^{48}Ca is dominated by a single resonant state at an excitation energy of 10.23MeV. Experiments disagree about B(M1) and this impacts our understanding of spin flips in nuclei. We performed abinitio computations based on chiral effective field theory and found that B(M1: 0^{+}→1^{+}) lies in the range from 7.0 to 10.2 μ_{N}^{2}. This is consistent with a (γ,n) experiment but larger than results from (e,e^{'}) and (p,p^{'}) scattering. Two-body currents yield no quenching of the B(M1) strength and continuum effects reduce it by about 10%. For a validation of our approach, we computed magnetic moments in ^{47,49}Ca and performed benchmark calculations in light nuclei.

Terahertz magnetic excitation in antiferromagnets: atomistic spin simulations versus a coupled pendulum model

Understanding and manipulating of the antiferromagnetic (AF) ultrafast spin dynamics in antiferromagnets (AFMs) is a crucial importance issue because of the promising applications in terahertz spintronic devices. In this study, an analytical theory extended from the classic coupled pendulum model has been developed to describe the intrinsic magnetic excitation of AFMs. The derived frequency dispersion of the AF resonances has been further checked by using the atomistic-level Landau–Lifshitz–Gilbert simulations. We show that the rutile crystalline AFM MnF2 possess two separate resonance modes at low magnetic fields: high frequency mode with right-handed polarization and low frequency mode with left-handed polarization. In the absence of magnetic field, these two resonance modes could degenerate into a single resonance state. When the applied magnetic field is higher than the spin-flip field, the system behaves a quasi-ferromagnetic mode. Both quantitative and qualitative agreement with atomistic simulation results confirm the theoretical picture of the AF resonance dynamics. This study provides a simple but physical understanding of the ultrafast dynamics of AF excitations.

Terahertz bandstop-to-bandpass converter based on VO2 hybrid metasurface

We experimentally demonstrated a terahertz (THz) bandstop-to-bandpass converter based on vanadium dioxide (VO2) hybrid metasurface. The unit cell consists of gold and VO2 hybrid structures standing on a high resistance silicon substrate. Simulation and experimental results show that for y-polarized waves, the device’s function converts from bandstop filtering to bandpass filtering when VO2 transits from insulating state to metallic state induced by the Joule heat produced by the heating wire on the surface. The operation principle is explained by combining the surface current distribution and the LC equivalent circuit model. For x-polarized waves, when VO2 changes from insulating state to metallic state, the response of the device to THz wave changes from single resonant state to double resonant state. This phenomenon originates from the conductance coupling effect in plasma optics. The proposed device realizes active control of THz waves and is expected to be used in the fields of THz communication, imaging and electromagnetic switching.

Capture reactions into borromean two-proton systems at rp-waiting points

We investigate even-even two-proton borromean systems at prominent intermediate heavy waiting points for the rapid proton capture process. The most likely single-particle levels are used to calculate three-body energy and structure as a function of proton-core resonance energy. We establish a linear dependence between two- and three-body energies with the same slope, but the absolute value slightly dependent on partial wave structure. Using these relations we estimate low-lying excited states in the isotones following the critical waiting points. The capture rate for producing a borromean bound state is described based on a full three-body calculation for temperatures about 0.1 – 10 GK. In addition, a simple rate expression, depending only on a single resonance state, is found to comply with the three-body calculation for temperatures between 0.1 and 4 GK. The rate calculations are valid for both direct and sequential capture paths. As a result the relevant path of the radiative capture reactions can be determined.

Phase-locking of multiple magnetic droplets by a microwave magnetic field

Manipulating dissipative magnetic droplet is of great interest for both the fundamental and technological reasons due to its potential applications in the high frequency spin-torque nano-oscillators. In this paper, a magnetic droplet pair localized in two identical or non-identical nano-contacts in a magnetic thin film with perpendicular anisotropy can phase-lock into a single resonance state by using an oscillating microwave magnetic field. This resonance state is a little away from the intrinsic precession frequency of the magnetic droplets. We found that the phase-locking frequency range increases with the increase of the microwave field strength. Furthermore, multiple droplets with a random initial phase can also be synchronized by a microwave field.

Duschinsky, Herzberg-Teller, and Multiple Electronic Resonance Interferential Effects in Resonance Raman Spectra and Excitation Profiles. The Case of Pyrene.

We show that a recently developed time-independent approach for the calculation of vibrational resonance Raman (vRR) spectra is able to describe Duschinsky and Herzberg-Teller (HT) effects acting on a single resonant state, together with interferential contributions arising from multiple electronic resonances, allowing us to investigate in detail how their interplay determines both the vRR spectra at selected wavelengths and the Raman excitation profiles. We apply this methodology to the study of the spectra of pyrene in acetonitrile, an ideal system since it exhibits three close-lying electronic transitions that are bright but also subjected to HT effects. To single out the different contributions to vRR line shapes we adopted two different adiabatic models for resonant-state potential energy surfaces, namely, Adiabatic Shift (only accounting from equilibrium geometry displacements) and Adiabatic Hessian (AH, including also the Duschinsky effects), and Franck-Condon (FC) or HT approximations for the transition dipole. We show that, on balance, FC+HT calculations within the AH model provide the best agreement with experiment. Moreover, our methodology permits to individuate bands in the experimental spectra due to the simultaneous contribution of more than one resonant state and to point out and analyze interferential effects between the FC and HT terms in each resonance Raman process, together with FC-HT and HT-HT interferences between different electronic states.

Ultrafast dynamics and photochemistry of π–π* excited trans‐azobenzene—a comprehensive analysis of resonance Raman intensities

AbstractAzobenzene and its derivatives have widely been recognized as systems with potential applications such as optical switching elements, and the mechanism of trans‐cis photoisomerization, which is the basis for all these applications, has long been sought. The long‐standing controversy on the relaxation mechanism of π–π* (S2) excited trans‐azobenzene (TAB) is addressed in this paper. The role of S1S2 intersection and the possible involvement of a third state in the S2 photochemistry are investigated. The excited state structure and dynamics of TAB are extracted by modeling the Raman excitation profiles (REPs) of the S1 and S2 states and the absorption spectrum of TAB by Heller's time‐dependent formalism. The REPs of S2 are modeled in the single resonant state approximation by explicitly including the eight most Franck–Condon (FC) active modes. The obtained NN and CN bond lengths of S2 and the intial dynamics in these coordinates following photoexcitation to S2 are in agreement with recent theoretical calculations. The structural parameters of S1 are obtained by exploiting the nature of interference in the REPs of S1. From these parameters a three‐mode–two‐state vibronic coupling model, based on harmonic diabatic potentials and linear coupling, is formulated to study the S2S1 internal conversion dynamics. The results indicate that the S1S2 intersection is not directly accessible within electronic dephasing time of S2 to cause significant population decay from S2 to S1. The current conception of π–π* relaxation mechanisms in TAB is reviewed and revised on the basis of our observations and also from recently reported results of time‐resolved and theoretical studies. The role of a third state (S3), which appears to be an integral part of in the relaxation mechanism of S2, is pointed out and the feasibility of two competing relaxation pathways for S2 state is identified. One is the direct rotationless S2S1(C2h) internal conversion and the other is S2S3 internal conversion along rotational pathway followed by a branching of the S3 population to S0 and S1. Copyright © 2008 John Wiley & Sons, Ltd.

Analysis of angular distributions in γN → πeta;N

Angular distributions in the final state of π0η photoproduction on nucleons are considered. As a formal base the familiar isobar model is used in which the π0ηN state is a product of the resonance decay into ηΔ(1232) and πS 11(1535) channels. One of the principal assumptions used is that in the actual energy region the reaction is dominated by a single resonance state. The developed formalism can serve as a tool for testing spin and parity of that resonance.

Thermal resonant tunneling rates by a generalized flux averaging method

AbstractThe calculation of the thermal rate constant as a time integral over flux‐flux correlation functions is a challenging task when the potential energy along the reaction coordinate cannot be associated with a distinctive single barrier. In the case of resonant tunneling through a double barrier potential, the calculations may become formidable due to the population of long‐lived resonance states and the corresponding long time‐decay of the flux‐flux correlation functions. The flux averaging method was introduced recently in order to circumvent this problem in cases where the long time dynamics is due to a single resonance state with the longest lifetime in the system. In this work we generalize the method for calculations of thermal resonant‐tunneling rates in systems of many resonances, where the long time‐decay is accompanied by an internal dynamics within the quasi‐bound system. This extra complication is handled by additional averaging of flux‐flux correlation funcation over the time period of the internal dynamics. The result is an exact expression for the rate constant in terms of a linear combination of time integrals over flux‐flux correlation functions, which reaches its asymptotic time limit in a short (direct scattering) time, regardless of the long time‐decay of the flux‐flux correlation functions. This is derived for an analytic model system, and demonstrated in a numerical simulation of resonant tunneling through a double barrier potential.

Hole states in GaAs/AlAs double-barrier structures and multiple quantum wells

We present a microscopic pseudopotential-based calculation including spin-orbit interaction for hole tunnelling in finite GaAs*AlAs multiple quantum wells. We have examined the light hole-heavy hole (mixing) in a GaAs/AlAs double-barrier structure and how the resonant states evolve into minibands in multiple quantum well structures. We find new features in the transmission characteristics which could not have been obtained from simple models. The double-barrier structure results show strong light-hole-heavy-hole mixing and quantum interference behaviour characterized by transmission 'antiresonances' as well as the usual resonances. More striking are the results for multiple quantum wells, where there is a loss in number of transmission resonant states for the heavy-hole minibands. For thicker barriers the heavy-hole miniband collapses and only a single resonant state is left, while the light-hole minibands remain intact. We interpret the lost resonances as being resonant contact interface states. This anomalous behaviour of formation of a contact interface state instead of a heavy-hole miniband is argued to be due to strong interface light-hole-heavy-hole mixing and the fact that for all but the thinnest AlAs barriers, only the evanescent light-hole barrier channel mediates the interwell coupling. This anomalous behaviour, which could not have been predicted from a calculation on an infinite superlattice, has consequences for transport measurements.

Detection and analysis of the 1P0 and 3P0 'shape' resonances of H- using stabilization

Stabilization techniques are used to investigate the existence of 'shape' resonances of H- above N=2. For each of the 1P0 and 3P0 symmetries, the authors only detect a single resonance state: the energy of the singlet one, -0.1244 Hartree, agrees with previous calculations and measurements; and the triplet resonance, of energy -0.1234 Hartree, has not yet been detected experimentally. Both states had been predicted using the hyperspherical coordinate method. On the other hand, further predictions of this method relating to series of (K=-1, T=0) resonances and on the 'size' of the 'shape' resonances are seen to be probably invalid. The 'shape' resonances have a complex configurational structure and a 'size' of about 80 a0.

Electron excitation of the 2p state of atomic hydrogen near threshold

The absolute total cross section for excitation of the 2p state of atomic hydrogen by electrons is examined in detail in the first electron volt above the 2p threshold. Only a single resonant state ('P) is found just above the threshold. An absolutevalue of the total 2p excitation cross section of 0*276_',4b"I," xuo2 at 11.0 eV is determined by calibration of the apparatus relative to the differential elastic scattering cross section value of 0.727 xaoa sr-' at 60" and 8.70 eV and by calibration of the Lyman alpha detector. These experimental measurements do not require any corrections for cascade radiation and do not depend upon any theoretical calculations for normalization. The present values agree well with the various three state close coupling calculations but disagree with the six-state close coupling calculations as well as all previous experimental values which were normalized to the Born approximation calculations.

(n, α) and (n, p) reactions in silicon at neutron energies up to 5.5 MeV

The energy spectra of the charged particles produced in the (n, p) and (n, α) reactions in silicon are analysed using silicon detectors as targets. The excitation functions for the Si 28(n, p) and Si 29(n, α) transitions are given from 3.7 to 5.5 MeV. The observed fluctuations can hardly be attributed to single resonant states; it is suggested to interpret them as due mainly to fluctuations in the level density of the intermediate nuclei.