Small Number Of Periods Research Articles

Narrative Restrictions and Proxies

We compare two approaches to using information about the signs of structural shocks at specific dates within a structural vector autoregression (SVAR): imposing “narrative restrictions” (NR) on the shock signs in an otherwise set-identified SVAR; and casting the information about the shock signs as a discrete-valued “narrative proxy” (NP) to point-identify the impulse responses. The NP is likely to be “weak” given that the sign of the shock is typically known in a small number of periods, in which case the weak-proxy robust confidence intervals in Montiel Olea, Stock, and Watson are the natural approach to conducting inference. However, we show both theoretically and via Monte Carlo simulations that these confidence intervals have distorted coverage—which may be higher or lower than the nominal level—unless the sign of the shock is known in a large number of periods. Regarding the NR approach, we show that the prior-robust Bayesian credible intervals from Giacomini, Kitagawa, and Read deliver coverage exceeding the nominal level, but which converges toward the nominal level as the number of NR increases.

Undulators Consisting of Magnetized Helices

A ferromagnetic helix magnetized axially or radially is apparently the simplest helical undulator. Four helices with alternating magnetization can provide a stronger helical field than a pair of conventional planar Halbach arrays. Such undulators and microundulators seem promising for various powerful high frequency sources. The fields of infinite helices uniformly magnetized in the longitudinal and radial directions are found analytically by integration of contributions from elementary dipoles. The analytical values of the field magnitudes are close to the numerically calculated ones for a helix with a relatively small number of periods five–ten. According to simulation, a magnitude of about 1 T can be obtained on the axis of a system of helices with small inner radii.

Waiting for the Next Factor Wave: Daily Rebalancing around Market Cycle Transitions

To deliver historically observed factor premiums, long-only factor investing relied heavily on a small number of periods, when factors realized outsized returns in the midst of changing market leadership. This article shows that by rebalancing factor funds more frequently during these periods—rebalancing on a daily basis instead of monthly or biannually—investors would have achieved significantly higher factor premiums, effectively doubling the historically observed premiums of many factors. These findings indicate that to harvest factor premiums to their maximal potential, skill is needed on the part of the fund manager—an ability to tell the right moment to aggressively rebalance. TOPICS:Analysis of individual factors/risk premia, factor-based models Key Findings ▪ Historically, long-only factor investing relied heavily on a small number of periods with outsized returns to deliver factor premiums. These periods coincided with market cycle transitions when the makeup of market leadership changed dramatically. ▪ By rebalancing frequently—daily instead of every month or six months—during market cycle transitions, a factor investor would have achieved significantly higher factor premiums, effectively doubling the historically observed premiums. This finding holds true even after accounting for much higher transaction costs driven by the higher turnover. ▪ Our findings indicate that an ability to tell the right moment to rebalance more frequently may be central to harvesting factor premiums to their maximal potential.

Comparison of the Efficiency of Promising Heterostructure Frequency-Multiplier Diodes of the THz-Frequency Range

The results of theoretical and experimental comparison of the signal-conversion efficiency by diodes based on superlattices with a small number of periods (N-like current–voltage characteristic) and promising frequency-multiplier diodes (varactors) (parabolic capacitance–voltage characteristic) are presented.

Reflectivity of 1D photonic crystals: A comparison of computational schemes with experimental results

We report the reflectivity of one-dimensional finite and semi-infinite photonic crystals, computed through the coupling to Bloch modes (BM) and through a transfer matrix method (TMM), and their comparison to the experimental spectral line shapes of porous silicon (PS) multilayer structures. Both methods reproduce a forbidden photonic bandgap (PBG), but slowly-converging oscillations are observed in the TMM as the number of layers increases to infinity, while a smooth converged behavior is presented with BM. The experimental reflectivity spectra is in good agreement with the TMM results for multilayer structures with a small number of periods. However, for structures with large amount of periods, the measured spectral line shapes exhibit better agreement with the smooth behavior predicted by BM.

Majority dynamics with one nonconformist

We consider a system in which a group of agents represented by the vertices of a graph synchronously update their opinion based on that of their neighbours. If each agent adopts a positive opinion if and only if that opinion is sufficiently popular among his neighbours, the system will eventually settle into a fixed state or alternate between two states. If one agent acts in a different way, other periods may arise. We show that only a small number of periods may arise if natural restrictions are placed either on the neighbourhood structure or on the way in which the nonconforming agent may act; without either of these restrictions any period is possible.

Cascade excitation mechanism of energy levels in the presence of a disordered photonic structure

The impact disordered coating has on the energy level excitation of its molecules via a cascade mechanism under the effects of terahertz pulses with a small number of periods is investigated. It is shown that the presence of a disordered structure can reduce the intensities of spectral lines induced by the cascade process even if the structure is behind the medium.

Simulation of electron transport in GaAs/AlAs superlattices with a small number of periods for the THz frequency range

The electron transport in superlattices based on GaAs/AlAs heterostructures with a small number of periods (6 periods) is calculated by the Monte Carlo method. These superlattices are used in terahertz diodes for the frequency stabilization of quantum cascade lasers in the range up to 4.7 THz. The band structure of superlattices with different numbers of AlAs monolayers is considered and their current–voltage characteristics are calculated. The calculated current–voltage characteristics are compared with the experimental data. The possibility of the efficient application of these superlattices in the THz frequency range is established both theoretically and experimentally.

Vertical Transport in GaN/AlGaN Resonant Tunneling Diodes and Superlattices

Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy (T–E) and the current density–voltage (J–V) characteristics. Calculations of both T–E and J–V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J–V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.

Low‐frequency symmetric waves in fluid‐filled boreholes and pipes with radial layering

ABSTRACTMany tasks in geophysics and acoustics require estimation of mode velocities in cylindrically layered media. For example, acoustic logging or monitoring in open and cased boreholes need to account for radial inhomogeneity caused by layers inside the borehole (sand screen, gravel pack, casing) as well as layers outside (cement, altered and unaltered formation layers). For these purposes it is convenient to study a general model of cylindrically layered media with inner fluid layer and free surface on the outside. Unbounded surrounding media can be described as a limiting case of this general model when thickness of the outer layer is infinite. At low frequencies such composite media support two symmetric modes called Stoneley (tube) and plate (extensional) wave. Simple expressions are obtained for these two mode velocities valid at zero frequency. They are written in a general form using elements of a propagator matrix describing axisymmetric waves in the entire layered composite. This allows one to apply the same formalism and compute velocities for n‐layered composites as well as anisotropic pipes. It is demonstrated that the model of periodical cylindrical layers is equivalent to a homogeneous radially transversely isotropic media when the number of periods increases to infinity, whereas their thickness goes to zero. Numerical examples confirm good validity of obtained expressions and suggest that even small number of periods may already be well described by equivalent homogeneous anisotropic media.

Analysis of a metallic reflector grating with the influence of the joule loss taken into account

Reflection characteristics of metallic waveguide gratings with rectangular grooves are investigated with the effect of the Joule loss taken into account. For this purpose, the scattering matrix for each step discontinuity is first found by using the mode-matching method with an impedance boundary condition at the surface of the conductor. Then, the scattering matrix for the overall waveguide grating is obtained by generalized transfer matrix methods. Numerical results show that Joule loss occurs mainly at horizontal wall sections. In order to suppress the loss of a reflector grating, it is shown that it should have a deep grating depth with a small number of periods. This configuration coincides with that for a compact and wideband reflector grating, which can well support single-mode operation, as was pointed out in our previous work.

Some causal lessons from macroeconomics

Causality has long been regarded among economists as a metaphysical mine'eld, best to be avoided. Yet, at the same time, the notion of cause and e*ect structures our ordinary lives and seems essential for understanding policy actions. Peter Adams et al.’s “Healthy, Wealthy, and Wise?” is welcome as an example of the relatively recent interest of microeconomists in overcoming their metaphysical aversions in order to say something useful about cause and e*ect. Their study is carefully executed and aims to resolve a well-posed causal question. The authors’ panel-data approach is closely analogous to the vector-autoregression (VAR) framework of macroeconomists. I want to pursue some of those analogies as they relate to causal inference. It is important to acknowledge that, because of the small number of periods, some of the technical issues in the estimation of panel data are di*erent from typical time-series data. The issues related to casual interpretation, however, do not depend on these di*erences. Adams et al.’s notion of cause can be summarized as time order plus explanatory power plus invariance. I have argued (Hoover, 2001) for a structural account of causation to which each of these three elements is related in a complex way. Macroeconomic examples demonstrate that none of these elements adequately de'nes causality (see Hoover, 2001, especially Chapters 6 and 7). But some of the special features of macroeconomics are less salient in microeconomic contexts, and it is at least plausible that Adams et al.’s criteria are discriminating enough for their application. To clarify ideas, consider a schematic structural model in which health (H) and wealth (W ) are related in time as

A simple recursive formula for calculating the S-parameters of finite periodic structures

A simple and efficient recursive formula to calculate the S-parameters of large, finite periodic structures is introduced. The technique utilizes measured or computed S matrices of structures with a small number of periods. The formula is verified on a simple rectangular waveguide with periodic gratings. Results from the formula are plotted against similar results obtained from finite-difference time-domain (FDTD) simulations, and they show excellent agreement. This formula can be used to compute the S-parameters of large microwave or optical periodic structures, which, by virtue of their finiteness may not be open to the application of periodic boundary conditions.

On the linkage of real interest rates between the US and Canada: some additional empirical evidence

This paper reexamines the linkage of real interest rates between the US and Canada. After examining the existence of a one-to-one long-run relationship between these two interest rates, we assess the degree of departure from the long-run relationship and the speed of adjustment to it following an exogenous shock in one of the markets. Our empirical results, based on data from approximately two decades, indicate that: (i) there exists a one-to-one long-run relationship and (ii) the extent of departure is small compared with the magnitude of a shock and the departure decays within a reasonably small number of periods.

Organic microcavities based on thermally evaporated TeO x-LiF dielectric mirrors

We report on the realization of high-quality organic microcavities consisting of distributed Bragg reflectors (DBRs) based on lithium fluoride (LiF) and tellurium dioxide (TeO x ) deposited by thermal evaporation. The materials are transparent in the range from 350 nm to 5 μm and have an evaporation temperature of about 1000 K . The large difference in the refractive index (about 0.9 in the visible and near infrared range) allows one to obtain a reflectivity higher than 99% over a spectral region about 200 nm wide with a small number of periods. The mirror deposition technique is suitable for the fabrication of organic quantum microcavities in a single deposition process. Three fully evaporated organic λ cavities with Phyrrometene 580 as active material are described. The cavities show Q-value up to 300, good uniformity and reproducibility.

Organic μ cavities based on thermally evaporated TeOx-LiF distributed Bragg reflectors

We report on the realization of high-quality organic microcavities consisting of distributed Bragg reflectors based on lithium fluoride (LiF) and tellurium dioxide (TeOx) deposited by thermal evaporation. The materials are transparent in the range of 350 nm–5 μm and have an evaporation temperature of about 1000 K. The large difference in the refractive index (about 0.9 in the visible and near-infrared range) allows one to obtain reflectivity higher than 99% over a spectral region about 200 nm wide with a small number of periods. The mirror deposition technique is suitable for the fabrication of organic quantum microcavities in a single deposition process. Three fully evaporated organic λ cavities with Phyrrometene 580 as the active material are described. The cavities show a Q value of up to 300, good uniformity, and reproducibility.

Electron escape rate from coupled multiple quantum wells: Effects of coherence length and hopping.

Electron escape rates from coupled quantum wells with a small number of periods have been determined by measuring the photoluminescence decay time. In the absence of an electric field, the escape time depends strongly on the coherence length of the wave function which is estimated to extend over six well layers. The escape time of electrons localized by an electric field reflects the hopping probability which is affected by the wave-function overlap between adjacent well layers. This probability is enhanced when LO-phonon-assisted hopping is possible.

Self-consistent calculation of spontaneous emission and small-signal gain in high-voltage free-electron lasers

Using the Weisskopf–Wigner technique, we develop a self-consistent quantum-electrodynamic (SCQED) theory of spontaneous emission of radiation and single-photon small-signal gain for high-voltage free-electron lasers (FEL’s). For the first time to our knowledge, excellent agreement is obtained between the predictions and the experimental observations simultaneously for line shift, linewidth, and gain. The SCQED theory predicts line shift and broadening caused by quantum-mechanical effects, which are not predicted by the classical or conventional FEL theories but have been observed. Excellent agreement is obtained between the spontaneous-emission spectra predicted by the SCQED theory and the 1980–1981 ACO [ J. Phys. Lett.41, L-549 ( 1980); Free Electron Generators of Coherent Radiation ( Addison–Wesley, Reading, Mass., 1981, 1982), Vols. 7, 8; the Stanford 10.6-μm [ Phys. Rev. Lett.36, 717 ( 1976)]; and the Stanford 3.4-μm [ Phys. Rev. Lett.38, 892 ( 1977)] FEL experimental spectra. This agreement is much better than that of the prediction from the classical or conventional FEL theory, which gives highly noticeable errors. We show that the spontaneous-emission spectrum that is obtained from classical or conventional FEL theories is valid only in the limit of a short undulator that contains a small number of periods. The small-signal gain derived from the SCQED theory is shown to reduce to Colson’s gain formula [ Phys. Lett.64A, 190 ( 1977)] in the classical limit. However, the SCQED theory predicts significant reductions in the small-signal gain that agree well with the ACO gain data [ Opt. Commun.40, 373 ( 1982)] and are not predicted well by Colson’s formula. Because of the nonnegligible finite electron state lifetime, it is discovered that a fundamental physical gain limit exists that is universal to all types of FEL within the limits of the single-photon transition scheme considered (i.e., if multiphoton effects are ignored). Finally, the implications of the theoretically obtained results are discussed for practical conditions of experimental interest. It is shown that under practical experimental conditions quantum effects can be quite important in FEL’s.

Optical and structural characterization of InGaAs/GaAs superlattices with increasing number of periods

We have studied the evolution of photoluminescence (PL) and photoluminescence excitation (PLE) spectra together with X-ray double diffraction of a series of MOVPE grown In 0.16Ga 0.84As/GaAs strained layer superlattices (SLS) as the number of periods increases from 5 to 20 while the composition and the thickness of the layers remain constant (70 Å). The structural quality of the samples with a small number of periods is excellent whereas for those with a large number of periods the rocking curves and the photoluminescence linewidth show a non-gaussian line broadening attributed to inhomogeneous strain relaxation. In the strained superlattice several well resolved interminiband transitions are observed and can be easily fitted using the experimental structural parameters.