Free Space Value Research Articles

Terahertz absorption properties of different graphene layers based on the Salisbury effect

Terahertz (THz) absorbers based on the Salisbury screen have attracted significant attention for high absorption performance and simple structure. Graphene is suitable for high-performance THz absorbers due to its extraordinary electronic and optical properties. The study of graphene THz absorbers based on Salisbury screens has attracted great interest, where the number of graphene layers significantly affects the interface impedance matching and absorption efficiency. In this work, we proposed a sandwich-structured graphene/Polyimide (PI) /Au THz absorber based on the Salisbury screen. The results show that the absorption peak tended to increase and then decrease with the increase in the number of graphene layers. The simulation demonstrates that the real and imaginary parts of the relative impedance of the 3-5 layer graphene absorber were 1.02 and 0.01, which achieved a better impedance matching with the free space. Meanwhile, the measured sheet resistance value of 426 Ω/sq was closest to the free-space impedance value of 377 Ω, consistent with the simulation results. The corresponding absorption reached a maximum value of 98.7% at 0.82 THz (measured). In addition, the absorption peak decreased from 98.7% to 86.7% as the angle of incidence increased from 0° to 60°. This demonstrates the advantage of wide-angle absorption. The proposed device is suitable for applications in electromagnetic shielding and imaging, while the suggested method can be employed for the fabrication of other graphene-based devices.

Dispersion of run-and-tumble microswimmers through disordered media.

Understanding the transport properties of microorganisms and self-propelled particles in porous media has important implications for human health as well as microbial ecology. In free space, most microswimmers perform diffusive random walks as a result of the interplay of self-propulsion and orientation decorrelation mechanisms such as run-and-tumble dynamics or rotational diffusion. In an unstructured porous medium, collisions with the microstructure result in a decrease in the effective spatial diffusivity of the particles from its free-space value. Here, we analyze this problem for a simple model system consisting of noninteracting point particles performing run-and-tumble dynamics through a two-dimensional disordered medium composed of a random distribution of circular obstacles, in the absence of Brownian diffusion or hydrodynamic interactions. The particles are assumed to collide with the obstacles as hard spheres and subsequently slide on the obstacle surface with no frictional resistance while maintaining their orientation, until they either escape or tumble. We show that the variations in the long-time diffusivity can be described by a universal dimensionless hindrance function f(ϕ,Pe) of the obstacle area fraction ϕ and Péclet number Pe, or ratio of the swimmer run length to the obstacle size. We analytically derive an asymptotic expression for the hindrance function valid for dilute media (Peϕ≪1), and its extension to denser media is obtained using stochastic simulations. As we explain, the model is also easily generalized to describe dispersion in three dimensions.

Minimizing the Ground Effect for Photophoretically Levitating Disks

Photophoretic levitation is a propulsion mechanism by which lightweight objects can be lifted and controlled through their interactions with light. Since photophoretic forces on macroscopic objects are usually maximized at low pressures, they may be tested in a vacuum chamber in close proximity to the chamber floor and walls. We report experimental evidence that the terrain under levitating microflyers, including the chamber floor or the launchpad from which the microflyer lifts off, can greatly increase the photophoretic lift forces relative to their free-space (midair) values. To characterize this so-called ``ground effect'' during vacuum-chamber tests, we introduce a miniature launchpad composed of three J-shaped (candy-cane-like) wires that minimize the microflyer's extraneous interactions with the underlying surfaces. We compare our J-shaped-wire launchpad with previously used wire-mesh launchpads for simple levitating Mylar-based disks with diameters of 2, 4, and 8 cm. Importantly, we discover that wire-mesh launchpads increase the photophoretic lift force by up to sixfold. A significant ground effect is also associated with the bottom of the vacuum chamber, particularly when the distance to the bottom surface is less than the diameter of the levitating disk. We provide guidelines to minimize the ground effect in vacuum-chamber experiments, which are necessary to test photophoretic microflyers intended for high-altitude exploration and surveillance on Earth or on Mars.

In-medium Λ isospin impurity from charge symmetry breaking in the Λ4H−Λ4He mirror hypernuclei

The $\Lambda$ separation energies in the mirror hypernuclei ${_{\Lambda}^4}{\rm H}-{_{\Lambda}^4}{\rm He}$ exhibit large charge symmetry breaking (CSB). Analyzing this CSB within pionless effective field theory while using partially conserved baryon-baryon SU(3) flavor symmetry, we deduce a $\Lambda -\Sigma^0$ induced in-medium admixture amplitude ${\cal A}_{I=1}\approx 1.5\%$ in the dominantly isospin $I=0$ $\Lambda$ hyperon. Our results confirm the free-space value ${\cal A}^{(0)}_{I=1}$ inferred directly within the SU(3) baryon octet by Dalitz and von-Hippel in 1964 and reaffirmed in a recent QCD+QED lattice calculation. Furthermore, exploring the consequences of SU(3) flavor symmetry on the $\Lambda$-nucleon interaction, we find that CSB is expected to impact the $S=1$ and $S=0$ spin channels in opposite directions, with the latter dominating by an order of magnitude. These observations explain a recent deduction of $\Lambda$-nucleon CSB strengths.

Magnetic and optical characteristics of Fe doped SrTiO3 perovskite compound: a first principle study

In this study, we theoretically probed the electronic, magnetic and optical characteristics of Sr1-xFexTiO3 and SrTi1-xFexO3 perovskite materials with the implementation of DFT based Wein2K code. In both doping atom is partially removed by (Fe) dopant. The substitution of Fe at A-site in SrTiO3 consequences the half-metallic ferromagnetic (FHM) character, while the substitution of Fe at B-site in SrTiO3 resulted into metallic character of the studied material. Moreover, In Sr0.5Fe0.5TiO3 the local magnetic moment of Fe is slightly decreased, whereas in SrTi0.5Fe0.5O3 the Fe magnetic moment is highly decreased from its free space value 4 µB. This reduction is due to p-d hybridization. In case of Sr0.5Fe0.5TiO3, the exchange constants N0α for s-d and N0β for p-d interaction are computed. The optical response of studied materials is described by jn.hat Sr0.5Fe0.5TiO3 is a prominent candidate for spintronics applications.

The Quantum Optics of Asymmetric Mirrors With Coherent Light Absorption

The local observables of the quantised electromagnetic field near a mirror-coated interface depend strongly on the properties of the media on both sides. In macroscopic quantum electrodynamics, this fact is taken into account with the help of optical Green’s functions which correlate the position of an observer with all other spatial positions and photon frequencies. Here we present an alternative, more intuitive approach and obtain the local field observables with the help of a quantum mirror image detector method. In order to correctly normalise electric field operators, we demand that spontaneous atomic decay rates simplify to their respective free space values far away from the reflecting surface. Our approach is interesting, since mirror-coated interfaces constitute a common basic building block for quantum photonic devices.

Strong coupling regime and bound states in the continuum between a quantum emitter and phonon-polariton modes.

We investigate the population dynamics of a two-level quantum emitter (QE) placed near a hexagonal boron nitride (h-BN) layer. The h-BN layer supports two energy phonon-polariton bands. In the case that the transition energy of the QE is resonant to them, its relaxation rate is enhanced several orders of magnitude compared to its free-space value and the population of the QE excited state shows reversible dynamics. We further show that for specific parameters of the QE/h-BN layer system, the QE population can be trapped in the excited state, keeping a constant value over long periods of time, thus demonstrating that the h-BN layer is a platform that can provide the strong light-matter interaction conditions needed for the formation of bound states in the electromagnetic continuum of modes. Semi-analytical methods are employed for determining whether such a bound state can be formed for given coupling conditions, as well as for computing the amount of initial population trapped in it. The bound states in the continuum are important for designing practical future quantum applications.

Quantum emitter interacting with a h -BN layer in the strong-coupling regime

We present that the hexagonal boron nitride ($h$-BN) layer is a platform that facilitates strong light-matter interactions. We calculate the emission spectrum of a quantum emitter (QE) interacting with the $h$-BN layer at the strong-coupling regime. The $h$-BN layer supports phonon polariton modes at the far-infrared part of the spectrum that lead to a ${10}^{4}$ confinement of light. The phonon polaritons are the main channel of relaxation of the QE, where the Purcell factor of the QE shows an enhancement of the relaxation rate above three orders of magnitude, compared to the free space value, when it is placed $100\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ above the $h$-BN layer. At smaller separation distances of the QE with the $h$-BN layer the total system operates in the strong-coupling regime, which appears as two peaks in the emission spectrum, splitting the single emission peak of the QE emitting in the free space. This energy splitting is the Rabi splitting $\ensuremath{\hbar}\mathrm{\ensuremath{\Omega}}$ that depends on the transition energy $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}$ of the QE, the position and the free-space relaxation rate of the QE, reaching values of $\mathrm{\ensuremath{\Omega}}/{\ensuremath{\omega}}_{0}=23.5%$.

Non-Markovian Spontaneous Emission Dynamics of a Quantum Emitter Near a Transition-Metal Dichalcogenide Layer

We investigate the spontaneous emission dynamics of a two-level quantum emitter near a transition-metal dichalcogenide layer, specifically, a tungsten disulfide (WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer. The optical response of the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer is obtained by electromagnetic calculations using parameters by experimental data. We find that depending on the distance between the quantum emitter, and the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer the spontaneous decay can be enhanced up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> compared with its free-space value for an emitter with transition dipole moment oriented perpendicular or tangential to the layer. We also perform quantum dynamical calculations, which indicate that the emitter's spontaneous emission dynamics changes from Markovian to weakly non-Markovian to clearly non-Markovian, featuring decaying population oscillations, as the coupling grows from relatively weak to strong, depending on the distance of the emitter to the layer, and the emitter's free-space decay rate. For quantum emitters with fast free-space decay times placed at close distances to the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer, the ultrastrong coupling regime can be reached, and we find that a significant amount of the initial population of the quantum emitter can remain trapped in it. We also quantify non-Markovianity using different measures. Similar results can also be obtained for other transition-metal dichalcogenide layers, like a tungsten diselenide (WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) layer.

Spontaneous Decay of a Dressed Harmonic Oscillator Inside a Spherical Cavity

We consider a complete study of the influence of the cavity size on the spontaneous decay of an atom excited state, roughly approximated by a harmonic oscillator. We confine the oscillator-field system in a spherical cavity of finite radius, perfectly reflective, and work in the formalism of dressed coordinates and states, which allows performing nonperturbative calculations for the probability of the oscillator to decay spontaneously from the first excited state to the ground state. In free space, we obtain known exact results and, for sufficiently small radii, we have developed a power expansion calculation on this parameter. Furthermore, for cavities of arbitrary size radius, we developed numerical computations and showed a complete agreement of this method with the exact one for free space and the power expansion results for small cavities, in this way showing the robustness of our numerical computations. We have found that, in general, the spontaneous decay of an excited state of the oscillator increases with the cavity size radius and vice versa. For sufficiently small cavities, the oscillator practically does not suffers spontaneous decay, whereas for large cavities, the spontaneous decay approaches the free-space value. On the other hand, for some particular values of the cavity radius, in which the cavity is in resonance with the natural frequency of the atom, the spontaneous decay transition probability is increased compared to the free-space case. Also, we showed how the probability of spontaneous decay goes from an oscillatory time behavior, for finite cavity radius, to almost exponential decay, for free space.

Analysis of Influence of the Dolomite Inactive Mineral Filler Quantity on Properties of Deep Cold Recycled Mixtures with Foamed Bitumen

The subject of research presented in article was an assessment of the impact of inactive mineral filler content, i.e. dolomite dust, on the recycled cold mix properties with foamed bitumen. The analysis of the impact of dolomite mineral dust was made on the basis of the experiment design type 33. It consisted of three levels of foamed bitumen factor in the range from 1.2% to 3.6% with a 1.2% step and the amount of mineral dust factor in range from 5% to 20% with a 7.5% step. Dolomite dust is a waste material derived from aggregate mining. Foamed bitumen was obtained from foaming process of the neat bitumen 50/70 at water temperature of 25ºC and its amount of 2.7%. In order to increase the cohesion of the recycled cold mix with foamed bitumen, Portland cement CEM I 32.5 R in amount of 2.0% was used. The assessment of the impact of quantitative variables was carried out in accordance with the adopted plan of the experiment that assumed the determination of physical and mechanical properties such as: resistance to climatic factors, dynamic modulus (E*) and phase angle. Analysis of the test results shown that factors quantity affects the variability of analysed features. In the entire scope of experiment plan, the obtained free space values met the requirements in required guidelines. The highest sealing of the recycled mixture was observed for the entire dolomite dust dosage but at maximum amount of foamed bitumen amounted to 3.6%. It was also observed that the maximum water resistance (TSR) was obtained for mixtures with a minimum amount of mineral dust (5%) as well as the maximum amount of dust in the recycled cold mix (20%). However, it was depended of the amount of foamed asphalt in a recycled mixture composition. The study also was considered the influence of the amount of dolomite mineral dust on the change in stiffness modulus tested by means of IT-CY and DTC-CY procedure. It was shown that either stiffness modulus measured by IT-CY or DTC-CY are foam bitumen and dust amount dependent. To assess the rheological properties, the direct compression-compression test (DTC-CY) was used. All samples were subjected to a cyclic sinusoidal constant strain with an amplitude of εo <25με. The tests were carried out in the temperature range (-7ºC, 5ºC, 13ºC, 25ºC, 40ºC) and loading time (0.1 Hz, 0.3 Hz, 1 Hz, 3 Hz, 10 Hz, 20 Hz). It was found a significant variability of recycled base stiffness depending on loading time and temperature.

DFT studies on nitrogen-rich pyrazino [2, 3-e] [1, 2, 3, 4] tetrazine-based high-energy density compounds.

By using the density functional theory method, we investigated the heats of formation (HOFs), electronic structure, detonation properties, thermal stability and sensitivity for a set of pyrazino [2, 3-e] [1, 2, 3, 4] tetrazine derivatives with different substituents and different numbers of N-oxides. Our findings reveal that the HOFs of the derivatives decrease dramatically with the increasing number of N-oxides. The effects of the substituents on the HOMO-LUMO gaps are coupled with those of the N-oxides. The calculated detonation properties point out that -NF2, -ONO2 and an increasing number of N-oxides are very helpful for improving the detonation performance of the designed derivatives. The bond dissociation energies of the weakest bonds indicate that a majority of our designed compounds have better thermal stability. The -NH2 group is very useful to decrease the free space value. Most of the derivatives have higher h50 values compared with parent molecules. Considering the sensitivity, thermal stability and detonation performance, four compounds could be considered as potential candidates of high-energy density compounds.

Yields of weakly bound light nuclei as a probe of the statistical hadronization model

The statistical hadronization model is a simple and efficient phenomenological framework in which the relative yields for very high energy heavy ion collisions are essentially determined by a single model parameter---the chemical freeze-out temperature. Recent measurements of yields of hadrons and light nuclei covering over 9 orders of magnitudes from the ALICE collaboration at the LHC were described by the model with remarkable accuracy with a chemical freeze-out temperature of 156.5 $\pm$ 1.5 MeV. A key physical question is whether the freeze-out temperature can be understood, literally, as the temperature at which the various species of an equilibrated gas of hadrons (including resonances) and nuclei chemically freeze out as the model assumes, or whether it successfully parametrizes the yield data for a different reason. The yields of weakly-bound light nuclei---the deuteron and the hypertriton---provide insights into this issue. The analysis indicates that a key assumption underlying the model---that hadrons (and nuclei), just prior to chemical freeze-out temperature, are in thermal equilibrium and are sufficiently dilute as to have particle distributions accurately described statistically by a nearly ideal gas of hadrons and nuclei with masses given by their free space values---appears to be inconsistent with the chemical freeze-out temperature output by the model, at least for these weakly-bound nuclei.

Average Linear and Angular Momentum and Power of Random Fields Near a Perfectly Conducting Boundary

The effect of a perfectly conducting planar boundary on the average linear momentum (LM), angular (momentum (AM), and power of a time-harmonic statistically isotropic random field is analyzed. These averages are purely imaginary and their magnitude decreases in a damped oscillatory manner with distance from the boundary. At discrete quasi-periodic distances and frequencies, the average LM and AM attain their free-space value. Implications for the optimal placement or tuning of power and field sensors are analyzed. Conservation of the flux of the mean LM and AM with respect to the difference of the average electric and magnetic energies and the radiation stresses via the Maxwell stress dyadic is demonstrated. The second-order spatial derivatives of differential radiation stress can be directly linked to the electromagnetic energy imbalance. Analytical results are supported by Monte Carlo simulation results. As an application, performance based estimates for the working volume of a reverberation chamber are obtained. In the context of multiphysics compatibility, mechanical self-stirred reverberation is proposed as an exploitation of electromagnetic stress.

Abnormal adsorption and desorption of nitrogen at 77 K on coals: Study of causes and improved experimental method

Low-pressure nitrogen adsorption at 77 K is a widely-used technique for determining surface area, pore structure, and pore size distribution of coal. However, some high-rank coal samples show inaccurate results when investigated by the traditional experimental method due to non-negligible amount of helium being captured in the extremely narrow micropores (< 0.7 nm) at 77 K, which affects measurement of the void volume. In this study, an improved experimental method, based on calibrating the free space value of the sample rather than using the measured value directly, was adopted to re-test the high-rank coal samples. The results showed that the adsorption/desorption isotherm of the high-rank coal samples was accurately obtained by the new experimental method, which was further used to calculate the Brunauer–Emmett–Teller specific surface area and the Barrett–Joyner–Halenda pore volume.

What is the maximum differential group delay achievable by a space-time wave packet in free space?

The group velocity of 'space-time' wave packets - propagation-invariant pulsed beams endowed with tight spatio-temporal spectral correlations - can take on arbitrary values in free space. Here we investigate theoretically and experimentally the maximum achievable group delay that realistic finite-energy space-time wave packets can achieve with respect to a reference pulse traveling at the speed of light. We find that this delay is determined solely by the spectral uncertainty in the association between the spatial frequencies and wavelengths underlying the wave packet spatio-temporal spectrum - and not by the beam size, bandwidth, or pulse width. We show experimentally that the propagation of space-time wave packets is delimited by a spectral-uncertainty-induced 'pilot envelope' that travels at a group velocity equal to the speed of light in vacuum. Temporal walk-off between the space-time wave packet and the pilot envelope limits the maximum achievable differential group delay to the width of the pilot envelope. Within this pilot envelope the space-time wave packet can locally travel at an arbitrary group velocity and yet not violate relativistic causality because the leading or trailing edge of superluminal and subluminal space-time wave packets, respectively, are suppressed once they reach the envelope edge. Using pulses of width ∼ 4 ps and a spectral uncertainty of ∼ 20 pm, we measure maximum differential group delays of approximately ±150 ps, which exceed previously reported measurements by at least three orders of magnitude.

The pion-nucleon σ term from pionic atoms

Earlier work suggested that the in-medium πN threshold isovector amplitude b1(ρ) gets renormalized in pionic atoms by ∼30% away from its ρ=0 free-space value, relating such renormalization to the leading low-density decrease of the in-medium quark condensate <q¯q> and the pion decay constant fπ in terms of the pion-nucleon σ term σπN. Accepting the validity of this approach, we extracted σπN from a large-scale fit of pionic-atom level shift and width data across the periodic table. Our fitted value σπN=57±7 MeV is robust with respect to variation of πN interaction terms other than the isovector s-wave term with which σπN was associated. Higher order corrections to the leading order in density involve some cancellations, suggesting thereby only a few percent overall systematic uncertainty. The value of σπN derived here agrees with values obtained in several recent studies based on near-threshold πN phenomenology, but sharply disagrees with values obtained in recent direct lattice QCD calculations.

The correlation between upper lip length, amount of free way space and visible portion of anterior teeth at rest position.

The aims of this study were to investigate the degree of visibility of maxillary and mandibular anterior teeth surfaces when the lips are at rest. And to evaluated its correlation with age and sex. As well as to determine the correlation of the upper lip length with the amount of clinical free way space.Two hundred adults (100 males and 100 females) were examined. All the subjects had maxillary and mandibular anterior teeth present with bilateral molar support. The measurement was taken at the midpoint of the each anterior tooth when the lips and lower jaw were at rest position. The length of upper lip was measured from the base of the columella to the tip of the philtrum at the midline of the face. Artificial landmarks (adhesive tape) were placed on the more prominent parts of the nose and chin of each subject. The clinical free way space value was obtained by subtracting measured vertical dimension of occlusion from postural vertical dimension.Females showed more of the maxillary central and lateral incisors than males. With increasing age, the amount of maxillary anterior teeth that was visible at rest position decreased. Statistically the amount of visible surface of maxillary central incisor showed significant differences with increasing age of subjects. The subjects with shorter upper lips displayed more of maxillary central and lateral incisors than those with longer upper lips. Significant differences were founded between the visible amount of maxillary central and lateral incisors and lip length. It was founded that the more increased in the length of upper lip, the more decreased in the vertical dimension as increased in the amount of clinical free way space which is founded within the recommended range (2-4mm). High significant differences were founded between the amount of clinical free way space and length of upper lip. Females showed shorter upper lip than males and high significant differences were founded between sexes and upper lip length.

The First-Principle Study of the Electronic Structure, Ferromagnetic and Thermoelectric Properties of Spinel Alloy FeAl2O4 Using mBJ Functional Approach

In this research work, the modified Becke and Johnson potential has been used to realize the actual electronic states and band gap properties of FeAl2O4. Ferromagnetism is illustrated in terms of John-Teller energy and exchange energies involved in the system due to electron spin. Moreover, the exchange constants also support the ferromagnetism in the studied compound. The reduction of the magnetic moment of Fe from its free space value and the shifting of the magnetic moment to nonmagnetic sites are the consequence of strong magnetization. In the end, the electrical conductivity and Seebeck coefficient incorporated to give a high value of the power factor and thermal efficiency for energy renewable devices.

Localized exciton modes and high quantum efficiency of a quantum emitter close to a MoS2 nanodisk

The total and radiative spontaneous emission (SE) rates of a quantum emitter (QE) are enhanced several orders of magnitudes in the presence of a ${\mathrm{MoS}}_{2}$ nanodisk, compared to their free space value, by exciting localized exciton polariton modes. The quantum efficiency can reach values above $24%$, at emission energies where the dipole resonance is excited, for small values of the material damping parameters. The radiative emission rate is enhanced 565 times compared to its free-space value. Two set of sharp resonances of the SE rate emerge, which are connected with the excitation of the exciton energies of the ${\mathrm{MoS}}_{2}$ nanodisk. When the emission energy is in the halo, between the first two peaks of the normalized SE rate, the quantum efficiency has its largest value of $53%$, where the radiative emission is enhanced 4.3 times compared to the free-space value. The high values for the quantum efficiency are sustained over distances up to $40\phantom{\rule{0.16em}{0ex}}\text{nm}$, where the radiative spontaneous emission can compete with the nonradiative emission.