Anisotropic Vacuum Research Articles

Atom-passivated GeC nanosheets for photocatalytic overall water splitting with high solar-to-hydrogen conversion efficiency

The high solar-to-hydrogen conversion efficiency (ηSTH′) of the photocatalytic overall water splitting can be achieved by well-designed nanostructures. Here, we describe that the high ηSTH′ of hydrogen evolution reaction (HER) can be achieved on the atom-passivated GeC nanosheets with the irradiation of sunlight. The suitable geometrical structures of eight GeC nanosheets passivated by different atoms are identified by systematic optimizations, and the stabilities are confirmed by ab initio molecular dynamics simulations. According to the anisotropic vacuum levels induced by the passivated atoms, 16 photocatalytic overall water splitting schemes are constructed for each nanosheet, and the qualified ones for HER and oxidation evolution reaction (OER) are recognized by the redox potentials and the charge density distributions of the band alignments. Then, the ηSTH′s are evaluated with bandgaps and overpotentials. Remarkably, a maximum ηSTH′ of 16.41% can be achieved for the F-GeC-SeF nanosheet, and a higher 20.74% is observed at 3% tensile strain. The optical absorption, mobility, and Gibbs free energy changes in HER and OER are calculated to understand the photocatalytic performances driven by the newfound nanosheets. All the results support that the atoms-passivated GeC nanosheets are the potential candidates for developing sunlight photocatalysts for producing hydrogen from overall water splitting.

High photocatalytic solar-to-hydrogen efficiency by the edge-passivated antimonene nanoribbons with anisotropic vacuum levels

The anisotropic vacuum levels on the different edges of antimonene nanoribbon are constructed by modifying the edges with different atoms, which simultaneously induces large overpotentials and narrows the bandgap, and results in high solar-to-hydrogen efficiency (η′STH). The optimized geometrical structures are recognized, and the stabilities are vindicated with the ab initio molecular dynamics simulations. Sixteen possible schemes of oxidation and reduction potentials are constructed for the hydrogen and halogen atoms modified antimonene nanoribbon based on the band edges and vacuum levels along with four directions of the nanoribbon. The achievable ones are confirmed by the charge density distributions and overpotentials of the band edges. The maximum η′STH of the best scheme can reach 32.11%, and the Gibbs free energy changes are 0.98–1.57 eV, which indicates that vacuum-level engineering is a promising way of designing catalysts for photocatalytic overall water-splitting to efficiently manufacture hydrogen.

Influence of Additional Micro-Sized Particles of Dy–Nd–Cu–Al on Magnetic Properties of Sintered Nd–Fe–B Magnets

In this work, we investigated the influence of concentration of the additional micro-sized particles of Dy40Nd30Al30 and Nd40Cu30Al30 on magnetic properties of the sintered Nd16.5Fe77B6.5 magnets. The additional particles with size in the range of 1-3 μm were prepared by ball milling method and then mixed into micrometer Nd16.5Fe77B6.5 master powder with different weight fractions before magnetic anisotropic pressing, vacuum sintering and annealing. The results show that the coercivity of the sintered Nd-Fe-B magnets can be improved considerably by introducing additional particles to the grain boundaries. The improvement of the coercivity Hc of the magnets is clearly dependent on the composition and concentration of the additional microparticles. The Hc increases linearly from 8.5 kOe to 17 kOe with increasing the weight fraction of the Dy40Nd30Al30 microparticles from 0 to 5%. Meanwhile, the coercivity of the magnet reaches a maximum value of 11.7 kOe with 4 wt% addition of Nd40Cu30Al30. The quite high maximum energy products, (BH)max > 30 MGOe, were also obtained for the magnets added with the microparticles. The obtained hard magnetic parameters of the magnets can be applied in practice.

Mirror symmetry of quantum Yang\u2013Mills vacua and cosmological implications

We find an argument related to the existence of a mathbb {Z}_2-symmetry for the renormalization group flow derived from the bare Yang–Mills Lagrangian, and show that the cancellation of the vacuum energy may arise motivated both from the renormalization group flow solutions and the effective Yang–Mills action. In the framework of the effective Savvidy’s action, two Mirror minima are allowed, with exactly equal and hold opposite sign energy densities. At the cosmological level, we explore the stability of the electric and magnetic attractor solutions, both within and beyond the perturbation theory, and find that thanks to these latter the cancellation between the electric and the magnetic vacua components is achieved at macroscopic space and time separations. This implies the disappearance of the conformal anomaly in the classical limit of an effective Yang–Mills theory. In this picture, the tunneling probability from the Mirror vacua to the other vacua is exponentially suppressed in the quantum non-thermal state – similarly to what happens for electroweak instantonic tunneling solutions. Specifically, we show that, in a dynamical Friedmann–Lemaître–Robertson–Walker (FLRW) cosmological background, the Nielsen–Olsen argument – on the instability of uniform chromo-electric and chromo-magnetic Mirror vacua – is subtly violated. The chromo-magnetic and chromo-electric uniform vacua are unstable only at asymptotic times, but at those times the attractor to a zero energy density is already reached. The two vacua can safely decay into one anisotropic vacuum that has zero energy-density inside the Fermi confinement volume scale. We also discover a new surprising pattern of solitonic and anti-solitonic space-like solutions, which are sourced by the Yang–Mills dynamics coupled to the Einstein’s equations in FLRW. We dub such non-perturbative configurations, which are directly related to dynamical cancellation mechanism of the vacuum energy, as chronons, or chi -solutions.

Generic Features of Thermodynamics of Horizons in Regular Spherical Space-Times of the Kerr-Schild Class

We present a systematic review of thermodynamics of horizons in regular spherically symmetric spacetimes of the Kerr-Schild class, d s 2 = g ( r ) d t 2 − g − 1 ( r ) d r 2 − r 2 d Ω 2 , both asymptotically flat and with a positive background cosmological constant λ . Regular solutions of this class have obligatory de Sitter center. A source term in the Einstein equations satisfies T t t = T r r and represents an anisotropic vacuum dark fluid ( p r = − ρ ), defined by the algebraic structure of its stress-energy tensor, which describes a time-evolving and spatially inhomogeneous, distributed or clustering, vacuum dark energy intrinsically related to space-time symmetry. In the case of two vacuum scales it connects smoothly two de Sitter vacua, 8 π G T ν μ = Λ δ ν μ as r → 0 , 8 π G T ν μ = λ δ ν μ as r → ∞ with λ < Λ . In the range of the mass parameter M c r 1 ≤ M ≤ M c r 2 it describes a regular cosmological black hole directly related to a vacuum dark energy. Space-time has at most three horizons: a cosmological horizon r c , a black hole horizon r b < r c , and an internal horizon r a < r b , which is the cosmological horizon for an observer in the internal R-region asymptotically de Sitter as r → 0 . Asymptotically flat regular black holes ( λ = 0 ) can have at most two horizons, r b and r a . We present the basic generic features of thermodynamics of horizons revealed with using the Padmanabhan approach relevant for a multi-horizon space-time with a non-zero pressure. Quantum evaporation of a regular black hole involves a phase transition in which the specific heat capacity is broken and changes sign while a temperature achieves its maximal value, and leaves behind the thermodynamically stable double-horizon ( r a = r b ) remnant with zero temperature and positive specific heat. The mass of objects with the de Sitter center is generically related to vacuum dark energy and to breaking of space-time symmetry. In the cosmological context space-time symmetry provides a mechanism for relaxing cosmological constant to a certain non-zero value. We discuss also observational applications of the presented results.

Influence of additional compounds on coercivity of sintered Nd-Fe-B magnets

In this paper, we investigated the influence of additional compounds of Dy-Nb-Al, Nd-Cu-Al… on the coercivity of the sintered Nd16.5Fe77B6.5 magnets. The additional compounds were first prepared by arc-melting method and then ground into particles with size in the range of 40 - 80 nm using a high energy ball milling method. After that, the additional powder were mixed with micrometer Nd-Fe-B powder before magnetic anisotropic pressing, vacuum sintering and annealing. The structure of the magnets was thoroughly analyzed using X-ray diffraction and electron microscopy techniques. The magnetic properties of the magnets were investigated on a pulsed field magnetometer. The results show that the coercivity of the sintered Nd-Fe-B magnets can be improved by introducing additional nanoparticles to their grain boundaries. The improvement of the coercivity of the magnets is clearly dependent on composition and fraction of the additional compounds. The coercivity has been enhanced 40% for the magnets by adding 3 wt% of the Dy-free compound of Nd40Cu30Al30.

On supersymmetric geometric flows and [formula omitted] inflation from scale invariant supergravity

Models of geometric flows pertaining to R2 scale invariant (super) gravity theories coupled to conformally invariant matter fields are investigated. Related to this work are supersymmetric scalar manifolds that are isomorphic to the Kählerian spaces Mn=SU(1,1+k)∕U(1)×SU(1+k) as generalizations of the non-supersymmetric analogs with SO(1,1+k)∕SO(1+k) manifolds. For curved superspaces with geometric evolution of physical objects, a complete supersymmetric theory has to be elaborated on nonholonomic (super) manifolds and bundles determined by non-integrable superdistributions with additional constraints on (super) field dynamics and geometric evolution equations. We also consider generalizations of Perelman’s functionals using such nonholonomic variables which result in the decoupling of geometric flow equations and Ricci soliton equations with supergravity modifications of the R2 gravity theory. As such, it is possible to construct exact non-homogeneous and locally anisotropic cosmological solutions for various types of (super) gravity theories modeled as modified Ricci soliton configurations. Such solutions are defined by employing the general ansatz encompassing coefficients of generic off-diagonal metrics and generalized connections that depend generically on all spacetime coordinates. We consider nonholonomic constraints resulting in diagonal homogeneous configurations encoding contributions from possible nonlinear parametric geometric evolution scenarios, off-diagonal interactions and anisotropic polarization/modification of physical constants. In particular, we analyze small parametric deformations when the underlying scale symmetry is preserved and the nontrivial anisotropic vacuum corresponds to generalized de Sitter spaces. Such configurations may mimic quantum effects whenever transitions to flat space are possible. Our approach allows us to generate solutions with scale violating terms induced by geometric flows, off-diagonal interactions and supersymmetric modifications of effective potentials. We study reconstructing the formalism for (super) geometric flows and modified gravity cosmological scenarios. We also analyze the conditions under which such modified mimetic type theories and solutions reproduce the Starobinsky inflationary models in the double scalar approach.

Cosmological model favored by the holographic principle

We present a regular spherically symmetric cosmological model of the Lemaitre class distinguished by the holographic principle as the thermodynamically stable end-point of quantum evaporation of the cosmological horizon. A source term in the Einstein equations connects smoothly two de Sitter vacua with different values of cosmological constant and corresponds to anisotropic vacuum dark fluid defined by symmetry of its stress-energy tensor which is invariant under the radial boosts. Global structure of space-time is the same as for the de Sitter space-time. Cosmological evolution goes from a big initial value of the cosmological constant towards its presently observed value.

Tests and prospects of new physics at very high energy. Beyond the standard basic principles, and beyond conventional matter and space-time. On the possible origin of Quantum Mechanics.

Recent results and announcements by Planck and BICEP2 have led to impor- tant controversies in the fields of Cosmology and Particle Physics. As new ideas and alter- native approaches can since then more easily emerge, the link between the Mathematical Physics aspects of theories and the interpretation of experimental results becomes more direct. This evolution is also relevant for Particle Physics experiments at very high energy, where the interpretation of data on the highest-energy cosmic rays remains a major theo- retical and phenomenological challenge. Alternative particle physics and cosmology can raise fundamental questions such as that of the structure of vacuum and space-time. In particular, the simplified description of the physical vacuum contained in standard quan- tum field theory does not necessarily correspond to reality at a deeper level, and similarly for the relativistic space-time based on four real variables. In a more general approach, the definition itself of vacuum can be a difficult task. The spinorial space-time (SST) we suggested in 1996-97 automatically incorporates a local privileged space direction (PSD) for each comoving observer, possibly leading to a locally anisotropic vacuum structure. As the existence of the PSD may have been confirmed by Planck, and a possible dis- covery of primordial B-modes in the polarization of the cosmic microwave background radiation (CMB) may turn out to contain new evidence for the SST, we explore other possible implications of this approach to space-time. The SST structure can naturally be at the origin of Quantum Mechanics at distance scales larger than the fundamental one if standard particles are dealt with as vacuum excitations. We also discuss possible implica- tions of our lack of knowledge of the structure of vacuum, as well as related theoretical, phenomenological and cosmological uncertainties. Pre-Big Bang scenarios and new ulti- mate constituents of matter (including superbradyons) are crucial open subjects, together with vacuum structure and the interaction between vacuum and standard matter.

Tests and prospects of new physics at very high energy. Beyond the standard basic principles, and beyond conventional matter and space-time. On the possible origin of Quantum Mechanics.

Recent results and announcements by Planck and BICEP2 have led to important controversies in the fields of Cosmology and Particle Physics. As new ideas and alternative approaches can since then more easily emerge, the link between the Mathematical Physics aspects of theories and the interpretation of experimental results becomes more direct. This evolution is also relevant for Particle Physics experiments at very high energy, where the interpretation of data on the highest-energy cosmic rays remains a major theoretical and phenomenological challenge. Alternative particle physics and cosmology can raise fundamental questions such as that of the structure of vacuum and space-time. In particular, the simplified description of the physical vacuum contained in standard quantum field theory does not necessarily correspond to reality at a deeper level, and similarly for the relativistic space-time based on four real variables. In a more general approach, the definition itself of vacuum can be a difficult task. The spinorial space-time (SST) we suggested in 1996-97 automatically incorporates a local privileged space direction (PSD) for each comoving observer, possibly leading to a locally anisotropic vacuum structure. As the existence of the PSD may have been confirmed by Planck, and a possible discovery of primordial B-modes in the polarization of the cosmic microwave background radiation (CMB) may turn out to contain new evidence for the SST, we explore other possible implications of this approach to space-time. The SST structure can naturally be at the origin of Quantum Mechanics at distance scales larger than the fundamental one if standard particles are dealt with as vacuum excitations. We also discuss possible implications of our lack of knowledge of the structure of vacuum, as well as related theoretical, phenomenological and cosmological uncertainties. Pre-Big Bang scenarios and new ultimate constituents of matter (including superbradyons) are crucial open subjects, together with vacuum structure and the interaction between vacuum and standard matter.

TRIPLE-HORIZON SPHERICALLY SYMMETRIC SPACETIME AND HOLOGRAPHIC PRINCIPLE

We present a family of spherically symmetric spacetimes, specified by the density profile of a vacuum dark energy, which have the same global structure as the de Sitter spacetime but the reduced symmetry which leads to a time-evolving and spatially inhomogeneous cosmological term. It connects smoothly two de Sitter vacua with different values of cosmological constant and corresponds to anisotropic vacuum dark fluid defined by symmetry of its stress–energy tensor which is invariant under the radial boosts. This family contains a special class distinguished by dynamics of evaporation of a cosmological horizon which evolves to the triple horizon with the finite entropy, zero temperature, zero curvature, infinite positive specific heat and infinite scrambling time. Nonzero value of the cosmological constant in the triple-horizon spacetime is tightly fixed by quantum dynamics of evaporation of the cosmological horizon.

Thermodynamics of horizons in a globally regular spherically symmetric spacetime

We address the question of thermodynamics of horizons in a globally regular spherically symmetric spacetime which is asymptotically de Sitter as r → 0 and as r → ∞. A source term in the Einstein equations smoothly connects two de Sitter vacua with different values of the cosmological constant and corresponds to an anisotropic vacuum fluid defined by symmetry of its stress-energy tensor, which is invariant under radial boosts. In the most general case, the spacetime has three horizons, an internal one, ra, which is a cosmological horizon for an observer in the R-region 0 ≤ r ≤ ra; the horizon rb > ra which is the boundary of the T-region ra < r < rb seen as a black or white hole by an observer in the R-region rb < r < rc, where rc is his cosmological horizon. We present a detailed analysis of the thermodynamics of horizons using the Padmanabhan approach relevant to the case of non-zero pressures.

Thermodynamics of Regular Cosmological Black Holes with the de Sitter Interior

We address the question of thermodynamics of regular cosmological spherically symmetric black holes with the de Sitter center. Space-time is asymptotically de Sitter as r → 0 and as r → ∞. A source term in the Einstein equations connects smoothly two de Sitter vacua with different values of cosmological constant: 8πGTμν = Λδμν as r → 0, 8πGTμν = λδμν as r → ∞ with λ &lt; Λ. It represents an anisotropic vacuum dark fluid defined by symmetry of its stress-energy tensor which is invariant under the radial boosts. In the range of the mass parameter Mcr1 ≤ M ≤ Mcr2 it describes a regular cosmological black hole. Space-time in this case has three horizons: a cosmological horizon rc, a black hole horizon rb &lt; rc, and an internal horizon ra &lt; rb, which is the cosmological horizon for an observer in the internal R-region asymptotically de Sitter as r → 0. We present the basicfeatures of space-time geometry and the detailed analysis of thermodynamics of horizons using the Padmanabhan approach relevant for a multi-horizon space-time with a non-zero pressure. We find that in a certain range of parameters M and q =√Λ/λ there exist a global temperature for an observer in the R-region between the black hole horizon rb and cosmological horizon rc. We show that a second-order phase transition occurs in the course of evaporation, where a specific heat is broken and a temperature achieves its maximal value. Thermodynamical preference for a final point of evaporation is thermodynamically stable double-horizon (ra = rb) remnant with the positive specific heat and zero temperature.

Intrinsic Quantum Beats of Atomic Populations and Their Nanoscale Realization Through Resonant Plasmonic Antenna

In the coherently trapped populations of a four-level atom, we demonstrated the quantum beats with different mechanism, which originate from the interference between transition channels with different dipole moments. The beat frequency is determined by the intrinsic atomic parameters, i.e., the spacing of upper levels and ratio of dipole moments. The resonant plasmonic nanoantenna, as a candidate for the creation of anisotropic vacuum, was proposed to achieve the nanoscale realization of the quantum beats, spontaneous emission cancellation, and Rabi oscillation in two-photon correlations through the enhanced near-field and modified decay rates.

An analytic approach to perturbations from an initially anisotropic universe

We present the analytic forms for the spectra of the cosmological perturbations from an initially anisotropic universe for the high momentum modes in the context of WKB approximations, as the continuation of the work [29]. We consider the Einstein gravity coupled to a light scalar field. We then assume that the scalar field has the zero velocity initially and then slowly rolls down on the potential toward the origin. In the slow-roll approximations, the Kasner-de Sitter universe with a planar symmetry is a good approximation as the background evolution. Quantization of the perturbations in the adiabatic vacuum, which we call the anisotropic vacuum, is carried out. For non-planar high momentum modes whose comoving momentum component orthogonal to the plane is bigger than the Hubble parameter at the inflationary phase, the WKB approximation is valid for the whole stage of the isotropization. On the other hand, the planar modes whose comoving momentum component orthogonal to the plane is comparable to the Hubble parameter, is amplified during the process of the anisotropic expansion. In the final gravitational wave spectra, we find that there is an asymmetry between the two polarizations of the gravitational wave because the initial mode mixing does not vanish.

Thermodynamics of regular cosmological black holes with de Sitter interior

We address the question of thermodynamics of a regular spherically symmetric cosmological black hole. The spacetime is asymptotically de Sitter as r → 0 and as r →∞ . A source term in the Einstein equations connects smoothly two de Sitter vacua with different values of the cosmological constant and corresponds to an anisotropic vacuum fluid defined by the symmetry of its stress-energy tensor, invariance under radial boosts. In the range of the mass parameter Mcrit1 ≤ M ≤ Mcrit2 it describes a regular cosmological black hole.

On the existence of anisotropic cosmological models in higher order theories of gravity

We investigate the behaviour on approach to the initial singularity in higher order extensions of general relativity by finding exact cosmological solutions for a wide class of models in which the Lagrangian is allowed to depend nonlinearly upon the three possible linear and quadratic scalars built from the Riemann tensor: R, RabRab and RabcdRabcd. We present new anisotropic vacuum solutions analogous to the Kasner solutions of general relativity and extend previous results to a much wider range of fourth-order theories of gravity. We discuss the implications of these results for the behaviour of the more general anisotropic Bianchi type VIII and IX cosmologies as the initial singularity is approached. Furthermore, we also consider the existence conditions for some other simple anisotropic Bianchi I vacuum solutions in which the expansion in each direction is of exponential, rather than power-law behaviour and their relevance for cosmic ‘no-hair’ theorems.

Scalar field in the anisotropic universe

We discuss the primordial spectrum of a massless and minimally coupled scalar field, produced during the initial anisotropic epoch before the onset of inflation. We consider two models of the anisotropic cosmology, the (planar) Kasner-de Sitter solution (Bianchi I) and the Taub-NUT-de Sitter solution (Bianchi IX), where the 3-space geometry is initially anisotropic, followed by the de Sitter phase due to the presence of a positive cosmological constant. We discuss the behavior of a quantized, massless and minimally coupled scalar field in the anisotropic stage. This scalar field is not the inflaton and hence does not contribute to the background dynamics. We focus on the quantization procedure and evolution in the preinflationary anisotropic background. Also, in this paper for simplicity the metric perturbations are not taken into account. The initial condition is set by the requirement that the scalar field is initially in an adiabatic state. Usually, in a quantum harmonic oscillator system, an adiabatic process implies the one where the potential changes slowly enough compared to its size, and the time evolution can be obtained from the zeroth order WKB approximation. In our case, such a vacuum state exists only for limited solutions of the anisotropic universe, whose spacetime structure is regular in the initial times. In this paper, we call our adiabatic vacuum state the anisotropic vacuum. In the Kasner-de Sitter model, for one branch of planar solutions there is an anisotropic vacuum unless ${k}_{3}\ensuremath{\ne}0$, where ${k}_{3}$ is the comoving momentum along the third direction, while in the other branch there is no anisotropic vacuum state. In the first branch, for the moderate modes, ${k}_{3}\ensuremath{\sim}k$, where $k$ is the total comoving momentum, the scalar power spectrum has an oscillatory behavior and its direction dependence is suppressed. For the planar modes, ${k}_{3}\ensuremath{\ll}k$, in contrast, the direction dependence becomes more important, because of the amplification of the scalar amplitude during this interval of the violation of WKB approximation in the initial anisotropic stage. The qualitative behaviors in the Taub-NUT-de Sitter models are very similar to the case of the first branch of the planar Kasner-de Sitter model.

Regular black hole remnants in de Sitter space

We address the question of thermodynamical evolution of regular spherically symmetric cosmological black holes with de Sitter center. Space–time is asymptotically de Sitter as r→0 and as r→∞. A source term in the Einstein equations connects smoothly two de Sitter vacua with different values of cosmological constant and corresponds to anisotropic vacuum dark fluid defined by symmetry of its stress–energy tensor. In the range of masses Mcr1⩽M⩽Mcr2 it describes a regular cosmological black hole with three horizons, an internal horizon ra, a black hole horizon rb>ra, and a cosmological horizon rc>rb. Thermodynamical preference for a final product of evaporation is a double-horizon (ra=rb) black hole remnant with the positive specific heat.

Geodesic portrait of de Sitter-Schwarzschild spacetime

De Sitter-Schwarzschild space-time is a globally regular spherically symmetric spacetime which is asymptotically de Sitter as r → 0 and asymptotically Schwarzschild as r →∞ .A source term in the Einstein equations smoothly connects de Sitter vacuum at the origin with Minkowski vacuum at infinity and corresponds to an anisotropic vacuum fluid defined by symmetry of its stress-energy tensor which is invariant under radial boosts. In the range of the mass parameter M ≥ Mcrit, de Sitter- Schwarzschild spacetime represents a vacuum nonsingular black hole, while M< M crit corresponds to a compact gravitationally bound vacuum object without horizons, called a G-lump. Masses of objects are related to both de Sitter vacuum trapped inside and to smooth breaking of the spacetime symmetry from the de Sitter group at the origin to the Poincar ´ eg roup at infi nity. We here present a geodesic survey of de Sitter-Schwarzschild spacetime.