Modern Physics Letters B

Earth’s core consists of a solid inner core and a liquid outer core, composed primarily of iron. The pressure in the solid inner core is about 330 gigapascals (GPa) at the temperature close to the melting point. Considering the extensive experimental and theoretical data, the shear wave ($s$-wave) velocity of the inner core is much lower than that of pure iron. Since the lower $s$-wave velocity has been observed in the seismic models, reasons have been widely discussed such as the premelting of iron in the Earth’s inner core. In this paper, a new explanation is expected to be proposed under the anisotropic stress. The calculated longitudinal wave and $s$-wave velocity of pure hexagonal close-packed iron (HCP-Fe) model based on the density functional theory (DFT) at the different density are matching with the seismic wave, the atomic distribution of HCP-Fe is obtained under the anisotropic stress. Unfortunately, it is unlikely conformed there was an inner-core condition due to the unreal anisotropic stress, although the lower $s$-wave velocity is. Somehow, this lower $s$-wave velocity may provide a new horizon to build mineralogical models for discussing. In addition, the $s$-wave and viscosity of iron are strongly dependent on shear stress, we then give a mathematical equation between the $s$-wave velocity and viscosity empirically by the shear behavior. It is revealed that the shear stress of iron has a positive influence on the $s$-wave and viscosity.

In this paper, we investigate the magnetic-domain wall (DW) dynamics in uniaxial/biaxial-nanowires under a thermal gradient (TG). The findings reveal that the DW propagates toward the hotter region in both nanowires. In uniaxial nanowire, the DW propagates accompanying a rotation of the DW-plane. In biaxial nanowire, the DW propagates in the hotter region, and the so-called Walker breakdown phenomenon is observed. The main physics of such DW dynamics is the magnonic angular momentum transfer to the DW. The hard (shape) anisotropy exists in biaxial-nanowire, which contributes an additional torque; hence DW speed is larger than that in uniaxial-nanowire. But the rotational speed is lower initially as hard anisotropy suppresses the DW-rotation. After certain TG, DW-plane overcomes the hard anisotropy and so the rotational speed increases slightly. With lower damping, the DW velocity is smaller and DW velocity increases with damping which is a contrary to usual desire. The reason is predicted as the formation of the standing spin-waves (by superposing the spin waves and its reflection from the boundary) which do not carry any net energy to DW. However, for larger damping, DW velocity decreases with damping since the magnon-propagation length decreases. Therefore, the above findings might be useful to realize the spintronics (i.e. racetrack-memory) devices.

In this paper, the AC susceptibility in superconducting film is studied by the extended exponential critical state model, in which the critical-current density is assumed to be inhomogeneous distributed, namely, space gradient dependent. The results show that it should be noted that existence of the gradient parameter produces a quantitative effect on the form of the dependence of AC susceptibility. The magnetization properties caused by the gradient parameter are also highly nonlinear, which should be useful for the design of thin superconducting film devices.

The dependence of the electrical durability ${\tau }_{\mathrm{\text{E}}}$ (time from the application of constant voltage until breakdown) on the temperature $T$ (in the range of 77–300 K) at different values of the electric field strength $E$ of polymer compositions of polypropylene and low-density polyethylene (PP–LDPE) with different supramolecular structures (SMS) was measured. At elevated temperatures ($\sim$ above 200 K), the dependence of lg${\tau }_{\mathrm{\text{E}}}(T^{-1})$ is linear, indicating a thermofluctuational mechanism of breakdown preparation; at low temperatures ($\sim$ below 200 K) ${\tau }_{\mathrm{\text{E}}}$ does not depend on $T$, which will allow us to conclude about its tunneling mechanism. The transition temperature of the linear dependence of ${\tau }_{\mathrm{\text{E}}}$ on $T$ lies in the $\sim 200$ K range and depends on the value of the applied constant electric field.

Silicon nitride (Si₃N${}_4)$ nanoparticles were irradiated with neutrons at different energy fluxes. Differential thermal analysis of non-irradiated and neutron irradiated samples was performed in the temperature range of 300–1400 K. Based on the obtained experimental results, relevant thermophysical parameters were calculated and discussed in detail. All analyses were comparatively discussed for non-irradiated and neutron irradiated nano-Si₃N₄ particles. On the basis of endotherms observed in DTA spectra, entropy and enthalpy in Si₃N₄ are interpreted in detail. Compared with the non-irradiated sample, slightly negative values of enthalpy indicate the formed ${}^{31}$P isotope due to neutron irradiation. In the mentioned temperature range, the values of the free Gibbs energy were calculated for all samples, and it was determined that it is varied around zero up to 600 K. Moreover, the effect of neutron transmutations on heat transfer in Si₃N₄ nanoparticles has been studied.

The structural, stability, and electronic properties and optimized inter-wall distances of double-walled boron nitride nanotubes (DWBNNTs) are investigated based on density functional theory (DFT) with the SIESTA code. The computations are done on the zigzag ($m$,0)@($n$,0) DWBNNTs with chirality of ($m=6$, 7 and $n=10$–18) and the armchair ($m,m)@(n,n)$ with chirality of ($m=5$, 6 and $n=7$–15). The calculated binding and formation energies revealed that the armchair and the zigzag DWBNNTs with chirality differences of ($n-m=6$ and 9) ($m,m)@(m+6,m+6$), ($m,0)@(m+9,0$) and inter-layer spacing of about 4.22Å and 3.62Å are the best favorable nanotubes, respectively. Analyzing the electronic structures revealed that all considered armchair and zigzag BNNTs are semiconductors. Furthermore, it is concluded that with increasing diameters of the tubes and the spaces between walls, the value of the band gap rises, and the change process is almost constant at larger distances between the walls. Also, compared to single-walled nanotubes, DWBNNTs have a narrower bandgap. Future empirical investigations can definitely benefit from the implications of this research.

Under the effect of uniaxial compressive strain along [001]-direction, the electronic, magneto-optical, and electronic transport properties of double perovskite oxide were realized by substituting the Ti atom by Cr atom in Ca₂TiMnO₆ (CTMO). A first-principles method within the various approximations (PBEsol-GGA, GGA+U, YS-PBE0 and TB-mBJ) has been employed. The analysis of the electronic structure reveals that the compound Ca₂CrMnO₆ (CCMO) has a half-metallic (HM) ferromagnet (FM) nature which attributes to hybridization between Cr-3$d$, Mn-3$d$ and O-2$p$ states. CTMO exhibits an integer value of magnetic moment 3 ${\mu }_B$. However, CCMO exhibits the half-metallicity (HM) under compressive strain from −2% to −5% with the total magnetic moment, a value of 5 ${\mu }_B$. CCMO possesses a mediocre spin-down bandgap ($\sim$2 eV) optimum for thermoelectricity and optoelectronics. The optical properties within GGA+U reveal that the CCMO can absorb light under all frequencies. We have calculated the Seebeck coefficient, and electrical and electronic thermal conductivities to determine the thermoelectric (TE) figure of merit (ZT), which is found to be approaching 1 at room temperature considering the spin-down electrons. This compound CCMO may be used for optoelectronic, solar cell, and TE applications due to its amazing properties.

In this paper, we investigate the optical analog to coherent population return (CPR) in coupled nonlinear waveguides. The nonlinearity sometimes plays a negative role in classical simulations of quantum adiabatic passages in waveguides. Nevertheless, it is interesting to note that the nonlinearity here helps simulating the adiabatic CPR technique. The CPR can still occur when the detuning (difference of the propagation constants) between the two nonlinear waveguides is moderate, which is a typical requirement of the technique. Moreover, the adiabatic process can appear when the detuning is small or zero. This is done by simply adjusting the incident light power, which is easy and of high accuracy compared with modulating the propagation constants. Therefore, the requirements towards the propagation constants of the waveguides are further relaxed in the nonlinear case. Finally, we point out that the CPR in nonlinear waveguides can be extended to design achromatic beam splitters.