Possible Link between the Distribution of Atomic Spectral Lines and the Radiation–Matter-Equilibrium in the Early Universe

The uncanny resemblance of the global distribution of all experimentally known atomic spectral lines to the Planckian spectral distribution associated with black body radiation at a temperature of $T\approx 9000\mathrm{K}$ is reported. This value coincides with the critical temperature of equilibrium between the respective densities of radiation and matter in the early universe.     

Efficient Ultraviolet Nanosources Based on Third-Harmonic Generation in Dielectric–Metal Composite Nanodisks

Dielectric materials with high indices have recently attracted much attention in the community of nanophotonics. Severe optical losses in visible–ultraviolet (UV) region, however, limit their applications. This article proposes dielectric–metal nanocomposites as alternative high-index materials for Mie-resonance-based applications. Such composite materials have high indices in the range of wavelength longer than plasmon resonance of inclusion metal nanoparticles, while they have much lower losses in the range from blue-violet down to near-UV compared with commonly used high-index materials such as silicon, enabling near-UV generation with high efficiency based on third-harmonic generation (THG). The numerical results show that ZnO nanodisk containing silver nanoparticles can generate near-UV radiation at 351.3 nm via THG with an efficiency about 20 times higher compared with silicon nanodisk under same pumping condition. Significantly high THG efficiency of 0.015% has been predicted with such a composite nanodisk supported by aluminum substrate under pumping with a peak intensity of 20 GW cm⁻², a spot size of 0.8 $\mu \mathrm{m}$, a duration of 50 fs at 1054 nm, respectively.     

Minimum Energy and Total Dipole Moment

Several magnetic materials consisting of dipoles owe their properties to the specific nature of the dipole–dipole interaction. In the present work, systems of particles possessing a dipole moment arranged on various types of 2D and 3D structures, completely arbitrary and, in some 2D instances, periodic (albeit finite), are studied. Noteworthy, the work is in the regime of strong dipole moments where a classical treatment is possible. The ultimate goal is to quantitatively address the unknown relation existing between the minimum possible energy of a system of dipoles and the concomitant total dipole moment. To such an end, classical numerical methods are used to the previous minimum energy–total dipole moment tandem for various magnetic configurations at zero temperature. An analytic bound for the minimal energy valid for any dimension is also obtained. With this exploration, new light is shed on the connection between the two former physical quantities, establishing an analytic inequality for $N=3$ particles, and describing other instances of physical interest.     

Large Magnetic Anisotropy Energy and Robust Half-Metallic Ferromagnetism in 2D MnXSe4 (X = As,Sb)

In recent years, intrinsic 2D magnetism has aroused great interest because of its potential application in spintronic devices. However, low Curie temperature (T_c) and magnetic anisotropy energy (MAE) limit its application prospects. Here, using first-principles calculations based on density-functional theory, a series of stable MnXSe₄ (X=As, Sb) single-layer is predicted. The MAE of single-layer MnAsSe₄ and MnSbSe₄ is 648.76 and 808.95 $\mu$eV per Mn atom, respectively. Monte Carlo simulations suggest the T_c of single-layer MnAsSe₄ and MnSbSe₄ is 174 and 250 K, respectively. The energy band calculation with hybrid Heyd–Scuseria–Ernzerhof (HSE06) function indicates the MnXSe₄ (X = As, Sb) is ferromagnetic half-metallic. Also, it has 100% spin-polarization ratio at the Fermi level. For MnAsSe₄ and MnSbSe₄, the spin-gaps are 1.59 and 1.48 eV, respectively. These excellent magnetic properties render MnXSe₄ (X = As, Sb) as promising candidate materials for 2D spintronic applications.     

Measurement of the Ratio between g‐Factors of the Ground States of 87Rb and 85Rb

The ratio between the Landé g‐factors of the ${}^{87}$Rb $F=2$ and ${}^{85}$Rb $F=3$ ground‐state hyperfine levels is experimentally measured to be $g_F(87)/g_F(85)=1.4988586(1)$, consistent with previous measurements. The g‐factor ratio is determined by comparing the Larmor frequencies of overlapping ensembles of ${}^{87}$Rb and ${}^{85}$Rb atoms contained within an evacuated, antirelaxation‐coated vapor cell. The atomic spins are polarized via synchronous optical pumping and the Larmor frequencies are measured by off‐resonant probing using optical rotation of linearly polarized light. The accuracy of this measurement of $g_F(87)/g_F(85)$ exceeds that of previous measurements by a factor of ≈50 and is sensitive to effects related to quantum electrodynamics.     

Atom‐Interferometry Measurement of the Fine Structure Constant

Using an atom interferometer to measure the quotient of the reduced Planck's constant and the mass of a cesium‐133 atom $?/m_{\mathrm{Cs}}$, the most accurate measurement of the fine structure constant $\alpha =1/137.035999046(27)$ is recorded, at an accuracy of 0.20 parts per billion (ppb). Using multiphoton interactions (Bragg diffraction and Bloch oscillations), the largest phase (12 million radians) of any Ramsey–Bordé interferometer and controlled systematic effects at a level of 0.12 ppb are demonstrated. Comparing the Penning trap measurements with the Standard Model prediction of the electron gyromagnetic anomaly $a_e$ based on the α measurement, a 2.5 $\sigma$ tension is observed, rejecting dark photons as the reason for the unexplained part of the muon's gyromagnetic moment discrepancy at a 99% confidence level according to frequentist statistics. Implications for dark‐sector candidates (e.g., scalar and pseudoscalar bosons, vector bosons, and axial‐vector bosons) may be a sign of physics beyond the Standard Model. A future upgrade of the cesium fountain atom interferometer is also proposed to increase the accuracy of $?/m_{\mathrm{Cs}}$ by 1 to 2 orders of magnitude, which would help resolve the tension.     

p‐Wave Superfluid Phases of Fermi Molecules in a Bilayer Lattice Array

The superfluid $\mathbf{p}=p_x+{\mathit{ip}}_y$ phases in an ultracold gas of dipolar Fermi molecules lying in two parallel square lattices in 2D are investigated. As shown by a two‐body study, dipole moments oriented in opposite directions in each layer are the key ingredients in our mean‐field analysis from which unconventional superfluidity is predicted. The $T=0$ phase diagram summarizes our findings: stable and metastable superfluid phases appear as a function of both, the dipole–dipole interaction coupling parameter and filling factor. A first‐order phase transition, and thus a mixture of superfluid phases at different densities, is revealed from the coexistence curves in the metastable region. The model predicts that these superfluid phases can be observed experimentally at 10 nK in molecules of NaK confined in optical lattices of size $a=532$ nm. Other routes to reach higher temperatures require the use of subwavelength confinement technique .     

Physical Dimensions/Units and Universal Constants: Their Invariance in Special and General Relativity

The theory of physical dimensions and units in physics is outlined. This includes a discussion of the universal applicability and superiority of quantity equations. The International System of Units (SI) is one example thereof. By analyzing mechanics and electrodynamics, it naturally leads one, besides the dimensions of length and time, to the fundamental units of action $\mathfrak{h}$, electric charge q, and magnetic flux ?. Also, $q\times ?=\text{action}$ and $q/?=1/\text{resistance}$ are known. These results of classical physics suggests to look into the corresponding quantum aspects of q and ? (and also of $\mathfrak{h}$): The electric charge occurs exclusively in elementary charges e, whereas the magnetic flux can have any value; in specific situations, however, in superconductors of type II at very low temperatures, ? appears quantized in the form of fluxons (Abrikosov vortices). And $\mathfrak{h}$ leads, of course, to the Planck quantum h. Thus, one is directed to superconductivity and, because of the resistance, to the quantum Hall effect. In this way, the Josephson and the quantum Hall effects come into focus quite naturally. One goal is to determine the behavior of the fundamental constants in special and in general relativity.     

Transmission in Graphene through Tilted Barrier in Laser Field

The transmission of Dirac fermions in graphene through a tilted barrier potential in the presence of a laser field of frequency ω is studied. By using Floquet theory, the Dirac equation is solved and then the energy spectrum is obtained. The boundary conditions together with the transfer matrix method allow to determine the transmission probabilities corresponding to all energy bands $E+l\hslash \omega$ $(l=0,\pm 1,\text{…})$. By limiting to the central band $l=0$ and the two first side bands $l=\pm 1$, it is shown that the transmissions are strongly affected by the laser field and barrier. Indeed, it is found that the Klein effect is still present, a variety of oscillations are inside the barrier, and there is essentially no transmission across all bands.     

Phase Diagrams of Periodically Driven Spin–Orbit Coupled 87Rb and 23Na Bose–Einstein Condensates

The phase boundaries of periodically driven spin–orbit coupled BECs with effective two‐body interactions are analytically calculated by using variational method. The phase diagrams of periodically driven ${}^{87}\mathrm{Rb}$ and ${}^{23}\mathrm{Na}$ systems present distinguished features from undriven systems, respectively. For the ${}^{87}\mathrm{Rb}$ BECs, the critical density $n_c$ (density at quantum tricritical point) will be dramatically reduced in some parameter regions, and the prospect of observing this intriguing quantum tricritical point is greatly enlarged. Moreover, a series of quantum tricritical points emerge quasi‐periodically when increasing the Raman coupling strength with fixed ${}^{87}\mathrm{Rb}$ density. In the ${}^{23}\mathrm{Na}$ BECs, two hyperfine states of ${}^{23}\mathrm{Na}$ atoms can be miscible within the suitable regions of driving parameter space. As a result, ${}^{23}\mathrm{Na}$ systems will stay in the stripe phase with small Raman frequency at typical density, which expands the region of stripe phase in the phase diagram. In addition, an absence of quantum tricritical point in such ${}^{23}\mathrm{Na}$ system is observed, which is very unlike ${}^{87}\mathrm{Rb}$ systems.