Band gap engineering, band edge emission, and p-type conductivity in wide-gap LaCuOS1–xSex oxychalcogenides

The preparation of LaCuOS1–xSex solid solutions (x = 0.0, 0.25, 0.5, 0.75, and 1.0) was attempted to control their energy gap and band edge emission energy. X-ray diffraction analysis revealed that the lattice constant of LaCuOS1–xSex increased linearly with increasing x, indicating the formation of a complete solid solution in the LaCuOS–LaCuOSe system. The energy gap estimated from the diffuse reflectance spectra varied continuously from ~3.1 eV for x = 0 to ~2.8 eV for x = 1. The sharp emission near the absorption edge was observed in all samples at room temperature under ultraviolet light irradiation. p-type electrical conduction in these materials was confirmed by Seebeck measurements, and the conductivity was enhanced by substitution of Sr for La. These results demonstrated that the formation of the solid solutions enabled band gap engineering in LaCuOS1–xSex oxychalcogenides keeping their band edge emission feature and p-type conductivity

Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0 to 0.2)

Thermoelectric properties of layered oxyselenides La1–xSrxCuOSe (x = 0.00 to 0.20) were investigated to evaluate the potential as thermoelectric material. Temperature dependence of the electrical conductivity and Seebeck coefficient measured in a temperature range of 373 to 673 K indicated that nondoped LaCuOSe was a p-type degenerate semiconductor due to Cu vacancies, while Sr-doped materials with x = 0.05 to 0.20 were p-type metals. The electrical conductivity increased and Seebeck coefficient decreased with increasing Sr concentration up to x = 0.10 in La1–xSrxCuOSe, suggesting that the effective hole carriers increase with increasing Sr content up to x = 0.10. Thermoelectric power factors were drastically enhanced by the Sr doping, and the value reached 1.0–1.4×10–4 W m–1 K–2 for La0.95Sr0.05CuOSe. Thermal conductivities measured for the materials with x = 0.00 and 0.05 were 2.1 W m–1 K–1 and 2.3 W m–1 K–1 at room temperature, respectively. These results lead to an estimation of Z value of 4.4×10–5 K–1 for La0.95Sr0.05CuOSe

Red photoluminescence in praseodymium-doped titanate perovskite films epitaxially grown by pulsed laser deposition

Intense red photoluminescence (PL) under ultraviolet (UV) excitation was observed in epitaxially grown Pr-doped Ca0.6Sr0.4TiO3 perovskite films. The films were grown on SrTiO3 (100) substrates by pulsed laser deposition, and their epitaxial growth was confirmed by x-ray diffraction and reflected high-energy electron diffraction. The observed sharp PL peak centered at 610 nm was assigned to the transition of Pr3+ ions from the 1D2 state to the 3H4 state. The PL intensity was markedly enhanced by postannealing treatments at 1000 °C, above the film-growth temperature of 600 or 800 °C. Because the excitation and absorption spectra are similar to each other, it was suggested that the UV energy absorbed by the host lattice was transferred to the Pr ions, resulting in the red luminescence

Electron paramagnetic centers in donor-doped CaTiO3 single crystals

Electron spin resonance signals observed in donor-doped CaTiO3 single crystals (Ca12xYxTiO3 orCaTi12xNbxO3) were analyzed and the results were discussed from a viewpoint of carrier generation. Severaltypes of signals were observed in insulating samples, and they were tentatively assigned to some acceptorssuch as Al impurity and the defects relating oxygen excess. Two types of signals with sharp and broad features,both of which are assignable to electron-trapped-type centers, were observed in conductive samples. Theorigins of the sharp and broad signals were attributed to electrons tightly trapped on Ti41 ions and electronsloosely localized around donors, respectively, from analysis of angular and temperature dependence of thesignals. The concentrations of the centers for the sharp and broad signals were estimated at ;3 K to be;331018 cm23 in the H2-reduced samples with x51024 and ;331019 cm23 in the as-prepared sampleswith x51022, respectively. Although no obvious correlation between electroconductive behavior and signalintensity was observed for the sharp signals, intensities of the broad signal increased as the electroconductivebehavior turned from metallic to semiconducting below ;25 K. Therefore, it was found that the electronsresponsible for the broad signals convert into conduction electrons when they are thermally released at hightemperatures, and their concentration is high enough to influence the electroconductive behavior

Valence-band structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), studied by ultraviolet photoemission spectroscopy and energy-band calculations

To examine the electronic structure of the valence band, ultraviolet photoemission spectra of a series of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), were measured. The measurements were conducted using He II, He I, and Ne I excitation lines to observe the excitation energy dependence of the spectral shape. Energy-band calculations based on a full-potential linearized augmented plain-wave method were performed. The calculated density of states and partial density of states were compared to the observed photoemission spectra. Five bands were observed in the valence band of LaCuOCh, and Ne I radiation remarkably enhanced two of them. The energy dependence of the photoionization cross section of atomic orbitals indicated that the two enhanced bands were due to the Ch p states. Energy calculations were used to assign the remaining bands. The electronic structure of LaCuOCh was further discussed using molecular-orbital diagrams to visualize the (La2O2)2+ and (Cu2Ch2)2– layers as large donor-acceptor pairs. The energy-band calculation and molecular-orbital diagram analyses suggested that the main difference among the valence-band structures of LaCuOCh (Ch=S, Se, and Te) originates from the variations in the energy position of the Ch p bands. The observed spectra are consistent with the results of the band calculations and clearly show the energy variations in the Ch p bands with respect to spectral shape and excitation energy dependence

Entanglement of the vacuum between left, right, future, and past spacetime states: The origin of entanglement-induced quantum radiation

The Minkowski vacuum state is expressed as an entangled state between the left and right Rindler wedges when it is constructed on the Rindler vacuum. In this paper, we further examine the entanglement structure and extend the expression to the future (expanding) and past (shrinking) Kasner spacetimes. This clarifies the origin of the quantum radiation produced by an Unruh–DeWitt detector in uniformly accelerated motion in the four-dimensional Minkowski spacetime. We also investigate the two-dimensional massless case where the quantum radiation vanishes but the same entanglement structure exists

A peak in the power spectrum of primordial gravitational waves induced by primordial dark magnetic fields

Dark gauge fields have been discussed as candidates for dark matter recently. If they existed, primordial dark magnetic fields during inflation would have existed. It is believed that primordial gravitational waves (PGWs) arise out of quantum fluctuations during inflation. We study the graviton-dark photon conversion process in the presence of background primordial dark magnetic fields and find that the process induces the tachyonic instability of the PGWs. As a consequence, a peak appears in the power spectrum of PGWs. It turns out that the peak height depends on the direction of observation. The peak frequency could be in the range from $10^{-5}$ to $10^{3}$ Hertz for GUT scale inflation. Hence, the observation of PGWs could provide a new window for probing primordial dark magnetic fields.Comment: 19 pages, 3 figures. arXiv admin note: text overlap with arXiv:2211.0557

Impact of quantum entanglement induced by magnetic fields on primordial gravitational waves

There exist observational evidence to believe the existence of primordial magnetic fields generated during inflation. We study primordial gravitational waves (PGWs) during inflation in the presence of magnetic fields sustained by a gauge kinetic coupling. In the model, not only gravitons as excitations of PGWs, but also photons as excitations of electromagnetic fields are highly squeezed. They become entangled with each other through graviton to photon conversion and vice versa. We derive the reduced density matrix for the gravitons and calculate their entanglement entropy. It turns out that the state of the gravitons is not a squeezed state but a mixed state.Comment: 27 pages, 6 figures, Comments upon replacement: Modified discussion in sections 3 and 4. Replaced perturbative solutions with analytical solutions. Removed arguments based on imprecise perturbation. Added an author who contributed to deriving analytical solutions, boundary conditions, and related topic

Anisotropic warm inflation

Anisotropic inflation is a model succeeded in explaining statistical anisotropy. Warm inflation is a model succeeded in providing a mechanism of reheating during inflation. We study anisotropic warm inflation focusing on the cosmic no-hair conjecture. In the anisotropic warm inflation, the condition for making anisotropy survived is clarified. We find exact solutions of power-law anisotropic warm inflation, and investigate the phase space structure of general solutions. It turns out that whether the anisotropy during inflation survives or not depends on the speed of decaying process from inflaton field into matter fields. If the process is very slow, the anisotropic warm inflation is realized. If the process is slow, the anisotropy tends to be hard to grow enough in the duration of inflation. If the process is rapid, no anisotropic inflation occurs.Comment: 22 pages, 3 figure