Ultra-thin van der Waals magnetic tunnel junction based on monoatomic boron vacancy of hexagonal boron nitride.

We present a novel strategy to create a van der Waals-based magnetic tunnel junction (MTJ) comprising a three-atom layer of graphene (Gr) sandwiched with hexagonal boron nitride (hBN) layers by introducing a monoatomic boron vacancy in each hBN layer. The magnetic properties and electronic structure of the system were investigated using density functional theory (DFT) and the transmission probability of the MTJ was investigated using the Landauer-Büttiker formalism within the non-equilibrium Green function method. The Stoner gap was created between the spin-majority channel and the spin-minority channel on the local density of states of the hBN monoatomic boron-vacancy (VB) near the Fermi energy, creating a possible control of the spin valve by considering two magnetic alignment of hBN(VB) layers, anti-parallel configuration (APC) and parallel configuration (PC). The transmission probability calculation results showed a high electron transmission in the PC of the hBN(VB) layers and a low transmission when the APC was considered. A high tunneling magnetoresistance (TMR) ratio of approximately 400% was observed when comparing the APC and PC of the hBN(VB) layers in hBN(VB)/Gr/hBN(VB), giving the highest TMR ratio for the thinnest MTJ system.

Computational study of III-V direct-gap semiconductors for thermoradiative cell applications.

We investigate the performance of thermoradiative (TR) cells using the III-V group of semiconductors, which include GaAs, GaSb, InAs, and InP, with the aim of determining their efficiency and finding the best TR cell materials among the III-V group. The TR cells generate electricity from thermal radiation, and their efficiency is influenced by several factors such as the bandgap, temperature difference, and absorption spectrum. To create a realistic model, we incorporate sub-bandgap and heat losses in our calculations and utilize density-functional theory to determine the energy gap and optical properties of each material. Our findings suggest that the absorptivity of the material, especially when the sub-bandgap and heat losses are considered, can decrease the efficiency of TR cells. However, careful treatment of the absorptivity indicates that not all materials have the same trend of decrease in the TR cell efficiency when taking the loss mechanisms into account. We observe that GaSb exhibits the highest power density, while InP demonstrates the lowest one. Moreover, GaAs and InP exhibit relatively high efficiency without the sub-bandgap and heat losses, whereas InAs display lower efficiency without considering the losses, yet exhibit higher resistance to sub-bandgap and heat losses compared to the other materials, thus effectively becoming the best TR cell material in the III-V group of semiconductors.

A New Spin-Correlated Plasmon in Novel Highly Oriented Single-Crystalline Gold Quantum Dots.

A concept of spin plasmon, a collective mode of spin-density, in strongly correlated electron systems has been proposed since the 1930s. It is expected to bridge between spintronics and plasmonics by strongly confining the photon energy in the subwavelength scale within single magnetic-domain to enable further miniaturizing devices. However, spin plasmon in strongly correlated electron systems is yet to be realized. Herein, we present a new spin correlated-plasmon at room temperature in novel Mott-like insulating highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Interestingly, the spin correlated-plasmon is tunable from the infrared to visible, accompanied by spectral weight transfer yielding a large quantum absorption midgap state, disappearance of low-energy Drude response, and transparency. Supported with theoretical calculations, it occurs due to an interplay of surprisingly strong electron-electron correlations, s-p hybridization and quantum confinement in the s band. The first demonstration of the high sensitivity of spin correlated-plasmon in surface-enhanced Raman spectroscopy is also presented.

Colossal in-plane magnetoresistance ratio of graphene sandwiched with Ni nanostructures.

In this study, we present a theoretical study on the in-plane conductance of graphene partially sandwiched between Ni(111) nanostructures with a width of ∼12.08 Å. In the sandwiched part, the gapped Dirac cone of the graphene was controlled using a pseudospin by changing the magnetic alignment of the Ni(111) nanostructures. Upon considering the antiparallel configuration of Ni(111) nanostructures, the transmission probability calculation of the in-plane conductance of graphene shows a gap-like transmission at E - E F = 0.2 and 0.65 eV from the pd-hybridization and controllable Dirac cone of graphene, respectively. In the parallel configuration, the transmission probability calculation showed a profile similar to that of the pristine graphene. High and colossal magnetoresistance ratios of 284% and 3100% were observed at E - E F = 0.65 eV and 0.2 eV, respectively. Furthermore, a magnetoresistance beyond 3100% was expected at E - E F = 0.65 eV when the width of the Ni(111) nanostructures on the nanometer scale was considered.

High magnetoresistance of a hexagonal boron nitride-graphene heterostructure-based MTJ through excited-electron transmission.

This work presents an ab initio study of a few-layer hexagonal boron nitride (hBN) and hBN-graphene heterostructure sandwiched between Ni(111) layers. The aim of this study is to understand the electron transmission process through the interface. Spin-polarized density functional theory calculations and transmission probability calculations were conducted on Ni(111)/nhBN/Ni(111) with n = 2, 3, 4, and 5 as well as on Ni(111)/hBN-Gr-hBN/Ni(111). Slabs with magnetic alignment in an anti-parallel configuration (APC) and parallel configuration (PC) were considered. The pd-hybridizations at both the upper and lower interfaces between the Ni slabs and hBN were found to stabilize the system. The Ni/nhBN/Ni magnetic tunnel junction (MTJ) was found to exhibit a high tunneling magnetoresistance (TMR) ratio at ∼0.28 eV for n = 2 and 0.34 eV for n > 2, which are slightly higher than the Fermi energy. The observed shifting of this high TMR ratio originates from the transmission of electrons through the surface states of the d z 2 -orbital of Ni atoms at interfaces which are hybridized with the p z -orbital of N atoms. In the case of n > 2, the proximity effect causes an evanescent wave, contributing to decreasing transmission probability but increasing the TMR ratio. However, the TMR ratio, as well as transmission probability, was found to be increased upon replacing the unhybridized hBN layer of the Ni/3hBN/Ni MTJ with graphene, thus yielding Ni/hBN-Gr-hBN/Ni. A TMR ratio as high as ∼1200% was observed at an energy of 0.34 eV, which is higher than the Fermi energy. Furthermore, a design is proposed for a device based on a new reading mechanism using the high TMR ratio observed just above the Fermi energy level.

Manifestation of charge/orbital order and charge transfer in temperature-dependent optical conductivity of single-layered Pr0.5Ca1.5MnO4.

Half-doped single-layered manganite, [Formula: see text] has shown a charge/orbital order of the e g electrons and CE-type spin order of the t 2g electrons of the Mn ions. A previous experimental study on that system, supported by a simple modelling, has suggested that the charge/orbital ordering play an important role in governing the temperature dependence of optical conductivity of a broad peak around 0.7-0.8 eV. In addition, another peak around 3.5 eV, which is less sensitive to temperature, has been attributed to the charge transfer from O-p  to Mn-e g orbitals. Nevertheless, the theoretical explanation was incomplete as the role of O-p  orbitals was not considered in the model. In this paper, we propose to improve the model by incorporating both Mn-e g and O-p  orbitals. We assume the existence of charge/orbital ordering and investigate how this ordering as well as the charge-transfer phenomenon control the temperature dependence of the optical conductivity. Our results reveal the charge/orbital-ordering peak in the region 0.7-1.2 eV, which is blue-shifted with decreasing temperature, and the charge-transfer peak around 3.5 eV, which is less sensitive to temperature. The capability of our model to capture the general profile and temperature dependence of the optical conductivity suggests the validity of our theory.

Modulation of Manganite Nanofilm Properties Mediated by Strong Influence of Strontium Titanate Excitons.

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic, and magnetic properties induced by the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic, and magnetic properties of La0.7Sr0.3MnO3 thin films on SrTiO3 (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2 nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin film nanostructure LSMO/STO at ∼4 eV, which could be attributed to the formation of anomalous charged excitonic complexes. On the basis of the spectral weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate plays a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

Tunable and low-loss correlated plasmons in Mott-like insulating oxides.

Plasmonics has attracted tremendous interests for its ability to confine light into subwavelength dimensions, creating novel devices with unprecedented functionalities. New plasmonic materials are actively being searched, especially those with tunable plasmons and low loss in the visible-ultraviolet range. Such plasmons commonly occur in metals, but many metals have high plasmonic loss in the optical range, a main issue in current plasmonic research. Here, we discover an anomalous form of tunable correlated plasmons in a Mott-like insulating oxide from the Sr1-xNb1-yO3+δ family. These correlated plasmons have multiple plasmon frequencies and low loss in the visible-ultraviolet range. Supported by theoretical calculations, these plasmons arise from the nanometre-spaced confinement of extra oxygen planes that enhances the unscreened Coulomb interactions among charges. The correlated plasmons are tunable: they diminish as extra oxygen plane density or film thickness decreases. Our results open a path for plasmonics research in previously untapped insulating and strongly-correlated materials.