In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here recent progress in understanding of the excitonic properties in monolayer TMDs is reviewed and future challenges are laid out. Discussed are the consequences of the strong direct and exchange Coulomb interaction, exciton light-matter coupling, and influence of finite carrier and electron-hole pair densities on the exciton properties in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

First-principles band theory, properly augmented by topological considerations, has provided a remarkably successful framework for predicting new classes of topological materials. This Colloquium discusses the underpinnings of the topological band theory and its materials applications.

Colloquium : Topological band theory

Importance of the field: Metal oxide nanoparticles, including zinc oxide, are versatile platforms for biomedical applications and therapeutic intervention. There is an urgent need to develop new classes of anticancer agents, and recent studies demonstrate that ZnO nanomaterials hold considerable promise.Areas covered in this review: This review analyzes the biomedical applications of metal oxide and ZnO nanomaterials under development at the experimental, preclinical and clinical levels. A discussion regarding the advantages, approaches and limitations surrounding the use of metal oxide nanoparticles for cancer applications and drug delivery is presented. The scope of this article is focused on ZnO, and other metal oxide nanomaterial systems, and their proposed mechanisms of cytotoxic action, as well as current approaches to improve their targeting and cytotoxicity against cancer cells.What the reader will gain: This review aims to give an overview of ZnO nanomaterials in biomedical applications.Take home...

/pdf/zinc-oxide-nanoparticles-for-selective-destruction-of-tumor-25sugjbm0z.pdf

Zinc oxide nanoparticles for selective destruction of tumor cells and potential for drug delivery applications.

We show that the pseudorelativistic physics of graphene near the Fermi level can be extended to three dimensional (3D) materials. Unlike in phase transitions from inversion symmetric topological to normal insulators, we show that particular space groups also allow 3D Dirac points as symmetry protected degeneracies. We provide criteria necessary to identify these groups and, as an example, present ab initio calculations of β-cristobalite BiO(2) which exhibits three Dirac points at the Fermi level. We find that β-cristobalite BiO(2) is metastable, so it can be physically realized as a 3D analog to graphene.

Dirac Semimetal in Three Dimensions

PbS quantum dots (QDs) embedded in a silicate glass matrix (Si02-Na 2 CO 3 -ZnO-Al 2 O 3 -PbO 2 -B 2 O 3 ) were synthesized by a fusion method. The QD size strongly depends upon thermal treatment process variables, such as annealing time and temperature. Two up-conversion photoluminescence emission bands at about 2.475 and 2.978 eV were observed. They are ascribed to the resonant Raman process and to the two-step two-photon absorption via a real intermediate state, respectively.

Optical Absorption and Photoluminescence in PbS Quantum Dots

Photoluminescence measurements performed in a wide range of temperature (10-100 K) and optical excitation intensity (0.03-90 W/cm2), and self-consistent numerical calculation is used to investigate n-doped GaAs/Al x Ga 1 − x As single asymmetric quantum wells. Under strong optical excitation intensity, a red-shift in the recombination energy with increasing temperature is observed. In the weak optical excitation condition, however, a breakdown of the recombination energy shift is found. The flat response was attributed to the combined effects of the temperature dependence of the bandgap energy, band bending, and bandgap renormalization.

https://iopscience.iop.org/article/10.1209/epl/i2001-00221-7/pdf

The breakdown of the temperature dependence of the photoluminescence peak position in single asymmetric quantum wells

We propose a physical model based on disordered (a hole punched inside a material) monolayer transition metal dichalcogenides (TMDs) to demonstrate a large-gap quantum valley Hall insulator. We find an emergence of bound states lying inside the bulk gap of the TMDs. They are strongly affected by spin-valley coupling, rest- and kinetic- mass terms and the hole size. In addition, in the whole range of the hole size, at least two in-gap bound states with opposite angular momentum, circulating around the edge of the hole, exist. Their topological insulator (TI) feature is analyzed by the Chern number, characterized by spacial distribution of their probabilities and confirmed by energy dispersion curves (Energy vs. angular momentum). It not only sheds light on overcoming low-temperature operating limitation of existing narrow-gap TIs, but also opens an opportunity to realize valley- and spin- qubits.

Topological nature of in-gap bound states in disordered large-gap monolayer transition metal dichalcogenides

Spin-split antibonding molecular ground state in manganese-doped quantum dot molecules

We propose a mechanism based on thermally assisted charge transfer (CT) to study the thermal effect on the formation of CT states and subsequent charge separation in an organic donor–acceptor solar cell. We reveal that the difference between phonons in the donor and acceptor caused by elastic energy acts as a thermally assisted driving force for charge transfer. It is found that the system exhibits a quite different CT process in the high and low temperature regions. Remarkably, combined with the entropy driving mechanics, the thermally assisted CT yields charge separation probability as high as 70% at room temperature. Our model and results provide a microscopic quantum understanding of the relevant recent experiments and open up a route to realize high-efficiency organic solar cells by effectively taking advantage of the thermal effect.

Fanyao Qu

Papers

Optical Absorption and Photoluminescence in PbS Quantum Dots

The breakdown of the temperature dependence of the photoluminescence peak position in single asymmetric quantum wells

Topological nature of in-gap bound states in disordered large-gap monolayer transition metal dichalcogenides

Spin-split antibonding molecular ground state in manganese-doped quantum dot molecules

Thermally assisted charge transfer and charge separation in organic donor–acceptor solar cells