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...

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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

We address the properties of excitons in monolayer ${\mathrm{MoS}}_{2}$ from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin-zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the ${(n\ensuremath{-}1/2)}^{\ensuremath{-}2}$ pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with nonanalytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer ${\mathrm{MoS}}_{2}$ is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.

Exciton band structure of monolayer MoS 2

We address the properties of excitons in monolayer MoS$_2$ from a theoretical point of view, showing that low-energy excitonic states occur both at the Brillouin zone center and at the Brillouin-zone corners, that binding energies at the Brillouin-zone center deviate strongly from the $(n-1/2)^{-2}$ pattern of the two-dimensional hydrogenic model, and that the valley-degenerate exciton doublet at the Brillouin-zone center splits at finite momentum into an upper mode with non-analytic linear dispersion and a lower mode with quadratic dispersion. Although monolayer MoS$_2$ is a direct-gap semiconductor when classified by its quasiparticle band structure, it may well be an indirect gap material when classified by its excitation spectra.

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Exciton band structure of monolayer MoS$_2$

We present a theory of magnetic exchange interactions in quantum dots containing electrons and magnetic ions. We find the interaction between the electron and Mn ion to depend strongly on the number of electrons. It can be switched off for closed shell configurations and maximized for partially filled shells. However, unlike the total electron spin S which is maximized for half-filled shells, we predict the exchange interaction to be independent of the filling of the electronic shell. We show how this unusual effect manifests itself in quantum dot addition and excitation spectrum.

Magnetic exchange interactions in quantum dots containing electrons and magnetic ions.

In this study the fusion method was used to synthesize PbS nanocrystal quantum dots (QDs) embedded in S-doped glass matrix (SiO2−Na2CO3−ZnO−Al2O3−PbO2−B2O3). Optical absorption, atomic force microscopy (AFM), and photoluminescence (PL) were used to investigate different samples. Experimental data indicate that the PbS QD-size is controlled by the different annealing times the S-doped glass matrix is submitted to. The size-dependence of the optical transitions in PbS QDs were obtained by numerical calculation and compared with the experimental data. A strong anti-Stokes photoluminescence (ASPL), from green (2.409 eV) to violet (2.978 eV), was found in all samples, at room temperature. A possible microscopic mechanism leading to the ASPL, which involves a two-photon absorption in two separate steps through a surface state, is proposed.

Anti-Stokes photoluminescence in nanocrystal quantum dots

A systematic study on the influence of two intense, long-wavelength, nonresonant laser fields on the electron energy levels and density of states (DOS) in $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum wells is performed within a Green's function approach. The carrier confinement pattern and its associated DOS are shown to be modified by the laser beams. For laser field polarizations parallel to the growth direction only the effective potential is changed whereas for in-plane polarizations only the DOS is altered in the sense that it is field-driven. The results show that for a $\mathrm{GaAs}∕\mathrm{AlGaAs}$ quantum well the effect of the laser field radiation is to induce strong blueshifts in the electronic energy levels. The DOS dependence on the laser-induced confinement characteristics changes from the usual ladder profile to a functional form that reminds us of a one-dimensional system.

Fanyao Qu

Papers

Exciton band structure of monolayer MoS 2

Exciton band structure of monolayer MoS$_2$

Magnetic exchange interactions in quantum dots containing electrons and magnetic ions.

Anti-Stokes photoluminescence in nanocrystal quantum dots

Electronic properties of a quasi-two-dimensional electron gas in semiconductor quantum wells under intense laser fields