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

The optical properties of n- and p-type, modulation- and delta-doped Al035Ga065As–GaAs single heterojunctions (SHs) have been studied by means of low-temperature photoluminescence measurement and self-consistent calculation A very strong, broad, and asymmetric H-band emission was observed only in p-type delta-doped SH When the optical excitation intensity increases by two orders of magnitude a redshift as large as 27 meV is observed in the H-band emission energy The large redshift observed is attributed to both inefficiency of screening in the two-dimensional-hole gas and effective band-gap renormalization It was also observed that the H-band emission strongly depends upon the doping type (n or p type) and doping profile (modulation doping or delta doping)

Optical properties of H-band emission in single heterojunctions

A pathway to design non-magnetic artificial molecules which display controllable magnetic properties is addressed theoretically by studying the effects of in-plane electrical field, spin–orbit interaction (SOI) and geometrical parameters on the magnetic phase transitions in few-electron lateral double quantum dots (DQDs). We demonstrate the tunability of the magnetic phase diagram of two-electron DQDs as the system is changed from a molecule to an atom, in both weak and strong SOI regimes. We find an unusual jump in the magnetization and an asymmetric peak of the magnetic susceptibility. In addition, both the asymmetric susceptibility peak position and the magnetic phase diagram are strongly dependent on the interdot tunnel coupling, which can be tuned effectively by changing repulsive barrier voltage and/or interdot distance, the number of electrons and the SOI strength. With increasing interdot tunnel-coupling strength, for instance, the rate of paramagnetic-to-diamagnetic phase area increases. The SOI makes the paramagnetic phase more stable under magnetic field. Moreover, the effects of geometry deviation on the electronic structure and magnetic property of the DQD are also discussed.

Magnetic phase diagram of non-magnetic few-electron quantum dot molecules

Energy transfer process and coupling-strength between PbS (PbSe) quantum dots have been studied by means of photoluminescence (PL), micro-PL, and spatially-resolved PL measurements. The energy-dependent transfer rate of excitons from smaller to larger dots via electronic coupling is observed. It has been shown that an efficient energy transfer occurs between quantum dots having a significant band gap energy difference, i.e., between dots of markedly different size. Thus, a promising way to enhance energy transfer between different subsets of quantum dots is to red-shift the emission sufficiently far from the host absorption so that self-absorption by the host becomes negligible. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Long-range interactions in PbS and PbSe nanocrystals: the problem of reabsorption

The Nd3+ Cr3+ single and codoped lead silicate glasses (SiO2-Na2O-Sb2O3- PbO2) were synthesized by means of fusion method. Their optical properties were studied by means of absorption and photoluminescence (PL), recorded at room temperature. The wear 4F3/2 yields 4I9/2 emission band centered at 890 nm observed in the Nd3+ single doped glasses was strongly enhanced in the Nd3+ and Cr3+ codoped glass. The effective mechanisms based on both direct donor-donor energy transfer processes assisted by localized phonons and energy cross-relaxation between donor-acceptor were proposed.

Fanyao Qu

Papers

Optical properties of H-band emission in single heterojunctions

Magnetic phase diagram of non-magnetic few-electron quantum dot molecules

Long-range interactions in PbS and PbSe nanocrystals: the problem of reabsorption

Energy transfer process in the rare-earth-ion-doped lead silicate glasses

WanTiBEXOS: A Wannier based Tight Binding code for electronic band structure, excitonic and optoelectronic properties of solids