Here we study subwavelength gratings for coupling into graphene plasmons by means of an analytical model based on transformation optics that is not limited to very shallow gratings. We consider gratings that consist of a periodic modulation of the charge density in the graphene sheet, and gratings formed by this conductivity modulation together with a dielectric grating placed in close vicinity of the graphene. Explicit expressions for the dispersion relation of the plasmon polaritons supported by the system, and reflectance and transmittance under plane wave illumination are given. We discuss the conditions for maximising the coupling between incident radiation and plasmons in the graphene, finding the optimal modulation strength for a conductivity grating. arXiv:1602.06812v1 [cond-mat.mes-hall]

Abstract

Si-Sb-Te materials including Te-rich Si₂Sb₂Te₆ and Si(x)Sb₂Te₃ with different Si contents have been systemically studied with the aim of finding the most suitable Si-Sb-Te composition for phase change random access memory (PCRAM) use. Si(x)Sb₂Te₃ shows better thermal stability than Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ in that Si(x)Sb₂Te₃ does not have serious Te separation under high annealing temperature. As Si content increases, the data retention ability of Si(x)Sb₂Te₃ improves. The 10 years retention temperature for Si₃Sb₂Te₃ film is ~393 K, which meets the long-term data storage requirements of automotive electronics. In addition, Si richer Si(x)Sb₂Te₃ films also show improvement on thickness change upon annealing and adhesion on SiO₂ substrate compared to those of Ge₂Sb₂Te₅ or Si₂Sb₂Te₆ films. However, the electrical performance of PCRAM cells based on Si(x)Sb₂Te₃ films with x > 3.5 becomes worse in terms of stable and long-term operations. Si(x)Sb₂Te₃ materials with 3 < x < 3.5 are proved to be suitable for PCRAM use to ensure good overall performance.

Lithium nickel manganese cobalt oxide (NMC) cathodes are of great importance for the development of lithium ion batteries with high energy density. Currently, most commercially available NMC products are polycrystalline secondary particles, which are aggregated by anisotropic primary particles. Although the polycrystalline NMC particles have demonstrated large gravimetric capacity and good rate capabilities, the volumetric energy density, cycling stability as well as production adaptability are not satisfactory. Well-dispersed single-crystalline NMC is therefore proposed to be an alternative solution for further development of high-energy-density batteries. Various techniques have been explored to synthesize the single-crystalline NMC product, but the fundamental mechanisms behind these techniques are still fragmented and incoherent. In this manuscript, we start a journey from the fundamental crystal growth theory, compare the crystal growth of NMC among different techniques, and disclose the key factors governing the growth of single-crystalline NMC. We expect that the more generalized growth mechanism drawn from invaluable previous works could enhance the rational design and the synthesis of cathode materials with superior energy density.

Ni-rich layered LiNi x Mn y Co z O2 (NMC) cathodes for lithium-ion batteries are receiving a lot of attention owing to their promising large capacity, whereas the high content of Ni results in several issues including poor thermal stability and serious Li/Ni disorder. Although a little degree of the Li/Ni disorder may be beneficial for the structural stability of NMC cathodes and even migration of Li ions, a high degree of the Li/Ni disorder certainly deteriorates their electrochemical performances. Therefore, tuning the Li/Ni disorder is of great interest in the development of safer NMC cathodes with larger accessible capacity. Post-synthesis annealing is a facile and low-cost way to manipulate lattice defects, yet has not been utilized to optimize the Ni-rich NMC cathodes. In this work, we report that post-synthesis annealing can induce the competition between lattice ordering and structure decomposition. The thermal annealing promoted that lattice ordering would prevail until the decomposition of oxygen lattice. Once the annealing temperature reaches the critical temperature to form oxygen vacancies, Ni ions can easily migrate into the Li slab. The Li/Ni disorder can be facilely tuned through post-synthesis annealing to optimize the electrochemical performances of NMC cathodes.

Compared with pure Sb2Te3, Ti0.32Sb2Te3 (TST) phase change material has larger resistance ratio, higher crystallization temperature and better thermal stability. The sharp decrease in mobility is responsible for the increasing amorphous and crystalline sheet resistance. The uniform crystalline structure of TST film is very benefit for the endurance characteristic. The Set and Reset operation voltages for TST-based phase change memory device are much lower than those of conventional Ge2Sb2Te5-based one. Remarkably, the device presents extremely rapid Set operation speed (∼6 ns). Furthermore, up to 1 × 106 programming cycles are obtained with stable Set and Reset resistances.

With a high crystallization temperature of 211 °C, Ti10Sb60Te30 phase change material exhibits a data retention of 10-yr at 137 °C, which is much better than that of usual Ge2Sb2Te5. No other phase is formed in Ti10Sb60Te30 film except hexagonal Sb2Te phase. For Ti10Sb60Te30-based phase change memory cell, as short as 6 ns electric pulse can fulfill the Set operation, demonstrating an extremely rapid crystallization speed of Ti10Sb60Te30. The programming cycles can reach 2.2 × 104 with very short Set/Reset pulses of 100 ns/50 ns.

A rigorous theory is developed to predict the radiation pressure force (RPF) exerted on a spheroid by an arbitrarily oriented and located shaped beam. Analytical expressions of RPF are derived for a homogeneous spheroid, which can be prolate or oblate, transparent or absorbing. Exemplifying calculations are performed and RPF calculations for spheroids are compared to RPF calculations for spheres. The "Optical Stretcher" is also numerically simulated to study the RPF exerted on a red blood cell during its deformation.