A new nanocomposite superionic system (CdHgI4/HgI2): Synthesis, characterization and experimental investigation

In nature, adaptive coloration has been effectively utilized for concealment and signaling. Various biological mechanisms have evolved to tune the reflectivity for visible and ultraviolet light. These examples inspire many artificial systems for mimicking adaptive coloration to match the visual appearance to their surroundings. Thermal camouflage, however, has been an outstanding challenge which requires an ability to control the emitted thermal radiation from the surface. Here we report a new class of active thermal surfaces capable of efficient real-time electrical-control of thermal emission over the full infrared (IR) spectrum without changing the temperature of the surface. Our approach relies on electro-modulation of IR absorptivity and emissivity of multilayer graphene via reversible intercalation of nonvolatile ionic liquids. The demonstrated devices are light (30 g/m2), thin (<50 μm), and ultraflexible, which can conformably coat their environment. In addition, by combining active thermal surfaces with a feedback mechanism, we demonstrate realization of an adaptive thermal camouflage system which can reconfigure its thermal appearance and blend itself with the varying thermal background in a few seconds. Furthermore, we show that these devices can disguise hot objects as cold and cold ones as hot in a thermal imaging system. We anticipate that, the electrical control of thermal radiation would impact on a variety of new technologies ranging from adaptive IR optics to heat management for outer space applications.

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Graphene-Based Adaptive Thermal Camouflage

Electrical currents flowing in ferromagnetic materials are spin-polarised as a result of the spin-dependent band structure. When the spatial direction of the polarisation changes, in a domain structure, the electrons must somehow accommodate the necessary change in direction of their spin angular momentum as they pass through the wall. Reflection, scattering, or a transfer of angular momentum onto the lattice are all possible outcomes, depending on the circumstances. This gives rise to a variety of different physical effects, most importantly a contribution to the electrical resistance caused by the wall, and a motion of the wall driven by the spin-polarised current. Historical and recent research on these topics is reviewed. Contents PAGE 1. Introduction5862. Spin-polarised current587 2.1. Tunnelling current spin polarisation589 2.2. Ballistic current spin polarisation592 2.3. Diffusive current spin polarisation5933. Magnetic Domain Walls598 3.1. Basics of domain walls598 3.1.1. Domain wall thickness and...

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Spin-polarised currents and magnetic domain walls

There are two main thrusts towards new permanent-magnet materials: improving extrinsic properties by nanostructuring and intrinsic properties by atomic structuring. Theory-both numerical and analytical-plays an important role in this ambitious research. Our analysis of aligned hard-soft nanostructures shows that soft-in-hard geometries are better than hard-in-soft geometries and that embedded soft spheres are better than sandwiched soft layers. Concerning the choice of the hard phase, both a high magnetization and a high anisotropy are necessary. As an example of first-principle research, we consider interatomic Mn exchange in MnAl and find strongly ferromagnetic intralayer exchange, in spite of the small Mn-Mn distances.

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Predicting the Future of Permanent-Magnet Materials

A theoretical study of Al1−xGaxN, based on the full-potential linearized augmented plane wave method, is used to investigate the variations in the bandgap, optical properties, and nonlinear behavior of the compound with the change in the Ga concentration. It is found that the bandgap decreases with the increase in Ga. A maximum value of 5.50 eV is determined for the bandgap of pure AlN, which reaches a minimum value of 3.0 eV when Al is completely replaced by Ga. The static index of refraction and dielectric constant decreases with the increase in the bandgap of the material, assigning a high index of refraction to pure GaN when compared to pure AlN. The refractive index drops below 1 for higher energy photons, larger than 14 eV. The group velocity of these photons is larger than the vacuum velocity of light. This astonishing result shows that at higher energies the optical properties of the material shifts from linear to nonlinear. Furthermore, frequency dependent reflectivity and absorption coefficients...

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Ab initio study of the bandgap engineering of Al1−xGaxN for optoelectronic applications

We present calculations of interband optical conductivity of the compound Ni 3 Al using the linear-muffin-tin-orbital method. Calculations are performed using constant matrix elements and calculated matrix elements. We find that matrix elements play a significant role in influencing the magnitude of optical conductivity σ(ω) and the position of peaks. Since Ni 3 Al is weakly ferromagnetic (magnetic moment equal to 0.31 μ B per unit cell), we have also calculated σ(ω) using the ferromagnetic band structure but without spin-orbit coupling. A comparison of our results with experimental data is presented

Interband optical properties of Ni3Al.

Using density functional theory implemented within a tight-binding linear muffin-tin orbital method we perform calculations of electronic, magnetic, and transport properties of ferromagnetic free-standing fcc Co wires with diameters up to 1.5 nm. We show that finite-size effects play an important role in these nanowires resulting in oscillatory behavior of electronic charge and the magnetization as a function of the wire thickness, and a nonmonotonic behavior of spin-dependent quantized conductance. We calculate the magnetoresistance (MR) of a domain wall (DW) modeled by a spin-spiral region of finite width sandwiched between two semi-infinite Co wire leads. We find that the DW MR decreases very rapidly, on the scale of a few interatomic layers, with the increasing DW width. The largest MR value of about 250% is predicted for an abrupt DW in the monatomic wire. We show that, for some energy values, the density of states and the conductance may be nonzero only in one spin channel, making the MR for the abrupt DW infinitely large. We also demonstrate that for the abrupt DW a large MR may occur due to the hybridization between two spin subbands across the DW interface. We do not find, however, such a behavior at the Fermi energy for the Co wires considered.

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Domain-wall magnetoresistance of Co nanowires

Three ordered compounds of the Co-Pt system, namely ${\mathrm{Co}}_{3}\mathrm{Pt},$ CoPt, and ${\mathrm{CoPt}}_{3},$ are ferromagnetic in nature. We have performed band-structure calculations for these compounds using the ab initio linear muffin-tin orbitals method. Our results are in agreement with the experimentally measured ground-state properties such as density of states, electronic specific-heat coefficient, and magnetic moments. The band-structure results along with the Mohn Wohlfarth theory are used to calculate the Curie temperature of these compounds. Our calculated values of the Curie temperature are in agreement with the experimental data of the Curie temperature. We also report calculations for the Fermi surface of ${\mathrm{Co}}_{3}\mathrm{Pt}$ and ${\mathrm{CoPt}}_{3}$ with the aim to ascertain how the Fermi surface is altered when Co(Pt) is replaced by Pt(Co), keeping the same crystal structure.

Electronic structure, Fermi surface, and Curie temperature calculations for the Co-Pt system

Optical properties of A1N

The electronic band structure of ordered equiatomic compounds of 3d transition elements (Mn, Fe, Co, Ni) with nonmagnetic 4d and 5d elements (Rh, Pd, Pt, Ir) are investigated by linear muffin-tin orbital calculations. The systematic study considers 3d and 4d/5d spin moments and interatomic exchange interactions, with emphasis on the comparison between ferromagnetic and antiferromagnetic order. Total and site-resolved exchange interactions are calculated from first principles, and the obtained exchange constants are used to estimate ordering temperatures on a mean-field level.

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A. K. Solanki

Papers

Interband optical properties of Ni3Al.

Domain-wall magnetoresistance of Co nanowires

Electronic structure, Fermi surface, and Curie temperature calculations for the Co-Pt system

Optical properties of A1N

Magnetism of L10 compounds with the composition MT (M=Rh, Pd, Pt, Ir and T=Mn, Fe, Co, Ni)