Antireflection coatings from analogy between electron scattering and spin precession

We use the analogy between scattering of a wave from a potential, and the precession of a spin-half particle in a magnetic field, to gain insight into the design of an antireflection coating for electrons in a semiconductor superlattice. It is shown that the classic recipes derived for optics are generally not applicable due to the different dispersion law for electrons. Using the stability conditions we show that a Poisson distribution of impedance steps is a better approximation than is a Gaussian distribution. Examples are given of filters with average transmissivity exceeding 95% over an allowed band

Ultrafast carrier relaxation in GaN, In_(0.05)Ga_(0.95)N and an In_(0.05)Ga_(0.95)/In_(0.15)Ga_(0.85)N Multiple Quantum Well

Room temperature, wavelength non-degenerate ultrafast pump/probe measurements were performed on GaN and InGaN epilayers and an InGaN multiple quantum well structure. Carrier relaxation dynamics were investigated as a function of excitation wavelength and intensity. Spectrally-resolved sub-picosecond relaxation due to carrier redistribution and QW capture was found to depend sensitively on the wavelength of pump excitation. Moreover, for pump intensities above a threshold of 100 microJ/cm2, all samples demonstrated an additional emission feature arising from stimulated emission (SE). SE is evidenced as accelerated relaxation (< 10 ps) in the pump-probe data, fundamentally altering the re-distribution of carriers. Once SE and carrier redistribution is completed, a slower relaxation of up to 1 ns for GaN and InGaN epilayers, and 660 ps for the MQW sample, indicates carrier recombination through spontaneous emission.Comment: submitted to Phys. Rev.

Low Temperature Properties of Anisotropic Superconductors with Kondo Impurities

We present a self-consistent theory of superconductors in the presence of Kondo impurities, using large-$N$ slave-boson methods to treat the impurity dynamics. The technique is tested on the s-wave case and shown to give good results compared to other methods for $T_K > T_c$. We calculate low temperature thermodynamic and transport properties for various superconducting states, including isotropic s-wave and representative anisotropic model states with line and point nodes on the Fermi surface.Comment: 21 pages, RevTeX 3.0, 12 figures available upon request, UF preprin

Realising superoscillations: A review of mathematical tools and their application

Superoscillations are making a growing impact on an ever-increasing number of real-world applications, as early theoretical analysis has evolved into wide experimental realisation. This is particularly true in optics: the first application area to have extensively embraced superoscillations, with much recent growth. This review provides a tool for anyone planning to expand the boundaries in an application where superoscillations have already been used, or to apply superoscillations to a new application. By reviewing the mathematical methods for constructing superoscillations, including their considerations and capabilities, we lay out the options for anyone wanting to construct a device that uses superoscillations. Superoscillations have inherent trade-offs: as the size of spot reduces, its relative intensity decreases as high-energy sidebands appear. Different methods provide solutions for optimising different aspects of these trade-offs, to suit different purposes. Despite numerous technological ways of realising superoscillations, the mathematical methods can be categorised into three approaches: direct design of superoscillatory functions, design of pupil filters and design of superoscillatory lenses. This categorisation, based on mathematical methods, is used to highlight the transferability of methods between applications. It also highlights areas for future theoretical development to enable the scientific and technological boundaries to be pushed even further in real-world applications

Anomalous Behavious of the Kondo Superconductor (La_1-xCe_x) Al₂

At low temperatures two phenomena of collective behavior of the conduction electrons in metals are known: superconductivity and the Kondo effect. With the system (La, Ce) Al2 we succeeded in demonstrating a strong interaction between these two phenomena as predicted by Müller-Hartmann and Zittartz in 1971.1 The most unusual behavior of this superconducting alloy is the vanishing of superconductivity below a second transition temperature at very low temperatures. This second transition back into the normal state was first observed by Riblet and Winzer2 in an adiabatic demagnetization cryostat with a standard ac mutual inductance technique. Figure 1 shows a typical example of these two transitions. The induction signal of the specimen compared to that of a clean LaAl2 probe of the same size is plotted vs. temperature. This specimen with 0.67 at. % Ce substitution of La in LaAl2 becomes superconducting between 1.5 and 1°K and normal again below 0.5°K. As a consequence of this second transition, the temperature dependence of all the other superconducting properties becomes strange. It is therefore proposed to call such an alloy a “Kondo superconductor.

Specific heat of a Kondo superconductor: (La, Ce) Al₂

The specific heat of LaAl2 and (La1-xCex)Al2 (x ⩽ 0.0064) has been measured between 0.3 and 5 K, both in the superconducting and in the normal state. For all samples the same values for the Debye temperature as well as for the electronic specific heat coefficient have been determined. LaAl2 shows an excellent BCS behavior. A remarkable excess specific heat at low temperatures due to the Kondo effect has been observed for all superconducting as well as for the normal conducting (La1-xCex) Al2 alloys. The specific heat jump ΔC at Tc depressed rapidly with increasing Ce concentration, allows the Kondo temperature TK ≅ 1 K to be determined. ΔC vanishes at finite temperatures