Inducement of Spin-pairing and Correlated Semi-metallic State in Mott-Hubbard Quantum Dot Array

We model a quantum dot-array (with one electron per dot) comprising of two (or more than two) coupled dots by an extended Hubbard Hamiltonian to investigate the role played by the inter-dot tunneling amplitude td, together with intra-dot (U) and inter-dot(U1) coulomb repulsions, in the singlet / triplet bound state formation and evolution of the system from the Mott-insulator-like state to a correlated semi-metallic state via charge-bond-order route. In the presence of magnetic field, td is complex due to the appearance of Peierls phase factor. We introduce a short-ranged inter-dot capacitive coupling U0, assumed to be non-zero for nearest-neighbor dots only, for the bound state analysis. The study indicates that, while for the tunable parameter d = (2td/U0) greater than unity only the possibility of the triplet bound state formation exists, for d less than one both triplet and singlet states are possible. The bound states are formed due to tunneling and capacitive dot-bondings with coulomb interactions (U,U1) playing marginal role. The interaction U, however, is found to play, together with complex td, an important role in the evolution of the double quantum dot system from the insulator-like state to that of a correlated semi-metallic state through charge-bond-ordering route.Comment: 18 pages,4 figure

Surface texturisation for the reduction of light reflection in ZnO/Si heterojunction

In this paper, the impact of pyramidal texture on a silicon substrate in ZnO/p-Si heterojunction was investigated. The texturisation of p-type silicon (100) substrate was obtained using the KOH anisotropic wet chemical etching method for different etching times. The RF magnetron sputtering technique was used to deposit ZnO thin films on textured Si substrates and planar Si substrates to form ZnO/Si heterojunction. The surface morphology was studied with field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Optical properties were investigated using UV-Visible spectroscopy and photoluminescence (PL). The results show that the PL intensity in the visible region of the electromagnetic spectrum increases with the etching time, while a significant reduction is observed in the reflectance. Due to impressive anti-reflection response, ZnO/Si (textured silicon-TS) heterojunction can be effective in improving the efficiency of solar cells

Laser damage studies of silicon surfaces using ultra-short laser pulses

Laser-induced damage morphology using femtosecond laser pulses on Si surfaces is reported. Damage morphology shows the ablation of material. A magnified view of the ablated portion shows a periodic surface structure in the form of ripples. The spacing of these ripples was between 0.5 and 2 μm and increased, on increasing the power density or number of pulses, and finally broke into parts, leaving well-ordered grains of approximate diameter 5 μm. Also for 100 or larger number of pulses, an amorphous ring in the periphery was formed. The diameter of this ring increased, on increasing either the laser fluence or the number of pulses. The formation of ripples has been explained with the help of the hypothesis of Boson condensation proposed by Van Vechten (Solid State Commun 39 (1981) 1285)

Effect of polarization on the surface damage morphology of GaAs single crystal during irradiation with picosecond laser pulses

A comparative study of damage morphology in GaAs induced by s- , p- and linearly polarized laser light (1.064 μm, 35 ps) is presented. For linearly polarized light damage initiates in the form of pits. This material damage occurs below the surface. For s- or p-polarized light material damage involves only the surface layer. For larger fluences or number of pulses the differences are less marked and the damage morphology occurs in a similar manner either for linearly polarized or s- or p-polarized light. Ripples are formed when multiple irradiation is used due to interference between the front and back faces of the test sample. The spacing of these ripples is 3 μm, which is in good accordance with the reported work of Guosheng et al

Effect of high index buffer layer in PbSe clad waveguide to design a visible range polarizer

Polarization properties of PbSe clad optical waveguide are exploited theoretically, in the visible range at λ = 0.6328 μm. Due to semiconducting nature, lead selenide (PbSe) supports lossy modes and showed oscillatory behavior in generated attenuation curves. It is observed that at different PbSe thicknesses, TM or TE guided modes may be selectively attenuated. High attenuation in waveguide modes mainly occurred due to the phase matching between lossy modes supported by PbSe cladding and lossless waveguide modes. It is proposed that by placing a high index buffer layer between PbSe cladding and waveguide, phase matching can be improved further. This property can be utilized to design highly efficient polarizers. Significant results are presented to show the effect of different buffer indices (1.55, 1.88 and 2.67). At the buffer index value  1.88, TE and TM pass polarizers can be designed with extinction ratios 501 dB and 316 dB respectively. Also, an extensively high attenuation loss of 243299 dBcm-1 can be achieved with buffer index 2.67 for TM mode polarization. Obtained results are well supported by generated field plots for the proposed polarizing structures