High-speed InGaAs metal-semiconductor-metal photodetectors with improved responsivity and process yield

High-performance In_0.9Ga_0.1P-InP-InGaAs metal-semiconductor-metal photodectors with semi-transparent Au Schottky contacts are fabricated and studied. The devices, with an active area of 50×50 m² and different finger spacings of 2, 3 and 4 m, all exhibit high responsivities over 0.7 AW^-1 and low dark currents below 10 nA. Extremely linear photoresponse without low frequency internal gain is also observed in these devices. The novel fabrication process used in this work is simple and has nearly 100% high yield. A device with a small finger spacing has also been demonstrated to have improved speed performance without sacrificing its responsivity.     

Far-infrared detection by GaAs-IREDs and GaAs0.6P0.4-LEDs

In GaAs-IREDs and GaAs_0.6P_0.4-LEDs a photoconductive and a bolometric FIR response has been observed at low temperatures by applying different pulsed optically pumped FIR lasers. The short circuit current responsivity peaks at =400 m being about 1.5 mA/W. By monitoring the rapid modulation of the FIR radiation due to mode beating of the pump laser, an upper limit of 1 ns was established to the time constant of the photoconductive signal.     

Seesaw mass matrix model of quarks and leptons with flavor-triplet Higgs scalars

In a seesaw mass matrix model with a universal structure of , as the origin of m_L (m_R) for quarks and leptons, flavor-triplet Higgs scalars whose vacuum expectation values v_i are proportional to the square roots of the charged lepton masses m_ei, i.e. , are assumed. Then, it is investigated whether such a model can explain the observed neutrino masses and mixings (and also quark masses and mixings) or not.     

Low lying structural instabilities and entropy of monoatomic liquids

The entropy of simple liquids is calculated within a model which reproduces the observed volume and entropy changes at the melting point reasonably well. The model is based on a sequeezed-atom instability originally proposed to explain low lying excitations in glasses. The process of flowing can be understood qualitatively in terms of formation and annihilation of these structural instabilities.     

Model for cometary grains to explain optical polarization

The observed optical polarizations for comets have been explained in past assuming cometary grains to be compact spheres, such that Mie theory could be applied to simulate the observed polarizations. However, recently other shapes like spheroids and then more realistic shapes like aggregates of monomers have been considered for cometary grains, to explain the observed polarizations. For this purpose T-matrix or DDA based light scattering technique was mostly used to simulate the observed polarizations. A number of authors have used T-matrix, DDA and various other techniques along with aggregate grain model to explain the polarizations of comets like 1P/Halley, C 1995/O1 Hale-Bopp, C/1990 K1 (Levy) and C/1996 B2 Hyakutake, etc. Recent STARDUST mission had suggested cometary grains to be mixtures of compact and porous aggregates. Accordingly, attempts have been made recently to reproduce the cometary polarization with mixtures of various compositions, shapes and porosity.The work presented here considers a model for cometary grains which contains (1) solid grains of pyroxene (silicate) and organic with various sizes of spheres, prolates and oblates and (2) aggregates with monomers of various sizes, with composition of pyroxene (silicate) and organic, having structures (shapes) defined by BCCA and BAM2 codes. It was found that the present model can explain the observed polarization data, especially the negative branch, for comet 1P/Halley at , more effectively as compared to other work done in past. Among the aggregates the BAM2 structure was found to play a key role, in deciding the cross-over angle and depth of negative polarization branch.     

A high-sensitivity near-millimeter-wave photoconductive detector using [Hg, Cd]Te

In an earlier paper [1], we reported the observation of photoconductivity from free-carrier absorption in [Hg, Cd]Te. By using samples of [Hg, Cd]Te with different electrical and alloy properties, we have improved the near-millimeter-wave (NMMW) responsivity by over two orders of magnitude. At 1.6 K a best sample responsivity of about 185 V/W and a bandwidth of over 5 MHz have been measured. This responsivity corresponds to a Johnson-noise-limited noise-equivalent-power (NEP) of 1.6 × 10^–12 . Another sample of similar compposition yielded an NEP of 1.8 × 10^–12 and a 25 MHz bandwidth. These results coupled with a wide spectral sensitivity [1] indicate that [Hg, Cd]Te NMMW detectors compare very favorably with similar InSb detectors [2].     

Dark matter in the framework of the shell-universe

We show that the shell-universe model (used to explain the observed expansion rate of the universe without a dark energy component) provides a natural mechanism for local increasing of the brane tension leading to the modified Newtons law alternative to galactic dark matter.     

A model of resonant Raman scattering via magnetic exciton in ferromagnetic phase

A semi-localized magneticd exciton model is proposed to explain the strong magnetic field dependence of the resonant Raman scattering observed in Eu-chalcogenides in ferro-magnetic phase. Choosing, in comparison with magneto-optical experiment, a single intermediate energy level in which all the spin and orbital angular momenta couple parallel, the observed frequency-, polarization- and magnetic field dependences are explained quantitatively in molecular field approximation. The discrepancy in the temperature dependence is discussed in terms of the short range order and the neglected intermediate energy levels.     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.