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We report experimental evidence that an ErAs quantum-dot array in a GaAs matrix under 1550 nm pulsed excitation produces cooperative spontaneous emission-Dicke superradiance-in the terahertz frequency region at room temperature., Comment: 8 pages, 4 figures

Here we present an original study of the effect of hydration on Terahertz absorption signatures in biomolecular (lactose monohydrate and biotin) and bioparticles( Bacillus thuringiensis and Bacillus cereus spores). We observe "read-shift" in center frequency with increasing hydration in all samples, consistent with Lorentzian-oscillator behavior. But the effect of hydration on linewidth is ambiguous, sometimes increasing the linewdith (consistent with Lorentzian behaviour) and sometimes decreasing the linewidth.

We report here the first RF noise measurements on two designs of n-doped GaN/AlN double-barrier resonant tunneling diodes (RTDs), each having a room-temperature negative differential resistance (NDR) and also strong near-UV light emission. The measurements are made with a standard, un-isolated RF receiver and calibration is made using a substitution-resistor/hot-cold radiometric technique which works in the positive differential resistance (PDR) region but not the NDR region. A high-quality InGaAs/AlAs double-barrier RTD is used as a control sample and displays shot noise suppression down to $\Gamma\approx$0.5 in the PDR region, as expected. The GaN/AlN RTDs display both shot-noise enhancement and suppression in the PDR regions, but no obvious sign of sudden shot-noise enhancement in the threshold bias region of light emission. This supports the hypothesis that the holes required for light emission are created by electronic (Zener) interband tunneling, not impact ionization. Further the minimum shot-noise factor of $\Gamma\sim$ 0.34 suggests that the GaN/AlN RTDs are acting like triple-barrier devices., Comment: 10 pages, 5 figures

We report strong light emission from a room-temperature n-type unipolar-doped In0.53Ga0.47As/AlAs double-barrier resonant-tunneling diode (DBRTD) precisely at the In0.53Ga0.47As band-edge near 1650 nm. The emission characteristics are very similar to what was observed recently in GaN/AlN DBRTDs, both of which suggest that the mechanism for emission is cross-gap electron-hole recombination via resonant- and Zener co-tunneling of electrons, the latter mechanism generating the required holes. Analysis shows that because of the relatively small bandgap, the Zener tunneling probability can be large in this In0.53Ga0.47As/AlAs DBRTD, and is a mechanism that may have been overlooked in the longstanding literature. The universal nature of the co-tunneling is best supported by the factor (EG)2/F in the Kane tunneling probability, which is nearly the same at the peak voltage of the In0.53Ga0.47As and GaN DBRTDs.

We report anomalous critical transport behavior in a GaAs structure containing a dense array of ErAs quantum dots. The structure displays a voltage (electric field)-controlled insulator-to-metal transition and strong hysteresis in the Kondo-like current-vs-temperature characteristic, with critical temperatures as high as 50 K. We attribute this behavior to a strong distributed Kondo resonance between the quantum dots after the Coulomb blockade of the array is lifted. This is consistent with a high sensitivity of the phase transition to a small external magnetic field that we have observed in the Voigt configuration, and a phenomenological model based on the RKKY interaction within a quantum dot and the cooperative Kondo-resonance amongst quantum dots., Comment: 15 pages, 6 figures

The photoconductions of ultrafast InGaAs:ErAs nanocomposites at low temperatures were investigated. The parabolic Tauc edge as well as the exponential Urbach tail are identified in the absorption spectrum. The Tauc edge supports that the density of states at the bottom of conduction band is proportional to the square root of energy. The Urbach edge is attributed to interband transition caused by smooth microscopic internal fields. The square root of mean-squared internal fields, whose distribution is Gaussian, is found in the order of $10^{5}$V/cm, agreeing very well with the theoretical predictions by Esser (B. ~ Esser, Phys. stat. sol. (b), vol. 51, 735 (1972)).

This article presents studies on low-field electrical conduction in the range 4-to-300 K for a ultrafast material: InGaAs:ErAs grown by molecular beam epitaxy. The unique properties include nano-scale ErAs crystallines in host semiconductor, a deep Fermi level, and picosecond ultrafast photocarrier recombination. As the temperature drops, the conduction mechanisms are in the sequence of thermal activation, nearest-neighbor hopping, variable-range hopping, and Anderson localization. In the low-temperature limit, finite-conductivity metallic behavior, not insulating, was observed. This unusual conduction behavior is explained with the Abrahams scaling theory.