Lifetime analysis of quasibound states in coupled quantum wells

A simple time-dependent model is presented to investigate lifetimes of the quasibound states in coupled quantum wells (CQWs). The transfer matrix approach is employed to discretize the conduction-band profile of the heterostructure and form a dispersion equation whose zeros correspond to the complex eigenenergies. Both the bound and quasibound states are extracted numerically in the complex plane by Newton's method. The lower and higher well subbands are found to have negative and positive energy shift, respectively, as following the no level crossing theorem. Besides, the decay rate of the quasibound state is approximately proportional to the absolute energy shift. The quasibound states, which have larger energy shift, have shorter lifetime and decay more quickly. Furthermore, the differences in lifetime between the quasibound states in CQWs can be easily realized as all the wave functions are specially adjusted to form the relative probability density distributions.     

Influence of phonon-associated tunneling rate on transport through a single-molecule transistor

We study the electronic transport through a single-molecule transistor (SMT) by considering the phonon-associated tunneling rate. We find that the electron-phonon interaction (EPI) changes the constant conductivities of the leads into a multi-channel structure of single vibration frequency. This interference of the multi-channel tunneling process results in a bias-dependent tunneling rate and obscures the conductance peaks at large bias voltage. The bias-dependent tunneling rate further causes a remarkable conductivity gap between the chemical potential of the leads (n=0) and the first phonon sideband (n=1). These anomalies are consistent with the experimental observations in transport experiments.     

Shell-model studies of the 90Y, 91Zr, 92Nb and 93Mo nuclei

A shell-model calculation of the N = 51, 39 ≦ Z ≦ 42 nuclei is presented. The ⁸⁸Sr nucleus is assumed to be an inert closed core. The extra-core protons are restricted to the ($\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$) configurations, and the active neutron is allowed to occupy the $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$, $\text{3}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{1}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ orbits. The proton-proton effective interaction is directly taken from the previous analysis on the energy levels for N = 50 isotones by Ball et al. The proton-neutron effective interaction is assumed to be of the form of the surface δ-interaction. The energy spectra are calculated from a least-squares fit to the experimental data, varying the T = 0 and T = 1 strengths of the surface δ-interaction. Spectroscopic factors, E2 transition rates and two-body matrix elements are also calculated and compared with the observed values and the previous theoretical results.     

Nanoindentation investigation of amorphous hydrogenated carbon thin films deposited by ECR-MPCVD

Nanomechanical properties of amorphous hydrogenated carbon thin films are performed by nanoindentation technique. The amorphous hydrogenated carbon films are produced on silicon substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-MPCVD). The effect of negative bias voltage on amorphous hydrogenated carbon films is examined by Raman spectroscopy and the results showed that the intensity ratio of D-peak to G-peak (I_D/I_G) of amorphous hydrogenated carbon films at various bias voltages, increased as the bias voltage increased. The results also showed that Young’s modulus and hardness also increased as the bias voltage increased. In addition, Young’s modulus and hardness both decreased as the indentation depth increased.     

Nanomechanical characterizations of InGaN thin films

In_xGa_1−xN thin films with In concentration ranging from 25 to 34 at.% were deposited on sapphire substrate by metal-organic chemical vapor deposition (MOCVD). Crystalline structure and surface morphology of the deposited films were studied by using X-ray diffraction (XRD) and atomic force microscopy (AFM). Hardness, Young's modulus and creep resistance were measured using a nanoindenter. Among the deposited films, In_0.25Ga_0.75N film exhibits a larger grain size and a higher surface roughness. Results indicate that hardness decreases slightly with increasing In concentration in the In_xGa_1−xN films ranged from 16.6 ± 1.1 to 16.1 ± 0.7 GPa and, Young's modulus for the In_0.25Ga_0.75N, In_0.3Ga_0.7N and In_0.34Ga_0.66N films are 375.8 ± 23.1, 322.4 ± 13.5 and 373.9 ± 28.6 GPa, respectively. In addition, the time-dependent nanoindentation creep experiments are presented in this article.     

Current detection of superradiance and induced entanglement of double quantum dot excitons

We propose to measure the superradiance effect by observing the current through a semiconductor double-dot system. An electron and a hole are injected separately into one of the quantum dots to form an exciton and then recombine radiatively. We find that the stationary current shows oscillatory behavior as one varies the interdot distance. The amplitude of oscillation can be increased by incorporating the system into a microcavity. Furthermore, the current is suppressed if the dot distance is small compared to the wavelength of the emitted photon. This photon trapping phenomenon generates the entangled state and may be used to control the emission of single photons at predetermined times.     

Cucurbitane Triterpenoids from the Fruit Pulp of Momordica charantia and Their Cytotoxic Activity

Two new cucurbitane‐type triterpenes, 25‐methoxycucurbita‐5,23(E)‐diene‐3β,19‐diol ( 1 ) and 7β‐ethoxy‐3β‐hydroxy‐25‐methoxycucurbita‐5,23(E)‐dien‐19‐al ( 2 ), together with three known cucurbitane‐type triterpenes, 3β,7β,25‐trihydroxycucurbita‐5,23(E)‐dien‐19‐al ( 3 ), (23E)‐3β‐hydroxy‐7β,25‐dimethoxycucurbita‐5,23‐dien‐19‐al ( 4 ), and 3β‐hydroxy‐25‐methoxycucurbita‐6,23(E)‐dien‐19,5β‐olide ( 5 ), were isolated from the fruit pulp of Momordica charantia. The structures of two new compounds were elucidated on the basis of 1D and 2D NMR, MS, IR, optical rotation. Among these isolates, compounds 1 , 2 , and 5 showed slight cytotoxic activity against the SK‐Hep 1 cell line with IC₅₀ values of 33.1, 24.3, and 38.7 μM, respectively.