arXiv subjects

Y. C. Chang

Publications and source records attributed to Y. C. Chang.

5 recordsLinked to original sources

arXiv2012

Effects of interdot hopping and Coulomb blockade on the thermoelectric properties of serially coupled quantum dots

We have theoretically studied the thermoelectric properties of serially coupled quantum dots (SCQD) embedded in an insulator matrix connected to metallic electrodes. In the framework of Keldysh Green's function technique, the Landauer formula of transmission factor is obtained by using the equation of motion method. Based on such analytical expressions of charge and heat currents, we calculate the electrical conductance, Seebeck coefficient, electron thermal conductance and figure of merit (ZT) of SCQD in the linear response regime. The effects of electron Coulomb interactions on the reduction and enhancement of ZT are analyzed. We demonstrate that ZT is not a monotonic increasing function of interdot electron hopping strength ($t_c$). We also show that in the absence of phonon thermal conductance, SCQD can reach the Carnot efficiency as $t_c$ approaches zero.

cond-mat.mes-hall↗

arXiv2011

An algorithm for decoherence analyses of lights through three-dimensional periodic microstructures

A transfer-matrix algorithm is presented herein as a beginning to study the transmission characteristics of coherent light through three-dimensional periodic microstructures, in which the structures are treated as two-dimensional-layer stacks and multiple reflections are considered negligible. The spatial-correlated noise is further introduced layer by layer to realize the actual decoherence of the light and allows for statistical investigation of the partial spatially coherent optics in transparent mediums. Numerical analyses show comparable results to the Gaussian Schell model in free-space cases, indicating the validity of the algorithms.

cond-mat.stat-mech↗

arXiv2001

Optical properties of self-assembled quantum wires for application in infra-red detection

The theoretical studies of optical properties of Ga$_{1-x}$In$_x$As self-assembled quantum-wires (QWR's) made of short-period superlattices with strain-induced lateral ordering are presented. Valence-band anisotropy, band mixing, and effects due to local strain distribution at the atomistic level are all taken into account. Using realistic material parameters which are experimentally feasible, we perform simulations of the absorption spectra for both inter-subband and inter-band transitions (including the excitonic effect) of this material.It is shown that the self-assembled QWR's have favorable optical properties for application in infra-red detection with normal incidence. The wavelength of detection ranges from 10 $μm$ to 20 $μm$ with the length of QWR period varying from 140Åto 300Å.

cond-mat.mtrl-sci↗

arXiv1999

Effects of microscopic strain distribution on Ga(1-x)In(x)As quantum wires grown by strain-induced lateral ordering

Band Structures and optical matrix elements of quantum wires(QWR's) made of short-period superlattices(SPS) with strain-induced lateral ordering(SILO) are investigated theoretically via an effective bond-orbital model(EBOM) combined with a valence-force field(VFF) model. Valence-band anistropy, band mixing, and effects due to local strain distribution at the atomistic level are all taken into account. In particular, Ga(1-x)In(x)As QWR's grown by SILO process are considered. A VFF model is used to find the equilibrium atomic positions in the SILO QWR structure by minimizing the lattice energy. The strain tensor at each atomic(In or GA) site is then obtained and included in the calculations of electronic states and optical peroperties. It is found that different local arrangement of atoms leads to very different strain distribution, which in term alters the optical properties. In particular, we found that the optical anisotropy can be reversed due to the change in shear strain caused by the inter-change of atomic positions. Good agreement with the existing experimental data on band gap and optical anisotropy can be obtained when a 2D alloy structure with lateral composition modulation in the InAs/GaAs interface planes of the SPS is used. Our studies revealed the possibility of "shear-strain engineering" in SILO QWR light-emitting devices to achieve desired optical anisotropy.

cond-mat.mes-hall↗

arXiv1996

The Role of Nonequilibrium Dynamical Screening in Carrier Thermalization

We investigate the role played by nonequilibrium dynamical screening in the thermalization of carriers in a simplified two-component two-band model of a semiconductor. The main feature of our approach is the theoretically sound treatment of collisions. We abandon Fermi's Golden rule in favor of a nonequilibrium field theoretic formalism as the former is applicable only in the long-time regime. We also introduce the concept of nonequilibrium dynamical screening. The dephasing of excitonic quantum beats as a result of carrier-carrier scattering is brought out. At low densities it is found that the dephasing times due to carrier-carrier scattering is in picoseconds and not femtoseconds, in agreement with experiments. The polarization dephasing rates are computed as a function of the excited carrier density and it is found that the dephasing rate for carrier-carrier scattering is proportional to the carrier density at ultralow densities. The scaling relation is sublinear at higher densities, which enables a comparison with experiment.

cond-mat↗