arXiv subjects

Yia-Chung Chang

Publications and source records attributed to Yia-Chung Chang.

79 records · Page 5Linked to original sources

arXiv1998

Parent-bilinear Sea-boson Correspondence with Spin

This is a sort of tutorial where we present many of the tedious details involved in deriving the correspondence between the number conserving product of two fermi/bose fields with spin and the corresponding sea/condensate displacements also with spin. This time we include finite temperature effects differently by coupling the sea-bosons to an inhomogeneous chemical potential. It is shown that many of the salient features of the free theory are reproduced properly by this approach. An attempt is made to be as detailed as possible, this includes providing formulas for the sea-bosons as well as computing the finite temperature Fermi correlators.

cond-mat.str-el↗

arXiv1998

Band structures and optical properties of GaInAs quantum wires grown by strain-induced lateral ordering

Band structures and optical matrix elements of strained multiple quantum-wires (QWR's) are investigated theoretically via the effective bond-orbital model, which takes into account the effects of valence-band anisotropy and the band mixing. In particular, the Ga$_{1-x}$In$_x$As QWR's grown by strain-induced lateral ordering (SILO) are considered. Recently, long wavelength Ga$_{1-x}$In$_x$As QWR lasers have been fabricated via a single step molecular beam epitaxy technique which uses the SILO process.[1] Low threshold current and high optical anisotropy have been achieved. Multi-axial strains [combinations of biaxial strains in the (001) and (110) planes] for QWR's are considered, Our calculated anisotropy in optical matrix elements (for light polarized parallel versus perpendicular to the QWR's axis) is in good agreement with experiment. We also find that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.

cond-mat↗

arXiv1998

Single-Particle Green Functions in Exactly Solvable Models of Bose and Fermi Liquids

Based on a class of exactly solvable models of interacting bose and fermi liquids, we compute the single-particle propagators of these systems exactly for all wavelengths and energies and in any number of spatial dimensions. The field operators are expressed in terms of bose fields that correspond to displacements of the condensate in the bose case and displacements of the fermi sea in the fermi case. Unlike some of the previous attempts, the present attempt reduces the answer for the spectral function in any dimension in both fermi and bose systems to quadratures. It is shown that when only the lowest order sea-displacement terms are included, the random phase approximation in its many guises is recovered in the fermi case, and Bogoliubov's theory in the bose case. The momentum distribution is evaluated using two different approaches, exact diagonalisation and the equation of motion approach. The novelty being of course, the exact computation of single-particle properties including short wavelength behaviour.

hep-th↗

arXiv1997

Exact Optical Dielectric Function of a Nearly Homogeneous Solid

Based on an exactly solvable model of a nearly homogeneous solid (jellium+phonons), we compute the optical dielectric function exactly at zero temperature in any number of spatial dimensions. It is argued as we have done elsewhere, that the answer for the optical dielectric function we obtain is in fact the exact answer for a real nearly homogeneous solid in the same limit in which RPA itself is exact.

cond-mat↗

arXiv1996

Interlayer exchange coupling in Fe/Cr multilayers

We investigate the origin of the long period oscillation of the interlayer exchange coupling in Fe/Cr trilayer systems. Within the stationary phase approximation the periods of the oscillations are associated with extremal vectors of the Fermi sphere of Cr. Using a realistic tight-binding model with spin-orbit interaction we calculate the coupling strength for each extremal vector based on the spin-asymmetry of the reflection amplitude for a propagating state impinging from the Cr to Fe layer. We find that for (001) and (110) growth direction the biggest coupling strength comes from the extremal vector centered at the ellipsoid N of the Fermi surface of Cr.

cond-mat.str-el↗

arXiv1996

Fine structure of excitons in Cu$_2$O

Three experimental observations on 1s-excitons in Cu$_2$O are not consistent with the picture of the exciton as a simple hydrogenic bound state: the energies of the 1s-excitons deviate from the Rydberg formula, the total exciton mass exceeds the sum of the electron and hole effective masses, and the triplet-state excitons lie above the singlet. Incorporating the band structure of the material, we calculate the corrections to this simple picture arising from the fact that the exciton Bohr radius is comparable to the lattice constant. By means of a self-consistent variational calculation of the total exciton mass as well as the ground-state energy of the singlet and the triplet-state excitons, we find excellent agreement with experiment.

cond-mat↗

arXiv1994

Nuclear Spin-Lattice Relaxation Induced by Thermally Fluctuating Flux Lines in Type-II Superconductors

Thermal motion of the flux lines (FL) gives rise to fluctuating magnetic fields. These dynamic fields couple to the nuclei in the sample and relax the nuclear spins. Based on a model of harmonic fluctuations, we provide a theoretical description of the nuclear spin-lattice relaxation (NSLR) process due to the fluctuating FLs in clean type-II superconductors. At low fields, the calculated longitudinal relaxation rate $T_1^{-1}$ is enormously enhanced at temperatures just below $T_c$. At intermediate fields, the resulting $T_1^{-1}$ exhibits a peak structure as a function of temperature, which is eventually suppressed as the field is increased. The vibrational modes which have components propagating along the FLs play an essential role in the $T_1^{-1}$ process.

cond-mat↗