Motivated by a recent experiment of spatial and temperature dependent average exciton energy distribution in coupled quantum wells [S. Yang textit{et al.}, Phys. Rev. B textbf{75}, 033311 (2007)], we investigate the nature of the interactions in indi
rect excitons. Based on the uncertainty principle, along with a temperature and energy dependent distribution which includes both population and recombination effects, we show that the interplay between an attractive two-body interaction and a repulsive three-body interaction can lead to a natural and good account for the nonmonotonic temperature dependence of the average exciton energy. Moreover, exciton energy maxima are shown to locate at the brightest regions, in agreement with the recent experiments. Our results provide an alternative way for understanding the underlying physics of the exciton dynamics in coupled quantum wells.
Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is show
n that in case of SC gap symmetry $Delta(-k_x,mathbf{k}_parallel)=Delta(k_x,mathbf{k}_parallel)$ with $k_x$ and $mathbf{k}_parallel$ respectively the normal and parallel component of electron Fermi wavevector, quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if $k_x x=npi$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $Delta(-k_x,mathbf{k}_parallel)=-Delta(k_x, mathbf{k}_parallel)$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multi-band pairings.
In the hole-doped $d_{x^{2}-y^{2}}$-wave cuprate superconductor, due to the midgap surface state (MSS), a zero bias conductance peak (ZBCP) is widely observed in [110] interface point contact spectroscopy (PCS). However, ZBCP of this geometry is rare
ly observed in the electron-doped cuprates, even though their pairing symmetry is still likely the $d_{x^{2}-y^{2}}$-wave. We argue that this is due to the coexistence of antiferromagnetic (AF) and the superconducting (SC) orders. Generalizing the Blonder-Tinkham-Klapwijk (BTK) formula to include an AF coupling, it is shown explicitly that the MSS is destroyed by the AF order. The calculated PCS is in good agreement with the experiments.
An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic d_{x^2-y^2}-wave and explains
why the observed gap function has a nonmonotonic d_{x^2-y^2}-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model.
The magnetic susceptibility of high-T_c superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in c-axis optical conductivity on underdoped sampl
es, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence suggest that the mechanism governing the c-axis optical pseudogap is different from that for the $a-b$ plane magnetic pseudogap.
