Modern Physics Letters B

The effect of nonmagnetic Zn impurity on superconductivity in iron-based superconductors is reviewed. Zn impurity can severely suppress the antiferromagnetic (AFM) order of Fe ions in the parent compound LaFeAsO. In the 1111 type F-doped LaFeAs(O, F) system, the superconducting transition temperature (T_c) increases in the underdoped regime, remains unchanged in the optimally-doped regime, and is severely suppressed in the overdoped regime in the presence of Zn impurity. The results suggest a switch of the symmetry of the superconducting order parameters from a s-wave to s±- or d-wave states as the charge carrier doping increases in this system. To our surprise, Zn impurity strongly suppresses T_c in the Co-doped LaFeAsO and BaFe₂As₂ systems despite of the Co doping level. The absence of universal Zn impurity effect implies that the pairing symmetry of the iron-based superconductors may be dependent on the details of the electronic structure.

Three systems of Broer–Kaup–Kupershmidt (BKK) equations, that describe nonlinear and dispersive long gravity waves, are investigated. The simplified form of the Hirota's bilinear method is employed for a reliable treatment of these three systems. Multiple soliton solutions are formally derived for each system.

Congruent Ho³⁺ (1 mol.%): LiNbO₃ crystals codoped with MgO (X mol.%, X = 1, 3, 5 and 7) were grown by the Czochralski technique. The ultraviolet-visible (UV-Vis) and infrared (IR) spectra were measured in order to analyze the defect structure of the crystals. The concentrations of Mg, Ho, Li and Nb in the crystals were carried out with an inductively coupled plasma atomic emission spectrometer. Experimental results indicates as Mg²⁺ doping concentration increases in melt, the distribution coefficients of Mg and Ho ions decrease, the Li/Nb ratio in the crystals decreases first and then increases, and the absorption edge shifts to a shorter wavelength. The light-induced scattering of Mg:Ho:LiNbO₃ crystals was quantitatively scaled via the incident exposure energy. The results demonstrated that Mg (7 mol.%): Ho:LiNbO₃ crystal had the weakest light-induced scattering and the mechanism related to their defect structures was discussed.

It is still a hot topic to catch the auto-dependence behavior of volatility. Here, based on the measurement of average volatility, under different observation window size, we investigated the dependence of successive volatility of several main stock indices and their simulated GARCH(1, 1) model, there were obvious linear auto-dependence in the logarithm of volatility under a small observation window size and nonlinear auto-dependence under a big observation. After calculating the correlation and mutual information of the logarithm of volatility for Dow Jones Industrial Average during different periods, we find that some influential events can change the correlation structure and the volatilities of different periods have distinct influence on that of the remote future. Besides, GARCH model could produce similar behavior of dependence as real data and long memory property. But our analyses show that the auto-dependence of volatility in GARCH is different from that in real data, and the long memory is undervalued by GARCH.

Within the kinetic energy driven superconducting mechanism, the dynamical spin response of the bilayer cuprate superconductors is studied. It is shown that there are two nondegenerate magnetic modes with odd and even symmetries, respectively, due to the bilayer interaction in the bilayer cuprate superconductors. Although the magnetic scattering in both odd and even symmetries shows an hour-glass-shaped dispersion, the commensurate magnetic resonance energy in the odd symmetry has a domelike shape of the doping dependence, while the commensurate magnetic resonance energy in the even symmetry decreases with increasing doping.

The Gaussian Effective Potential (GEP) is shown to be a useful variational tool for the study of the magnetic properties of strongly correlated electronic systems. The GEP is derived for a single band Hubbard model on a two-dimensional bi-partite square lattice in the strong coupling regime. At half-filling the antiferromagnetic order parameter emerges as the minimum of the effective potential with an accuracy which improves over RPA calculations and is very close to that achieved by Monte Carlo simulations. Extensions to other magnetic systems are discussed.

Photoelectrochemical etched Si layers were prepared on n-type (110) oriented silicon wafer. The photoluminescence (PL), Fourier transformed infrared (FTIR) absorption and Raman spectroscopies of etched Si (110) at two different current densities were studied. Both samples showed PL peak in the visible spectral range situated from 650 nm to 750 nm. The corresponding changes in Raman spectra at different current density are discussed. The blue shift in the PL and Raman peaks is consequent of the quantum confinement effect and defect states of surface Si nanocrystallites complexes and hydrogen atoms of the photoelectrochemical etched Si (110) samples. The attenuated total reflection (ATR) results show both hydrogen and oxygen related IR modes in the samples which can be used to explain the PL effect.

Using the first-principles method based on the density functional theory, we have calculated electronic structure of zinc blende AlN doped with 6.25% of V. The V dopants are found spin polarized and the calculated band structures suggest a 100% polarization of the conduction carriers. The ferromagnetic ground state in V-doped AlN can be explained in terms of double-exchange mechanism, and a Curie temperature above room temperature can be expected. These results suggest that the V-doped AlN may present a promising dilute magnetic semiconductor and find applications in the field of spintronics.