Anomalous optical and electronic properties of dense sodium

Based on the density functional theory, we systematically study the optical and electronic properties of the insulating dense sodium phase (Na-hp4) reported recently (Ma et al., 2009). The structure is found optically anisotropic. Through Bader analysis, we conclude that ionicity exists in the structure and becomes stronger with increasing pressure.     

Electronic properties of hydrogen-related complexes in pure semiconductors

Hydrogen has been shown to activate the neutral impurities carbon, silicon and oxygen in ultra-pure germanium and form shallow level complexes. The double acceptors beryllium and zinc in silicon and germanium, as well as the triple acceptor copper in germanium, can be partially passivated, leading to single hole acceptors. The study of the electronic level spectrum of the single carrier bound to these centers at low temperatures has provided much information on symmetry and composition. Most centers reveal a symmetry axis along [1 1 1] and are static. In some cases hydrogen has been found to tunnel between equivalent real space positions. hoto hermal onization pectroscopy (PTIS) has been the most important tool for the study of the optical transitions of the hole (electron) in these hydrogen containing complexes. This photoconductivity technique combines high sensitivity with high resolution and permits the study of shallow acceptors or donors present at concentrations as low as 10⁸ cm^-3. Even lower limits may be attained under favorable circumstances.     

Theory of hydrogen-related metastability in disordered silicon

Density functional electronic structure calculations are employed to examine hydrogen for a variety of configurations in silicon. A novel complex is found for hydrogen in amorphous silicon. The complex involves the breaking of a weak silicon bond to form two Si-H bonds with both hydrogens in between the original silicon atoms. This complex provides a microscopic model for new metastable complexes observed in amorphous silicon. Mechanisms for hydrogen-related metastability are discussed for amorphous and polycrystalline silicon.     

Anomalous properties of 2-D electrons in a magnetic field

In strong magnetic field and at low temperature, 2-D electron systems are expected to show distinctive temperature oscillations when the field is changed adiabatically.     

Anomalous electronic state in CaCrO3 and SrCrO3

Comprehensive measurements of electrical transport properties under pressure, thermal conductivity, magnetic susceptibility, and room-temperature compressibility have been used to characterize SrCrO3 and CaCrO3 perovskites synthesized under high pressure. Comparison with other narrow-band perovskite oxides suggests that their anomalous physical properties are correlated with bond-length instabilities caused by the crossover from localized to itinerant electronic behavior.     

Anomalous properties of cobalt diselenide

From N.M.R. and X-rays experiments we show that the anomalous temperature dependence of the bulk magnetic susceptibility of cobalt diselenide which was first attributed to the occurence of antiferromagnetism is rather due to some structural instability. This is supported by an observed anomaly in the thermal variation of the lattice parameter and a (Tm-T)${}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ thermal variation of the ⁷⁷Se N.M.R. linewidth (Tm ～ 50K is the temperature at which the instability occurs). Such behaviour has already been observed in CuS₂ and layered NbSe₂ where structural instabilities are known to occur. Some off-stoichiometry effects are     

Anomalous magnetic and superconducting properties in a Ru-based double perovskite

We have studied the double perovskite [1] structure Sr₂Y(Ru_1-x Cu_x)O₆ system. The parent compound is an antiferromagnetic insulator with Neel temperature ～ 26 K. Partially substituted the Ru ion by Cu the compounds increase their conductivity drastically and eventually become superconducting. More intriguingly is the observation of the coexistence of superconductivity and magnetic ordering. The superconducting transition temperature T _c and the magnetic ordering temperature T _m are of the same order. The observed magnetic structure and superconductivity of these compounds can be understood in terms of a plausible theoretical model based on the double exchange idea.     

Anomalous electronic transport in quasicrystals and related complex metallic alloys

We analyse the transport properties in approximants of quasicrystals α-AlMnSi, 1/1-AlCuFe and for the complex metallic phase λ-AlMn. These phases present strong analogies in their local atomic structures and are related to existing quasicrystalline phases. Experimentally, they present unusual transport properties with low conductivities and a mix of metallic-like and insulating-like characteristics. We compute the band structure and the quantum diffusion in the perfect structure without disorder and introduce simple approximations that allow us to treat the effect of disorder. Our results demonstrate that the standard Bloch–Boltzmann theory is not applicable to these intermetallic phases. Indeed their dispersion relations are flat, indicating small band velocities, and corrections to quantum diffusion, which are not taken into account in the semi-classical Bloch–Boltzmann scheme, become dominant. We call this regime the small velocity regime. A simple relaxation time approximation to treat the effect of disorder allows us to reproduce the main experimental facts on conductivity qualitatively and even quantitatively.     

Optical properties of a donor impurity in a two-dimensional quantum pseudodot

We investigate the linear and nonlinear optical properties of a donor impurity confined by a two-dimensional pseudoharmonic potential both including harmonic dot and antidot potentials in the presence of a strong magnetic field. Calculations are made by using the perturbation method and the compact density-matrix approach within the effective-mass approximation. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index changes have been examined. The results are presented as a function of the incident photon energy for the different values of the chemical potential of the electron gas and the zero point of the pseudoharmonic potential. The results show that the optical properties of a donor impurity in a two-dimensional pseudoharmonic QD are strongly affected by the zero point of the pseudoharmonic potential, the chemical potential of the electron gas and the Coulomb interaction.     

Anomalous behavior of shallow donor ground state levels in Ge under pressure

The crossing between absolute (L₁) and secondary (Δ₁) minima which occurs in Ge under the application of a hydrostatic pressure has been found to evidence the strong multi-valley interactions responsible of an anomalous pressure dependence of the A₁ level. Two alternative experimental investigations are suggested to evidence these expectations.     

Anomalous electrical transport properties of a quenched iron film system

An iron film system, deposited on glass surfaces by thermal evaporation method and quenched with a floating oil layer immediately after the deposition, has been fabricated. The temperature dependence of the resistance and the transversal magnetoresistance (MR) of the iron films have been studied. The experiment shows that, as the temperature decreases, the sample resistance increases first and then drops monotonically, finally it increases again. Furthermore, a crossover of MR from positive to negative is observed as the magnetic field increases. It is proposed that these anomalous phenomena originate from the characteristic microstructure of the samples.     

重费米子材料中的反常物性

重费米子研究对探索强关联电子体系新奇量子现象的组织原则具有重要的意义。文章介绍了作者近年来对重费米子材料正常态反常性质的研究进展,揭示了重电子的普适温度演化行为和Fano干涉效应,为理解重费米子物理和探索非常规超导机理提供了新的思路。     

Anomalous electronic Raman scattering in NaxCoO2.yH2O

Raman scattering experiments on NaxCoO2.yH2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm(-1) and a cutoff at approximately 560 cm(-1) over a wide range of doping levels. The electronic Raman spectra in superconducting and nonsuperconducting samples are similar at room temperature, but evolve in markedly different ways with decreasing temperature. For superconducting samples, the low-energy spectral weight is depleted upon cooling below T* approximately 150 K, indicating the opening of a pseudogap that is not present in nonsuperconducting materials. Weak additional phonon modes observed below T* suggest that the pseudogap is associated with charge ordering.     

Nonlinear optical properties of a hydrogenic donor quantum dot

A investigation of the nonlinear optical properties of a hydrogenic donor in a disc-like parabolic quantum dot has been performed by using the matrix diagonalization method. The optical absorption coefficient between the ground (L=0) and the first excited state (L=1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the incident optical intensity and the confinement strength. We found the total optical absorption coefficient is strongly affected by the incident optical intensity and the confinement strength.     

Thermoelectric properties of a dilute graphite donor intercalation compound

Data on the in-plane thermal conductivity and thermoelectric power of a stage-5 potassium donor graphite intercalation compound are reported in the temperature range 3 < T < 300 K. In the lowest temperature range the electronic contribution dominates the thermal conductivity, while at higher temperature there is a dominant lattice contribution, which is much smaller than pristine graphite. The thermopower is negative in the whole temperature range.     

Resonant electronic Raman scattering on donor levels in cubic semiconductors

Inelastic resonant electronic Raman scattering (ERS) on donor levels in direct band gap cubic semiconductors is studied. The theory of three-branch and five-branch excitonic polaritons for ERS is developed. The excplicit expressions and angular dependences are derived for the scattering cross section.The authors would like to thank Prof. Nguyen Van Hieu for suggesting the problem and for continue support. Many thanks are due to the Research Grant No 86–87 of the Third World Academy of Science for the support.     

Anomalous properties of absorbed water films in layered silicates

The electrical properties of water films in residual electric fields on the surface of crystals and in slitlike capillaries of layered silicates are investigated. The results obtained indicate a substantial change in the water structure, degradation of the dipole-orientation polarization of molecules, and a decrease in the crystallization temperature.