Electronic structure of strained Sin/Gen(001) superlattices

Using the empirical tight binding method we have investigated the electronic properties of the Si_n/Ge_n(001) strained superlattices as a function of the superlattice periodicity and the band misfit. For n ≥ 4 we have found that first and second conduction band states are localized in Si. The hole states localized in Ge appear for n ≥ 4. The difference between the direct and indirect band gaps is reduced from 2.01 eV for bulk Si to 0.01 eV for n=6 which can be considered to be quasi-direct. For the cases n=6 and n=8, the band gap might become direct for large values of band misfit.     

Interfacial segregation and electro-diffusion of dopants in superlattices

A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.     

Structural, energetic, and electronic properties of Sin, Gen, and SinGen clusters

Results of a theoretical study of the properties of Si_n, Ge_n, and Si_nGe_n clusters are presented. An approximate density-functional method in combination with genetic algorithms have been used in an unbiased determination of the structures of the lowest total energy. The resulting structural, energetic, and electronic properties are analysed and compared with each other for the different systems.     

Interfacial roughness and angle dependence of giant magnetoresistance in magnetic superlattices

Using the two-point conductivity formula, we numerically evaluate the giant magnetoresistance (GMR) in magnetic superlattices with currents in the plane of the layers (CIP), from which the effect of the interfacial roughness and magnetization configuration on the GMR is studied. With increasing interfacial roughness, the maximal GMR ratio is found to first increase and then decrease, exhibiting a peak at an optimum strength of interfacial roughness. For systems composed of relatively thick layers, the GMR is approximately proportional to ,where is the angle between the magnetizations in two successive ferromagnetic layers, but noticeable departures from this dependence are found when the layers become sufficiently thin. Received 21 September 1998 and Received in final form 22 December 1998     

Ordering energy and stability of perfect superlattices

The rigorous solution to the problem of stability of stochiometric ordered phases is considered. Conditions of stability are found relative to phases with maximal degree of order for given stochiometry. The treatment is given for all known superlattices that arise from an fcc solid solution.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 22–26, September, 1980.     

Cyclotron masses in InGaAs/GaAs superlattices and InGaAs/AlAs superlattices

Cyclotron resonance (CR) measurements have been carried out to evaluate the effective mass of electron in (InGaAs)_n/(GaAs)_nsuperlattices (SLs) and (InGaAs)_n/(AlAs)_nSLs. To clarify the dependence of cyclotron mass on the monolayer numbern , we measured CR signals using pulsed high-magnetic fields up to 150 T and a far-infrared laser. We found clear cyclotron resonances in the transmission of 10.6 μ m at 75 T at room temperature in (InGaAs)_n/(GaAs)_nSLs and little dependence on the monolayer number n in the SLs. However, for (InGaAs)_n/(AlAs)_nSLs, a large dependence of cyclotron mass on the monolayer number n was observed. We consider that these dependencies are related to the difference between the barrier height in the SLs and the influence of nonparabolicity on the conduction subbands in the SLs.     

Electronic stability of magnetic Fe/Co superlattices with monatomic layer alternation

We report a surprising observation that the growth of the [Fe(1 ML)/Co(1 ML)](n) superlattice of L1(0) structure on Cu(100) is stable only up to six atomic layers (n=3), which cannot be rationalized by stress arguments. Instead, first-principles calculations reveal a transition from the L1(0) to the B2 structure due to the effect of dimensionality on the stability of the electronic structure of the superlattice. Whereas the majority-spin electrons are energetically insensitive to the layer thickness, the minority-spin electrons induce the transition at n=3.     

Interfacial electronic structure and magnetoelectric effect in M/BaTiO3 (M=Ni,Fe) superlattices

Using first-principles density functional theory, we investigate the interfacial electronic structure and magnetoelectric effect in M/BaTiO₃ (M=Ni, Fe) superlattices, and find a novel type of interfacial magnetoelectric coupling mechanism in the Ni/BaTiO₃ interface. This magnetoelectric effect is determined by the change of magnetic moments on Ni atoms near the interface, instead of the induced moments on interfacial Ti atoms in Fe/BaTiO₃ system, which is also distinguished from the spin-polarized carriers screening mechanism. The underlying physics is the strong interface bonding and the pdσ-type magnetic interactions between Ni 3d and O 2p spins. Furthermore, there exists an extraordinary intralayer oscillation of magnetic moments within the Ni layers, which may be observed in experiments.     

On the stability of bcc Co in Co/Fe superlattices an NMR and XRD study

While RHEED observations show that 10 to 11 As is the stability limit for an open bcc Co layer when grown on an Fe substrate, our XRD and NMR studies have shown that, in MBE grown Co/Fe superlattices, cobalt can be stabilised in a bcc structure up to a critical Co thickness of 21 Ås. In order to understand this apparent discrepancy, NMR experiments have been carried out in Co_x/Fe_y multilayers with thickness varying in the range 5 Å < x < 42 Å and 24 Å < y < 60 Å, grown on GaAs (1 1 0) as well as on MgO (1 0 0) substrates. The analysis of the chemical short range order by NMR concludes that the larger bcc Co thickness observed in superlattices results from the formation of a rather homogeneous CoFe_20% bcc alloy which contains the supplementary 10–11 As of Co and which coexists with pure Co grains. The concentration of about 20% Fe in the alloyed part of the Co layer happens to be close to the stability limit for a bcc structure in the equilibrium phase diagram of bulk CoFe alloys. However, while a mixture of bcc and fcc phases is observed in bulk alloys, the bcc structure is preserved in all phases under the MBE growth conditions and below the critical thickness. Above the critical thickness amixture of bcc Co, bcc CoFe and hcp Co is observed.     

基于密度泛函理论的SinCN团簇结构及稳定性的研究

利用密度泛函理论(DFT)中的B3LYP/6-311G*方法,对SinCN(n=2～6)团簇的几何结构、振动频率和基态能量等性质进行了研究,分别讨论了其基态和亚稳态结构的能量及自旋多重态.振动频率和振动强度被用来判断体系的稳定性.结果表明,随着Si原子数的增加,零点振动能、热容量和熵近似线性增长,其平均增幅分别为0.80 kcal/mol、5.20 cal/mo1?K和12.72 cal/mol?K.n为偶数的SinCN团簇比n为奇数的更为稳定.     

Hydrogen in Nb/Ta superlattices

The structural and thermodynamic properties of hydrogen dissolved in Nb/Ta superlattices are studied by in-situ x-ray scattering techniques. From the x-ray satellite intensities, it is found that H induces a strain modulation exhibiting a Curie-Weiss temperature dependence. The intensities are analyzed in terms of a mean field model of the modulated lattice gas, yielding quantitative information on the hydrogen-metal and hydrogen-hydrogen interaction energies. Critical behavior associated with a gas-liquid transition is also observed. Hydrogen density fluctuations with wavelengths shorter than a superlattice period are, however, suppressed by the superlattice, which represents a novel manifestation of a coherent phase transition. These experiments provide new and fundamental insight into the role of spacially varying two body interactions in critical phenomena.