Nature of perpendicular-to-parallel spin reorientation in a Mn-doped GaAs quantum well: canting or phase separation?

It is well known that the magnetic anisotropy in a compressively strained Mn-doped GaAs film changes from perpendicular to parallel with increasing hole concentration p. We study this reorientation transition at T=0 in a quantum well with delta-doped Mn impurities. With increasing p, the angle theta that minimizes the energy E increases continuously from 0 (perpendicular anisotropy) to pi/2 (parallel anisotropy) within some range of p. The shape of E(min)(p) suggests that the quantum well becomes phase separated with regions containing low hole concentrations and perpendicular moments interspersed with other regions containing high hole concentrations and parallel moments. However, because of the Coulomb energy cost associated with phase separation, the true magnetic state in the transition region is canted with 0相似文献   

Counting Contingency Tables via Multistage Markov Chain Monte Carlo

This article describes a multistage Markov chain Monte Carlo (MSMCMC) procedure for estimating the count, , where denotes the set of all a × b contingency tables with specified row and column sums and m total entries. On each stage s = 1, …, r, Hastings–Metropolis (HM) sampling generates states with equilibrium distribution , with inverse-temperature schedule β₁ = 0 < β₂ < ??? < β _r < β _{r + 1} = ∞; nonnegative penalty function H, with global minima H(x) = 0 only for ; and superset , which facilitates sampling by relaxing column sums while maintaining row sums. Two kernels are employed for nominating states. For small β _s , one admits moderate-to-large penalty changes. For large β _s , the other allows only small changes. Neither kernel admits local minima, thus avoiding an impediment to convergence. Preliminary sampling determines when to switch from one kernel to the other to minimize relative error. Cycling through stages in the order r to 1, rather than 1 to r, speeds convergence for large β _s . A comparison of estimates for examples, whose dimensions range from 15 to 24, with exact counts based on Barvinok’s algorithm and estimates based on sequential importance sampling (SIS) favors these alternatives. However, the comparison strongly favors MSMCMC for an example with 64 dimensions. It estimated count with 3.3354 × 10^{? 3} relative error, whereas the exact counting method was unable to produce a result in more than 182 CPU (computational) days of execution, and SIS would have required at least 42 times as much CPU time to generate an estimate with the same relative error. This latter comparison confirms the known limitations of exact counting methods and of SIS for larger-dimensional problems and suggests that MSMCMC may be a suitable alternative. Proofs not given in the article appear in the Appendix in the online supplemental materials.     

Quantum accelerator modes from the Farey tree

We show that mode locking finds a purely quantum nondissipative counterpart in atom-optical quantum accelerator modes. These modes are formed by exposing cold atoms to periodic kicks in the direction of the gravitational field. They are anchored to generalized Arnol'd tongues, parameter regions where driven nonlinear classical systems exhibit mode locking. A hierarchy for the rational numbers known as the Farey tree provides an ordering of the Arnol'd tongues and hence of experimentally observed accelerator modes.     

Study of Formation Mechanisms of Photo-Induced Dichroism in Azo-Containing Polymer Films

Photoinduced dichroism (PD) in azo-containing polymers can result from chromophore orientation and/or cis-isomer accumulation. The PD formation mechanism of azochrommophores DO3 (DisperseOrange 3) doped in polymethyl methacrylate (PMMA) at a temperature varying from –20 to +120 °C is studied by UV-visible spectroscopy. It is shown that the main PD formation mechanism at T < 0°C and T > 60°C is cis-isomer accumulation and trans-isomer photoorientation, respectively.     

TRIS (perfluoroalkylethyl)silyl alkyl amines as calibration standards for electron ionization mass spectrometry in the mass range of 100–3000 Da

A new fluorinated compound mixture has been developed for the calibration of the double focusing mass spectrometer in the mass range of 100–3000 Da in the positive electron ionization (EI) mode. Current calibration standards for EI have either limited mass range [perfluorotributylamine (PFTBA), perfluorokerosene (PFK), s-triazines (TRIS)] or poor peak intensities with significant chemical background in the instrument for several days (perfluoroalkyl phosphazine). The newly synthesized fluorinated silyl alkyl amines mixture is proposed as a reference/calibration standard for EI-MS. This standard produced abundant parent and fragment ions across the entire mass range without any memory effect.     

Excitons in quantum well wires: V-shaped wire,T-shaped wire and strain-induced lateral confinement

Il Nuovo Cimento D - We calculated the energy and the wave function of the exciton for i) a V-shaped quantum wire, ii) a T-shaped quantum wire and iii) a quantum wire resulting from strain-induced...