Pulsed EPR studies of polycrystalline cesium hexamethyl hexacyclen sodide

The electron spin echo envelope modulation (ESEEM) technique of pulsed EPR spectroscopy has been used to measure weak¹³³Cs hyperfine couplings to trapped electrons in polycrystalline cesium hexamethyl hexacyclen sodide. Two magnetically distinct groups of weakly coupled¹³³Cs nuclei were found — one with an isotropic electron-nuclear hyperfine (Fermi contact) coupling of 0.34 MHz and the other with a contact coupling less than 0.1 MHz. Analysis of these data, by computer simulation, shows that the number of Cs nuclei that give rise to these interactions and their distance from the paramagnetic center is consistent with the hypothesis that the trapped electrons occupy vacant anion sites. The results indicate that the spread of unpaired electron spin density, along the axis formed by the “contact ion pair,” may be greater than that in the plane perpendicular to this axis.     

On the number of guard edges of a polygon

This paper establishes tight bounds on the number of edges of a polygon from which every point in the polygon is visible; we call them guard edges. For a nonstarshaped polygon, there can be at most three guard edges. For a polygon with holes, there may be at most six; three on the outer boundary and three on one of the holes. The results give new insights into the structure of visibility in polygons and shed light on developing an efficient algorithm for finding all guard edges of a polygon with or without holes. This work was partially supported by the Korea Science and Engineering Foundation (Project No. 91-01-01).     

The quenching of nucleon yields in the nonmesonic weak decay of Λ -hypernuclei and the three-body weak decay process

Recent exclusive coincidence measurements of non-mesonic weak decays (NMWD) reported for the ratio of the partial decay width of neutron-induced-to-proton-induced NMWD, Γ_n/Γ_p , values of 0.45±0.11±0.03 and 0.51±0.13±0.04 for 5lam and ¹² _Λ C , respectively. These observations agree well with the improved theoretical Γ_n/Γ_p ratios which are in the range of 0.3-0.7. It appears that the long-standing discrepancy between the experimental and theoretical values of Γ_n/Γ_p has finally been solved. However, when compared to the results of intra-nuclear cascade (INC) calculations, the observed numbers of both single nucleons and coincident nucleon pairs are strongly quenched. The quenching of the proton yield observed previously has been interpreted as an increase of the Γ_n/Γ_p ratio. On the other hand, significant contributions from the two-nucleon-induced three-body process ΛNN → nNN are predicted. Indeed, the angular correlation of the emitted nucleon pairs in the NMWD of ¹² _Λ C showed not only decay events in back-to-back kinematics, but also events with non-back-to-back kinematics. In this paper we show that the difficulties to extract the correct Γ_n/Γ_p ratio from the proton spectra is related to the three-body weak-interaction process which strongly quenches the nucleon yields.     

Error estimates for a single phase quasilinear stefan problem with a forcing term

In this paper, we apply finite element Galerkin method to a singlephase quasi-linear Stefan problem with a forcing term. We consider the existence and uniqueness of a semidiscrete approximation and optimal error estimates inL ₂, L_∞,H ₁ andH ₂ norms for semidiscrete Galerkin approximations are derived.     

Polarization selective magneto-optical study on the coupled quantum dots using resonant excitation

We have studied a double-layer self-assembled quantum dot (QD) structures consisting of non-magnetic CdSe and magnetic CdMnSe. Transmission electron microscopy image shows that QDs are formed within the CdSe and CdMnSe layers, and they are vertically correlated in the system. The strong interband ground state transition was observed in magneto-photoluminescence (PL) experiments. In contrast to a typical behavior for many low-dimensional systems involving diluted magnetic semiconductors (DMSs), where PL signal dramatically increases when an external magnetic field is applied, we have observed a significant decrease of the PL intensity as a function of magnetic field in the double-layer structures where the alternating QD layers contain the DMS and non-DMS QDs. We attribute such effect to carrier transfer from non-magnetic CdSe dots to magnetic CdMnSe dots due to the large Zeeman shift of the band edges of DMS QDs in magnetic field. Since the band alignment of QD structure strongly depends on the spin states of system, we performed polarization-selective PL measurement to identify spin-dependent carrier tunneling in this coupled system.     

Error estimates for mixed finite element approximations to a linear Stefan problem

We introduce a semidiscrete mixed finite element approximation for the single-phase linear Stefan problem and show the unique existence of the approximation. And the optimal rate of convergence inL ² andH ¹ norms are derived.     

A simple method for synthesis of amides and peptides through acyl chlorides : A rapid synthesis of thyrotropin releasing hormone

By improvement of the classical SOCl₂-pyridine method for the preparation of acid chlorides, amides and optically pure peptides were synthesized rapidly in a simple manner from DCHA dicyclohexylammonium) salts of carboxylic acids. This modified SOCl₂-pyridine method was applied to a rapid synthesis of TRH.     

An Assmus–Mattson-Type Approach for Identifying 3-Designs from Linear Codes over Z 4

The complete weight enumerator of the Delsarte–Goethals code over Z ₄ is derived and an Assmus–Mattson-type approach at identifying t-designs in linear codes over Z ₄ is presented. The Assmus–Mattson-type approach, in conjunction with the complete weight enumerator are together used to show that the codewords of constant Hamming weight in both the Goethals code over Z ₄ as well as the Delsarte–Goethals code over Z ₄ yield 3-designs, possibly with repeated blocks.