Using Lorenz microscopy and small-angle electron diffraction, we directly present that the chiral magnetic soliton lattice (CSL) continuously evolves from a chiral helimagnetic structure in small magnetic fields in Cr(1/3)NbS2. An incommensurate CSL undergoes a phase transition to a commensurate ferromagnetic state at the critical field strength. The period of a CSL, which exerts an effective potential for itinerant spins, is tuned by simply changing the field strength. Chiral magnetic orders observed do not exhibit any structural dislocation, indicating their high stability and robustness in Cr(1/3)NbS2.

Data have been compiled on the cross sections for collisions of electrons and photons with oxygen molecules (O2). For electron collisions, the processes included are: total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational, and electronic states, dissociation, ionization, and attachment. Ionization and dissociation processes are considered for photon impact. Cross-section data selected are presented graphically. Spectroscopic and other properties of the oxygen molecule are summarized for understanding of the collision processes. The literature was surveyed through August 1987, but some more recent data are included when available to the authors.

Data have been compiled on the cross sections for collisions of electrons and photons with nitrogen molecules (N2). For electron collisions, the processes considered are: total scattering, elastic scattering, momentum transfer, excitations of rotational, vibrational and electronic states, dissociation, and ionization. Ionization and dissociation processes are discussed for photon impact. Cross section data selected are presented graphically. Spectroscopic and other properties of the nitrogen molecule are summarized. The literature was surveyed through the end of 1984, but some more recent data are included when useful.

Background: Effective interactions, either derived from microscopic theories or based on fitting selected properties of nuclei in specific mass regions, are widely used inputs to shell-model studies of nuclei. The commonly used unperturbed basis functions are given by the harmonic oscillator. Until recently, most shell-model calculations have been confined to a single oscillator shell like the sd shell or the pf shell. Recent interest in nuclei away from the stability line requires, however, larger shell-model spaces. Because the derivation of microscopic effective interactions has been limited to degenerate models spaces, there are both conceptual and practical limits to present shell-model calculations that utilize such interactions. Purpose: The aim of this work is to present a novel microscopic method to calculate effective nucleon-nucleon interactions for the nuclear shell model. Its main difference from existing theories is that it can be applied not only to degenerate model spaces but also to nondegenerate model spaces. This has important consequences, in particular for intershell matrix elements of effective interactions. Methods:The formalism is presented in the form of a many-body perturbation theory based on the recently developed extended Kuo-Krenciglowa method. Our method enables us to microscopically construct effective interactions not only in one oscillator shell but also for several oscillator shells. Results: We present numerical results using effective interactions within (i) a single oscillator shell (a so-called degenerate model space) like the sd shell or the pf shell and (ii) two major shells (nondegenerate model space) like the sdf 7 p 3 shell or the pf g 9 shell. We also present energy levels of several nuclei that have two valence nucleons on top of a given closed-shell core. Conclusions: Our results show that the present method works excellently in shell-model spaces that comprise several oscillator shells, as well as in a single oscillator shell. We show, in particular, that the microscopic intershell interactions are much more attractive than has been expected by degenerate perturbation theory. The consequences for shell-model studies are discussed.

We present the first application of the newly developed EKK theory of the effective nucleonnucleon interaction to shell-model studies of exotic nuclei, including those where conventional approaches with fitted interactions encounter difficulties. This EKK theory enables us to derive the interaction suitable for several major shells (sd+pf in this work). By using such an effective interaction obtained from the Entem-Machleidt QCD-based χN 3 LO interaction and the Fujita-Miyazawa three-body force, the energies, E2 properties and spectroscopic factors of low-lying states of neutronrich Ne, Mg and Si isotopes are nicely described, as the first shell-model description of the "island of inversion" without fit of the interaction. The long-standing question as to how particle-hole excitations occur across the sd-pf magic gap is clarified with distinct differences from the conventional approaches. The shell evolution is shown to appear similarly to earlier studies. Introduction. -The nuclear shell model [1, 2] provides a unified and successful description of both stable and exotic nuclei, as a many-body framework which can be related directly to nuclear forces. Exotic nuclei are located far from the β-stability line on the Segrè chart, exhibiting very short life times, mainly due to an unbalanced ratio of proton (Z) and neutron (N ) numbers. Exotic nuclei differ remarkably in some other aspects from their stable counterparts, providing us with new insights in understanding atomic nuclei and nuclear forces [3][4][5]. As experimental data on exotic nuclei are, in general, less abundant compared to stable nuclei, theoretical calculations, interpretations and predictions play an ever increasing role.Shell-model (SM) calculations handle the nuclear forces in terms of two-body matrix elements (TBMEs). In the early days, TBMEs were empirically determined in order to reproduce certain observables. A well-known example is the effective interaction for p-shell nuclei by Cohen and Kurath [6]. A breakthrough towards more microscopically-derived TBMEs was achieved by Kuo and Brown for sd-shell nuclei [7]. Although basic features of the nucleon-nucleon (N N ) force for the SM calculation are included in these effective interactions, empirical adjustments of TBMEs were needed in order to reproduce various observables [8][9][10].These effective interactions were all derived for a Hilbert space represented by the degrees of freedom of one major (oscillator) shell. As we move towards ex-

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