Energy inversion of thef 7/2 andh 9/2 neutrons in yrast states of154Yb

The N=84 nucleus¹⁵⁴Yb was investigated through the¹⁰⁶Cd(⁵⁴Fe,2p) reaction. We found the 16⁺ yrast level in this nucleus to be formed by the (h_11/2 ² f_7/2h_9/2) configuration, in contrast to the lower-Z isotones where (h_11/2 ² f_7/2 ²)16⁺ is the yrast state. Manybody shell model calculations with empirical two-body interactions support our conclusion.Dedicated to Prof. Dr. O. Schult on the occasion of his 60^th birthday     

Calculation of energy levels with the hybrid multi-determinant method in the fp region

We compare the recent exact and coupled-cluster calculations (Phys. Rev. Lett. 98, 112501 (2007)) of energy levels using the GXPF1A Hamiltonian modified with variable shell gap with the corresponding results obtained with the hybrid multi-determinant scheme. We show that the hybrid multi-determinant scheme is highly cost-efficient and very accurate for several angular momenta and variable shell gap of the f5/2, p3/2, p1/2 shells relative to the f7/2 shell.     

High-spin states in the A= 39 mirror nuclei 39Ca and 39K

High-spin states of the mass A= 39 mirror pair ³⁹K and ³⁹Ca were investigated via the fusion-evaporation reaction ²⁸Si +¹⁶O at 125 MeV beam energy. The Gammasphere array in conjunction with the 4π charged-particle detector array Microball and neutron detectors was used to detect γ rays in coincidence with evaporated light particles. The results of the first high-spin study of the T _z=−1/2 nucleus ³⁹Ca are discussed in terms of mirror symmetry and compared to spherical shell-model calculations in the 1d _3/2–; 1f _7/2 configuration space. Received: 18 August 1999     

Ferromagnetic-to-antiferromagnetic transition in (Hf1−xTix)Fe2 intermetallic compounds induced by geometrical frustration of the Fe(2a) sites

The ferromagnetic-to-antiferromagnetic transition in the hexagonal (Hf_1−xTi_x)Fe₂ (0?x?1) intermetallic compounds has been investigated by ⁵⁷Fe Mössbauer spectroscopy. At 10 K, the transition occurs within rather narrow concentration limits, around x=0.55–0.65. We found that the key factor governing the unexpected quick change of the magnetic structure is the magnetic frustration of the Fe(2a) sites. The magnetic frustration is caused by the noncollinearity of the Fe(6h) magnetic sublattice. The noncollinearity arises from the rotation of the magnetic moments due to the competition between the ferromagnetic exchange interactions and the antiferromagnetic Fe(6h)–Ti–Fe(6h) interaction. In the compounds with x=0.4–0.6, the temperature transitions to the antiferromagnetic state are observed. As an example, the Hf_0.4Ti_0.6Fe₂ compound is completely antiferromagnetic above 200 K.     

Study of the first-excited K π=0+ and 2+ bands in 228Th in the 226Ra(α,2n) reaction

Excited bands of ²²⁸Th were studied in the ²²⁶Ra(α,2n) reaction by gamma-ray and conversion-electron spectroscopy. The first-excited K ^π=0⁺ and 2⁺ bands were identified up to I ^π= 12⁺ and 10⁺, respectively. The spin dependence of the moments of inertia is discussed. The experimental data indicate a structural change of these bands from ²²⁸Th to ²³⁰Th and ²³²Th. Received: 11 August 1997 / Revised version: 19 September 1997     

Gamma-ray spectroscopy of the doubly magic nucleus 56Ni

The doubly magic N = Z nucleus ⁵⁶Ni has been investigated with two fusion-evaporation reactions; ⁴⁰Ca(²⁸Si, 3α)⁵⁶Ni at a beam energy of 122MeV and ²⁸Si(³²S, 2p2n)⁵⁶Ni at 130MeV. To detect γ-rays in coincidence with evaporated particles the Ge-detector array Gammasphere was used in conjunction with the charged-particle detector system, Microball and a 1π neutron detector array. Results include a significantly extended level scheme of ⁵⁶Ni, which is compared to large-scale shell model calculations in the fp shell. The experimental and theoretical results agree to a large extent, with one notable exception; the theoretical model fails to predict the proper sequence of the yrast and yrare 8⁺ states.     

Measurement of Angular Distribution for the ^8Li（p,d）^7Li Reaction

The ^8Li（p, d）^7 Li reaction plays an important role in the inhomogeneous Big Bang nucleosynthesis and in the seed-nuclide production phase for the r-process. For the first time, its angular distribution at backward angles was measured in inverse kinematics at Ec.m.=4.0 MeV by using an ^8Li secondary beam. The result of measurement includes the contributions of ^8Li（p, d0）^7Li and ^8Li（p, d1）^7Li^＊. The ^8Li（p, d0）^7Li component is estimated to be 40%-58% in the mixture angular distribution by analysing the measured result.     

γ-ray spectroscopy of excited states in 61 30Zn31

The ⁶¹ ₃₀Zn₃₁ isotope has been produced at the Oak Ridge National Laboratory in the fusion-evaporation reaction ⁴⁰Ca(²⁴Mg, 2pn)⁶¹Zn at 104MeV. The experimental set-up allowed γ-rays to be detected in the CLARION Ge detector array in coincidence with the detection of recoiling nuclei in the focal plane at the end of the recoil mass spectrometer. This provides a unique identification of γ-rays belonging to ⁶¹Zn. The excited states have been explored by means of recoil- γγ coincidences, and the resulting decay scheme comprises almost 70 transitions. The data reveal numerous non-yrast states and suggest a revised spin and parity assignment for a previously observed superdeformed band. The resulting decay scheme is compared to predictions from different sets of large-scale shell model calculations.     

New states in 44,46Ar isotopes from deep-inelastic heavy ion reaction studies

In-beam γ-rays in neutron-rich sdfp shell nuclei, produced in deep-inelastic collisions of ⁴⁸Ca projectiles on ⁴⁸Ca target nuclei, have been studied using Ge multidetector array EU-ROBALL III. New yrast states in heavy argon isotopes have been identified. Received: 11 November 1999     

The 3701 keV neutron g 9/2 single-particle state in 57Ni

The neutron g _9/2 single-particle state in ⁵⁷Ni has been unambiguously identified in a combined measurement of the linear polarization, angular distribution, and angular correlation of γ rays following the fusion-evaporation reaction ²⁸Si(³²S,2pn)⁵⁷Ni at a beam energy of 90 MeV. The linear polarization was measured with a Euroball cluster detector and a prototype of an encapsulated six-fold segmented hexaconical Ge-detector. The spin of the 3701 keV level in ⁵⁷Ni was confirmed to be I= 9/2 and its parity determined to be positive. Received: 24 September 1999 / Revised version: 22 October 1999     

The use of symmetry in the search for canted ferromagnetism, its application to the molecular magnets Mn[N(CN)2]2 and Fe[N(CN)2]2

Canted ferromagnets present a important class of molecular magnets. In these materials the uncompensated magnetisation, created by the DzyaloshinskyMoriya interaction, has the particular property of being compatible with the symmetry of the underlying antiferromagnetic Hamiltonian. If the ordering transition is continuous, Landau theory and simple symmetry arguments about the crystal structure can be used to determine whether such parasitic ferromagnetism is possible, and therefore to aid the systematic search for new molecular ferromagnets.     

Effect of Co-Substitution on Magnetic Properties in Spinels GeNi2O4 Systems

The magnetic properties of spinel GeNi_2-2xCo_2xO₄ systems in the range 0≤x≤1 are studied by mean field theory and high-temperature series expansions. The nearest neighbouring and the next-neighbouring super-exchange interactions J₁ (x) and J₂(x) are evaluated for these systems in the range 0≤x≤1, by using the first theory. The intra-planar and the inter-planar interactions and the exchange energy are deduced. The second theory is applied in the spinel GeNi_2-2xCo_2xO₄ systems, combined with the Padé approximants method, to determine the magnetic phase diagrams (T_N versus dilution x) in the range 0≤x≤1. The obtained theoretical results are in agreement with experimental data obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility γ and the correlation lengths ν are deduced.     

Observation of the 7 MeV excited spin-flip and non-spin-flip partners in 16 ΛO by γ -ray spectroscopy

We have observed three γ -ray transitions in _Λ ¹⁶ O from both 7MeV excited spin-flip and non-spin-flip partners ( 2^-, 1^- ₂ to the ground-state doublet ( 1^- ₁, 0^- via the ¹⁶O(K ^-,π^-) reaction. The 7MeV excitation energies of the spin-doublet members ( 2^-, 1^- ₂ were determined to be 6784±4±4 keV and 6562±1±2 keV, respectively, and thus the spacing was obtained to be 222±4±5 keV. This is the first observation of the spin-flip state directly populated by the ( K ^-,π^- reaction. Moreover, such directly populated spin-flip and non-spin-flip partners were resolved for the first time.     

Study of 227Th in the 226Ra(α,3n) reaction

The nuclear structure of ²²⁷Th was studied in the ²²⁶Ra (α,3n) reaction by gamma-ray and conversion-electron spectroscopy. Six rotational bands were identified with spins upto approximately 39/2. Attempts to connect these bands to the low-lying levels known from radioactive-decay work were not fully successful. Received: 25 June 1998     

Polarized SSXANES study of spin ordering in ferromagnetic and paramagnetic phases of La1.2Sr1.65Ca0.15Mn2O7

Spin-selected polarized X-ray absorption near-edge structures (SSXANES) at the Mn K-edge from a bilayer La_1.2Sr_1.65Ca_0.15Mn₂O₇ single crystal have been studied with high resolution, both in the ferromagnetic (15 K) as well as paramagnetic phase (300 K). The orientation-dependent SSXANES spectra show unmistakable temperature dependence as the system makes the ferromagnetic to paramagnetic phase transition. The pre-edge structures are too intense to be ascribed to weak quadrupole transitions and are interpreted in terms of hybridization of Mn 3d orbitals with O2p and Mn 4p orbitals over and above similar onsite hybridization. The results also indicate possible existence of a small local (time-frozen) ferromagnetic ordering in the macroscopically disordered state. Need for further experimental and theoretical work on the SSXANES spectra from the bilayer system is emphasized.     

Evidence for an isomeric 3/2- state in 53Co

The fragmentation of a 550MeV/u primary beam of ⁵⁸Ni on a ⁹Be target has been used to measure time- and energy-correlated γ decays following the implantation of event-by-event discriminated secondary fragments into a ⁹Be stopper plate. A new isomeric γ decay with T _1/2 = 14( ) ns and E _γ = 646.2(2) keV is observed and attributed to the decay of the yrast 3/2^- state in ⁵³ ₂₇Co₂₆ . This short-lived isomeric state has been populated by means of nuclear reactions during the stopping process of the secondary fragments. The experimental findings are discussed in the framework of large-scale spherical shell model calculations in conjunction with isospin symmetry-breaking residual interactions for the A = 53 , T _z = ±1/2 mirror nuclei ⁵³Co and ⁵³Fe .     

The influence of the O–O isotope substitution on spin dynamics of (La0.25Pr0.75)0.7Ca0.3MnO3: La NMR data

The ¹³⁹La NMR spectra and spin–spin relaxation times have been measured for the ¹⁶O and ¹⁸O isotope-substituted manganite (La_0.25Pr_0.75)_0.7Ca_0.3MnO₃ in the external magnetic field of 5 T. The NMR signal wipe-out has been observed in the ¹⁸O-enriched sample in the charge-ordered state. This phenomenon is connected with a sharp increase in the spin–spin relaxation rate. The great isotope-effect observed provides a clear evidence of an essential role of oxygen motion in controlling the long-range magnetic order in manganites.     

Orientifolds and slumps in G2 and Spin(7) metrics

We discuss some new metrics of special holonomy, and their roles in string theory and M-theory. First we consider Spin(7) metrics denoted by , which are complete on a complex line bundle over . The principal orbits are S⁷, described as a triaxially squashed S³ bundle over S⁴. The behaviour in the S³ directions is similar to that in the Atiyah–Hitchin metric, and we show how this leads to an M-theory interpretation with orientifold D6-branes wrapped over S⁴. We then consider new G₂ metrics which we denote by , which are complete on an bundle over T^1,1, with principal orbits that are S³×S³. We study the metrics using numerical methods, and we find that they have the remarkable property of admitting a U(1) Killing vector whose length is nowhere zero or infinite. This allows one to make an everywhere non-singular reduction of an M-theory solution to give a solution of the type IIA theory. The solution has two non-trivial S² cycles, and both carry magnetic charge with respect to the R–R vector field. We also discuss some four-dimensional hyper-Kähler metrics described recently by Cherkis and Kapustin, following earlier work by Kronheimer. We show that in certain cases these metrics, whose explicit form is known only asymptotically, can be related to metrics characterised by solutions of the su(∞) Toda equation, which can provide a way of studying their interior structure.     

The magnetic behavior of La1−xTbxMn2Si2 (0≤x≤0.3) in weak magnetic fields

The structure and magnetic properties of La_1−xTb_xMn₂Si₂ (0≤x≤0.3) were studied by X-ray powder diffraction and DC magnetization measurements. All the compounds crystallize in ThCr₂Si₂-type structure. Substitution of Tb for La led to a linear decrease in the lattice constants and the unit-cell volume. A ferromagnetic phase for x≤0.15, and an antiferromagnetic phase for x=0.3 have been observed at about room temperature, whereas the compounds with x=0.2 and 0.25 exhibit a magnetic phase transition from ferromagnetism to antiferromagnetism.