Magnetic order and transport in the heavy-fermion system CeCuAu

We report on extensive elastic neutron scattering to determine the wave vector of the magnetic order in CeCu_6-xAu_x single crystals for x > 0.1. For all values of x investigated (0.2, 0.3, 0.5, 1.0) we find long-range incommensurate antiferromagnetic order with an ordering vector (0.625 0.275) for x =0.2, nearly unchanged for x =0.3, and (0.59) for x =0.5, staying roughly the same for x =1.0. In addition, short-range correlations are observed for x =0.2, reminiscent of those found previously for x =0.1. The ordered magnetic moment is found to increase rapidly for small x, and more slowly for the larger x values. The increase of the specific-heat anomaly at the ordering temperature with x is in qualitative accord with this behavior. Finally, data of the electrical resistivity for current flow along the three crystallographic directions are presented, showing a clear signature of the magnetic order. A theoretical interpretation of the interplay of magnetic order and transport in terms of (i) the partial suppression of the Kondo effect by the staggered magnetization and (ii) the anisotropic band structure induced by the staggered field is shown to account well for the data, provided the ordering vector is close to 2 k F, where k F is a typical Fermi momentum. Received: 20 February 1998 / Accepted: 17 April 1998     

Hydrogen atom transfer in indole clusters: formation dynamics of , n = 1-6, fragments

The H atom transfer reaction in electronically excited indole(NH ₃ ) _n clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products (NH ₃ ) _{n - 1} NH ₄ for n = 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of NH ₃ molecules. The formation times of the products are ca. 140ps for n = 2-4 and about 80ps for n = 5, 6. Received 30 April 2002 / Received in final form 29 May 2002 Published online 13 September 2002     

Lifetime measurements for some levels in Be-like Cl XIV and S XIII

Lifetime measurements have been carried out for some low lying (n = 2) levels in the four-electron ions Cl XIV and S XIII using the beam-foil technique. Accurate oscillator strengths for the 2 s2 ¹ S - 2 s 2 p ¹ P o transition have been determined by the inclusion of prominent cascades in the analysis. Lifetimes of the levels of the 2 p2 ³ P j multiplet have also been measured for both ions. The results are compared with theoretical predictions and earlier measurements. Received: 29 October 1997 / Revised: 10 February 1998 / Accepted: 5 March 1998     

Anomalous magnetic behavior of superconducting thin films in small magnetic fields [4] close to the transition temperature

Using a specially designed SQUID magnetometer we measured the temperature dependence of the critical current density in a ring patterned thin film for magnetic fields parallel to the c-axis. In addition, the temporal relaxation of the remanent state as prepared by field cooling in an external field of 100 Oe at different temperatures is determined. The j c ( T ) data show a field-dependent anomalous kink close to T_c pointing to reduced dissipation with increasing temperature allowing to construct a corresponding H-T borderline. A similar behavior is observed for the normalized relaxation rate S ( T ) as extracted from the temporal behavior of the remanent state, which, at low temperatures, exhibits the expected increase for increasing T-values, while an anomalous decrease of S ( T ) is found for temperatures above 85 K. While the low-T regime is attributed to creep of 2D pinned single vortex lines, the high-T behavior is suggested to be dominated by collective motion with a more sluggish dynamics. This change in dynamics is also reflected by the activation barriers for flux creep U ( j ), which show a corresponding crossover in μ from 0.06 to 0.99. An additional scaling analysis of the E-j characteristics for according to vortex glass theory reveals quasi-2D collective creep behavior with . Received: 8 April 1998 / Revised: 15 July 1998 / Accepted: 2 September 1998     

Magnetization and neutron scattering studies of the pressure effect on the magnetic transition in Er Y Co

Neutron powder diffraction was employed to study the pressure effect on the magnetic transition in the pseudobinary Laves-phase compound Er_0.57Y_0.43Co₂ and to determine the magnetic moments of the Er- and Co-subsystems. Our studies reveal that the onset of long-range magnetic order for both the localized 4 f (Er) and itinerant 3 d (Co) electron moments appears at about the same temperature at ambient pressure. The pressure effect on T_c is found to be negative and equal for both sublattices, namely T _c / p ∼ - 0.4 K/kbar. The values of the magnetic moments of the Er and the Co ions are found = 5.40±0.15μ _B /atom, = 0.50±0.07μ _B /atom and 5.35±0.15μ _B /atom, 0.37±0.09μ _B /atom, for p = 0 and 6 kbar, respectively. Our experimental results give evidence for short-range magnetic order formation at temperatures already above T_c and for a coexistence short- and long-range order below T_c down to 4 K. Received 20 December 2001 / Received in final form 12 June 2002 Published online 31 October 2002 RID="a" ID="a"e-mail: andrew.podlesnyak@psi.ch     

Localization effects in the charge density wave state [4]of the quasi-two-dimensional monophosphate tungsten bronzes {(P{O_2})_4}{(W{O_3})_{2m}}(m = 7,8,9)

The monophosphate tungsten bronzes (PO ₂ ) ₄ (WO ₃ ) _2m are quasi-two-dimensional conductors which show charge density wave type electronic instabilities. We report electrical resistivity and magnetoresistance measurements down to 0.30 K and in magnetic fields up to 16 T for the m = 7, 8 and 9 members of this family. We show that these compounds exhibit at low temperature an upturn of resistivity and field dependences of the magnetoresistance characteristic of localization effects. We discuss the dimensionality of the regime of localization as m is varied. We show that for m =7, the regime is quasi-two-dimensional and three-dimensional for m = 8, 9. Received 16 September 1999     

Study of excitation transfer Li (3 D → 3 P ) occurring in optical collisions with rare gas atoms experimentally

By selective optical excitation of collision pairs and observation of the reemitted fluorescence information is obtained on the role of the molecular channels involved in inelelastic collisions. As an example case we have studied experimentally the Li( 3 D → 3 P) excitation transfer in Li(3D)X systems with X = Ne, Ar by means of the optical collision process Li (2 P ) + X + h ν→ LiX (3 D Λ) → Li (3 P , 3 D ) + X where LiX (3 D Λ) collision molecules dissociate into Li(3P, 3D) atoms following laser excitation h ν of Li (2 P ) + X pairs. For this purpose we measured the Li 3P/3D population ratio by the fluorescence from these levels as function of the laser detuning Δν from the Li(2P-3D) transition and the rare gas pressure, and determined from this the 3P/3D excitation ratio B (Δν) for single collision conditions. The experiments were performed using two step cw laser excitation of gaseous mixtures Li + X at temperatures around 600 K in the detuning range |Δν| ? 100 cm^-1. The B (Δν) profiles obtained display strong blue-red wing asymmetries both for Li ^* Ne and Li ^* Ar. This reflects different dissociation probabilities from the 3 D Σ or 3 D (Π,Δ) states that are initially prepared by blue wing or red wing excitation, respectively. The results are qualitatively discussed in terms of new ab initio potentials for the two systems. Received 23 February 2000 and Received in final form 5 July 2000     

Isotope dependent predissociation in the C 1 Σ + , v =0 and v =1 states of CO

Rotationally resolved spectral lines in the C - X (1, 0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm^-1 in the vacuum ultraviolet. Improved molecular constants for the excited state are derived and no effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the C ¹ Σ ⁺ , v = 1 state for five natural isotopomers of carbon monoxide: τ( ¹² C ¹⁷ O ) = 280 ps, τ( ¹² C ¹⁸ O ) = 210 ps, τ( ¹³ C ¹⁶ O ) = 295 ps, τ( ¹³ C ¹⁷ O ) = 160 ps, and τ( ¹³ C ¹⁸ O ) = 150 ps. Within the accuracy of the present measurements no effects of J-dependent lifetimes were observed, for neither of the isotopomers. In addition direct time domain measurements of the lifetime of the C ¹ Σ ⁺ , v = 0 and v = 1 states of the main isotopomer are performed in a pump-probe experiment using a picosecond VUV-laser, yielding τ( ¹² C ¹⁶ O ) = 1780 ps for v = 0 and τ( ¹² C ¹⁶ O ) = 625 ps for v = 1. For C ¹ Σ ⁺ , v = 0 in ¹²C¹⁶O and ¹³C¹⁶O the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay in this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C ¹ Σ ⁺ , v = 0 state of the ¹³C¹⁸O isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for ¹²C¹⁶O.) From the combined data predissociation yields upon excitation of the C ¹ Σ ⁺ , v = 1 state are derived, lying in the range 0.84-0.91 for the five less abundant isotopomers; for the main ¹²C¹⁶O isotopomer a strongly deviating predissociation yield of 0.65 is deduced. Received 21 December 2000 and Received in final form 26 March 2001     

Electron-pair densities of group 2 atoms in their and terms

Electron-pair intracule (relative motion) and extracule (center-of-mass motion) densities are studied in both position and momentum spaces for the ¹ P and ³ P terms of the group 2 atoms Be (atomic number Z =4), Mg (Z =12), Ca (Z =20), Sr (Z =38), Ba (Z =56), and Ra (Z =88). In position space, the ¹ P - ³ P difference in the intracule densities shows that the probability of a small interelectronic distance is larger in the triplet for all the six atoms, as reported for the lightest Be atom in the literature. The position-space extracule density clarifies that the triplet electrons are more likely to be at opposite positions with respect to the nucleus than the singlet electrons for all the atoms. In momentum space, the singlet generally has a larger probability of a small relative momentum between two electrons as a na?ve manifestation of the Fermi hole in the triplet. The extracule density in momentum space shows that the ¹ P term has a distribution larger in a large center-of-mass momentum region than the ³ P term. Received: 26 August 1998 / Received in final form: 1 February 1999     

On the existence of <Emphasis Type="Bold">1, 3</Emphasis>P <Emphasis Type="Bold">o</Emphasis> resonances in H<Superscript>-</Superscript> between n = 2 and 3 H thresholds

We examine the influence of relativistic and QED effects on the existence of the ^1,3P ^o H^- resonances between n = 2 and 3 hydrogen thresholds, the relativistic and QED corrections and the coupling effects between the high singlet and triplet states are considered as first-order perturbations. We firstly obtain accurate non-relativistic resonant energies and widths of fifteen ¹P ^o resonances, and fifteen ³P ^o resonances. The fifteen ¹P ^o resonances are classified to be ₃ (2, 0) ^- _n ( 4 ? n ? 12) and ₃ (1, 1) ⁺ _n ( 3 ? n ? 8). The fifteen ³P ^o resonances are classified to be ₃ (2, 0) ⁺ _n ( 3 ? n ? 12) and ₃ (1, 1) ^- _n ( 4 ? n ? 8). We found there exist six Feshbach resonances for ₃ (2, 0) ^- _n (¹P ^o ) series, four Feshbach resonances for ₃ (1, 1) ⁺ _n (¹P ^o ) series, seven Feshbach resonances for ₃ (2, 0) ⁺ _n (³P ^o ) series, and three Feshbach resonances for ₃ (1, 1) ^- _n (¹P ^o ) series. Received 22 February 2002 Published online 24 September 2002     

Study of : pronounced differences between the and ground states

We present electrical resistivity and specific heat measurements of alloys on the Rh rich side of the phase diagram of the Ce(Rh_1-xPd_x)₂Si₂ system. We compare these results with those obtained at intermediate and low Rh concentrations. The analysis of the concentration and temperature dependence of the entropy and of the scaling behaviour of C _el ( T ) and ρ( T ) clearly confirm a separation of the magnetic phase diagram into two regions: the region x ≤0.3, showing a concentration independent characteristic temperature for the 4 f-electrons with T ₀ ≈ 45 K, while for x > 0.3, T₀ decreases to T ₀ ( x = 1) ≈ 15 K. At low Pd-content, T_N decreases very rapidly from T _N = 36 K in pure CeRh₂Si₂ to T _N = 18 K at x = 0.1. With higher Pd concentration T_N stabilizes at T _N ≈ 15 K whereas the magnitude of the anomalies in C _el ( T ) and in the susceptibility around T_N are further reduced and disappear at x ≈ 0.3. This differs from the behavior found on the Pd-rich side, where T_N decreases continuously to zero with increasing Rh content. The pronounced differences observed between both phase boundaries and the drastic effect of doping on the Rh rich side suggest an itinerant character in CeRh₂ Si₂, in contrast with the localized character of CePd₂Si₂. Further evidence for the itinerant character of CeRh₂Si₂ is given by the ρ( T ) dependence observed for x ≤0.3, which scales with ρ( T ) of the prototype itinerant compound YCo₂. Received 31 December 2001 / Received in final form 6 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: berisso@cab.cnea.gov.ar     

Finestructure, hyperfine structure and isotope shift of 4f 6s7s in Er I

The isotope shift (IS) and hyperfine structure (hfs) of nine levels (31720 to 38921 cm^-1) assigned to the configuration 4 f ¹² 6 s 7 s in neutral erbium have been determined experimentally using Doppler-reduced saturation absorption spectroscopy in a gas discharge. We performed a fine structure analysis in the SL-coupling scheme of the single configuration 4 f ¹² 6 s 7 s, confirming and extending the classification of even parity Er I levels. We discriminated the different hfs contributions of the 4f¹² core and the (6 s +7 s) outer electrons of the shell in a non-relativistic JJ-coupling approach and in the relativistic effective tensor operator formalism in SL-coupling. The relativistic one-electron parameters of the hfs for ¹⁶⁷Er were fitted to the experimental data by a least squares fit procedure: [0pt] a ⁰¹ _4f =-147(3) MHz, [0pt] a ¹⁰ _6s + a ¹⁰ _7s =-1840(30) MHz, [0pt] b ⁰² _4f =6560(80) MHz. The level dependencies of the isotope shift were evaluated based on crossed second order (CSO) effects. We obtained the following results for the CSO parameters for the isotope pairs ^170-168Er: d _6s7s =-740(30) MHz, z _4f = 0(5) MHz, ( g _3,6s ( f , 6 s )+ g _{3, 7s} ( f , 7 s ))= -24(15) MHz and for ^170-166Er: d _6s7s =-1500(50) MHz, z _4f =0(10) MHz, ( g _3,6s ( f ,6 s )+ g _3,7s ( f +7 s ))=-50(29) MHz. The resulting parameters for the hfs are compared with those known for other configurations of the Er atom and ion. Received 16 May 1999 and Received in final form 31 January 2000     

High accuracy Dirac-finite-element (FEM) calculations for H 2 + and Th 2 179 +

A two-dimensional, fully numerical approach to the four-component first-order Dirac-differential-equation utilizing the Finite-Element-Method (FEM) is employed for H ₂ ⁺ and Th ₂ ^{179 +} . Using elliptic-hyperbolic coordinates and further one-dimensional singular transformations, scaling transformations and extrapolation techniques (geometrical over iteration steps and logarithmic over grid points) we achieve for the molecules H ₂ ⁺ and Th ₂ ^{179 +} relative accuracies better than 10^-12 for 1(1/2) g energies. Received 16 February 2001 and Received in final form 12 July 2001     

Improving Visible Light Sensitization of Luminescent Europium Complexes

The synthesis and characterization of the new ligands L ₁ , L ₂ and L ₄ are described with the series of four europium complexes of formula [EuL _n (TTA)₃] in which TTA refers to 2-thenoyltrifluoroacetonate and L _n to tridentate ligands with nitrogen containing heterocyclic structure, such as a 2,6-bis(3-methyl-pyrazolyl)-4-(p-toluyl-ethynyl)-triazine for L ₁ , or terpyridines functionalized at the 4′ position by a phenyl-vinylene for L ₂ , a p-dimethylamino-phenylene for L ₃ , or a p-aminophenyl-ethynylene for L ₄ . The spectroscopic properties of the ligands and of the complexes are studied by means of UV–Vis absorption spectroscopy, as well as steady-state and time-resolved luminescence spectroscopy. All complexes display europium centred luminescence upon ligand excitation. Careful examination of the excitation spectra revealed differences in the ligand based sensitization efficiencies. For complexes of L ₁ and L ₂ , excitation of europium is mainly achieved through the TTA moieties and the photo-physical studies on [EuL ₁ (TTA)₃] evidenced a weaker coordination of the bispyrazolyltriazine tridentate ligand, resulting from a partial decomplexation upon dilution. Complexes of L ₃ and L ₄ display intense excitation through the tridentate units, which extend down to 460 nm in the visible region. In the case of L ₃ , selective excitation reveals the presence of a ligand-centred emission band at 520 nm which is likely ascribed to a L ₃ centred charge transfer state.     

Effects of Ca substitution on the phonon anomalies in the Raman scattering of YBa<Subscript>2</Subscript>Cu<Subscript>4</Subscript>O<Subscript>8</Subscript>

We report on the temperature dependence of the frequencies and linewidths in the phonon Raman scattering for Y _1-y Ca y Ba ₂ Cu ₄ O ₈ system ( y = 0 - 0.15). The phonon anomalies above T _c of the frequencies are observed for the out-of-phase O(2)-O(3) A _g and O(4) A _g modes, and these onset temperatures decrease with increasing Ca content. These features are consistent with the spin-gap behaviors associated with Ca doping reported previously. Furthermore, we find that the smaller gap exists near or just above the Ba phonon frequency at in the undoped samples and its energy increases with Ca doping. Received 9 June 1999     

Unconventional antiferromagnetic correlations of the doped Haldane gapsystem Y 2 BaNi 1 - x Zn x O 5

We make a new proposal to describe the very low temperature susceptibility of the doped Haldane gap compound Y₂BaNi_1-xZn_xO₅. We propose a new mean field model relevant for this compound. The ground state of this mean field model is unconventional because antiferromagnetism coexists with random dimers. We present new susceptibility experiments at very low temperature. We obtain a Curie-Weiss susceptibility χ( T ) ∼ C /(Θ + T ) as expected for antiferromagnetic correlations but we do not obtain a direct signature of antiferromagnetic long range order. We explain how to obtain the “impurity” susceptibility ( T ) by subtracting the Haldane gap contribution to the total susceptibility. In the temperature range [1 K, 300 K] the experimental data are well fitted by T ( T ) = C _imp 1 + T _imp / T . In the temperature range [100 mK, 1 K] the experimental data are well fitted by T ( T ) = A ln( T / T _c ), where T _c increases with x. This fit suggests the existence of a finite Néel temperature which is however too small to be probed directly in our experiments. We also obtain a maximum in the temperature dependence of the ac-susceptibility ( T ) which suggests the existence of antiferromagnetic correlations at very low temperature. Received 17 July 2001     

Evolution of ( La 1 - y Pr y ) 0.7 Ca 0.3 MnO 3 crystal structure with A-cation size, temperature, and oxygen isotope substitution

The atomic structure of ( La _{1 - y} Pr _y ) _0.7 Ca _0.3 MnO ₃ compound with 0.5≤ y ≤1 has been systematically studied by neutron powder diffraction in the temperature range from 15 to 293 K. For composition with y = 0.75, the structural analysis was performed on two samples, one containing the natural mixture of oxygen isotopes and the other one 75% enriched by ¹⁸ O. The room temperature structural characteristics of the series, including cell volume, average Mn-O bond distance, and average Mn-O-Mn bond angle, are the linear functions of the < r _A >. Temperature dependencies of these parameters are quite smooth, except for the point T = T _FM , where a jump like changes occur. The isotope enriched samples have been found identical in crystal and magnetic structure down to the temperature of transition of the sample with ¹⁶ O into the metallic ferromagnetic phase. It confirms that different transport and magnetic properties of the samples with ¹⁶ O and ¹⁸ O at low temperature are driven by the different oxygen atoms dynamics solely. Temperature dependencies of the CO and AFM diffraction peak intensities and of the peak widths for compositions close to the metal-insulator boundary ( y ≈ 0.75) indicate the macroscopically phase separated AFM-dielectric + FM-metallic state below T _FM . Received 28 April 2000     

Temperature and size dependence of the optical response of small Ag clusters

The absorption spectra of small Ag ⁺ n clusters are calculated at finite vibrational temperature by using a microscopic tight-binding RPA method. We consider free clusters with sizes between n = 3 and n =13 and take into account explicitly the degrees of freedom corresponding to the 4 d-electrons. We analyze the optical absorption as a function of the cluster size. We show that the contribution of the d-electrons has an important influence on the size dependence of the energy of the Mie plasmon. We also perform ensemble averages to obtain the absorption spectra for different vibrational temperatures. We obtain relatively good agreement with experiment for a temperature . The dynamics of the 4 d-electrons, which shows in small clusters an incipient delocalized character for n >7, yields an important contribution to the absorption spectrum already for n =13. We find that the strength of this contribution can be controlled by varying the vibrational temperature. Received: 4 January 1999 / Received in final form: 12 May 1999     

Ferromagnetism in lacunar perovskite manganites Pr 0.7 Sr 0.3 - x x MnO 3 and Pr 0.7 - x x Sr 0.3 MnO 3

Deficiency effects in the A site upon the structural, magnetic and electrical properties in the lacunar perovskite manganite oxides Pr_0.7Sr_{0.3-x x}MnO₃ ( 0 ? x ? 0.3) and Pr_{0.7-x x}Sr_0.3MnO₃ ( 0 ? x ? 0.23) have been investigated. This study focuses on the different parameters which govern the magnetic and electrical properties in such samples. The powder X-ray diffraction patterns for all samples could be indexed either with a rhombohedral perovskite structure and R c space group for x ? 0.2 in strontium deficient samples and for x ? 0.1 for praseodymium deficient ones. For other values of x the samples could be indexed in the orthorhombic structure with Pbnm space group. Magnetic and electrical investigations show that praseodymium and strontium vacancies do not have similar effects on the lacunar compounds. Magnetization measurements versus temperature show that all our samples exhibit a magnetic transition when the temperature decreases. All the praseodymium deficient samples exhibit a paramagnetic-ferromagnetic transition when the temperature decreases while the strontium deficient ones exhibit this transition only for low x values. The magnetic transition temperature shifts to lower values as the strontium deficiency increases (from 265 K for x = 0 to 90 K for x = 0.3) and to higher values with the praseodymium deficiency increase (from 265 K for x = 0 to 315 for x = 0.23). Resistivity measurements as a function of temperature show a semiconducting-metallic transition for all x values in the praseodymium lacunar samples and only for low x values ( 0 ? x ? 0.1) in the strontium lacunar ones when the temperature decreases. Received 12 April 2000 and Received in final form 8 January 2001     

Properties of doubly excited states of H- and He associated with the manifolds from N = 6 up to N = 25

This paper discusses theory and results on ¹P⁰ doubly excited states (DES) in He and in H^- of very high excitation, up to the N = 25 manifold. Our calculations employed full configuration interaction (CI) with large hydrogenic basis sets and produced correlated wavefunctions for the four lowest roots at each hydrogenic manifold by excluding open channels and the small contribution of series belonging to lower thresholds. The suitability of the hydrogenic basis sets for such calculations is justified, apart from their practicality, by the fact that, by computing from them natural orbitals, the results were shown to be the same with those of earlier multiconfigurational Hartree-Fock (MCHF) calculations on low-lying DES. In total, 160 states were computed, most of them for the first time. Their energy spectrum should be of use to possible future photoabsorption experiments. For certain low-lying DES up to N = 13, for which previous reliable results are available, comparison of the calculated energies shows good agreement. The correlated wavefunctions contain systematically chosen single and double excitations from each hydrogenic manifold of interest. From their analysis, we determined the “goodness" of different quantum numbers and the geometry (average angles and radii) as a function of excitation. For the Sinano lu-Herrick ( K , T ) classification scheme, whose basis is a restricted CI with hydrogenic functions and which has thus far been tested only on low-lying DES, we established that, whereas T remains a good index as energy increases, K does not. Consequently, a more flexible than K quantum number is needed in order to account for most of the additional correlation. This number, represented by F = N - K - 1, where N and K are not good numbers anymore, produces consistently a much higher degree of purity than the ( K , T ) scheme does, especially as N increases and as the relative significance of various virtual excitations due to electron correlation increases. Among the four states of each manifold, in all cases in H^- and in most cases in He, the three are of the intrashell type and one is of the intershell type with ( F , T ) = (0, 0). The lowest intrashell states and the lowest intershell states exhibit a wide angle geometry tending to 180 ^° as N ↦∞. Received 10 September 2001 and Received in final form 12 November 2001