Magnetic susceptibility in quasi one-dimensional Ba<Subscript>a2</Subscript>V<Subscript>3</Subscript>O<Subscript>9</Subscript>: chain segmentation versus the staggered field effect

A pronounced Curie-like upturn of the magnetic susceptibility χ( T ) of the quasi one-dimensional spin chain compound Ba₂V₃O₉ has been found recently [#!kaul:02!#]. Frequently this is taken as a signature for a staggered field mechanism due to the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We calculate this contribution within a realistic structure of vanadium 3 d- and oxygen 2 p-orbitals and conclude that this mechanism is far too small to explain experimental results. We propose that the Curie term is rather due to a segmentation of spin chains caused by broken magnetic bonds which leads to uncompensated S = ? spins of segments with odd numbers of spins. Using the finite-temperature Lanczos method we calculate their effective moment and show that ∼ 1% of broken magnetic bonds is sufficient to reproduce the anomalous low-T behavior of χ( T ) in Ba₂V₃O₉. Received 19 December 2002 / Received in final form 29 January 2003 Published online 14 March 2003     

( Sr / Ca ) 14 Cu 24 O 41 spin ladders studied by NMR under pressure

⁶³Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr_14-xCa_xCu₂₄O₄₁ single crystals 0 ? x ? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved. The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under pressure and confirm the onset of low lying spin excitations around P _c as previously reported. The spin gap in Sr ₂ Ca ₁₂ Cu ₂₄ O ₄₁ is strongly reduced above 20 kbar. However, the data of an experiment performed at P = 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes. We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower than the pressure corresponding to the maximum of the superconducting critical temperature. Received 8 March 2001 and Received in final form 27 July 2001     

A theoretical analysis of the magnetic properties of La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The magnetic properties of the La₂CuO₄ are analyzed by means of the paramagnetic solution of the Hubbard model within the composite operator method. The experimental findings of the inelastic neutron magnetic scattering [R. Coldea et al., Phys. Rev. Lett. 86, 5377 (2001)] for the spin spectrum, the spin-wave intensity and the behavior of the dispersion at the zone boundary are well described by our results although the difference in phase. The Hubbard model emerges has a minimal model capable to describe the anomalous magnetic behavior of such a strongly correlated material. Received 29 July 2002 / Received in final form 2 January 2003 Published online 14 March 2003     

DFT studies of the molecular nanomagnet Fe 8 and the V 15 spin system

Based on first-principles all-electron density-functional calculations we report the electronic structure and magnetic ordering of the molecular magnet Fe₈ and the V₁₅ spin system. The ferrimagnetic ordering with total spin S = 10 of the eight iron atoms in the Fe₈ cluster agrees well with experimental results from polarized neutron data. In comparison the low spin system V₁₅ shows a spin S = 1/2 ground state which is also found from our calculations. Received 30 October 2000     

Structural phase transition in the 2D spin dimer compound SrCu 2 ( BO 3 ) 2

A displacive, 2nd order structural phase transition at T _s = 395 K from space group I 2 m below T _s to I 4/m c m above T _s has been discovered in the two-dimensional spin dimer compound SrCu₂(BO₃)₂. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed. Received 24 July 2000 and Received in final form 2 November 2000     

Magnetic contributions to the low-temperature specific heat of the ferromagnetic insulator Pr 0.8 Ca 0.2 MnO 3

The Pr _1-x Ca_xMnO₃ system exhibits a ferromagnetic insulating state for the composition range x ? 0.25. A metallic ferromagnetic state is never realized because of the low hole concentration and the very small averaged A-site cation radius. In the present study, the nature of the magnetic excitations at low temperature has been investigated by specific heat measurements on a Pr _0.8 Ca_0.2MnO₃ single crystal. The decrease of the specific heat under magnetic field is qualitatively consistent with a suppression of ferromagnetic spin waves in a magnetic field. However, at low temperature, the qualitative agreement with the ferromagnetic spin waves picture is poor. It appears that the large reduction of the specific heat due to the spin waves is compensated by a Schottky-like contribution possibly arising from a Zeeman splitting of the ground state multiplet of the Pr³⁺ ions. Received 21 May 2001 and Received in final form 14 December 2001     

Experimental determination of spin-rotation and spin-spin magnetic interactions in by nuclear spin conversion

When a gas sample of ¹³CH₃F is prepared with a population of isomers (ortho and para forms) far from the equilibrium given by nuclear spin statistics, it relaxes towards this equilibrium with an exponential decay rate. This phenomenon called nuclear spin conversion is mainly governed by intramolecular spin-spin and spin-rotation interactions. In the quantum relaxation model [P.L. Chapovsky, Phys. Rev. A 43, 3624 (1991)], two pairs of ortho-para levels (J = 9, K = 3; J' = 11, K' = 1) and (J = 20, K = 3; J' = 21, K' = 1) are principally responsible for the conversion. The levels of the second pair are coupled by both spin-spin and spin-rotation interactions. The application of an electric field (up to 10 kV/cm) induces a crossing of the Stark components of this pair, which is observed for the first time. A specific experimental set-up based on an electric field of alternating triangular shape is used, which allows the determination of the strength of both interactions via the measurement of the spin conversion decay rates. This work yields the first experimental value for the electronic contribution to the spin-rotation interaction in ¹³CH₃F. Received 23 May 2002 / Received in final form 18 September 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: Patrice.Cacciani@univ-lille1.fr     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

Phase diagram of the spin extended model

The quantum phase transition in the ground state of the extended spin S = 1/2 XY model has been studied in detail. Using the exact solution of the model the low temperature thermodynamics, as well as the ground state phase diagram of the model in the presence of applied uniform and/or staggered magnetic field are discussed. Received 29 November 2002 / Received in final form 24 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: japa@iph.hepi.edu.ge     

Pseudo-gap and possible Spin-Peierls transition in the vanadium oxide VOSb 2 O 4

We report on the magnetic susceptibility and electron spin resonance measurements on polycrystalline samples of the vanadium oxide VOSb₂O₄, a quasi-one-dimensional S = 1/2 Heisenberg system. The susceptibility vanishes at zero temperature, but there is no cusp at the onset of the susceptibility drop, and the ESR linewidth exhibits an increase characteristic of a phase transition at a much lower temperature. We show that this behaviour is consistent with the formation of a pseudo-gap in a spin-Peierls system in the adiabatic limit. Received 7 February 2001 and Received in final form 24 April 2001     

Structure and magnetism on iron oxide clusters Fe nO m ( n = 1-5): Calculation from first principles

We have studied structural and magnetic properties in small iron oxide clusters, Fe_nO_m (n = 1-5), by means of the first-principles calculation based on the density functional theory. We have used not only the usual spin polarized scheme, but also the scheme for noncollinear magnetism to carry out efficient optimization in magnetic structure. The result of FeO_m (m = 1-4) is in good agreement with the previous work. We found the stable adduct clusters in FeO₅ and FeO₆. The bridge site of oxygen atom is more favorable in energy than any other site for the clusters of Fe_nO (n = 2-5). As increasing the number of oxygen atoms, the alignment of Fe magnetic moments changes from ferromagnetic configuration to antiferromagnetic one at Fe_nO_n (n = 2-4). Received 10 September 2002 Published online 3 July 2003     

Location of the minimum of the differential tunneling resistance in a superconductor-degenerate semiconductor Schottky contact

Measurements of differential resistance in a superconductor-degenerate semiconductor junction Nb - n ^{+ +} GaAs at T = 1.6 K show close similarity to those for a conventional superconductor-insulator- normal metal junction, except for the position of the minimum which is located at 3.6 meV. Using a simple model for the charge screening at the Schottky barrier, we give an argument why this minimum is by far displaced with respect to the superconducting gap energy ( Δ _g = 1.5 meV for bulk Nb). We argue that a rebuilding of the density of states takes place at the barrier, due to the imperfect metal screening in the degenerate semiconductor. Energy states close to the degenerate semiconductor Fermi energy are depleted at the barrier and are not available for tunneling, up to an energy E_g which adds to the superconducting gap Δ _g . Received 11 November 2002 / Received in final form 21 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: c.nappi@cib.na.cnr.it     

Insulator-metal transition shift related to magnetic polarons in La<Subscript>0.67-x</Subscript>Y<Subscript>x</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

The magnetic transport properties have been measured for La_0.67-xY_xCa_0.33MnO₃ ( 0 ⩽ x ⩽ 0.14) system. It was found that the transition temperature T _p almost linearly moves to higher temperature as H increases. Electron spin resonance confirms that above T _p , there exist ferromagnetic clusters. From the magnetic polaron point of view, the shift of T _p vs. H was understood, and it was estimated that the size of the magnetic polaron is of 9.7 ∼ 15.4 ? which is consistent with the magnetic correlation length revealed by the small-angle neutron-scattering technique. The transport properties at temperatures higher than T _p conform to the variable-range hopping mechanism. Received 27 August 2002 / Received in final form 2 December 2002 Published online 14 March 2003     

The <Emphasis Type="Bold">±</Emphasis><Emphasis Type="Bold">J</Emphasis> spin glass in Migdal-Kadanoff approximation

We study the low-temperature phase of the three-dimensional ± J Ising spin glass in Migdal-Kadanoff approximation. At zero temperature, T = 0, the properties of the spin glass result from the ground-state degeneracy and can be elucidated using scaling arguments based on entropy. The approach to the asymptotic scaling regime is very slow, and the correct exponents are only visible beyond system sizes around 64. At T > 0, a crossover from the zero-temperature behaviour to the behaviour expected from the droplet picture occurs at length scales proportional to T ^-2/ds where d_s is the fractal dimension of a domain wall. Canonical droplet behaviour is not visible at any temperature for systems whose linear dimension is smaller than 16 lattice spacings, because the data are either affected by the zero-temperature behaviour or the critical point behaviour. Received 18 February 2001     

Covalent magnetism in the RFe <Emphasis Type="Bold">6</Emphasis>Ge <Emphasis Type="Bold">6</Emphasis> series

The electronic structure of the RFe ₆ Ge ₆ compounds ( R = Sc, Lu, Ti, Zr, Hf and Nb) of HfFe ₆ Ge ₆ -type structure has been studied using the muffin-tin Korringa-Kohn-Rostoker method in a non-relativistic approach. The chemical bonding is analyzed based on the l-decomposed site projected densities of states. Spin-dependent changes in the R nd- Fe 3d covalent bond are shown to be responsible for the experimentally observed rise in the Fe moment and hyperfine field upon increasing the R valency. The limited quantitative agreement between theoretical and experimental values is interpreted as being due to a non-negligible orbital moment and to a significant asphericity in the spin density at the iron site. The theoretical results also forecast a strong increase of the Ge(2e) transferred hyperfine field with the R valency. Received 20 December 2002 Published online 4 June 2003 RID="a" ID="a"e-mail: Thomas.Mazet@lcsm.uhp-nancy.fr RID="b" ID="b"Associé au CNRS (UMR 7555)     

Orbital-selective Mott-insulator transition in Ca 2 - x Sr x RuO 4

The electronic structures of the metallic and insulating phases of the alloy series Ca_2-xSr_xRuO₄ ( 0 ? x ? 2) are calculated using LDA, LDA+U and Dynamical Mean-Field Approximation methods. In the end members the groundstate respectively is an orbitally non-degenerate antiferromagnetic insulator (x = 0) and a good metal (x = 2). For x > 0.5 the observed Curie-Weiss paramagnetic metallic state which possesses a local moment with the unexpected spin S = 1/2, is explained by the coexistence of localized and itinerant Ru-4d-orbitals. For 0.2 < x < 0.5 we propose a state with partial orbital and spin ordering. An effective model for the localized orbital and spin degrees of freedom is discussed. The metal-insulator transition at x = 0.2 is attributed to a switch in the orbital occupation associated with a structural change of the crystal. Received 27 July 2001     

Aging dynamics of Edwards-Anderson spin glasses

We analyze by means of extensive computer simulations the out of equilibrium dynamics of Edwards-Anderson spin glasses in d = 4 and d = 6 dimensions with ± J interactions. In particular, we focus our analysis on the scaling properties of the two-time autocorrelation function in a wide range of temperatures from T = 0.07 T _c to T = 0.75 T _c in both systems. In both the 4 d and 6 d models at very low temperatures we study the effects of discretization of energy levels. Strong sub-aging behaviors are found. We argue that this is because in the times accessible to our simulations the systems are only able to probe activated dynamics through the lowest discrete energy levels and remain trapped around nearly flat regions of the energy landscape. For temperatures T ≥ 0.5 T _c in 4 d and 6 d we find logarithmic scalings that are compatible with simple dynamical ultrametricity. Nevertheless the behaviour of the systems, even in 6 d is very different from the mean field SK model results. Received 21 October 2002 / Received in final form 13 January 2003 Published online 11 April 2003 RID="a" ID="a"Associate researcher of the Abdus Salam International Centre for Theoretical Physics; e-mail: stariolo@if.ufrgs.br; http://www.if.ufrgs.br/stariolo RID="b" ID="b"Present address: The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11, 34014 Trieste, Italy e-mail: mmontemu@ictp.trieste.it RID="c" ID="c"e-mail: tamarit@famaf.unc.edu.ar     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

Soft X-ray magnetic circular dichroism in Fe and Fe 0.50 Co 0.48 V 0.02 films: quantitative analysis of transmission

Magnetic circular dichroism (MCD) in X-ray absorption has been measured at the L _{2, 3} edges of Fe in ex-situ grown Fe and Fe _0.50 Co _0.48 V _0.02 films by means of the transmission method. A new approach is developed for fitting the observed transmittance, which describes the resonance lineshapes as (generalized) Fano profiles. Analytical integration of each single resonance allows a more reliable determination of the orbital and spin magnetic moments based on the MCD sum rules. The results are consistent with an increase of the Fe spin and orbital magnetic moment in Fe-Co alloys as obtained by other experiments and band structure calculations. Received 15 August 2000 and Received in final form 11 June 2001     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000