186 research outputs found

    11^{11}B NMR detection of the magnetic field distribution in the mixed superconducting state of MgB2_2

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    The temperature dependence of the magnetic field distribution in the mixed superconducting phase of randomly oriented MgB2_2 powder was probed by 11% ^{11}B NMR spectroscopy. Below the temperature of the second critical (Bc2B_{% {c2}}) field, Tc2≈27T_{{c2}}\approx 27K, our spectra reveal two NMR signal components, one mapping the magnetic field distribution in the mixed superconducting state and the other one arising from the normal state. The complementary use of bulk magnetization and NMR measurements reveals that MgB2_2 is an anisotropic superconductor with a Bc2c<2.35B_{c2}^c<2.35 Tesla anisotropy parameter γ≈6\gamma\approx 6

    11^{11}B and 27^{27}Al NMR spin-lattice relaxation and Knight shift study of Mg1−x_{1-x}Alx_xB2_2. Evidence for anisotropic Fermi surface

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    We report a detailed study of 11^{11}B and 27^{27}Al NMR spin-lattice relaxation rates (1/T11/T_1), as well as of 27^{27}Al Knight shift (K) of Mg1−x_{1-x}Alx_xB2_2, 0≤x≤10\leq x\leq 1. The obtained (1/T1T1/T_1T) and K vs. x plots are in excellent agreement with ab initio calculations. This asserts experimentally the prediction that the Fermi surface is highly anisotropic, consisting mainly of hole-type 2-D cylindrical sheets from bonding 2px,y2p_{x,y} boron orbitals. It is also shown that the density of states at the Fermi level decreases sharply on Al doping and the 2-D sheets collapse at x≈0.55x\approx 0.55, where the superconductive phase disappears

    1H NMR investigation of the magnetic spin configuration in the molecule-based ferrimagnet [MnTFPP][TCNE]

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    Journal Article1H NMR line-shape measurements have been performed in the linear chain molecule-based ferrimagnet [MnTFPP][TCNE].xPhMe (TFPP=tetrakis(4-fluorophenyl)porphinato; TCNE=tetracyanoethylene) as a function of temperature. Hyperfine shifts of opposite sign were observed indicating the presence of two oppositely hyperfine couplings, predominantly between the Mn ions and the 1H nuclei of the planar ring of the [MnTFPP]+ unit. The observation of the hyperfine shifts strongly suggests the existence of two oppositely directed [MnTFPP]+ sites. Since there is no direct exchange coupling between adjacent [MnTFPP] spins, then it can be argued that an ordered spin configuration with antiparallel components may arise from antisymmetric spin-spin exchange interactions. The field independence of the hyperfine shifts at low temperatures and for applied magnetic fields in the range 2-5 T, indicates the presence of ferromagnetic interactions in the ground state of the ferrimagnetic chain

    139La NMR evidence for phase solitons in the ground state of overdoped manganites

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    Hole doped transition metal oxides are famous due to their extraordinary charge transport properties, such as high temperature superconductivity (cuprates) and colossal magnetoresistance (manganites). Astonishing, the mother system of these compounds is a Mott insulator, whereas important role in the establishment of the metallic or superconducting state is played by the way that holes are self-organized with doping. Experiments have shown that by adding holes the insulating phase breaks into antiferromagnetic (AFM) regions, which are separated by hole rich clumps (stripes) with a rapid change of the phase of the background spins and orbitals. However, recent experiments in overdoped manganites of the La(1-x)Ca(x)MnO(3) (LCMO) family have shown that instead of charge stripes, charge in these systems is organized in a uniform charge density wave (CDW). Besides, recent theoretical works predicted that the ground state is inhomogeneously modulated by orbital and charge solitons, i.e. narrow regions carrying charge (+/-)e/2, where the orbital arrangement varies very rapidly. So far, this has been only a theoretical prediction. Here, by using 139La Nuclear Magnetic Resonance (NMR) we provide direct evidence that the ground state of overdoped LCMO is indeed solitonic. By lowering temperature the narrow NMR spectra observed in the AFM phase are shown to wipe out, while for T<30K a very broad spectrum reappears, characteristic of an incommensurate (IC) charge and spin modulation. Remarkably, by further decreasing temperature, a relatively narrow feature emerges from the broad IC NMR signal, manifesting the formation of a solitonic modulation as T->0.Comment: 5 pages, 4 figure

    Orbital domain state and finite size scaling in ferromagnetic insulating manganites

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    55Mn and 139La NMR measurements on a high quality single crystal of ferromagnetic (FM) La0.80Ca20MnO3 demonstrate the formation of localized Mn(3+,4+) states below 70 K, accompanied with strong anomalous increase of certain FM neutron Bragg peaks. (55,139)(1/T1) spin-lattice relaxation rates diverge on approaching this temperature from below, signalling a genuine phase transition at T(tr) approx. 70 K. The increased local magnetic anisotropy of the low temperature phase, the cooling-rate dependence of the Bragg peaks, and the observed finite size scaling of T(tr) with Ca (hole) doping, are suggestive of freezing into an orbital domain state, precursor to a phase transition into an inhomogeneous orbitally ordered state embodying hole-rich walls.Comment: 4 pages, 4 figure

    Spin-polarized oxygen hole states in cation deficient La(1-x)CaxMnO(3+delta)

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    When holes are doped into a Mott-Hubbard type insulator, like lightly doped manganites of the La(1-x)CaxMnO3 family, the cooperative Jahn-Teller distortions and the appearance of orbital ordering require an arrangement of Mn(3+)/Mn(4+) for the establishment of the insulating canted antiferromagnetic (for x<=0.1), or of the insulating ferromagnetic (for 0.1<x<= 0.2) ground state. In the present work we provide NMR evidence about a novel and at the same time puzzling effect in La(1-x)CaxMnO(3+delta) systems with cation deficience. We show that in the low Ca-doping regime, these systems exhibit a very strong hyperfine field at certain La nuclear sites, which is not present in the stoichiometric compounds. Comparison of our NMR results with recent x-ray absorption data at the Mn K edge, suggests the formation of a spin-polarized hole arrangement on the 2p oxygen orbitals as the origin of this effect.Comment: 10 pages, 4 Figures, submitted to PR
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