A. Wisniewski

Temperature and field dependence of the anisotropy of MgB2

The anisotropy gamma of the superconducting state of high quality single crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from gamma approximately 6 at 15 K to 2.8 at 35 K. Reversible torque data near T(c) reveal a field dependent anisotropy, increasing nearly linearly from gamma approximately equal to 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the k--> dependence of the effective mass tensor.

Mixed-state properties of superconducting MgB2 single crystals

We report on measurements of the magnetic moment in superconducting MgB 2 single crystals. We find μ 0 H c c 2 (0)=3.2 T, μ 0 H a b c 2 (0) = 14.5 T, γ=4.6, μ 0 H c (0)=0.28 T, and κ(T c ) = 4.7. The standard Ginzburg-Landau andLondon model relations lead to a consistent data set and indicate that MgB 2 is a clean limit.

Magnetic properties of nanocrystalline La1- xMnO3+δ manganites : Size effects

The magnetic properties of nanocrystalline manganites La1?xMnO3+? with particle size of 20 (LMO20), 25 (LMO25), and 30 nm (LMO30), prepared by the citrate method, have been investigated in the temperature range 5?320?K, magnetic field up to 90?kOe and under quasi-hydrostatic pressures up to 14.5?kbar. The studies involve sequential zero-field-cooled magnetization (M) measurements followed by magnetization measurements during cooling in the same magnetic field (H) and complementary measurements of ac susceptibility. Additional measurements of M versus H were carried out at ambient and applied pressures. All nanoparticles exhibit a paramagnetic to ferromagnetic transition (PFT) at a Curie temperature TC>200?K. It was found that the relative volume of the ferromagnetic phase increases for larger particle size and approaches a value of about 93% for LMO30. The real part of the ac susceptibility of sample LMO20 exhibits strong frequency dependence in a wide temperature range below TC, whereas for sample LMO30 only relatively weak frequency dependence was observed. The magnetization of sample LMO30 exhibits a PFT of second order; the type of transition could not be established for the smaller particles. It was found that an applied pressure enhances the TC of La1?xMnO3+? nanoparticles with a pressure coefficient of dTC/dP?1.9?K?kbar?1 for LMO20 and dTC/dP?1.4?K?kbar?1 for LMO25 and LMO30 samples. Peculiar magnetic memory effects observed for sample LMO20 are discussed.

MgB2 single crystals: high pressure growth and physical properties

Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.

MgB2 single crystals: high pressure growth and anisotropic properties

Single crystals of MgB2 with a size up to 1.5 × 0.9 × 0.2 mm3 have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB9. Single crystals of BN and MgB2 grow simultaneously by a peritectic decomposition of MgNB9. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ~0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H//cc2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H//abc2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H//abc2/H//cc2 was found to be temperature dependent (decreasing from γ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γeff, as calculated from reversible torque data near Tc, is field dependent (increasing roughly linearly from γeff 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB2 with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H//cc2.

Crystal structure, magnetic susceptibility and thermopower of superconducting and non-superconducting Nd1.85Ce0.15CuO4+y

Abstract An experimental study of superconducting and non-superconducting Nd1.85Ce0.15CuO4+y, including structure determination by neutron powder diffraction, recording of oxygen changes by gas volumetry, and susceptibility and thermoelectric measurements, is reported. Difference neutron diffraction patterns from samples prepared on-line at the spectrometer show that the structures of superconducting and non-superconducting samples are identical within the limits set by the statistical errors of our data. Simultaneous gas volumetric measurements reveal that Δy

Strong magnetic pair breaking in Mn-substituted MgB2 single crystals

Magnetic ions (Mn) were substituted in MgB2 single crystals resulting in a strong pair-breaking effect. The superconducting transition temperature, Tc, in Mg1−xMnxB2 has been found to be rapidly suppressed at an initial rate of 10 K∕%Mn, leading to a complete suppression of superconductivity at about 2% Mn substitution. This reflects the strong coupling between the conduction electrons and the 3d local moments, predominantly of magnetic character, since the nonmagnetic ion substitutions, e.g., with Al or C, suppress Tc much less effectively (e.g., 0.5 K∕%Al). The magnitude of the magnetic moment (≃1.7 μB per Mn), derived from normal state susceptibility measurements, uniquely identifies the Mn ions to be divalent, and to be in the low-spin state (S=1∕2). This has been found also in x-ray absorption spectroscopy measurements. Isovalent Mn2+ substitution for Mg2+ mainly affects superconductivity through spin-flip scattering reducing Tc rapidly and lowering the upper critical field anisotropy Hc2ab∕Hc2c at T=0 from 6 to 3.3 (x=0.88% Mn), while leaving the initial slope dHc2∕dT near Tc unchanged for both field orientations.

Single crystal growth of MgB2 and thermodynamics of Mg-B-N system at high pressure

Abstract Single crystals of MgB 2 have been grown at high pressure via the peritectic decomposition of MgNB 9 . The crystals are of a size up to 1.5×0.9×0.2 mm 3 with a weight up to 230 μg, and typically have transition temperatures between 37 and 39 K with a width of 0.3–0.5 K. Investigations of the P – T phase diagram prove that the MgB 2 phase is stable at least up to 2190 °C at high hydrostatic pressure in the presence of Mg vapor under high pressure. Small variations of T c are caused by doping with metal elements from the precursor or annealing of defects during the crystal growth process.

High-pressure synthesis, crystal growth, phase diagrams, structural and magnetic properties of Y2Ba4CunO2n+x, HgBa2Can-1CunO2n+2+delta and quasi-one-dimensional cuprates

In this paper we present a review of high-gas-pressure single crystal growth studies of YBa2Cu4O8 and Y2Ba4Cu7O15-x performed in oxygen pressure up to 3000 bar and Hg1-xMxBa2Can-1CunO2n+2+ (M = Pb, Re; n = 1-7) compounds in argon pressure up 11 000 bar at temperature up to 1200 °C. Chain compounds A1-xCuO2 (A = Sr, Ca, Ba) have been synthesized at high oxygen pressure up to 2000 bar. High-pressure phase diagram studies of the investigated systems are also discussed. Structure analyses of Y2Ba4CunO2n+x (n = 6-8), Hg1-xMxBa2Can-1CunO2n+2+ (M = Pb, Re; n = 1-8), Sr0.73CuO2 and (Sr,Ca)4Cu6O10 single crystals have been performed. The effects of substitutions and trends in bondlengths are discussed. The vortex state properties of HgBa2Ca2Cu3O8+ and YBa2Cu4O8 single crystals are compared. For Hg-based compounds, the influence of oxygen content, chemical substitutions and radiation defects on vortex pinning were determined. The quasi-one-dimensional cuprates Ca0.83CuO2 and Sr0.73CuO2 show an antiferromagnetically ordered state of long-range 3D character at T < 10 K. The spin dynamics of Sr0.73CuO2, measured by inelastic neutron scattering, indicate that this ordered state coexists with a dimerized singlet ground state.

Magnetotransport in granular LaMnO3+δ manganite with nano-sized particles

Transport and magnetic properties of compacted LaMnO3+? manganite nanoparticles of an average size of 18?nm have been investigated in the temperature range 5?300?K. The nanoparticles exhibit a paramagnetic-to-ferromagnetic (FM) transition at the Curie temperature TC ~ 246?K. However, the spontaneous magnetization disappears at a higher temperature of about 270?K. It was found that at low temperatures the FM core occupies about 50% of the particle volume. The temperature dependence of the resistivity shows a metal?insulator transition and a low-temperature upturn below the resistivity minimum at T ~ 50?K. The transport at low temperatures is controlled by the charging energy and spin-dependent tunnelling through grain boundaries. It has been found that the charging energy decreases monotonically with increasing magnetic field. The low temperature I?V characteristics are well described by an indirect tunnelling model while at higher temperatures both direct and resonant tunnelling dominates. The experimental features are discussed in the framework of a granular ferromagnet model.