S. Blanchard, T. Klein, J. Marcus, I. Joumard, A. Sulpice, P. Szabo, P. Samuely, A.G.M. Jansen and C. Marcenat 1Laboratoire d’Etudes des Propriétés Electroniques des Solides, Centre National de la Recherche Scientifique, BP166, 38042 Grenoble Cedex 9, France 2Centre de Recherche sur les Très Basses Températures, Centre National de la Recherche Scientifique, BP166, 38042 Grenoble Cedex 9, France 3Institute of Experimental Physics, Slovak Academy of Sciences, SK-04353 Košice, Slovakia 4Grenoble High Magnetic Field Laboratory, Max-Planck-Institut für Festkörperforschung and Centre National de la Recherche Scientifique, B.P. 166, F-38042 Grenoble Cedex 9, France 5Commissariat à l’Energie Atomique Grenoble, Département de Recherche Fondamentale sur la Matière Condensée, SPSMS, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France (today)

The introduction of columnar defects in (K,Ba )Bi O3 single crystals shifts both the irreversibility and thermodynamic transition lines, respectively, deduced from ac susceptibility (and/or transport) and specific heat measurements, upwards. This shift can be attributed to the defect-induced decrease of the difference (Delta F) between the free energies in the superconducting and the normal states, assuming that the position of the superconducting transition is given by the condition absolute value Delta F approximately k(B )T/xi(3 ). This criterion also perfectly reproduces the influence of the angle between the tracks and the external field. This result suggests that no vortex liquid phase exists in this system.

We report on specific heat measurements in high quality (K,Ba)BiO3 single crystals (Tc∼31.5 K). A well defined specific heat jump is clearly visible at TCp(H) in the entire investigated field range (up to 13 T). However, the corresponding TCp(H) exhibits an anomalous positive curvature and the amplitude of the jump rapidly decreases with field suggesting a non-linear increase of the Sommerfeld coefficient (γ(H)). This anomalous behaviour is confirmed by low temperature measurements which show that γ(H)∝Hα with α∼0.65.

Two heptacoordinated Mn(II) complexes are isolated and X-ray characterized using the well-known tpen ligand (tpen = N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine): [(tpen)Mn(OH(2))](ClO(4))(2) (1(ClO(4))(2)) and [(tpen)Mn(micro-OAc)Mn(tpen)](ClO(4))(3).2H(2)O (2(ClO(4))(3).2H(2)O). Crystallographic data for 1(ClO(4))(2) at 110(2) K (respectively at 293(2) K): monoclinic, space group C2/c, a = 15.049(3) A (15.096(3) A), b = 9.932(2) A (10.105(2) A), c = 19.246(4) A (19.443(4) A), beta = 94.21(3) degrees (94.50(3) degrees ), Z = 4. Crystallographic data for 2(ClO(4))(3).0.5(C(2)H(5))(2)O at 123(2) K: triclinic, space group P, a = 12.707(3) A, b = 12.824(3) A, c = 19.052(4) A, alpha = 102.71(3) degrees, beta = 97.83(3) degrees, gamma = 98.15(3) degrees, Z = 2. Investigation of the variation upon temperature of the molar magnetic susceptibility of compound 2(ClO(4))(3).2H(2)O reveals a weak antiferromagnetic exchange interaction between the two high-spin Mn(II) ions (J = -0.65 +/- 0.05 cm(-)(1), H = -JS(1).S(2)). EPR spectra are recorded on powder samples and on frozen acetonitrile solutions, demonstrating the maintenance upon dissolution of the heptacoordination of Mn in complex 1 while complex 2 partially dissociates. Electrochemical responses of complexes 1 and 2 are investigated in acetonitrile, and bulk electrolyses are performed at oxidative potential in the presence of various amounts of 2,6-lutidine (0-2.65 equiv per Mn ion). The formation from either 1 or 2 of the mixed-valent complex [(tpen)Mn(III)(micro-O)(2)Mn(IV)(tpen)](3+) (3) is established from mass spectrometry and EPR and IR spectroscopy measurements. When reaction is started from 2, formation of [(tpen)Mn(IV)(micro-O)(2)(micro-OAc)Mn(IV)](3+) (4) is evidenced from cyclic voltammetry, EPR, and UV-vis data. The Mn vs tpen ratio in the electrogenerated complexes is accurately controlled by the quantity of additional 2,6-lutidine. The role of tpen as a base is discussed.

We have measured the specific heat, resistivity, and ac susceptibility of (K,Ba)BiO3 single crystals before and after introduction of either point or columnar defects by electron (EI) or heavy-ion irradiation (HII). While the magnetic field dependence of these properties remains mainly unaffected by EI, the irreversibility line and the location of the specific heat anomaly are both shifted up in temperature after HII. The shift is apparent only if the magnetic field is applied parallel to the ion tracks. For perpendicularly applied fields, both lines lie at the same field as in the pristine sample. These experiments call the nature of the vortex liquid state into question.

The upper critical field Hc2 for the fields perpendicular and parallel to Mg and B planes of the magnesium diboride is presented in the temperature range from 5.4 K up to Tc as obtained by comparison of high-field magnetotransport, ac-susceptibility and specific heat measurements. The onset of the finite resistivity yields Hc2, while the end of the resistive transition to the normal state is due to surface effects. Hc2 perpendicular to the planes reveals a conventional temperature dependence with μ0Hc2⊥ab(0) = 3.5 T but the parallel critical field with μ0Hc2||ab(0) = 17 T has a positive curvature at temperatures above 20 K. Consequently, the anisotropy factor Γ = Hc2||ab/Hc2⊥ab is temperature dependent with Γ equal to 4.8 at low temperatures and about 2 near Tc. The angular dependence of the upper critical field of an elliptic form is obtained at several temperatures.

A new mu-phenoxo-bis-mu-acetato di-Mn(II) complex using the BpmpH ligand was isolated as a perchlorate salt (BpmpH = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenol). The X-ray structure has been solved showing that the two Mn(II) ions are in a distorted octahedral environment. Investigation of the variation of the molar magnetic susceptibility upon temperature reveals an antiferromagnetic exchange interaction between the two high-spin Mn(II) ions. Fitting of the experimental data led to g = 1.99 and J = 9.6 cm(-1) (H(HDvV) = JS(A).S(B)). EPR spectra recorded on a powder sample of [(Bpmp)Mn(2)(mu-OAc)(2)](ClO(4)).0.5H(2)O at X-band between 4.3 K and room temperature and at Q-band between 5 and 298 K are presented. A new method based on a scrupulous examination of the variation upon temperature of these experimental spectra is developed here to first assign the transitions to the relevant spin states and second to determine the associated spin parameters. This approach is compared to the deconvolution process that has been previously applied to dinuclear Mn(II) complexes or metalloenzyme active sites. Crystallographic data is as follows: triclinic, space group P one macro, a = 10.154(2) A, b = 12.0454(2) A, c = 17.743(4) A, alpha = 101.69(3) degrees, beta = 93.62(3) degrees, gamma = 94.67(3) degrees, Z = 2.

A new dinuclear manganese(II) complex was synthesised with the biscompartimental ligand 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-nitrophenol (NO(2)BpmpH) and characterised by X-ray crystallography. Magnetic susceptibility measurements revealed that the two high-spin Mn(II) ions are antiferromagnetically coupled with a singlet-to-triplet separation of 7.2 cm(-1). The powder EPR spectra were recorded for both X- and Q-bands between 1.8 K and 35 K. A detailed analysis of these spectra led to the determination of three out of five individual spin-state zero-field splitting parameters. From the proposed simulations, the exchange coupling constant J and the intermetallic distance have been computed.

We report on specific-heat, high-magnetic-field transport, and ac-susceptibility measurements on MgB2 single crystals. The upper critical field for magnetic fields perpendicular and parallel to the basal planes is presented in the entire temperature range. A very different temperature dependence has been observed in the two directions, which yields a temperature-dependent anisotropy with Gammasimilar to5 at low temperatures and similar to2 near T-c. A peak effect is observed for mu(0)Hsimilar to2 T parallel to the c axis, and the critical current density presents a sharp maximum for H parallel to the ab plane.

The influence of the magnetic history on the structure of the vortex solid has been investigated by small angle neutron scattering in the cubic (K,Ba)BiO3 superconductor. The diffracted intensity strongly depends on the cooling procedure for 0.3<μ0H<0.7T (at 2.5 K) showing that defects can be quenched into the vortex solid in this field range. For all cooling procedures, the diffracted intensity tends towards zero for μ0H=BN≪Bc2 (BN∼1.5T at 2.5 K) for all cooling procedures. The corresponding BN(T) line is discussed in comparison with the field induced order–disorder transition theory.

Specific heat, transport and reversible magnetization measurements have been performed on high quality (K,Ba)BiO3 single crystals (Tc∼31 K). Calorimetric measurements up to 16 T show that the transition line between the superconducting and normal states has a strong anomalous positive curvature. The reversible magnetization deviates strongly from the standard London model above 0.8Tc emphasizing the original nature of the superconducting transition in this system.

Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K, Ba)BiO3 single crystals. The temperature dependence of the magnetic field H-Cp corresponding to the onset of the specific heat anomaly presents a clear positive curvature. H-Cp is significantly smaller than the field H-Delta for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization presents a strong deviation from the Ginzburg-Landau theory emphasizing the peculiar nature of the superconducting transition in this material.

Although crystals are usually quite stable, they are sensitive to a disordered environment: even an infinitesimal amount of impurities can lead to the destruction of crystalline order. The resulting state of matter has been a long-standing puzzle. Until recently it was believed to be an amorphous state in which the crystal would break into 'crystallites'. But a different theory predicts the existence of a novel phase of matter: the so-called Bragg glass, which is a glass and yet nearly as ordered as a perfect crystal. The 'lattice' of vortices that contain magnetic flux in type II superconductors provide a good system to investigate these ideas. Here we show that neutron-diffraction data of the vortex lattice provides unambiguous evidence for a weak, power-law decay of the crystalline order characteristic of a Bragg glass. The theory also predicts accurately the electrical transport properties of superconductors; it naturally explains the observed phase transitions and the dramatic jumps in the critical current associated with the melting of the Bragg glass. Moreover, the model explains experiments as diverse as X-ray scattering in disordered liquid crystals and the conductivity of electronic crystals.