J. C. Jol

Single crystal 205Tl NMR in the normal and superconducting state of the high-Tc superconductor Tl2Ba2CaCu2O8: A study of the Knight and chemical shift

Abstract We have performed 205 Tl NMR in c -axis aligned single crystals of Tl 2 Ba 2 CaCu 2 O 8 ( T c ≈101 K ) in a field of 4.7 T. There are two Tl-lines with, at room temperature, distinct intensities, chemical and Knight shifts. One line arises from the normal (N) Tl-sites in the Tl-double layers and mimics the Cu-nuclear spin. The other one arises from impurity (I) Tls replacing part of the Ca-sites within the Cu double layer, i.e. the same locations as for the Y-ions in YBa 2 Cu 3 O 7 . Because it appeared possible to follow the dependence of the shifts of the two lines on temperature ( T ) down to T / T c ≤0.1, (for H ∥ c and H ∥ a, b ) the chemical and the Knight shifts could be determined accurately. The T -dependences of the Knight shift of the N(I)-line in the present sample with about 10% Ca-replacement are analogous to those of Cu(Y) in YBa 2 Cu 3 O 6.6 : the shift starts to decrease already above T c without an abrupt change at T c . Only below T / T c ≈0.6 do the resonance lines start to broaden.

The conductivity and band structure of some synthetic semiconductors based on M(dmit)2 (M=Ni,Pd,Pt)

The conductivity of the following six new low-dimensional compounds is reported: (Me4N)(Ni(dmit)2), (Et4N)(Ni(dmit)2), (Pr4N)(Ni(dmit)2), (Bu4N)(Ni(dmit)2), (Bu4N)(Pt(dmit)2) and (Et4N)0.5(Pd(dmit)2). Semiconducting behaviour is found for all the compounds investigated, with activation energies between 0.1 and 0.5 eV. The crystal structures have been established by X-ray diffraction. Using these, the band structure has been determined. It is shown that the compounds investigated are semiconductors each having a correlation gap resulting from an effective electron-electron repulsion of 1.0 eV.

Vortex structure in the high-Tc superconductor Tl2Ba2CaCu2O8; A 205Tl NMR study on single crystals

Abstract We have performed 205 Tl NMR in c-axis aligned single crystals of Tl 2 Ba 2 CaCu 2 O 8 (T c (0) ≈ 101 K) in a field of 4.7 T for H ∥ c and H ⊥ c. Below 50 K the linewidth increases by more than a factor 3. There is a direct relation between the onset temperature for the broadening and the Kosterlitz-Thouless transition temperature. Using the linewidth anisotropy and the maximum angle of misalignment we calculate a lower bound for the effective mass anisotropy of about 10 2 .

Similarity between the Cu(2)-O-planes in tetragonal and orthorhombic YBa2Cu3O7−δ as probed by 89Y NMR

89 Y-NMR linewidth and relaxation rate data are reported for YBa 2 Cu 3 O 7−δ in the orthorhombic metallic and the tetragonal nonmetallic modifications. The temperature dependence of the observed relaxation rates is Korringa like with almost equal Korringa constants for both compounds. These results are correlated to the Cu(2) NMR data of Warren et al. and might be seen as a support of any theory which decouples charge transport from spin fluctuations.

89Y NMR as probe for the spin dynamics in the Cu(2)O-planes

Abstract The 89Y nuclear relaxation rates in the orthorhombic 90 K superconducting and the tetragonal semiconducting modification of YBa2Cu3O7−δ are almost similar. These results are correlated to Y line shift data and Cu- NMR results. A relaxation process via magnetic excitations in active regions of the Cu(2)O planes allows a consistent picture.

Spin dynamics in YBa2Cu3O7-δ with 0.0 ≤ δ ≤ 0.6 studied by 89Y-NMR

Abstract In YBa2Cu3O6.6, as in YBa2Cu3O7, the relaxation rate (T−11n) above 90 K is found to be Korringa-like. In YBa2Cu3O6.6, a change in the line shift as a function of temperature gives a corresponding change in T1nT. In YBa2Cu3O7 the line shift is temperature independent. Between 90 K and Tc = 60 K T1n is shortened by an order of magnitude. An initial drop in T−11n below Tc is followed by a subsequent rise. Magnetic excitations between localized spins in the superconducting state are thought to be essential for an explanation.