G. Collin

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.

Lattice dynamical studies of HTSC materials

Abstract A survey is presented on recent progress in the understanding of the lattice dynamics in Nd2CuO4, (La,Sr)2CuO4 and YBa 2 Cu 3 O 6 7 . Classical anharmonicity and twinning were found to be major complications for the interpretation of the data. The lattice vibrations of the cuprates can now largely be described within the framework of shell models for strongly ionic compounds. Phonon anomalies inferred from a comparison of doped and undoped compounds resemble those found in classical superconductors.

Etude structurale et proprietes physiques de CuCrS2

Abstract Two series of single crystals of CuCrS were prepared: series I single crystals were quenched from 850°C to room temperature, series II were slowly cooled (50°C/day). In the series II-CuCrS2, Cu atoms are ordered and occupy half of the tetrahedral sites. Series I-CuCrS2 is non-stoichiometric and 10% of the Cu atoms are in the interstatial position. Transport properties indicate a semi-conducting comportement of both phases. Magnetic susceptibility measurements confirm an antiferromagnetic ordering (TN = 40K). The resolved structures and observed properties are compared to the related Ti and V derivates (CuTiS2, Cu0.75VS2 and Cu0.65VS2) which are metallic.

Normal State Magnetic Properties of Ni and Zn Substituted in YBa2Cu3O6+x: Hole-Doping Dependence

We present SQUID susceptibility data on Zn and Ni substituted YBa2Cu3O6 + x. Cross-checks with NMR yield an unprecedented accuracy in the estimate of the magnetic susceptibility associated with the substituents, from the underdoped to the lightly overdoped case. This allows us to determine the Weiss temperature θ for YBCO: its value is very small for all hole dopings nh. Since in conventional metals the Kondo temperature TK > θ; in contrast, increasing nh produces a reduction of the small moment induced by Zn2+ and a nearly constant effective moment for Ni2+ corresponding to a spin 1/2 rather than to a spin 1.

Preparation et structure de Ga2S3α type wurtzite lacunaire

Abstract Preparations of powdered and crystalline αGa 2 S 3 are described. Structure of αGa 2 S 3 is established from single crystal determination (R = 0.058). These is a superstructure of wurtzite type, with ordered vacancies on gallium positions. A polymorphism of Ga 2 S 3 is described.

Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates

Using 7Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below T(c) in YBa 2Cu 3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on first near neighbor Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.

Absence of static phase separation in the high T(c) cuprate YBa(2)Cu(3)O(6+y).

We use 89Y NMR in YBa(2)Cu(3)O(6+y) in order to evaluate with high sensitivity the distribution of hole content p in the CuO2 planes. For y=1 and y=0.6, this hole doping distribution is found narrow with a full width at half maximum smaller than Deltap=0.025. This rules out any large static phase separation between underdoped and optimally doped regions in contrast with the one observed by STM in Bi2212 and by NQR in LaSrCuO. This establishes that static electronic phase separation is not a generic feature of the cuprates.

Structural aspects of the α transition in stoichiometric FeS: Identification of the high-temperature phase

Abstract Compounds of the Fe 1− x S system ((0 ≤ x ≤ 0.125) are of NiAs type ( a and c being the substructure lattice constants). For0 ≤ x ≤ 0.05, the temperature-induced metal-nonmetal α transition is associated with a structural change. Powder studies identify the influence of x on the transition and especially on the superstructure reflection intensities of the hexagonal√3 a ,2 c low-temperature ( T α ) phase. Crystal growth for compositions in the transition range is detailed. The structural study of a stoichiometric FeS crystal at various temperatures gives evidence for disorder phenomena in the pretransition range and for the exact nature of the hexagonal HT phase (T > T α ) 2a,c (SG P6 3 mc ). Its structure refinement ( R = 6.0% ) rules out the previous tritwinned model of MnP type. The cationic lattice of this 2a,c modification is intermediate between the lattices of the LT √3a,2c phase and those of a pure NiAs phase.

Effect of Tethered Chains on Adhesion

The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in YBa2(Cu0.97Ni0.03)3O7 (Tc=80 K) and YBa2(Cu0.99Zn0.01)3O7 (Tc=78 K). In the Ni-substituted system, the magnetic resonance peak (which is observed at Er approximately 40 meV in the pure system) shifts to lower energy with a preserved Er/Tc ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.

Evidence of a single nonmagnetic Co 3 + state in the Na 1 CoO 2 cobaltate

Macroscopic magnetization, muon spin rotation (muSR), and NMR measurements were carried out to study magnetism in the Na1CoO2 cobaltate. Using SQUID measurements, Na1CoO2 is shown to have a bulk magnetic susceptibility much lower and flatter than that of NaxCoO2 with x=0.7-0.9. In fact, muSR yields a signal of mostly nonmagnetic origin, which is attributed to the x=1 phase. The intrinsic cobalt spin susceptibility corresponding to this x=1 phase is measured using Na NMR. It is indeed found to be almost zero, in agreement with a low-spin 3+ charge state of all cobalt atoms. This single state of Co ions in CoO2 planes is confirmed by Co NMR, whose determination of cobalt shift and quadrupolar parameters allows us to give a reference value of the Co3+ orbital shift in cobaltates.