P. Ségransan

NMR study of 17O in high Tc superconducting oxides

Abstract We present the temperature dependence of the Magnetic Hyperfine Shift tensors of 17 O nuclei for the sites within the CuO 2 plane (site O(I)) and in the BaO, LaO, SrO plane (O(II) site) in the compounds YBa 2 Cu 3 O 7 , YBa 2 Cu 3 O 6.65 , La 1.85 Sr 0.15 CuO 4 and Bi 2 Sr 2 CaCu 2 O 8 . For O(I) sites, the isotropic part of the tensor reflects the polarisation of the Oxygen 2s orbitals by the neigboring Cu d-holes and the temperature dependence of the susceptibility. We show that in YBa 2 Cu 3 O 6.65 , the spins of the oxygen p-holes have their own degree of freedom above 120 K.

63Cu and 199Hg NMR in overdoped HgBa2CaCu2O6+δ

Abstract We report on 63 Cu and 199 Hg NMR spectra and nuclear spin-lattice relaxation rate (1/ T 1 ) measurements, and the Cu NQR spectrum, in HgBa 2 CaCu 2 O 6+δ powder sample with T c = 101 K. The results agree with previous NMR studies in the overdoped regime of the cuprates, showing antiferromagnetic correlations and no spin-gap behaviour in the normal state. The Cu quadrupole coupling frequency 63 ν Q and orbital magnetic hyperfine shift 63 K orb c are relatively low, which might be connected to a strong in-plane CuO hybridization. The position of the 199 Hg NMR spectrum is essentially determined by an orbital shift. However, the observed identical T -dependence for both 199 T 1 and 63 T 1 reveals the presence of an hyperfine field transferred from Cu spins, whose fluctuations are essentially responsible for the nuclear relaxation of 199 Hg. We discuss the implications of these findings, compared to recent NMR results in Hg-1201 [23].

NMR investigation of low energy excitations in high Tc superconductors

Abstract The difference between the magnetic properties in the overdoped and the underdoped regime, that is mainly the opening of a spin gap well above Tc in underdoped samples, is discussed on the basis of NMR data obtained in YBCO single crystals. We also make a quantitative comparison between NMR and inelastic neutron scattering obtained on the same single crystals.

From underdoped to overdoped regime in YBa2Cu3O6+x, an NMR investigation of single crystals

We present17O and63Cu NMR results obtained in three single crystals of YBa2 Cu3O6+x: YBa1.92Sr0.08Cu3O7 (Tc=89 K) which is overdoped, and YBa1.93Sr0.07Cu3O6.92 (Tc=91 K) and YBa2Cu3O6.52 (Tc=59 K), both of which are underdoped. We show that Cu(2) nuclear spin-lattice relaxation rate (NSLRR), the static spin susceptibility, the NSLRR for O(2,3) site as well as its anisotropy, have completely different temperature dependence according to the regime (overdoped or underdoped). The O(2,3) NSLRR and magnetic hyperfine shift (MHS) data seem to confirm that in the overdoped regime, a Fermi liquid description is correct, while in the underdoped regime, a pseudo-gap in the magnetic excitations always opens well aboveTc. We also show that both neutron inelastic scattering results and Cu(2) and O(2,3) NSLRR results can be explained using the same χ″(q, ω).

Normal state spin susceptibility in YBa2Cu3O6.92 single crystal from 63Cu and 89Y nuclear magnetic resonance

Abstract We present an extensive nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) study of the normal state (T>Tc) of an YBa1.93Sr0.07Cu3O6.92 single crystal (Tc=90 K). The NMR data is interpreted using a model for the imaginary part of the dynamical electron spin susceptibility χ″(q,ω) that is consistent with the inelastic neutron scattering (INS) experiments (carried out on the same sample investigated here): the q-dependence is assumed to be Gaussian (commensurate), with short and T independent coherence length ξ. This model enabled us to perform a quantitative analysis of the planar copper spin–lattice and spin–spin relaxation rates which evidenced that a spin pseudogap starts to open at T*≈130 K, confirming the INS results. Moreover, we show that the AF fluctuations contribution to yttrium relaxation is not negligible due to the dipolar coupling of the yttrium with the neighboring copper spins. The main signature of this feature is that a relation KααT1T=const does not hold for the yttrium site at higher temperatures. Our main conclusion is that, in order to explain the yttrium (and the planar oxygen) relaxation coherently with the INS results and the widely accepted one-band picture, the characteristic energy Γ0 of the spin dynamics near q=0 must be strongly T dependent, otherwise the one-band description is not feasible. All the NMR results are interpreted within the ionic model of the hyperfine Hamiltonian taking into account the anisotropy of the g-factor and the static spin susceptibility χs.

NMR study of 63,65Cu in YBa2Cu3O7to 6.65 single crystals

Abstract We report 63 Cu NMR nuclear spin-lattice relaxation rates (NSLRR) and magnetic hyperfine shift (K cc ) of copper Cu(2) in the CuO 2 planes of YBa 2 Cu 3 O 7−δ as a function of temperature and δ. The independence of K cc and NSLRR (above 120 K) on δ associated with the existence of localized Cu(3d) holes. We comment on the difference in the NSLRR between the δ = 0 and δ ≠ 0 samples which appear below 120 K. It is interpreted as an indirect consequence of the changes in the oxygen 2p electronic band caused by the introduction of oxygen vacancies in the system.

NMR and NQR study of La1.85Sr0.15CuO4 and YBa2Cu3O6+x (x=0, 0.25)

The spin lattice relaxation rate of La in La 1.85 Sr 0.15 CuO 4 has been measured in the temperature range 12–300K at 5.75 Teslas. A peak in the relaxation rate has been found 15K above the superconducting transition. NQR of 63,65 Cu in antiferromagnetic YBa 2 Cu 3 O 6+x for x=0.25 and x=0 is also presented. Pairs of lines corresponding to Cu[1] have been observed at 30.1–27.8 MHz for x = 0.25 and 29.9-27.6 MHz for x=0.

Spin susceptibility in underdoped YBaCuO single crystals from NMR

Abstract We report on 89 Y, 63 Cu and 17 O NMR measurements in the normal state of YBaCuO single crystals (T c =91 K and T c =48 K). A comprehensive description of all NMR results is obtained starting from a spin susceptibility with a contribution from antiferromagnetic spin fluctuations delineated by the inelastic neutron scattering data, and a contribution from itinerant quasi-particles with a characteristic frequency increasing at low temperatures. For the first time a complete analysis is given for yttrium spin-lattice relaxation data.

NMR Investigation of Low-Energy Excitations in YBa2Cu3O6+x Single Crystals

17O and 63Cu NMR results obtained in three single crystals of YBa2Cu3O6+x, show that the static spin susceptibility and (T1T)−1 for Cu(2) and O(2,3) have completely different temperature dependence according to the regime — overdoped for x > 0.94, underdoped for x < 0.94. In the overdoped regime a Fermi liquid description seems to be correct, while in the underdoped regime a pseudo-gap in the magnetic excitations opens well above Tc, supporting the phase diagram based on the spin-charge separation. We also show that both neutron inelastic scattering results and Cu(2) and 0(2,3) NSLRR results can be explained using the same χ″(q, ω).

17O and 63Cu NMR study of anisotropic magnetic fluctuations in a single crystal of YBa2Cu3O6+x: Comparison with neutron diffraction

Abstract NMR measurements of 63 Cu and 17 O are reported for two single crystals of YBa 2 Cu 3 O 6+ x ( x =0.52, 0.92) and compared with neutron diffraction measurements on the same or equivalent samples. Comparison with the model of Millis, Monien and Pines shows that it is not consistent with the measurements. The contribution from the antiferromagnetic fluctuations is not enough to explain the 63 Cu spin relaxation.