Using Ni Substitution and17ONMR to Probe the Susceptibilityχ′(q)

Abstract

We present CuO 2 plane 17 O NMR measurements for Ni-substituted YBa 2 Cu 3 O 61y . The Ni moment induces an oscillatory spin density responsible for the broadening of the NMR line. In slightly overdoped y 1 compounds, this broadening scales with the Ni susceptibility. In contrast, such a scaling is not found in underdoped y 0.6 samples. We associate it with the T dependence of the antiferromagnetic staggered spin susceptibility x 0 ͑Q AF ͒ intrinsic to the CuO 2 planes. Discussion with respect to 63 Cu NMR transverse relaxation (T 2G ) and neutron scattering data shows that the AF correlation length j is T independent. [S0031-9007 (97)03937-9] PACS numbers: 74.25.Nf, 74.62.DhThe normal state of high-T c superconductors exhibits a peculiar magnetic behavior, distinct from that of a normal metal [1]. In the underdoped regime, well above T c , the static susceptibility x͑q 0͒ shows an anomalous "pseudogap" decrease with decreasing temperature T [2]. Simultaneously, antiferromagnetic (AF) correlations occur within the CuO 2 planes, as evidenced by an enhancement of the imaginary part of the susceptibility x 00 near the AF wave vector seen both in neutron scattering [3] and 63 Cu NMR longitudinal relaxation time [4]. These AF correlations might play a crucial role in the mechanism of superconductivity in these materials [5]. Thus, the AF correlation length j and both the imaginary and the real part of the staggered susceptibility at Q AF are major parameters for any coherent description of the cuprates. No clear experimental agreement on the T dependence of j and x 0 ͑q͒ is yet achieved. An integral information on x 0 ͑q͒ is available through 63 Cu transverse relaxation data ( 63 T 2G ) [6-9]. These data are usually analyzed as an increase of j at low T [10], whereas neutron experiments for x 00 suggest that j is T independent in YBa 2 Cu 3 O 61y [3]. No other technique has been used up to now to determine x 0 ͑q͒ at q fi 0.We propose here a new approach to probe x 0 ͑q͒, using magnetic impurity substitution effects in the CuO 2 planes. The impurity magnetic moment gm B ͗S Z ͘ acts as a local field H͑r͒~͗S Z ͘d͑r͒, which induces an in-plane spin polarization s͑r͒~x 0 ͑r͒͗S Z ͘. This polarization may be probed by the NMR of nuclei coupled to the planes: at r from the impurity, the NMR frequency shift dnH hf x 0 ͑r͒͗S Z ͘ is due to the hyperfine coupling H hf with the spin density. Hence, the broadening due to the shift distribution among the nuclei yields information on x 0 ͑r͒. We will show here that the use of the 17 O NMR probe allows us, for the first time, a thorough investigation of x 0 ͑r͒. This approach contrasts with the usual impurity studies of the specific properties of the magnetic defects [11] and their influence on superconductivity [12]. We find an anomalously large T variation of the 17 O linewidth for the underdoped composition. The specific geometry of the 17 O nuclei within the planes allows us through extended numerical simulations to demonstrate that the 17 O linewidth probes the amplitude of x 0 ...

Universal Alternating Order around Impurities in Antiferromagnets

“…Physical predictions for finite doping are made, which are relevant for experiments probing Knight shifts and the order parameter. The intentional doping of antiferromagnetic materials has become a useful tool in order to study the complicated physics in the context of high temperature superconductivity and quantum magnetism [1,2,3,4,5,6]. Large alternating magnetic moments around static nonmagnetic impurities are observed in Knight shift experiments when a uniform field is applied [3,4,5,6].…”

“…Jm, 74.25.Nf, 75.20.Hr, 75.40.Mg The intentional doping of antiferromagnetic materials has become a useful tool in order to study the complicated physics in the context of high temperature superconductivity and quantum magnetism [1,2,3,4,5,6]. Large alternating magnetic moments around static nonmagnetic impurities are observed in Knight shift experiments when a uniform field is applied [3,4,5,6]. Theoretical studies have shown that vacancies in low-dimensional antiferromagnetic backgrounds give rise to locally enhanced antiferromagnetic correlations [7,8,9,10,11,12,13,14], which strongly depend on the microscopic model and temperature in the low dimensional models.…”

Dynamic correlated Cu(2) magnetic moments in superconductingYBa2(Cu0.96Ni0.04Hodges1, Bonville2, Forget3 et al. 2009Phys. Rev. B2030View full textAdd to dashboardCite

“…The structure remains orthorhombic and there is essentially no change in the oxygen level nor in the doping level 9,22,23,24,25,26 . Penetration depth measurements, made down to 1.5 K, show that the superconducting condensate density increases progressively as the temperature is lowered 27 .…”

“…The sea of spinons which couple to the localised impurity spin is polarised by an external field and leads to a staggered Cu(2) spin polarisation. The polarisation decreases as r −3 with distance from the Ni impurity and the correlation length could be adjusted to reproduce the observed 17 O NMR line broadening 9 . This treatment pertains to underdoped cuprates where there is a pseudo-gap in the spin excitation spectrum.…”

“…Superconducting samples of YBa 2 (Cu 1−x Ni x ) 3 O y have been examined both above and below T sc using NMR in 6 T on 17 O substituted in the Cu(2)-O planes 9,10 . The observed hyperfine coupling is dominated by coupling to the spin polarisation of the two nearest in-plane Cu(2) 32,33 .…”