Anisotropy of the antiferromagnetic spin correlations in the superconducting state of YBa₂Cu₃O₇ and YBa₂Cu₄O₈

Abstract

Abstract. We present evidence that the antiferromagnetic spin correlations in optimally doped YBa 2 Cu 3 O 7 and underdoped YBa 2 Cu 4 O 8 develop a surprisingly strong anisotropy in the superconducting state. Comparing the ratio of the nuclear spin-lattice relaxation rates of the planar copper and oxygen, measured at the lowest and highest temperatures as well as at Tc, we conclude that the antiferromagnetic in-plane correlations vanish as the temperature goes to zero. This observation is corroborated by the measurement of the copper linewidth in YBa 2 Cu 4 O 8 . In contrast, the out-of-plane correlations do not change appreciably between T = Tc and T = 0. Within a model of fluctuating fields this extreme anisotropy of the antiferromagnetic correlations also explains the observed temperature dependence of the anisotropy of the copper relaxation measured in a low external magnetic field.Nuclear magnetic/quadrupole resonance (NMR/NQR) experiments have shed light on the antiferromagnetic (AFM) spin correlations in the normal state of hightemperature superconductors. It was found that the correlations become progressively stronger as the temperature is reduced towards the superconducting transition temperature T c . The question is to what extent these correlations persist below T c . As the temperature is lowered in the superconducting state, the spin-lattice relaxation1α of the planar copper (k = 63) and planar oxygen (k = 17) nuclei decrease rapidly, with the applied field either parallel to the CuO 2 plane (α = ab) or perpendicular to it (α = c). This decrease goes approximately like T 3 , which is the temperature dependence expected for d-wave orbital pairing [1]. Great emphasis has been put on the different temperature behaviour of the copper and oxygen relaxation rates in the normal state. However, less attention has been payed on the ratios of these rates [2][3][4][5][6][7][8][9] in the superconducting state. A difficulty arises since in the normal state, the spin-lattice relaxation is largely insensitive to the strength of the applied magnetic field [10,11] [5][6][7]12]. In order to draw any conclusions about magnetism in the superconducting state it is therefore very important to look for intrinsic effects which can only be obtained from experiments done in weak magnetic fields so as to minimise the flux line influence. We will consider four NMR/NQR experimental results: the ratio 63 T

Spin anisotropy of the magnetic excitations in the normal and superconducting states of optimally doped YBa2Cu3O6.9Headings1, Hayden2, Kulda3 et al. 2011Phys. Rev. B322261View full textAdd to dashboardCite

“…However, the present measurements show that there is also an significant intrinsic anisotropy in χ ′′ α (q, ω) with respect to the spin direction which must be considered. It is interesting to note that Uldry et al [61] have extracted the intrinsic anisotropy from NMR data and concluded that the out-of-plane correlations do not change appreciably on entering the superconducting state, in contrast to our results. This may be because NMR measurements probe the excitations at much lower frequencies than our measurements.…”

Anisotropic spin fluctuations and multiple superconducting gaps in hole-doped Ba_0.7K_0.3Fe_2As_2: NMR in a single crystal

“…Namely, the susceptibility at the AF wave vector is larger in the a-axis direction than in the c-axis direction. This property resembles the case of the cobaltate superconductor Na x CoO 2 •1.3H 2 O [10] but is different from the high-T c cuprates [11]. An NMR measurement using a powder sample was recently reported [9], but the measured temperature was not low enough to discuss the superconducting gap feature.…”

“…Namely, χ ′′ ± (Q) is much larger than χ ′′ zz (Q), where z is along the c-axis direction. This is in contrast to the high-T c cuprates where the dif-ference between the spin fluctuations along the c-axis and the ab-plane is small [11,21], but similar situation was encountered in cobaltate superconductor [10]. The relationship between the energy-and q-dependences of the spin fluctuations (SF) and possible SF-induced superconductivity has been studied both theoretically [22] and experimentally [23].…”

NMR Spin-Lattice Relaxation Rates in Cuprates

“…The measured NMR spin-lattice relaxation rates thus provide rich information about the temperature dependence of the electronic and spin excitations which, however, still awaits a complete understanding. In this respect, it should also be mentioned that the analysis [17] of k T −1 1α data in the superconducting state within the frame of our model reveals surprising results about the symmetry of spin correlations.…”

“…The term τ eff (T ) is an effective electronic spin-spin correlation time, and k V β (T ) and k V γ (T ) correspond to the square of the components β and γ of the hyperfine fields at the nucleus. For a planar oxygen, the fluctuating fields stem from the localized moments on the two copper moments (at site 0 and 1) adjacent to the O and one gets 17…”