Spin dynamics and ordering of a cuprate stripe antiferromagnet

Abstract

In La 1.48 Nd 0.4 Sr 0.12 CuO 4 the 139 La and 63 Cu nuclear quadrupole resonance relaxation rates and signal wipeout upon lowering temperature are shown to be due to purely magnetic fluctuations. They follow the same renormalized classical behavior as seen in neutron data, when the electronic spins order in stripes, with a small spread in spin stiffness ͑15% spread in activation energy͒. The La signal, which reappears at low temperatures, is magnetically broadened and experiences additional wipe out due to slowing down of the Nd fluctuations. DOI: 10.1103/PhysRevB.63.020507 PACS number͑s͒: 74.72.Dn, 76.60.Ϫk, 75.30.Ds, 75.40.Gb Strongly correlated electron systems such as layered cuprates exhibit very unusual properties. One of the most interesting among them is the coexistence of superconductivity with local antiferromagnetism ͑AF͒-a fingerprint of the topological effects of doping of AF insulators by holes. The charges segregate into a periodical array of stripes separating antiphase antiferromagnetic domains. Experimental evidence for stripe correlations has been provided by neutron studies in Nd-doped La 1.875 Sr 0.125 CuO 4 and in other cuprates and nickelates. 1,2 The spatial organization of the stripe structures is a subject of much debate. 3-8 Stripe formation is characterized by the temperatures of charge (T charge ) and spin T spin ordering with T charge ϾT spin . Since these different types of order coexist on the microscopic level, local methods of analysis, like NMR or nuclear quadrupole resonance ͑NQR͒, are well suited to see their interrelation. One striking feature in the NMR data is the wipe-out effect. In Cu-NQR experiments on a number of Sr doped La 2 CuO 4 samples, Hunt et al. 6 showed a correlation between the amount of the intensity loss and the development of charge order of the stripe phase. Curro et al. 7 found strong Cu wipe-out effect in their NMR experiments on La 2ϪyϪx Eu y Sr x CuO 4 and showed that this effect could be accounted for by a wide ͑100%͒ distribution in the energy of the thermally activated correlation times that determine the relaxation processes-so-called glassy behavior.In this communication the role of slow magnetic fluctuations is elucidated. We show that the variation of the line shape as well as wipe-out and relaxation effects probe the growing spin order in the stripe phase. Our investigation takes profit of the NQR frequency range of 139 La and 63 Cu, and especially of the low frequencies and relatively small line widths of La NQR in La 1.48 Nd 0.4 Sr 0.12 CuO 4 . In this compound both Cu and La exhibit strong wipe-out effects. Because La ͑contrary to Cu͒ nuclei are relatively weakly coupled to the electronic spins in the CuO 2 planes, La NQR signals can be followed down to the spin-ordering temperature, as seen by SR. Using the spin correlation times extracted from the activated La spin-lattice relaxation rates we are able to predict precisely these wipe-out features by introducing a spread of only 15% in the activation energy. Within experimental error t...

How to detect fluctuating stripes in the high-temperature superconductors

“…The work by Imai's group suggesting that local stripe order in variants of La 2−x Sr x CuO 4 could be detected through the "wipeout effect" of Cu NQR has motivated a considerable number of further studies using Cu and La NMR and NQR (Curro et al, 2000a;Hunt et al, 2001;Julien et al, 1999Julien et al, , 2001Simovič et al, 2002;Singer et al, 1999;Suh et al, 1999Suh et al, , 2000Teitel'baum et al, 2001Teitel'baum et al, , 2000. Although there has been some controversy over details of the interpretation (and in certain cases, there may be differences between samples), it is now generally agreed that NQR and NMR are sensitive to the onset of slow spin fluctuations whose appearance tends to correlate with the onset of local charge stripe pinning as determined by diffraction.…”

Nuclear-Magnetic-Resonance Evidence for Charge Inhomogeneity in Stripe OrderedLa1.8−xEu0.2SrxCuO4

“…Structurally, this material is almost identical to the prototypical high temperature superconductor La 2−x Sr x CuO 4 , but undergoes a subtle phase transition to the low temperature tetragonal (LTT) structure below T LT = 135K [12]. Instead of superconducting below T c ∼ 35K, this system exhibits glassy magnetic order below T N ∼ 25K [12,13,14,15,16,17,18], and elastic NS measurements have identified static long-range spin and structural modulations that are likely produced by stripe order [5]. The slow spin fluctuations in this system dominate the NMR response of the La and Cu nuclei [14,16,19].…”

“…Furthermore, 17 O (I=5/2) has a quadrupolar moment ( 17 Q = -2.56 ×10 −26 cm 2 ), so it is sensitive to the EFG at the nuclear site, which is a measure of the local hole doping [21]. Although previous NMR studies have shown the presence of spatial modulations due to doping inhomogeneities in superconducting La 2−x Sr x CuO 4 [6,8] 17 O by annealing in 17 O 2 gas at 600 • C for 24 hours. The powder samples were then mixed with epoxy and aligned along the c-axis by curing in an external field.…”

“…We report 17 O Nuclear Magnetic Resonance (NMR) results in the stripe ordered La1.8−xEu0.2SrxCuO4 system. Below a temperature Tq ∼ 80K, the local electric field gradient (EFG) and the absolute intensity of the NMR signal of the planar O site exhibit a dramatic decrease.…”

“…2b) implies either (i) the center n p of the distribution P(n p ) is reduced below T q , or (ii) n p remains temperature independent, but the upper end of the distribution does not contribute to the NMR signal below T q . To distinguish between these two scenarios, we measured the absolute strength of the NMR signal as a function of temperature [14,17]. The temperature dependence of the measured number of spins, N 0 , shown in Fig.…”