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

Abstract

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. We interpret these results as microscopic evidence for a spatially inhomogeneous charge distribution, where the NMR signal from O sites in the domain walls of the spin density modulation are wiped out due to large hyperfine fields, and the remaining signal arises from the intervening Mott insulating regions.PACS numbers: 74.72. Dn, 75.10.Nr, Several doped transition metal oxides exhibit inhomogeneous charge stripe order on a mesoscopic scale due to competing long and short range interactions acting on the charge carriers [1,2]. In the cuprates, the doped charge carriers (holes) are expected to form one-dimensional channels (charge stripes) separating regions of insulating antiferromagnetic order of the Cu spins (spin stripes) [3]. The presence of this inhomogeneity may be vital to the mechanism of d-wave superconductivity [4], however direct experimental evidence for such structures has been elusive. To date, the only observations have been via techniques that probe either the spin density or the charge inhomogeneity: Neutron Scattering (NS) experiments provided the first evidence for the modulation of spin density that is expected in a stripe lattice [5], whereas NMR and Scanning Tunnelling Microscopy (STM) experiments indicate the presence of inhomogeneous doping distributions [6,7,8,9,10]. In this Letter we discuss new NMR data that provide direct evidence for a correlation between the local charge and spin density maps by taking advantage of the unique properties of the planar oxygen to probe simultaneously both the local spin structure as well as the local hole doping in the O p-orbitals. Our data reveal that not only is the charge spatially inhomogeneous, but that the regions of excess charge are correlated with the domain walls of the spin order, exactly as expected for a stripe pattern [3,11].The rare-earth co-doped La 1.8−x Eu 0.2 Sr x CuO 4 series is ideal for NMR investigations of the spin and charge inhomogeneity. 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]. However, the planar oxygen does not suffer the same fate: it experiences an isotropic transferred hyperfine coupling (129 kOe/µ B ) to the two nearest neighbor Cu spins, so for antiferromagnetically correlated neighbors, the hyperf...

NMR studies of pseudogap and electronic inhomogeneity in Bi2Sr2CaCu2OCrocker1, Dioguardi2, apRoberts-Warren3 et al. 2011Phys. Rev. BSelf Cite

“…In other cuprates oxygen NMR is usually only possible in aligned samples, in which a powder sample (with greater surface area) is isotopically enriched, then mixed with epoxy and cured in an external field. 17,21,30 Aligned powder samples have well resolved spectra for the applied field along the alignment axis (usually the c-direction), but exhibit a random powder pattern for perpendicular fields. 31 Although there are ways to extract parameters of the EFG and T −1 1 tensors from these powder patterns, there are usually unavoidable baseline and orientation distribution corrections.…”

“…1,16 Nuclear magnetic resonance (NMR) of the oxygen nuclei in the cuprates offers unique insight into the interplay of charge and spin degrees of freedom. [17][18][19][20][21][22] Unlike STM, NMR probes the bulk of the material, and both the quadrupolar moment and nuclear magnetic moment of the 17 O couple to the local charge and spin environments, respectively. 17 phases.…”

“…[17][18][19][20][21][22] Unlike STM, NMR probes the bulk of the material, and both the quadrupolar moment and nuclear magnetic moment of the 17 O couple to the local charge and spin environments, respectively. 17 phases. 17 In order to investigate the local environment at the oxygen site, we have measured NMR spectra and the nuclear spin-lattice-relaxation rate (T −1 1 ) of a single overdoped crystal of Bi 2 Sr 2 CaCu 2 O 8+δ (T c = 82 K) isotopically enriched with 17 O.…”

“…The dominant contribution arises from this on-site term, and is a direct measure of the doping in the cuprates, which enables us to quantitatively extract the local hole concentration in the oxygen orbitals. 17,34 Susceptibility and resistivity measurements indicate T c = 82 K corresponding to a doping level of p ≈ 0.21. 35 The temperature dependence of the shift, EFG and linewidths were determined by measuring the centers of gravity and second moments n = ±1 satellite transitions.…”

“…17 phases. 17 In order to investigate the local environment at the oxygen site, we have measured NMR spectra and the nuclear spin-lattice-relaxation rate (T −1 1 ) of a single overdoped crystal of Bi 2 Sr 2 CaCu 2 O 8+δ (T c = 82 K) isotopically enriched with 17 O. This compound has been studied extensively via surface probes such as STM and angle resolved photoemission (ARPES) because as it cleaves easily revealing well formed surfaces normal to the c−direction.…”