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NMR evidence for spatial correlations between spin and charge order in (La,Eu) 2-x Sr x CuO 4

NMR evidence for spatial correlations between spin and charge order in (La,Eu) 2-x Sr x CuO 4. Nicholas Curro curro@lanl.gov. Hans-Joachim Grafe , Los Alamos National Laboratory Markus H ücker , Brookhaven National Laboratory Bernd Büchner , Leibniz Instit ü t, Dresden.

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NMR evidence for spatial correlations between spin and charge order in (La,Eu) 2-x Sr x CuO 4

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  1. NMR evidence for spatial correlations between spin and charge order in (La,Eu)2-xSrxCuO4 Nicholas Curro curro@lanl.gov Hans-Joachim Grafe,Los Alamos National Laboratory Markus Hücker, Brookhaven National Laboratory Bernd Büchner, Leibniz Institüt, Dresden "The STM and neutron scattering experiments have broadened our knowledge of high-Tc materials, but it's not clear how their separate findings are related to one other. Only when several different techniques are brought to bear on the same material will researchers get some insight into how the spin and charge structures influence one other." -- Physics Today, Sept. 2004

  2. Evidence for Spin Inhomogeneity J. Tranquada et al., Nature 429, 534 (04) J. Tranquada et al., Nature 375, 561 (95) Inelastic NS in La2-xSrxCuO4 (R. Birgeneau et al.): dynamic incommensurate AF correlations K. Yamada et al., PRB 57 6165 (98) Elastic / Inelastic NS in La2-x-yREySrxCuO4 and La1.875Ba0.125CuO4 (Tranquada et al.): LTT phase stabilizesstatic incommensurate AF below TN ~ 50K and spin excitations suggestive of 1D spin ladders

  3. Evidence for Charge Inhomogeneity STM (Kapiltunik et al., Davis et al., Yazdani et al.)- inhomogeneous surface states in Bi2Sr2CaCuO8-x and Ca2-xNaxCuO2Cl2 : modulations of LDOS at length scales ~ 4a0 T. Hanaguri et al., Nature 430, 1001 (04) Cu NQR in La2-xSrxCuO4 (Imai et al.): Local hole doping variations at nm level P. M. Singer et al., PRL 88, 47602 (02) O NMR in La2-xSrxCuO4 (Haase, Slichter et al.): Spatial variations of local spin susceptibility and local EFG J. Haase et al., J. Supercond. 13, 723 (00)

  4. NMR as Probe of Spin and Charge Quadrupolar Interaction- alignment with EFG (nQ,h) ~ 10-6 eV - - + + Zeeman Interaction- alignment in external field ~ 10-6 eV (5 mK) Hyperfine Interaction - alignment with electron spin ~ 10-8 eV

  5. Rare-Earth Co-doping and LTT La1.8-xEu0.2SrxCuO4 INS mSR H.-H. Klaubet al., Hyperfine Int. (2000) M. Braden, unpublished (1999) • Superconductivity suppressed • Glassy spin freezing in LTT phase H.-H. Klaubet al., PRL 85 4590 (2000)

  6. Spin Response in LTT phase ESR V. Kataev et al., PRB 55 , 3394 (97) NMR M. Hucker., Ph. D. Thesis (1999) N. Curro et al., PRL 85 642 (2000) Susceptibility dominated by Van Vleck term from Eu3+ and from CuO2 plane La NMR, Cu NMR, mSR and Gd ESR dominated by glassy spin fluctuations

  7. Oxygen NMR in Cuprates • Hyperfine coupling at O site is to the two nearest neighbor Cu spins • Vanishes for AF correlations (Filtered out by form factor) O Cu p/2

  8. Quadrupolar Splitting 5/2 +3/2 +1/2 Frequency -1/2 -3/2 Satellite Splitting proportional to local EFG, nc -5/2 17O NMR in La1.8-xEu0.2SrxCuO4 allows one to probe the EFG in the limit of slow spin dynamics

  9. Oxygen Electric Field Gradient • 2p6 does not create an EFG at the nucleus, but 2p5 does La2-xSrxCuO4 La1.8-xEu0.2SrxCuO4 T > 150K • EFG is a direct measure of the number of holes in the O 2p orbital

  10. NMR Spectra on Aligned Powder • powder sample necessary to enrich with 17O • Aligned and mixed with epoxy • Enriched and non-enriched spectra are subtracted

  11. Spectra La1.67Eu0.2Sr0.13CuO4 • From the planar O spectra, we observe: • T dependent Knight shift • Magnetic broadening below 20K • Strongly T dependent nc!

  12. Temperature Dependence • EFG is strongly temperature dependence below T ~ 60K • Never been seen previously in superconducting cuprates Effective number of holes at the O sites has decreased!

  13. Missing Signal Intensity O Intensity x=0.13 La Intensity x=0.20 Some of the oxygen sites do not contribute to signal: remaining sites experience reduced hole doping Where do the holes go?

  14. NEXAFS and O hole doping X-ray absorption fluorescence spectroscopy of the O 1s  2p transition: intensity proportional to number of holes in oxygen 2p orbitals La1.8-xEu0.2SrxCuO4 No observable change of holes in 2p orbitals of LESCO! J. Fink et al., J. Elec. Spec. 66 395 (1994)

  15. Implications of NEXAFS and NMR • Breadth of the local hole distribution increases at low temperatures for both LTO and LTT • For LTT, an unknown mechanism wipes-out regions of high hole doping • What is this mechanism? T > Tq T > Tq T < Tq

  16. Hyperfine Field and Wipeout 1 ~ Hhyp2 t T1 La NMR La Cu P(T1-1) PRL 85 642 (2000) ln(T1-1) Detection window set by spectrometer - maximum detectableT1-1 SiteHhyp (kOe/mB) La ~ 1 Cu ~ 100 O ~ 0-50

  17. Hyperfine Fields at Oxygen Cu Cu Cu O O O Cu Cu Cu 1 1 ~ Hhyp2 t ~ large ~ Hhyp2 t ~ 0 T1 T1 Hhyp ~ 0 Hhyp ~ large These sites wiped-out!

  18. Spin Density Modulation S(r) Cu O Cu Cu Cu O O Cu Cu • Hhyp ~ S(r) • T1-1 is largest near nodes of S(r): wiped out • The NMR signal showing reduced hole concentration comes from regions far from nodes! • Is hole concentration correlated with the nodes?

  19. Charged Domain Walls Charged Domain-Walls:Zaanen et al. (PRB 40 7391 (89)) Bishop et al. (cond-mat/0306672) • Hyperfine fields wipe out regions of high hole density • Spatial correlation between np(r) and S(r)

  20. Checkerboard Topology • 1D stripes: 25% of signal is lost • Site-centered checkerboard: 38% lost • Experiment: ~ 50% lost • Random disorder: r0 ~ 15 Å

  21. An Interesting Question LTO LTT Conventional Picture of Stripe Pinning: TLT Why is the width of the local doping distribution the same in both La2-xSrxCuO4 and La1.8-xEu0.2SrxCuO4, and there are no dramatic changes at TLT ? • Perhaps the charge inhomogeneity is already set in at a high temperature • Small fluctuations (local phonons – Bishop) give rise to spin fluctuations that are gapped and exhibit glassy behavior in the LTT phase

  22. Glassy Behavior • Inhomogeneity remains spatially disordered • Gives rise to slow spin (and possibly charge) fluctuations

  23. In-plane Doping: La2Cu1-xLixO4 • Li1+ in-plane doping ~ Zn2+ & hole • No incommensurate splitting • Curie-Weiss susceptibility from FM response surrounding Li sites • Holes “bound” to Li sites J. L. Sarrao et al., PRB 54 12 014 (1996)

  24. NMR in La2Cu1-xLixO4 La NMR • Glassy spin and charge dynamics • O spectra show large magnetic broadening, possibly changes in EFG as well Suh et al., PRL 81 2791 (98) Park et al., PRL 94 17002 (05) O Zn impurity in YBCO Julien et al., PRL 84 3422 (2000)

  25. Summary • O NMR observes a distribution of local charges, that is spatially correlated with the local spin density • The LTT structure suppresses spin fluctuations, rather than “pins” stripes • The spin fluctuations must be important for superconductivity • The stripe “template” may exist at high temperatures, and the spin and charge fluctuations observed are only excitations associated with this heterogeneity

  26. Resistivity

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