NMR evidence for spatial correlations between spin and charge order in (La,Eu)2-xSrxCuO4

Nicholas Curro curro@lanl.govHans-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

Evidence for Spin InhomogeneityInelastic 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 stabilizes static incommensurate AF below TN ~ 50K and spin excitations suggestive of 1D spin ladders

J. Tranquada et al., Nature 375, 561 (95)

J. Tranquada et al., Nature 429, 534 (04)

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)

NMR as Probe of Spin and Charge

Zeeman Interaction - alignment in external field ~ 10-6 eV (5 mK)

Hyperfine Interaction - alignment with electron spin ~ 10-8 eV

Quadrupolar Interaction - alignment with EFG (Q, ~ 10-6 eV

-

+

+

-

Rare-Earth Co-doping and LTTLa1.8-xEu0.2SrxCuO4

H.-H. Klau et al., PRL 85 4590 (2000)

•Superconductivity suppressed

•Glassy spin freezing in LTT phase

SR

H.-H. Klau et al., Hyperfine Int. (2000)

M. Braden, unpublished (1999)

INS

Spin Response in LTT phase

NMR

N. Curro et al., PRL 85 642 (2000)

ESR

V. Kataev et al., PRB 55 , 3394 (97)

M. Hucker., Ph. D. Thesis (1999)

Susceptibility dominated by Van Vleck term from Eu3+ and from CuO2 plane

La NMR, Cu NMR, SR and Gd ESR dominated by glassy spin fluctuations

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)

/2

CuO

Quadrupolar Splitting

Frequency

Satellite Splitting proportional to local EFG, c

+3/2

+1/2

-1/2

-3/2-5/2

5/2

17O NMR in La1.8-xEu0.2SrxCuO4 allows one to probe the EFG in the limit of slow spin dynamics

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

NMR Spectra on Aligned Powder

• powder sample necessary to enrich with 17O

•Aligned and mixed with epoxy

•Enriched and non-enriched spectra are subtracted

Spectra

From the planar O spectra, we observe:

•T dependent Knight shift

•Magnetic broadening below 20K

•Strongly T dependent c !

La1.67Eu0.2Sr0.13CuO4

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!

Missing Signal Intensity

Some of the oxygen sites do not contribute to signal: remaining sites experience reduced hole doping

Where do the holes go?

x=0.13

x=0.20

O Intensity

La Intensity

NEXAFS and O hole doping

J. Fink et al., J. Elec. Spec. 66 395 (1994)

La1.8-xEu0.2SrxCuO4

X-ray absorption fluorescence spectroscopy of the O 1s 2p transition: intensity proportional to number of holes in oxygen 2p orbitals

No observable change of holes in 2p orbitals of LESCO!

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

Hyperfine Field and Wipeout

-1 0 1 2 3 4 5

0

2

4

6

8

10

P(T1-1)

ln(T1-1)

Detection window set by spectrometer - maximum detectable T1

-1

La NMR

1T1

~ Hhyp2

Site Hhyp (kOe/B)La ~ 1Cu ~ 100O ~ 0-50

PRL 85 642 (2000)

La Cu

Hyperfine Fields at Oxygen

O CuCu

Hhyp ~ 0

O CuCu O CuCu

Hhyp ~ large

1T1

~ Hhyp2

~ large

These sites wiped-out!

1T1

~ Hhyp2

Spin Density Modulation

O CuCuO CuCu O CuCu

S(r)

•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?

Charged Domain WallsCharged 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)

Checkerboard Topology

•1D stripes: 25% of signal is lost

•Site-centered checkerboard: 38% lost

•Experiment: ~ 50% lost

•Random disorder: r0 ~ 15 Å

An Interesting Question

LTO LTT

Conventional Picture of Stripe Pinning:

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

TLT

Glassy Behavior

•Inhomogeneity remains spatially disordered

•Gives rise to slow spin (and possibly charge) fluctuations

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)

NMR in La2Cu1-xLixO4

La NMR

O

Suh et al., PRL 81 2791 (98) Park et al., PRL 94 17002 (05)

• Glassy spin and charge dynamics

• O spectra show large magnetic broadening, possibly changes in EFG as well

Zn impurity in YBCOJulien et al., PRL 84 3422 (2000)

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

Resistivity