1. Physics Department, Brookhaven National Laboratory, Upton, New York 11973, USA
2. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK
3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Correspondence to: J. M. Tranquada¹ Email: jtran@bnl.gov

Abstract

In the copper oxide parent compounds of the high-transition-temperature superconductors¹ the valence electrons are localized—one per copper site—by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism², where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations³. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes⁴, which would necessitate an unconventional mechanism of superconductivity⁵. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa₂Cu₃O_6+x near optimum doping⁶ are incompatible with the naive expectations^{7, 8} for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La_1.875Ba_0.125CuO₄. We show that the measured excitations are, surprisingly, quite similar to those in YBa₂Cu₃O_6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations^{7, 8} is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity¹¹.

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