Condensed Matter and Materials Physics(DCMMP)
Physique de la matière condensée et des matériaux (DPMCM)

---

W.J.L. BUYERS
National Research Council

Spins and paired carriers in a superconductor that is nearly an antiferromagnet - who pushes who?

---

Near the onset of superconductivity, where there is competition with the nearby antiferromagnetic phase, we find that the spin resonance in YBCO6+x for x= 6.35 (Tc = 19 K) is at low energy, is intense and is overdamped. Its relaxation rate slows to ~3 meV below Tc, an order of magnitude lower than the resonance energy in x=6.50 (Tc=59 K). It appears that this spectral feature will turn into the spin wave of the antiferromagnetic insulator once it is no longer held up by interaction with the gapped quasiparticles. At a further order of magnitude lower in energy we find a hitherto unobserved non-divergent central peak. Although we expect it to be related to the AF Bragg peak of the undoped insulator, there is no long-range order since the spins at all energies are spatially confined to a region with a correlation range of 2 nm or about 5 cells. Despite the low ~6% doping of holes the superconducting order causes a gradual freezing of the spins, but without a sudden onset at Tc. The results demonstrate the strong interaction between spins and carriers, and suggest that they spontaneously reassemble to cause spin confinement. [1] C. Stock, W.J.L. Buyers, et al., Phys. Rev. B 69, 014502 (2004)