Spin dynamics near the critical doping in weakly superconducting underdopedYBa2Cu3O6.35

Chris Stock

¹

,

W. J. L. Buyers

²

,

Z. Yamani

³

et al.

Abstract

From neutron inelastic and elastic scattering we have determined the magnetic structure and fluctuations in the YBa 2 Cu 3 O 6.35 ͑YBCO͒ superconductor ͑T c =18 K͒ near the boundary of the superconducting phase. The long-range ordered collinear spins of the insulating antiferromagnet are replaced by a commensurate central mode arising from slow, isotropically polarized, short-range spin correlations extending over four planar unit cells. The inelastic spectrum up to 30 meV is also broad in wave vector and commensurate. In contrast to the resonance peak of higher-T c superconductors, the spins exhibit a single overdamped spectrum whose rate of relaxation ⌫ decreases on cooling and saturates at 2⌫ =5Ϯ 1 meV below ϳ50 K. As the relaxation rate saturates, the quasistatic spin correlations grow and become resolution limited in energy. The spin susceptibility above ϳ50 K follows the same / T scaling relation found for the monolayer La 2−x Sr x CuO 4 system, indicating that the dominant energy scale is set by the temperature. Below 50 K the scale length is geometric and not linked by velocity to dynamic widths. Despite the large differences from an antiferromagnet, we show that integrated intensity conserves the total moment sum rule, and that on cooling the spectral weight transfers from the inelastic spin relaxation to the elastic central peak. There is no observable suppression of the spin fluctuations or central mode upon the onset of superconductivity. The spins respond not to coherent charge pairs but to hole doping, allowing coexistence of glassy short-range spin order with superconductivity. Since the physics of the weakly superconducting system YBCO 6.35 must connect continuously with that in more strongly superconducting YBCO 6.5 , we find that neither incommensurate stripe-like spin modulations nor a well-defined neutron spin resonance are essential for the onset with doping of pairing in a high-temperature cuprate superconductor.

Separation of magnetic and superconducting behavior inYBa2Cu3O6.33(Tc=Yamani1, Buyers2, Wang3 et al. 2015Phys. Rev. BSelf Cite5150View full textAdd to dashboardCite

“…This is in agreement with previous conclusions 22,23 for the slightly higher doped YBCO6.35 (T c =18.5 K). It appears that for very low concentrations of hole doping in YBCO, an arrangement of antiferromagnetically correlated quasistatic spins in three dimensions over a finite range is more favored than alternating quasi-one-dimensional spin and charge regions.…”

“…Although observations are qualitatively similar to higher doped YBCO6.35 (T c =20 K) [22][23][24] , these are the first measurements on YBCO6.33 with such a low T c revealing that even for such low doping, long-range antiferromagnetic order and superconductivity do not coexist. Rather the state coexisting with superconductivity is best regarded as a textured spin phase with short-range AF correlations.…”

“…We will present clear evidence for the existence of spatially highly-correlated and slow spin fluctuations. Although the observations are qualitatively similar to higher doped YBCO6.35 (T c =18 K) [22][23][24] , these are the first measurements on YBCO6.33 with such a low T c , and we will show that even for such low doped yet superconducting materials, long-range antiferromagnetic order and superconductivity do not coexist.…”

“…17 lie on a single trend line with doping that is the same for the YBCO and LSCO 30 systems. The value of the observed moment is derived by calibrating magnetic intensities from the observed intensities for several nuclear Bragg peaks 23,75 . We have verified that integration of the elastic line when put on absolute scale by phonon calibration also results in a similar value to within error bars (0.08±0.02 µ B compared to 0.11±0.02 µ B from Bragg peaks).…”

“…1). Our earlier experiments on YBCO6.35 (T c =18 K, p=0.06) revealed that no longrange Bragg-ordered AF phase coexists with superconductivity [22][23][24] . The short-range static spin correlations observed for YBCO6.35 leaves open the possibility that an AF quantum critical point or novel phase might occur at lower doping.…”