Quantum spin excitations through the metal-to-insulator crossover inYBa2Cu3O6+y

Abstract

We use inelastic neutron scattering to study the temperature dependence of the spin excitations of a detwinned superconducting YBa2Cu3O6.45 (Tc = 48 K). In contrast to earlier work on YBa2Cu3O6.5 (Tc = 58 K), where the prominent features in the magnetic spectra consist of a sharp collective magnetic excitation termed "resonance" and a large (hω ≈ 15 meV) superconducting spin gap, we find that the spin excitations in YBa2Cu3O6.45 are gapless and have a much broader resonance. Our detailed mapping of magnetic scattering along the a * /b * -axis directions at different energies reveals that spin excitations are unisotropic and consistent with the "hourglass"-like dispersion along the a * -axis direction near the resonance, but they are isotropic at lower energies. Since a fundamental change in the low-temperature normal state of YBa2Cu3O6+y when superconductivity is suppressed takes place at y ∼ 0.5 with a metal-to-insulator crossover (MIC), where the ground state transforms from a metallic to an insulating-like phase, our results suggest a clear connection between the large change in spin excitations and the MIC. The resonance therefore is a fundamental feature of metallic ground state superconductors and a consequence of high-Tc superconductivity.

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

“…The stark difference we see between the properties of the low-doped yet superconducting YBCO6.33 and the higher doped superconductors may be related to the presence of a critical hole concentration for a metal-toinsulator crossover (MIC) in the superconducting dome as suggested earlier 19 from neutron scattering studies 19 on YBCO6.45. The magnetic excitations were found to be gapless with a resonance much broader in energy than at the higher doping.…”

“…Moreover, providing the integrated intensity of the resonance is related to the superfluid density 99 , one might argue that because YBCO6.33 is such a weak superconductor (T c ∼8.4 K), the intensity of the resonance may be too small to observe. The modified Lorentzian we observe, however, does not display the properties typical of a resonance [18][19][20][83][84][85][86] neither an increase in its intensity below T c , nor an hour-glass dispersion nor a decrease with applied magnetic field (see Section III D). Instead the overdamped exciations decline on cooling as the spins drop into the static ground state revealed by the growing central peak.…”

“…For higher doped YBCO6+x with x≥0.45, the doped holes frustrate any AF order causing the entire scattering to be dynamic (no elastic magnetic scattering). The spectrum consists of a prominent resolution limited peak at the resonance energy, E res , located at the saddle point of the hourglass-shaped dispersion 4,[18][19][20][83][84][85][86][87][88] . The observed hourglass magnetic spectrum and presence of incommensurate peaks around AF zone centre in cuprates are explained in terms of collective modes of an underlying stripe state [89][90][91][92][93][94][95][96][97] .…”

Glassy low-energy spin fluctuations and anisotropy gap in La1.88Sr0.12CuO4<

“…2 Undoped cuprates are charge transfer insulators that upon charge-carrier doping of the CuO 2 layers become superconducting. For carrier concentrations lower than optimal for superconductivity, i.e., in the underdoped regime, several competing or coexisting order parameters have been identified, such as circulating orbital currents, [3][4][5] incommensurate spin and charge stripe ordering, 6,8 and recently, charge density wave order. 9,10 Stripe order in cuprates was originally discovered when the hole-doping level p was tuned to p = x = 1/8 in La 1.6−x Nd 0.4 Sr x CuO 4 (Nd-LSCO).…”

Electron-spin excitation coupling in an electron-doped copper oxide superconductor

“…While doped electrons reside in the Cu 3d orbitals with correlations involving primarily d-electrons 2 , doped holes form Zhang-Rice singlets moving in the background of Cu spins 1 and are subject to stronger correlations [1][2][3][4][5][6][7] . In electron-doped copper oxides, there are bulk signatures of coexisting AF and superconducting phases [8][9][10][11][12] . These measurements, however, cannot distinguish nanoscale spatial coexistence from larger scale phase segregation.…”