Emergence of low-energy spin waves in superconducting electron-doped cuprates

Abstract

In order to fully utilize the technological potential of unconventional superconductors, an enhanced understanding of the superconducting mechanism is necessary. In the best performing superconductors, the cuprates, superconductivity is intimately linked with magnetism, although the details of this coupling remain elusive. In search of clarity in the magnetism-superconductivity relationship, we focus on the electron-doped cuprate $\mathrm{Nd}_{1.85}\mathrm{Ce}_{0.15}\mathrm{CuO}_{4-δ}$ (NCCO). NCCO has an antiferromagnetic ground state when synthesized, and only becomes superconducting after a reductive annealing process. This makes NCCO an ideal template to study how the magnetism differs in the superconducting and non-superconducting state, while keeping the material template as constant as possible. Using neutron spectroscopy, we reveal that the as-grown crystal exhibits a large spin pseudogap in the magnetic fluctuation spectrum. Upon annealing, defects that are introduced by the commonly employed synthesis method are removed and the spin pseudogap is significantly reduced. While the spin pseudogap in the annealed sample is likely an effect of superconductivity, we argue that the spin pseudogap in the as-grown sample is caused by the absence of long-wavelength spin waves. The defects in as-grown NCCO thus play the dual role of suppressing both superconductivity and low-energy spin waves, highlighting a potential connection between these two phenomena.

Figures (7)

• Figure 1: Magnetization as a function of temperature (ZFC: zero-field cooled), measured at 10 Oe field, for the as-grown (AG) and reductively annealed, superconducting (SC) NCCO single crystals, depicted in blue pentagons and orange triangles, respectively. $T_c$ is defined as the onset temperature of superconductivity. Insert: crystal structure of NCCO,Momma_VESTA_softwarebelokoneva1991preparation with Cu, O and Nd depicted in blue, red and green, respectively. The 15% Ce doping on the Nd site is denoted as a pink slice on the green Nd atoms.
• Figure 2: Representative magnetic response of the as-grown (AG) NCCO sample at two temperatures, measured with neutron spectroscopy. At high temperature (27 K), a peak indicates the presence of magnetic excitations at this temperature. The peak, and therefore the excitations, vanish at low temperature (2 K). The $q$-resolution of the neutron instrument is sufficiently broad to cover the full magnetic response. (Insert) Illustration of the scanning direction (red arrow) across the commensurate magnetic peak (black circle). The solid lines are Gaussian fits to the data. A hypothesis test, using Wilk's theorem, is used to determine whether or not a peak is present, as outlined in the Supplemental Material.
• Figure 3: Dynamic susceptibility $\chi"(\omega)$, as a function of energy transfer. (a) as-grown (AG) sample. (b) annealed, superconducting (SC) sample. The black outlined points indicate 3-point scans, while colored outlined points indicate $q$-scans. The solid lines are fits to the response described in the Supplemental Material. The dashed lines are drawn as guide to the eye, while the colored vertical dotted lines are the estimate of the spin pseudogap onset with the faded area representing the uncertainty.
• Figure 4: Shift in $\chi"(\omega)$ determined by subtraction of the 2 K data from the 27 K data depicted in Fig. \ref{['fig:E_IN20']}, for both the as-grown (AG) and annealed, superconducting (SC) samples. The dashed lines are drawn as guide to the eye and The solid lines are fits to the response, the dashed lines are drawn as guide to the eye, while the colored vertical dotted lines are the estimate of the spin pseudogap onset with the faded area representing the uncertainty.
• Figure 5: $\chi"(\omega)$ shown as a function of temperature. (a) as-grown sample (AG) and (b) annealed, superconducting (SC) sample. The black outlined points indicate 3-point scans while colored outlined points indicate $q$-scans. Square blue points falling below this line indicate the onset of a spin pseudogap. The dashed blue and yellow lines are guides to the eye.