Growth and structural characterization of large superconducting crystals of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$

Large crystals of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (La-Ca-2126) with $x=0.10$ and 0.15 have been grown and converted to bulk superconductors by high-pressure oxygen annealing. The superconducting transition temperature, $T_c$, is as high as 55 K; this can be raised to 60 K by postannealing in air. Here we present structural and magnetic characterizations of these crystals using neutron scattering and muon spin rotation techniques. While the as-grown, nonsuperconducting crystals are single phase, we find that the superconducting crystals contain three phases forming coherent domains stacked along the $c$ axis: the dominant La-Ca-2126 phase, very thin (1.5 unit-cell) intergrowths of ${\mathrm{La}}_2{\mathrm{CuO}}_4$, and an antiferromagnetic ${\mathrm{La}}_8{\mathrm{Cu}}_8{\mathrm{O}}_{20}$ phase. We propose that the formation and segregation of the latter phases increases the Ca concentration of the La-Ca-2126, thus providing the hole doping that supports superconductivity.

Figure 1

Volume magnetic susceptibility for samples SC45 (blue circles) and SC55 (red squares). Open symbols: Meissner fraction (measured on field cooling); filled symbols: shielding fraction (zero-field cooling).

Figure 2

Normalized susceptibility data for pieces of the $\mathrm{SC}55\mu$ sample. Red line: zero-field-cooled bulk susceptibility; blue circles: diamagnetic shift of the internal field observed by muons in a field of 30 mT.

Figure 3

Neutron diffraction intensities measured along $\mathbf{Q}=(0,0,L)$ for the SC55 (red circles, measured on HB-1A) and SC45 (blue circles, measured on HB-1) crystals at $T=4$ K. Intensity scale is in arbitrary units; data for the two samples have been normalized at the (004) peak. Counting time was $\sim 5$ s/pt, with an attenuator in the beam (to avoid detector saturation). Top (bottom) panel uses a linear (logarithmic) intensity scale. The solid lines are model calculations as described in the text.

Figure 4

(a) Elastic scattering obtained at SEQUOIA on the SC55 sample at $T=4$ K. Rings of scattering correspond to powder diffraction from the Al sample holder and from the sample itself. (b) Calculated diffraction pattern for the La-8-8-20 phase using the structural parameters from Ref. [27].

Figure 5

Magnetic volume fraction in the NSC and $\mathrm{SC}55\mu$ samples as determined from $\mu \mathrm{SR}$ measured in a weak transverse field of 3 mT.

Figure 6

Neutron diffraction scans along $\mathbf{Q}=(\frac{1}{2},\frac{1}{2},L)$ measured at $T=4$ K (blue points) and 150 K (violet points) on HB-1A. Background data (black points) measured at $\mathbf{Q}=(0.45,0.45,L)$ and $T=4$. Three of the $L$ values at which the temperature dependence was followed in detail are indicated by arrows.

Figure 7

Temperature dependence of intensities for $\mathbf{Q}=(0.5,0.5,L)$ at several values of $L$, after appropriate background subtraction. Red squares: $L=0$; black circles: $L=2.53$; violet diamonds: $L=3.2$; green triangles: $L=6$.

Figure 8

Results obtained from a series of 24h annealings in air at steadily increasing temperatures. (a) Cumulative change in oxygen content, normalized to the La-Ca-2126 formula unit, estimated from mass changes for a piece of SC55 and an as-grown, nonsuperconducting piece of La-Ca-2126 with $x=0.15$. (b) Resulting $T_c$ after each annealing, determined from ZFC susceptibility at $\chi =-0.1$. (c) Superconducting (shielding) volume at 40 K, relative to that of the $700{}^{\circ }\mathrm{C}$ sample at 5 K, from the ZFC susceptibility data.