Growth and characterization of ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ and ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$ crystals

We report the successful growth of sizable crystals of the cuprate superconductors ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ and ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$. These compounds are well known for their high optimal superconducting critical temperatures of $T_{\mathrm{c}}=128$ K and 134 K at ambient pressure, respectively, and for their challenging synthesis. Using a conventional quartz-tube encapsulation method and a two-layer encapsulation method that utilizes custom-built high-pressure furnaces, we are able to grow single crystals with linear dimensions up to several millimeters parallel to the ${\mathrm{CuO}}_2$ planes. Extended postgrowth annealing is shown to lead to sharp superconducting transitions, indicative of high macroscopic homogeneity. X-ray diffraction and polarized Raman spectroscopy are identified as viable nondestructive methods to help separate the two compounds from synthesis products, as the latter often contain intergrowths of the former. Our work helps to remove obstacles toward the study of these model cuprate systems with experimental probes that require sizable high-quality crystals.

Figure 1

Schematic comparison of the crystal structures of the first three members of the Hg-family of cuprates.

Figure 2

(a) Magnetic susceptibility measurement of a Hg1212 crystal grown with the conventional method (see text) in a 5 Oe $c$-axis field, upon cooling in a magnetic field (FC) and after cooling in zero field (ZFC). (b) Photograph of the crystal. (c) $c$-axis x-ray Laue pattern obtained for the crystal.

Figure 3

(a) Sketch of custom-designed furnace with two layers of encapsulation. As temperature increases, the heated nitrogen that surrounds the sealed quartz tube provides a counterpressure to prevent the quartz tube from exploding. (b) Black: Temperature profile for the crystal growth of Hg1212 and Hg1223. Red: Total pressure of Hg and ${\mathrm{O}}_2$ inside the quartz tube, generated by a mixture (see text) that contains 0.75 g of excess HgO, estimated from the ideal-gas equation. In the “synthesis” step, the pressure decreases because some of the HgO reacts with the rest of the materials. Blue: ${\mathrm{N}}_2$ pressure read from the gauge.

Figure 4

(a) Magnetic susceptibility measurements of an as-grown Hg1223 single crystal in a 5 Oe $c$-axis field, after zero-field cooling (red) and field cooling (blue). (b) Photograph of single crystals with naturally-cleaved surfaces. Inset: Raw product from the growth containing single crystals. (c) $c$-axis x-ray Laue pattern for a Hg1223 single crystal.

Figure 5

Annealing history of a Hg1223 crystal in air at ${480}^{\circ }\mathrm{C}$. The measurements were performed in a 5 Oe $c$-axis field after zero-field cooling.

Figure 6

Single-crystal x-ray diffraction data for Hg1201, Hg1212 and Hg1223, with momentum transfer along the $c$ axis. The common peaks around 43 degrees originate from the sample plate. The (0 0 6) reflection of Hg1212 and the (0 0 8) reflection of Hg1223 are too weak and/or too close to these peaks to be visible. Inset: Illustration of the diffraction geometry.

Figure 7

(a)–(c) $A_{1g}$ Raman spectra for Hg1201, Hg1212 and Hg1223 measured at different temperatures. The data are normalized and offset for clarity. Symbols indicate different $A_{1g}$ phonon modes and are coded with the assigned dominant atomic motion (see text) in the crystal structures in (d).