In most high-T_c cuprates, charge order (CO) coexists with the pseudogap, but develops gradually at lower temperatures, likely hampered and pinned by chemical defects. Whether charge order causes the pseudogap, or is simply an instability within the pseudogap phase, remains one of the central questions in the physics of cuprates. To isolate intrinsic electronic behaviour, recent investigations focused on the stoichiometric cuprate YBa₂Cu₄O₈ (Y124). This is a rare system, free of substitutional disorder, and with exceptional electronic homogeneity. It also happens to have a self-doping level exactly in the underdoped regime where charge order is strongest. In particular, a resonant x-ray scattering campaign revealed that, differently from all other cuprates, the temperature dependence of the CO signal in Y124 displays a concave, order-parameter-like growth with an abrupt onset at T ≈ 200 K, identical to the pseudogap onset temperature T* established by NMR and Raman spectroscopy [1]. This coincident onset strongly suggests a causal connection between CO and pseudogap, and revived the interest both in the role of disorder in the physics of cuprates, and in the use of Y124 as a clean, electronically homogenous cuprate model system to be investigated with other techniques.

Unfortunately, a significant practical limitation in this approach, prohibiting neutron scattering and quantum oscillation experiments, to name a few, is the need for bigger and better samples of Y124. Partially due to the high oxidation state of Cu in this incongruently-melting compound, the samples are originally grown by KOH-flux method, which usually produces high quality and unstrained single crystals, and do not exceed typical dimensions of hundreds of micrometres. In order to tackle this problem, we revisited the KOH flux synthesis and nucleation of Y124 [3-5].

A careful thermogravimetric and differential thermal analysis (TGA-DTA) revealed that the flux stoichiometry for the growth of Y124 shows a series of transitions, and that crystallization occurs over a wide temperature range. Subsequent systematic studies of varying flux growth attempts reveal that the current growth process is controlled by supersaturation. This imposes an intrinsic limit to the size of flux crystals, as well as an inherent source of contamination from KOH. These findings prompted us to turn to the floating zone method, known to typically yield large and clean single crystals, also allowing flexibility with chemical substitutions (thus controlled introduction of chemical disorder) [6]. We present successful preliminary results of Y124 synthesis under 300 bar oxygen pressure, from which we can extract few mm-large single crystals, already showing a promising alternative in this method.

^{[1] Betto, D., et al. "Coincident onset of charge order and pseudogap in a homogeneous high-temperature superconductor." Nature Communications 16.1 (2025): 3579.}

^{[2] Tachibana, Makoto. Beginner's guide to flux crystal growth. Tokyo: Springer Japan, 2017.}

^{[3] Sun, G. L., Y. T. Song, and C. T. Lin. "Investigation of YBa2Cu4O8 single crystal growth by KOH flux." Superconductor Science and Technology 21.12 (2008): 125001.}

^{[4] Song, Y. T., et al. "Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux." Journal of crystal growth 300.2 (2007): 263-266.}

^{[5] Dabrowski, B. "Single-crystal growth and properties of substituted." Superconductor Science and Technology 11.1 (1998): 54-58.}

^{[6] Schmehr, Julian L., and Stephen D. Wilson. "Active crystal growth techniques for quantum materials." Annual Review of Materials Research 47 (2017): 153-174.}