Growth of ultra-clean oxide single crystals of the altermagnet candidate RuO2

We report the details of the growth of ultra-clean oxide single crystals of RuO 2 , a candidate material for altermagnetism. By using a necking-structure crystal-growth tube and finely tuning the conditions of the sublimation transport method, we demonstrate a systematic control of the crystal morphology. We obtained mainly three kinds of morphologies: thick plate-like crystals up to 10 × 5 × 2 mm 3 with a large (101) facet, rhombohedral columnar crystals elongating along the [001] direction, and fiber and needle crystals of length up to 8 mm. These crystals show residual resistivity of about 30 n Ω cm and a residual resistivity ratio (RRR) up to 1200, substantially higher than previous reports. We extended the resistivity modeling and resolved the discrepancy between Debye temperatures from specific heat and resistivity. These ultra-clean oxide crystals with much higher RRR than other recent studies lead us to make a decisive conclusion that the intrinsic ground state of bulk RuO 2 is paramagnetic. • Ultra-clean oxide crystals of RuO 2 , exhibiting a residual resistivity ratio (RRR) up to 1200, have been grown by the sublimation transport method. • Using a neck-shaped growth tube, we demonstrated precise and reproducible control of RuO 2 crystal morphologies via temperature — an aspect rarely addressed before. • We resolved the discrepancy over the Debye temperature values deduced from resistivity and specific heat by analyzing the resistivity using various combinations of Bloch–Grüneisen and Einstein terms. • Availability of these crystals with significantly higher RRR values than those used in recent reports lead us to make a decisive conclusion that the ground state of bulk RuO 2 is paramagnetic.