Praseodymium and manganese doped Ca2RuO4: crystals growth and physical properties

Abstract

Layered ruthenates have received a great attention in condensed matter physics due to the richness of their physical properties and the related phase diagram. Ruthenate compounds show a strong interplay between electronic, structural, magnetic, and orbital degrees of freedom. One representative material for this class of systems is the Ca2RuO4, a Mott insulator at room temperature and becomes an antiferromagnetic (AFM) insulator at TN  113 K. In Ca2RuO4 the delicate balance between competing microscopic interactions can be overturned by a slight external influence. Several experimental and theoretical studies have shown that the insulator-to-metal transition (IMT) in Ca2RuO4 is sensitive to external perturbations and can be tuned by temperature, electrical current, pressure, strain, and chemical substitution. The present work was motivated by the expectation to clarify some open issues related to the IMT induced by different stimuli as well as the different magnetic ordering that emerges in Ca2RuO4 with doping. Two different routes have been explored to tune the IMT in this system, one based on chemical substitution and the other one based on current-induced transition. Regarding the chemical substitution approach, either rare earth or transition metal site doping has been used. For this study, pure Ca2RuO4 crystals as well as Ca2-xPrxRuO4 and Ca2Ru1-xMnxO4 doped crystals have been grown. The praseodymium doping of Ca2RuO4 offered an additional route to tune the IMT in this system. Compositional and structural studies of crystals synthesized by varying the excess of Ru in the starting melted material showed the formation of pure Pr-doped crystals using an excess of Ru of about 23%. Regarding the electronic properties, transport measurements showed a decrease in the IMT temperature in the Pr-doping samples, while V-I characteristics at different temperatures showed

similar behavior to that observed in Ca2RuO4, suggesting the emergence of a metastable current- induced metallic phase also in Pr-doped crystals.

Additionally, since slightly substituting Ru with a 3d transition metal ion induces exotic magnetic behavior below TN, by resonant X-Ray spectroscopy (REXS) the evolution of the structural, magnetic, and orbital degrees of freedom of Mn-doped Ca2RuO4 has been studied. Through this substitutional doping it has found a way to control the subtle change in magnetic ordering induced without changing the symmetry of the crystal structure. REXS measurements allowed to determine the magnetic ordering under a wide range of different Mn doping. Using angle-resolved photoemission nano-spectroscopy (with a very small light spot), the evolution of the electronic structure in Ca2RuO4 as the crystal is driven across the IMT by the electrical current has been explored. With this experiment it has been shown that the current-driven IMT in Ca2RuO4 leads to the expected increase of the in-gap spectral weight, but at the same time the Mott state appears to be largely retained. [edited by Author]