Charge density wave and superconductivity in perovskite bismuthates
Dr Minu Kim
The perovskite bismuthates have attracted widespread interest since superconductivity was discovered in hole-doped compounds BaPb1-xBixO3 and Ba1-xKxBiO3 with moderately high transition temperature of 13 and 30 K, respectively [1,2]. The parent compound BaBiO3 (BBO) is an insulator, generally believed to exhibit charge disproportionation into the Bi3+ and Bi5+ valence states (6s2 and 6s0 configurations). This disproportionation leads to lattice (breathing) distortion originated from variations in the Bi-O bond length, and the formation of charge density wave which creates a gap at Fermi energy . Whereas phase diagram of the bismuthate compounds was explored via chemical substitution using Pb or K , the complexities associated with chemical doping have so far presented a difficult experimental challenge.
Here, we study epitaxial BBO thin films grown via pulsed laser deposition without any insertion of chemical dopants. Surprisingly, spectroscopic ellipsometry reveals the suppression of the charge density wave as film thickness decreases. X-ray diffraction and Raman spectroscopy confirms that thick films have a tetragonal structure with lattice doubled due to the breathing distortion, while thin films have a cubic without any evidence of lattice doubling. A critical thickness of the concurrent suppression of the charge density wave and the breathing distortion is found to be about 10 unit cells. These results suggest that dimensionality can be an ideal tuning parameter to study the electronic phase of charge-density-wave materials without dopant substitution.
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