Observation of two-dimensional superconductivity in bilayers of ${\mathrm{BaBiO}}_3$ and ${\mathrm{BaPbO}}_3$

Lead-doped barium bismuthate is intriguing because of its relatively high critical temperature and the nature of its superconductivity in the vicinity of a charge-density-wave-ordered topological insulator. Here we present bilayers of barium bismuthate and barium leadate showing emergent superconductivity as a function of the barium leadate top layer thickness. Magnetotransport studies allow for the characterization of the superconducting properties in these bilayers to be two-dimensional. In addition current-voltage characteristics of superconducting bilayers reveal signatures of a Berezinskiĭ-Kosterlitz-Thouless transition. The particular observed dependence of the superconducting transition temperature on the thickness of the barium leadate top layer suggests the formation of the superconducting state originating from the interface.

Figure 1

Normalized resistances of pulsed-laser-deposition (PLD)-grown thin films of ${\mathrm{BaPbO}}_3$ (BPO) and ${\mathrm{BaBi}}_{0.25}{\mathrm{Pb}}_{0.75}{\mathrm{O}}_3$ and ${\mathrm{BaBiO}}_3$ (BBO)/BPO bilayers (BLs) as a function of temperature. Whereas BBO thin films are highly insulating (not shown), BPO thin films show metallic behavior (gray hexagons). Thin films of nominal ${\mathrm{BaBi}}_{0.25}{\mathrm{Pb}}_{0.75}{\mathrm{O}}_3$ are superconducting at low temperatures. BLs with a fixed thickness of BBO [100 laser pulses (LPs)] and variable thickness of the BPO top layer (controlled by the number $N$ of LPs) undergo superconducting transitions (samples with $N=\{40,60,80,100\}$).

Figure 2

Observed dependence of the upper critical magnetic field $H_{\mathrm{c}2}$ of a superconducting BL ($N=60$) on the direction $\theta$ of the externally applied swept magnetic field (diamonds). The data are well represented by Eq. (1) (dashed line) which applies to a 2D superconducting sheet in the GL regime [40, 41] with $H_{\mathrm{c}2,\parallel }=5.6\mathrm{T}$ and $H_{\mathrm{c}2,\perp }=1.1\mathrm{T}$.

Figure 3

Measured current-voltage ($I\text{−}V$) characteristics of a BBO/BPO BL ($N=60$) at temperatures ranging from 2 K (blue) to 4 K (red). Straight black lines are polynomial fits following the measured data directly at the transition. At 3.2 K the $I\text{−}V$ curve is best represented by a cubic fit (straight blue line) identifying 2D superconductivity following a Berezinskiĭ-Kosterlitz-Thouless (BKT) transition. The deviation of the fit for small currents is attributed to finite size effects [53].

Figure 4

XRD studies of BLs of BBO (100 LP)/BPO ($N$ LP) grown on (001)-oriented STO substrates. Reciprocal space maps of BLs for (a) $N=50$ (superconducting) and (b) $N=150$ (metallic). Whereas BBO grows completely relaxed on STO substrates, following the relaxation line (black), the locations of both peaks of BPO are clear indicators for strained growth of BPO on BBO. The observed shift of the BPO peak away from BBO peak for larger $N$ supports the idea that the concurrent observed changes in superconductivity are affected by strain. (c) Obtained (004) peak positions of BBO and BPO in BBO/BPO BLs from a series of $\theta /2\theta$ scans and critical temperatures for selected BLs (black lines serve as guides to the eye). Whereas BBO shows no significant shift in its peak positions, the BPO peaks move to lower angles with decreasing $N$, supporting ongoing strain-induced growth. This shift of the BPO peak is accompanied by the occurrence of superconductivity in the BLs (see blue squares and corresponding temperature axis on the right).