Superconductivity in spin-orbit coupled BaBi$_3$ formed by in situ reduction of bismuthate films

TL;DR

The paper addresses stabilizing superconductivity in bismuthate perovskites by leveraging in situ chemical reduction at BaBiO3 interfaces. Using Eu or Al overlayers, BaBiO3 is reduced to BaBi3 (and BaO), with XRD and STEM confirming phase decomposition into superconducting BaBi3. Transport measurements reveal a superconducting transition near 6 K with quasi-two-dimensional character, including anisotropic upper critical fields and indications of a BKT-type transition. The authors discuss the role of Bi spin-orbit coupling and disorder, suggesting this approach as a platform to explore high-Tc topological superconductivity in Bi-based layered systems.

Abstract

Oxygen-scavenging at oxide heterointerfaces has emerged as a powerful route for stabilizing metastable phases that exhibit interesting phenomena, including high-mobility two-dimensional electron gases and high T$_{c}$ superconductivity. We investigate structural and chemical interactions at the heterointerface formed between Al or Eu and the charged-ordered insulator, BaBiO$_3$, leading to emergent superconductivity at 6 K. A combination of X-ray diffraction and electron microscopy measurements shows that oxygen scavenging by the Eu and Al adlayers leads to the formation of superconducting intermetallic BaBi$_3$ in nominal Eu/BaBiO$_3$ and Al/BaBiO$_3$ bilayers. Anisotropic magnetotransport measurements and current-voltage signatures of quasi two-dimensional superconductivity are observed. The mechanisms behind quasi-two-dimensional superconductivity and the role of disorder remain to be clarified. These findings highlight the potential for the use of in situ reduction of bismuthate heterostructures as a platform for stabilizing materials with exotic functional properties. Additionally, the strong spin-orbit coupling at the Bi sites may pave the way for the realization of high T$_{c}$ topological superconductivity.

Figures (12)

• Figure 1: Crystal structure of BaBiO$_3$ and BaBi$_3$(A) Crystal structure of charged ordered insulating monoclinc BaBiO$_3$(B) Crystal structure of superconducting tetragonal BaBi$_3$.
• Figure 2: Structural properties of nominal Eu/BaBiO$_3$ heterostructures grown by molecular beam epitaxy.(A) In situ reflection high-energy electron diffraction (RHEED) images measured during the growth of BaBiO$_3$ and Eu metal along the SrTiO$_3$ [100] zone axis. (B) Comparison of X-ray diffraction measurements for a BaBiO$_3$ film on (001)-oriented SrTiO$_3$, a nominal Eu/BaBiO$_3$, and Al/BaBiO$_3$ heterostructures on SrTiO$_3$. (C) Scanning transmission electron microscope high-angular annular dark field (HAADF) image of cross-section of nominal Eu/BaBiO$_3$ heterostructure on SrTiO$_3$. (D) Energy-dispersive X-ray spectroscopy maps for cross-section in (C) showing elemental distributions of Ba, Bi, O, and Eu.
• Figure 3: Transport properties of nominal Eu/BBO heterostructures (Sample 1) grown by molecular beam epitaxy.(A) Resistance versus temperature. The inset graph shows the low temperature resistance as a function of magnetic fields applied perpendicular to the sample surface. (B) Magnetoresistance measured as a function of temperature above and below the superconducting transition. (C) Hall coefficient as a function of temperature above the superconducting transition temperature (T$_c$). The inset shows the Hall resistance vs magnetic field. (D) Thickness dependence of superconducting transition.
• Figure 4: Magnetotransport measurements of Eu/BBOx (Sample 2) on STO (001)(A) Resistance versus temperature for Sample 2. (B) Magnetoresistance measured as a function of temperature for out-of-plane B configuration. (C) Magnetoresistance measured as a function of temperature for the in-plane B configuration. (D) The upper critical field (B$_{c2}$(T)) as a function of temperature for in-plane and out-of-plane configurations. The upper critical field (B$_{c2}$(T)) were determined using 50 % and 90 % of normal-state resistance criterion. The green and dark yellow solid lines represent the fit using the 2D Ginzburg-Landau (GL) equations for in-plane and out-of-plane configurations: $B_{c2\perp}(T)= B_{c2\perp} (0) \left( 1-\frac{T}{T_c} \right)$ and $B_{c2\parallel}(T)= B_{c2\parallel} (0) \left( 1-\frac{T}{T_c} \right)^{1/2}$.
• Figure 5: Current-Voltage (I-V) measurements of EuO/BBOx (Sample 2) on STO (001)(A) I-V characteristics measured at various temperatures. (B) Critical current (I$_{c}$), determined from the derivative of the I-V characteristics, plotted as a function of temperature, demonstrating superconducting behavior. (C) I-V curves plotted on a logarithmic scale, to highlight non-linear behaviour more distinctly. (D) Exponent $\alpha$ (extracted from (C)) as a function of temperature: The horizontal solid line indicates the Berezinskii-Kosterlitz-Thouless transition temperature $T_{BKT}\approx$ 5.34 K for $\alpha = 3$. The solid black line is a guide to the eye.