# Co-appearance of superconductivity and ferromagnetism in a Ca$_2$RuO$_4$ nanofilm crystal

## TL;DR

This study demonstrates high-temperature superconductivity co-occurring with ferromagnetism in Ca$_2$RuO$_4$ nanofilm crystals, revealing novel quantum states and superconductor-insulator transitions induced by film thickness and bias current.

## Contribution

It reports the first observation of high-temperature superconductivity coexisting with ferromagnetism in Ca$_2$RuO$_4$ nanofilms, highlighting the effects of reduced dimensionality.

## Key findings

- Superconductivity with $T_c$ up to 64 K in Ca$_2$RuO$_4$ nanofilms.
- Coexistence of ferromagnetism at 180 K and superconductivity.
- Superconductor-insulator transition controlled by bias current and film thickness.

## Abstract

By tuning the physical and chemical pressures of layered perovskite materials we can realize the quantum states of both superconductors and insulators. By reducing the thickness of a layered crystal to a nanometer level, a nanofilm crystal can provide novel quantum states that have not previously been found in bulk crystals. Here we report the realization of high-temperature superconductivity in Ca$_2$RuO$_4$ nanofilm single crystals. Ca$_2$RuO$_4$ thin film with the highest transition temperature $T_c$ (midpoint) of 64~K exhibits zero resistance in electric transport measurements. The superconducting critical current exhibited a logarithmic dependence on temperature and was enhanced by an external magnetic field. Magnetic measurements revealed a ferromagnetic transition at 180~K and diamagnetic magnetization due to superconductivity. Our results suggest the co-appearance of superconductivity and ferromagnetism in Ca$_2$RuO$_4$ nanofilm crystals. We also found that the induced bias current and the tuned film thickness caused a superconductor-insulator transition. The fabrication of micro-nanocrystals made of layered material enables us to discuss rich superconducting phenomena in ruthenates.