Electronic inhomogeneity and phase fluctuation in one-unit-cell FeSe films
Dapeng Zhao, Wenqiang Cui, Yaowu Liu, Guanming Gong, Liguo Zhang, Guihao Jia, Yunyi Zang, Xiaopeng Hu, Ding Zhang, Yilin Wang, Wei Li, Shuai‐Hua Ji, Lili Wang, Ke He, Xu-Cun Ma, Qi‐Kun Xue
Abstract
Abstract One-unit-cell FeSe films on SrTiO 3 substrates are of great interest owing to significantly enlarged pairing gaps characterized by two coherence peaks at ±10 meV and ±20 meV. In-situ transport measurement is desired to reveal novel properties. Here, we performed in-situ microscale electrical transport and combined scanning tunneling microscopy measurements on continuous one-unit-cell FeSe films with twin boundaries. We observed two spatially coexisting superconducting phases in domains and on boundaries, characterized by distinct superconducting gaps ( $${\Delta }_{1}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>Δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> </mml:math> ~15 meV vs. $${\Delta }_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>Δ</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> ~10 meV) and pairing temperatures ( T p1 ~52.0 K vs. T p2 ~37.3 K), and correspondingly two-step nonlinear $$V \sim {I}^{\alpha }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>V</mml:mi> <mml:mo>~</mml:mo> <mml:msup> <mml:mrow> <mml:mi>I</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>α</mml:mi> </mml:mrow> </mml:msup> </mml:math> behavior but a concurrent Berezinskii–Kosterlitz–Thouless (BKT)-like transition occurring at $${T}_{{{{{{\rm{BKT}}}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>BKT</mml:mi> </mml:mrow> </mml:msub> </mml:math> ~28.7 K. Moreover, the onset transition temperature $${T}_{{{{{{\rm{c}}}}}}}^{{{{{{\rm{onset}}}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>onset</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ~54 K and zero-resistivity temperature $${T}_{{{{{{\rm{c}}}}}}}^{{{{{{\rm{zero}}}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>zero</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> ~31 K are consistent with T p1 and $${T}_{{{{{{\rm{BKT}}}}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>T</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>BKT</mml:mi> </mml:mrow> </mml:msub> </mml:math> , respectively. Our results indicate the broadened superconducting transition in FeSe/SrTiO 3 is related to intrinsic electronic inhomogeneity due to distinct two-gap features and phase fluctuations of two-dimensional superconductivity.