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Superconductivity in Isolated Single Copper Oxygen Plane

Youngdo Kim, Byeongjun Gil, Sehoon Kim, Yeonjae Lee, Donghan Kim, Jaeung Lee, Jinyoung Kim, Younsik Kim, Miyoung Kim, Changyoung Kim

TL;DR

This work addresses whether superconductivity can exist in an isolated $CuO_2$ plane by constructing a heterostructure that isolates a single $CuO_2$ plane in LSCO and measuring its electronic structure with in-situ ARPES. The monolayer exhibits a $d$-wave–like gap with $ abla E_F$ features and a maximal gap around $\,Delta \,sim 10$ meV, with the gap closing between $40$ and $80$ K, a temperature range somewhat above the bulk $T_c$ of similar doping. Importantly, the monolayer's electronic structure and gap behavior closely resemble those of bulk LSCO, indicating that cuprate superconductivity is essentially a two-dimensional phenomenon and does not require interlayer coupling. This establishes a platform for studying purely 2D cuprate physics and motivates future doping-tuning experiments to map out the 2D phase diagram, including potential pseudogap or charge-order phenomena.

Abstract

One of the central questions in cuprate superconductivity is if superconductivity can exist in an isolated single CuO$_2$ plane without any interlayer coupling. There have been numerous experimental efforts to answer this question, but it still has not been clearly resolved. Here we present a heterostructure system with an isolated half-unit-cell La$_{2-x}$Sr$_x$CuO$_4$ which has a single CuO$_2$ plane. Using in-situ angle-resolved photoemission spectroscopy, we measured the electronic and gap structures of a single CuO$_2$ plane. We observed a \textit{d}-wave-like gap which closes somewhat above the bulk T$_c$. Moreover, almost identical gap properties are seen for both single CuO$_2$ plane and bulk. These observations lead us to the conclusion that the d-wave superconductivity of cuprates also exists in a single CuO$_2$ plane. Our results demonstrate that cuprate superconductivity is essentially a two-dimensional phenomenon and provide a platform to study cuprate superconductivity in a purely two-dimensional system.

Superconductivity in Isolated Single Copper Oxygen Plane

TL;DR

This work addresses whether superconductivity can exist in an isolated plane by constructing a heterostructure that isolates a single plane in LSCO and measuring its electronic structure with in-situ ARPES. The monolayer exhibits a -wave–like gap with features and a maximal gap around meV, with the gap closing between and K, a temperature range somewhat above the bulk of similar doping. Importantly, the monolayer's electronic structure and gap behavior closely resemble those of bulk LSCO, indicating that cuprate superconductivity is essentially a two-dimensional phenomenon and does not require interlayer coupling. This establishes a platform for studying purely 2D cuprate physics and motivates future doping-tuning experiments to map out the 2D phase diagram, including potential pseudogap or charge-order phenomena.

Abstract

One of the central questions in cuprate superconductivity is if superconductivity can exist in an isolated single CuO plane without any interlayer coupling. There have been numerous experimental efforts to answer this question, but it still has not been clearly resolved. Here we present a heterostructure system with an isolated half-unit-cell LaSrCuO which has a single CuO plane. Using in-situ angle-resolved photoemission spectroscopy, we measured the electronic and gap structures of a single CuO plane. We observed a \textit{d}-wave-like gap which closes somewhat above the bulk T. Moreover, almost identical gap properties are seen for both single CuO plane and bulk. These observations lead us to the conclusion that the d-wave superconductivity of cuprates also exists in a single CuO plane. Our results demonstrate that cuprate superconductivity is essentially a two-dimensional phenomenon and provide a platform to study cuprate superconductivity in a purely two-dimensional system.
Paper Structure (5 sections, 3 equations, 4 figures)

This paper contains 5 sections, 3 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Growth of LSCO monolayers
  3. Electronic structure measurement
  4. Superconducting gap structure
  5. Discussion

Figures (4)

  • Figure 1: Construction of single CuO$_2$ plane system. A schematic illustration of (a) the heterostructure with single CuO$_2$ plane used in the study and (b) ARPES measurements on isolated single CuO$_2$ plane. (c) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and (d) energy-dispersive X-ray spectroscopy (EDX) images of the heterostructure.
  • Figure 2: Electronic structure of monolayer and 30-layer La$_{2-x}$Sr$_x$CuO$_4$. Fermi surface maps of (a) monolayer and (b) 30 layer (15 UC) of LSCO with LSAO buffer layer. (c) Fermi surface of a monolayer and 30-layer LSCO fitted with tight-binding model. (d-e) Off-nodal and nodal cuts of a monolayer along the white and red dashed lines in (a), respectively. (f-g) Off-nodal and nodal cuts of 30-layer LSCO along the white and red dashed lines in (b), respectively.
  • Figure 3: Superconducting gap analysis. (a) Fermi surface of LSCO monolayer. White dots indicate the Fermi momenta k$_F$ where k$_F$ energy distribution curves (EDCs) are taken. (b) EDCs at k$_F$'s in (a), with each color corresponding to the line color in (a). The inset zooms in near the Fermi level to show the leading-edge shift. (c) EDCs from a 30-layer LSCO obtained with the same method as in the monolayer LSCO case. (d-g) Symmetrized EDCs of monolayer and 30-layer LSCO. Black lines are fitting results. (d) Angle-dependent symmetrized EDCs of the monolayer LSCO. (e) Temperature-dependent symmetrized EDCs of monolayer LSCO. EDCs are extracted at k$_F$ of off-nodal cut (gray line in (a)). (f-g) Angle- and temperature-dependent symmetrized EDCs of 30-layer LSCO, respectively. All EDCs were processed in the same way as in case of the monolayer LSCO.
  • Figure 4: Momentum- and temperature-dependent gap behavior. Red squares and blue triangles represent gap behavior determined by fitting and leading-edge shifts, respectively. Black dashed lines are guides for the d-wave gap (momentum-dependence) and BCS gap function (temperature-dependence). (a) Momentum- and (b) temperature-dependent gaps for the monolayer. (c) Momentum- and (d) temperature-dependent gaps for the 30-layer LSCO.