Evidence for magnetic-field-induced decoupling of superconducting bilayers in ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$

We report a study of magnetic susceptibility and electrical resistivity as a function of temperature and magnetic field in superconducting crystals of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ with $x=0.10$ and 0.15 and transition temperature $T_c^{\mathrm{m}}=54$ K (determined from the susceptibility). When an external magnetic field is applied perpendicular to the ${\mathrm{CuO}}_2$ bilayers, the resistive superconducting transition measured with currents flowing perpendicular to the bilayers is substantially lower than that found with currents flowing parallel to the bilayers. Intriguingly, this anisotropic behavior is quite similar to that observed for the magnetic irreversibility points with the field applied either perpendicular or parallel to the bilayers. We discuss the results in the context of other studies that have found evidence for the decoupling of superconducting layers induced by a perpendicular magnetic field.

Figure 1

(a) Crystal structure of the ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$. (b) As-grown ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ single-crystal rods grown by the traveling-solvent floating-zone method. (c) Laue back-diffraction image of ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ single crystal with (001) direction ($c$ axis) pointing along the x-ray beam. Color points in (c) correspond to the calculated diffraction pattern, generated by software QLaue [22].

Figure 2

Mass magnetic susceptibility ($\chi =M/H$) of ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (top row) and ${\mathrm{La}}_{1.85}{\mathrm{Ca}}_{1.15}{\mathrm{Cu}}_2{\mathrm{O}}_6$ (bottom row), measured with a magnetic field of 10 Oe applied (a), (d): perpendicular to the ${\mathrm{CuO}}_2$ planes or (b), (c), (e)–(f): parallel to the planes. Measurements were performed both in zero-field cooling (ZFC, filled circles) and field-cooling (FC, open circles) modes. The insets illustrate the relative directions of ${\mathrm{CuO}}_2$ planes and the magnetic field $H$. The criteria to determine $T_c^{\mathrm{m}}$ is shown in (c) and (f).

Figure 3

(a) The hysteretic curves of magnetic susceptibility versus temperature in sample ${\mathrm{La}}_{1.9}{\mathrm{Ca}}_{1.1}{\mathrm{Cu}}_2{\mathrm{O}}_6$ under an in-plane field ($H\parallel ab$) up to 7 T. (b) Similar hysteretic curves obtained under a perpendicular field ($H\perp ab$). (c) The $T$ dependences of irreversibility field $H_{\mathrm{irr}}\left(T\right)$ in both directions relative to the ${\mathrm{CuO}}_2$ planes for the ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ ($x=0.10$ and 0.15) single crystals.

Figure 4

(a) The in-plane resistivity ${\rho }_{ab}\left(T\right)$ and interlayer resistivity ${\rho }_c\left(T\right)$ of the superconducting ${\mathrm{La}}_{2-x}{\mathrm{Ca}}_{1+x}{\mathrm{Cu}}_2{\mathrm{O}}_6$ single crystals. Different $y$-axis values are used to illustrate the normal-state resistivity in both directions. (b) A zoomed-in figure showing the superconducting transition near 54 K. (c) A zoomed-in figure showing the finite resistivity in ${\rho }_c$ at low temperature.

Figure 5

Electrical resistivity of (a) ${\rho }_{ab}$ and (b) ${\rho }_c$ as a function of temperature, obtained under a perpendicular field up to 7 T. (c) Phase diagram indicating the boundaries between the normal state, 2D and 3D superconductivity of ${\mathrm{La}}_{1.85}{\mathrm{Ca}}_{1.15}{\mathrm{Cu}}_2{\mathrm{O}}_6$. Large squares (circles) represent the superconducting transition determined from ${\rho }_c$ (${\rho }_{ab}$) with $\mathbf{H}\parallel \mathbf{c}$. Small squares (circles) represent $H_{\mathrm{irr}}$ determined from $\chi$ with $\mathbf{H}\parallel \mathbf{c}$ ($\mathbf{H}\perp \mathbf{c})$.