Molecular beam epitaxy of electron-doped infinite-layer ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ thin films

Thin films of the electron-doped infinite-layer ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$, with $R={\mathrm{La}}^{3+}$, ${\mathrm{Nd}}^{3+}$, and ${\mathrm{Ce}}^{4+}$, have been synthesized using molecular beam epitaxy. The solubility limits of $R$ in ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ are $0.060\pm 0.002$, $0.080\pm 0.010$, and $<0.01$ for $R={\mathrm{Nd}}^{3+}$, ${\mathrm{La}}^{3+}$, and ${\mathrm{Ce}}^{4+}$, respectively. Using high-resolution reciprocal space mapping we show that the in-plane lattice constants of ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ follow the same trend as is observed for other cuprates with square-planar coordinated copper, i.e., elongation of the Cu-O bond length upon electron doping. We measured the temperature dependencies of the resistivity and the Hall coefficient to trace the influence of the doped charge carriers on the electronic response. We show that the Hall coefficient is negative below 300 K and that the $R$ substitution level is insufficient to achieve a positive Hall coefficient, a necessary prerequisite for superconductivity.

Figure 1

(a) Crystal structure of infinite-layer ${\mathrm{Ca}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$. (b), (c) Infinite-layer structure in the [110] and [100] projections, respectively. In the [110] projection, the stacking sequence along the in-plane direction at each atomic site is homologous. In the case of the [100] projection, 50% O [denoted as $\mathrm{O}{1}^{\prime }$ in (c)] and Cu are located at same atomic position, while the other 50% O [denoted as O1 in (c)] are independent. (d) Cross-sectional inverted ABF-STEM image of the interface of infinite-layer ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ thin film grown on (001)LSAT substate taken along the [110] direction. (e) The intensity profiles of the inverted ABF image taken along the dashed (Ca/Nd layer) and dotted (${\mathrm{CuO}}_2$ layer) lines in (d). (f) HAADF-STEM image of ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ thin film along [100] direction and EELS elemental maps of Ca $L$, Cu $L$, Nd $M$, and O $K$ edges.

Figure 2

High-resolution reciprocal space maps of inifinite-layer ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ (a) and ${\mathrm{Ca}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{CuO}}_2$ (b) thin films grown on (001)LSAT substrates.

Figure 3

High-resolution XRD patterns around the (001) and (002) reflections of infinite-layer ${\mathrm{Ca}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{CuO}}_2$ (a), (d), ${\mathrm{CaCuO}}_2$ (b), (e), and ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ (c), (f) thin films grown on (001)LSAT substrates. Cross-sectional transmission electron microscopy (TEM) images of ${\mathrm{Ca}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{CuO}}_2$ (g) and ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ (h) thin films grown on (001)LSAT substrates. The film thicknesses ($t$) calculated from the Laue fringes in XRD peaks are 55, 76, and 60 nm for ${\mathrm{Ca}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{CuO}}_2$, ${\mathrm{CaCuO}}_2$, and ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ thin films, respectively, which agrees well with $t$ estimated from the TEM images [$t=58,70$, and 62 nm for ${\mathrm{Ca}}_{0.96}{\mathrm{La}}_{0.04}{\mathrm{CuO}}_2$, ${\mathrm{CaCuO}}_2$ (not shown), and ${\mathrm{Ca}}_{0.96}{\mathrm{Nd}}_{0.04}{\mathrm{CuO}}_2$ thin films, respectively].

Figure 4

$2\theta$/$\theta$-scanned x-ray diffraction patterns around the (002) reflections of infinite-layer ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ ($x$ = 0.04, 0.07, 0.08, and 0.09) (a) and ${\mathrm{Ca}}_{0.94}{\mathrm{Nd}}_{0.06}{\mathrm{CuO}}_2$ (b). In (b) spectra for the as-grown and annealed films are shown. The latter was annealed with atomic oxygen (rf power of 300 W and oxygen flow rate of 0.8 sccm) at $280{}^{\circ }\mathrm{C}$ for 10 min.

Figure 5

Temperature dependencies of resistivity for infinite-layer ${\mathrm{Ca}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$ ($x$ = 0.04, 0.06) (a) and ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ ($x$ = 0.07, 0.08) (b) thin films grown on (001)LSAT substrates. The inset in (a) shows enlarged view of $\rho -T$ curves of ${\mathrm{Ca}}_{0.94}{\mathrm{Nd}}_{0.06}{\mathrm{CuO}}_2$ thin film down to 200 mK under magnetic field of 0, 1, 3, 5, 12, and 14 T applied perpendicular to the $ab$ planes. For comparison, the $\rho -T$ curve for ${\mathrm{CaCuO}}_2$ is shown in the inset in (b). $T^{*}$ and $T_{\text{min}}$ are defined as onset temperatures for the pseudogap and resistivity minimum, respectively. $T_{\text{c}}$ is superconducting critical temperature. The current density used is 30 A/${\mathrm{cm}}^2$, which is far below the superconducting critical current density of infinite-layer cuprates where $J_{\text{c}}$ = 5.9 MA/${\mathrm{cm}}^2$ [56].

Figure 6

Magnetoresistance $\mathrm{\Delta }{\rho }_{xx}/{\rho }_{xx}(0,T)$ of ${\mathrm{Ca}}_{0.94}{\mathrm{Nd}}_{0.06}{\mathrm{CuO}}_2$ (a) and ${\mathrm{Ca}}_{0.92}{\mathrm{La}}_{0.08}{\mathrm{CuO}}_2$ (b) thin films grown on LSAT substrates, where $\mathrm{\Delta }{\rho }_{xx}={\rho }_{xx}({\mu }_{0}H,T)$–${\rho }_{xx}(0,T)$. The temperature ranges below 100 K. Magnetic field is applied perpendicular to the $ab$ planes. Linear interpolated contour plot of $\mathrm{\Delta }{\rho }_{xx}/{\rho }_{xx}(0,T)$ as a function of magnetic field ($-14$ T $\le {\mu }_{0}H\le$ 14 T) and temperature ($T$ = 2, 4, 7, 10, 12, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, and 100 K) for corresponding ${\mathrm{Ca}}_{0.94}{\mathrm{Nd}}_{0.06}{\mathrm{CuO}}_2$ (c) and ${\mathrm{Ca}}_{0.92}{\mathrm{La}}_{0.08}{\mathrm{CuO}}_2$ (d) films.

Figure 7

Temperature dependencies of $R_{\text{H}}$ for infinite-layer ${\mathrm{Ca}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$ ($x$ = 0.04, 0.06) (a) and ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ ($x$ = 0.07, 0.08) (b) thin films grown on (001) LSAT substrates. For comparison, $R_{\text{H}}-T$ for ${\mathrm{CaCuO}}_2$ is also shown (c). The insets are $R_{xy}-{\mu }_{0}H$ curves at 2 K. Black and red solid lines correspond to measurement data and linear fitted curves, respectively.

Figure 8

Cross-sectional TEM (a) and HAADF-STEM (b) images on ${\mathrm{Ca}}_{1-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_2$ ($x$ $<0.01$) thin film grown on (001)LSAT substrate. (c), (d) The fast Fourier transform patterns taken from domains A and B in (b). (e) Temperature dependence of resistivity of the ${\mathrm{Ca}}_{1-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_2$ thin film.

Figure 9

(a)-(d) Counter plots of the (001) XRD peak intensities of infinite-layer ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ ($R$ = ${\mathrm{La}}^{3+}$, ${\mathrm{Nd}}^{3+}$, ${\mathrm{Ce}}^{4+}$) thin films grown on (001)LSAT substrates as a function of the total pressures of atomic oxygen ${\mathrm{O}}^{*}$ and ${\mathrm{O}}_2$ ($p_{{\text{O}}^{*}+{\text{O}}_2}$) and $T_{\text{s}}$. Optimal growth conditions of ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ where the improvements of the crystal quality and conductivity coincide, are for ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ (a) $590{}^{\circ }\mathrm{C}$ with rf power of 300 W and ${\mathrm{O}}_2$ flow rate of 0.8 sccm, for ${\mathrm{CaCuO}}_2$ (b) $590{}^{\circ }\mathrm{C}$ with rf power of 300 W and ${\mathrm{O}}_2$ flow rate of 1.5 sccm, for ${\mathrm{Ca}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$ (c) $590{}^{\circ }\mathrm{C}$ with rf of 300 W and ${\mathrm{O}}_2$ flow rate of 2.5 sccm, and for ${\mathrm{Ca}}_{1-x}{\mathrm{Ce}}_x{\mathrm{CuO}}_2$ (d) $590{}^{\circ }\mathrm{C}$ with rf of 300–350 W and ${\mathrm{O}}_2$ flow rate of 2.0 sccm. The conditions where the infinite-layer (IL) phase is stabilized are shown by red. In the blue region, impurity CaO coexists with the infinite-layer phases. (a), (b), (c), and (d) contain 36, 34, 78, and 29 different samples, respectively.

Figure 10

$R_{\text{H}}$ taken at 300 K as a function of $x$ for infinite-layer ${\mathrm{Ca}}_{1-x}{R}_x{\mathrm{CuO}}_2$ thin films. For comparison, data for superconducting ${\mathrm{Sr}}_{0.9}{\mathrm{La}}_{0.1}{\mathrm{CuO}}_2$ (hollow circle) [61] are also shown. The dashed red line is a linear fit to the four $R_{\text{H}}$ values. For $x_{\text{cri}}$ = 0.096, the $R_{\text{H}}$ is expected to become positive. The solubility limits are also shown for ${\mathrm{Ca}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CuO}}_2$ and ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{CuO}}_2$ at 0.06 and 0.08, respectively.