Probing the connections between superconductivity, stripe order, and structure in La${}_{1.905}$Ba${}_{0.095}$Cu${}_{1-y}$Zn${}_y$O${}_4$

The superconducting system La${}_{2-x}$Ba${}_x$CuO${}_4$ is known to show a minimum in the transition temperature $T_c$ at $x=\frac{1}{8}$ where maximal stripe order is pinned by the anisotropy within the CuO${}_2$ planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For $x=0.095$, where $T_c$ reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here, we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1$\%$ Zn substituted for Cu, which reduces $T_c$ to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO${}_2$ layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.

Figure 1

(a) Plots of ${\rho }_{ab}$ (diamonds) and ${\rho }_c$ (circles, line) for LBCO $x=0.095$ in ${\mu }_0{H}_{\perp }=0$ T (open symbols), 1 T (line), and 3 T (filled symbols), from paper I. (b) Field-cooled susceptibility measured in ${\mu }_0{H}_{\perp }=0.01$ T, from Ref. 34. The vertical dashed lines mark 27 and 35 K.

Figure 2

Peak intensity (left axis, circles) and half width at half maximum (right axis, diamonds) as a function of temperature for the (032) superlattice peak at temperatures near 300 K for the Zn0 (open symbols) and Zn1 (filled symbols) samples. Error bars represent $\pm$1 standard deviation throughout the paper.

Figure 3

(a) and (b) Two scans through the LTO peaks of the Zn0 crystal at 54 K with trajectories indicated in (d). (c) Schematic of the orthorhombic (solid line) and high-temperature tetragonal (dashed line) unit cells; large and small circles indicate Cu and O sites, respectively. In this work, the orthorhombic notation is used. In (d), the filled and open circles indicate the strong and weak, respectively, (200) and (020) Bragg peaks that are present simultaneously due to twinning. The strong twin pair is labeled T${}_{1a}$ and T${}_{1b}$; the weak pair are T${}_{2a}$ and T${}_{2b}$. Arrows indicate directions of scans in (a) and (b); the dashed lines connect a pair of (200) and (020) peaks belonging to a single domain.

Figure 4

Diagonal scans through the T${}_1$ peaks of the LTO phase of the Zn0 crystal at temperatures of (a) 54 K, (b) 33 K, (c) 27 K, and (d) 12 K, showing the evolution from the split peaks of the LTO phase to the single peak of the LTLO phase. Insets show the schematics of the peak patterns, and the scan direction; closed circles: strong peaks; gray circles: weaker peaks; open circles, weakest peaks.

Figure 5

(a) Normalized intensities of the (200) peaks [LTO T${}_{1b}$ peak (triangles), LTLO peak (diamonds)] and the (110) superlattice peak of the LTLO phase (circles) as a function of temperature. Vertical dashed lines indicate the main region of the transition. (b) Orthorhombic strain vs temperature. (c) Temperature dependence of the inelastic scattering intensity measured at $\mathbf{Q}=(0.01,3,2)$ and $\hslash \omega =1$ meV. The solid line is a guide to the eye.

Figure 6

LTO to LTLO transition in the Zn1 sample. Normalized peak intensities for a ${\left(200\right)}_{\mathrm{LTO}}$ peak (triangles) and the ${\left(200\right)}_{\mathrm{LTLO}}$ (diamonds) as a function of temperature.

Figure 7

(a) Scans through one of the spin-stripe-order peaks along the direction shown in the inset for Zn1 (diamonds) and Zn0 (circles) at $T=5$ K. To remove background, scans performed at 50 K have been subtracted. Lines through data are Gaussian peak fits. (b) Integrated intensity of the spin-stripe-order peak as a function of temperature for the two samples. The intensities for the Zn0 sample in a $c$-axis magnetic field are indicated by filled circles (from paper I). Error bars reflect counting statistics. Lines through the data are guides to the eye.

Figure 8

Scans through $(0.9,0.1,0)$ for the Zn1 sample under zero (diamonds) and 7-T magnetic field (triangles) at 4 K with intensities normalized to the results of the La${}_{1.875}$Ba${}_{0.125}$CuO${}_4$ sample in zero field (Ref. 47). Background measured at 50 K has been subtracted. Lines through data are fitted Gaussians.

Figure 9

Bulk susceptibility for Zn0 (circles) and Zn1 (diamonds) obtained under zero-field-cooling conditions in a 2-Oe field perpendicular to the $ab$ plane.

Figure 10

Fluctuation diamagnetism obtained using Eq. (2), based on bulk susceptibility measurements performed with a field of 100 Oe. Inset shows same data shifted by $T_c=32$ K for Zn0 and 17 K for Zn1. The unit emu/mol is equivalent to 4$\pi$cm${}^3/$mol.

Figure 11

Plot of $\Delta {\chi }^s$, as given by (3), denoted by (red) circles. Solid line is a calculated curve corresponding to a constant plus a Curie term (described in text), with the constant equal to $-2\times {10}^{-6}$ emu/mol. The solid and dashed lines correspond to $S_{\mathrm{eff}}(S_{\mathrm{eff}}+1)=0.14$ and 0.25, respectively.

Figure 12

Thermal conductivity measured parallel to the planes ${\kappa }_{ab}$ with field perpendicular to the planes for (a) Zn0, (c) Zn1. Thermoelectric power $S_{ab}$ divided by temperature $T$ for (b) Zn0, (d) Zn1. Vertical dashed lines denote the characteristic temperature 27 K, as explained in the main text.

Figure 13

(a) Comparison of ${\rho }_c\left(T\right)$ (circles, data from Ref. 51) and $S_{ab}/T$ [diamonds, from Fig. 12b] for Zn0 in zero magnetic field. (b) Temperature derivatives of ${\rho }_c\left(T\right)$ (circles) and of $S_{ab}/T$ (diamonds).