Paramagnetically driven superconducting re-entrance in Eu-doped infinite layer nickelates

TL;DR

The paper demonstrates field-induced re-entrant superconductivity in Eu-doped NdNiO$_2$ infinite-layer nickelates arising from a delicate balance between Eu$^{2+}$ and Nd$^{3+}$ magnetic moments. It combines magnetotransport, Hall effect analysis, and X-ray magnetic dichroism with a two-component spin model and Gor'kov-based pair-breaking theory to connect rare-earth magnetism to the superconducting state via a total field $B_{ m tot}$ that includes exchange contributions. The results imply a novel Jaccarino-Peter-like compensation mechanism involving two distinct rare-earth species, and they reproduce the observed nonmonotonic $B_{c2}(T)$ behavior and re-entrant superconductivity. This work advances understanding of magnetism–superconductivity interplay in nickelates and suggests routes to tune superconductivity via magnetic rare-earth ions.

Abstract

The breakthrough discovery of superconductivity in infinite-layer nickelates, and subsequently in several superconducting nickelates with more complex layered structures, capped a search spanning more than two decades and opened an entirely new field of research. Significant efforts aim to increase the critical temperature, to determine the electronic structure of the system, the underlying pairing mechanism, and the similarities between this system and cuprates - Ni1+ in infinite-layer nickelates being isoelectronic to Cu2+ in high-Tc cuprates. Here, we explore the unique role of magnetic rare earth ions in superconducting Eu-doped NdNiO2. We show that the field-induced re-entrant superconductivity which we evidence in this compound is the result of a delicate balance between the competing effects of the Eu2+ and Nd3+ ions. Our analyses of the extraordinary Hall effect and modeling of the superconducting critical fields demonstrate that the influence of these ions on magneto-transport is only felt when they are polarized by a magnetic field.

Figures (5)

• Figure 1: (a) High-resolution X-ray diffraction scan around the (002) reflection of an optimally doped Nd$_{0.7}$Eu$_{0.3}$NiO$_2$ film grown on LSAT. The scan reveals an intense film peak, and the pronounced finite-size oscillations, marked by arrows, indicate high crystalline quality and smooth interfaces. The fit (red dashed line) yields an out-of-plane lattice parameter of 3.329 Å and an estimated film thickness of approximately 22 unit cells. The determined c-axis value is confirmed by fitting the atomic positions and measuring the out-of-plane lattice spacing in a high magnification STEM image shown in the inset. (b) Resistivity versus temperature measurements for an optimized Nd$_{0.7}$Eu$_{0.3}$NiO$_2$ film on LSAT showing a sharp superconducting transition with T$_c^{\%50 }$ = 9.5 K and a zero-resistance state below $\sim$ 6 K.
• Figure 2: (a) Resistivity versus temperature curves for an optimized Nd$_{0.7}$Eu$_{0.3}$NiO$_2$ film on LSAT under perpendicular magnetic fields ranging from 0 T to 3.5 T in the top panel and 4 to 12 T in the bottom one. A conventional suppression of the superconducting transition temperature $T_c$ is observed in the lower field range, followed by a re-entrant enhancement of superconductivity at higher fields. (b) Analogous measurement performed in the parallel magnetic field configuration, displaying the same re-entrance at around 3.5 T. This sample is from a different batch than the one whose properties are shown in Fig. \ref{['fig:1']}b).
• Figure 3: (a) and (b): Magnetic phase diagrams for out-of-plane (perpendicular) (a) and in-plane (parallel) (b) magnetic field configurations. Blue and red markers correspond respectively to the superconducting transition temperatures extracted using the 20% and 50% criteria (temperatures at which the resistivity is 20% and 50% of its value in the normal state at 15 K, as indicated by the dashed lines in Figure \ref{['fig:2']}). For both criteria, the solid markers correspond to measurements performed at the LNCMI-EMFL high-magnetic-field facility in Grenoble, while the open markers refer to measurements carried out at UNIGE on a different sample with the same composition. In both configurations, a low-field re-entrant superconducting behavior is observed, suggesting the presence of two superconducting regions, the second persisting up to the measured field of 30 T, separated (in some temperature range) by an intermediate dissipative phase. As can be seen and as expected, the field is less effective in suppressing superconductivity in the in-plane geometry. (c): Magnetoresistivity curves at fixed temperatures of 2 K and 4.2 K, for perpendicular and parallel geometries, respectively. Together with the data, a schematic physical picture representing the rare-earth spins configuration is presented. The light blue and purple insets illustrate respectively the Eu$^{2+}$ and Nd$^{3+}$ spin polarizations in each regime and their compensation as the external field is increased.
• Figure 4: Hall resistance R$_{xy}$(B) measured as a function of out-of-plane magnetic field in optimally doped Nd$_{0.7}$Eu$_{0.3}$NiO$_2$ thin films grown on different substrates. (a) Data from a non-superconducting optimally doped film grown on NdGaO$_3$, measured between 2.5 K and 100.5 K. The curves show a pronounced, temperature-dependent non-linearity, ascribed to a magnetic (extraordinary) Hall component possibly arising from the field-polarized Nd$^{3+}$ and Eu$^{2+}$ moments. Dashed lines represent fits using a two-component magnetic model described in the main text. (b) Data from a superconducting optimally doped film grown on LSAT, measured between 10 K and 120 K. Inset: temperature dependence of the extracted Hall coefficient $R_H$, obtained from fits (dashed lines) of the antisymmetrized data.
• Figure 5: Experimental and theoretical parallel and perpendicular critical fields versus temperature for the 50% resistivity criterion. The experimental data are shown in the main panel while the inset shows the theoretical $B_{c2}(T)$ obtained by solving the modified gap equation.