Bosonic phases across the superconductor-insulator transition in infinite-layer samarium nickelate

Superconductivity arises from the global phase coherence of Cooper pairs. Modulation of phase coherence leads to quantum phase transitions, serving as an important tool for studying unconventional superconductivity. Here, we demonstrate bosonic phases across the superconductor-insulator transition in infinite-layer nickelate superconducting films by the control of spatially periodic network patterns. Magnetoresistance oscillations with a periodicity of h/2e provide direct evidence of 2e Cooper pairing in nickelates. The phase transition is predominantly driven by enhanced superconducting fluctuations, and Cooper pairs are involved in charge transport across the transition. Notably, we observe two types of anomalous metallic phases, emerging respectively at finite magnetic fields and down to zero magnetic field. They can be characterized by bosonic excitations, suggesting the dynamic roles of vortices in the ground states. Our work establishes nickelates as a key platform for investigating the rich landscape of bosonic phases controlled via the phase coherence of Cooper pairs.

💡 Research Summary

In this work the authors investigate the superconductor‑insulator transition (SIT) in infinite‑layer samarium nickelate (Sm₀.₉₅₋ₓEuₓCa₀.₀₅NiO₂) thin films by fabricating spatially periodic nanostructured networks. The films, about 9 nm thick, are grown by pulsed‑laser deposition and topotactic reduction on (LaAlO₃)₀.₃(Sr₂TaAlO₆)₀.₇ substrates. Anodized‑aluminum‑oxide (AAO) masks with a triangular array of 50 nm holes spaced 100 nm apart are transferred onto the films, and reactive‑ion etching (RIE) is used to progressively remove material. By repeating the etch‑measure cycle, a series of samples with increasing disorder and decreasing inter‑island coupling are obtained, labelled S1#0 (pristine) through S1#5 (highly etched).

Transport measurements reveal that the pristine film shows a conventional superconducting transition with an onset around 20 K and zero‑resistance at ≈7.5 K. As the etching time increases, the sheet resistance at 50 K (Rₛ,50 K) rises dramatically, while the onset temperature only modestly drops to ≈15 K. The temperatures at which the resistance reaches 50 % and 1 % of the normal‑state value collapse to zero once Rₛ,50 K exceeds a critical value, indicating that the pairing amplitude within each superconducting island remains robust whereas the global phase coherence between islands is progressively weakened. Kinks in the Rₛ(T) curves near 15–20 K persist even after zero resistance disappears, suggesting that Cooper pairs survive in the islands throughout the transition.

A key discovery is the observation of magnetoresistance (MR) oscillations with a period of ≈0.23 T, corresponding precisely to one superconducting flux quantum (Φ₀ = h/2e) per unit cell of the honey‑comb network (area ≈8 660 nm²). These oscillations are seen both in the metallic regime (sample S1#3) and in the insulating regime (S1#4). The amplitude grows as the temperature is lowered from 3 K, saturating below 0.5 K. By fitting the amplitude to a standard expression for phase‑coherent transport, the authors extract a phase‑coherence length ξφ of about 65 nm in the metallic state and ~40 nm in the insulating state. The magnitude and temperature dependence of the oscillations are far larger than expected from the Little‑Parks effect, leading the authors to attribute them to a Josephson‑junction‑array (JJA) mechanism in which the tunnelling rate of Cooper pairs is periodically modulated by the applied magnetic field.

Two distinct anomalous metallic (AM) phases are identified. The first appears under finite magnetic fields (0.2–2 T) in sample S1#2. In this regime the resistance saturates to a finite value as temperature approaches zero, and an Arrhenius plot shows a linear high‑temperature region consistent with thermally activated vortex creep. The activation energy follows a power‑law dependence on field, and Hall measurements confirm that the carrier type does not change across the transition. The second AM phase emerges at zero field in the intermediate region of the SIT (sample S1#3). Here the resistance exhibits a linear temperature dependence over a broad range, reminiscent of a “bosonic strange metal” where transport is dominated by incoherent Cooper pairs. Upon further cooling, enhanced superconducting fluctuations eventually drive the system into a Cooper‑pair insulator, where transport is governed by localized bosons. Both AM states involve Cooper pairs, but differ in the balance between phase coherence and vortex dynamics.

Overall, the study demonstrates that disorder‑induced fragmentation of an infinite‑layer nickelate film into a periodic network provides a powerful knob to tune the competition between pairing amplitude, phase coherence, and vortex excitations. The h/2e MR oscillations give unambiguous evidence that Cooper pairs participate in transport even when global superconductivity is destroyed. Enhanced superconducting fluctuations are shown to be the primary driver of the SIT, giving rise to a rich landscape of bosonic phases, including two anomalous metallic states with distinct microscopic origins. The work positions infinite‑layer nickelates as a versatile platform for exploring quantum phase transitions, bosonic transport, and the interplay of Cooper‑pair and vortex physics in a high‑temperature superconducting material.