Bosonic Phases Demonstrate 2e Cooper Pairing across Superconductor-Insulator Transitions

Scientists are increasingly focused on understanding the complex interplay between superconductivity and insulating behaviour in novel materials. Menghan Liao from the Beijing Key Laboratory of Fault-Tolerant Quantum Computing, Heng Wang from the Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area, and Mingwei Yang from the Department of Physics, City University of Hong Kong, et al., have now demonstrated bosonic phases emerging across the superconductor-insulator transition in infinite-layer samarium nickelate films by carefully controlling spatially periodic network patterns. This research is significant because it provides direct evidence of 2e Cooper pairing in nickelates , observed through magnetoresistance oscillations with a periodicity of h/2e , and reveals the dynamic roles of vortices in the material’s ground states. By establishing nickelates as a key platform for investigating bosonic landscapes controlled via Cooper pair coherence, this work promises to advance our understanding of unconventional superconductivity itself.

The research team achieved precise control over the superconducting state by fabricating nanopatterned films, inducing superconductor-insulator transitions and observing unique magneto-resistance oscillations. Experiments show that these phase transitions are predominantly driven by enhanced superconducting fluctuations, with Cooper pairs actively participating in charge transport across the transitions. One phase appears in finite magnetic fields, while the other remarkably persists down to zero magnetic field, both characterized by bosonic excitations which suggest the dynamic roles of Vortices at the ground states.

These observations indicate that the behaviour of Cooper pairs is intricately linked to the emergence of these metallic states and the underlying bosonic excitations. Researchers fabricated ~9nm-thick Sm0.95-xEuxCa0.05NiO2 films using pulse laser deposition and topotactic reduction on (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates, then employed anodized aluminum oxide masks and reactive ion etching to create the desired network patterns. Furthermore, the study demonstrates that enhanced superconducting fluctuations significantly modulate the transport properties of these Cooper pairs, leading to the observation of these intriguing bosonic phases. The discovery of two anomalous metallic states, one sensitive to magnetic fields and another persisting even at zero field, provides valuable insights into the interplay between superconductivity, bosonic excitations, and vortex dynamics within nickelate superconductors. This research opens new avenues for understanding and potentially harnessing the unique properties of these materials for future technological applications.

Nickelate Networks Fabricated via Reactive Ion Etching exhibit

The research team fabricated networks of superconducting islands connected by weak links, meticulously controlling the degree of etching to tune the film’s connectivity and induce the SIT. Initially, pristine Sm0.95-xEuxCa0.05NiO2 films were grown on (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates, then covered with AAO masks before undergoing RIE to pattern the nickelate material. Following the etching process, electrical transport measurements were performed in a cryostat, alternating between etching and measurement cycles to obtain a comprehensive dataset of resistance versus temperature (Rs(T)) at varying etching levels, specifically, samples labelled S1#0 through S1#5, with S1#0 representing the unetched film. This iterative approach enabled the team to track the evolution of superconductivity as the film transitioned from a continuous superconducting state to an insulating state, meticulously documenting changes in the superconducting transition temperature (Tc).

The team measured Rs(T) for each etching step, identifying kinks in the curves to pinpoint Tc, onset around 20 K for the pristine sample, and observing zero resistance at Tc0 ≈ 7.5 K. Sheet resistance at 50 K (Rs, 50 K) was then plotted against total etching time, revealing a clear correlation between increased etching and suppressed superconductivity, the data showed that as etching time increased, resistance rose and superconductivity diminished, with S1#3 exhibiting metallic behaviour and S1#4 entering a fully insulating state. X-ray diffraction (XRD) measurements confirmed that the crystal structure of the superconducting islands remained largely unaffected by the etching process, indicating that the SIT was driven by disorder rather than structural changes.

2e Cooper pairing and bosonic transport observed

Experiments revealed that these oscillations are predominantly driven by enhanced superconducting fluctuations, with Cooper pairs actively involved in charge transport across the networks. Researchers fabricated nanopatterned nickelate networks using ~9nm-thick Sm0.95-xEuxCa0.05NiO2 films (with x = 0.24, 0.2 for sample S1 and S2, and x = 0.22 for samples S3 and S4) on (LaAlO3)0.3(Sr2TaAlO6)0.7 substrates via pulse laser deposition and topotactic reduction. Electrical transport measurements were performed on the films as etching progressed, revealing a clear superconductor-insulator transition. The pristine film (S1#0) exhibited a superconducting transition with Tc, onset around 20 K, achieving zero resistance at Tc0 ≈ 7.5 K.

As etching time increased, the resistance rose, and superconductivity was suppressed; S1#3 displayed metallic behaviour below Tc, onset, while S1#4 entered a fully insulating state. The study meticulously tracked the evolution of resistance with etching, demonstrating that the sheet resistance at 50 K (Rs, 50 K) increased as etching progressed, while superconducting transition temperatures decreased. Specifically, Tc0 and Tc, 50%Rn rapidly dropped to zero as Rs, 50 K increased, whereas Tc, onset experienced a milder decrease from 20 K to 15 K despite a 20-fold increase in Rs, 50 K. These measurements confirm that etching primarily weakens the coupling between superconducting islands, while the pairing strength within the islands remains relatively unaffected.

Further investigations into the magnetic field dependence of resistance revealed that the pristine sample (S1#0) maintained zero resistance below 0.8 T. In contrast, S1#2 showed a suppression of dissipationless transport with a small magnetic field. Remarkably, S1#3, exhibiting finite resistance, displayed resistance oscillations with a period of approximately 0.23 T, remaining visible even in the insulating regime (S1#4). These oscillations, also observed in sample S2, provide compelling evidence for the bosonic nature of charge carriers and the crucial role of Cooper pairs in the nickelate networks.

Nickelate Superconductivity via Bosonic Excitations and Pairing reveals

The findings reveal a phase diagram dependent on disorder and temperature, showcasing transitions from dissipationless superconductivity to an anomalous metallic state, then to Cooper pair insulating behaviour. The authors acknowledge limitations related to estimating phase boundaries based on experimental data from a single sample, and suggest further research could explore the behaviour of samples with varying stoichiometries. Future studies might also investigate the precise mechanisms driving the observed quantum creep of vortices and the coupling between bosonic and fermionic modes to fully understand the observed phenomena.