Counterintuitive inverse superconducting transition beyond 4He-cooling limit

Summary

This paper reports inverse superconducting transitions realized beyond the liquid-helium cooling limit in Eu-based infinite-layer nickelates (Eu_xNd₁‑xNiO₂ and Eu_xPr₁‑xNiO₂). Through magnetic-field tuning, the zero-resistance superconducting state is observed to be confined between a lower critical temperature (T_c‑inv ≈ 2.6–5.4 K) and a higher normal T_c in both overdoped and underdoped regions; raising the temperature or increasing the current density can drive the system from a resistive state into superconductivity, which then vanishes again at higher temperatures and currents. Systematic temperature-dependent transport measurements reveal that this inverse superconducting transition in the Kelvin range arises from the temperature-driven alternating dominance of a compensating effective magnetic field associated with Eu²⁺ 4f⁷ moments and the upper critical field, supported by a temperature-induced re-entrant superconductivity phenomenon where superconductivity reappears at around 300 mK under an applied magnetic field. This work establishes a high-temperature superconductor system with magnetically reconstructed interactions as a platform for exploring quantum phenomena that reverse the paradigm of thermal decoherence, and opens application avenues for the inverse design of quantum phase-transition devices.

Materials

• Eu_xNd_1-xNiO₂
• Eu_xPr_1-xNiO₂

Methods

• Electrical resistivity measurements
• Magnetic field-dependent transport
• Current-voltage (I-V) measurements
• High-field magneto-transport
• X-ray diffraction (XRD)
• X-ray photoelectron spectroscopy (XPS)
• STEM
• Ginzburg-Landau modeling
• Fischer model fitting
• Werthamer-Helfand-Hohenberg (WHH) theory

Keywords

• inverse superconducting transition
• temperature induced reentrant superconductivity
• jaccarino peter compensation
• upper critical field
• thermal decoherence reversal

Highlights

• First realization of inverse superconducting transitions above the liquid-helium cooling limit (T_c-inv ≈2.6–5.4 K) in Eu-based infinite-layer nickelate thin films.
• Superconductivity is confined between a lower inverse critical temperature and a higher conventional critical temperature, and can be activated by both temperature rise and increasing current density.
• Direct observation of temperature-induced reentrant superconductivity—superconductivity reappears upon cooling to ≈100–300 mK after vanishing at a lower temperature—unprecedented in known superconducting systems.
• The inverse transition emerges at both overdoped and underdoped boundaries, while optimum doping screens it, highlighting the role of magnetic Eu₂+ 4f7 moments and exchange-field compensation.
• The work establishes magnetic-interaction-reconfigured high-T_c superconductors as a platform for quantum phenomena that reverse the thermal decoherence paradigm.

Conclusions

• Field-modulated inverse superconducting transitions above the 4He-cooling limit are identified in Eu_xRE_1-xNiO₂ thin films at both overdoped and underdoped phase boundaries of the superconducting dome.
• The inverse superconducting transitions are driven by a lower-threshold critical temperature T_c-inv to reach zero resistance, which is eliminated at the normal critical temperature T_c.
• Superconductivity can also be triggered by current density from a resistive state below T_c-inv, and further vanishes at a higher current threshold.
• The temperature-induced alternating dominance between the effective magnetic field and the upper critical field Hc₂ plausibly explains the inverse superconducting transition in the Kelvin range.
• Reemerged superconductivity below sub-Kelvin temperatures reveals an extended framework, giving rise to temperature-induced reentrant superconducting phenomenon.
• These collective findings establish magnetic ordering intertwined high-T_c systems as a fertile platform for exploring quantum phenomena that overcome thermal decoherence, redefining architectures for quantum-phase-transition devices.

Main claims

• Field-modulated inverse superconducting transitions are realized above the 4He-cooling limit in Eu-based infinite-layer nickelates.
  • Evidence: ρ-T curves show zero resistance confined between T_c-inv and T_c under magnetic fields (Fig. 1a,b).,The inverse transition is reversible and path-independent (inset of Fig. 1b, Suppl. Fig. 5).
• Superconductivity can be triggered not only by cooling below T_c but also by heating above T_c-inv or by increasing current density.
  • Evidence: Temperature-dependent resistivity shows superconducting state emerging upon warming through T_c-inv (Fig. 1b).,Current-voltage characteristics demonstrate a drop to zero voltage at a finite current threshold (Extended Data Fig. 3).
• The inverse transition is plausibly explained by temperature-driven alternating dominance of the effective magnetic field (Htot) relative to the upper critical field (Hc₂).
  • Evidence: Plots of Htot(T) and Hc₂(T) intersect twice at fields 1–2 T, defining T_c-inv and T_c (Fig. 2b).,Fischer-type WHH fitting of high-field phase boundaries (Fig. 2a).
• A temperature-induced reentrant superconductivity appears below ≈100–300 mK under magnetic fields, extending beyond the Jaccarino-Peter compensation framework.
  • Evidence: Low-temperature ρ-T measurements reveal re-emergence of superconductivity below T c' ≈100–300 mK (Fig. 4a).,The H-T phase diagram deviates from the J-P + mean-field prediction at low T (Fig. 4b).
• Magnetic-interaction-reconfigured high-T_c systems serve as fertile platforms for exploring quantum phenomena that reverse the thermal decoherence paradigm.
  • Evidence: The observation of both inverse and reentrant superconductivity in a high-T_c system implies new design principles for quantum devices.,The negative pairing strength associated with the inverse transition (−0.19 T/K) hints at magnetically promoted pairing.

Workflow

• sample_synthesis — Epitaxial thin films of Eu-based infinite-layer nickelates were successfully grown on a special substrate.
  • Materials: Eu; Nd; Pr; perovskite precursor films; special substrate
  • Methods: thin film growth; soft chemical reduction
  • Observations: synthesized Eu_xNd_1-xNiO₂ and Eu_xPr_1-xNiO₂ thin films
• transport_measurement — Inverse superconducting transitions were observed above the liquid-helium cooling limit, driven by temperature, magnetic field, and current density.
  • Materials: synthesised thin films; cryostat; superconducting magnet
  • Methods: temperature-dependent resistivity (ρ-T); field-dependent resistivity (ρ-H); current-voltage (I-V) characteristics
  • Observations: inverse superconducting transition at T_c-inv ≈2.6–5.4 K with zero resistance between T_c-inv and T_c; field-reentrant superconductivity; current-driven transition from resistive to superconducting state; temperature-induced reentrant superconductivity below ≈100–300 mK
• phenomenological_analysis — The inverse superconducting transition is explained by a temperature-dependent crossover in dominance between effective magnetic field and upper critical field.
  • Materials: high-field magnetotransport data
  • Methods: Fischer-type modified Ginzburg-Landau fitting; Brillouin function for exchange field HJ(T); Werthamer-Helfand-Hohenberg (WHH) theory for Hc₂(T)
  • Observations: two intersection points between Htot(T) and Hc₂(T) at fields 1–2T; Hc₂ upturn at low temperatures
• phase_diagram_construction — Inverse superconducting transition emerges near the edges of the superconducting dome where magnetic interactions are enhanced.
  • Materials: transport data for multiple Eu compositions
  • Methods: construction of superconducting phase diagram in temperature-field-composition space
  • Observations: inverse superconducting transitions occur only at underdoped and overdoped boundaries; optimally doped samples show no inverse transition or field-reentrance