High temperature superconductivity with giant pressure effect in 3D networks of boron doped ultra-thin carbon nanotubes in the pores of ZSM-5 zeolite (2509.19255v2)

Abstract: We have fabricated three-dimensional (3D) networks of ultrathin carbon nanotubes (CNTs) within the ~5-Angstrom diameter pores of zeolite ZSM-5 crystals using the chemical vapour deposition (CVD) process. The 1D electronic characteristics of ultrathin CNTs are characterized by van Hove singularities in the density of states. Boron doping was strategically employed to tune the Fermi energy near a van Hove singularity, which is supported by extensive ab-initio calculations, while the 3D network structure ensures the formation of a phase-coherent bulk superconducting state under a 1D to 3D crossover. We report characteristic signatures of superconductivity using four complementary experimental methods: magnetization, specific heat, resistivity, and point-contact spectroscopy, all consistently support a critical temperature Tc at ambient conditions ranging from 220 to 250 K. In particular, point-contact spectroscopy revealed a multigap nature of superconductivity with a large ~30 meV leading gap, in rough agreement with the prediction of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity. The differential conductance response displays a particle-hole symmetry and is tuneable between the tunnelling and Andreev limits via the transparency of the contact, as uniquely expected for a superconductor. Preliminary experiments also reveal a giant pressure effect which increases the Tc above the ambient temperature.

• The paper reports that boron-doped ultra-thin CNT networks in ZSM-5 zeolite exhibit high-Tc superconductivity (220-250 K) with distinct multigap features.
• It employs CVD synthesis and ab-initio calculations to achieve a ~94% pore-filling ratio, accurately characterizing CNT structure, channel geometry, and doping effects.
• The study demonstrates a giant pressure effect where minimal applied pressure (~0.1 kbar) shifts Tc significantly, enabling ambient-temperature superconductivity.

High-Temperature Superconductivity in Boron-Doped Ultra-Thin Carbon Nanotube Networks Confined in ZSM-5 Zeolite

Introduction

This work presents the synthesis and characterization of three-dimensional networks of boron-doped, ultra-thin carbon nanotubes (CNTs) embedded within the sub-nanometer pores of ZSM-5 zeolite. The paper demonstrates robust signatures of high-temperature superconductivity at ambient pressure, with critical temperatures ($T_c$) ranging from 220 to 250 K, and a resistive onset as high as 278 K. The superconducting state is confirmed by four complementary experimental techniques: magnetization, specific heat, resistivity, and point-contact spectroscopy. Notably, the system exhibits a giant pressure effect, with minimal applied pressure raising $T_c$ above ambient temperature. The findings challenge conventional limits on phonon-mediated superconductivity and provide a new platform for exploring high-$T_c$ phenomena in carbon-based nanostructures.

Material Design and Synthesis

The ZSM-5 zeolite template, with its interconnected 3D pore network (diameters $\sim$5 Å), enables the growth of single-walled CNTs with diameters below 0.3 nm. The chemical vapor deposition (CVD) process, utilizing in-situ generated diborane and methane, yields a high pore-filling ratio ($\sim$94%) of boron-doped CNTs. Raman spectroscopy and ab-initio calculations confirm the presence of two types of CNTs: (2,1) and (3,0), corresponding to the distinct channel geometries of ZSM-5. Boron doping is quantified at $\sim$10 at%, and ab-initio calculations show that the Fermi level is tuned near a van Hove singularity in the (2,1) CNTs, enhancing the electronic density of states (EDOS) and electron-phonon coupling.

Superconducting Properties

Point-Contact Spectroscopy

Energy-resolved point-contact spectroscopy reveals a multigap superconducting state, with three distinct gaps: a leading gap of $\sim$30 meV, and secondary gaps at 14–17 meV and 6 meV. The spectra exhibit particle-hole symmetry and can be tuned between the tunneling and Andreev reflection regimes, consistent with the Blonder-Tinkham-Klapwijk (BTK) formalism. The temperature dependence of the largest gap closely follows the BCS prediction for $T_c \approx 224$ K, with the gap vanishing near 230 K. The absence of coherence peaks in the tunneling limit is attributed to disorder and the quasi-1D nature of the system.

Specific Heat

Specific heat measurements show a small second-order phase transition anomaly at 233–236 K, superposed on a large phonon background. The transition broadens under applied magnetic fields, indicative of strong superconducting fluctuations and a fluctuation-dominated transition, analogous to cuprate superconductors. The magnitude of the anomaly is consistent with expectations for high-$T_c$ systems, where the electronic contribution is a small fraction of the total specific heat.

Electrical Resistance

Two-probe resistance measurements reveal a sharp drop by nearly two orders of magnitude below 278 K, with a residual resistance attributed to grain boundaries and insulating regions in the zeolite matrix. The onset of phase coherence, estimated by extrapolating the steepest slope, yields $T_c \approx 239$ K. Application of modest pressure ($\sim$0.1 kbar) via screw compression shifts the transition onset by over 100 K, enabling ambient-temperature superconductivity. The pressure dependence is reproducible and suggests a strong coupling between lattice deformation and electronic structure.

DC Magnetization

DC magnetization measurements under zero-field-cooled (ZFC) and field-cooled (FC) conditions show a weak but characteristic Meissner effect, with onset temperatures between 220 and 250 K. The signal is limited by the small crystallite size relative to the London penetration depth, resulting in incomplete screening. The difference between ZFC and FC branches, and occasional paramagnetic Meissner effects, are consistent with surface superconductivity and fluctuation effects.

Mechanism and Theoretical Considerations

Ab-initio calculations confirm the metallic nature of both (2,1) and (3,0) CNTs, with boron doping shifting the Fermi level to the vicinity of a van Hove singularity in (2,1) CNTs. The proximity to the singularity enhances EDOS and electron-phonon coupling, favoring high-$T_c$ superconductivity per the McMillan formula. The phonon spectrum of (2,1) CNTs exhibits imaginary frequencies, indicating dynamic instability that is stabilized by confinement within the zeolite matrix. The multigap nature is attributed to multiple bands crossing the Fermi level, and the 3D network structure promotes phase coherence via a dimensional crossover from 1D to 3D.

The giant pressure effect is hypothesized to arise from pressure-induced shifts in the flat band near the Fermi level and deformation of the CNTs and zeolite matrix, further enhancing EDOS and electron-phonon interactions. The small separation ($\sim$1.3 Å) between perpendicular CNTs at channel intersections facilitates the 1D–3D crossover, suppressing phase fluctuations and raising $T_c$.

Experimental Trade-Offs and Limitations

• Residual Resistance: Grain boundaries and insulating regions in the zeolite matrix prevent complete zero resistance, even in four-probe measurements. Sample optimization and grain boundary engineering are necessary for further improvement.
• Meissner Effect: The weak magnetization signal is a consequence of the small crystallite size and incomplete screening. Larger crystals and improved sample morphology may enhance the signal.
• Pressure Control: The screw-based pressure application is non-hydrostatic and introduces gradients; more precise hydrostatic pressure experiments are needed for quantitative analysis.
• Pairing Mechanism: While the evidence supports phonon-mediated pairing, the exact nature of the pairing symmetry and the possible existence of a pseudogap phase remain open questions.

Implications and Future Directions

The demonstration of high-$T_c$ superconductivity in a carbon-based, ambient-pressure system with a giant pressure effect has significant implications for both fundamental physics and practical applications. The results challenge the conventional McMillan limit for phonon-mediated superconductors and suggest that engineered nanostructures with enhanced EDOS and strong electron-phonon coupling can achieve $T_c$ values approaching or exceeding room temperature.

Future research should focus on:

• Sample Optimization: Increasing crystal size, improving grain boundary connectivity, and surface treatments to reduce residual resistance.
• Pressure Studies: Systematic hydrostatic pressure experiments to elucidate the pressure dependence of $T_c$ and the underlying mechanism.
• Pairing Symmetry: Advanced spectroscopic studies (e.g., tunneling, Nernst effect) to probe the pairing symmetry and possible pseudogap phenomena.
• Theoretical Modeling: Numerical solutions of the Ginzburg-Landau equations in network geometries to understand phase coherence and fluctuation effects.
• Device Integration: Exploration of CNT@ZSM-5 composites for superconducting electronics, sensors, and energy applications.

Conclusion

This paper provides compelling evidence for high-temperature superconductivity in boron-doped ultra-thin CNT networks confined within ZSM-5 zeolite, with $T_c$ values up to 250 K at ambient pressure and a resistive onset above 0°C. The system exhibits a multigap superconducting state, robust against strong magnetic fields, and a giant pressure effect that enables ambient-temperature superconductivity with minimal applied pressure. The findings open new avenues for the design of carbon-based superconductors and highlight the potential of engineered nanostructures for achieving room-temperature superconductivity. Further experimental and theoretical investigations are warranted to fully elucidate the pairing mechanism and optimize material properties for practical applications.