Soft point-contact Andreev reflection spectroscopy in a palm-type cubic anvil-pressure cell

摘要

研究人员将软点接触安德列夫反射谱技术集成到棕榈型立方压砧高压腔中，通过基底锚定与外部导线分线策略，在高达15 GPa的静水压下稳定构建多个点接触结。对单质超导体Nb的基准测量验证了方法的可靠性，得到零温超导能隙比2Δ(0)/k_B T_c≈3.3。进一步应用于kagome金属超导体CsCr₃Sb₅和双层镍酸盐超导体La₂PrNi₂O₇，观测到与常规BCS超导体截然不同的尖锐零偏压电导峰，并系统研究了其随温度、磁场和压力的演化。分析表明，这些谱学特征与非常规超导电性及可能的d波配对对称性相符，为理解其配对机制提供了直接谱学证据。该工作成功搭建了连接宏观电输运和微观光谱探针的高压实验平台，为广泛探索压力诱导非常规超导体的配对对称性开辟了新途径。

材料

• Nb
• CsCr₃Sb₅
• La₂PrNi₂O₇

方法

• Soft point-contact Andreev reflection spectroscopy
• Cubic-anvil pressure cell
• Substrate anchoring
• External wire-splitting
• Resistivity measurements
• Lock-in amplifier modulation technique

关键词

• unconventional superconductivity
• zero bias conductance peak
• d wave pairing symmetry
• pressure induced superconductivity
• point contact spectroscopy
• andreev reflection

亮点

• Achieves nearly 100% success rate for stable junction formation under pressure.
• First implementation of PCARS in a cubic-anvil cell, extending the technique to 15 GPa.
• Sharp zero-bias conductance peaks in CsCr₃Sb₅ and La₂PrNi₂O₇ indicate unconventional pairing, possibly d-wave.
• The method provides direct spectroscopic insight into pairing symmetry of pressure-induced superconductors.

结论

• Soft PCARS implemented in a cubic-anvil cell enables stable formation of multiple point-contact junctions under hydrostatic pressures up to 15 GPa.
• Benchmark measurements on Nb reproduce known gap parameters with 2Δ(0)/k_B T_c ≈ 3.3.
• Pronounced zero-bias conductance peaks in CsCr₃Sb₅ and La₂PrNi₂O₇ provide spectroscopic evidence consistent with unconventional superconductivity.
• The platform bridges macroscopic transport and microscopic spectroscopic probes for pressure-induced unconventional superconductors.

主要论断

• Soft PCARS can be stably implemented in a CAC using substrate anchoring and external wire-splitting.
  • 证据: Benchmark measurements on Nb demonstrate high reproducibility and yield a zero-temperature gap with gap ratio 3.3.,Stable formation of multiple point-contact junctions under hydrostatic pressures up to 15 GPa, with nearly 100% success rate.
• PCARS spectra on CsCr₃Sb₅ show a zero-bias conductance peak, providing spectroscopic evidence consistent with unconventional superconductivity.
  • 证据: Pronounced ZBCP observed, evolution with temperature, magnetic field and pressure investigated.,Fits using a phenomenological d-wave BTK model capture essential spectral features.
• PCARS on La₂PrNi₂O₇ at 13.5 GPa reveals a ZBCP, suggesting unconventional pairing.
  • 证据: Well-defined Andreev reflection spectra displaying a sharp ZBCP.,Fits using a nodal d-wave model capture the ZBCP reasonably well; extracted gap Δ(0)≈6 meV.
• The CAC-based PCARS platform bridges macroscopic transport and microscopic spectroscopic probes, opening new avenues for investigating pairing symmetry in pressure-induced superconductors.
  • 证据: The method extends high-pressure studies from macroscopic transport to microscopic gap structure and pairing symmetry, demonstrated on multiple unconventional superconductors.

研究流程

• technique_integration — Soft PCARS is successfully integrated into a CAC, enabling stable junction formation under high pressure.
  • 材料: palm-type cubic anvil pressure cell; insulating substrate (Al₂O₃, SiO₂); gold wires; silver paste
  • 方法: substrate anchoring; external wire-splitting
  • 观察: stable multiple point-contact junctions; nearly 100% success rate
• benchmark_measurements_on_Nb — Extracted gap Δ(0)=1.3 meV, ratio 2Δ/kTc≈3.3, consistent with known s-wave superconductivity, validating the setup.
  • 材料: elemental superconductor Nb
  • 方法: temperature-dependent PCARS; s-wave BCS model fitting
  • 观察: coherence peaks at ±1.3 meV; dip features at higher bias; spectra vanish above T_c
• spectroscopy_of_CsCr3Sb5 — ZBCP suggests unconventional superconductivity, possibly d-wave pairing, with Δ(0)=2.3 meV and strong-coupling ratio 2Δ/kTc≈8.2.
  • 材料: kagome superconductor CsCr₃Sb₅
  • 方法: PCARS under pressure; temperature sweeps; d-wave BTK model fitting
  • 观察: sharp zero-bias conductance peak (ZBCP); broadening and decreasing amplitude with temperature
• spectroscopy_of_La2PrNi2O7 — ZBCP indicates unconventional pairing; extracted gap Δ(0)≈6 meV, though possibly from filamentary superconductivity.
  • 材料: polycrystalline bilayer nickelate La₂PrNi₂O₇
  • 方法: pressure-dependent PCARS; background normalization; d-wave model fitting
  • 观察: ZBCP at 13.5 GPa; hump persists to ≈80K