[Paper Review] Point-contact Andreev reflection spectroscopy of candidate topological superconductor Cu0.25Bi2Se3

X. Chen, Chao Huan+3 more|arXiv (Cornell University)|Oct 22, 2012

Topological Materials and Phenomena3 references10 citations

TL;DR

This study uses point-contact Andreev reflection spectroscopy with a ballistic gold tip to investigate superconducting properties in Cu₀.₂₅Bi₂Se₃. It identifies a robust zero-bias conductance peak (ZBCP) on superconducting regions, attributed to topological surface Andreev bound states, and shows that an effective single-band p-wave pairing model with Z = 10 and Δ₀ = 0.32 meV quantitatively explains the ZBCP and differential conductance minima, supporting unconventional, likely p-wave, superconductivity in this candidate topological superconductor.

ABSTRACT

We perform a point-contact Andreev reflection spectroscopic study of the topological superconducting material, Cu0.25Bi2Se3, in the ballistic regime using a normal-metal gold tip. We observe distinct point-contact spectra on the superconducting and non-superconducting regions of the crystal surface: the former shows a marked zero-bias conductance peak, indicative of unconventional superconductivity, while the latter exhibits a pseudogap-like feature. In both cases the measured differential conductance spectra exhibit a large linear background, preventing direct quantitative comparison with theory. We attribute this background to inelastic scattering at the tip-sample interface, and compare the background-subtracted spectra with a single-band p-wave model.

Motivation & Objective

• To investigate the pairing symmetry and superconducting gap structure in the candidate topological superconductor Cu₀.₂₅Bi₂Se₃ using ballistic point-contact Andreev reflection spectroscopy.
• To resolve the origin of the large linear background in previous point-contact spectra, which obscured quantitative comparison with theory.
• To determine whether the observed zero-bias conductance peak (ZBCP) arises from unconventional superconductivity, such as p-wave pairing, rather than conventional s-wave or other scattering mechanisms.
• To test the validity of a single-band p-wave model in explaining the differential conductance spectra, particularly the ZBCP and gap structure.

Proposed method

• Point-contact Andreev reflection spectroscopy was performed using a high-purity gold tip (diameter <100 nm) on freshly cleaved Cu₀.₂₅Bi₂Se₃ single crystals in a 3He cryostat with magnetic field control.
• Differential conductance (dI/dV) spectra were measured at multiple surface locations to compare superconducting and non-superconducting regions.
• A linear background in dI/dV spectra was identified as arising from inelastic scattering at the tip-sample interface, independent of temperature and not related to superconducting properties.
• Background-subtracted spectra were compared with theoretical models using the BTK formalism for p-wave pairing, with parameters D₀ = 0.071, Δ₀ = 0.32 meV, and Z = 10.
• Theoretical tunneling conductance was calculated using the normalized conductance σ(E) from the p-wave model of Yamashiro et al., incorporating Fermi-Dirac statistics and energy-dependent transmission.
• Temperature-dependent measurements of the gap parameter Δ₀(T) were analyzed and found to exhibit linear T-dependence, inconsistent with BCS theory.

Experimental results

Research questions

• RQ1 What causes the large linear background in point-contact dI/dV spectra of Cu₀.₂₅Bi₂Se₃, and can it be separated from intrinsic superconducting features?
• RQ2 Does the observed zero-bias conductance peak (ZBCP) in superconducting regions indicate unconventional pairing, such as p-wave symmetry?
• RQ3 Can a single-band p-wave model quantitatively reproduce the main features of the measured dI/dV spectra, including the ZBCP and minima?
• RQ4 Is the temperature dependence of the superconducting gap consistent with conventional s-wave BCS theory or indicative of unconventional pairing?

Key findings

• A robust zero-bias conductance peak (ZBCP) was observed exclusively on superconducting regions of the Cu₀.₂₅Bi₂Se₃ surface, indicating the presence of midgap Andreev bound states.
• The linear background in dI/dV spectra is attributed to inelastic scattering at the tip-sample interface, not to superconducting properties, and is temperature-independent.
• After background subtraction, the measured spectra show excellent qualitative agreement with a single-band p-wave pairing model using D₀ = 0.071, Δ₀ = 0.32 meV, and Z = 10.
• The superconducting gap parameter Δ₀(T) exhibits a linear temperature dependence, inconsistent with conventional BCS theory and supporting unconventional pairing.
• The absence of coherence peaks near the gap edge and the presence of a ZBCP rule out conventional s-wave tunneling, reflectionless tunneling, and magnetic/Kondo scattering as explanations.
• The data support the existence of topological surface Andreev bound states due to unconventional, likely p-wave, superconductivity in Cu₀.₂₅Bi₂Se₃.