Point-contact Andreev reflection spectroscopy of layered superconductors with device-integrated diamond anvil cells

TL;DR

The paper tackles the challenge of obtaining spectroscopic information on superconductivity in mechanically exfoliable layered materials under pressure by combining point-contact Andreev reflection spectroscopy (PCAR) with a device-integrated diamond anvil cell (DIDAC). Using FeSe thin flakes, the authors demonstrate ambient and high-pressure PCAR measurements, extracting a dominant superconducting gap that follows a BC S-like temperature dependence (Δ(0) ≈ 1.25 meV at ambient and ≈ 2.23 meV under ≈18 kbar, with Tc ≈ 6.9 K and ≈ 11.5 K, respectively). The gap-to-Tc ratios are ~4.18 and ~4.53, and while a two-gap model can describe some features, a single dominant gap description suffices for the presented data, with notable spectral broadening discussed. This work establishes a practical route for multiprobe, pressure- and thickness-tunable investigations of thin-layer superconductors, enabling deeper insights into pairing mechanisms under extreme conditions.

Abstract

Superconductors that can be mechanically exfoliated are an interesting platform for exploring superconducting properties tuned by layer thickness. These layered superconductors are also expected to exhibit sensitivity to applied pressure. While pressure has been demonstrated to be an effective way of tuning bulk superconductors, analogous studies on superconducting thin flakes have been limited due to technical challenges. In particular, spectroscopic measurements under pressure remain insufficiently explored. In this work, we functionalized the diamond anvil cell technique for point-contact Andreev reflection spectroscopy (PCAR) measurement on thin-flake materials under pressure, offering the opportunity to obtain spectroscopic information on superconductivity. To validate the feasibility of this method, we have conducted PCAR measurements on iron-selenide thin flakes to extract temperature-dependent superconducting gap values under ambient and high pressure. Combine with the proven magnetotransport capability, our method provides a conceptually simple tool for a detailed examination of thin-flake superconductors under pressure.

Figures (7)

• Figure 1: A schematic diagram (with false color) showing the configuration of device-integrated diamond anvil cells for point-contact spectroscopy measurement.
• Figure 2: Temperature dependence of the electrical resistance of FeSe thin flake at ambient pressure (S1). Inset: Temperature derivative of electrical resistance for determining nematic transition temperature (T$_{\rm s}$ = 90 K).
• Figure 3: Normalized PCAR spectra of FeSe thin flake at ambient pressure (S1). (a) presents the temperature dependence of the normalized PCAR spectra. In (b), the normalized PCAR spectrum at 50 mK is presented by the symbols while the solid and dashed lines represent the fits by the BTK model assuming a single or double $s$-wave superconducting gap, respectively.
• Figure 4: Temperature dependence of the electrical resistance of FeSe thin flake at 18 kbar (S2). Inset (top): Superconducting transition in electrical resistance at 18 kbar under different magnetic fields. Inset (bottom): Temperature derivative of electrical resistance, rescaled for determining nematic transition temperature (T$_{\rm s}$ = 50 K).
• Figure 5: Normalized PCAR spectra of FeSe thin flake at 18 kbar (S2). (a) Temperature dependence at zero magnetic field. (b) Field dependence at 2 K.