[Paper Review] Heat capacity evidence for conventional superconductivity in the Type-II Dirac semi-metal PdTe$_2$

Amit, Yogesh Singh|arXiv (Cornell University)|Jan 10, 2018

Topological Materials and Phenomena4 citations

TL;DR

This study investigates superconductivity in the Type-II Dirac semi-metal PdTe₂ using heat capacity, electrical transport, and magnetoresistance measurements on high-quality single crystals. Despite the material's topologically non-trivial Fermi surface, heat capacity data reveal a BCS-like electronic specific heat anomaly (ΔC/γTc ≈ 1.52), exponential low-T dependence, and a conventional H–T phase diagram, demonstrating that superconductivity in PdTe₂ is conventional s-wave in nature, not topological.

ABSTRACT

We use electrical transport, magnetoresistance, and heat capacity measurements on high quality single crystals of the recently discovered superconducting Type-II Dirac semi-metal PdTe$_2$, to probe the nature of it's superconducting phase. The magnitude of the electronic heat capacity anomaly at $T_c$, the low temperature exponential $T$ dependence of the heat capacity, and a conventional $H - T$ phase diagram establish that the superconductivity in PdTe$_2$ is conventional in nature despite the presence of a topologically non-trivial Fermi surface band which contributes to the electrical conduction.

Motivation & Objective

• To determine whether superconductivity in the Type-II Dirac semi-metal PdTe₂ is conventional or topological in nature.
• To investigate the interplay between topological electronic band structure and superconducting pairing in a bulk material.
• To establish the pairing symmetry and gap structure of superconductivity in PdTe₂ using thermodynamic measurements.
• To rule out topological superconductivity by examining the heat capacity anomaly and critical field behavior.

Proposed method

• Heat capacity measurements were performed down to 0.4 K using a He3 system to probe the superconducting transition and electronic specific heat.
• Electrical resistivity and magnetoresistance were measured to confirm superconductivity and identify the critical temperature Tc ≈ 1.7 K.
• The electronic contribution to heat capacity was isolated by subtracting the lattice (βT³) term from total C(T) data at zero field.
• Low-temperature C(T) data were fitted to C_el/T = γ_res + A exp(–Δ/T) to probe the superconducting gap structure.
• The H–T phase diagram was constructed using equal entropy construction on C(T) data at various magnetic fields.
• The critical field H_c(T) was fitted to the BCS expression H_c(T) = H_c(0)[1 – (T/T_c)²] to assess conventional behavior.

Experimental results

Research questions

• RQ1 Does the superconducting state in PdTe₂ exhibit signatures of topological superconductivity despite its Type-II Dirac band structure?
• RQ2 What is the magnitude of the electronic specific heat anomaly ΔC at T_c, and how does it compare to BCS expectations?
• RQ3 Does the low-temperature heat capacity exhibit exponential T dependence, indicating a fully gapped s-wave superconductor?
• RQ4 Is the H–T phase diagram consistent with conventional BCS superconductivity?

Key findings

• The electronic specific heat anomaly at T_c yields ΔC/γTc ≈ 1.52, close to the BCS value of 1.43 for weak-coupling single-gap superconductors.
• Low-temperature C_el/T data show excellent agreement with an exponential fit C_el/T = γ_res + A exp(–Δ/T), indicating a fully gapped s-wave superconducting state.
• The residual Sommerfeld coefficient γ_res = 0.4 mJ/mol K² suggests ~7% of the sample volume is non-superconducting.
• The H–T phase diagram follows the BCS form H_c(T) = H_c(0)[1 – (T/T_c)²], with H_c(0) = 195(2) Oe and T_c = 1.78 K from fitting.
• The superconducting transition is sharp and un-broadened, with T_c = 1.72 K from equal entropy construction, indicating high sample quality and bulk superconductivity.
• All thermodynamic evidence—specific heat anomaly, gap structure, and H–T phase diagram—points to conventional s-wave superconductivity in PdTe₂.