Superconductivity with a Violation of Pauli Limit and Evidences for Multigap in η-Carbide Type Ti₄Ir₂O

Abstract

We report the synthesis, crystal structure, and superconductivity of Ti4Ir2O. The title compound crystallizes in an η-carbide type structure of the space group F d 3 ¯ m (No. 227), with lattice parameters a = b = c = 11.6194(1) Å. The superconducting temperature T c is found to be 5.1–5.7 K. Most surprisingly, Ti4Ir2O hosts an upper critical field of 16.45 T, which is far beyond the Pauli paramagnetic limit. Strong coupled superconductivity with evidences for multigap is revealed by the measurements of heat capacity and upper critical field. First-principles calculations suggest that the density of states near the Fermi level originates from the hybridization of Ti-3d and Ir-5d orbitals, and the effect of spin-orbit coupling on the Fermi surfaces is prominent. Large values of the Wilson ratio (R W ∼ 3.9), the Kadowaki–Woods ratio [A/γ 2 ∼ 9.0 × 10−6 μΩ⋅cm/(mJ⋅mol−1⋅K−1)2], and the Sommerfeld coefficient (γ = 33.74 mJ⋅mol−1⋅K−2) all suggest strong electron correlations (similar to heavy fermion systems) in Ti4Ir2O. The violation of Pauli limit is possibly due to a combination of strong-coupled superconductivity and large spin-orbit scattering. With these intriguing behaviors, Ti4Ir2O serves as a candidate for unconventional superconductor.

Observation of superconductivity and enhanced upper critical field of η-carbide-type oxide Zr4Pd2O

“…( 16 ) does not consider the SOC. The importance of SOC was pointed out by Ruan et al 32 and Shi et al 33 in Ti 4 Ir 2 O, exhibiting the violation of the Pauli limit. Similarly, we can expect that the enhanced μ 0 H c2 (0) of Zr 4 Pd 2 O would be raised from strong SOC.…”

“…From the expression, we can understand the strength of SOC proportions to Z 4 within the same electronic orbital. In the case of the Ti 4 Ir 2 O superconductor, we can expect the Ir-5 d orbital hosting enhanced SOC should play an important role for the large μ 0 H c2 (0), and it was found that Ti-3 d and Ir-5 d orbitals hybridize near its Fermi level and the violation of the Pauli limit is a result of a combination of strong-coupled superconductivity, SOC, and strong electron correlation 32 . Furthermore, the large SOC splitting a band structure along Γ-K lines due to the Ir-5 d electrons was weakened by applying pressures, and large μ 0 H c2 (0) undergoes a crossover at 35.6 GPa from well beyond to less than the μ 0 H P 33 .…”

Superconductivity of anomalous pseudospin

“…While we have emphasized nodal planes on which anomalous pseudospin exists, there are also materials for which anomalous pseudospin develops on nodal lines and not on nodal planes. Some such materials also exhibit unusual response to magnetic fields [78][79][80], suggesting a broader range of applicability for anomalous pseudospin superconductivity.…”