Group-9 Transition Metal Suboxides Adopting the Filled-Ti2Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields
Abstract
The Ti2Ni and the related η-carbide structure are known to exhibit various intriguing physical properties. The Ti2Ni structure with the cubic space group Fd3m is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related η-carbide compounds correspond to a filled version of the Ti2Ni structure. Here, we report on the structure and superconductivity in the η-carbide type suboxides Ti4M2O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of T c = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of C/γ T c = 1.65, 1.28, and 1.80 for Ti4Co2O, Ti4Rh2O, and Ti4Ir2O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is Ti4Ir2O with an upper critical field of μ0 H c2 (0) =~16.06~T, which exceeds by far the weak-coupling Pauli limit of 9.86~T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti2Co. In contrast to the metallic η-carbide type suboxides Ti4M2O, we find that Ti2Co displays a semimetallic behavior. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti2Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.
Turn this paper into a lesson
ArcXiv compiles a structured reading guide from this paper's metadata: plain-English importance, contributions, prerequisite concepts, which sections to read first, flashcards, and a quiz. Grounded in the abstract, never invented.