Pressure-Induced Transition from Spin to Superconducting States in Novel MnN₂

Abstract

The connection between magnetism and superconductivity has long been discussed since the discovery of Fe-based superconductors. Here, we report the discovery of a pressure-induced transition from a spin to a superconducting state in novel MnN 2 based on ab initio calculations. The superconducting state can be obtained in two ways: the first is the pressure-induced transition from an AFM- P 2 1 / m to an NM- I 4/ mmm phase at 30 GPa, while the other is the pressure-induced transition from an FM- I 4/ mmm phase to magnetic vanishing at 14 GPa, which leads to a structural transition with the distortion of octahedrons to tetragonal pyramids. NM- I 4/ mmm- MnN 2 is superconductive with T c ≈ 17.6 K at 0 GPa. In the second way, electronic structure calculations indicate that the system transforms from a high-spin state to a low-spin state due to increasing crystal-field splitting, causing disappearance of magnetism; more electron occupancy around the Fermi level drives the emergence of superconductivity. Remarkably, I 4/ mmm- MnN 2 can achieve mutual spin-to-superconducting state transformation by pressure. Moreover, the AFM- P 2 1 / m- MnN 2 phase is extremely incompressible with the hardness above 20 GPa. Our results provide a reasonable and systematic interpretation for the connection between magnetism and superconductivity and give clues for achieving spin-to-superconducting switching materials with certain crystal features.

Development of novel superconductivity with higher T c

“…No anionic electrons are found to confine in interspaces in the tetragonal I4/m structure, indicating the formation of a non-electride phase. We attribute the development of superconductivity to the suppression of magnetism by destroying the spin-polarized quasi-2D anionic electrons, accompanied with the changes of electron-confinement topology under high pressures, which is different from the mechanism of increasing of crystal field splitting reported in MnN 2 [34]. The unprecedentedly high T c value of Y 2 C is found to relate to the increase of density of states near Fermi level induced by the complete delocalization of excess electrons in the low-pressure electride phases.…”

“…In both of these two structures, the results show that the vibrations of Y atoms dominate the low frequency region (<8 THz for Pnma and < 10 THz for I4/m), whereas the C atoms dominate the high end of the spectra. Their Eliashberg phonon spectral functions are similar below 20 THz, whereas highfrequency vibrations (32)(33)(34)(35) exist in I4/m (figure 4(b)) due to the appearance of molecular C 2 units. We find that the low frequency Y vibrations contribute largely to the overall λ, i.e.…”

High-Pressure Synthesis of a High-Pressure Phase of MnN Having NiAs-Type Structure

“…Thus, NiAs-type MnN appeared regardless of the pressure medium at a pressure of at least 54.2 GPa. The XRD profiles of Sample #2 and #3 showed several peaks that are different from known Mn–N compounds − and theoretically predicted Mn pernitrides and polynitrides. − This result suggests that nitrogen-rich MnN x may have been synthesized, but the details are undetermined, and further analysis and experimentation are currently underway.…”

“…These nitrides were synthesized by gas reaction or DC reactive spattering and have crystal structures based on a NaCl-type structure with nitrogen defects or lattice distortion caused by magnetic ordering. High-pressure experiments have reported the synthesis of η-Mn 3 N 2 and θ-MnN at approximately 10 and 30 GPa, respectively, which can also be synthesized at ambient pressure. , However, theoretical predictions reported manganese mononitrides with crystal structures different from θ-MnN, pernitrides, and polynitrides, which have not been synthesized yet. − Thus, further exploration of manganese nitrides under high pressure is required.…”