Pressure induced superconductivity in MnSe

The study of iron-based superconductors, particularly FeSe and related compounds, has been crucial in understanding high-temperature superconductivity. These materials exhibit a complex interplay of structural distortions, magnetic and orbital ordering, and electronic nematicity. FeAs-based materials typically undergo a tetragonal-to-orthorhombic structural transition accompanied by antiferromagnetic (AFM) ordering. Doping suppresses these transitions, inducing superconductivity. In FeSe, a similar transition occurs without magnetic order at ambient pressure, with superconductivity emerging below 8 K and related to the orthorhombic distortion. Applying pressure to FeSe further affects these properties, suppressing the structural transition, inducing a magnetically ordered phase, and enhancing Tc to approximately 37 K at ~6 GPa. Prior research explored substituting Fe in FeSe with other transition metals, revealing that Mn substitution had a limited impact, while Cu substitution completely suppressed superconductivity. MnSe, with a cubic structure and anomalous magnetic behavior at ambient pressure, showed no superconductivity. This study, inspired by the superconductivity observed in MnP under pressure, investigates whether comparable conditions could induce superconductivity in MnSe. Previous attempts involved replacing Se with S in MnSe to create internal pressure, resulting in some suppression of magnetic properties but no superconductivity. However, under high pressure, MnSe exhibits a structural transition to an orthorhombic phase, similar to the superconducting phase of MnP, and shows metallic behavior at ~30 GPa, motivating the more detailed investigation presented in this paper.

The rich phenomenology of Fe-based superconductors, particularly FeSe and its analogs, has fueled extensive research into the mechanisms underlying high-temperature superconductivity. The interplay between structural transitions, magnetic ordering, and electronic nematicity are key aspects explored in this field. Studies on FeAs-based materials show a correlation between tetragonal-to-orthorhombic structural transitions, antiferromagnetic order, and superconductivity. Doping these materials can suppress the magnetic and structural transitions, leading to the emergence of superconductivity. In contrast, FeSe displays a similar structural transition without magnetic order at ambient pressure, highlighting the intricate relationship between structural changes and superconductivity. Pressure studies on FeSe reveal that applying pressure suppresses the structural transition, induces a magnetic phase, and increases Tc. Substitution experiments in FeSe have yielded insights into the role of different elements in modulating superconductivity. Mn substitution showed minimal impact, while Cu substitution completely suppressed it. MnSe, with its inherent cubic structure and anomalous magnetism, presented a potential challenge to induce superconductivity. The success of inducing superconductivity in MnP under pressure provided a roadmap for investigating similar possibilities in MnSe. Prior research explored creating internal pressure within MnSe by substituting Se with S. While this did suppress some magnetic phases, it did not induce superconductivity, setting the stage for a high-pressure approach.

The study employed various experimental techniques to investigate the effects of high pressure on MnSe. Polycrystalline MnSe samples were synthesized via solid-state reaction of Mn and Se elements in an evacuated quartz ampoule. High-pressure structural characterization utilized angle-dispersion X-ray diffraction (ADXRD) using a symmetric diamond anvil cell (DAC) with a rhenium gasket and helium gas as a pressure-transmitting medium (PTM). High-pressure resistivity measurements were performed using a DAC with 400 µm culets, a rhenium gasket insulated with cubic boron nitride (c-BN) powder, and hexagonal boron nitride (h-BN) as the PTM. Gold foils and wires were used for electrical connections, and the Van der Pauw method was used to measure resistance at low temperatures. Pressure was calibrated using the ruby fluorescence method. High-pressure magnetic susceptibility measurements were conducted using a mini-DAC adapted to a Quantum Design MPMS, with BeCu alloy, non-magnetic Ni-Cr-Al alloy gaskets, and a 4:1 methanol-ethanol mixture (M:E 4-1) as the PTM. Low-pressure measurements used a piston-cylinder-type high-pressure cell with Daphne-7373 oil as the PTM. Theoretical calculations used Quantum Espresso with norm-conserving local density approximation (LDA) pseudopotentials to determine the energetically favored phases under pressure. Coulomb U (5 eV) was included for calculations at low pressures to account for the correlation of localized 3d orbitals, but not at higher pressures where the system is metallic.

Resistive measurements revealed a transition to metallic behavior at ~16 GPa, with a superconducting transition appearing above 20 GPa. The highest Tc observed resistively was 6.5 K at ~40 GPa. Magnetic measurements detected a diamagnetic drop indicating superconductivity at pressures as low as 11.75 GPa, reaching a maximum Tc of 9 K at approximately 35 GPa. Magnetic measurements revealed that the two magnetic anomalies observed in MnSe at ambient pressure were suppressed with increasing pressure. In situ synchrotron XRD experiments showed that MnSe undergoes two structural phase transitions: a partial cubic-to-hexagonal transformation at 12.2 GPa and a complete transition to an orthorhombic (MnP-type, Pnma) phase at 30.5 GPa. Between 16 and 30 GPa, a mixed-phase state of cubic, hexagonal, and orthorhombic structures was observed. Theoretical calculations supported the experimental observation of the orthorhombic phase transition at approximately 40 GPa and indicated a low-spin state (S=0.5) for Mn in the orthorhombic phase. The upper critical field (Hc2) was estimated to be ~3463 Oe at 36 GPa, and the superconducting coherence length was estimated to be ~308 Å. The onset Tc from magnetic measurements generally showed higher values than from resistive measurements. The pressure dependence of volume from XRD shows the volume decrease with pressure, however, no direct relationship of Tc and pressure is identified. The higher Tc from magnetic measurements, especially at higher pressures (above 30 GPa) might be attributed to the interfacial effects and Josephson-junction coupling between superconducting grains.

The observation of pressure-induced superconductivity in MnSe is a significant finding. The suppression of anomalous magnetic behavior and the emergence of superconductivity around the pressure where the orthorhombic phase appears strongly suggest a link between the structural transformation and the superconducting state. However, the disparity between Tc values obtained from magnetic and resistive measurements, particularly above 30 GPa, indicates that the orthorhombic phase alone may not be the sole determinant of superconductivity. The possibility of interface effects between metallic and insulating regions, leading to Josephson junction coupling between superconducting grains, requires further investigation. This is supported by the relatively low upper critical field. The inconsistent Tc values between different measurements suggest pressure inhomogeneity might play an important role. The decrease in volume with pressure indicates a compression in the lattice which is not consistent with the superconductivity increase with pressure in other materials. Therefore, further research is needed to understand the detailed mechanism of the pressure-induced superconductivity in MnSe.

This study definitively demonstrates pressure-induced superconductivity in MnSe, a material with complex magnetic behavior at ambient conditions. Superconductivity emerges at approximately 12 GPa (magnetic measurements) and 16 GPa (resistive measurements), coinciding with the appearance of an orthorhombic phase. While the orthorhombic structure likely plays a role, the discrepancy between magnetic and resistive Tc values suggests interfacial effects significantly contribute to the observed superconductivity. Future studies should focus on clarifying the interplay between structural transitions, pressure homogeneity, and interfacial effects in determining the superconducting properties of MnSe.

The study's primary limitation is the potential for pressure inhomogeneity within the DAC, particularly at higher pressures, which can affect the accuracy of Tc determination and interpretation of phase transitions. The discrepancy between resistive and magnetic measurements highlights the need for more precise control of pressure uniformity across the sample. Additionally, further studies are needed to fully characterize the interface effects and their contribution to superconductivity, particularly in separating intrinsic properties of the orthorhombic phase from interfacial contributions. The quantitative analysis of the XRD data might be improved by higher resolution and refined fitting parameters.