Pressure induced superconductivity in MnSe

The study explores whether superconductivity can be induced in MnSe under pressure, motivated by the rich physics and pressure-enhanced superconductivity observed in FeSe and iron-based superconductors. Fe-based systems display multiple intertwined orders (structural, magnetic, nematic), and FeSe shows enhanced T_c under pressure up to ~37 K near 6 GPa. MnSe at ambient conditions is cubic and magnetically complex, and does not superconduct. Given that MnP becomes superconducting under pressure and MnSe shares magnetic behaviors with MnP, the authors probe if pressure can suppress MnSe’s magnetic anomalies and drive structural transitions favorable to superconductivity. The purpose is to map the pressure–temperature–structure relationships in MnSe and identify the emergence and characteristics of superconductivity, clarifying whether structural phase transitions (notably to orthorhombic MnP-type) correlate with the onset of superconductivity.

- Fe-based superconductors display structural distortions, magnetism, and nematicity; in FeAs systems, superconductivity emerges upon suppressing orthorhombicity and antiferromagnetism via doping. FeSe undergoes a tetragonal-to-orthorhombic transition (~90 K) without long-range magnetic order at ambient pressure, with superconductivity at ~8 K; pressure suppresses the structural transition, introduces magnetism around ~1 GPa, and enhances T_c to ~37 K near 6 GPa. - Substitution studies: Up to 6% Mn in FeSe retains superconductivity, while ~3% Cu suppresses it. Superconductivity in FeSe is linked to the tetragonal structure; hexagonal phases (e.g., NiAs-type) are magnetic and non-superconducting. - MnSe is cubic at ambient conditions with anomalous magnetic behavior; prior work on Mn(Se,S) showed sulfur substitution (internal pressure) suppresses cubic-to-hexagonal transformation, with an estimated equivalent compression pressure for MnS relative to MnSe of ~13.2 GPa, but MnS remains cubic and magnetic without superconductivity. - High-pressure studies reported lattice collapse and a distortion to orthorhombic Pnma phase near ~30 GPa in MnSe with metallic transport and low-spin states, but earlier work covered limited pressure/temperature ranges. MnP (orthorhombic) exhibits superconductivity (~1 K) at ~8 GPa, motivating analogous exploration in MnSe.

- Material synthesis: Polycrystalline MnSe was synthesized by solid-state reaction using Mn (99.95%) and Se (99.95%). Stoichiometric mixtures were sealed in evacuated quartz ampoules, heated to 750 °C, annealed for several hours, and furnace-cooled to room temperature. - High-pressure X-ray diffraction (HP-XRD): Angle-dispersive XRD performed in a symmetric diamond anvil cell (DAC) with 300 µm culets. Rhenium gasket pre-indented to ~50 µm; 150 µm diameter hole served as sample chamber. Ruby spheres were used as pressure gauges. Primary measurements used helium gas as the pressure transmitting medium (PTM) due to excellent hydrostaticity up to at least 50 GPa; additional HP-XRD used methanol:ethanol 4:1 (M:E 4-1) as PTM for comparison. Experiments conducted at NSRRC BL01C2 with 20 keV X-rays. Lattice parameters and phase identification were derived; volume vs pressure trends analyzed. Scherrer and Williamson–Hall methods were applied to estimate mean grain size and micro-strain in the single orthorhombic phase region. - Electrical transport under pressure: Four-probe resistivity in a DAC with 400 µm culets; rhenium gasket insulated by c-BN powders. Sample dimensions ~70×70×15 µm^3 loaded with hexagonal BN as PTM. Gold foils/wires used as electrodes. Pressure determined by ruby fluorescence. Low-temperature resistance measured via Van der Pauw method in a 4He cryostat. T_c defined from resistivity data using intersections of adjacent linear fits in dρ/dT. - Magnetization under pressure: Ultrasensitive magnetization in a mini-DAC (BeCu) adapted to a Quantum Design MPMS. 300 µm culet diamonds; nonmagnetic Ni-Cr-Al gaskets pre-indented to ~20 µm with ~120 µm sample chamber. M:E 4-1 served as PTM; pressure measured by ruby fluorescence. A piston-cylinder cell (up to ~1.3 GPa) with Daphne-7373 oil was used for low-pressure measurements, with Pb manometer. T_c from magnetization was determined using dM/dT vs T criteria. Field dependence of T_c was mapped to extract H_c2. - Theoretical calculations: First-principles DFT with Quantum ESPRESSO using norm-conserving LDA pseudopotentials, 60 Ry plane-wave cutoff. At low pressures (semiconducting cubic MnSe), a Hubbard U=5 eV was included for Mn 3d correlations; for metallic hexagonal/orthorhombic phases at high pressure, U was omitted. Relative phase energies vs pressure, crystal structures, and band structures (orthorhombic MnSe at ~40 GPa) were computed. - Data analysis specifics: Upper critical field H_c2(0) estimated via Werthamer–Helfand–Hohenberg formula using the slope dH_c2/dT near T_c; coherence length ξ from Ginzburg–Landau relation μ0H_c2(0)=Φ0/(2πξ^2).

- Emergence of superconductivity under pressure: Diamagnetic drop in magnetization appears at ≥11.75 GPa, indicating onset of superconductivity. Resistivity shows a small low-temperature drop above ~16 GPa, evolving into clear superconducting transitions above 20 GPa, with zero resistance above 2 K for P ≥36 GPa. - Transition temperatures: From resistivity, T_c (onset defined via dρ/dT line-intersection) initially increases with pressure, reaching a maximum of ~6.5 K near ~40 GPa, then decreases at higher pressures. From magnetization, the highest onset T_c reaches ~9 K around ~35 GPa, with a local minimum in T_c(P) near ~26 GPa. - Normal-state transport: Below ~16 GPa, MnSe exhibits semiconducting behavior with a decreasing gap under pressure. Room-temperature resistivity exhibits abrupt drops near ~10 GPa, ~16 GPa, and above ~20 GPa. Metallic behavior sets in at ~16 GPa. - Upper critical field and coherence length: At 36 GPa, dH_c2/dT ≈ 806 Oe/K with T_c ≈ 6.2 K gives H_c2(0) ≈ 3463 Oe (WHH). The corresponding superconducting coherence length is ξ ≈ 308 Å (from μ0H_c2(0)=Φ0/2πξ^2). - Magnetism under pressure: Ambient-pressure MnSe shows two magnetic anomalies (T_N between 100–200 K; T_S ~266 K). Up to 1.2 GPa, T_N and T_S increase with pressure at rates dT_N/dP ≈ 18.9 K/GPa and dT_S/dP ≈ 34.3 K/GPa. In DAC measurements, these anomalies are suppressed by ~2.68 GPa. A small AFM-like hump near ~150 K appears between ~11.75 and ~25.92 GPa and is suppressed at higher pressures as the diamagnetic signal strengthens. - Structural phase evolution (HP-XRD): Starting cubic (Fm3m, a≈5.4697 Å) at ambient pressure. At ~12.2 GPa, partial transformation to hexagonal phase (coexistence with cubic). Around ~16 GPa, an orthorhombic phase (MnP-type, Pnma) appears, yielding a three-phase coexistence (cubic, hexagonal, orthorhombic) from ~16–30 GPa. By ~30 GPa, MnSe fully transforms to single-phase orthorhombic (Pnma) with lattice parameters a=5.7527 Å, b=3.1045 Å, c=6.0434 Å (helium PTM; slightly lower transition pressure with M:E 4-1). Volume vs pressure trends corroborate transitions. Micro-strain in the single orthorhombic phase is ~1% and increases with pressure; mean grain size remains roughly constant or slightly increases. - Theory: DFT indicates cubic is energetically favored below ~10 GPa; hexagonal and orthorhombic are nearly degenerate and close to cubic between ~10–30 GPa, consistent with observed phase coexistence; orthorhombic becomes clearly favored near ~40 GPa. Orthorhombic MnSe at high pressure exhibits a low-spin state (S≈0.5). - Relationship of superconductivity to structure: Onset of superconductivity coincides with the emergence of the orthorhombic phase (~16 GPa by transport; ~12 GPa by magnetization/HP-XRD with M:E 4-1). However, the diamagnetic signal magnitude does not increase dramatically upon reaching single-phase orthorhombic, and discrepancies between T_c from magnetization and resistivity persist/increase above 30 GPa, suggesting additional factors (e.g., interfacial superconductivity, pressure inhomogeneity) beyond bulk orthorhombic phase alone.

The findings demonstrate that applying pressure suppresses MnSe’s anomalous magnetic behavior and induces superconductivity coincident with structural transitions. The emergence of the orthorhombic MnP-type phase aligns with the onset of metallicity and superconductivity, implying a structural origin. Yet, comparable diamagnetic signals below and above the completion of the orthorhombic transition, together with the persistent higher onset T_c from magnetization than from resistivity (especially above 30 GPa), indicate that superconductivity may not be uniformly bulk. Instead, it may be significantly influenced by interfacial effects between coexisting metallic and insulating/magnetic domains or grain boundaries, forming Josephson-coupled networks that yield magnetic signatures before full percolative zero resistance. Pressure homogeneity and the choice of PTM affect both structural transition pressures and superconducting signatures, consistent with prior observations in other pressure-induced superconductors. The relatively low H_c2 and broad transitions support the interfacial/granular superconductivity scenario. Comparisons with other systems (e.g., FeSe under pressure, MgB2 with strain effects, Re under shear) suggest that lattice strain and microstructural factors can modulate T_c, and in MnSe, increasing micro-strain in the orthorhombic phase correlates with T_c evolution, though lattice contraction with pressure differentiates it from some strain-enhanced cases. Overall, pressure-induced superconductivity in MnSe likely arises from a combination of structural phase transition to orthorhombic symmetry and interfacial effects across mixed-phase or grain boundary regions.

This work establishes pressure-induced superconductivity in MnSe. Magnetic measurements detect the onset near ~12 GPa, while resistivity shows superconducting drops above ~16 GPa and zero resistance for P ≥36 GPa. T_c reaches up to ~9 K (magnetization, ~35 GPa) and ~6.5 K (resistivity, ~40 GPa). Structural analysis reveals a cubic-to-hexagonal transition near ~12 GPa, the emergence of an orthorhombic (MnP-type, Pnma) phase around ~16 GPa, and a complete transformation to the orthorhombic phase by ~30 GPa. The superconducting onset correlates with appearance of the orthorhombic phase, but the behavior of diamagnetism, T_c discrepancies between techniques, and sensitivity to pressure media indicate that interfacial effects between metallic and insulating domains likely play a crucial role. Future studies should aim to: (i) improve pressure homogeneity and use single-crystalline samples to disentangle intrinsic from interfacial effects; (ii) perform spatially resolved probes (e.g., micro-XRD, scanning SQUID/STM) under pressure to map superconducting domains; (iii) extend high-field measurements to refine H_c2 and anisotropy; and (iv) conduct detailed electronic structure and pairing mechanism studies in the orthorhombic phase.

- Broad superconducting transitions and discrepancies between T_c from magnetization and resistivity suggest pressure inhomogeneity and/or nonuniform superconductivity (granular/interfacial) influenced the measurements. Different DAC setups and PTMs (helium vs methanol:ethanol 4:1) yielded different transition pressures and T_c values. - Mixed-phase coexistence (cubic, hexagonal, orthorhombic) from ~12–30 GPa complicates attribution of superconductivity to a single crystallographic phase. - Limited upper magnetic field range constrains precise determination of H_c2, and coherence length estimates rely on extrapolation (WHH) assumptions. - Polycrystalline samples and possible grain boundary effects may enhance interfacial superconductivity, differing from intrinsic bulk behavior in single crystals. - Theoretical calculations use LDA (with/without U) and may not capture all correlation effects; calculated lattice parameters differ slightly from experiment.