Enhanced Superconductivity on the Tetragonal Lattice in FeSe under Hydrostatic Pressure

Abstract

Superconductivity under pressure in FeSe (T c ∼7.5 K) has been investigated using singlecrystal specimens through the measurements of DC magnetization and electrical resistivity.A characteristic three-step increase in T c has been found under hydrostatic pressure up to ∼34 K above 7 GPa. The structural transition from a tetragonal phase to an orthorhombic phase (T s ∼87 K) is found to disappear at P ∼2.3 GPa, above which T c increases rapidly, suggesting that the superconductivity is enhanced by the tetragonal environment. Under nonhydrostatic pressure, the increase in T c is suppressed and the superconductive volume fraction is considerably reduced above 2 GPa, probably owing to the breaking of the tetragonal lattice symmetry by the uniaxial stress. The intimate correlation between the enhanced (suppressed) superconductivity and the tetragonality (orthorhombicity) in the phase diagram is a common feature of FeSe and other iron-pnictide superconductors.

Complex biphase nature of the superconducting dome of the FeSe phase diagram

“…Before the disappearance of superconductivity it is clearly established that the Tc of FeSe first exhibits a plateau around 25 K in the 3-5 GPa range in polycrystalline samples, as concluded from magnetization and electrical resistivity measurements with the Daphne 7474 or Daphne 7373 oil as PTM 35 , or at 20 K in the 2.5-5 GPa range in single crystals pressurized with glycerol as PTM 20 . Then follows a strong increase, reported in several studies, with a maximum Tc around 6 GPa 36,20 (glycerol as PTM) or in the 6-8 GPa range 25,22 (argon gas as PTM). In all cases a decrease of Tc is found by further pressurization above this critical pressure 11,20,35 .…”

Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe

“…For example, chemical substitution with sulfur or tellurium also suppresses T s , but slightly increases T c for low substitution values 30 . Similarly, hydrostatic pressure decreases T s and increases T c initially 27,31,32 . Recently, it was shown that a dose of 2.5 MeV electron irradiation producing approximately ∼ 0.1% Frenkel pairs per formula unit, decreases T s by 0.9 K and increases T c by 0.4 K (Ref.…”

Enhancement of superconducting transition temperature by pointlike disorder and anisotropic energy gap in FeSe single crystals

“…The superconducting transition temperature, T c , is non-monotonic, -increasing initially up to 0.8 GPa, then decreasing, reaching a minimum at 1. 3 GPa and increasing again [24][25][26]. Despite the absence of LRMO, a strong nematic response is found in FeSe, and has been discussed in terms of both spin and orbital fluctuations [27][28][29][30][31].…”