Conventional empirical law reverses in the phase transitions of 122-type iron-based superconductors (original) (raw)
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Iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ has been investigated under high pressure using synchrotron based x-ray diffraction and mid-infrared reflectance measurements at room temperature. Pressure dependence of the superconducting transition temperature (T$_c$) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. Although the high pressure orthorhombic phase ($\textit{Pbnm}$) starts emerging at 4 GPa, structural transition becomes clearly observable above 10 GPa. A strong correlation is observed between the Fe(Se,Te)$_{4}$ tetrahedral deformation in the tetragonal phase ($\textit{P4/nmm}$) and the sharp rise of T$_c$ up to sim\simsim4 GPa, above which T$_c$ is found to be almost pressure independent at least up to 10 GPa. A subtle structural modification of the tetragonal phase is noticed above 10 GPa, suggesting a structural transition with possible Fe$^{2+}$ spin-state transition. The evolution with pressure of t...
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Collapsed tetragonal phase and superconductivity ofBaFe2As2under high pressure
Physical Review B, 2010
High pressure x-ray diffraction and electrical resistance measurements have been carried out on BaFe 2 As 2 to a pressure of 35 GPa and temperature of 10 K using a synchrotron source and designer diamond anvils. At ambient temperature, a phase transition from the tetragonal phase to a collapsed tetragonal ͑CT͒ phase is observed at 17 GPa under nonhydrostatic conditions as compared to 22 GPa under hydrostatic conditions. The superconducting transition temperature increases rapidly with pressure up to 34 K at 1 GPa and decreases gradually with a further increase in pressure. Our results suggest that T C falls below 10 K in the pressure range of 16-30 GPa, where CT phase is expected to be stable under high-pressure and low-temperature conditions.
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We studied the effect of hydrostatic pressure (P) on the structural phase transitions and superconductivity in the ternary and pseudo-ternary iron arsenides CaFe 2 As 2 , BaFe 2 As 2 , and (Ba 0.55 K 0.45)Fe 2 As 2 , by means of measurements of electrical resistivity (ρ) in the 1.8-300 K temperature (T) range, pressures up to 20 kbar, and magnetic fields up to 9 T. CaFe 2 As 2 and BaFe 2 As 2 (lightly doped with Sn) display structural phase transitions near 170 K and 85 K, respectively, and do not exhibit superconductivity in ambient pressure, while K-doped (Ba 0.55 K 0.45)Fe 2 As 2 is superconducting for T < 30 K. The effect of pressure on BaFe 2 As 2 is to shift the onset of the crystallographic transformation down in temperature at the rate of ~-1.04 K/kbar, while shifting the whole ρ(T) curves downward, whereas its effect on superconducting (Ba 0.55 K 0.45)Fe 2 As 2 is to shift the onset of superconductivity to lower temperatures at the rate of ~-0.21 K/kbar. The effect of pressure on CaFe 2 As 2 is first to suppress the crystallographic transformation and induce superconductivity with onset near 12 K very rapidly, i.e., for P < 5 kbar. However, higher pressures bring about another phase transformation characterized by reduced resistivity, and the suppression of superconductivity, confining superconductivity to a narrow pressure dome centered near 5 kbar. Upper critical field (H c2) data in (Ba 0.55 K 0.45)Fe 2 As 2 and CaFe 2 As 2 are discussed.
EPL (Europhysics Letters), 2012
PACS 62.50.-p-High-pressure effects in solids and liquids PACS 74.62.Fj-Effects of pressure on superconducting transition temperature PACS 61.50.Ks-Crystallographic aspects of phase transformations; pressure effects Abstract-Simultaneous high pressure x-ray diffraction and electrical resistance measurements have been carried out on a PbO type α-FeSe 0.92 compound to a pressure of 44 GPa and temperatures down to 4 K using designer diamond anvils at synchrotron source. At ambient temperature, a structural phase transition from a tetragonal (P4/nmm) phase to an orthorhombic (Pbnm) phase is observed at 11 GPa and the Pbnm phase persists up to 74 GPa. The superconducting transition temperature (T C) increases rapidly with pressure reaching a maximum of ~28 K at ~ 6 GPa and decreases at higher pressures, disappearing completely at 14.6 GPa. Simultaneous pressure-dependent x-ray diffraction and resistance measurements at low temperatures show superconductivity only in a low pressure orthorhombic (Cmma) phase of the α-FeSe 0.92. Upon increasing pressure at 10 K near T C , crystalline phases change from a mixture of orthorhombic (Cmma) and hexagonal (P63/mmc) to a high pressure orthorhombic (Pbnm) phase near 6.4 GPa where T C is maximum. Introduction.-The pressure variable has always played a pivotal role in the discovery and optimization of novel superconducting materials. Discovery of high temperature superconductivity in a new class of iron-based layered compounds has received extensive attention recently [1-6]. Undoped iron-based layered compounds like REOFeAs (RE = trivalent rare earth metal), and AFe 2 As 2 (A = divalent alkaline earth metal) are non-superconducting at ambient pressure and are known to exhibit tetragonal to orthorhombic structural transition and antiferromagnetic (AFM) ordering on cooling. The AFM ordering and structural transition is suppressed under high pressure or chemical doping and superconductivity is induced [1-4]. However, the critical relationships between structure, magnetism, and superconductivity still remain unresolved. More recently, superconductivity was reported at 8 K in α-FeSe 1-δ samples with PbO-type tetragonal structure [5]. At ambient conditions, α-FeSe 1-δ has a structure composed of stacks of edge-sharing FeSe 4tetrahedral layers stacked along c-axis [5-7] while, the structure of FeAs-based superconductors consists of edge sharing FeAs 4tetrahedra stacked layer by layer with separating elements like REO in REOFeAs or A in AFe 2 As 2 between the FeAs 4 layers [1, 2]. The tetragonal α-FeSe undergoes a structural phase transition to an orthorhombic (Cmma) below 70 K upon cooling [7]. One remarkable aspect of superconductivity in binary FeSe-system is the strong relationship between the superconducting state and pressure. Recently, the T C onset was shown to increase at huge rate of dTc/dP = 9.1 K/GPa (the largest for any of the known FeAs-compounds), dramatically reaching an onset of 27 K at 1.5 GPa [6]. This sensitivity of T C to pressure convincingly indicates that there is a strong correlation between the superconducting properties and changes in the crystal structure of FeSe-system under pressure. A number of pressure-dependent structural and resistance measurements of the layered Fe-based systems have been reported that are aimed at understanding the critical relationship between compression behavior of crystal structure and superconductivity [6-11]. However, none of the previous works reported simultaneous high pressure and low temperature resistivity and x-ray diffraction measurements on the same sample in the same experiment. Here, we report simultaneous high pressure resistance and x-ray diffraction experiments using a designer-DAC to precisely elucidate the effect of pressure on the observed superconducting properties and the local crystallographic modulations of FeSe 1-δ. In addition, there have been some disagreements on structural phase modulations in FeSe 1-δ system under pressure. While some report a tetragonal to a hexagonal phase (P63/mmc) transition above 12 GPa [8, 9], a phase transition from the
Journal of Materials Research, 2021
We report high pressure structural studies (52 GPa) at room temperature combined with magnetic [(M(T):1GPa] and electrical resistivity [(ρ(T):0-21GPa)] measurements down to 2K on Fe0.99Ni0.01Se0.5Te0.5 superconductor using designer diamond anvils (D-DAC) pressure cell. The M(T) data shows huge enhancement of superconducting transition temperature (Tc) from 8.62 to 14.8 K (1 GPa) and ρ(T) reveals maximum enhancement of Tc ~ 30.5 K at 3 GPa (dTc/dP= ~ 7.19 K/GPa) followed by moderate decrease of Tc up to 19 K at 7.5 GPa, further increasing pressure Tc gets vanished at 10.6 GPa. The reduction of Tc due to the occurrence of structural transition that is likely associated with possible reduction of charge carriers in the density of states in Fermi surface. The high pressure XRD measurements shows tetragonal phase exists up to 7 GPa, followed by mixed-phase which is visible between 7.5 GPa to 14.5 GPa. The structural transformation occurs at 15 GPa from tetragonal (P4/nmm) to NiAs-type hexagonal phase (P63/mmc) and it is stable up to 52 GPa were confirmed from the equation of state (EOS) and it can be correlated with variation of Tc under pressure for Fe0.99Ni0.01Se0.5Te0.5 chalcogenide superconductors.
Role of the 245 phase in alkaline iron selenide superconductors revealed by high-pressure studies
Physical Review B, 2014
There is considerable interest in uncovering the physics of iron-based superconductivity from the alkaline iron selenides, a materials class containing an insulating phase (245 phase) and a superconducting (SC) phase. Due to the microstructural complexity of these superconductors, the role of the 245 phase in the development of the superconductivity has been a puzzle. Here we demonstrate a comprehensive high-pressure study on the insulating samples with pure 245 phase and biphasic SC samples. We find that the insulating behavior can be completely suppressed by pressure in the insulating samples and also identify an intermediate metallic (M) state. The Mott insulating (MI) state of the 245 phase and the M state coexist over a significant range of pressure up to ß10 GPa, the same pressure at which the superconductivity of the SC samples vanishes. Our results reveal the M state as a pathway that connects the insulating and SC phases of the alkaline iron selenides and indicate that the coexistence and interplay between the MI and M states is a necessary condition for superconductivity. Finally, we interpret the M state in terms of an orbital selectivity of the correlated 3d electrons.