Pressure-induced effects on the structure of the FeSe superconductor (original) (raw)
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Physical Review B, 2009
FeSe with the PbO structure is a key member of the family of new high-Tc iron pnictide and chalcogenide superconductors, as while it possesses the basic layered structural motif of edge-sharing distorted FeSe4 tetrahedra, it lacks interleaved ion spacers or charge-reservoir layers. We find that application of hydrostatic pressure first rapidly increases Tc which attains a broad maximum of 37 K at ∼7 GPa (this is one of the highest Tc ever reported for a binary solid) before decreasing to 6 K upon further compression to ∼14 GPa. Complementary synchrotron X-ray diffraction at 16 K was used to measure the low-temperature isothermal compressibility of α-FeSe, revealing an extremely soft solid with a bulk modulus, K0 = 30.7(1.1) GPa and strong bonding anisotropy between inter-and intra-layer directions that transforms to the more densely packed β-polymorph above ∼9 GPa. The non-monotonic Tc(P ) behavior of FeSe coincides with drastic anomalies in the pressure evolution of the interlayer spacing, pointing to the key role of this structural feature in modulating the electronic properties. PACS numbers: 74.70.Dd, 74.25.Ha, 61.05.C-
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
Pressure effect on superconductivity in FeSe0.5Te0.5
physica status solidi (b), 2016
Due to the simple layered structure, isostructural FeSe and FeSe 0.5 Te 0.5 are clue compounds for understanding the principal mechanisms of superconductivity in the family of Fe-based superconductors. High-pressure magnetic, structural and Mössbauer studies have been performed on single-crystalline samples of superconducting FeSe 0.5 Te 0.5 with T c = 13.5 K. Susceptibility data have revealed a strong increase of T c up to 19.5 K for pressures up to 1.3 GPa, followed by a plateau in the T c (p) dependence up to 5.0 GPa. Further pressure increase leads to a disappearance of the superconducting state around 7.0 GPa. X-ray diffraction and Mössbauer studies explain this fact by a tetragonal-to-hexagonal structural phase transition. Mössbauer parameters of the non-superconducting high-pressure phase indicate less covalency of Fe-Se bonds. Based on structural and susceptibility data we conclude about a common character of T c (p) diagrams for both FeSe and FeSe 0.5 Te 0.5 superconductors.
FAST TRACK COMMUNICATION: Superconducting and normal phases of FeSe single crystals at high pressure
Journal of Physics-condensed Matter, 2009
We report on the synthesis of superconducting single crystals of FeSe, and their characterization by X-ray diffraction, magnetization and resistivity. We have performed ac susceptibility measurements under high pressure in a hydrostatic liquid argon medium up to 14 GPa and we find that TC increases up to 33-36 K in all samples, but with slightly different pressure dependences on different samples. Above 12 GPa no traces of superconductivity are found in any sample. We have also performed a room temperature high pressure X-ray diffraction study up to 12 GPa on a powder sample, and we find that between 8.5 GPa and 12 GPa, the tetragonal PbO structure undergoes a structural transition to a hexagonal structure. This transition results in a volume decrease of about 16%, and is accompanied by the appearance of an intermediate, probably orthorhombic phase.
High-Pressure Synthesis and the Enhancement of the Superconducting Properties of FeSe0.5Te0.5
Materials
A series of FeSe0.5Te0.5 bulk samples have been prepared using the high gas pressure and high-temperature synthesis (HP-HTS) method to optimize the growth conditions for the first time and investigated for their superconducting properties using structural, microstructure, transport, and magnetic measurements to reach the final conclusions. Ex situ and in situ processes are used to prepare bulk samples under a range of growth pressures using Ta-tube and without Ta-tube. The parent compound synthesized by convenient synthesis method at ambient pressure (CSP) exhibits a superconducting transition temperature of 14.8 K. Our data demonstrate that the prepared FeSe0.5Te0.5 sealed in a Ta-tube is of better quality than the samples without a Ta-tube, and the optimum growth conditions (500 MPa, 600 °C for 1 h) are favorable for the development of the tetragonal FeSe0.5Te0.5 phase. The optimum bulk FeSe0.5Te0.5 depicts a higher transition temperature of 17.3 K and a high critical current dens...
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...
The Pressure Effects on Electronic Structure of Iron Chalcogenide Superconductors FeSe1-xTex
Acta Physica Polonica A, 2012
We study the electronic structure of iron-based superconductors F eSe1−xT ex within the density functional theory. We pay particular attention to the pressure effects on the Fermi surface (FS) topology, which seem to be correlated with a critical superconducting temperature TC of iron chalcogenides and pnictides. A reduction of the FS nesting between hole and electron cylinders with increasing pressure is observed, which can lead to higher values of TC . The tellurium substitution into selenium sites yields FS changes similar to the pressure effect.
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.
Interrelation of superconductivity and magnetism in FeSe1−xTex compounds. Pressure effects
Low Temperature Physics, 2014
The effect of isotropic pressures P up to 5 kbar on the superconducting transition temperature T c of the FeSe 1Àx Te x system (x ¼ 0, 0.85, 0.88, 0.90) is studied. For the first time, a change in the sign of the effect of pressure on T c on going from FeSe to the tellurium-rich alloys is observed. This makes it possible to specify more precisely the form of the dependence of the pressure derivative dT c /dP on composition in this system. This dependence is compared with first principles calculations of the electron structure and magnetism of FeSe, FeTe, and FeSe 0.5 Te 0.5 as functions of pressure, as well as with our earlier experimental data on the effect of pressure on the magnetic susceptibility of the normal state in FeSe and FeTe. This comparison is indicative of a competitive interrelationship between superconductivity and magnetism in tellurium rich FeSe 1Àx Te x compounds. V C 2014 AIP Publishing LLC.[http://dx.