6 research outputs found
Effects of Disorder in FeSe : An Ab Initio Study
Using the coherent-potential approximation, we have studied the effects of
excess Fe, Se-deficiency, and substitutions of S, Te on Se sub-lattice and Co,
Ni and Cu on Fe sub-lattice in FeSe. Our results show that (i) a small amount
of excess Fe substantially disorders the Fe-derived bands while Se-deficiency
affects mainly the Se-derived bands, (ii) the substitution of S or Te enhances
the possibility of Fermi surface nesting, specially in FeSeTe,
in spite of disordering the Se-derived bands, (iii) the electron doping through
Co, Ni or Cu disorders the system and pushes down the Fe-derived bands, thereby
destroying the possibility of Fermi surface nesting. A comparison of these
results with the rigid-band, virtual-crystal and supercell approximations
reveals the importance of describing disorder with the coherent-potential
approximation.Comment: Redone VCA calculations, and some minor changes. (Accepted for
publication in Journal of Physics:Condensed Matter
Pressure evolution of the low-temperature crystal structure and bonding of the superconductor FeSe (Tc=37 K)
FeSe with the PbO structure is a key member of the family of new high-
iron pnictide and chalcogenide superconductors, as while it possesses the basic
layered structural motif of edge-sharing distorted FeSe tetrahedra, it
lacks interleaved ion spacers or charge-reservoir layers. We find that
application of hydrostatic pressure first rapidly increases which attains
a broad maximum of 37 K at 7 GPa (this is one of the highest 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 -FeSe,
revealing an extremely soft solid with a bulk modulus, = 30.7(1.1) GPa
and strong bonding anisotropy between inter- and intra-layer directions that
transforms to the more densely packed -polymorph above 9 GPa. The
non-monotonic () 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
High temperature superconductivity (Tc onset at 34K) in the high pressure orthorhombic phase of FeSe
We have studied the structural and superconducting properties of tetragonal
FeSe under pressures up to 26GPa using synchrotron radiation and diamond anvil
cells. The bulk modulus of the tetragonal phase is 28.5(3)GPa, much smaller
than the rest of Fe based superconductors. At 12GPa we observe a phase
transition from the tetragonal to an orthorhombic symmetry. The high pressure
orthorhombic phase has a higher Tc reaching 34K at 22GPa.Comment: 15 pages, 4 figure
Superconducting and normal phases of FeSe single crystals at high pressure
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
Interplay between magnetism and superconductivity and appearance of a second superconducting transition in alpha-FeSe at high pressure
We synthesized tetragonal alpha-FeSe by melting a powder mixture of iron and
selenium at high pressure. Subsequent annealing at normal pressure results in
removing traces of hexagonal beta- FeSe, formation of a rather sharp transition
to superconducting state at Tc ~ 7 K, and the appearance of a magnetic
transition near Tm = 120 K. Resistivity and ac-susceptibility were measured on
the annealed sample at hydrostatic pressure up to 4.5 GPa. A magnetic
transition visible in ac-susceptibility shifts down under pressure and the
resistive anomaly typical for a spin density wave (SDW) antiferromagnetic
transition develops near the susceptibility anomaly. Tc determined by the
appearance of a diamagnetic response in susceptibility, increases linearly
under pressure at a rate dTc/dP = 3.5 K/GPa. Below 1.5 GPa, the resistive
superconducting transition is sharp; the width of transition does not change
with pressure; and, Tc determined by a peak in drho/dT increases at a rate ~
3.5 K/GPa. At higher pressure, a giant broadening of the resistive transition
develops. This effect cannot be explained by possible pressure gradients in the
sample and is inherent to alpha-FeSe. The dependences drho(T)/dT show a
signature for a second peak above 3 GPa which is indicative of the appearance
of another superconducting state in alpha-FeSe at high pressure. We argue that
this second superconducting phase coexists with SDW antiferromagnetism in a
partial volume fraction and originates from pairing of charge carriers from
other sheets of the Fermi surface