58 research outputs found
Interplay between Superconductivity and Magnetism in Rb0.8Fe1.6Se2 under Pressure
High-pressure magnetization, structural and 57Fe M\"ossbauer studies were
performed on superconducting Rb0.8Fe1.6Se2.0 with Tc = 32.4 K. The
superconducting transition temperature gradually decreases on increasing
pressure up to 5.0 GPa followed by a marked step-like suppression of
superconductivity near 6 GPa. No structural phase transition in the Fe
vacancy-ordered superstructure is observed in synchrotron XRD studies up to
15.6 GPa, while the M\"ossbauer spectra above 5 GPa reveal the appearance of a
new paramagnetic phase and significant changes in the magnetic and electronic
properties of the dominant antiferromagnetic phase, coinciding with the
disappearance of superconductivity. These findings underline the strong
correlation between antiferromagnetic order and superconductivity in
phase-separated AxFe2-x/2Se2 (A = K, Rb, Cs) superconductors
Pressure-induced magnetic collapse and metallization of
The crystal structure, magnetic ordering, and electrical resistivity of
TlFe1.6Se2 were studied at high pressures. Below ~7 GPa, TlFe1.6Se2 is an
antiferromagnetically ordered semiconductor with a ThCr2Si2-type structure. The
insulator-to-metal transformation observed at a pressure of ~ 7 GPa is
accompanied by a loss of magnetic ordering and an isostructural phase
transition. In the pressure range ~ 7.5 - 11 GPa a remarkable downturn in
resistivity, which resembles a superconducting transition, is observed below 15
K. We discuss this feature as the possible onset of superconductivity
originating from a phase separation in a small fraction of the sample in the
vicinity of the magnetic transition.Comment: 12 pages, 5 figure
Exotic magnetism in the alkali sesquoxides Rb4O6 and Cs4O6
Among the various alkali oxides the sesquioxides Rb4O6 and Cs4O6 are of
special interest. Electronic structure calculations using the local
spin-density approximation predicted that Rb4O6 should be a half-metallic
ferromagnet, which was later contradicted when an experimental investigation of
the temperature dependent magnetization of Rb4O6 showed a low-temperature
magnetic transition and differences between zero-field-cooled (ZFC) and
field-cooled (FC) measurements. Such behavior is known from spin glasses and
frustrated systems. Rb4O6 and Cs4O6 comprise two different types of dioxygen
anions, the hyperoxide and the peroxide anions. The nonmagnetic peroxide anions
do not contain unpaired electrons while the hyperoxide anions contain unpaired
electrons in antibonding pi*-orbitals. High electron localization (narrow
bands) suggests that electronic correlations are of major importance in these
open shell p-electron systems. Correlations and charge ordering due to the
mixed valency render p-electron-based anionogenic magnetic order possible in
the sesquioxides. In this work we present an experimental comparison of Rb4O6
and the related Cs4O6. The crystal structures are verified using powder x-ray
diffraction. The mixed valency of both compounds is confirmed using Raman
spectroscopy, and time-dependent magnetization experiments indicate that both
compounds show magnetic frustration, a feature only previously known from d-
and f-electron systems
Structural phase transitions in the Ag2Nb4O11 – Na2Nb4O11 solid solution
The phase transitions between various structural modifications of the natrotantite-structured system
xAg2Nb4O11 – (1-x)Na2Nb4O11 have been investigated and a phase diagram constructed as a function
of temperature and composition. This shows three separate phase transition types: (1) paraelectric –
ferroelectric, (2) rhombohedral – monoclinic and (3) a phase transition within the ferroelectric
rhombohedral zone between space groups R3c and R3. The parent structure for the entire series has
space group R-3c. Compositions with x > 0.75 are rhombohedral at all temperatures whereas
compositions with x < 0.75 are all monoclinic at room temperature and below. At x = 0.75,
rhombohedral and monoclinic phases coexist with the phase boundary below room temperature being
virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic
phase field. No evidence was found for the R3–R3c phase boundary extending into the monoclinic
phase field and it is concluded that a triple point is formed
Upper critical field, lower critical field and critical current density of FeTe0.60Se0.40 single crystal
The transport and magnetic studies are performed on high quality
FeTe0.60Se0.40 single crystals to determine the upper critical fields (Hc2),
lower critical field (Hc1) and the Critical current density (Jc). The value of
upper critical field Hc2 are very large, whereas the activation energy as
determined from the slope of the Arrhenius plots are was found to be lower than
that in the FeAs122 superconductor. The lower critical field was determined in
ab direction and c direction of the crystal, and was found to have a anisotropy
of 'gamma'{=(Hc1//c) / (Hc1//b)} ~ 4. The magnetic isotherms measured up to 12
Tesla shows the presence of fishtail behavior. The critical current densities
at 1.8K of the single crystal was found to almost same in both ab and c
direction as 1X105 Amp/cm2 in low field regime.Comment: 9 pages, 6 figure
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
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