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ABSTRACT: We report neutron inelastic scattering measurements on the normal and superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with previous measurements on RbxFe2−ySe2, we observe two distinct spin excitation signals: (i) spin-wave excitations characteristic of the block antiferromagnetic order found in insulating AxFe2−ySe2 compounds, and (ii) a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane wave-vector of (0.25,0.5). The resonance peak increases below Tc=27 K, and has a similar absolute intensity to the resonance peaks observed in other Fe-based superconductors. The existence of a magnetic resonance in the spectrum of RbxFe2−ySe2 and now of CsxFe2−ySe2 suggests that this is a common feature of superconductivity in this family. The low-energy spin-wave excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity, consistent with the notion of spatially separate magnetic and superconducting phases.
Physical Review B 09/2012; 86:094528. · 3.69 Impact Factor
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ABSTRACT: We report neutron inelastic scattering measurements on the normal and
superconducting states of single-crystalline Cs0.8Fe1.9Se2. Consistent with
previous measurements on Rb(x)Fe(2-y)Se2, we observe two distinct spin
excitation signals: (i) spin-wave excitations characteristic of the block
antiferromagnetic order found in insulating A(x)Fe(2-y)Se2 compounds, and (ii)
a resonance-like magnetic peak localized in energy at 11 meV and at an in-plane
wave vector of (0.25, 0.5). The resonance peak increases below Tc = 27 K, and
has a similar absolute intensity to the resonance peaks observed in other
Fe-based superconductors. The existence of a magnetic resonance in the spectrum
of Rb(x)Fe(2-y)Se2 and now of Cs(x)Fe(2-y)Se2 suggests that this is a common
feature of superconductivity in this family. The low energy spin-wave
excitations in Cs0.8Fe1.9Se2 show no measurable response to superconductivity,
consistent with the notion of spatially separate magnetic and superconducting
phases.
08/2012;
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ABSTRACT: We have studied the high temperature multiferroic cupric oxide using
polarized neutron diffraction as a function of temperature and applied electric
field. We find that the chiral domain population can be varied using an
external electric field applied along the b axis. Using representation analysis
we derive the incommensurate magnetic structure in the multiferroic phase. The
origin of the magnetoelectric coupling is consistent with models based on the
inverse Dzyaloshinskii-Moriya interaction, but is different from the simple
cycloidal mechanism.
01/2012;
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ABSTRACT: We have used polarized-neutron inelastic scattering to resolve the spin fluctuations in superconducting FeSe0.5Te0.5 into components parallel and perpendicular to the layers. A spin resonance at an energy of 6.5 meV is observed to develop below Tc in both fluctuation components. The resonance peak is anisotropic, with the in-plane component slightly larger than the out-of-plane component. Away from the resonance peak, the magnetic fluctuations are isotropic in the energy range studied. The results are consistent with a dominant singlet pairing state with s± symmetry, with a possible minority component of different symmetry.
Physical Review B 05/2011; 83(18):180506. · 3.69 Impact Factor
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ABSTRACT: Superconductivity in layered copper oxide compounds emerges when charge carriers are added to antiferromagnetically ordered CuO(2) layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to superconductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual 'hour-glass' feature in the momentum-resolved magnetic spectrum that is present in a wide range of superconducting and non-superconducting materials. There is no widely accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, and this idea is supported by measurements on stripe-ordered La(1.875)Ba(0.125)CuO(4) (ref. 15). Many copper oxides without stripe order, however, also exhibit an hour-glass spectrum. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means that its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper oxide superconductors arises from fluctuating stripes.
Nature 03/2011; 471(7338):341-4. · 36.28 Impact Factor
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ABSTRACT: The superconducting and magnetic properties of FeySe0.25Te0.75 single crystals (0.9≤y≤1.1) were studied by means of x-ray diffraction, superconducting quantum interference device magnetometry, muon-spin rotation, and elastic neutron diffraction. The samples with y<1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y≥1 but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the FeySe1−xTex can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.
Phys. Rev. B. 12/2010; 82(21).
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ABSTRACT: We have used polarized-neutron inelastic scattering to resolve the spin
fluctuations in superconducting FeSe0.5Te0.5 into components parallel and
perpendicular to the layers. A spin resonance at an energy of 6.5 meV is
observed to develop below T_c in both fluctuation components. The resonance
peak is anisotropic, with the in-plane component slightly larger than the
out-of-plane component. Away from the resonance peak the magnetic fluctuations
are isotropic in the energy range studied. The results are consistent with a
dominant singlet pairing state with s^{\pm} symmetry, with a possible minority
component of different symmetry.
Physical Review B 10/2010; 83:180506(R). · 3.69 Impact Factor
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ABSTRACT: The superconducting and magnetic properties of Fe$_{y}$Se$_{0.25}$Te$_{0.75}$ single crystals ($0.9\leq y \leq1.1$) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with $y<1$ exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for $y\geq 1$, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fe$_{y}$Se$_{1-x}$Te$_{x}$ can be tuned not only by varying the Se/Te ratio but also by changing the Fe content.
10/2010;
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ABSTRACT: We report measurements of the magnetic excitation spectrum of the layered antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0–250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small (∼1 meV) dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of Co2+ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.
Phys. Rev. B. 09/2010; 82(18).
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ABSTRACT: We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe(1.01)Se(0.50)Te(0.50) and non-superconducting Fe(1.10)Se(0.25)Te(0.75). We confirm the existence of a spin resonance in the superconducting phase of Fe(1.01)Se(0.50)Te(0.50), at an energy of 7 meV and a wavevector of (1/2, 1/2, 0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2, 1/2, 0) ± (0.18, - 0.18, 0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low energy magnetic excitation is also observed in Fe(1.10)Se(0.25)Te(0.75) close to the commensurate antiferromagnetic ordering wavevector (1/2 - δ, 0, 1/2), where δ≈0.03. The magnetic correlations in both samples are found to be quasi-two-dimensional in character and persist well above the magnetic (Fe(1.10)Se(0.25)Te(0.75)) and superconducting (Fe(1.01)Se(0.50)Te(0.50)) transition temperatures.
Journal of Physics Condensed Matter 04/2010; 22(14):142202. · 2.55 Impact Factor
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ABSTRACT: We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2, 1/2, 0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2, 1/2, 0) ± (0.18, − 0.18, 0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low energy magnetic excitation is also observed in Fe1.10Se0.25Te0.75 close to the commensurate antiferromagnetic ordering wavevector (1/2 − δ, 0, 1/2), where δ≈0.03. The magnetic correlations in both samples are found to be quasi-two-dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition temperatures.
Journal of Physics Condensed Matter 03/2010; 22(14):142202. · 2.55 Impact Factor
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ABSTRACT: We have used inelastic neutron scattering and muon-spin rotation to compare the low energy magnetic excitations in single crystals of superconducting Fe1.01Se0.50Te0.50 and non-superconducting Fe1.10Se0.25Te0.75. We confirm the existence of a spin resonance in the superconducting phase of Fe1.01Se0.50Te0.50, at an energy of 7 meV and a wavevector of (1/2,1/2,0). The non-superconducting sample exhibits two incommensurate magnetic excitations at (1/2,1/2,0)\pm(0.18,-0.18,0) which rise steeply in energy, but no resonance is observed at low energies. A strongly dispersive low-energy magnetic excitation is also observed in Fe1.10Se0.25Te0.75 close to the commensurate antiferromagnetic ordering wavevector (1/2-\delta,0,1/2) where \delta \approx 0.03. The magnetic correlations in both samples are found to be quasi-two dimensional in character and persist well above the magnetic (Fe1.10Se0.25Te0.75) and superconducting (Fe1.01Se0.50Te0.50) transition temperatures. Comment: 10 pages, 4 figures
03/2010;
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R. Khasanov,
M. Bendele,
A. Amato, P. Babkevich,
A. T. Boothroyd,
A. Cervellino,
K. Conder,
S. N. Gvasaliya,
H. Keller,
H. -H. Klauss,
H. Luetkens,
E. Pomjakushina,
B. Roessli
[show abstract]
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ABSTRACT: We report an investigation into the superconducting and magnetic properties of Fe_{1+y}Se_{x}Te_{1-x} single crystals by magnetic susceptibility, muon spin rotation, and neutron diffraction. We find three regimes of behavior in the phase diagram for 0\leq x\leq 0.5: (i) commensurate magnetic order for x< 0.1, (ii) bulk superconductivity for $x\lesssim 0.1$, and (iii) a range \sim 0.25\leq x\leq 0.45 in which superconductivity coexists with static incommensurate magnetic order. The results are qualitatively consistent with a two-band mean-field model in which itinerant magnetism and extended s-wave superconductivity are competing order parameters. Comment: 4 pages, 4 figures
07/2009;
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R. Khasanov,
M. Bendele,
A. Amato, P. Babkevich,
A. T. Boothroyd,
A. Cervellino,
K. Conder,
S. N. Gvasaliya,
H. Keller,
H.-H. Klauss,
H. Luetkens,
V. Pomjakushin,
E. Pomjakushina,
B. Roessli
[show abstract]
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ABSTRACT: We have studied the superconducting and magnetic properties of Fe1+ySexTe1−x single crystals (0≤x≤0.5) by magnetic susceptibility, muon-spin rotation, and neutron diffraction. We find three regimes of behavior: (i) commensurate magnetic order for x≲0.1, (ii) bulk superconductivity for x∼0.5, and (iii) a range x≈0.25–0.45 in which superconductivity coexists with incommensurate magnetic order. The results are qualitatively consistent with two-band mean-field models in which itinerant magnetism and extended s-wave superconductivity are competing order parameters.
Phys. Rev. B. 80(14).
