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ABSTRACT: We use inelastic neutron scattering to study the effect of an in-plane
magnetic field on the magnetic resonance in optimally doped superconductors
FeSe$_{0.4}$Te$_{0.6}$ ($T_c=14$ K) and BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$
($T_c=20$ K). While the magnetic field up to 14.5 Tesla does not change the
energy of the resonance, it particially suppresses $T_c$ and the corresponding
superconductivity-induced intensity gain of the mode. However, we find no
direct evidence for the field-induced spin-1 Zeeman splitting of the resonance.
Therefore, it is still unclear if the resonance is the long-sought
singlet-triplet excitation directly coupled to the superconducting electron
Cooper pairs.
Physical Review B 07/2011; 84:024518. · 3.69 Impact Factor
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ABSTRACT: We report a systematic investigation on c-axis point-contact Andreev
reflection (PCAR) in BaFe$_{2-x}$Ni$_x$As$_2$ superconducting single crystals
from underdoped to overdoped regions (0.075 $\leq x\leq 0.15$). At optimal
doping ($x=0.1$) the PCAR spectrum feature the structures of two
superconducting gap and electron-boson coupling mode. In the $s\pm$ scenario,
quantitative analysis using a generalized Blonder-Tinkham-Klapwijk (BTK)
formalism with two gaps: one isotropic and another angle dependent, suggest a
nodeless state in strong-coupling limit with gap minima on the Fermi surfaces.
Upon crossing above the optimal doping ($x > 0.1$), the PCAR spectrum show an
in-gap sharp narrow peak at low bias, in contrast to the case of underdoped
samples ($x < 0.1$), signaling the onset of deepened gap minima or nodes in the
superconducting gap. This result provides evidence of the modulation of the gap
amplitude with doping concentration, consistent with the calculations for the
orbital dependent pair interaction mediated by the antiferromagnetic spin
fluctuations.
06/2011;
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ABSTRACT: We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe0.4Te0.6 (Tc=14 K) and BaFe1.9Ni0.1As2 (Tc=20 K). While the magnetic field up to 14.5 T does not change the energy of the resonance, it partially suppresses Tc and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore it is still unclear if the resonance is the long-sought singlet-triplet excitation directly coupled to the superconducting electron Cooper pairs.
Physical Review B 05/2011; 84(2):024518. · 3.69 Impact Factor
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Shiliang Li,
Chenglin Zhang,
Meng Wang, Hui-Qian Luo,
Xingye Lu,
Enrico Faulhaber,
Astrid Schneidewind,
Peter Link,
Jiangping Hu,
Tao Xiang,
Pengcheng Dai
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ABSTRACT: We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe(1-x)Tex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.
Physical Review Letters 10/2010; 105(15):157002. · 7.37 Impact Factor
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Shiliang Li,
Chenglin Zhang,
Meng Wang, Hui-qian Luo,
Xingye Lu,
Enrico Faulhaber,
Astrid Schneidewind,
Peter Link,
Jiangping Hu,
Tao Xiang,
Pengcheng Dai
[show abstract]
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ABSTRACT: We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe$_{0.4}$Te$_{0.6}$ and essentially non-superconducting (NSC) FeSe$_{0.45}$Te$_{0.55}$. In contrast to BaFe$_{2-x}$(Co,Ni)$_{x}$As$_2$, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe$_{0.4}$Te$_{0.6}$ are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe$_{0.45}$Te$_{0.55}$, we argue that the observed incommensurate spin excitations in FeSe$_{1-x}$Te$_{x}$ are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling. Comment: 4 pages, 4 figures
01/2010;
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ABSTRACT: We report a detailed investigation on the lower critical field H{c1} of the superconducting Ba0.6K0.4Fe2As2 (122) single crystals. A pronounced kink is observed on the H{c1}(T) curve, which is attributed to the existence of two superconducting gaps. By fitting the data H{c1}(T) to the two-gap BCS model in the full temperature region, a small gap of Delta{a}(0)=2.0+/-0.3 meV and a large gap of Delta{b}(0)=8.9+/-0.4 meV are obtained. The in-plane penetration depth lambda{ab}(0) is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in 122 superconductors.
Physical Review Letters 01/2009; 101(25):257006. · 7.37 Impact Factor
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ABSTRACT: The temperature dependent resistivity of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ (x = 0.23, 0.25, 0.28 and 0.4) single crystals and the angle dependent resistivity of superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystals were measured in magnetic fields up to 9 T. The measurements of temperature dependent resistivity for samples with different doping levels revealed very high upper critical fields which increase with the transition temperature monotonously, and a very low superconducting anisotropy ratio $\Gamma=H_{c2}^{ab}/H_{c2}^c \approx$ 2. By scaling the resistivity in the frame of the anisotropic Ginzburg-Landau theory, the angle dependent resistivity of the Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ single crystal measured with different magnetic fields at a certain temperature collapsed onto one curve. As the only scaling parameter, the anisotropy $\Gamma$ was determined alternatively for each temperature and was found to be between two and three. Comment: 5 pages, 5 figures
08/2008;
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ABSTRACT: We report a detailed investigation on the lower critical field $H_{c1}$ of the superconducting Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (FeAs-122) single crystals. A pronounced kink is observed on the $H_{c1}(T)$ curve, which is attributed to the existence of two superconducting gaps. By fitting the data $H_{c1}(T)$ to the two-gap BCS model in full temperature region, a small gap of $\Delta_a(0)=2.0\pm 0.3$ meV and a large gap of $\Delta_b(0)=8.9\pm 0.4$ meV are obtained. The in-plane penetration depth $\lambda_{ab}(0)$ is estimated to be 105 nm corresponding to a rather large superfluid density, which points to the breakdown of the Uemura plot in FeAs-122 superconductors. Comment: 5 pages, 4 figures
08/2008;
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ABSTRACT: We report an investigation on c-axis point-contact Andreev reflection (PCAR) spectroscopy of optimally doped BaFe1.9Ni0.1As2 single crystals. The PCAR spectrum features the structures of a conductance peak with the energy scale of an electron-boson coupling mode. Quantitative analysis using a generalized Blonder-Tinkham-Klapwijk formalism with two gaps—one isotropic and another angle dependent—resolves a nodeless state in the strong-coupling limit with gap minima on the Fermi surfaces. Using the extracted superconducting energy gap value, a characteristic boson energy Ωb is obtained. Our results are consistent with the pairing interaction mediated by the antiferromagnetic spin fluctuations.
Phys. Rev. B. 86(6).
