Publications (11)22.11 Total impact
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Article: Microscopic Coexistence of Superconductivity and Antiferromagnetism in Underdoped Ba(Fe_{1-x}Ru_{x})_{2}As_{2}
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ABSTRACT: We use ^{75}As nuclear magnetic resonance to investigate the local electronic properties of Ba(Fe_{1-x}Ru_{x})_{2}As_{2} (x=0.23). We find two phase transitions: to antiferromagnetism at T_{N}≈60 K and to superconductivity at T_{C}≈15 K. Below T_{N}, our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4μ_{B}/Fe. The spin-lattice relaxation rate 1/^{75}T_{1} is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below T_{C}, 1/^{75}T_{1} on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.Physical Review Letters 11/2012; 109(19):197002. · 7.37 Impact Factor -
Article: Microscopic coexistence of superconductivity and antiferromagnetism in underdoped Ba(Fe1-xRux)2As2
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ABSTRACT: We use $^{75}$As nuclear magnetic resonance (NMR) to investigate the local electronic properties of Ba(Fe$_{1-x}$Ru$_{x}$)$_2$As$_2$ ($x =$ 0.23). We find two phase transitions, to antiferromagnetism at $T_N \approx$ 60 K and to superconductivity at $T_C \approx$ 15 K. Below $T_N$, our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4 $\mu_B$/Fe. The spin-lattice relaxation rate $1/^{75}T_1$ is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below $T_C$, $1/^{75}T_1$ on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.05/2012; -
Article: Electron spin resonance in Eu based Fe pnictides
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ABSTRACT: The phase diagrams of EuFe$_{2-x}$Co$_x$As$_2$ $(0 \leq x \leq 0.4)$ and EuFe$_2$As$_{2-y}$P$_y$ $(0 \leq y \leq 0.43)$ are investigated by Eu$^{2+}$ electron spin resonance (ESR) in single crystals. From the temperature dependence of the linewidth $\Delta H(T)$ of the exchange narrowed ESR line the spin-density wave (SDW) $(T < T_{\rm SDW})$ and the normal metallic regime $(T > T_{\rm SDW})$ are clearly distinguished. At $T > T_{\rm SDW}$ the isotropic linear increase of the linewidth is driven by the Korringa relaxation which measures the conduction-electron density of states at the Fermi level. For $T < T_{\rm SDW}$ the anisotropy probes the local ligand field, while the coupling to the conduction electrons disappears. With increasing substitution $x$ or $y$ the transition temperature $T_{\rm SDW}$ decreases linearly accompanied by a linear decrease of the Korringa-relaxation rate from 8 Oe/K at $x=y=0$ down to 3 Oe/K at the onset of superconductivity at $x \approx 0.2$ or at $y \approx 0.3$, above which it remains nearly constant. Comparative ESR measurements on single crystals of the Eu diluted SDW compound Eu$_{0.2}$Sr$_{0.8}$Fe$_2$As$_2$ and superconducting (SC) Eu$_{0.22}$Sr$_{0.78}$Fe$_{1.72}$Co$_{0.28}$As$_2$ corroborate the leading influence of the ligand field on the Eu$^{2+}$ spin relaxation in the SDW regime as well as the Korringa relaxation in the normal metallic regime. Like in Eu$_{0.5}$K$_{0.5}$Fe$_2$As$_2$ a coherence peak is not detected in the latter compound at $T_{\rm c}=21$ K, which is in agreement with the expected complex anisotropic SC gap structure.05/2012; -
Article: Anisotropic Dirac fermions in a Bi square net of SrMnBi2.
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ABSTRACT: We report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi(2), based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the (SrBi)(+) layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi(2) is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of ~8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions.Physical Review Letters 09/2011; 107(12):126402. · 7.37 Impact Factor -
Article: Anisotropic Dirac Fermions in a Bi Square Net of SrMnBi 2
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ABSTRACT: We report the observation of highly anisotropic Dirac fermions in a Bi square net of SrMnBi 2 , based on a first-principles calculation, angle-resolved photoemission spectroscopy, and quantum oscillations for high-quality single crystals. We found that the Dirac dispersion is generally induced in the ðSrBiÞ þ layer containing a double-sized Bi square net. In contrast to the commonly observed isotropic Dirac cone, the Dirac cone in SrMnBi 2 is highly anisotropic with a large momentum-dependent disparity of Fermi velocities of $8. These findings demonstrate that a Bi square net, a common building block of various layered pnictides, provides a new platform that hosts highly anisotropic Dirac fermions.Physical Review Letters 09/2011; 20. · 7.37 Impact Factor -
Article: Evolution of transport properties of BaFe2-xRuxAs2 in a wide range of isovalent Ru substitution
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ABSTRACT: The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are investigated by measuring resistivity and Hall coefficient on high-quality single crystals in a wide range of doping (0 < x < 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behaviors with a linear-temperature dependence of resistivity and a strong temperature dependence of Hall coefficient with a sign change. Upon higher doping, however, both of them recover conventional Fermi-liquid behaviors. Strong doping dependence of Hall coefficient together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multi-band scenario.09/2011; -
Article: Electron-hole asymmetry in Co- and Mn-doped SrFe2As2
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ABSTRACT: Phase diagram of electron and hole-doped SrFe2As2 single crystals is investigated using Co and Mn substitution at the Fe-sites. We found that the spin-density-wave state is suppressed by both dopants, but the superconducting phase appears only for Co (electron)-doping, not for Mn (hole)-doping. Absence of the superconductivity by Mn-doping is in sharp contrast to the hole-doped system with K-substitution at the Sr sites. Distinct structural change, in particular the increase of the Fe-As distance by Mn-doping is important to have a magnetic and semiconducting ground state as confirmed by first principles calculations. The absence of electron-hole symmetry in the Fe-site-doped SrFe2As2 suggests that the occurrence of high-Tc superconductivity is sensitive to the structural modification rather than the charge doping. Comment: 7 pages, 6 figures04/2010; -
Article: Coupling of localized moments and itinerant electrons in EuFe2As2 single crystals studied by Electron Spin Resonance
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ABSTRACT: Electron spin resonance measurements in EuFe2As2 single crystals revealed an absorption spectrum of a single resonance with Dysonian lineshape. Above the spin-density wave transition at T_SDW = 190 K the spectra are isotropic and the spin relaxation is strongly coupled to the CEs resulting in a Korringa-like increase of the linewidth. Below T_SDW, a distinct anisotropy develops and the relaxation behavior of the Eu spins changes drastically into one with characteristic properties of a magnetic insulating system, where dipolar and crystal-field interactions dominate. This indicates a spatial confinement of the conduction electrons to the FeAs layers in the SDW state. Comment: 4 pages, 4 figures09/2009; -
Article: Strong reduction of the Korringa relaxation in the spin-density wave regime of EuFe_ {2} As_ {2} observed by electron spin resonance
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ABSTRACT: Electron spin resonance measurements in EuFe2As2 single crystals revealed an absorption spectrum of a single resonance with Dysonian line shape. Above the spin-density wave (SDW) transition at TSDW=190 K the spectra are isotropic and the Eu spins relax via the conduction electrons resulting in a Korringa-type increase in the linewidth. Below TSDW, a distinct anisotropy develops and the relaxation behavior of the Eu spins changes drastically into one with characteristic properties of a magnetic insulating system, where dipolar and crystal-field interactions dominate. This indicates a spatial confinement of the conduction electrons to the FeAs layers in the SDW state.Phys. Rev. B. 81(2). -
Article: Electron-hole asymmetry in Co-and Mn-doped SrFe_ {2} As_ {2}
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ABSTRACT: Phase diagrams of electron- and hole-doped SrFe2As2 single crystals are investigated using Co and Mn substitution at the Fe sites. We find that the spin-density-wave state is suppressed by both dopants but the superconducting phase appears only for Co (electron) doping, not for Mn (hole) doping. Absence of the superconductivity by Mn doping is in sharp contrast to the hole-doped system with K substitution at the Sr sites. First-principles calculations based on detailed structural investigations reveal that a distinct structural change, i.e., the increase in the Fe-As distance by Mn doping is the most decisive factor to induce a magnetic and semiconducting ground state in SrFe2−xMnxAs2. The absence of electron-hole symmetry in the phase diagrams of the Fe-site doped SrFe2As2 suggests that the occurrence of high-Tc superconductivity is sensitive to the structural modification rather than the carrier doping.Phys. Rev. B. 82(2). -
Article: Evolution of transport properties of BaFe_ {2− x} Ru_ {x} As_ {2} in a wide range of isovalent Ru substitution
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ABSTRACT: The effects of isovalent Ru substitution at the Fe sites of BaFe2−xRuxAs2 are investigated by measuring resistivity (ρ) and Hall coefficient (RH) on high-quality single crystals in a wide range of doping (0 ≤ x ≤ 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 ≤ x ≤ 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behavior with a linear-temperature dependence of ρ and a strong temperature dependence of RH with a sign change. Upon higher doping, however, both ρ and RH recover conventional Fermi-liquid behavior. Strong doping dependence of RH together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multiband scenario.Phys. Rev. B. 85(2).
Top co-authors
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Institutions
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2009–2012
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Pohang University of Science and Technology
- Department of Physics
Andong, North Gyeongsang, South Korea
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