Showing papers by "Zhu-An Xu published in 2009"
TL;DR: The strong pairing strength is found to switch to the nested β FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.
Abstract: The discovery of high-temperature superconductivity in iron pnictides raised the possibility of an unconventional superconducting mechanism in multiband materials. The observation of Fermi-surface (FS)-dependent nodeless superconducting gaps suggested that inter-FS interactions may play a crucial role in superconducting pairing. In the optimally hole-doped Ba0.6K0.4Fe2As2, the pairing strength is enhanced simultaneously (2Δ/Tc≈7) on the nearly nested FS pockets, i.e., the inner hole-like (α) FS and the 2 hybridized electron-like FSs, whereas the pairing remains weak (2Δ/Tc≈3.6) in the poorly nested outer hole-like (β) FS. Here, we report that in the electron-doped BaFe1.85Co0.15As2, the FS nesting condition switches from the α to the β FS due to the opposite size changes for hole- and electron-like FSs upon electron doping. The strong pairing strength (2Δ/Tc≈6) is also found to switch to the nested β FS, indicating an intimate connection between FS nesting and superconducting pairing, and strongly supporting the inter-FS pairing mechanism in the iron-based superconductors.
275 citations
TL;DR: It is found that EuFe2(As0.7P0.3)(2) undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu2+ moments at 20 K, and supplies a rare example showing the coexistence of superconductivity and ferromagnetism in the ferroarsenide family.
Abstract: We have studied ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}{)}_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance, and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that ${\mathrm{EuFe}}_{2}({\mathrm{As}}_{0.7}{\mathrm{P}}_{0.3}{)}_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of ${\mathrm{Eu}}^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing the coexistence of superconductivity and ferromagnetism in the ferroarsenide family.
225 citations
TL;DR: In this article, the authors report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe$2}$(As$1-x}$P${x} $)$)
Abstract: We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. The P-for-As substitution results in shrinkage of lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with the maximum $T_c$ of 30 K emerges at $x$=0.32, coinciding with a magnetic quantum critical point (QCP) which is evidenced by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The $T_c$ values were found to decrease with further P doping, and no superconductivity was observed down to 2 K for $x\geq$ 0.77. The appearance of superconductivity in the vicinity of QCP hints to the superconductivity mechanism in iron-based arsenides.
161 citations
TL;DR: In this paper, the electric field effect on superconductivity in atomically thin flakes of NbSe, prepared by mechanical exfoliation, has been studied and it has been shown that these flakes are superconducting down to a thickness of a single unit cell consisting of two molecular layers.
Abstract: We studied the electric field effect on superconductivity in atomically thin flakes of ${\text{NbSe}}_{2}$ prepared by mechanical exfoliation. We found that these ${\text{NbSe}}_{2}$ flakes are superconducting down to a thickness of a single unit cell consisting of two molecular layers of ${\text{NbSe}}_{2}$ with an onset superconducting transition temperature $({T}_{c})$ up to 2.5 K. We demonstrated that the ${T}_{c}$ of the thinnest flakes can be modulated by an applied gate voltage with a ${T}_{c}$ shift as large as 200 mK, which is larger than expected from a simple consideration based on Bardeen-Cooper-Schrieffer theory. We discuss the possible reasons for the observed enhanced electric field modulation of ${T}_{c}$.
148 citations
TL;DR: The results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.
Abstract: We use inelastic neutron scattering to study magnetic excitations of the FeAs-based superconductor BaFe1.9Ni0.1As2 above and below its T-c (=20 K). In addition to gradually open a spin gap at the in-plane antiferromagnetic ordering wave vector (1, 0, 0), the effect of superconductivity is to form a three-dimensional resonance with clear dispersion along the c axis. The intensity of the resonance develops like a superconducting order parameter, and the mode occurs at distinctively different energies at (1, 0, 0) and (1, 0, 1). If the resonance energy is associated with the superconducting gap energy Delta, then Delta is dependent on the wave vector transfers along the c axis. These results suggest that one must be careful in interpreting the superconducting gap energies obtained by surface sensitive probes such as scanning tunneling microscopy and angle resolved photoemission.
111 citations
TL;DR: In this article, the authors reported the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO and showed that the Fe2As2 layers were squeezed while the La2O2 layers are stretched along the c-axis.
Abstract: We report the realization of superconductivity by an isovalent doping with phosphorus in LaFeAsO. X-ray diffraction shows that, with the partial substitution of P for As, the Fe2As2 layers are squeezed while the La2O2 layers are stretched along the c-axis. Electrical resistance and magnetization measurements show emergence of bulk superconductivity at ~10 K for the optimally doped LaFeAs1- xPxO (x=0.25–0.3). The upper critical field at zero temperature is estimated to be 27 T, much higher than that of the LaFePO superconductor. The occurrence of superconductivity is discussed in terms of chemical pressures and bond covalency.
97 citations
TL;DR: In this article, high-resolution angle-resolved photoemission spectroscopy was performed on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2.
Abstract: We have performed high-resolution angle-resolved photoemission spectroscopy on heavily electron-doped non-superconducting (SC) BaFe1.7Co0.3As2. We find that the two hole Fermi surface pockets at the Brillouin zone center observed in the hole-doped superconducting Ba0.6K0.4Fe2As2 are absent or very small in this compound, while the two electron pockets at the zone corner significantly expand due to electron doping by the Co substitution. Comparison of the Fermi surface between non-SC and SC samples indicates that the coexistence of hole and electron pockets connected via the antiferromagnetic wave vector is essential in realizing the mechanism of superconductivity in the iron-based superconductors.
91 citations
TL;DR: In this article, the magnetic and superconducting phase diagrams of a cobalt-doped single crystal were analyzed under magnetic fields parallel and perpendicular to the basal plane, respectively.
Abstract: We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped $\text{Eu}{({\text{Fe}}_{0.89}{\text{Co}}_{0.11})}_{2}{\text{As}}_{2}$ single crystal. We observed a resistivity drop at ${T}_{c}\ensuremath{\sim}21\text{ }\text{K}$, which shifts toward lower temperatures under external fields, suggesting a superconducting transition. The upper critical fields near ${T}_{c}$ show large anisotropy, in contrast with those of other ``122'' FeAs-based superconductors. Low-field magnetic susceptibility data also show evidence of superconductivity below 21 K. Instead of the expected zero resistance below ${T}_{c}$, however, a resistivity re-entrance appears at 17 K under zero field, coincident with the magnetic ordering of ${\text{Eu}}^{2+}$ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the ${\text{Eu}}^{2+}$ spins is proposed. External magnetic fields easily change the helimagnetism into ferromagnetism with fully polarized ${\text{Eu}}^{2+}$ spins, accompanying by disappearance of the resistivity re-entrance. Therefore, superconductivity coexists with ferromagnetic state of ${\text{Eu}}^{2+}$ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$.
83 citations
TL;DR: In this paper, measurements of the anisotropic magnetization and magnetoresistance on single crystals of EuFe2As2, a parent compound of ferro-arsenide high-temperature superconductor, were reported.
Abstract: We report the measurements of the anisotropic magnetization and magnetoresistance (MR) on single crystals of EuFe2As2, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin density wave (SDW) transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu2+ spins (J=S=7/2). The latter AFM state exhibits metamagnetic (MM) transition under magnetic fields. Upon applying magnetic field with H∥c at 2 K, the magnetization increases linearly to 7.0 μB f.u.−1 at μ0H=1.7 T and then remains at this value for saturated Eu2+ moments under higher fields. In the case of H∥ab, the magnetization increases step-like to 6.6 μB f.u.−1 with small magnetic hysteresis. An MM phase was identified with the saturated moments of 4.4 μB f.u.−1. The MM transition accompanies negative in-plane MR, reflecting the influence of Eu2+ moments ordering on the electrical conduction of FeAs layers. These results were explained in terms of spin-reorientation and spin-reversal based on an A-type AFM structure for Eu2+ spins. The magnetic phase diagram has been established.
81 citations
TL;DR: In this article, the Ni-substitution effect in the LaFeAsO system was studied by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity.
Abstract: We have studied Ni-substitution effect in ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Ni}}_{x}\text{AsO}$ $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03\ensuremath{\le}x\ensuremath{\le}0.06$ with the maximum ${T}_{c}$ of 6.5 K at $x=0.04$, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered ${T}_{c}$ in comparison with ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system is discussed.
67 citations
TL;DR: In this paper, the authors used neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe1.9Ni0.1As2(T-c=20 K).
Abstract: We use neutron spectroscopy to determine the nature of the magnetic excitations in superconducting BaFe1.9Ni0.1As2(T-c=20 K). Above T-c the excitations are gapless and centered at the commensurate antiferromagnetic wave vector of the parent compound, while the intensity exhibits a sinusoidal modulation along the c axis. As the superconducting state is entered a spin gap gradually opens, whose magnitude tracks the T dependence of the superconducting gap as observed by angle-resolved photoemission. Both the spin-gap and magnetic-resonance energies are temperature and wave-vector dependent, but their ratio is the same within uncertainties. These results suggest that the spin resonance is a singlet-triplet excitation related to electron pairing and superconductivity.
TL;DR: In this paper, the physical properties of the undoped compound EuFe$2-x}$Ni$(x)$As$2}$ were studied by electrical resistivity, magnetic susceptibility and thermopower measurements.
Abstract: We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$\leq$\emph{x}$\leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $x\geq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $x\geq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.
TL;DR: In this article, the effect of Zn doping on the parent and F-doped LaFeAsO oxy-arsenides was investigated and it was shown that Zn-doping significantly suppresses the resistivity anomaly associated with the antiferromagnetic (AFM) spin density wave (SDW) in the parent compound.
Abstract: We report Zn-doping effect in the parent and F-doped LaFeAsO oxy-arsenides Slight Zn doping in LaFe1- xZnxAsO drastically suppresses the resistivity anomaly around 150 K associated with the antiferromagnetic (AFM) spin density wave (SDW) in the parent compound The measurements of magnetic susceptibility and thermopower confirm further the effect of Zn doping on AFM order Meanwhile Zn doping does not affect or even enhances the Tc of LaFe1-xZnxAsO09F01, in contrast to the effect of Zn doping in high-Tc cuprates We found that the solubility of Zn content (x) is limited to less than 01 in both systems and further Zn doping (ie x ≥01) causes phase separation Our study clearly indicates that the non-magnetic impurity of Zn2+ ions doped in the Fe2As2 layers affects selectively the AFM order and superconductivity remains robust against the Zn doping in the F-doped superconductors
TL;DR: In this paper, the structural phase transition of four parent compounds RFeAsO (R=La, Sm, Gd, and Tb) was measured by low-temperature x-ray diffraction (XRD).
Abstract: We report measurements of structural phase transition of four parent compounds RFeAsO (R=La, Sm, Gd, and Tb) by means of low-temperature x-ray diffraction (XRD). Magnetic transition temperatures associated with Fe ions (T(N1)) are also determined from the
TL;DR: The in-plane thermal conductivity of electron-doped iron-arsenide superconductor BaFe was measured down to 70 mK in zero field and showed a slow field dependence up to 14.5 T in magnetic field as mentioned in this paper, which is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments.
Abstract: The in-plane thermal conductivity $\kappa$ of electron-doped iron-arsenide superconductor BaFe$_{1.9}$Ni$_{0.1}$As$_2$ ($T_c$ = 20.3 K) single crystal was measured down to 70 mK. In zero field, the absence of a residual linear term $\kappa_0/T$ at $ T \to 0$ is strong evidence for nodeless superconducting gap. In magnetic field, $\kappa_0/T$ shows a slow field dependence up to $H$ = 14.5 T ($\approx$ 30% $H_{c_2}$). This is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments in BaFe$_{1.85}$Co$_{0.15}$As$_2$ ($T_c$ = 25.5 K), where isotropic superconducting gaps with similar size on hole and electron pockets were observed.
TL;DR: The in-plane thermal conductivity of electron-doped iron-arsenide superconductor BaFe1.9Ni0.1As2 (Tc=20.3?K) single crystal was measured down to 70?mK.
Abstract: The in-plane thermal conductivity ? of electron-doped iron?arsenide superconductor BaFe1.9Ni0.1As2 (Tc=20.3?K) single crystal was measured down to 70?mK. In zero field, the absence of a residual linear term ?0/T at T?0 is strong evidence for a nodeless superconducting gap. In magnetic field, ?0/T shows a slow field dependence up to H=14.5?T (?30%?Hc2). This is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments in BaFe1.85Co0.15As2 (Tc=25.5?K), where isotropic superconducting gaps with similar size on hole and electron pockets were observed.
TL;DR: In this article, the authors presented a study of resistivity and Seebeck coefficient extended down to sub-Kelvin temperature range uncovering a singular case of anisotropic superconductors.
Abstract: In contrast to almost all anisotropic superconductors, the upper critical field of ${\text{URu}}_{2}{\text{Si}}_{2}$ is larger when the field is oriented along the less conducting direction. We present a study of resistivity and Seebeck coefficient extended down to sub-Kelvin temperature range uncovering a singular case of anisotropy. When the current is injected along the $c$ axis ${\text{URu}}_{2}{\text{Si}}_{2}$ behaves as a low-density Fermi liquid. When it flows along the $a$ axis, even in presence of a large field, resistivity remains T-linear down to ${T}_{c}$ and the Seebeck coefficient undergoes a sign change at very low temperatures. We conclude that the characteristic energy scale is anisotropic and vanishingly small in the basal plane.
TL;DR: In this article, the quasi-particle dynamics of the charge/orbital ordered (COO) and ferromagnetic clusters in the optimally doped manganite, La 0.7Ca 0.3 (LCMO) single crystal by time-resolved two-color pump?probe spectroscopy were investigated.
Abstract: We report on the quasi-particle dynamics of the charge/orbital ordered (COO) and ferromagnetic clusters in the optimally doped manganite, La0.7Ca0.3MnO3 (LCMO) single crystal by time-resolved two-color pump?probe spectroscopy. Pump photons with energies of ~1.55 and ~0.21?eV were employed in our transient optical spectroscopy to investigate the percolative phase separation including the COO and ferromagnetic clusters from 4 to 480?K. At 1.55?eV, the transient reflectivity change, ?R/R, at ?t=0 shows a similar temperature dependence as that in resistivity and in neutron scattering intensity. We attribute the reflectivity signal to the characteristic optical response of the COO domains. We identify a new temperature scale T*~400?K for the clean limit to the formation of COO clusters in LCMO. In contrast, the temperature-dependent amplitude of the transient reflectivity change in the mid-infrared (IR) absorption band (~5??m) scales with the volume fraction of the ferromagnetic metallic (FM) phases. Our results suggest ultrafast optical spectroscopy to be a powerful probe to reveal the correlated polarons and charge disorders in phase-separated manganites.
TL;DR: In this paper, the magnetotransport properties of self-assembled Co@CoO nanoparticle arrays at temperatures below 100 K were investigated and a giant positive magnetoresistance (MR) of >1400% was observed, which decreases with increasing temperature.
Abstract: We report the magnetotransport properties of self-assembled Co@CoO nanoparticle arrays at temperatures below 100 K. Resistance shows thermally activated behavior that can be fitted by the general expression of R∝ exp{(T0/T)ν}. Efros–Shklovskii variable range hopping (ν=1/2) and simple activation (hard gap, ν=1) dominate the high and low temperature region, respectively, with a strongly temperature-dependent transition regime in between. A giant positive magnetoresistance (MR) of >1400% is observed at 10 K, which decreases with increasing temperature. The positive MR and most of its features can be explained by the Zeeman splitting of the localized states that suppresses the spin dependent hopping paths in the presence of on-site Coulomb repulsion.
TL;DR: The structural, magnetic, and transport properties of the manganite system have been investigated by x-ray diffraction, scanning electron microscopy, magnetization, and magnetoresistance measurements.
Abstract: The structural, magnetic, and transport properties of Mo doping La0.67Sr0.33Mn1−xMoxO3 (x=0–0.04) manganite system have been investigated by x-ray diffraction, scanning electron microscopy, magnetization, and magnetoresistance measurements. The Mo doping in Mn site is found to lower the Curie temperature Tc slightly and induce the cluster spin glass behavior in ferromagnetic state of La0.67Sr0.33MnO3. The room temperature magnetoresistance of Mo doping La0.67Sr0.33Mn1−xMoxO3 is found to be 50% higher than that of La0.67Sr0.33MnO3 without Mo doping. The significant enhancement for room temperature magnetoresistance could likely be attributed to the presence of the cluster spin glass state caused by the Mo doping.
TL;DR: In this paper, the effect of magnetic field on the transport properties of polycrystalline parent compound LaFeAsO was investigated and it was shown that magnetic field has no significant influence on the structural phase transitions or spin density wave (SDW) ordering of small moments on iron, but causes large positive MAGento-resistance below 100 K.
Abstract: We report the effect of magnetic field on the transport properties of polycrystalline parent compound LaFeAsO. The magnetic field has no significant influence on the structural phase transitions or spin density wave (SDW) ordering of small moments on iron, but causes large positive magento-resistance below 100 K. Meanwhile, the absolute magnitude of thermopower, which is negative, increases remarkably with increasing magnetic field below 100 K. Such a large positive magneto-thermopower was first reported in this iron arsenide. The result suggests that there could exist an enhancement of SDW gapping in holelike Fermi surface.
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TL;DR: In this paper, the spin-flip scattering between the charge carriers and the local moments in the undoped FeAs layer where Kondo effect coexists with the SDW was investigated.
Abstract: The normal state of the iron arsenides shows the poor metallic behavior mixed with strong magnetic fluctuations. In particular, some FeAs-1111 and FeAs-122 compounds show the linear-T dependence of susceptibility above the spin-density wave (SDW) transition and the logarithmic upturn of resistivity at low temperatures. We suggest that this is due to the spin-flip scattering between the charge carriers and the local moments in the undoped FeAs layer where Kondo effect coexists with the SDW. This scenario is also accounted for the change of the magnetoresistance from positive to negative in the Sr3Sc2O5Fe2As2 compound.