Chenglin Zhang

Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides

High-temperature superconductivity in iron pnictides occurs when electrons and holes are doped into their antiferromagnetic parent compounds. Since spin excitations may be responsible for electron pairing and superconductivity, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe2As2 parent compound modifies the low-energy spin excitations and their correlation with superconductivity (<50 meV) without affecting the high-energy spin excitations (>100 meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.Meng Wang∗,1 Chenglin Zhang∗,2 Xingye Lu∗,1, 2 Guotai Tan,2 Huiqian Luo,1 Yu Song,2 Miaoyin Wang,2 Xiaotian Zhang,1 E. A. Goremychkin,3 T. G. Perring,3 T. A. Maier,4 Zhiping Yin,5 Kristjan Haule,5 Gabriel Kotliar,5 and Pengcheng Dai2, 1 1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China 2 Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA 3 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK 4 Center for Nanophase Materials Sciences and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6494, USA 5 Department of Physics, Rutgers University, Piscataway, NJ 08854, USA

Spin waves and magnetic exchange interactions in insulating Rb 0.89 Fe 1.58 Se 2

The parent compounds of iron pnictide superconductors are bad metals with a collinear antiferromagnetic structure and Néel temperatures below 220 K. Although alkaline iron selenide A(y)Fe(1.6+x)Se(2) (A=K, Rb, Cs) superconductors are isostructural with iron pnictides, in the vicinity of the undoped limit they are insulators, forming a block antiferromagnetic order and having Néel temperatures of roughly 500 K. Here we show that the spin waves of the insulating antiferromagnet Rb(0.89)Fe(1.58)Se(2) can be accurately described by a local moment Heisenberg Hamiltonian. A fitting analysis of the spin wave spectra reveals that the next-nearest neighbour couplings in Rb(0.89)Fe(1.58)Se(2), (Ba,Ca,Sr)Fe(2)As(2), and Fe(1.05)Te are of similar magnitude. Our results suggest a common origin for the magnetism of all the Fe-based superconductors, despite having different ground states and antiferromagnetic orderings.

Electron-doping evolution of the low-energy spin excitations in the iron arsenide superconductor BaFe2-xNixAs2

BaFe1.9Ni0.1As2 Tc =2 0 K and overdoped BaFe1.85Ni0.15As2 Tc =1 4 K superconductors, the static AF longrange order is completely suppressed and the spin excitation spectra are dominated by a resonance and spin gap at lower energies. We determine the electron-doping dependence of the neutron spin resonance and spin gap energies and demonstrate that the three-dimensional nature of the resonance survives into the overdoped regime. If spin excitations are important for superconductivity, these results would suggest that the threedimensional characters of the electronic superconducting gaps are prevalent throughout the phase diagram and may be critical for superconductivity in these materials.

Neutron Scattering Studies of spin excitations in hole-doped Ba0.67K0.33Fe2As2 superconductor

We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.

Spin Excitation Anisotropy as a Probe of Orbital Ordering in the Paramagnetic Tetragonal Phase of Superconducting BaFe1:904Ni0:096As2

We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconducting BaFe1.904Ni0.096As2 (Tc=19.8  K) change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic (AFM) ordering and tetragonal-to-orthorhombic lattice distortion temperatures (TN≈Ts=33±2  K) without an uniaxial pressure. While the anisotropic spin excitations are not sensitive to the AFM order and tetragonal-to-orthorhombic lattice distortion, superconductivity induces further anisotropy for spin excitations along the [110] and [110] directions. These results indicate that the spin excitation anisotropy is a probe of the electronic anisotropy or orbital ordering in the tetragonal phase of iron pnictides.

Critical quadrupole fluctuations and collective modes in iron pnictide superconductors

The multiband nature of iron pnictides gives rise to a rich temperature-doping phase diagram of competing orders and a plethora of collective phenomena. At low dopings, the tetragonal-to-orthorhombic structural transition is closely followed by a spin-density-wave transition both being in close proximity to the superconducting phase. A key question is the nature of high-

Anisotropic neutron spin resonance in superconducting BaFe(1.9)Ni(0.1)As(2)

{T}_{c}

In-plane spin excitation anisotropy in the paramagnetic state of NaFeAs

superconductivity and its relation to orbital ordering and magnetism. Here we study the

Distinguishing s(+/-) and s(++) electron pairing symmetries by neutron spin resonance in superconducting NaFe0.935Co0.045As

{\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}

Antiferromagnetic order and superlattice structure in nonsuperconducting and superconducting RbyFe1.6+xSe2

superconductor using polarization-resolved Raman spectroscopy. The Raman susceptibility displays critical enhancement of nonsymmetric charge fluctuations across the entire phase diagram, which are precursors to a