J. T. Park

Commensurate antiferromagnetic excitations as a signature of the pseudogap in the tetragonal high-Tc cuprate HgBa2CuO4+δ

Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa2CuO4+δ. Here we report neutron-scattering results for HgBa2CuO4+δ (superconducting transition temperature Tc≈71u2009K, pseudogap temperature T*≈305u2009K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped ‘hourglass response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped and significantly enhanced below T*, and hence a prominent signature of the pseudogap state.

Impact of uniaxial pressure on structural and magnetic phase transitions in electron-doped iron pnictides

We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural (Ts) and antiferromagnetic (AF) phase transitions in iron pnictides BaFe2−xNixAs2 (x = 0,0.03,0.12), SrFe1.97Ni0.03As2 ,a nd BaFe 2(As0.7P0.3)2. In antiferromagnetically ordered BaFe2−xNixAs2 and SrFe1.97Ni0.03As2 with TN and Ts (TN Ts), a uniaxial pressure necessary to detwin the sample also increases TN , smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

Transition from Sign-Reversed to Sign-Preserved Cooper-Pairing Symmetry in Sulfur-Doped Iron Selenide Superconductors.

An essential step toward elucidating the mechanism of superconductivity is to determine the sign or phase of the superconducting order parameter, as it is closely related to the pairing interaction. In conventional superconductors, the electron-phonon interaction induces attraction between electrons near the Fermi energy and results in a sign-preserved s-wave pairing. For high-temperature superconductors, including cuprates and iron-based superconductors, prevalent weak coupling theories suggest that the electron pairing is mediated by spin fluctuations which lead to repulsive interactions, and therefore that a sign-reversed pairing with an s_{±} or d-wave symmetry is favored. Here, by using magnetic neutron scattering, a phase sensitive probe of the superconducting gap, we report the observation of a transition from the sign-reversed to sign-preserved Cooper-pairing symmetry with insignificant changes in T_{c} in the S-doped iron selenide superconductors K_{x}Fe_{2-y}(Se_{1-z}S_{z})_{2}. We show that a rather sharp magnetic resonant mode well below the superconducting gap (2Δ) in the undoped sample (z=0) is replaced by a broad hump structure above 2Δ under 50% S doping. These results cannot be readily explained by simple spin fluctuation-exchange pairing theories and, therefore, multiple pairing channels are required to describe superconductivity in this system. Our findings may also yield a simple explanation for the sometimes contradictory data on the sign of the superconducting order parameter in iron-based materials.

Orbital selective neutron spin resonance in underdoped superconducting NaFe 0.985 Co 0.015 As

We use neutron scattering to study the electron-doped superconducting NaFe

Neutron spin resonance as a probe of superconducting gap anisotropy in partially detwinned electron underdoped NaFe 0.985 Co 0.015 As

_{0.985}

Structure of spin excitations in heavily electron-doped Li0.8Fe0.2ODFeSe superconductors

Co

Direct observation of spin excitation anisotropy in the paramagnetic orthorhombic state of BaFe2−xNixAs2

_{0.015}

Coexistence of Ferromagnetic and Antiferromagnetic Fluctuations in Iron-germanide Superconductor

As (

Spin reorientation transition in Na‐doped BaFe2As2 studied by single‐crystal neutron diffraction

T_c=14

Effect of nematic order on the low-energy spin fluctuations in detwinned BaFe

K), which has co-existing static antiferromagnetic (AF) order (