d=3 random field behavior near percolation

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering for H>0. A low-temperature (T<11K), low-field (H<1T) pseudophase transition boundary separates a partially antiferromagnetically ordered phase from the paramagnetic one. For 1<H<7T at low temperatures, a region of antiferromagnetic order is field induced but is not enclosed within a transition boundary.Comment: 9 pages, 4 figure

Evolution of spin-wave excitations in ferromagnetic metallic manganites

Neutron scattering results are presented for spin-wave excitations of three ferromagnetic metallic $A_{1-x}A^{\prime}_{x}$MnO$_3$ manganites (where $A$ and $A^\prime$ are rare- and alkaline-earth ions), which when combined with previous work elucidate systematics of the interactions as a function of carrier concentration $x$, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series. The ratio of fourth to first neighbor exchange ($J_4/J_1$) that controls the zone boundary magnon softening changes systematically with $x$, but does not depend on the other parameters. None of the prevailing models can account for these behaviors.Comment: Submitted to Phys. Rev. Let

Zn-induced spin dynamics in overdoped La2−x_{2-x}Srx_xCu1−y_{1-y}Zny_yO4_4

Spin fluctuations and the local spin susceptibility in isovalently Zn-substituted La$_{2-x}$Sr$_{x}$Cu$_{1-y}$Zn$_y$O$_4$ ($x=0.25$, $y\approx0.01$) are measured via inelastic neutron scattering techniques. As Zn$^{2+}$ is substituted onto the Cu$^{2+}$-sites, an anomalous enhancement of the local spin susceptibility $\chi^{\prime\prime}(\omega)$ appears due to the emergence of a commensurate antiferromagnetic excitation centered at wave vector \textbf{Q}$=(\pi, \pi, 0)$ that coexists with the known incommensurate SDW excitations at \textbf{Q}$_{HK}=(\pi\pm\delta,\pi), (\pi,\pi\pm\delta)$. Our results support a picture of Zn-induced antiferromagnetic (AF) fluctuations appearing through a local staggered polarization of Cu$^{2+}$-spins, and the simultaneous suppression of T$_c$ as AF fluctuations are slowed in proximity to Zn-impurities suggests the continued importance of high energy AF fluctuations at the far overdoped edge of superconductivity in the cuprates.Comment: 10 pages, 8 figure

Electron doping evolution of the neutron spin resonance in NaFe1−x_{1-x}Cox_{x}As

Neutron spin resonance, a collective magnetic excitation coupled to superconductivity, is one of the most prominent features shared by a broad family of unconventional superconductors including copper oxides, iron pnictides, and heavy fermions. In this work, we study the doping evolution of the resonances in NaFe$_{1-x}$Co$_x$As covering the entire superconducting dome. For the underdoped compositions, two resonance modes coexist. As doping increases, the low-energy resonance gradually loses its spectral weight to the high-energy one but remains at the same energy. By contrast, in the overdoped regime we only find one single resonance, which acquires a broader width in both energy and momentum, but retains approximately the same peak position even when $T_c$ drops by nearly a half compared to optimal doping. These results suggest that the energy of the resonance in electron overdoped NaFe$_{1-x}$Co$_x$As is neither simply proportional to $T_c$ nor the superconducting gap, but is controlled by the multi-orbital character of the system and doped impurity scattering effect.Comment: accepted by PR