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Recent observations of selective emergence (suppression) of superconductivity in the uncollapsed (collapsed) tetragonal phase of ${\mathrm{LaFe}}_{2}{\mathrm{As}}_{2}$ has rekindled interest in understanding what features of the band structure control the superconducting ${T}_{c}$. We show that the proximity of the narrow $\mathrm{Fe}\text{\ensuremath{-}}{d}_{xy}$ state to the Fermi energy emerges as the primary factor. In the uncollapsed phase this state is at the Fermi energy, and is most strongly correlated and a source of enhanced scattering in both single and two particle channels. The resulting intense and broad low energy spin fluctuations suppress magnetic ordering and simultaneously provide glue for Cooper pair formation. In the collapsed tetragonal phase, the ${d}_{xy}$ state is driven far below the Fermi energy, which suppresses the low-energy scattering and blocks superconductivity. A similar source of broad spin excitation appears in uncollapsed and collapsed phases of ${\mathrm{CaFe}}_{2}{\mathrm{As}}_{2}$. This suggests controlling coherence provides a way to engineer ${T}_{c}$ in unconventional superconductors primarily mediated through spin fluctuations.

We experimentally demonstrate the generation of heralded bi-chromatic single photons from an atomic collective spin excitation (CSE). The photon arrival times display collective quantum beats, a novel interference effect resulting from the relative motion of atoms in the CSE. A combination of velocity-selective excitation with strong laser dressing and the addition of a magnetic field allows for exquisite control of this collective beat phenomenon. The present experiment uses a diamond scheme with near-IR photons that can be extended to include telecommunications-wavelengths or modified to allow storage and retrieval in an inverted-Y scheme., 11 pages, 6 figures

Carrier doping effects on spin dynamics in ${\mathrm{Sr}}_{14\ensuremath{-}x}{A}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ ( $A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Ca}$, Y, and La) are investigated by Cu NMR and NQR. The energy gaps in the spin excitation spectra (spin gap) are confirmed to be $\ensuremath{\Delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}140\mathrm{K}$ for the Cu${\mathrm{O}}_{2}$ chain and $\ensuremath{\Delta}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}470\mathrm{K}$ for the C${\mathrm{u}}_{2}$${\mathrm{O}}_{3}$ ladder in S${\mathrm{r}}_{14}$C${\mathrm{u}}_{24}$${\mathrm{O}}_{41}$. The spin gap for the ladder increases with the Y and La substitutions for Sr (up to $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$), while it decreases with increasing x in ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ and seems to collapse around $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}13$ of the Ca substitution. Hole-doping effects on the spin gaps for the ladder configuration are interpreted in terms of magnon and hole-depairing excitations for the spin $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{1}{2}$ ladder.

Polarized and unpolarized neutron scattering have been used to determine the effect of superconductivity on the magnetic excitation spectra of YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{6.5}$ $({T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}52\phantom{\rule{0ex}{0ex}}\mathrm{K})$ and YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{6.7}$ $({T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}67\phantom{\rule{0ex}{0ex}}\mathrm{K})$. Pronounced enhancements of the spectral weight centered around 25 and 33 meV, respectively, are observed below ${T}_{c}$ in both crystals, compensated predominantly by a loss of spectral weight at higher energies. The data provide important clues to the origin of the 40 meV magnetic resonance peak in YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{7}$.

Using data recorded by the CLEO-II detector at CESR, we report the first observation of a narrow state decaying into $\Xi_c^+\pi^-$. The state has mass difference $M(\Xi_c^+\pi^-)-M(\Xi_c^+)$ of $178.2\pm0.5\pm1.0$ $\rm{MeV/c^2}$, and a width of $

Magnetic properties of Sr${\mathrm{Cu}}_{2}$${\mathrm{O}}_{3}$ and ${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{5}$, containing two-leg and three-leg $S=\frac{1}{2}$ ladders made of antiferromagnetic Cu-O-Cu linear bonds, were investigated. The susceptibility of the two-leg ladder material, Sr${\mathrm{Cu}}_{2}$${\mathrm{O}}_{3}$, is characteristic of thermal excitation from a nonmagnetic ground state with a spin gap of about 420 K, while the susceptibility of ${\mathrm{Sr}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{5}$, containing three-leg ladders, reflects a gapless spin excitation spectrum. The temperature dependence of the nuclear-spin lattice relaxation rate, $\frac{1}{{T}_{1}}$, of $^{63}\mathrm{Cu}$ NMR also indicates the existence of a spin gap only for Sr${\mathrm{Cu}}_{2}$${\mathrm{O}}_{3}$. The spin gap we observed for Sr${\mathrm{Cu}}_{2}$${\mathrm{O}}_{3}$ confirms a recent theoretical prediction for the magnetic behavior of spin = \textonehalf{} antiferromagnetically coupled Heisenberg chains.

By resonant inelastic x-ray scattering in the soft x-ray regime we probe the dynamical multiple-spin correlations in the antiferromagnetic cuprates La2CuO4 and CaCuO2. High resolution measurements at the copper L3 edge allow the clear observation of dispersing bimagnon excitations. Theory based on the ultrashort core-hole lifetime expansion fits the data on these coherent spin excitations without free parameters.

The recent observation of resonance spin excitation at (1/2, 1/2, 1/2) in the superconducting state of CeCoIn5 [C. Stock, Phys. Rev. Lett. 100, 087001 (2008)10.1103/Phys. Rev. Lett.100.087001] was interpreted as evidence for d{x{2}-y{2}} gap symmetry, by analogy with cuprates. This is true if the resonance is a spin exciton. We argue that such a description is undermined by the three dimensionality of CeCoIn5. We show that in 3D systems the excitonic resonance only emerges at strong coupling, and is weak. We argue in favor of the alternative, magnon scenario, which does not require a d{x{2}-y{2}} gap.

There was a slight error in calculating the nonlinear sigma-model prediction for the 2-magnon scattering continuum shown in Fig. 2 of cond-mat/0010049, and Phys. Rev. Lett. {\bf 87}, 017202 (2001). Present erratum explains what was incorrect, and shows a corrected version of the figure. The correction is minor and does not affect main conclusions drawn in the original paper.

In Sr2RuO4 the spin excitation spectrum is dominated by incommensurate fluctuations at q = (0.3 0.3q(z)), which arise from Fermi-surface nesting. We show that upon Ti substitution, known to suppress superconductivity, a short range magnetic order develops with a propagation vector (0.307 0.307 1). In Sr2Ru0.91Ti0.09O4, the ordered moment points along the c direction. This finding shows that superconducting Sr2RuO4 is extremely close to an incommensurate spin density wave instability.